Contrib Mineral Petrol (1996) 125: 406–420 C Springer-Verlag 1996

Yigang Xu ? Jean-Claude C. MercierMartin A. Menzies ? John V. Ross? Ben HarteChuanyong Lin ? Lanbin Shi

K-rich glass-bearing wehrlite xenoliths from Yitong, Northeastern China:petrological and chemical evidence for mantle metasomatism

Received: 1 August 1995y Accepted: 19 June 1996

Abstract Ultramafic xenoliths in Cenozoic alkalibasalts from Yitong, northeast China comprise threetypes in terms of their modal mineralogy: lherzolite, py-roxenite and wehrlite. The wehrlite suite always containsinterstitial paleybrown glass which occupies several percent by volume of the whole rock. The texture of thewehrlites is porphyroclastic with some large strainedgrains of olivine (0.5–1 mm) scattered in a very finegrained matrix (0.1 mm), implying a metamorphic orig-in for the protolith rather than an igneous origin. The hostminerals are compositionally zoned, showing evidenceof reaction with a melt. Petrological evidence for resorp-tion of spinel (lherzolite) and orthopyroxene (wehrlite)by infiltrating melt further supports the hypothesis thatthe wehrlites result from interaction between a partialmelting residue and a melt, which preferentially re-

placed primary spinel, Cr-diopside and enstatite to pro-duce secondary clinopyroxene (cpx)1 olivine (ol)+ chromite + feldspar (fd). The composition of themineral phases supports this inference and, further indi-cates that, prior to melt impregnation, the protoliths ofthese wehrlites must have been subjected to at least oneearlier Fe-enrichment event. This explanation is consis-tent with the restricted occurrence of glasses in thewehrlite suite. The glass is generally associated withfine-grained (0.1 mm) minerals (cpx1ol1chromite+fd). Electron microprobe analyses of these glassesshow them to have high SiO2 content (54–60 wt%), a highcontent of alkalis (Na2O, 5.6–8.0%; K2O, 6.3–9.0%),high Al2O3 (20–24%), and a depletion in CaO (0.13–2.83%), FeO (0.89–4.42%) and MgO (0.29–1.18%). Ionprobe analyses reveal a light rare earth element-enrich-ment in these glasses with chondrite normalised(La)n 5 268–480. The high K2O contents in these glassesand their mode of occurrence argue against an origin byin-situmelting of pre-existent phases. Petrographic char-acteristics and trace element data also exclude the possi-bility of percolation of host-basalt related melts for theorigin of these glasses. Thus the glasses must have re-sulted from local penetration of mantle metasomaticmelts which may have been produced by partial meltingof peridotites with involvement of deep-seated fluids.Such melts may have been significantly modified by sub-sequent fractional crystallization of ol, cpx and sp, exten-sive reaction with the mantle conduit and the xenolithtransport process.

Introduction

Glasses in high-temperature ultramafic xenoliths havegenerally been interpreted as decompression melts ofphases such as amphibole, mica or pyroxene formed dur-ing rapid ascent to the Earth’s surface after entrainmentin the host basalt (Frey and Green 1974; Francis 1976).This interpretation, however, has been questioned(Maaloe and Printzlau 1979; Jones et al. 1983; Kuo and

Y. Xu (✉) ? J.-C.C. Mercier1

Laboratoire de pe´trologie physique,UniversiteParis 7 and I.P.G.P., 2 Place Jussieu,F-75252, Paris, France

Y. Xu ? M.A. MenziesDepartment of Geology, Royal Holloway University of London,Egham, Surrey, TW20 OEX, UK

Y. XuGuangzhou Institute of Geochemistry,Chinese Academy of Sciences, 510640 Guangzhou, P.R. China

J.V. RossDepartment of Geoscience, University of British Columbia,Vancouver, Canada VGT 1Z4

B. HarteDepartment of Geology andGeophysics,University of Edinburgh, Edinburgh, EH9 3JW, UK

C. Lin ? L. ShiInstitute of Geology, State Seismological Bureau,100029 Beijing, P.R. China

Present address:1 Departement des Siences de la Terre,Universitede la Rochelle, Rue Marillac,F-17000 La Rochelle, France

Editorial responsibility: T. L. Grove

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Essene 1985; Gamble and Kyle1987; Ionov et al. 1994),because glasses are rarely reported in xenoliths despitethe fact that mantle xenoliths are brought up at similarvelocities by alkali-basalt volcanism. Textural and chem-ical evidence has been called upon to suggest that someglasses could have formed through partial melting in theupper mantle prior to entrainment of the xenoliths. Withthe possibility that some glasses within mantle xenolithsmay indeed represent quenched upper mantle melts, in-terest in glasses in xenoliths has been growing rapidly,with the hope that they can provide information on incip-ient partial melting and, alternatively, on the nature ofmantle metasomatism. Wehrlite xenoliths from Yitong,northeast China contain glass that is particular in severalaspects: (1) the relatively large percentage of glass in thewhole rock; (2) the remarkable homogeneity of the glasscompared to cross-cutting host-basalt veins; (3) the glassis scattered throughout the rock despite locally variablemodes; (4) a peculiar chemical composition extremelyenriched in alkali elements. These features suggest anorigin for the glass in the Yitong wehrlites significantlydifferent from other xenolith glasses reported in the lit-erature (e.g. Frey and Green 1974; Francis 1976; Maaloeand Printzlau 1979). Clear petrographic evidence for re-action between melt and peridotite (in some samplestransitional between melt-free lherzolite and melt-richwehrlite) provides new information on possible interac-tions between percolating melts and upper mantle peri-dotites (e.g. Green and Wallace 1988; Kelemen 1990;Yaxley et al. 1991) as well as direct evidence for someprocesses of formation of ultramafic rock types such aswehrlites andyor dunites. In this paper, we present thetextural and chemical features of the Yitong wehrlitesand of their glasses. The following questions will be ad-dressed. (1) Do the wehrlite and glass assemblages rep-resent the products of a mantle metasomatic event and, ifso, (2) what is the nature of that event?

Petrography of the glass-bearing wehrlites

At Yitong, Jilin province, NE China (Fig. 1), the ultramafic xeno-liths found in late Miocene basaltic centres can be ascribed to threedistinct series according to their modal composition. Thelherzo-lite suitetypically includes spinel (sp) lherzolites, whose texture,mineral chemistry and trace element composition (Xu et al. 1993,1996) compare favourably to group I xenoliths (Frey and Prinz1978). Thepyroxenite suiteis characterized by coarse-grainedigneous textures and distinct mineral compositions [Mgy(Mg1Fe)5 Mg'ol 5 0.80, Cry(Cr1A1) 5 Cr'sp 5 0.03). Thesepyroxenites are considered to be crystallisation products from analkali parent magma in the upper mantle. Lastly, thewehrlite suiteis characterized by an olivine (ol)-clinopyroxene (cpx) mineralassemblage and is distinguished petrographically from the lherzo-lite series by the usual lack of orthopyroxene (opx), a large modalproportion of ol, a very small grain size (, 1 mm; often even, 0.2 mm; Fig. 2), and conspicuous interstitial glass (light brownto colourless).

Fourteen of these wehrlites have been analysed in detail. Thetextures of the wehrlites are granuloblastic to highly strained por-phyroclastic (Harte 1977). Abundant glass and cpx phases (50%modal) are unevenly distributed on the scale of a thin section.Some glass is scattered throughout the ol matrix, but most appears

Fig. 1 Simplified geological map of Yitong region

as large ol-free clusters (3 mm) or “melt pockets”, where thephases (cpx and glass) tend to display a concentric distribution.For convenience, the ol-rich matrix and the melt pockets will bediscussed separately.

Matrix ol of the porphyroclastic samples consists of two gener-ations with non overlapping ranges of grain sizes (ca 1.5–2 mm forkinked porphyroclasts versus ca. 0.2 mm for neoblasts; Fig. 2B).The granuloblastic samples (Fig. 2A,C) display grain sizes rangingfrom 0.1 to 0.5 mm with virtually no optically visible deformationof the crystals. Thematrix cpxhas grain sizes similar to that of theol neoblasts. These cpx display numerous minute glassy blebs scat-tered throughout the crystals which gives them a spongy appear-ance. Some samples display a well developed foliation (Fig. 2D),involving lamination of the cpx and melt pockets. In addition tothese major phases, the matrix also contains trace amounts ofmagnetite and some interstitial glass, wetting indifferently eithercpx-ol or ol-ol boundaries.

Opx has been found in only one of the wehrlite samples (3–22).Thisopx wehrlitehas a granuloblastic texture similar to that of theordinary wehrlites described above, but with a slightly coarsergrain size (1.1 mm). Opx is scattered through the sample as crys-tals (1.4 mm), and is a minor phase (4 crystals in a thin section).More interesting is the fact that it appears as a relict phase withreaction corona of ol and cpx similar to the pyroxene of the com-mon wehrlites or of the matrix of this sample (Fig. 3C). By estab-lishing a qualitative budget for the reaction, it is evident that incon-gruent melting may only be part of the reaction and that Ca is beingadded to the system. Hence, this sample demonstrates open-systemresorption of a former opx.

Glass in the wehrlites attains 10% by volume of the whole rockand is usually over 5%. These glasses are associated with the greencpx and fine-grained zones (Fig. 4A). Some glasses also containround vesicles. The very low totals for the analyses by electron

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Fig. 2A–D Photomicrographs of glass-bearing wehrlite xenolithsfrom Yitong, NE China:A fine-grained wehrlite with no, or slight,foliation (3–15, crossed polarizers);B deformed olivine (kinkband) in wehrlite, similar to those found in common tectonites(3–24, crossed polarizers);C fine-grained wehrlite with partiallymelted clinopyroxenes and some infiltrated melts (3–15, transmit-ted light);D fine-grained wehrlite with a well-developed foliationof the partially melted clinopyroxenes and glasses (3–24, trans-mitted light)

elongate and define a well-defined foliation, the glass must havebeen present during mantle deformation. All these characteristicsimply, firstly, that these two melts are unrelated and, secondly, thatthe penetrative glass phase of the wehrlites existed before the erup-tion started.

In addition to wehrlites, a peculiar lherzolite (3–2) of potentialsignificance for establishing the origin of the wehrlites has alsobeen analysed. It has a granuloblastic texture with an ol grain sizeof 1.5 mm. It has brown amoeboid sp similar to those in the lherzo-lites, but these sp (0.5 mm) are surrounded by feldspars and cpx,plus minor chromite (Fig. 3A). The opx is similar to those in thelherzolites. Most of the cpx (0.7 mm) resemble those of thewehrlites (no exsolutions, similar ragged contours, cloudiness).Whatever their origin (thermal destablization, reaction with animpregnating melt, or both) they are clearly a late phase if onlybecause of the existence of examples of a former cpx undergoingtransformation (0.1–0.2 mm rims) into the now dominant cpxphase (Fig. 3B). This sample and the above-mentioned 3–22 repre-sent examples of the petrogenetic “links” one might expect be-tween mantle lherzolite and wehrlite if the latter were formedthrough reaction with a melt.

Analytical methods

Minerals and glasses were analysed with the CAMEBAX mi-croprobe at the CAMPARIS analytical centre (University Paris-6,France), using an automatic four-spectrometer wavelength disper-sive system. For mineral analyses, the acceleration voltage appliedwas 15 kV with a beam current of 40 nA and a beam diameter of

microprobe suggest that these vesicles may result from degassingof the liquid. The coarse (up to 3 mm) melt pockets (Fig. 4A) showvariable modal compositions from sample to sample, with a glass-poor end-member with interstitial glass concentrated at the core ofclusters of nebulous cpx. Concentrations of small chromite crys-tals appear superimposed onto the glass-pyroxene structure. Inglass-rich samples, the melt pockets include microphenocrysts(, 0.1 mm; Fig. 4A) of diopside, ol, opaques and, sometimes,feldspar. The melt pockets do not have clear, “crystal-shape”boundaries. Rather, some of the glass wets the grain boundariesthroughout the matrix, and the melt pockets consist of homoge-neous and pale brown glass in their centre with a peripheral con-centration of microphenocrysts with either euhedral crystallinehabits or embayed shapes (Fig. 4B).

It is worth emphasizing the peculiar relationships between thisglass and the host magma, as exemplified by one of the wehrlites(3–34; Fig. 4C). In contrast to the glass, cross-cutting basalticveins contain abundant phenocrysts and microlites, and a deepcolour, whereas the glasses appear homogeneous, with many crys-tallites and crystals. Moreover, since the glasses are sometimes

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Fig. 4A–C Some characteristics of the impregnating glasses inthe wehrlites from Yitong, China:A a large melt pocket in sample1–45 composed of fine-grained clinopyroxenes, olivines andopaques;B glasses associated with the partially molten clinopy-roxenes (3–15);C intruding and infiltrating host-basalt vein cross-cutting the glass-bearing wehrlite (3–34)

1–2 mm. The counting time varied between 10 and 20 secondsdepending upon the elements. For the feldspar and glass analyses,either the beam was defocused to about 10mm using fluorescenceor the current intensity was reduced to 10 nA. The data werecollected using the Bence-Albee correction method and precisionwas evaluated on synthetic oxide calibrations and natural mineralstandards PHN-1569.

Trace element (Ti, Zr, Nb, Hf, Sr, Y) and rare earth element(REE) analyses were performedin-situ in polished, gold-coatedthin sections using a Cameca ims-4f ion microprobe at the Univer-sity of Edinburgh, with a 10 nA O- primary beam of 10 keV energyfocused to ca. 20mm in diameter. Positively charged secondaryions were measured in the mass spectrometer with an energy offset

Fig. 3A–C The petrological reactions related to the transforma-tion of lherzolite into wehrlite by the impregnating liquid:A re-sorption of spinel in a “transitional” lherzolite (3–2); relict spinelis surrounded by a “cloud” of secondary clinopyroxenes, plagio-clases and chromites;B alteration of Cr-clinopyroxene (3–2);C resorption of the primary orthopyroxenes by interaction withimpregnating liquids; secondary small clinopyroxenes andolivines are produced at their periphery (3–22)

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of 75 eV from the secondary accelerating voltage of 4500 eV (e.g.energy filtering). This technique minimised molecular ion inter-ferences in the mass spectrum. However, barium oxides still sig-nificantly interfered with151Eu and153Eu, particularly when Bacontents in samples were relatively high (. 200 ppm). A correc-tion was made using BaOyBa ratio determined for each analysis.Corrections have further been made for complexes of the majorelements (Si1Al) and LREE, since there are significant overlapsof LaSi and CeAl on atomic mass 167 (Er) in the case of glassanalysis. The ion intensities were normalised to Si1 and conversionfrom measured ion intensities to element concentrations of un-known samples was based on a calibration using the syntheticsilicate glass NBS-610 as reference standard (Hinton 1990).This standard was analysed under the same analytical conditionsas the unknown samples during each day. At least three and upto six analyses have been performed in each sample. The overallaccuracy and precision estimated from replicate analyses are+10–15% for Sr, Zr,+5–10% for light REE (LREE),+10–15%for heavy REE (HREE). The accuracy and precision are alsochecked against the secondary standard of cpx from KilbourneHole (KH1).

Results

Electron microprobe data

Fourteen wehrlites have been analysed in detail and rep-resentative data are listed in Tables 1–5.

Olivine

On a CaOversusMg' diagram (Fig. 5A), four sampleshave lower Mg' (ca 0.78–0.86) than the bulk of the othersamples (0.88–0.91; median value of 0.898). In theselatter samples, calcium in ol is clearly not related toMg', but varies within all the samples. There is no dif-ference between porphyroclast and neoblast cores: theirCaO contents are less than 0.1%. However, ol adjacent toglass or cpx is zoned and is significantly enriched in Catowards the grain boundary. Within 10mm values are ashigh as 0.4wt%. Routine analyses of Cr, Al and Ti showno significant concentrations in the ol cores, but olivinerim enrichment in CaO is accompanied by a slight in-crease in Cr, Al, and Ti. The concentrations of Ni and Mnare about 0.006 (with just a few values reaching 0.009 atMg' of 0.90), and 0.0045, respectively. These values arein the range of those for common group I lherzolites(Frey and Prinz 1978). For reference, median values forMg' and Mn of ol in lherzolites from the present locali-ties are 0.89 and 0.003, respectively.

Euhedral ols within the melt pockets have cores withMg' similar to those of the matrix ol, though they de-fine a slightly broader range towards lower values as aresult of zoning: detailed stepped profiles across ol ofsample 3–34 showed continuous significant variationsfrom 0.903 to 0.864, from core to rim, respectively. Azonation is also found for CaO, with higher values at therims, as has been confirmed through a detailed trace ele-ment study on the microprobe on mounted separatedgrains. They show markedly higher CaO contents in theircores than the matrix ol.

Fig. 5 Variation in Mg-number with CaO (A) and Mn (B) ofolivines from Yitong wehrlite xenoliths. Lherzolite and pyroxenitedomains according to Xu et al. (1993). Four samples significantlyenriched in Fe relative to normal mantle values (e.g. group I xeno-liths, Frey and Prinz 1978). For all samples, the rim area ofolivines in matrix and crystals in glass (solid symbolin A) haveremarkably high CaO values

Comparing the bulk of the analytical data from mag-nesian samples to those for the Fe-rich samples, only Mnappears to be strongly correlated to Mg', with a varia-tion from 0.011 to 0.004 for Mg' increasing from 0.78to 0.90, respectively (Fig. 5B). The coarse-grained py-roxenites from the same localities plot conspicuouslyoutside the trend defined by the wehrlites, whereas thelherzolites fall at the high-Mg end of it. Concentrationsof Ni (0.006) are remarkably constant and independent ofMg', though slightly lower values (0.003) are observedfor ol in melt pockets (3–30) or those associated withresorption of opx (3–22), relative to the matrix ol. As forCaO, the ol of each of these Fe-rich samples behave asdescribed above, with similar concentrations and com-positional variations.

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Table 1 Representative olivine analyses from glass-bearing wehrlite xenoliths from Yitong, NE China [PC, PRporphyroclast cores andrims respectively,GC andGR cores and rims of euhedral olivines in glass,Mg' Mgy(Mg1Fe)]

Specimen 3–2 3–24 3–26 3–34 MA91-4 1–45

Core PC PR PC GC GR PC GC GR PC PR GC GR PC GC

SiO2 40.99 40.65 39.43 40.32 40.23 40.28 40.52 39.89 39.21 40.98 40.51 40.21 40.09 40.14 40.23FeO 9.11 10.38 10.51 11.39 11.02 11.11 10.45 10.72 12.58 9.92 10.20 10.87 10.44 10.64 10.41NiO 0.09 0.19 0.27 0.20 0.22 0.23 0.19 0.25 0.21 0.33 0.20 0.27 0.15 0.22 0.28MnO 0.31 0.27 0.28 0.30 0.31 0.39 0.36 0.26 0.17 0.00 0.33 0.37 0.15 0.19 0.34MgO 49.16 48.71 47.06 48.09 48.79 48.92 48.56 47.80 44.94 49.18 49.38 48.81 48.41 49.14 49.51CaO 0.03 0.03 0.37 0.05 0.10 0.21 0.04 0.30 1.08 0.05 0.20 0.32 0.31 0.00 0.03

Total 99.77 100.28 98.14 100.37 100.92 101.41 100.13 99.38 98.29 100.47 100.90 100.97 99.92 100.70 100.86

100*Mg' 90.16 89.32 88.87 88.27 88.75 88.70 89.23 88.82 86.43 89.84 89.61 88.90 89.21 89.18 89.46

Specimen 3–61 3–15 3–22 3–30 3–40

PC PR GC GR PC GC PC PR Rim§ PC PR GC PR GC GR

SiO2 40.26 38.96 39.75 39.89 38.47 38.45 40.22 39.71 39.61 40.02 39.78 39.92 37.69 39.30 37.88FeO 11.25 12.81 10.72 12.21 20.38 18.65 13.32 13.61 13.17 14.24 13.56 13.96 18.47 17.49 19.06NiO 0.33 0.28 0.03 0.32 0.45 0.13 0.27 0.24 0.20 0.28 0.23 0.32 0.38 0.39 0.38MnO 0.19 0.30 0.32 0.21 0.26 0.43 0.18 0.22 0.20 0.27 0.28 0.24 0.28 0.29 0.30MgO 47.53 44.82 48.42 47.28 40.24 40.25 45.91 45.67 45.10 45.71 46.45 45.45 41.82 41.84 40.49CaO 0.05 0.38 0.32 0.32 0.05 0.11 0.04 0.16 0.45 0.04 0.18 0.40 0.02 0.38 0.41

Total 99.63 98.66 99.65 100.31 99.88 98.03 99.97 99.68 99.14 100.60 100.56 100.45 98.69 99.84 98.60

100*Mg' 88.27 86.19 88.10 87.35 77.88 79.34 86.00 85.69 85.93 85.13 85.92 85.42 80.30 81.00 78.80

a Olivine in replacement rim of orthopyroxene. Note that CaO is always distinctly higher in rims and cores of euhedral crystals in glass,compared to olivine porphyroclast cores

Clinopyroxene

Analyses of the cpx show some compositional complexi-ties. The “matrix” cpxs have a high and restricted Mg'(0.902–0.928) and are characterized by relatively highSiO2 (52.2–54.6%) and Cr2O3 (1% up to 3.5%, while theCr-diopsideseries, 1.5%); and low Al2O3 for somesamples (, 2% for 3–26, 3–34). However, the cpx incontact with glasses shows a strong chemical zonation asillustrated by sample 3–24: TiO2 and CaO increase fromcore to rim, whereas Mg', SiO2 and Cr2O3 contents havedecreasing trends (Table 2). The cpx in glasses except forsample 3–24 is higher in Al2O3 and TiO2 contents andslightly lower in Mg' than the matrix cpx. When com-pared to the average composition of the cpx in the lherzo-lite suite from the same volcanic area, Yitong wehrlitecpx (including matrix and euhedral crystals) shows somesimilarities, but is characterized by high Cr and low Alcontents (Fig. 6A), which perhaps accounts for the cleargreen colour observed in the hand specimens. Thesegreen cpx differ significantly from those in the pyroxen-ite suite (Fig. 6A). Crystals in the glasses are composi-tionally zoned with increasing Al2O3, TiO2 and FeO fromcore to rim, whereas the reverse holds for SiO2 and Cr2O3

(Fig. 6B).The Mg's of cpxs from the Fe-rich samples, are cor-

related with those of ol (e.g. samples 3–15, 3–40). How-ever, even the samples with relatively high Fe contents(Mg' values are 0.835 and 0.866 for 3–15 and 3–40,

respectively) are compositionally not similar to Al-augite or megacrysts (Wass 1979). In fact, their highCr2O3 content may indicate that their parent mineralsalso belong to theCr-diopsideseries (Wilshire and Sher-vais 1975) and that Fe-enrichment may be caused by an-other separate event.

The cpxs of the two transitional samples (3–2 and3–22; Table 2) have compositions which vary consider-ably from grain to grain. Compared to clean cpx, thespongy textured ones in sample 3–2 are enriched in Caand highly deficient in Al and Na which must have en-tered incongruent melts. The cpxs associated with spinelresorption zones (3–2), are also characterised by an in-crease in Ca and a decrease in Na and Al. The latter couldbe related to either the leaching effect of impregnatingliquid or the buffering effect of coexisting newly formedfeldspars. A similar chemical feature is found for the cpxin replacement rims of opx (3–22) with a notable defi-ciency in Al. Another important characteristic of cpxs inreaction zones is that they have values of Mg' virtuallyidentical to those of the original ones.

Orthopyroxene

Only one wehrlite (3–22) contains opx. Its compositionis virtually identical to those for lherzolites, except fora relatively low Mg' of 0.876 (Table 3). However,this value is fully consistent with that for ol (Fig. 5)

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Table 2 Representative electron microprobe analyses of clinopyroxenes in wehrlite xenoliths from Yitong. [PC, PRporphyroclast coresand rims respectively,GC andGR cores and rims of euhedral clinopyroxenes in glass)

Specimen 3–2 3–24 3–26 3–34 3–61 M-4

Clear Spongy Reactedcore rim zone PC PR GC GR PC PR PC GC GR PC GC GR GC

SiO2 54.39 53.05 51.57 51.54 48.55 52.27 50.48 52.44 53.47 53.42 53.47 49.49 54.09 53.02 45.65 53.65TiO2 0.05 0.07 0.49 0.33 1.98 0.57 0.91 0.32 0.19 0.16 0.03 0.84 0.02 0.29 2.75 0.18Al2O3 5.22 3.19 4.94 5.28 5.71 2.91 5.18 3.39 1.23 1.56 2.19 3.82 1.16 3.52 7.93 1.91Cr2O3 1.15 1.89 1.67 1.56 0.91 0.54 0.25 2.09 1.42 1.24 1.99 0.55 1.03 1.87 0.12 1.51FeO 2.51 2.57 2.17 2.15 3.67 2.86 3.64 2.45 3.24 2.67 2.59 4.27 2.92 2.45 6.96 2.49MnO 0.08 0.10 0.01 0.0 0.05 0.20 0.16 0.05 0.06 0.08 0.12 0.04 0.07 0.06 0.12 0.00MgO 15.31 17.33 15.77 15.55 14.15 16.31 14.27 15.72 16.30 16.50 16.21 14.79 17.57 15.96 11.85 17.08CaO 17.85 20.24 20.88 20.92 22.35 22.64 22.31 21.35 22.29 23.12 21.49 22.70 21.47 21.41 22.78 23.23Na2O 2.52 0.83 0.98 1.07 0.71 0.48 0.51 1.36 0.65 0.62 1.10 0.71 0.78 1.18 0.63 0.80K2O 0.00 0.00 0.00 0.00 0.01 0.09 0.06 0.00 0.01 0.01 0.05 0.02 0.00 0.00 0.01 0.05Total 99.09 99.34 98.52 98.54 98.09 98.95 97.86 99.20 98.93 99.38 99.35 97.27 99.13 99.78 98.83 100.90100*Mg' 91.58 92.32 92.82 92.79 87.30 91.07 87.48 91.95 89.98 92.92 91.77 86.06 91.47 92.07 75.21 92.06

Specimen M-4 1–45 3–15 3–22 3–30 3–40

GC GC GR PC GC GR PC Rim§ PC PR GC GR PC PR GC GR

SiO2 51.57 54.28 54.64 51.99 51.18 51.05 52.66 54.39 51.87 46.57 50.23 49.02 51.98 50.23 49.49 46.31TiO2 0.76 0.09 0.07 0.14 0.48 0.41 0.09 0.15 0.47 2.51 0.96 1.95 0.49 0.59 1.55 2.91Al2O3 3.78 1.04 0.68 2.13 3.09 2.42 2.46 0.82 3.31 7.79 4.41 6.68 3.01 2.22 3.87 7.38Cr2O3 1.10 3.06 1.66 1.36 1.44 1.00 2.98 2.95 1.24 0.43 1.36 1.09 1.95 1.10 0.56 0.41FeO 2.55 2.11 2.17 4.87 5.10 5.14 3.23 3.19 3.12 4.64 3.30 3.72 4.27 4.47 5.64 6.98MnO 0.23 0.14 0.03 0.09 0.17 0.14 0.12 0.14 0.16 0.11 0.05 0.06 0.10 0.11 0.03 0.15MgO 15.57 16.43 17.31 15.01 14.52 14.53 16.40 17.19 16.15 13.31 15.70 13.45 15.53 15.33 14.02 11.94CaO 23.70 20.58 21.23 22.03 22.21 22.65 19.44 19.39 22.65 22.92 22.99 22.30 20.87 22.27 22.72 22.81Na2O 0.78 1.56 0.89 0.71 0.83 0.60 1.37 1.17 0.57 0.56 0.51 0.88 1.01 0.52 0.52 0.62K2O 0.08 0.08 0.03 0.00 0.03 0.05 0.00 0.00 0.00 0.00 0.01 0.26 0.00 0.00 0.01 0.03Total 100.42 99.24 98.86 98.48 99.13 97.98 98.60 99.41 99.62 98.90 99.52 99.43 99.26 96.89 98.45 99.60100*Mg' 91.59 93.30 93.43 84.60 83.52 83.43 90.64 90.53 90.22 83.62 89.45 86.57 86.63 85.93 79.11 75.30

a Clinopyroxene in replacement rim of orthopyroxene

and remains relatively close to normal upper mantle val-ues.

Feldspar

Table 3 shows that the feldspars in reaction zones of lher-zolite sample 3–2 are not only plagioclase, but sanidine.Such-high K feldspars have rarely been reported frommantle peridotites. Their existence implies that an infil-trated K-rich liquid must have been involved in the reac-tion with spinel (Fig. 3), because it cannot be accountedfor in the mass balance. Detailed probe analyses showthat these feldspars are relatively heterogeneous increas-ing in Al2O3, CaO, TiO2 and decreasing in K2O and SiO2

from core to the rim adjacent to cpx. Only a few glassescontain quench plagioclase crystals (Table 3).

Spinel

The Cr'sp of lherzolite 3–2 has a range of 0.31–0.32,typical of mantle xenoliths from alkali basalts (Frey andPrinz 1978). The coexisting black sps are much richer in

Cr (Cr' 5 0.46). Core to rim analyses in steps of a fewmicrometers have been completed on brown sp (3–2) butno chemical zonation was found.

There is no primary brown sp left in the wehrlites.The small size of the sps in wehrlites makes it difficultto study any potential zonation and representative resultsare given in Table 4 and illustrated in Fig. 7. Ferric ironwas calculated by assuming that sp represents solid solu-tion between R21R31O4 and R21R41O4. It is clear thatthese sps are different from typical lherzolite sp, beingenriched in Cr, Fe31 and Ti. All these Cr-sp lie outside ofthe areas defined by Dick and Bullen (1984) for abyssalperidotites. Samples (3–15, 3–30, 3–40) lie under Dickand Bullen’s compositional variation trend calculatedusing the method of Irvine (1965) by fixing the Fo con-tent of the coexisting ol (5 0.90). Hence, the observeddiscrepancies mainly reflect the Fe-enrichment alreadyrecognized for these samples.

Glass

Representative glass analyses are listed in Table 4. Thecomposition of these glasses is characterised by moder-ate SiO2 (54.9–59.6 wt%), high Al2O3 (20.3–24.2 wt%)

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nificant inter-element correlations have been detected,neither within nor between xenoliths.

Compared to the glasses in ultramafic xenoliths fromworld-wide alkali basalts (e.g. Francis 1976; Maaloe andPrinzlau 1979; Jones et al. 1983; Kuo and Essene 1985;Gamble and Kyle1987), the Yitong glasses are muchricher in K2O and poorer in MgO, FeO and CaO, and theydefine a distinctive group in a Ca-Na-K triangle (Fig. 8).They do not resemble most partial melts experimentallygenerated at high pressure under anhydrous conditions(Table 4). Partial melts of peridotite1H2O1CO2 arecharacterised by high SiO2, Al2O3 and low FeO, MgOcontents (Table 4, Mysen and Boettcher 1975). Recentmelting experiments of Baker et al. (1995) show that thenear-solidus melts (low melt fraction F,0.02) of “dry”fertile peridotite can be enriched in SiO2, Al2O3 andNa2O, and depleted in FeO, MgO and CaO. Nevertheless,these melts are only qualitatively similar to the Yitongglasses and are quite different from them in absoluteconcentrations, especially in K content. The melts withcompositions similar to the Yitong glasses (high K, Na,Al and low Mg, Fe) have recently been found in continen-tal and oceanic mantle xenoliths (Draper 1992; Hauri etal. 1993) and in melt inclusions trapped in xenolith min-erals from world-wide continental and oceanic alkalibasalt sites (Schiano and Clocchiatti 1994). All theseglasses of peculiar composition have been interpreted tobe related directly or indirectly to mantle metasomatism,in some cases involving carbonate melts.

Calculated Fe*-Mg exchange coefficients for ol-liq-uid [Kd 5 Fe'olMg'liqy(Fe'liqMg'ol)], using compo-sitions of euhedral olivine in glasses or porphyroclastolivine rims adjacent to glasses, range from 0.07 to 0.19(Table 5), which are lower than the canonical value of0.33 (Roeder and Emslie 1970). Similar low Kds are alsocharacteristic of melt inclusions analysed by Schiano etal. (1992) and Hauri et al. (1993). The low olivineymeltKds may be interpreted either as an equilibrium charac-teristic of melts with high alkali content (Baker et al.1995), or as an analytical artefact due to lack of consid-

Fig. 6 A Major-element composition of clinopyroxenes in Yitongwehrlites normalised to that of Cr-diopside of lherzolite suite fromsame volcanic region. The normalised data for the pyroxenite suiteare shown for comparison. Clinopyroxene data for lherzolites andpyroxenites are from Xu et al. (1993).B Correlation of Cry(Cr1Al) versus Mgy(Mg1Fe) for clinopyroxenes in Yitongwehrlites. Two successive processes can be recognized: a variableFe-enrichment of the source material (melt andyor reacting solid)and then a fractional crystallization trend.

and alkali (Na2O, 5.2–9.4 wt%; K2O, 5.5–9.1wt%) con-tents, low FeO (0.9–4.42 wt%), MgO (0.27–1.18 wt%)and CaO (0.13–2.83 wt%) contents. Analyses of some ofthe glasses yield low oxide-weight totals, suggesting thata small amount of volatile components (CO2+H2O) ispresent. All glasses are nepheline normative and no sig-

Table 3 Representative analy-ses of orthopyroxenes andfeldspars in glass-bearingwehrlite xenoliths from Yi-tong, northeastern China

Specimen Orthopyroxene Feldspar

1–31 3–2 3–22 3–2 3–2 1–31 3–24 3–30 3–34in glass in glass in glass in basalt

SiO2 55.84 56.56 56.68 66.14 56.12 53.96 56.67 54.77 55.38TiO2 0.02 0.00 0.06 0.07 0.12 0.16 0.29 0.13 0.31Al2O3 2.58 2.84 1.29 18.87 27.41 28.92 26.84 28.73 27.97Cr2O3 0.27 0.50 0.34 0.01 0.01 0.02 0.03 0.03 0.00FeO 6.67 5.59 8.23 0.14 0.14 0.25 0.46 0.33 0.17MgO 33.52 33.37 32.39 0.03 0.16 0.07 0.13 0.06 0.01CaO 0.70 0.65 0.66 0.32 9.63 10.81 8.08 10.52 9.49Na2O 0.12 0.19 0.20 6.21 5.45 4.81 5.57 4.80 5.25K2O 0.00 0.00 0.00 7.79 0.69 0.43 1.17 0.54 0.74

Total 99.91 99.98 100.16 99.57 99.74 99.49 99.44 99.91 99.40

100*Mg' 89.96 91.40 87.54Or 45.54 4.06 2.56 7.00 3.23 7.00Ab 53.95 48.50 43.47 52.15 43.74 48.32An 1.51 47.45 53.97 40.48 53.02 44.69

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surely not in the cpx crystal structure, since cpxymeltpartition coefficients for Rb and Ba are on the order of0.001 (Hart and Dunn 1993), suggesting that cpx com-monly has,0.1 ppm Rb and,1 ppm Ba. Given thepresence of spongy texture in many Yitong wehrlitecpxs, it is possible that these high Rb and Ba contents aredue to the numerous micro-inclusions which wereanalysed with the host cpx in ion probe spots. This istested by correcting all ion probe analyses with anoma-lously high Ba contents with the assumption that the in-clusions in the cpx are similar to the interstitial glassesand that all the Ba in the cpx analyses comes from theseinclusions. An example of such an exercise is shown forMA91-4 in Table 6. The broad consistency between cor-rected values and those obtained by analysing the crystalfree of inclusions thus supports our speculation. Never-theless, such a correction can only yield an approximateestimate because the cpx micro-inclusions are likely tobe quite different in composition from the glasses, andeven the behaviour of elements during the melt incorpo-ration may be different.

It is clear from Fig. 9 that all the cpxs are character-ised by LREE-enrichment with convex profiles with Prand Nd at the maximum. Values of (La)N in those cpxvarying from 40 to 88 are significantly higher than in thecpx from pyroxenites from the same localities (Xu et al.1996) and other localities of the world (Roden and

Fig. 7A–C Compositional variation of chromian spinels from thewehrlite suite from Yitong.A Fey(Mg1Fe) versusCry(Cr1Al).The line is calculated using the method of Irvine (1965) whenfixing Fo of coexisting olivine (5 0.90). Thedashed areais forabyssal peridotites based on Dick and Bullen (1984);B CryCr1Al) versusTi; C Fe versus Fe31. The composition of spinelsfrom the lherzolite and pyroxenite suites in the same region (Xu etal. 1993) are also given for comparison

eration of Fe2O3 content of the melt (electron microprobeanalyses). The distinction between these two possibili-ties can be achieved if we can accurately estimate Fe2O3

contents of melts. Kilinc et al. (1983) experimentallydemonstrated that the ferric-ferrous ratio of silicate liq-uids is dependent on temperature,fO2 and bulk composi-tion of melts. It is doubtful, however, that such an estima-tion can be applied correctly in the case of Yitong glassesbecause it is difficult to obtain two important parameters(T andfO2) due to the general absence of orthopyroxenein the Yitong wehrlites.

Ion microprobe data

Trace element analyses have been undertaken on cpx andthe glasses from five wehrlites. Despite the effort toanalyse the cpx free of inclusions, we found that manyanalyses on cpx yield anomalously high Rb and Ba con-tents (from several to hundred ppm). This Rb and Ba is

Table 4 Representative spinelanalyses from glass-bearingwehrlite xenoliths from Yi-tong, NE China

Sample 3–2a 1–45 3–15 3–22 3–26 3–30 3–34 3–40 3–61 MA91-4

SiO2 0.06 0.05 0.04 0.09 0.02 0.08 0.09 0.03 0.01 0.02TiO2 0.07 0.54 0.23 0.78 1.41 2.47 0.76 0.64 0.11 0.47Al2O3 41.20 7.79 13.40 16.31 15.69 26.98 12.73 19.35 9.71 16.66Cr2O3 28.01 55.34 48.35 45.25 42.37 28.78 51.24 42.45 56.45 46.12Fe2O3

b 1.15 7.35 7.06 6.88 8.42 8.54 3.92 6.75 5.85 7.18FeO 10.75 18.85 19.98 16.75 18.40 20.30 17.97 19.66 16.66 16.09NiO 0.23 0.46 0.51 0.27 0.42 0.39 0.33 0.34 0.38 0.35MnO 0.23 0.12 0.16 0.19 0.19 0.22 0.19 0.14 0.29 0.00MgO 17.87 9.21 8.60 11.36 10.31 11.30 10.03 9.98 10.68 11.89CaO 0.00 0.17 0.00 0.09 0.05 0.03 0.14 0.05 0.01 0.02

Total 99.58 99.90 98.32 98.01 97.30 99.15 97.44 99.42 100.15 98.83

CryCr1Al 0.687 0.827 0.708 0.651 0.644 0.417 0.730 0.595 0.796 0.769MgyMg1Fe 0.748 0.479 0.434 0.547 0.500 0.498 0.499 0.475 0.533 0.524

a Lherzoliteb Fe2O3 calculated by stoi-chiometry

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In contrast to cpx, glasses exhibit relatively homoge-neous trace element compositions. They have HREEcontents similar to (or lower than) their co-existing cpx,but have high LREE concentrations [(La)n 5 268–480].They define a chondrite-normalised pattern with a steepnegative slope. The Ti and Hf contents in all glasses aredepleted relative to their neighbouring REE (Fig. 9C).Glasses in two samples (MA91-4 and 3–34) are also de-pleted in Zr but this is not observed in the other twosamples (3–26 and 3–40).

Discussions

Evidence for metasomatic origin of the K-rich glasses

The microscopic spongy texture of the cpxs and theirassociation with glass could be taken as evidence that thepartial breakdown of cpx resulting from decreasingpressure may account for the origin of glass in thewehrlites. However, the decompressional melting of cpxand other hydrous phases (amphibole and phlogopite)typically produces isolated blebs or melt pockets (Fran-cis 1976; Mercier 1977; Chazot et al. 1995). The absenceof such a textural feature in the Yitong wehrlites pro-vides evidence againstin-situ melting. Moreover, a hy-pothesis involving decompression melting is not consis-tent with the high K contents in the glasses and the totalabsence of hydrous phases in the wehrlite suite.

An origin involving injection of an extraneous meltinto the mantle matrix is therefore preferred for the orig-in of the Yitong glasses. However, these glasses couldnot be trapped host basaltic magma, given the followingtextural and chemical evidence. (1) Petrographic differ-ences exist between the host basalt and the glasses

Fig. 8 Ca-Na-K (atomic fraction) triangle plot for the glassesfrom Yitong wehrlite xenoliths. Also shown various compositionsof glasses and melt inclusions in mantle xenoliths from the litera-ture for comparison

Shimizu 1993; Sen et al. 1993). All the cpx but one (3–15) exhibit strong negative depletions in Ti, Zr and Hfrelative to the adjacent REE (Fig. 9C). The Fe-rich cpx in3–15 also exhibits a large negative anomaly for Ti but noZr anomaly.

Table 5 Representative analyses of the glasses in wehrlite xeno-liths from Yitong, NE China. Total iron as FeO*. Kd(Fe2Mg)

olygl 5(Fe'olMg'liq) (Fe'liqyMg'ol). 1 Francis (1976): composition ofa glass from a fine-grained zone in a lherzolite from NunivakIsland, Alaska,2 Maaloe and Printzlau (1979): composition of aglass bleb in a spinel lherzolite from Dreiser Weiher, Germany,3Girod et al. (1981): Averaged composition of glasses in a lherzolitefrom southern Algeria,4 Draper (1992): interstitial glass in a

lherzolite from Lorena Butte, Simcoe, southern Washington,USA, 5 Jaques and Green (1980): experimental melts at 13008Cand 5 kbar under anhydrous conditions,6 Takahashi and Kushiro(1983): experimental liquid composition equilibrated with peri-dotite at 12258C and 5 kbar,7 Mysen and Bottcher (1975): liquidformed by partial melting of peridotite1H2O1CO2 at 11008C and15 kbar, 8 Nicholls (1974): experimental melts in equilibriumwith a peridotite at 10008C and 15 kbar, water-saturated

Specimen 1–45 3–15 3–22 3–24 3–26 3–30 3–34 3–40 3–61 MA Composition Experimental melts91-4 of glasses in literature anhydrous hydrous

1 2 3 4 5 6 7 8

SiO2 59.63 54.90 57.51 54.41 57.33 56.26 55.45 57.02 57.45 54.72 58.99 53.19 57.02 62.60 52.60 54.40 58.40 55.90TiO2 0.61 0.55 1.80 0.93 0.78 0.96 0.84 0.50 0.62 0.58 0.16 0.27 1.23 0.34 2.30 1.93 1.00 0.40Al2O3 20.46 20.33 21.59 23.57 23.11 21.76 24.18 22.93 22.02 23.52 20.87 20.93 19.23 20.05 11.30 16.20 20.10 21.30FeO* 2.18 4.42 3.53 2.14 1.23 2.72 1.12 1.04 1.62 1.69 2.77 3.93 2.46 1.37 8.80 6.64 4.50 4.10MnO 0.00 0.10 0.02 0.09 0.05 0.08 0.04 0.00 0.02 0.00 0.07 0.0 0.05 0.10 0.10 0.03 0.10 0.20MgO 1.00 1.18 1.81 1.08 0.63 1.71 0.48 0.17 1.04 0.71 3.60 3.58 4.02 0.99 12.90 7.91 2.40 5.20CaO 0.13 2.83 3.92 1.74 0.84 0.84 1.05 1.58 2.14 1.16 6.58 8.86 8.64 1.86 9.70 8.78 9.80 5.80Na2O 9.44 7.65 3.19 5.60 5.21 5.81 6.58 8.55 7.41 6.85 4.57 4.73 6.45 6.16 1.90 2.03 3.30 5.60K2O 6.61 6.18 5.44 8.38 7.24 9.35 8.99 6.87 7.06 9.33 2.10 1.74 0.90 7.06 0.40 0.93 0.10 1.40

Total 100.06 98.14 98.81 97.94 96.42 99.49 98.73 98.66 99.38 98.56 99.71 97.23 100.0 100.74 99.40 99.44 100.0 99.90

Mgy(Mg1Fe) 0.45 0.32 0.48 0.48 0.48 0.53 0.44 0.23 0.53 0.43 0.70 0.62 0.75 0.56 0.72 0.68 0.49 0.69Kd(Fe-Mg)olygl 0.10 0.12 0.15 0.12 0.12 0.19 0.12 0.07 0.17 0.08

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Table 6 Representative ion microprobe analyses of clinopyroxenes and glasses in the Yitong wehrlites (in ppm). (Cpxanalyse of cleancrystal,cpx* corrected compositions (see text for correction method),cpx§raw analyses of crystals with micro-inclusions)

Samples Mg–wehrlites Fe–wehrlite

3–26 3–34 MA91–4 3–15 3–40

Cpx* Glass Cpx Glass Cpx § Cpx* Cpx Glass Cpx* Cpx* Glass

Ti 1175 2953 733.8 2430 1277 1076.98 1507 2781 1294 1988 7907Rb – 94.04 – 52.96 2.78 – 0.01 41.56 – – 80.84Sr 289.37 734.3 192.70 1319 308.10 212.08 216.2 1335 143 136.74 1777Y 30.42 24.83 25.53 23.15 32.72 30.72 31.28 27.75 30.20 16.51 14.49Zr 76.51 339.00 67.55 64.82 43.74 33.81 35.84 138.07 357.00 19.97 314.70Hf 0.25 4.35 0.86 1.01 0.90 0.78 0.47 1.64 8.62 1.45 6.09Nb – 153.00 1.57 34.49 7.97 1.96 2.88 83.51 – – 143.60Ba – 92.66 1.62 420.15 28.20 1.08 1.08 377.07 – – 854.80

La 16.25 63.01 9.28 113.00 27.25 19.73 20.61 104.57 11.50 10.33 81.67Ce 56.64 108.10 35.01 200.00 74.19 60.62 59.81 188.63 42.30 35.33 122.90Pr 9.12 12.86 6.17 17.56 9.51 8.29 8.79 16.96 7.54 5.58 10.99Nd 47.15 45.03 31.17 58.33 44.5 40.67 43.66 53.22 3.96 29.88 35.61Sm 11.41 8.17 9.75 7.69 9.59 9.03 9.56 7.77 11.40 6.6 5.62Eu 4.31 1.86 2.74 2.37 3.29 3.13 3.38 2.24 3.88 2.24 1.46Gd 11.43 5.40 7.98 7.51 9.31 8.82 7.50 6.87 12.30 8.15 4.90Dy 9.10 5.29 7.38 6.06 7.84 7.39 8.74 6.29 8.82 5.26 3.59Ho 1.49 1.00 1.12 1.07 1.53 1.45 1.32 1.15 1.45 0.84 0.66Er 4.15 2.92 2.81 2.66 4.26 4.01 3.46 3.53 4.22 2.19 1.88Yb 2.55 2.05 2.85 2.16 4.00 3.79 3.22 2.97 2.98 2.35 1.03

(Fig. 2D). The geometric cross-cutting relations be-tween some basaltic veins and glasses is consistent witha slight foliation (observed in 1–45, 3–24) indicating thatthe glasses and the “basalt” vein represent two differentmagmatic events, and that the glass-producing eventmust have happened prior to entrainment. (2) The glassesare compositionally different from the host basalts bybeing deficient in Fe and high in K, Na and Al contents,and by their high LREE contents and the presence of aHFSE-depletion. (3) A chemical difference does existbetween the crystals in the host basalt and those in theglasses. For example, the cpx in the glasses is character-ised by high Cr' and Mg' values (except for the Fe-richsamples), whereas the phenocrysts in the host basalt havemuch lower Cr' and Mg' (0.86–0.80) values, but highTiO2 contents.

The Yitong glasses are similar to the glasses in mantlexenoliths from other localities (see Ionov et al. 1994 fora review) in that they are rich in Si and Al, poor in Mgand Fe, but differ from them in surprisingly high K con-tents. Glasses with similar high K contents are also re-ported in several recent studies on mantle xenoliths andmelt inclusions in xenolith minerals (Draper 1992; Hauriet al. 1993; Schiano et al. 1992 1994; Schiano and Cloc-chiatti 1994). All these authors have ruled out the simplemelting of either mica or amphibole for these silica andK-rich glasses, and suggested that the melting may havebeen assisted by volatile-rich fluids. For example, Schi-ano et al. (1992) analysed the interstitial glasses and meltinclusions in peridotite xenoliths from Tahaa, SocietyIsland, and obtained very high SiO2 (63%) and K2O (6%)contents. Together with the results of homogenizationexperiments, they concluded these melts were formed byreaction of a metasomatic fluid with peridotites. Specifi-

cally, Hauri et al. (1993) attributed the glasses in peri-dotite xenoliths from the islands of Savai’i and Tubuai tothe products of carbonate metasomatism. Many experi-mental studies have revealed the potential for the for-mation of such a potassic melt in the upper mantle. Forexample, Schneider and Eggler (1986) have determinedthe solute content of an H2O-rich fluid which is in equi-librium with various mantle assemblages atT . 8508Cand P . 17 kbar: the mantle fluid tends to be rich ina normative feldspar component (i.e. in Si, Al, Na andK, but poor in Fe and Mg, notably Ti). The presenceof H2O and CO2 in the upper mantle is evidenced bythe presence of hydrous mineral assemblages (Menziesand Hawkesworth 1987) and is also supported byfluid inclusion studies in natural samples (Navon et al.1988; Schiano and Clocchiatti 1994). Although the re-cent experiments of Baker et al. (1995) demonstratedthat melts with high silica and alkali (Na) contents can begenerated under dry conditions through near-solidusmelting of fertile lherzolite, high K and incompatibleelement contents in the Yitong glasses, combined withthe foregoing consideration, lead us to believe that theyare more likely related to melting of peridotites with thepresence of deep-seated fluids. This is substantiated bythe low electron microprobe totals of these glasses whichimply the presence of volatiles in the original metaso-matic melt.

Such an interpretation is similar to those put forwardby Jones et al. (1983) and Schiano et al. (1994) in whichthe glasses represent the metasomatic melts migratingthrough upper mantle. However, it is difficult to estimatean unmodified melt composition from these glasses,since these interstitial glasses must have been subjectedto the modifications caused by fractional crystallisation,

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because the glasses in Fe-rich wehrlites (e.g. 3–15) tendto be also rich in Fe. The presence of vesicular texturesand vugs in the Yitong glasses suggests that someamount of volatile could have been lost prior to or duringthe xenolith transport to the surface. Amundsen (1987)and Schiano et al. (1994) have demonstrated that majorvolatile phases may escape from the metasomatic liquidthrough an immisibility process under upper mantle con-ditions. However, it is also possible that the exsolutionandyor outgassing of volatiles from melts took place as aresult of decompression during xenolith transport. Schi-ano et al. (1992) noted that Cl concentrations in intersti-tial glasses are much lower than in melt inclusions, prob-ably because of this effect. This would be not only forvolatiles, but also for major and trace elements to someextent. Since we do not know the composition of thevolatile phase lost and the behaviour of major and traceelements during such a process, the quantitative assess-ment of chemical modification due to this effect is nearlyimpossible.

Origin of the wehrlite suite

Wehrlites are commonly considered as polybaric segre-gations of alkaline magmas (Frey and Prinz 1978; Irving1980; Roden and Shimizu 1993). A straightforward in-terpretation of the petrogenesis of the Yitong wehrlites isthat these rocks represent cumulates from parental mag-mas and that the coexisting glasses are the residual liquidof this magma after extensive crystallization of mineralassemblages. However a simple cumulate process cannotexplain the fact that virtually identical glass composi-tions occur in both iron rich and magnesian wehrlites andis not consistent with peculiar composition of the Yitongglasses. If the wehrlite minerals and glasses represent thecomplementary parts of a crystallisation process, theresidual liquids (now represented by glasses) should ex-hibit some correlations between major oxides given therelatively wide composition range of the Yitongwehrlites (Fo5 79–91). But no such compositional cor-relation has been observed. Moreover, 75% of thewehrlites are magnesian, similar to that of the lherzolitesuite in the same volcanic area. This, together with thepresence of high temperature deformed olivines(Fig. 2B), strongly argues against such an hypothesis,and instead favours a metamorphic origin for the pro-tolith of these wehrlites. A large depletion in Ti and Zr inthe cpx of the wehrlite cannot be due to partitioning intoa coexisting Ti-bearing phase since no such phases havebeen found in the rocks. This contrasts with the smallHFSE-depletion commonly observed in group II wehrlitecpx (Roden and Shimizu 1993; Xu et al. 1996). More-over, the cpxs in Yitong wehrlites are anomalously en-riched in incompatible elements. The estimated compo-sition of hypothetical melts in equilibrium with thesecpxs are considerably different from those of the hostbasalts (Fig. 10). In fact, the textural and petrologicalevidence presented in previous sections suggests that the

Fig. 9 Chondrite-normalised REE patterns of clinopyroxenes andglasses inA magnesian andB Fe-rich wehrlites.C Chondrite-nor-malised element profiles for Zr and Ti in addition to the REE.Chondrite values are from Anders and Grevesse (1989)

interaction with peridotite and xenolith transport to thesurface. The extensive crystallisation of a cpx1ol1spassemblage would have resulted in a strong depletion ofMgO, FeO and CaO and an enrichment of SiO2, Al2O3

and alkalis in the residual liquid. However, major oxidesin the glasses are not correlated between different sam-ples and show no clear evolution trend consistent withfractional crystallization from a primary melt. Moreover,the glasses containing abundant microphenocrysts (e.g.1–45, Fig. 4A) have higher Mg-numbers than those poorin microphenocrysts or devoid of them (e.g. 3–15, 3–34),while their alkali and Al-contents are virtually identical(Table 4). All this suggests that the crystallisation pro-cess probably is not the only process responsible forchemical evolution of the glasses. Extensive reactionwith the host mineral assemblage could be another im-portant process, given the petrographic evidence inFig. 3. The dissolution of opx into the melt, and precipi-tation of cpx and ol resulted in silica enrichment andCa-depletion in the melt (Kelemen 1990). The migratingmelts seem to have been buffered, to various degrees, bythe peridotite assemblage through which they passed,

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previous sections, the metasomatic agent more likely re-sembles a volatile-rich melt(s). The extremely low FeOand MgO contents in such melts due to extensive crys-tallisation of cpx1ol1sp make them incapable of caus-ing significant FeyMg fractionation in the metasoma-tised peridotites.

A more complex history for the Yitong wehrlites isrevealed by the presence of several Fe-rich samples. Thecpxs in these samples have LREE-enriched and convex-upward REE patterns somewhat similar to group II py-roxenites, but differ from them in their higher concentra-tion of REE, the larger degree of HFSE-depletion rela-tive to the REE and the lack of an igneous texture whichis commonly observed in group II pyroxenites from thesame volcanic area (Xu et al. 1993). While a cumulateorigin cannot fully be ruled out, we favour a metasomaticorigin for these Fe-wehrlites, as proposed for magnesianwehrlites. This inference is based on the presence of thesame K-glasses in both groups of samples and on texturalevidence for a reaction between lherzolite and K-glassesas observed in an Fe-rich, opx-bearing wehrlite (3–22).The K-rich glasses occur in both Fe-rich and magnesianwehrlite xenoliths without any correlation between themodal amount of glass and the Mg-value of the rock,thereby suggesting that the impregnating K-rich liquidwas not responsible for the Fe-enrichment (Fig. 5). Suchan inference is in agreement with the nature of glasseswhich are too deficient in Fe to cause Fe-enrichment.The transitionalopx-bearing wehrlite(3–22) is moder-ately rich in Fe (Fo5 86), strongly suggesting that Fe-enrichment and K-metasomatism are two independent,successive events with the Fe-enrichment occurring ear-lier than the K-enrichment. Further support for this ideacan be found in the ol and cpx compositions (Figs. 5, 6).Chemical zonation in the microphenocrysts can be at-tributed to fractional crystallization processes. Relative-ly high Ca concentrations in ol phenocrysts could be in-terpreted as reflecting the high content of CaO inparental melts andyor low pressure and rapid crystalliza-tion (Simkin and Smith 1970; Stormer 1973; Adams andBishop 1986). However, the rims of ol within the matrixare also enriched in Ca and this enrichment in the rimareas is unrelated to the degree of Fe or Mn enrichment(Fig. 5). In the Fe-rich wehrlites, phenocrysts (cpx1ol)in the glasses are also rich in Fe and have virtually iden-tical compositions compared to matrix minerals(Fig. 6B). To account for this compositional heritage,another process must be called upon because crystalliza-tion from the same metasomatic melt would producecrystals with similar compositions. Buffering of the liq-uid by the local mantle through which the melt migratedis a more likely process and this mantle could have beenpreviously affected by some extraneous silicate melts.

In summary, the formation of the Yitong wehrlitesinvolves a two-stage process, with a petrological conver-sion from lherzolite to wehrlite during K-metasomatism,but a former Fe-enrichment process causing no signifi-cant modal modification. Such an Fe-enrichment hasbeen widely described in the literature, principally in-

Fig. 10 Trace element composition of hypothetical melts in equi-librium with LREE-enriched clinopyroxenes. The partition coef-ficients used in this calculation are after Hart and Dunn (1992).The compositions of the host basalts and continental carbonatitesare from Fan and Liu (1992) and Nelson et al. (1988)

Yitong wehrlite suite represent the “end-product” of lo-cal metasomatism of the mantle lithosphere unrelated tothe host basalt. The processes that accompanied the up-ward movement of a metasomatic melt (now K-glass) inthe mantle during crack propagation might include frac-tional crystallisation and chemical exchange with themantle matrix. This metasomatic melt firstly replacedthe primary sp, cpx, and then the low-Ca opx, as evi-denced by rim replacement textures in two intermediarysamples (3–2 and 3–22), and then crystallised as an as-semblage of cpx1ol1sp, thereby changing the mantle(lherzolite) modal composition towards that of thewehrlite suite.

A similar explanation has been proposed to accountfor the formation of some magnesian wehrlites. For ex-ample, Yaxley et al. (1991) described similar rim re-placement textures of opx in sp-peridotite xenoliths inAustralia. On the basis of experimental results, they pro-posed that apatite-bearing magnesian wehrlites formedthrough the reaction of lherzolites with permeating car-bonatite melts. Since the carbonatite melts cannot causesignificant FeyMg fractionation (Green and Wallace1988), the newly formed wehrlites inherited the highmagnesian characteristics of the mantle lherzolites. Fig-ure 10 shows that the hypothetical melts in equilibriumwith the Yitong wehrlite cpx do resemble carbonatites(Nelson et al. 1988) in terms of trace element abun-dances. However, there are some major petrological dif-ferences between the Australian and the Yitongwehrlites. Firstly, apatite has never been found in Yitong.Secondly, the low P2O5 contents in glasses and the lowNa2O in cpx rimming opx suggest that the original meta-somatic melts were probably compositionally differentfrom “carbonatite” or carbonate melt. As discussed in

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ferred from composite xenoliths (e.g. Irving 1980;Kempton 1987). Among the Yitong peridotite xenoliths,some Fe-lherzolites (Fo5 80) have been detected byelectron microprobe analyses and have been consideredas the result of metasomatism by an Fe-melt, possiblybasaltic in composition (Xu et al. 1993). Such Fe-lherzo-lites (perhaps group II pyroxenites as well) would thusrepresent the protoliths of the Fe-rich wehrlites. The lat-er K-metasomatism responsible for the petrologicaltransformation clearly overprinted an already heteroge-neous mantle, which accounts for Fe-rich wehrlites like3–15 and 3–40.

Concluding remarks

K-rich glass-bearing wehrlite xenoliths from Yitongrecord important information on several mantle process-es, including textural and mineralogical evidence for aprogressive metasomatic transformation in the sequence:sp-lherzolite ! opx-bearing wehrlite ! opx-freewehrlite. Thus, the wehrlite suite at Yitong originatedfrom the interaction between the lherzolite mantle and aninfiltrating metasomatic melt, rather than through ig-neous processes. Mineral chemistry and trace elementdata not only support this conclusion, but make it possi-ble to decipher the details of a more complex chemicalevolution history. The upper mantle beneath Yitong hadpreviously been subjected to infiltration of silicatemelts, which caused local Fe and Mn enrichment of theadjacent mantle, though without significant modalchanges. Subsequently, a K-rich liquid impregnated thismantle. This metasomatic melt is not related to the hostbasalt and is believed to form from the low degree ofpartial melting of peridotites involving deep-seated flu-ids. Subsequent fractional crystallisation of cpx, ol andsp, extensive reaction with the mantle lithosphere and therapid xenolith transport to the surface have resulted inthe significant modification in the composition of glass-es.

Acknowledgements Financial support from INSU-CNRS ofFrance, SSB and NNSF of R.P. China, NSERC of Canadaand the Royal Society (London) is gratefully acknowledged.Mr. X.E. Zhang and Q.F. Yang are thanked for their assistancein the field. Special thanks are due to Dr. R. Hinton for helpwith the ion microprobe, and to Dr. M. Fialin for assistancewith the electron microprobe. Critical and thoughtful commentsof three anonymous reviewers have greatly improved themanuscript.

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