E. Wul�-Pedersen á E.-R. Neumann á R. VannucciP. Bottazzi á L. Ottolini

Silicic melts produced by reaction between peridotiteand in®ltrating basaltic melts: ion probe data on glasses and mineralsin veined xenoliths from La Palma, Canary Islands

Received: 15 November 1998 /Accepted: 17 May 1999

Abstract Mantle xenoliths hosted by the historic Vol-can de San Antonio, La Palma, Canary Islands includeveined spinel harzburgites and spinel dunites. Glassesand associated minerals in the vein system of veinedxenoliths show a gradual transition in composition frombroad veins to narrow veinlets. Broad veins contain al-kali basaltic glass with semi-linear trace element pat-terns enriched in strongly incompatible elements. As theveins become narrower, the SiO2-contents in glass in-crease (46 ® 67 wt% SiO2 in harzburgite, 43 ® 58 wt%in dunite) and the trace element patterns changegradually to concave patterns depleted in moderatelyincompatible elements (e.g. HREE, Zr, Ti) relative tohighly incompatible ones. The highest SiO2-contents (ca.68% SiO2, low Ti-Fe-Mg-Ca-contents) and most ex-treme concave trace element patterns are exhibited byglass in unveined peridotite xenoliths. Clinopyroxenesshift from LREE-enriched augites in basaltic glass, toREE-depleted Cr-diopside in highly silicic glass. Esti-mates indicate that the most silicic glasses representmelts in, or near, equilibrium with their host peridotites.The observed trace element changes are compatible withformation of the silicic melts by processes involvingin®ltration of basaltic melts into mantle peridotitefollowed by reactions and crystallization. The Fe-Mginterdi�usion pro®les in olivine porphyroclasts adjacent

to the veins indicate a minimum period of di�usion of600 years, implying that the reaction processes have ta-ken place in situ in the upper mantle. The CaO-TiO2-La/Nd relationships of mantle rocks may be used to dis-criminate between metasomatism caused by carbonatiticand silicic melts. Unveined mantle xenoliths from LaPalma and Hierro (Canary Islands) show a wide range inLa/Nd ratios with relatively constant, low-CaO contentswhich is compatible with metasomatism of ``normal''abyssal peridotite by silicic melts. Peridotite xenolithsfrom Tenerife show somewhat higher CaO and TiO2

contents than those from the other islands and may havebeen a�ected by basaltic or carbonatitic melts. Theobserved trace element signatures of ultrama®c xenolithsfrom La Palma and other Canary Islands may beaccounted for by addition of small amounts (1±7%) ofhighly silicic melt to unmetasomatized peridotite. Alsoultrama®c xenoliths from other localities, e.g. easternAustralia, show CaO-TiO2-La/Nd relationships com-patible with metasomatism by silicic melts. These resultssuggest that silicic melts may represent important meta-somatic agents.

Introduction

Metasomatism is generally recognized as a very impor-tant process in the upper mantle. However, the transportagents causing di�erent types of mantle metasomatismhave been the subject of considerable debate over theyears. H2O- and CO2-rich ¯uids as well as basaltic andcarbonatitic melts have been proposed as media capableof causing cryptic metasomatism in mantle peridotites(e.g. Francis 1976; Harte 1983; Dawson 1984; Eggler1987; Menzies et al. 1987; Wilshire 1987; Nielson andNoller 1987). Lately it has been recognized that in addi-tion to such ¯uids and melts, silicic melts (60±72 wt%SiO2) are frequently present in the upper mantle. Silicicmelts are observed as glass inclusions and interstitial glasspockets in spinel-bearing harzburgite, lherzolite and

Contrib Mineral Petrol (1999) 137: 59 ± 82 Ó Springer-Verlag 1999

E. Wul�-Pedersen á E.-R. Neumann (&)Mineralogical-Geological Museum, University of Oslo,Sarsgt. 1, N-0562 Oslo,Norway

R. VannucciDipartimento di Scienze della Terra,UniversitaÁ di Pavia,via Ferrata 1, I-27100 Pavia,Italy

R. Vannucci á P. Bottazzi á L. OttoliniCNR-Centro di Studio per la Cristallochimica e la Cristallogra®a,via Ferrata 1, I-27100 Pavia,Italy

Editorial responsibility: J. Touret

dunite xenoliths in numerous localities around the world(e.g. Frey andGreen 1974; Francis 1976, 1987; Jones et al.1983; Siena et al. 1991; Ionov et al. 1994; Schiano et al.1992, 1994; Schiano and Clocchiatti 1994; Neumann andWul�-Pedersen 1995; Schiano et al. 1995; Zinngrebe andFoley 1995; Wul�-Pedersen et al. 1996a). It has also beenproposed that silicic melts may act as metasomatictransport agents (Zinngrebe and Foley 1995; Wul�-Pedersen et al. 1996a; Draper and Green 1997).

Mantle xenoliths from di�erent Canary Islands showextensive evidence of pervasive metasomatism in theunderlying lithospheric mantle, including enrichment inincompatible elements (cryptic metasomatism) (Neu-mann 1991; Neumann et al. 1995; Whitehouse andNeumann 1995; Wul�-Pedersen et al. 1996a; Neumannand Wul�-Pedersen 1997; E.-R. Neumann, unpublisheddata). These peridotite xenoliths also frequently exhibitbasaltic to silicic glasses, representing trapped melts(Neumann and Wul�-Pedersen 1997). Silicic glasses arefar more abundant than basaltic ones. Wul�-Pedersenet al. (1996a) presented major element data on mineralsand glasses in veined spinel peridotites from La Palma,Canary Islands. The glasses in these xenoliths show agradual transition from alkali basaltic (ca. 43 wt% SiO2,Ti-Fe-rich, TiO2/Al2O3 > 0.15) in broad veins, to highlysilicic (ca. 67% SiO2, Ti-Fe-Mg-Ca-poor, TiO2/Al2O3< 0.08) in narrow veinlets penetrating harzburgitefragments. The mineral-melt relations and contact rela-tions between glass and peridotite minerals found in thevein systems of veined xenoliths closely resemble thosefound for unveined Canary Islands xenoliths. The ob-served shift in glass composition and accompanyingchanges in phase assemblage and mineral compositionswere interpreted as the results of reactions between in-®ltrating TiO2-rich alkali basaltic melts and refractoryperidotite wall-rock (IRC-processes = in®ltration ± re-action ± crystallization). Neumann and Wul�-Pedersen(1997) concluded that IRC-processes represent the gen-eral mode of formation of silicic melts in the uppermantle under the Canary Islands, and that these meltsmay represent important metasomatic agents. The IRC-processes are thus clearly widespread and important inthe upper mantle under the Canary Islands, and may alsorepresent important mantle processes in general. Theveined xenoliths are believed to demonstrate on a smallscale general reaction processes which take place whenTiO2-rich alkali basaltic melts in®ltrate and react withrefractory peridotite mantle wall-rock.

The petrography and major element chemistry of theveined xenoliths, and the sequence of reactions takingplace between in®ltrating melts and mantle wall-rockswere described in detail by Wul�-Pedersen et al. (1996a).That paper also included major and trace elementwhole-rock data and major element mineral data onunveined mantle xenoliths from La Palma. Vannucciet al. (1998) discussed the partitioning of REE (rareearth elements), Y, Sr, Zr and Ti between clinopyroxeneand silicate melts of di�erent compositions in veined andunveined mantle xenoliths from La Palma. However,

neither of those papers presented a general discussion ofthe trace element compositions of glasses and coexistingminerals in mantle xenoliths from the Canary Islands,nor their implications for IRC-processes and mantlemetasomatism. The present investigation was undertak-en in order to: (1) describe the change in trace elementcompositions of basaltic to highly silicic glasses and as-sociated minerals in the vein system of the veined xeno-liths from La Palma; (2) use this information to obtaininformation about the e�ect of IRC-processes on traceelements; (3) compare the trace element compositions ofglasses in the vein systems of veined xenoliths with thoseof glasses in unveined xenoliths; (4) compare the expectedmetasomatic e�ect of silicic melts with that of othertransport agents believed to cause mantle metasomatism;(5) discuss the transport agent(s) causing metasomaticenrichment in the upper mantle under La Palma.

Analytical methods

Minerals and glasses in polished thin sections were analysed formajor elements using an automatic wavelength-dispersiveCAMECA electron microprobe ®tted with a LINK energy-dis-persive system at the Mineralogical-geological Museum in Oslo.Analyses were run with an acceleration voltage of 15 keV, samplecurrents of 20 nA for Na-poor (olivine, pyroxene, spinel) and10 nA for Na-rich phases (glass), and counting times of 100 sec-onds. Oxides and natural and synthetic minerals were used asstandards. Matrix corrections were performed by the PAP-proce-dure in the CAMECA software. Analytical precision and accuracy(2r) evaluated by repeated analysis of individual grains and stan-dards are better than �1% for elements in concentrations of> 20 wt% oxide, better than �2% for elements in the range 10±20 wt% oxide, better than 5% for elements in the range 2±10 wt%oxide, and better than 10% for elements in the range 0.5±2 wt%oxide. The low sums obtained for some of the glasses (Table 1)cannot be accounted for by their H2O-contents.

Trace elements (K, Sc, Ti, V, Cr, Rb, Sr, Y, Zr, Nb, Cs, Ba, and9 selected REE), and F and H2O content in minerals and glasseswere determined using the Cameca IMS 4f ion microprobe atCSCC, Pavia. The so-called ``energy-®ltering'' technique (Shimizuet al. 1978) was applied by analysing high-energy ions consistingmostly of monoatomic species (see Bottazzi et al. 1994, for details).Residual BaO+ and NdO+ interferences on Eu+ and Gd+, re-spectively, were removed using the peak-stripping procedures de-scribed in Bottazzi et al. (1994). Both H2O and F were acquiredunder the same measurement conditions and quanti®ed accordingto the analytical protocol reported in Ottolini et al. (1995). Con-version of ion intensities into concentrations was accomplished byusing both a major element as internal standard (Si) and severalsilicate reference samples as external standards. Accuracy of ionprobe data is better than 10% for concentrations greater thanabout 2 times the chondritic value of the element, with the excep-tions F, Rb, and Nb (accuracy �25%). Below this concentrationlevel, precision and hence accuracy are limited by counting statis-tics; for instance, the standard deviation is �30% at 0.2 times thechondritic value. For H2O, accuracy is 15% for samples withSiO2<55%, and �25% for high-silica glasses (Ottolini et al. 1995).The REE in phlogopite are below detection limits.

Petrography and major element relations

The xenoliths discussed in this paper were collected fromalkaline basaltic lavas from the historic volcano, Volcan

60

de San Antonio, situated in the southernmost part of LaPalma. The dominating type of peridotitic xenolithsfrom Volcan de San Antonio is refractory spinel ha-rzburgite, but also spinel lherzolite, spinel dunite andwehrlite are found. The harzburgites and lherzolitesconsist of olivine (Fo89.8±91.2; 63±87 vol.%), orthopy-roxene (6±34%), Cr-diopside (1.2±6.6%), and chromite(0.2±2.5%); about half the xenoliths contain minoramounts of phlogopite (0.0±0.7%). In addition to oli-vine (Fo88.6±90.7), the dunites contain 0.2±4.0% Cr-diopside and 0.5±4.8% chromite; the majority carriesphlogopite (up to 4.0%), and orthopyroxene is presentin a few samples (up to 3.4%). The wehrlites consist of52±73% olivine (Fo79.7±89.4), 13±29% clinopyroxene, 7±23% kaersutite and 0.7±6.4% spinel. Petrographic rela-tions, major element compositions of minerals andglasses, and whole-rock major and trace element chem-istry were presented and discussed by Wul�-Pedersenet al. (1996a). In addition to the presence of hydrousminerals in many samples, they found the di�erent typesof peridotites to be enriched in highly lithophile elementsand depleted in moderately incompatible elements ascompared to N-type (``normal MORB'')(mid oceanridge basalt)-mantle. On the basis of modal relations,and whole rock and mineral chemistry Wul�-Pedersenet al. (1996a) concluded that spinel harzburgites, lher-zolites and dunites from La Palma represent old, oceanicmantle lithosphere which has been metasomatized dur-ing the Canary Islands magmatism. The Fe-Ti-rich we-hrlites are interpreted as cumulates and reactionproducts formed from melts and ¯uids associated withthe Canary Islands magmatism (Wul�-Pedersen et al.1996a).

The description of mantle xenoliths from La Palmagiven by Wul�-Pedersen et al. (1996a) includes detaileddata on two veined xenoliths, one spinel harzburgite(PAT2-4), and one spinel dunite (PAT2-62). The phaseassemblage and composition of each phase changegradually through the vein system as the diameter ofthe vein decreases. On the basis of phase assemblages thevein systems have been divided into ®ve main types. Thepetrography and relationship between the di�erent typesare described below and presented schematically inFig. 1. Major and trace element data on glasses andminerals are listed in Table 1.

Veined spinel harzburgite (PAT2-4)

This consists of spinel harzburgite (Fo89.9) cut by anetwork of broad and narrow veins (Fig. 2a):

1. Amphibole veins (amph-veins) are 1±3 mm wide(marked I in Fig. 2a) and contain mainly euhedralkaersutite, and minor amounts of augite, olivine, Fe-Ti-oxide, phlogopite, traces of apatite, and brownish ba-saltic glass.

2. Branching o� from the amph-veins are 0.1±2.0 mmwide phlogopite-amphibole veins (phlog-amph veins;marked II in Fig. 2a) which are made up by granular

phlogopite, euhedral to subhedral kaersutitic to par-gasitic amphibole, minor amounts of clinopyroxene, andpale brown glass (Figs. 2a±c).

3. The contacts between amph-veins and phlog-amphveins and peridotite fragments are marked by reactionzones consisting of intergrowths of olivine + Cr-diop-side + phlogopite or olivine + Cr-diopside along or-thopyroxene porphyroclasts.

4. As the phlog-amph veins become narrower withincreasing distance from their intersections with amph-veins, amphibole, augite and Fe-Ti-oxide disappear, andthe phase assemblage changes to phlogopite, Cr-diop-side, chromite, and colourless glass with numerous ¯uidvesicles (<0.1 mm long, now empty). These narrowveinlets are called phlogopite-veins (phlog-veins;Figs. 2d, e).

5. Very narrow, glass-dominated phlog-veins appearto continue into thin glass ``®lms'' along grain bound-aries (Fig. 2f) and interstitial ``glass pockets'' insideperidotite fragments. Such glass occurrences are referredto as INF-v (in®ltrated peridotite in veined xenoliths;marked III in Fig. 2a).

As far as we have been able to ascertain, ¯uid inclusionsin the peridotite fragments, like ¯uid inclusions in gen-eral in unveined xenoliths from the Canary Islands,contain pure CO2.

From amph-veins to INF-v the SiO2-Al2O3-Na2O-K2O contents of glasses increase, and Ti-Fe-Mg-Ca de-crease. However, at SiO2 contents above about 53 wt%,Al2O3, Na2O, and K2O tend to decrease with increasing

amph-veinkaers(+aug+ol+mt

phlog ap hau)+brown glass

+ + +

phlog-amph veinphlog+kaers(+aug)+ pale brown glass

phlog-vein

phlog-vein

phlog( Cr-di chr)+ colorless glass

+ +INF-vcolorlessglass

INF-v

phlog-vein reaction zone

++

+

ol+Cr-di kaers phlog

reaction zone

PERIDOTITEWALL-ROCK

+mt+pale glass

Fig. 1 Schematic presentation of the vein system (dark gray) withreaction zones (light gray) in the veined xenoliths (host peridotite iswhite)

61

SiO2 (Wul�-Pedersen et al. 1996a; Table 1). ColourlessINF-v glass in central parts of xenolith fragments con-tains about 67 wt% SiO2, 2% MgO, 7% K2O, 5%Na2O, and <1% TiO2 (Table 1). Signi®cant changes inglass compositions are seen over distances >1 mm. In-creasing SiO2 concentrations in glass are accompaniedby decreasing Ti-Fe, and increasing Mg-Cr-contents incoexisting vein minerals, and the most silicic glassescontain Ti-Al-poor Cr-diopside and chromite of com-positions typical of refractory spinel harzburgites.Phlogopite shows a gradual shift in composition from3.9 wt% TiO2 and mg#[Mg/(Mg+Fe)]=0.75 in amph-veins to 0.35 wt% TiO2 and mg# = 0.90 in the nar-rowest phlog-veins.

Veined spinel dunite (PAT2-62)

This consists of fragments of olivine porphyroclasts andolivine aggregates (Fo91.1) cut by glass-bearing veins.The vein types in this sample are the same as for PAT2-4, but the phase assemblages show some di�erences:

1. Amph-veins contain coarse-grained, euhedral to sub-hedral kaersutite (up to 4.0 mm long, with small,equant oxide inclusions), apatite, Fe-Ti-oxides, andbrown glass; hauÈ yne is present locally.

3. Contacts between amph-veins and phlog-amph veinsand peridotite fragments are marked by an outerreaction zone composed of medium- to ®ne-grainedamphibole-augite aggregates (against the vein); and

Glass Spinel harzburgite

Sample PAT2-4Veined

PAT2-68Unveined

PAT2-41Unveined

Locationtype

Phlogamphvein

Phlogamphvein

Phlogamphvein

Reactionzone

Reactionzone

Reactionzone

Phlogvein

INF-v INF-u INF-u INF-u

Point 6±22 6±19 3±16 2±17 2±13 2±16 14 15wr 1 2 1

SiO2 46.55 47.49 53.45 54.59 58.89 61.18 56.05 66.56 67.42 66.69 65.64TiO2 3.45 3.15 1.12 0.81 0.38 0.16 0.79 0.85 ND ND 0.06Al2O3 17.34 17.33 19.85 20.56 19.25 14.91 18.43 13.74 17.09 17.37 16.64FeO 10.23 10.22 4.21 1.53 2.18 2.67 3.15 2.46 1.57 1.34 1.76MnO 0.19 0.19 0.21 0.01 ND ND 0.10 0.01 ND 0.01 0.12MgO 3.22 3.36 2.56 2.03 1.36 3.85 2.55 1.73 1.43 1.30 1.38CaO 8.68 8.75 3.80 2.41 2.18 0.67 2.21 1.15 1.19 1.14 1.24Na2O 5.30 4.91 5.88 5.10 4.21 6.35 6.11 4.65 7.16 7.31 7.51K2O 3.09 3.13 8.13 7.69 7.15 6.98 7.52 7.32 4.64 4.45 4.65P2O5 0.61 0.66 0.17 ND 0.08 ND 0.17 0.04 ND ND 0.06F 0.11 0.08 0.04 0.04Cl 0.16 0.09 0.10 0.02 0.07 0.19H2O 0.36 0.34 0.21 0.24Sum 98.82 99.28 99.48 94.75 95.75 96.96 97.55 98.93 100.75 99.89 99.06

ppmNa 44508 36719 52816 53781K 67379 59451 41410 40179 37906Sc ND 16 11 9 11Ti 4293 4957 146 138 453V 89 148 108 105 131Cr 17 516 30 25 29Rb 274 208 131 124 124Sr 582 333 497 458 596Y 9.5 4.3 0.7 0.7 3.3Zr 1076 346 2.5 2.2 14Nb 110 41 1.1 1 11Cs 2.9 1.4 1.4 1 2.8Ba 2503 973 420 375 579La 114 64 5 4.5 25Ce 128 67 4.1 3.4 37Nd 24 8.8 0.73 0.62 9.9Sm 2.9 1.16 0.07 0.05 1.3Eu 0.65 0.27 0.04 0.02 0.45Gd 1.8 1.05 0.18 0.06 0.63Dy 1.7 0.80 0.15 0.10 0.52Er ND ND ND ND 0.41Yb 0.97 0.84 0.11 0.12 0.18

Table 1 Major and trace element compositions of glasses in veinedmantle xenoliths (PAT2-4, PAT2-62), and selected unveined mantlexenoliths (PAT2-68, PAT2-41, PAT2-42) from La Palma. Amph-vein, phlog-amph vein, reaction zone, phlog-vein, and INF-v re-

present di�erent stages of melt evolution from broad veins to in-terstitial positions in the veined xenoliths (see Fig. 1 and text foradditional explanation). INF-u represents glasses in unveined xe-noliths. (ND not detected, open space not analysed)

62

an innermost reaction zone consisting of ®ne-grained,vermicular intergrowths of clinopyroxene + oxide,or ®ne-grained intergrowths of phlogopite � clin-opyroxene � oxide � amphibole + glass. The lat-ter also form contacts against phlog-amph veins.

4. With decreasing diameter phlog-amph veins turn intophlog-veins (like in PAT2-4).

5. The most silicic glasses are represented by glass ®lmsalong grain boundaries (INF-v). and glass inclusionsin olivine porphyroclasts.

Like sample PAT2-4, the glass in the vein system ofsample PAT2-62 changes from brown, basaltic glass(about 43 wt% SiO2, 5% MgO, 5% Na2O and 2.4%

K2O) in amph-veins to colourless, more Si-rich glassfound as inclusions in olivine porphyroclasts (Table 1;about 58 wt% SiO2, 1% MgO, 7±8% Na2O, 4±6%K2O). The most evolved glass in the vein system insample PAT2-62 is thus considerably less Si-rich thanthat in sample PAT2-4. Vein minerals in PAT2-62change from Ti-Al-Fe-rich compositions in amph-veinsto Ti-Al-Fe-poor, and Mg-Cr-rich compositions innarrow phlog-veins (Table 2).

For comparison with the veined xenoliths, we chosethree representative unveined xenoliths from La Palma(out of a selection of 46 analysed samples). The selectedsamples comprise one anhydrous spinel harzburgite(PAT2-68, Fo91.0), one hydrous spinel harzburgite

Spinel dunite

PAT2-62Veined

PAT2-42Unveined

Amphvein

Amphvein

Phlogamphvein

Reactionzone

Phlogvein

Phlogvein

Phlogvein

Phlogvein

Phlogvein

Phlogvein

Inclusionolivine

Inclusionolivine

INF-u INF-u INF-u

III VI-5 14±21 15±1 5±11a V-2 V-4 V-7 V-11 5±11e 12±11 12±12 1 2 3

43.07 42.70 45.28 45.49 49.39 53.23 53.47 53.37 53.37 54.57 57.66 58.08 52.18 51.69 51.893.72 4.64 2.41 3.60 1.95 1.30 0.93 1.10 1.10 1.11 0.60 0.78 0.89 0.94 0.9115.30 17.62 17.77 17.45 21.45 20.78 20.58 20.34 20.34 20.85 20.93 21.94 22.20 21.79 21.6011.29 11.60 8.39 9.22 3.15 2.66 2.83 2.69 2.69 2.44 2.17 1.54 3.14 3.14 3.500.41 0.34 0.32 0.15 0.12 0.03 ND 0.03 0.03 0.15 0.04 ND ND ND 0.015.44 3.29 3.49 3.81 2.70 2.21 2.14 2.33 2.33 2.02 0.95 1.21 1.52 1.34 2.2311.12 11.21 10.70 9.55 6.41 4.51 4.34 4.85 4.85 4.37 1.10 1.17 7.31 7.44 8.345.02 4.05 5.19 4.97 6.04 6.29 6.35 6.29 6.29 6.76 8.65 7.11 7.69 7.82 7.392.15 2.39 2.60 2.46 6.41 7.46 7.18 6.98 6.98 7.30 4.24 5.87 3.67 3.47 3.011.46 0.73 0.43 1.87 0.49 0.38 0.47 0.53 0.53 0.59 0.26 0.29 0.14 0.27 0.260.22 0.20 0.17 0.17

0.10 0.11 0.23 0.23 0.30 0.300.06 0.06 0.35 0.3199.26 98.83 96.68 96.68 98.34 99.37 98.77 98.51 98.51 100.39 96.90 98.29 98.74 97.90 99.14

37240 30045 46661 4710716652 18283 54632 57648 59674 60170 31401 25786 23691

11 8 12 11 11 12 19 21 2220239 18464 7399 5778 7154 6424 3873 4747 4681282 184 168 145 158 157 274 389 38023 42 54 87 70 101 ND 30 2737 29 168 181 201 195 68 60 54

1924 2045 919 857 609 889 956 1268 122258 47 21 15.7 15.9 16.6 12 18 18

443 446 505 556 479 550 34 44 41135 137 99 92 96 97 12 23 21

0.6 0.7 1.3 2.2 1.6 2.7 1.2 2.3 1.2993 1116 1360 1429 1209 1555 569 639 589180 155 131 112 101 113 33 44 42315 262 176 146 136 150 32 51 49137 113 55 43 39 44 6.3 12 1123 18.2 8.1 6.2 6.1 6.6 1.2 2.2 1.96.4 5.1 1.94 1.37 0.87 1.5 0.58 0.92 0.7817 13.9 6.7 4.7 4.2 5.2 1.3 2.1 2.511.8 9.9 4.2 3.1 3.1 3.8 1.7 2.8 2.65.2 4.7 ND ND ND ND 1.2 2.3 1.83.7 2.8 1.2 1.0 0.68 0.9 1.1 2.0 1.8

63

(PAT2-41, Fo90.8, traces of phlogopite), and one hy-drous spinel dunite (PAT2-42, Fo89.8, traces of amphi-bole) (analytical data are given by Wul�-Pedersen et al.1996a). Like other mantle xenoliths from La Palma,these xenoliths are enriched in strongly incompatibleelements compared to moderately incompatible ones. Inthese xenoliths glass is found in inclusions in minerals(commonly with ¯uid and/or daughter minerals), in in-terstitial glass pockets (glass and microlites), and as thin®lms along grain boundaries (referred to as INF-u). Thecompositions of glasses in unveined xenoliths from theCanary Islands show clear correlations with type of hostrock. Harzburgites and lherzolites from La Palma showa relatively limited range in glass compositions (62±69 wt% SiO2) and tend towards silica-oversaturation,whereas glasses in dunites and wehrlites are less silicic(46±61 wt% SiO2) and are generally silica-undersatu-rated (Neumann and Wul�-Pedersen 1997). Glasses inthe unveined spinel harzburgites PAT2-68 and PAT2-41contain 66±68 wt% SiO2, <0.1% TiO2, 1.3±1.8% MgO,6.3±7.5% Na2O, and 4.4±4.9% K2O (Neumann andWul�-Pedersen 1997; Table 1). The glasses in these xe-noliths thus belong to the most Si-rich ones found in LaPalma harzburgites and lherzolites. Glasses in unveineddunite PAT2-42 carry 50±57 wt% SiO2, <0.1% TiO2,1.3±2.3% MgO, 6.3±8.0% Na2O, and 3.0±7.5% K2O(Neumann and Wul�-Pedersen 1997; Table 1). Theseglasses fall in the middle of the SiO2-range covered byglasses in spinel dunites.

Trace element relations

Trace element compositions of glasses and minerals arelisted in Tables 1 and 2, and abundances normalized toaverage C1 chondrite composition (Anders and Gre-vesse 1989) are presented in Fig. 3. Unfortunately, manyof the glass domains were too small to be analysed byion probe. We are therefore not able to present a com-plete record of the trace element variations in the glassesfor either of the veined xenoliths. In order to give a morecomplete picture of the chemical variations in the veinsystems, major element data are added for some glassdomains which were too small for ion probe analyses.

Veined xenoliths

The analysed glasses show clear relations between majorand trace element compositions. In veined spinel dunitePAT2-62, the least silicic glasses (basaltic glass with ca.43 wt% SiO2 in amph-veins) are highly enriched in themost incompatible elements (enrichment factors for Nband La equal to 556 and 770 times chondrite, respec-tively), and enriched in L (light) REE relative to H(heavy) REE (Fig. 3b). The chondrite-normalized traceelement patterns are near linear for REE, and roughlyparallel to those of alkali basalts from the Canary Islands(Fig. 3b). With increasing SiO2 and decreasing MgO, the

concentrations of strongly incompatible elements (Cs-Srin Figs. 3b, 4) in the glasses increase mildly and reach amaximum in glass in phlog-veins. Other incompatibleelements (Nd-Yb in Fig. 3b) decrease with decreasingMgO (Fig. 4). Glasses in the veined spinel harzburgitePAT2-4 show decreasing Ba and La contents from phlog-veins to in®ltrated wall-rock and very low concentrationsin the most Si-rich, Mg-poor glasses (Fig. 4) suggesting apossible maximum at about 1±2 wt%MgO. This is mostpronounced in the Ti-MgO plot for which most data areavailable. The most silicic glasses exhibit concave traceelement patterns (Fig. 3a, b). It is to be noticed that thehost basalt has a di�erent composition from the mostMg-rich glasses in the vein systems.

Also the vein minerals show systematic shifts in traceelement composition with mode of occurrence. Clino-pyroxenes in amph-veins, phlog-amph veins and phlog-veins in both the veined dunite and the veined ha-rzburgite are highly enriched in REE, and in LREErelative to HREE, and show negative Sr-anomalies anddi�erent degrees of negative Ti-anomalies (Table 2;Fig. 3c, d). Moderately incompatible elements like REEand Ti decrease, and Sc, V and Cr increase from amph-veins and phlog-amph veins, through phlog-veins, to re-action zones and INF-v (Fig. 5). Diopsides in reactionzones and in INF-v are generally depleted in incompat-ible elements and show similar enrichment factors for allREE (Fig. 3d).

Phlogopites are highly enriched in e.g. Rb, Ba (705±6047 ppm), Nb (11±157 ppm), Sr, Zr and Ti, whereasthe REE are below detection limit (Table 1). Phlogopitesin the veined spinel harzburgite (PAT2-4) show stronglydecreasing concentrations of Ba, Nb, Sr, Zr, Ti and Yfrom phlog-amph veins to in®ltrated wall-rock (INF-v;Table 2; Fig. 3e). Most phlogopites analysed in the du-nite (PAT2-62) are from phlog-veins, so the pattern thereis not clear. Apatite and amphibole coexisting with ba-saltic glass in amph-veins are highly enriched in REE andLILE elements (Fig. 3f).

Unveined xenoliths

Silicic glasses in unveined xenoliths are more stronglydepleted in intermediate and heavy REE relative to Rb

Fig. 2a±f Petrographic photographs of veined spinel harzburgitePAT2-4: a Overview showing basaltic vein in®ltrating the xenolith.Black circles and lines are ink marks for microprobe analyses. Lettersindicate di�erent types of veins (see text for explanation) (I amph-veinand phlog-amph vein, II phlog-vein, III in®ltrated wall-rock). b Phlog-amph vein with reaction zone against olivine porphyroclast in theharzburgite wall-rock. c Schematic drawing of phlog-amph vein withreaction zones against harzburgite wall-rock. Amphibole is onlypresent in the central part of the vein. The area shown in b is outlinedby dashed lines. d Phlog-vein with reaction zone against wall-rockminerals (crossed nicols). e Narrow glass-rich phlog-vein withnumerous empty vesicles. f Highly silicic glass forming thin ®lmalong grain boundaries within harzburgite fragment, termed in®ltrat-ed wall-rock (crossed nicols). (Ol olivine, sp spinel, amph amphibole,phlog phlogopite, au augite, gl glass)

c

64

and Ba than any of the vein glasses (Table 1, Fig. 4).Their chondrite-normalized trace element patterns aremarkedly concave, and show positive Sr-anomalies(Fig. 3a, b). The most extreme trace element composi-tions are exhibited by the highly silicic glass (67 wt%SiO2) in the unveined, anhydrous harzburgite PAT2-68.

Similarly, Cr-diopside in the unveined xenoliths ex-hibits the lowest concentrations in incompatible ele-

ments and ¯at trace element patterns (Ce/Yb= 2.6±3.7;Figs. 3d, 5). Chrome-diopside in unveined samples alsoexhibits negative Ti-anomalies, and weak negative Zr-anomalies. According to Johnson et al. (1990), suchanomalies are typical for clinopyroxene in peridotitesfrom hotspot regions, whereas clinopyroxene in non-hotspot peridotites is commonly depleted in LREE rel-ative to heavy REE (Ce/Yb between 0.01 and 0.3).

65

Orthopyroxene is not present in the vein systems, buttrace element compositions of orthopyroxeneporphyroclasts in unveined harzburgite PAT2-68 arelisted in Table 1 and shown in Fig. 3e.

Discussion

Mode of formation of silicic glasses

The origin of silicic glasses ( >60 wt% SiO2) in mantlexenoliths has been the subject of considerable debate.Two studies of silicate glasses in mantle peridotites in theCanary Islands led Wul�-Pedersen et al. (1996a) and

Neumann and Wul�-Pedersen (1997) to propose thatthe silicic melts which gave rise to these glasses formedas the result of in®ltration of basaltic melts into spinelperidotite, followed by reactions and crystallization[model (1)]. One of these studies (Neumann and Wul�-Pedersen 1997), based on 674 analyses of silicate glassinclusions and interstitial glasses in about 50 spinel-bearing peridotite xenoliths from La Palma, Hierro,Tenerife and Lanzarote, show a range in glass compo-sitions from basaltic (ca. 44 weight percent SiO2), tohighly silicic (up to 71 weight percent SiO2, low con-centrations of TiO2, FeO, MgO, CaO, and P2O5). Theother study (Wul�-Pedersen et al. 1996a) comprises dataon spinel peridotite xenoliths and their minerals in La

Table 2 Major and trace element concentrations in di�erent mi-neral phases in veined and unveined xenoliths. Amph-vein, amph-phlog vein, reaction zone, phlog-vein, INF-v: di�erent stages of melt

evolution from broad veins to interstitial positions in the xenoliths(see text for discussion); INF-u: unveined xenoliths. (ND not de-tected)

Phase Clinopyroxene

Spinel harzburgiteVeined

Sample PAT2-4 PAT2-68

Locationtype

Phlog amphvein

Phlog amphvein

Reactionzone

Phlog vein Phlog vein Phlog vein INF-v INF-u INF-u

Point 13cpx 6cpx 3cpx 2cpx 5cpx 7cpx 11cpx 3cpx 6cpx

SiO2 48.37 47.77 55.61 53.62 53.93 53.21 52.91 53.22 53.82TiO2 1.36 1.26 0.06 0.16 0.09 0.24 0.36 0.05 0.02Al2O3 4.99 4.75 0.50 1.70 1.32 2.41 2.45 2.38 2.80Cr2O3 ND ND 0.04FeO 10.86 10.41 3.21 5.18 4.60 5.32 2.62 2.25 2.30MnO 0.55 0.46 0.23 0.31 0.28 0.26 0.04 0.08 NDMgO 9.96 9.99 16.94 14.47 15.61 14.26 16.13 16.75 16.45NiO ND 0.10 ND 0.09 0.07 0.05 0.04 0.03 0.01CaO 22.19 21.82 21.97 23.46 22.90 23.06 23.29 23.53 22.34Na2O 1.39 1.52 0.73 0.92 0.95 1.00 0.58 0.49 0.73K2O ND ND ND 0.01 ND ND 0.01 0.01 0.02FH2OSum 99.67 98.08 99.29 99.92 99.75 99.81 98.43 98.79 98.49

ppmNa 6862 7047 7678 3932 3781 4673K 5 6 6 70 134 41Sc 9 9 9 116 93 74Ti 954 531 1465 1931 164 140V 84 62 102 249 209 205Cr 75 93 81 7052 7839 8108Rb ND ND ND ND ND NDSr 371 496 396 16.6 19.5 20Y 36 31 43 11.9 3.9 3.2Zr 342 382 479 18.4 3.8 3Nb 0.5 0.5 0.9 0.3 0.3 0.3Cs ND ND ND ND ND NDBa 0.65 1.05 0.9 1.3 2.9 0.25La 63 107 80 3 0.53 0.56Ce 134 197 172 6.1 1.03 1.16Nd 62 65 77 4.5 1.06 1.07Sm 11 8.7 13.6 1.15 0.4 0.36Eu 2.9 3.5 3.7 0.44 0.18 0.13Gd 8.6 5.5 9.4 1.54 0.4 0.33Dy 6.7 5 7.8 1.89 0.61 0.48Er 3.1 3 3.5 1.17 0.36 0.33Yb 3 3.4 3.5 1.01 0.4 0.31

66

Palma, including major element data on glasses andminerals in the veined xenoliths PAT2-4 and PAT2-62which are also discussed in this paper. An in®ltration-reaction model was also proposed by Zinngrebe andFoley (1995) to explain the generation of silicic glasses inmantle xenoliths from Gees, West Eifel, Germany. Othermodels which have been proposed include: (2) metaso-matic reaction between an alkali-carbonatitic melt andmantle minerals (Coltorti et al. 1999); (3) the breakdownof amphibole in response to decompression (e.g. Freyand Green 1974; Francis 1976); (4) immiscible separa-tion of a single melt into coexisting silicic and carbonatemelts (Schiano et al. 1994); (5) small degrees of partialmelting of subducted crust followed by percolation ofthe melts into the overlying depleted mantle wedge in

volcanic arcs (Schiano et al. 1995); (6) in®ltration bymigrating metasomatic melt phases genetically unrelatedto the mantle rock in which they are found (Edgar et al.1989; Schiano et al. 1992, 1994, 1995; Schiano andClocchiatti 1994); (7) in situ melting processes(Amundsen 1987; Francis 1987; Hauri et al. 1993;Chazot et al. 1996). Recently, experimental investiga-tions have supported the proposition that highly silicicmelts may form by small degrees of in situ partialmelting in the upper mantle (melt fraction F < 0.05;Hauth 1991; Johnson and Kushiro 1992; Baker et al.1995; Drury and Fitz Gerald 1996; Draper and Green1997).

The petrographic relations in veined and unveinedxenoliths in the Canary Islands strongly suggest that the

Spinel duniteVeined Unveined

PAT2-62 PAT2-42

Amph vein Amph vein Amph vein Phlog amphvein

Phlog amphvein

Phlog amphvein

Phlog vein Reactionzone

Reactionzone

INF-v INF-u

VI-1 VI-7 VI-8 II II-1 II-2 V-9 IV-1 IV-3 V-6 lb

48.17 46.91 48.17 46.49 48.00 49.47 53.25 52.35 52.29 53.62 53.181.85 2.25 1.76 2.52 1.89 1.35 0.35 0.43 0.55 0.33 0.155.57 6.54 5.32 5.93 4.91 3.59 2.33 2.95 3.20 1.50 2.33

8.18 8.64 7.05 7.61 6.52 6.27 3.22 3.31 3.06 2.99 2.660.21 0.20 0.15 0.20 0.19 0.19 0.15 0.13 0.06 0.11 0.0611.49 11.28 13.04 12.81 13.46 14.66 16.12 15.90 15.83 17.23 16.35ND 0.02 0.01 ND 0.01 0.04 0.09 ND 0.04 0.1122.73 23.11 23.05 22.92 22.79 23.08 22.31 22.81 22.94 23.18 23.371.25 1.06 0.73 0.84 0.86 0.63 0.81 0.70 0.83 0.67 0.70ND ND ND ND ND ND ND ND ND ND ND

99.45 100.01 99.28 99.32 98.63 99.28 98.63 98.58 98.80 99.74 98.80

9273 7864 5415 6231 6380 4674 6009 5193 6157 519323 13 23 18 30 30 ND 38 22 ND 1958 61 86 54 60 43 44 84 60 142 115

12096 11339 9825 12180 11377 9317 839 969 1523 1311 1719271 266 216 250 235 201 68 96 89 211 215366 380 1668 114 213 163 130 2076 2863 5354 6879

ND ND ND ND ND ND ND ND ND ND 2350 367 214 245 238 41 423 236 255 41 17433 28 33 29 36 33 40 18 18 8.5 16

460 443 212 259 238 240 259 70 74 14 153.2 2.5 1.5 2.3 2 1.5 ND 0.4 0.4 ND 0

ND ND ND ND ND ND ND ND ND ND 0.10.6 0.5 0.6 ND ND ND ND ND 0.6 ND 0.3939 32 23 24 26 27 50 17.7 16.5 5.8 8.394 76 69 65 74 75 114 45 43 12.7 1359 44 56 49 62 58 69 26 26 6 5.412.8 10.6 13.4 12.1 15 13.5 14.2 4.6 5.6 1.27 1.73.8 3.1 4 3.7 4.5 3.7 4.3 2 2.1 0.36 0.89.3 7.3 10.5 9.4 12.2 8 10.2 5.2 4.3 1.41 2.07.1 5.8 7.6 7.3 9 6.7 8.1 3.3 3.3 1.4 2.32.7 2.6 2.6 3.2 2.8 3.9 3.5 1.7 1.5 0.88 1.72.2 2.1 1.9 2 2.1 2.2 2.7 1.2 1.3 0.72 1.4

67

most highly silicic melts in each rock group are in, orclose to, equilibrium with the spinel peridotites in whichthey are found, whereas basaltic melts are not (Neu-mann and Wul�-Pedersen 1997; this study). The evi-dence includes: (a) no reaction zones between highlysilicic melts and peridotite wall-rock, in contrast to thereaction-type contacts between xenoliths and basalticmelts seen in the veined xenoliths, and along the surfacesof unveined xenoliths; (b) daughter minerals in highlysilicic glass have compositions similar to correspondingphases in the host peridotites (Fo90±92 in olivine, Cr-diopside, chromite), whereas daughter minerals in theless silicic glasses include Ti-Al-Fe-rich phases such asaugite and titanomagnetite (Tables 1, 2; Neumann andWul�-Pedersen 1997); (c) similar low TiO2/Al2O3 ratios(<0.08) in highly silicic glasses and spinel peridotite

xenoliths, and high TiO2/Al2O3 ratios (>0.15) in low-SiO2 glasses, similar to those that characterize ma®cCanary Islands lavas in general. Petrographic evidencefor equilibrium between silicic melts and their hostperidotites, and disequilibrium between basaltic meltsand peridotite was also reported by Zinngrebe and Foley(1995). The evidence of equilibrium, or near equilibrium,between highly silicic melts and the host peridotites isnot compatible with models (5) and (6) above, each ofwhich assumes that there exists no genetic relationshipbetween the silicic glasses and their host rocks. The lackof amphibole in the unveined harzburgites and lherzo-lites also renders model (3) unlikely. Similarly, the ap-parent lack of carbonatite melt inclusions in the studiedxenoliths, and the wide range in glass compositionsmake model (4) (immiscible separation) highly unlikely.

Table 2 (Continued)

Phase Phlogopite

Spinel harzburgiteVeined

Spinel dunite

Sample PAAT2-4 PAT2-62

Locationtype

Phlog amphvein

Phlog vein Reactionzone

INF-v Phlog vein Phlog vein Phlog vein Phlog vein INF-v

Point 1 phl 6 phl 8 phl 10 phl V-1 V-3 V-8 V-10 IV-2

SiO2 38.58 39.25 41.61 41.42 40.94 39.92 40.63 40.99 37.63TiO2 2.93 1.68 0.33 0.35 1.49 1.64 1.57 1.53 1.72Al2O3 15.14 15.09 13.27 13.73 14.63 15.03 14.54 14.78 15.5Cr2O3

FeO2 8.68 6.66 4.75 4.12 3.92 4.21 3.83 4.04 4.59MnO 0.18 0.10 0.01 0.06 0.00 0.00 0.02 0.02 0.02MgO 20.05 22.59 25.02 24.95 22.25 21.88 22.23 22.06 22.05NiO 0.11 0.21 0.21 0.12 0.34 0.24 0.22 0.18 0.19CaO 0.00 0.00 0.00 0.03 0.03 0.05 0.01 0.00 0.00Na2O 1.12 1.20 1.11 1.10 1.10 1.00 0.85 0.81 1.32K2O 8.50 8.94 9.01 9.07 9.07 8.62 9.05 8.44 8.54F 0.50 0.55 0.50 0.45 0.76 0.68H2O 2.89 2.69 3.05 2.68 3.10 2.95Sum 98.68 98.96 98.87 98.08 97.63 92.59 96.58 92.85 91.56

ppmNa 8531 8716 7863 7863 7863 7418 6306 6009 9792K 73460 67380 63340 64910 64040 60360 65710 60580 61340Sc ND ND ND 5 6 6 7 5 9Ti 18060 10010 3245 1859 8450 8040 9093 7135 10450V 203 142 134 187 199 187 190 156 211Cr ND ND 4585 2258 333 236 307 221 3705Rb 316 291 237 227 194 199 293 196 186Sr 171 179 45 32 68 70 92 58 148Y 0.11 0.12 0.05 0.03 0.06 0.09 0.13 0.09 0.11Zr 71 148 54 12 46 51 65 56 56Nb 134 157 50 11 78 74 87 75 80Cs 1.3 1.7 1.7 2.1 1.4 2.1 2.8 2.8 1.4Ba 5560 6050 899 705 931 1090 1619 836 2531La ND ND ND ND ND ND ND ND NDCe ND ND ND ND ND ND ND ND NDNd ND ND ND ND ND ND ND ND NDSm ND ND ND ND ND ND ND ND NDEu ND ND ND ND ND ND ND ND NDGd ND ND ND ND ND ND ND ND NDDy ND ND ND ND ND ND ND ND NDEr ND ND ND ND ND ND ND ND NDYb ND ND ND ND ND ND ND ND ND

68

We are left with models involving reactions betweendi�erent types of in®ltrating melts and mantle minerals[models (1) and (2)], and low degrees of partial melting[model (7)]. The possibility that some of the most silicicglasses occurring in unveined peridotite xenoliths in theCanary Islands represent quenched melts formed by lowdegrees of partial melting of their host peridotite [model(7)] cannot be excluded. However, low degrees of partialmelting can neither account for the compositional rangefrom basaltic to silicic glasses found in the unveined andveined Canary Islands xenoliths, nor for the fact that theleast silicic glasses appear to be out of equilibrium withtheir host peridotites. One might argue that melts ofintermediate compositions have formed by mixing be-tween in®ltrating host basalt and highly silicic meltsgenerated by in situ partial melting. However, the

chemical relations among glasses in unveined and veinedxenoliths are not in agreement with simple mixing(Wul�-Pedersen 1996a; Figs. 3, 4). As we show below,an IRC-model is strongly supported by the trace elementdata on the veined xenoliths. Below we also discuss thetype of melt which may have in®ltrated the upper mantlebeneath the Canary islands in general.

The data presented above demonstrate that thegradual changes in phase assemblage and major elementcompositions observed in glasses and minerals in theveined xenoliths from La Palma are accompanied bysigni®cant changes in trace element characteristics.These changes are best expressed by glasses in spineldunite PAT2-62 for which most data are available. Themost prominent change is a progressive, strong decreasein Nb, intermediate REE and Zr, with only minor

Amphibole Apatite Orthopyroxene

Spinel harzburgiteVeined

Spinel dunite Spinel duniteVeined

Spinal harzburgiteUnveined

PAT2-4 PAT2-62 PAT2-62 PAT2-68

Amphvein

Phlog amphvein

Amphvein

Amphvein

Amphvein

Amphvein

Amphvein

Amphvein

Amphvein

Amphvein

INF-u INF-u

13am 6am III VI-3 VI-2 VI-6 II III-1 III-2 III-3 4r 5c

40.27 39.64 39.26 37.24 37.48 36.79 37.86 56.7 56.593.79 4.22 4.22 5.81 4.59 5.07 5.34 0.00 0.0112.56 12.69 12.08 12.71 12.40 12.83 12.59 1.99 2.070.02 0.0013.31 12.00 8.81 10.49 12.29 12.02 9.85 6.08 6.270.31 0.38 0.18 0.21 0.13 0.22 0.13 0.14 0.1111.74 12.07 13.71 11.84 11.51 11.03 12.93 Not analysed 33.53 33.520.00 0.00 0.01 0.04 0.05 0.00 0.02 0.20 0.0512.01 11.46 11.72 11.98 11.73 11.87 11.85 0.99 1.042.99 2.89 2.80 2.64 2.67 2.65 2.69 0.01 0.051.30 1.16 1.12 1.20 1.14 1.10 1.13 0.00 0.00

0.230.52

98.30 96.51 94.66 94.16 93.99 93.58 94.39 99.64 99.71

20770 19590 19810 19660 19960 111 44510140 9300 10170 9880 9250 ND ND

42 49 79 41 41 28 2634100 28080 32400 27870 27410 Not anlaysed 57 55

411 335 374 353 292 95 98161 228 234 485 82 3472 425912 12 7 12 10 ND ND

1864 956 1474 1397 1053 0.1 0.362 36 31 41 35 0.3 0.3346 199 127 308 197 0.2 0.2139 59 36 100 55 ND ND

ND ND ND ND ND ND ND1036 458 751 722 506 ND ND

63 30 23 50 30 980 1252 698 0.003 0.007161 79 61 118 76 1779 2228 1460 0.005 ND102 59 51 74 56 772 889 733 0.02 0.01922 14 13 15 13 128 136 126 ND 0.0086.6 4.0 3.9 4.3 4.1 31 33 31 ND 0.00717 11 11 12 11 98 99 91 0.023 0.03513 7.7 6.8 8.8 7.6 52 53 49 0.022 0.0415.5 2.9 2.4 3.5 3.0 17 18 15 0.016 0.0513.9 1.9 1.6 2.1 1.7 10 11 9 0.038 0.091

69

changes in Cs, Rb, Ba and K, in glass and clinopyrox-ene, from broad amph-veins to in®ltrated wall-rock(Figs. 3±5). The strongest depletion in moderately in-compatible elements relative to strongly incompatibleones is exhibited by glasses in the unveined spinel ha-rzburgite xenoliths (INF-u). Similar decreases in traceelement concentrations are exhibited in phlogopites inthe veined spinel harzburgite (PAT2-4) for the elementsBa, Nb, Sr, Zr, Ti and Y (Table 2; Fig. 3).

Furthermore, glasses in the veined dunite (PAT2-62)show strong fractionation of incompatible trace ele-

Cs Rb Ba K Nb La Ce Sr Nd SmEuZr Gd Ti Dy Y Er YbSc V

Gla

ss/C

1

Spinel harzburgite

Cs Rb Ba K Nb La Ce Sr Nd SmEuZr Gd Ti Dy Y Er YbSc V

Gla

ss/C

1

Spinel dunite

Clin

opyr

oxen

e/C

1

Cs Rb Ba K Nb La Ce Sr Nd SmEuZr Gd Ti Dy Y Er YbSc V

Spinel harzburgite

Cs Rb Ba K Nb La Ce Sr Nd SmEuZr Gd Ti Dy Y Er YbSc V

Spinel dunite

Cs Rb Ba K Nb La Ce Sr Nd SmEuZr Gd Ti Dy Y Er YbSc V

Min

eral

/C1

Cs Rb Ba K Nb La Ce Sr Nd SmEuZr Gd Ti Dy Y Er YbSc V

Min

eral/C

1

Phlogopite

Orthopyroxene

Amphibole

Apatite

amph-veinsphlog-amph vphlog-veinsreaction zone

INF-vINF-u hydrINF-u anhhost basalt

104

10-2

Clin

opyr

oxen

e/C

1

a b

dc

e f

104

10-2

10-210-2

10-1 10-1

10-1 10-1

10-110-1

100 100

100 100

100100

101101

101 101

101101

102 102

102 102

102102

103103

103 103

103103

Fig. 3 Trace element patterns of glasses and minerals in veined andunveined xenoliths from La Palma normalized to average C1chondrite (Anders and Grevesse 1989). In order to make thepresentation more complete, Ti-data (recalculated from major elementdata) are shown for glasses and clinopyroxenes for which traceelement data are lacking. The trace element pattern of the host basalt(dashed) (Wul�-Pedersen et al. 1996b) is included in (a) and (b) forcomparison. Note the systematic shift in trace element patterns forglasses and clinopyroxenes, from enriched patterns in amph veins, toREE-depleted patterns in in®ltrated wall-rock and unveined spinelharzburgite. See text for further comments

70

ments relative to one another throughout the vein sys-tem. For example, La/Nd and Ti/Sm ratios in glass in-crease regularly whereas Sm/Ba decreases with

decreasing MgO (and increasing SiO2) from amph-veinsto INF-v and INF-u (Fig. 6). The Zr/Sm ratio appears toshow a maximum for phlog-veins and INF-v. For thespinel harzburgites only high-Si, low-Mg glasses couldbe analysed for trace elements. These exhibit higher La/

Clinopyroxene

0

20

40

60

80

100

0

4

8

12

0

4000

8000

12000

10 12 14 16 18MgO (wt%)

Ti (p

pm)

Sm

(pp

m)

La (

ppm

)B

a (p

pm)

Hz Dunamph-v

reac z

INF-v

INF-u

phl-a-v

phlog-v

3

2

1

0

Fig. 5 Concentrations of selected trace elements in clinopyroxenes inveined and unveined xenoliths from La Palma plotted against MgO.The clinopyroxenes show similar variation patterns from amph-veins toin®ltrated wall-rock (INF-v and INF-u) as do the glasses (Fig. 4). Itshould be noticed that as the glasses become progressively depleted inMgO, the coexisting clinopyroxenes become more MgO-rich. The Ti-MgO ®gure includes Ti-data recalculated from major element analysesin order to give the most complete picture possible of thecompositional changes

Glass

0

2

4

6

8

0

5

10

15

20

0 2 4 6 8 10MgO (wt%)

20000

15000

10000

5000

0

Sm

(ppm

)T

i (ppm

)

10

2000

1500

1000

500

0

2500

3000

La (

ppm

)B

a (

ppm

)

host basalt

host basalt

host basalt

host basalt

Hz Dun

amph-vphl-a-vphlog-vreac zINF-vINF-u

Fig. 4 Concentrations of selected trace elements in glasses in veinedand unveined xenoliths from La Palma plotted against MgO. Ba andLa in the glasses increase with decreasing MgO and reach a maximumin phlog-veins, after which their concentrations decrease, whereas Smand Ti decrease regularly with decreasing MgO. The Ti-MgO plotincludes Ti-data recalculated from major element analyses in order togive the most complete picture possible of the compositional changes

71

Nd, Zr/Sm and Ti/Sm ratios, and lower Sm/Ba ratiosthan glasses in similar setting in the dunites (Figs. 3, 6).Clinopyroxenes in the veined spinel dunite show a reg-

ular decrease in Sm/Ba, Zr/Sm and Ti/Sm ratios fromamph-veins to INF-v and INF-u, phlog-veins and reactionzones. The Cr-diopside in phlog-veins in the spinel ha-rzburgite appears to fall on the same trend for Sm/Ba,but shows higher Zr/Sm and lower Ti/Sm than those inphlog-veins in the veined dunite. The lowest Sm/Ba andZr/Sm ratios in any of the analysed clinopyroxenes arefound in the unveined peridotites, but these show con-siderable scatter in Ti/Sm ratios (INF-v and INF-u;Fig. 7). The amphiboles (amph-veins) are highly enrichedin incompatible elements (Fig. 3), as is typical for am-phiboles crystallizing from enriched melts.

The observed changes in trace (and major) elementconcentrations and ratios in glasses and minerals in theveined xenoliths support the IRC-model. The IRC-processes lead to the formation of new phases, e.g.amphibole, apatite, phlogopite and consumption ofothers, for example formation of olivine � phlogopite� amphibole + Si-rich melt at the expense of ortho-pyroxene + in®ltrated melt. Mass balance modelling byWul�-Pedersen et al. (1996a) indicates that almost asmuch melt is released through breakdown of minerals,as is consumed through the reactions. As the concen-trations of lithophile trace elements in orthopyroxeneare very low, a Si-rich melt component released throughthe breakdown of orthopyroxene must be highly re-fractory. This partly explains the general negative cor-relation between SiO2 and lithophile elementconcentrations in the vein glasses (e.g. Fig. 3, Table 1).Furthermore, elements highly compatible with any ofthe newly formed phases selectively will be partitionedinto, or trapped by, this phase, and will consequentlybecome more strongly depleted in the residual, pro-gressively Si-enriched melt than elements with lowermineral/melt partition coe�cients. Examples are trap-ping of Nb and Ti in titanomagnetite, Ti and REE inamphibole, and REE in apatite in amph-veins. Barium isexpected to be enriched in the residual melt until for-mation of phlogopite becomes important (in the phlog-veins), after which stage Ba also is expected to becomedepleted in the residual melt. This explains partitioningof the incompatible trace elements relative to one an-other. It may also explain the apparent non-linear be-havior of Ba in the vein-systems.

Equilibrium conditions between silicic meltsand enclosing spinel peridotite

It is very di�cult to model the trace element changes in amelt evolving in response to IRC-processes. One prob-lem is that the veins clearly represent open systems withelement exchange through the open veins before, during,and after the reactions. Another problems is that min-eral/melt partition coe�cients change signi®cantly as afunction of melt temperature and degree of polymer-ization of the melt. (e.g. Ryerson and Hess 1978; Greenand Pearson 1985; Sisson 1991; Green 1994). In accor-dance with this, Vannucci et al. (1998) found partition

0

1000

2000

3000

4000

0 1 2 3 4 5 6 7 8 9 10

0

2

4

6

8Glass

MgO

Ti/S

mS

m/B

aZ

r/S

mLa

/Nd

0

0.01

0.02

Hz Dunamph-v

phlog-v

INF-v

INF-u

host basalt

host basalt

host basalt

0

100

200

300

host basalt

Fig. 6 Variations in incompatible element ratios with decreasingMgO in glasses in veined and unveined xenoliths from La Palma. Theglasses show di�erent patterns of variation for di�erent ratios. See textfor discussion

72

coe�cients cpx/melt to increase signi®cantly from ba-saltic amph-veins (e.g. cpx/liqDSm= 0.6 and cpx/liqDYb=0.8 in in sample PAT2-62), through phlog-veins (e.g.cpx/liqDSm= 3.8 and cpx/liqDYb= 3.1 in PAT2-4), to Cr-diopside/highly silicic glass in unveined harzburgitePAT2-68 (e.g. cpx/liqDSm= 6.1±7.1 and cpx/liqDYb= 2.7±3.5). The highest cpx/glass ratios obtained by Vannucciet al. (1998) for La Palma xenoliths are close to valuesobtained for rhyolitic systems. However, an importantconsequence of the IRC-model is that it implies equi-

librium between the melt representing the ``end-prod-uct'' of these processes, and their host peridotites; the``end-product'' being the most highly silicic melt. Thequestion of equilibrium, or near equilibrium, betweenthe most highly silicic melts and the host peridotite thusrepresents an important test for the IRC-model. Thetrace element composition of a melt in equilibrium witha given rock may be estimated directly from the simplerelation Cr/Cl =

sol/liqD, where sol/liqD = RXi � sol/liqDi,Cr and Cl are the concentrations of the element in rockand melt, respectively, Xi is the modal proportion of agiven phase in the rock, and sol/liqDi the correspondingmineral/melt partition coe�cients.

As starting materials we have used the modal andtrace element compositions (whole-rock analyses; Ta-ble 3) of: (1) average spinel harzburgite/lherzolite xe-noliths from La Palma; (2) average spinel dunite fromLa Palma; (3) the anhydrous, highly refractory spinelharzburgite PAT2-68; (4) the hydrous spinel harzburgitePAT2-41 (Table 3). On the basis of the above discussionof the relations between partition coe�cients and meltcompositions, we chose partition coe�cients represen-tative of highly silicic mineral-melt systems (Table 3) asa basis for estimates of the trace element compositions ofmelts in equilibrium with the spinel peridotites.

Our calculations (Fig. 8) imply that melts in equilib-rium with the starting materials (1) to (3) are highlyenriched in strongly incompatible elements relative tomoderately incompatible ones, and have distinctly con-cave REE patterns. The estimated patterns are thussigni®cantly di�erent from those of the less silicicglasses, but closely resemble those of the most silicicones (glasses in unveined spinel harzburgites PAT2-41and PAT2-68; Fig. 3a, b Table 1). For most elementsthe compositional range of the analysed silicic glassesoverlaps the trace element compositions estimated forequilibrium melts (Fig. 8). Negative anomalies for Zrand Ti, as obtained in the estimated patterns, are alsoobserved in glass in xenolith PAT2-41. All in all, anddespite some deviation between calculated and observedtrace element concentrations due to the assumed parti-tion coe�cients, the results support our suggestion thatthe glasses in spinel harzburgites PAT2-41 and PAT2-68are in, or close to equilibrium with their host peridotites.

A di�erent conclusion is reached for the low-SiO2

glasses in spinel dunite PAT2-42. With 52 wt% SiO2 theglass analysed for trace elements in spinel dunite PAT2-42 falls in the lower part of the SiO2-range for the glassesof this sample (52±60 wt% SiO2), and in the middle partof the range exhibited by La Palma dunites in general(46±60 wt% SiO2; Neumann and Wul�-Pedersen 1997).Equilibrium between the 52 wt% SiO2-glass and thehost dunite is not expected. This glass is richer in mod-erately incompatible elements by more than one order ofmagnitude relative to the estimated trace element com-position of melt in equilibrium with average La Palmadunites, whereas estimated and observed concentrationsof Cs, Rb and Ba are similar. The estimated trace ele-ment patterns support the impression based on major

0

5

10

15

20

25

0

400

800

1200

1600

10 11 12 13 14 15 16 17 18MgO

Ti/S

mZr

/Sm

Sm

/Ba

Clinopyroxene

10

20

30

40

50

0

Hz Dunamph-v

reac zoneINF-v

INF-u

phl-amph-vphlog-v

Fig. 7 Variations in incompatible element ratios with decreasingMgO in clinopyroxenes in veined and unveined xenoliths from LaPalma. The glasses show di�erent patterns of variation for di�erentratios. See text for discussion

73

elements that the low-SiO2 PAT2-42 glasses do notrepresent melts close to equilibrium with their host rock.

Depth of formation of the vein systemin the veined xenoliths

An important question which has to be addressed is thedepth at which the melt-peridotite reactions in theveined xenoliths took place. Do they represent mantleprocesses, or processes in crustal magma chambers priorto eruption?

There is considerable evidence that the silicic melts( >60% SiO2) found as inclusions and melt pockets inmantle xenoliths form at mantle depths. In many mantle

xenoliths colourless glass (relatively Si-rich) coexistswith CO2 in inclusions. Microthermometry on CO2 insuch inclusions gave trapping pressures of up to 1.2 GPaon xenoliths from Hierro (Hansteen et al. 1991), and0.6±0.8 GPa for Lanzarote (Neumann et al. 1995).Mantle trapping pressures (0.7±1.25 GPa) have alsobeen reported for silicic glasses (60 wt% SiO2) inperidotite xenoliths from other areas, such as Tahaa

Cs Rb Ba K Nb La Ce Sr Nd SmEuZr Gd Ti Dy Y Er YbSc V

Gla

ss/C

1

Cs Rb Ba K Nb La Ce Sr Nd SmEuZr Gd Ti Dy Y Er YbSc V

Gla

ss/C

1

av harzPAT2-68PAT2-41

Estim. Anal.av dunPAT2-42

Estim. Anal.

Spinel harzburgite Spinel dunite

PAT2-4

101 101

102 102

103 103

100 100

102 102

Fig. 8 a Trace element patterns of melts expected to be in equilibriumwith La Palma harzburgites, compared to analysed glasses in unveinedspinel harzburgite xenoliths (INu). b Trace element patterns of meltsexpected to be in equilibrium with spinel dunites from La Palmacompared to analysed glasses in unveined spinel dunite xenolithPAT2-41 (INu). See text for additional information

Table 3 Partition coe�cients mineral/silicic melt and whole-rocktrace element data used to estimate the trace element compositionsof melts in equilibrium with La Palma peridotites. Partition coef-®cients are based on data from Mahood and Hildreth (1983), Nashand Crecraft (1985), Sisson (1991), Ionov et al. (1994), Nielsenet al. (1994), and La Tourette et al. (1995). Where published par-tition coe�cients are not available for a given element, they have

been interpolated on the basis of relative di�erences in magnitudeamong neighbouring elements. Partition coe�cients phlogopite/melt for REE have tentatively been set to 0.01. (Av hz/lz theaverage of 34 hydrous and anhydrous spinel harzburgite/lherzolitexenoliths from La Palma, Av dun the average of 14 hydrous spineldunite xenoliths from La Palma, PAT2-68 anhydrous spinel harz-burgite, La Palma)

Whole-rock analyses Partition coe�cients

Av hz/lz PAT2-68 PAT2-41 Av dun PAT2-42 Olivine/melt

Orthopyroxene/melt

Clinopyroxene/melt

Spinel/melt

Phlogopite/melt

Cs 0.3 0.03 0.02 0.02 0.02 0.01 0.025 0.001 0.001 1Rb 2.21 1.58 ND 0.53 0.44 0.01 0.005 0.03 0.001 2Ba 12.2 4.9 3.7 4 5.5 0.01 0.003 0.001 0.01 5K 643 390 74 180 415 0.01 0.02 0.001 0.001 2Nb 2.07 0.24 5.2 0.91 0.5 0.4 0.015 0.2 0.7 6La 0.8 0.34 1.01 0.47 0.26 1 0.002 0.8 0.8 0.01Ce 1.43 0.4 1.73 0.94 0.2 0.8 0.006 1.6 1.2 0.01Sr 15.2 9.2 11 9.9 7 0.01 0.009 0.8 0.01 0.5Nd 0.46 0.13 0.41 0.53 1.3 0.7 0.03 2.8 1.5 0.01Zr 2 0.06 1.3 1.7 0.1 0.009 0.5 0.2 1.2Sm 0.085 0.02 0.06 0.05 0.02 0.6 0.09 4.7 1.9 0.01Eu 0.035 0.01 0.02 0.04 0.09 0.6 0.07 4.7 0.5 0.01Gd 0.095 0.03 0.09 0.05 0.6 0.6 4.7 2.9 0.01Ti 46 10 40 92 90 0.1 0.7 2.2 4 2Dy 0.065 0.021 0.038 0.03 0.7 0.52 4.7 1.6 0.01Y 0.76 0.2 0.25 0.24 0.8 0.5 2.6 1.4 0.01Er 0.05 0.016 0.021 0.018 0.8 0.85 3.5 1.2 0.01Yb 0.055 0.027 0.031 0.028 0.9 0.48 1.8 1 0.01V 33 43 37 28 25 0.8 5.8 2.8 100 1.5Sc 8.1 9.5 4 5.4 5.6 5 26 130 14 15

74

Islands, Society archipelago, and Kerguelen (Schianoet al. 1992, 1994).

In order to be able to use the information about IRC-processes provided by the veined xenoliths from LaPalma as model for upper mantle processes, we needevidence that the vein-reactions also took place atmantle depths. One line of approach is to estimate theperiod of time during which the IRC-processes havebeen in operation. Pro®les through olivine porphyroc-lasts adjacent to veins, taken parallel to, and normal, tomineral/vein contacts, show increasing mg# with in-creasing distance from the porphyroclast/vein contact.Figure 9 shows a representative core-rim cross section(1.4 mm) of a large olivine porphyroclast in contact witha basaltic vein in veined spinel dunite PAT2-62. Theobserved marked change in Fo-content is clearly theresult of Fe-Mg interdi�usion between olivine and melt.A semi-in®nite, one dimensional model for Fe-Mg in-terdi�usion has been used to estimate the minimumlength of time from melt in®ltration until the system``froze''. The time (t) may be estimated from the ap-proximate relation

v ����

pDt�

where v is the penetration depth of the di�usion and D isthe di�usion coe�cient for a given element (Crank1975). The penetration depth, v, is measured as thedistance from the olivine/vein contact at which theconcentration of the di�using element is the averagebetween initial and ``new'' concentration, the lattermeasured at the mineral surface. The model is based onthe assumptions that: (1) dissolution of olivine along the

contact with the vein is negligible; (2) the process isisothermal; (3) the surface of the olivine is held at con-stant composition, instantaneously imposed on the oli-vine at the start of the di�usion process; (4) di�usioncoe�cients are independent of composition. Di�usion ofCa, Mn and Ni is neglected. The Fe-Mg interdi�usioncoe�cients decrease with increasing molar concentrationof Mg2SiO4 (Misener 1974; Morioka and Nagasawa1991); the di�usion coe�cient have therefore been cho-sen on the basis of olivine composition. Di�usion coef-®cients also increase with increasing pressure, but thee�ect is minor (Misener 1974). Temperature, in contrast,has a signi®cant e�ect on the interdi�usion coe�cients,which necessitates appropriate temperature corrections.Coexisting clinopyroxene ± orthopyroxene pairs fromthe reaction zones along veins in harzburgite samplePAT2-4 give temperatures of 1015±1023 °C by theWood and Banno (1973), 910±928 °C by the Wells(1977) and 900±1000 °C by the Lindsley (1983) geo-thermometers. Based on these results we have assumed atemperature of 1000 °C for the the olivine-melt inter-di�usion in question in dunite PAT2-62 (Fig. 9). TheFe-Mg di�usion in olivine is also dependent on thecrystallographic orientation. According to Chakrabortyet al. (1994) the Fe-Mg di�usion coe�cient for olivineFo92±93 is 4.53 � 10)19 m2 s)1 at 1000 °C, and increaseswith increasing temperature to 1.22 � 10)17 at 1200 °Cand 5.2 � 10)17 at 1300 °C.

Penetration depth (v) is normally estimated on thebasis of the compositional di�erence between the rimand a chemically consistent plateau in the center of theporphyroclast. Our pro®le does not show a central pla-teau. However, the core composition of this olivine(Fo=90.2) corresponds to core compositions of olivineporphyroclasts in central parts of peridotite fragments,and lies within the range of olivine grains in unveinedspinel harzburgites from La Palma (range 89.8±91.5;Wul�-Pedersen et al. 1996a), suggesting that the core isundisturbed by di�usion. On this basis, the pro®le ex-hibited in Fig. 9 gives a penetration depth of about 500lm for both MgO and FeO. Using di�usion coe�cientslisted above, we obtain a period of time for melt±wall-rock di�usion of 17500 years at 1000 °C, and about 600years at 1200 °C. The temperature estimates for the re-action (1000 °C) support a period of time >104 years,rather than <103 years.

Eruption times of xenolith-bearing magmas are esti-mated to be up to 60 hours (Kushiro et al. 1976; Mercier1979; O'Reilly 1989). Studies on dissolution rates indi-cate that at temperatures >1200 °C ultrama®c xeno-liths can survive in a host lava for only a few days beforethey are dissolved and consumed, but the dissolutionrate decreases with decreasing temperature (e.g. Scarfeand Brearley 1987, and references therein). On the basisof chemical zoning in rims of mantle xenoliths from LaPalma, KluÈ gel et al. (1996) estimated that these xeno-liths had resided up to a few years in crustal magmachambers. Reaction periods signi®cantly longer than afew years may thus be taken as strong evidence that the

16

46

8

0

0.2

0.4

0.6

200 400 600 800 1000 1200

MnO

NiO

FeO

Core

Wei

ght %

oxi

de

Distance (microns) Rim

CaO

MgOZoned olivine

10

12

14

48

50

Fig. 9 Cross section rim±core through a zoned olivine porphyroclastin contact with basalt vein (veined spinel dunite PAT2-62). The zoningpattern results from Fe-Mg interdi�usion between olivine in thedunite, and in®ltrated basaltic melt. v is the penetration depth (1400±900 = 500 lm) used for calculating the period during which di�usionwas allowed to proceed. Di�usion of Ni, Mn and Ca is shown to belimited

75

IRC-processes re¯ected in the veined xenoliths tookplace in situ in the upper mantle, rather than in crustalmagma chambers. The estimated period of time formelt/peridotite di�usion of >104 years is far too longfor the di�usion to be related to ascent processes, or toshort residence times in crustal magma chambers.

Metasomatic potential of silicic melts

We showed above that in®ltration of alkali basalticmelts into mantle peridotite, and reactions betweenmelts and peridotite (IRC-processes) will produce silicicmelts with trace element characteristics which are sig-ni®cantly di�erent from the initial compositions of thein®ltrating melts. The IRC-processes involve chemicalexchange between in®ltrating melt and peridotite wall-rock, and will also cause compositional changes (meta-somatism) in the peridotite wall-rock. It is clear thatwhen equilibrium between a speci®c highly silicic meltand the metasomatized host peridotite has been reached,the silicic melt no longer has a potential as metasomaticagent relative to that particular rock, or rocks of thesame composition. However, if this silicic melt migratesinto parts of the mantle where the peridotite is unmet-asomatized, or less metasomatized, the silicic melt is nolonger in trace element equilibrium with the surroundingrock, and may cause metasomatism in new parts of themantle.

It may be argued that highly silicic, polymerizedmelts are too viscous to be able to move through mantlerocks and cause penetrative metasomatism. However,the mobility of a silicic melt is highly dependant on itswater content. Addition of water will depolymerize themelt, lower its viscosity and increase its mobility. Waterhas not been detected in ¯uid inclusions in mantle xe-noliths from the Canary Islands (e.g. Hansteen et al.1991; Siena et al. 1991; Andersen et al. 1995; Neumannet al. 1995; Wul�-Pedersen et al. 1996a; Neumann andWul�-Pedersen 1997). However, in the islands of LaPalma and Tenerife silicic glass coexists with phlogopitewhich contains up to 3% H2O. The water contents ofsilicic glasses are 0.06±0.37% in veined xenoliths and0.2% in unveined ones (Table 1). Recently NaCldaughter crystals and very thin layers of NaCl, oftencoexisting with orthopyroxene, spinel and other phases,have been discovered in ¯uid inclusions in mantle xe-noliths from Tenerife (E.-R. Neumann and S.L. Si-monsen, unpublished data). This implies that ¯uidspenetrating the upper mantle under Tenerife included anH2O-solution rich in Si, Na and Cl. Vein glasses andinterstitial glasses contain numerous rounded, relativelylarge vesicles (Fig. 2d, e) which indicate that signi®cantamounts of volatiles were unmixed from the melt priorto, or during, decompression. Prior to unmixing of a¯uid phase the water content of the melts must thereforehave been considerably higher than the measured values,and probably high enough to signi®cantly reduce theirviscosity and increase their mobility.

The presence of silicic glasses with similar composi-tions in all unveined mantle xenoliths from La Palma(hydrous and anhydrous; Neumann and Wul�-Pedersen1997) is strong evidence that silicic melts have been ableto move on a scale exceeding the diameter of single xe-noliths, that is 20±30 cm. Draper and Green (1997)proposed that highly silicic melts may move through themantle without reacting or crystallizing, and act asmetasomatic agents. Electron backscatter photos ofspinel lherzolite xenoliths from Lorena Butte, Wash-ington (Fig. 5 in Draper 1992) show silica-, alkali-,alumina-rich glass as a thin ®lm wetting the surfaces ofnearly all grains in the rock. Below we present datawhich strongly support the suggestions of Wul�-Peder-sen et al. (1996a) that silicic melts have indeed acted asmetasomatic agents in the upper mantle beneath theCanary Islands, as documented also elsewhere byZinngrebe and Foley (1995).

Metasomatism in the upper mantle underthe Canary Islands

As indicated above, mantle xenoliths from La Palmaand the other Canary Islands (spinel harzburgites,lherzolites and dunites) show clear evidence of pervasivecryptic or patent metasomatism (patent metasomat-ism = enrichment in strongly incompatible elementsaccompanied by introduction of hydrous minerals;Wilshire 1987). This is re¯ected in whole rock trace el-ement patterns which are strongly enriched in highlyincompatible elements relative to moderately incom-patible ones, and show negative Ti-anomalies (Wul�-Pedersen et al. 1996a; E. Wul�-Pedersen and E.-R.Neumann, unpublished data). Metasomatism is alsore¯ected in their Sr-Nd-Pb isotopic ratios which aresimilar to those of Canarian basaltic lavas, that is, mildlydepleted (Whitehouse and Neumann 1995). Similar re-sults were reported by Vance et al. (1989) and Sienaet al. (1991). The trace element patterns clearly indicatemetasomatism by an agent which transported stronglyincompatible elements much more easily than moder-ately incompatible ones (e.g. Ti and HREE).

A broad range of ¯uids and melts appear to havebeen present in the upper mantle under the Canary Is-lands at some time. Fluid inclusion data on mantle xe-noliths indicate that with few exceptions thecomposition worldwide is pure CO2 (e.g. Roedder 1984).This is also true for the Canary Islands (e.g. Hansteenet al. 1991; Siena et al. 1991; Andersen et al. 1995;Neumann et al. 1995; Wul�-Pedersen et al. 1996a;Neumann and Wul�-Pedersen 1997), although CO2-N2-bearing inclusions have been found in a few spinel du-nites from Lanzarote (Andersen et al. 1995). As shownabove, an H2O-solution rich in Si, Na and Cl must alsohave been present. With respect to melts, glass inclusionsand interstitial glasses in Canarian peridotite xenolithsrange from basaltic to highly silicic compositions, al-though highly silicic glass is by far the most abundant

76

(Neumann and Wul�-Pedersen 1997). Also rare occur-rences of carbonatitic melts have been reported from theCanary Islands (Frezzotti et al. 1994). The Ti-Fe-richcumulate xenoliths from Gomera contain mixed ultra-ma®c silicate + carbonate inclusions (Frezzotti et al.1994). Interstitial silicate glass (ca. 64 wt% SiO2, mildlysilica oversaturated) with carbonate and sulphide glob-ules was found in Ti-Fe-rich wehrlitic alteration zones ina harzburgite xenolith (Fo91) from the Montana ClaraIsland (Kogarko et al. 1995). Finally, carbonatitic dykesare present in Fuerteventura (Barrera et al. 1981;Cantagrel et al. 1993). Based on these observations,both basaltic, highly silicic, carbonatitic melts, and H2Oor H2O-CO2 solutions may be regarded as potentialmetasomatic agents for the observed trace element en-richment in the upper mantle under the Canary Islands.

The metasomatic e�ect of di�erent transport agentshas been the subject of considerable debate. Agents be-lieved to cause cryptic and patent metasomatism includeascending CO2 � H2O ¯uids (e.g. Dawson 1984;Kempton 1987), gas/¯uid released from in®ltrating ba-saltic melts (e.g. Wilshire 1987), carbonatitic melts (e.g.Meen 1987; Green and Wallace 1988; Hauri et al. 1993),and highly silicic melts (Zinngrebe and Foley 1995;Wul�-Pedersen et al. 1996a; Neumann and Wul�-Pedersen 1997; Vannucci et al. 1998). It is generallyagreed that H2O-CO2-¯uids transport LREE muchmore easily than HREE, and that for H2O-rich ¯uids thesolubility increases dramatically with increasing pressure(e.g. Mysen 1977, 1979, 1983; Wendlandt and Harrison1979; Schneider and Eggler 1986; Eggler 1987; Meenet al. 1989). According to Meen et al. (1989) CO2-rich¯uids have very low solubility for REE at mantle con-ditions, and cannot be responsible for cryptic metaso-matism. Carbonatitic melts, and unaltered magmaticcarbonatitic rock are typically highly enriched in CaOand REE, relatively depleted in elements such as Ti andZr relative to basaltic melts, and have high LREE/HREE ratios (e.g. Barrera et al. 1981; Hornig 1988; Pelland HoÈ y 1989; Green et al. 1992; Klemme et al. 1995;Sweeney et al. 1994; Hornig-Kjarsgaard 1998). StrongREE-enrichment combined with relative Zr, Ti-deple-tion in mantle rocks has been interpreted as the result ofcarbonatite metasomatism (e.g. Hauri et al. 1993;Klemme et al. 1995). Highly silicic melts (Figs. 3a, b, 4,6) have a much higher potential for transport of themost strongly incompatible elements and LREE than forthe moderately incompatible ones. The di�erence intransport potential for these two groups of trace ele-ments is much less pronounced for basaltic melts.

In Fig. 10 CaO-La/Nd and Ti-La/Nd relations areused to di�erentiate between metasomatism by carbon-atitic (Ca-rich), alkali basaltic, and highly silicic melts.The upper mantle under the Canary Islands is believedoriginally to have consisted of refractory oceanic mantle(e.g. Neumann et al. 1995; Wul�-Pedersen et al. 1996a).Xenoliths from La Palma, Hierro and Lanzarote showclear trends of increasing La/Nd with near-constantCaO- and TiO2-contents, from the vicinity of normal

abyssal peridotite (A in Fig. 10) towards the ®eld ofhighly silicic glass. Trends leading towards the ®elds ofbasaltic and Ca-rich carbonatitic melts are lacking.Peridotites from Tenerife, in contrast, show somewhathigher CaO- and TiO2-contents than those from theother islands and de®ne an area close to that of abyssalperidotite. The trends de®ned by peridotites from LaPalma, Hierro and Lanzarote are in agreement withmetasomatism by highly silicic melts, whereas the uppermantle beneath Tenerife may have been a�ected by ba-saltic or carbonatitic melts. Some contribution by CO2-H2O ¯uids released from the silicate melts cannot bedisregarded.

Only small volumes of highly silicic melts are neededin order to obtain the metasomatic imprint observed inCanarian mantle xenoliths. This is demonstrated bymixing calculations in Fig. 11. Major and trace elementcompositions of mantle xenoliths suggest that the mantlelithosphere beneath the Canary Islands was considerablymore depleted than ``normal'' abyssal upper mantleprior to metasomatism (Fig. 11; Neumann et al. 1995;Wul�-Pedersen et al. 1996a). Assuming the HREEconcentrations not to have been signi®cantly changed bythe metasomatic processes, we chose as a model for thepre-metasomatic upper mantle a refractory peridotitewith HREE concentrations similar to those of mantlexenoliths from La Palma. These requirements are met bya peridotite (called ``Depleted peridotite'' in Fig. 11)with trace element concentrations equal to 25% of thosein normal abyssal peridotite, as estimated by Wood(1979). The most highly silicic glass in the vein system inspinel harzburgite PAT2-4 was used as a model for themetasomatic agent (High-silica glass in Fig. 11). Fig-ure 11 shows that addition of small volumes of silicicmelts to a refractory peridotite will signi®cantly increaseits content of strongly incompatible elements withoutcausing signi®cant changes in the least incompatibleones. The trace element pattern of average hydrousspinel harzburgite from La Palma falls within the rangedisplayed by Depleted peridotite mixed with 1±7% silicicmelt.

Repeated episodes of basaltic melt in®ltration andIRC-processes in a certain volume of peridotite are ex-pected progressively to increase the degree of metaso-matism. Pervasive cryptic metasomatism of oceanicmantle peridotite on a large scale, as re¯ected in CanaryIslands xenoliths (see below), is most likely the result ofrepeated metasomatic events.

Silicic melts as metasomatic agents in other regions

Available information indicates that silicic melt inclu-sions (60±72 wt% SiO2) are common also in mantlexenoliths from other areas, and are considerably morecommon than carbonatitic melt inclusions (e.g. Sienaet al. 1991; Draper 1992; Schiano et al. 1992, 1994, 1995;Hauri et al. 1993; Schiano and Clocchiatti 1994;I. Ryabchikov, personal communication, 1995; Draper

77

and Green 1997; Zinngrebe and Foley 1995). Most ofthese authors have models for the formation of silicicmelts that di�er from ours (see discussion above). In

spite of this, it is possible that silicic melts representimportant metasomatic agents on a general basis.Zinngrebe and Foley (1995) and Draper and Green(1997) proposed silicic melts as a possible agent causingcryptic metasomatism in the upper mantle under Gees,Eifel, Germany, and in general, respectively.

We have tested silicic, basaltic and carbonatitic meltsas potential metasomatic agents against chemical dataon mantle xenoliths from Victoria, Australia, andMongolia. Like Canary Islands xenoliths, anhydrousand hydrous spinel lherzolite xenoliths from Victoria,Australia (data from O'Reilly and Gri�n 1988) show a

Cs Rb Ba K Nb La Ce Sr Nd Sm EuZr Gd Ti Dy Y

Sam

ple/

C1

10-3

7%

Depleted peridotite

High-silica glass

Average hydrous harzburgite from La Palma

1%

3%

10-2

10-1

100

101

103

La Palma

Carbonatite

BASALT Phlog-veinglass

Si-rich glassin unveinedxenoliths

Hierro

LanzaroteSi-RICHGLASS

Amph-veinglass

Tenerife

Tenerife

La Palma

Amph-vein glassPhlog-veinglass

HierroLanzarote

CARBONATITE

M

M

Si-RICHGLASS

CARBONATITE

BASALT

Si-rich glassin unveinedxenoliths

0

0

1

1

2

2

3

3

4

4

5

5

6

6

7

7

8

8La/Nd

105

104

103

102

102

101

101

100

100

Ti (

ppm

)C

aO (

wt%

)

10-1

Fig. 10 CaO-La/Nd and Ti-La/Nd relations (whole-rockdata) among peridotite xeno-liths from La Palma, Hierro,Lanzarote and Tenerife, com-pared to potential metasomaticagents: highly silicic melts (thisstudy), Canarian basaltic lavas(this study; E.-R. Neumann andE. Wul�-Pedersen unpublisheddata), carbonatitic melts (datafrom: Barrera et al. 1981; Pelland HoÈ y 1989, and referencestherein; Green et al. 1992;Sweeney et al. 1994; Klemmeet al. 1995; Hornig-Kjarsgaard1998), and abyssal peridotite(M La and Nd data for N-typeMORB mantle from Wood1979; CaO data estimated forprimitive MORB residue byGreen et al. 1979; and forabyssal spinel lherzolite, Atlan-tic ocean by Dick and Fisher1984). Wide ranges in La/Ndratios with near constant, lowCaO in peridotite xenolithsfrom La Palma, Hierro andLanzarote suggest that highlysilicic melts are much morelikely metasomatic agents thanare basaltic or carbonatiticmelts. The data on Tenerifexenoliths are inconclusive

Fig. 11 Estimated trace element patterns of highly refractoryperidotite (Depleted peridotite) mixed with 1, 3 and 7% of highlysilicic melt (dotted and dashed lines marked 1%, 3%, and 7%). Thecomposition of Depleted peridotite corresponds to 25% of theconcentrations estimated by Wood (1979) for N-type MORB sourcemantle (Table 1). For comparison is shown the composition ofaverage hydrous spinel harzburgite from La Palma (Wul�-Pedersenet al. 1996a). Concentrations are normalized to average C1 chondritecomposition (Anders and Grevesse 1989). See text for discussion

b

78

wide range in La/Nd ratios with near-constant CaOand Ti (Fig. 12), de®ning a trend from depletedperidotite towards the ®eld of highly silicic glass.O'Reilly and Gri�n (1988) attributed the LREE-en-richment in the Australian lherzolites to CO2-rich ¯u-ids. However, Fig. 12 implies that metasomatism bysilicic melts is an alternative possibility. Spinel lherzoliteand harzburgite xenoliths from Mongolia (data fromWiechert et al. 1997), in contrast, cover an area of re-stricted La/Nd ratios, and CaO and Ti concentrationssimilar to that covered by Tenerife xenoliths. Wiechertet al. (1997) related enrichment in these xenoliths tocarbonate melts.

Concluding remarks

Veined xenoliths from La Palma showing a gradualchange from Ti-Fe-Mg-Ca-rich alkali basaltic glass inbroad veins, to Si-Na-K-rich, Ti-Fe-Mg-Ca-poor glassin narrow veins, have been used to study the e�ects of

IRC-processes (basaltic melt In®ltration, Reaction andCrystallization) on the trace element compositions ofglasses and minerals produced through such processes.Increasing SiO2- and decreasing MgO-contents, frombasaltic glass in broad amphibole-veins, to ®lms of highlysilicic glass along grain boundaries (INF-v) is accom-panied by a progressive depletion in moderately litho-phile elements relative to highly lithophile ones in glassesand coexisting clinopyroxenes. The most extreme traceelement patterns are exhibited by the most silicic glassinclusions and interstitial glasses in unveined peridotitexenoliths. Clinopyroxenes show decreasing REE con-centrations and decreasing LREE/HREE ratios withincreasing SiO2-contents in the coexisting glass.

The observed changes in trace element compositionsthroughout the vein system are compatible with changesexpected from IRC-processes, as proposed earlier on thebasis of petrographic and major element data. Further-more, estimates indicate equilibrium, or near-equilibri-um, between the most highly silicic glasses and their hostperidotites, as implied by the IRC-model.

Mongolia

Australia

Mongolia

Si-RICHGLASS

CARBONATITE

BASALT

BASALT

Si-RICHGLASSS

CARBONATITE

Tenerife vein-glass trend

Tenerife vein-glass trend

M

AustraliaM

0

0

1

1

2

2

3

3

4

4

5

5

6

6

7

7

8

8

La/Nd

105

104

103

102

102

101

101

100

100

Ti (

ppm

)C

aO (

wt%

)

10-1

Fig. 12 CaO-La/Nd and Ti-La/Nd relations among perido-tite xenoliths from Victoria,Australia (data from O'Reillyand Gri�n 1988) and Mongolia(Wiechert et al. 1997) comparedto di�erent metasomatic agents(references in Fig. 10). Thetrends de®ned by Australianxenoliths suggest metasomatismby highly silicic melts ratherthan basaltic or carbonatiticones

79

The zoning patterns of olivine adjacent to veins in-dicate that di�usion processes associated with the veinprocesses lasted for at least 600 years, implying that theveining processes studied in the veined xenoliths cannothave taken place during transport of the xenoliths to thesurface, but must represent mantle processes.

Metasomatism of the mantle by silicic melts may bedi�erentiated from metasomatism by basaltic or car-bonatitic melts on the basis of whole-rock CaO-TiO2-La/Nd relations in metasomatized peridotite. Metaso-matism by basaltic and carbonatitic melts is expected tolead to increasing TiO2 and CaO with negligible tomoderate increases in the La/Nd ratio, whereas meta-somatism by silicic melts is expected to lead to strongenrichment in La/Nd, with negligible changes in CaO orTiO2.

Metasomatized oceanic peridotite xenoliths from LaPalma, Hierro and Lanzarote show clear trends of near-constant CaO and TiO2 with increasing La/Nd, from the®eld of normal abyssal peridotite towards that of highlysilicic glasses. This strongly suggests metasomatism bysilicic melts. The compositional relations of peridotitesfrom Tenerife, in contrast, are compatible with meta-somatism by basaltic or carbonatitic melts. The meta-somatism re¯ected in the trace element signatures ofultrama®c xenoliths from La Palma may be accountedfor by addition of small amounts (<7%) of highly silicicmelts. Also peridotites from other areas show CaO-TiO2-La/Nd relations suggesting metasomatism by si-licic melts.

Acknowledgements This project has been ®nanced through grantsfrom the Norwegian Research Council for Sciences and Humani-ties (NAVF), Nansenfondet and associated funds, and the Com-mission of the European Communities, DGXII, EnvironmentProgramme, Climatology and Natural Hazards Unit, under thecontract EV5V-CT-9283. We gratefully acknowledge the permis-sion from the Ayuntamiento de Fuencaliente de La Palma (toE.-R.N. in 1988) to collect xenolith samples from the volcanoes ofSan Antonio and Tenegua. This manuscript has improved signi®-cantly through constructive criticism and suggestions from TimElliot, Trevor Green, Bill Gri�n, Sue OÔReilly, Jacques Touret,Reidar Trùnnes, and Marjorie Wilson.

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