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Geochimica et Cosmochimica Acta, Vol. 59, No. I, pp. 59-75, 1995 Copyright 0 1995 Elsevier Science Ltd Printed in the USA. All rights reserved

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An ion microprobe study on trace element composition of clinopyroxenes from blueschist and eclogitized Fe-Ti-gabbros, Ligurian Alps, northwestern Italy:

Some petrologic considerations

BRUNO MESSIGA, ‘,’ RICCARDO TRIBUZIO, ’ PIERO BOTTAZZI,’ and LUISA O~~OLINI’

‘Dipartimento di Scienze della Terra, Universith di Pavia, Via Abbiategrasso 209, I-27100 Pavia, Italy *C.N.R.-Centro di Studio per la Cristallochimica e la Cristallografia (CSCC), Via Abbiategrasso 209, I-27100 Pavia, Italy

(Received October 27, 1993; accepted in revised form August 29, 1994)

Abstract-Detailed microtextural investigations coupled with ion microprobe trace element determinations have been performed on clinopyroxenes from blueschist and eclogitized Fe-Ti-gabbros from Ligurian Alps (northwestern Italy). Ion microprobe analyses have also been carried out on garnets from eclogitized Fe- Ti-gabbros. The aim is to understand the relations between metamorphic reactions and trace element com- positions of clinopyroxenes in high pressure/low temperature terranes.

Trace and major element analyses on relicts of igneous diopsides indicate similar protoliths for both eclogites and blueschists. Metamorphic clinopyroxenes have variable trace element compositions which reflect the compositions of original mineral phases. In particular, the clinopyroxenes replacing the igneous diopside commonly retain geochemical signatures of the precursor mineral, such as relatively high SC, Y, and HREE contents.

The trace element variations among high pressure clinopyroxenes in different textural domains are greater for blueschists than for eclogites. This reflects the more limited major element equilibration of blueschists relative to eclogites. In deformed eclogites, the clinopyroxenes lack the geochemical signatures of the original igneous minerals, most probably due to the enlargement of reaction domains caused by synmeta- morphic deformation.

Trace element partitioning between omphacite and garnet suggests that in both undeformed and deformed eclogites, chemical equilibrium is locally approached, and, therefore, the trace element composition of clinopyroxenes is controlled by the coexisting minerals. For instance, clinopyroxenes in eclogites are strongly HREE- and LREE-depleted (i.e., with bell-shaped pattern) due to the equilibration with garnet and allanite, respectively. The partition coefficients for REEs between omphacite and garnet are markedly lower than those reported in the literature. On the basis of crystal-chemical considerations, we suggest that this is probably due to lower temperature conditions during formation.

INTRODUCTION

Published data on the trace element compositions of high pressure Na-clinopyroxenes are scarce and concern mainly the concentration of rare earth elements (REEs) in clinopy- roxenes from high temperature eclogites included in kimber- lites (Philpotts et al., 1972; Shervais et al., 1988; Caporuscio and Smyth, 1990; Jerde et al., 1993a,b). A few data on REE compositions of clinopyroxenes from eclogites in medium and high grade basement complexes were presented by Thoni and Jagoutz ( 1992), and Griffin and Brueckner ( 1985), re- spectively. The trace element compositions of high pressure clinopyroxenes from low temperature terranes are still un- known despite their importance, for example, in modeling the metasomatic effects of a subducting slab on magma genesis at destructive plate margins.

In high pressure/low temperature terranes, metamorphic recrystallization is controlled by heterogeneous deformation patterns (e.g., Pognante, 1985; Rubie, 1986, 1990). The ab- sence of synmetamorphic deformation commonly produces the following disequilibrium features: ( 1) metastable relicts of protolith minerals; (2) microdomains related to the proto- lith minerals, in which the composition of high pressure/low temperature minerals reflects the local chemical environment; and (3 ) coronitic reactions between microdomains. Dynamic recrystallization enhances the rates of element diffusion, thus

enlarging the equilibrium domains (e.g., Brodie and Rutter, 1985; Dipple and Ferry, 1992).

In this paper, we provide new trace element data (REE, Sr, Y, SC, V, Zr, Ti) on clinopyroxenes and garnets from rocks reequilibrated under high pressure/low temperature conditions. Trace element analyses have been performed by means of an ion microprobe. Microbeam analysis tech- niques eliminate uncertainty related to compositional zon- ing and contamination by solid or fluid microinclusions that arise by analyzing mineral separates.

We have selected the blueschist and eclogitized Fe-Ti- gabbros from Jurassic ophiolites of Ligurian Alps (north- western Italy), since their bulk-rock chemistry and meta- morphic evolution have been thoroughly investigated (e.g., Mottana and Bocchio, 1975; Ernst, 1976; Beccaluva et al., 1979; Morten et al., 1979; Messiga, 1987; Messiga and Scambelluri, 1991). Blueschist Fe-Ti-gabbros are corona- textured, whereas eclogitized Fe-Ti-gabbros display vari- ous textures (from coronitic to mylonitic) caused by the stress gradient during synmetamorphic deformation. We, therefore, compared corona-textured rocks reequilibrated under blueschist and eclogite facies conditions, and inves- tigated an eclogite sequence with different deformation ef- fects, i.e., presumably characterized by enlargement of re- action domains.

59

60 B. Messiga et al

This work focuses on the relations between microtextures and trace element composition of clinopyroxenes. We will show that, due to the lack of bulk-rock chemical equilibrium, the trace element compositions of high pressure/low temper- ature minerals are controlled by a large number of variables. We will also indicate that local equilibrium is approached in eclogitized Fe-Ti-gabbros, as testified by the trace element partitioning between omphacite and garnet. On the basis of crystal-chemical considerations and of comparison with lit- erature data, we will explore the controls on REE partitioning between clinopyroxene and garnet. Details of the analytical techniques used are given in the Appendix.

GEOLOGIC AND PETROLOGIC OUTLINE

Geology and Field Relations

The ophiolites of Northern Apennine, Corsica, Western, and Lig- urian Alps represent lithosphere remnants of the Jurassic Liguriar- Piedmontese basin (e.g., Lemoine et al., 1987; Lagabrielle and Can- nat, 1990). The intrusive sequence of these ophiolites were derived by crystal fractionation of tholeiitic melts, whose composition is thought to be similar to primitive basalts from present-day oceanic basins (Hebert et al., 1989). Fe-Ti-gabbros, crystallized from evolved liquids of ferrobasaltic composition, are composed of cu- mulus plagioclase and clinopyroxene, interstitial Fe-Ti oxides, minor Ti-pargasite, and accessory apatite (Semi, 1980; Pognante et al., 1982).

The ophiolitic units of Ligurian Alps (Fig. 1) were subjected to low pressure ocean-floor metamorphism and, subsequently, to per- vasive re-equilibration under high pressure metamorphic conditions as a result of a subduction event probably of Cretaceous-Eocene age (Ernst, 1976; Messiga, 1987; Messiga and Scambelluri, 1989; Scam- belluri et al., 1991).

The Fe-Ti-gabbros from the Voltaggio-Montenotte unit display a static re-equilibration under blueschist facies conditions, with a para- genesis of lawsonite, Na-amphibole, Na-clinopyroxene, and titanite (Beccaluva et al., 1979; Messiga, 1987). Temperatures lower than 450°C and pressures higher than 1 .O GPa have been inferred on the basis of phase relations (Tribuzio, 1992). Blueschist Fe-Ti-gabbros in places display a decompressional greenschist assemblage (Bec- caluva et al., 1979).

In the Beigua-Ponzema unit, belonging to the so-called Gruppo di Voltri (Chiesa et al., 1975), the Alpine metamorphism reached eclo- gitic conditions and was accompanied by deformation along ductile shear zones. The Fe-Ti-gabbros developed the bimineralic assem- blage of omphacite and garnet, with accessory rutile (Mottana and Bocchio, 1975; Ernst, 1976; Messiga and Scambelhni, 1991). The eclogitized Fe-Ti-gabbros may be subdivided in two main textural types: ( 1) undeformed coronitic eclogites, which allow the recogni- tion of the original intrusive texture; (2) deformed eclogites, char- acterized by foliation and grain size reduction resulting from recrys- tallization. Due to strain gradients, deformed eclogites range from low-strain flaser eclogites to high-strain mylonitic eclogites. Both deformed and undeformed eclogites are locally cut by veins (mn- wide by m-long) filled with garnet. Geothermometers based on the Fe-Mg exchange between garnet and omphacite (KD = 30-40) yield temperatures of 430-500°C (Ernst, 1976; Messiga and Scambelluri, 1991). Minimum pressure estimates of 1.2 GPa (Messiga and Scam- belluri, 1991) were obtained on the basis of the coexistence of om- ohacite (JddO) with quartz (Holland, 1983) ; the stability of paragonite and epidote, together with the absence of kyanite, indicate that pres- sure conditions did not exceed 2.0 GPa (Holland, 1979). The eclo- gitized Fe-Ti-gabbros frequently show the development of amphibole replacing the eclogite assemblage, indicating a complex decompres- sional evolution towards greenshist facies conditions (Ernst, 1976; Messiga and Scambelluri, 1991)

Plagioclase has been replaced by aggregates of lawsonite, jadeite- rich (65-75 mol%) clinopyroxene, and crossitic to glaucophanic am- phiboles. These aggregates in places overgrow relicts of chlorite and subordinate albite. Igneous ilmenite is partly replaced by fine grained aggregates of titanite developed in fractures and coronas. Na-amphi- boles or aegirines grew, in both plagioclase and clinopyroxene do- mains, as outer portions of coronas around Fe-Ti-oxides. The coron- itic amphiboles display a strong increasing Al at the expense of Fe’+ (up to 1 .O apfu) from the oxide to the plagioclase domain.

Bulk-Rock Chemistry Undeformed eclogites

Blueschist and eclogitized Fe-Ti-gabbros display extremely high Undeformed, or coronitic, eclogites (Fig. 2b) developed garnet contents of FeO,,, TiOl, and MnO (ml7 wt%, -5 wt% and ~0.3 coronas along the boundaries of the original igneous minerals. The

FIG. 1. Index map of the main ophiolitic Units from Ligurian Alps (north western Italy). 1 = Erro-Tobbio Unit; 2 = Voltaggio-Monten- otte Unit; 3 = Beigua-Ponzema Unit. Units 1 and 3 belong to the so- called Gruppo di Voltri Group (Chiesa et al., 1975), in which the Alpine peak metamorphism reached eclogite facies conditions (see also Scambelluri et al., 1991). The Voltaggio-Montenotte Unit (2) reequilibrated under blueschist facies metamorphic conditions. A, B = sampling locations; GE = Genova; SV = Savona.

wt%, respectively), which derive from the high modal Fe-Ti oxides in the primary igneous assemblage. Blueschists differ from eclogites in the higher Fe oxidation ratios (Fe203/Fe0 IJ 0.9 and -0.3, re- spectively) and higher loss on ignition (L.O.I.) values (Mottana and Bocchio, 1975; Boy et al., 1976; Beccaluva et al., 1979; Tribuzio, 1992). The Cl-normalized patterns of apatite-free lithologies are characterized by LREE-depletion (La,/Yb, = 0.5), flat HREE in the range lo-30 X Cl, and a positive Eu anomaly. With increasing apatite amounts, the Cl-normalized patterns become slightly MREE- enriched (up to 150 X Cl ) and show an inversion in the Eu anomaly (Mot-ten et al., 1979; Ernst et al., 1983; Tribuzio, 1992). Among the analyzed samples, deformed eclogites show higher Fe,,,/Mg values, higher P and lanthanides, and lower Ti and V contents than blue- schists and undeformed eclogites (Tribuzio, 1992).

Petrography and Major Element Mineral Chemistry

Blueschists

Blueschist Fe-Ti-gabbros generally preserve relicts of igneous cli- nopyroxene and ilmenite (Table 1; Fig. 2a). Igneous clinopyroxenes display thin ( a3 pm) lamellae of aegirine-rich clinopyroxene or Na- amphibole along ( lOO), which originally were probably of exsolved Ca-poor pyroxene. These lamellae are about 5% of the original cli- nopyroxene. The igneous clinopyroxene relicts are diopsides (Fig. 3) with low Mg/(Mg + Fe’+) ratios (a0.7) and relatively high TiO* contents ( -0.7 wt%). These igneous clinopyroxene relicts are par- tially replaced by variable amounts of aegirine-rich (50-65 mol%) clinopyroxene and minor crossite. Along the contact with the original plagioclase, igneous clinopyroxene relicts are bounded by a thin rim (~500 pm thick) of aegirine-rich clinopyroxene which grows en- tirely in the plagioclase domain.

Trace-element geochemistry of cpx 61

Table 1. Main petrographic features of undeformed blueschist and eclogitized Fe-Ti-gabbros from Ligurian Alps. Mineral abbreviations here and in all tables and figures according to Kreta (1983).

BLUESCHISTS ECLOGITES

Igneous relicts frequent (Di, Ilm) rare (Di, Ilm)

Pseudomorphs

plagioclase Na-Amp + Jd + Omp + Grt k Ep Lws f Par

diopside Aeg + Na-Amp Agt + Rt

Fe-Ti-oxides W Rt + Agt

coronas

plagic&se/diopside Aeg Grt

plagioclas&Fe-Ti- Na-Amp + Sph Glt oxides

au&e/Fe-T&oxides Aeg/Na-Amp + Glt W

primary intrusive texture is thus recognizable, even if the original igneous minerals are rarely preserved. The garnet coronas developed to different extents and generally result from the coalescence of gar- net crystals, each displaying dusted inclusion-rich cores and clear rims.

The plagioclase domains consist of fine grained equigranular ag- gregates of omphacite, which coexist with scattered garnet idioblasts and rare quartz. The plagioclase domains are sometimes character- ized by the presence of symplectitic epidote-omphacite intergrowths, associated with paragonite porphyroblasts, which are interpreted as pseudomorphs after earlier lawsonite (Messiga and Scambelluri, 1991).

Igneous clinopyroxene relicts display thin lamellae of aegirine- augite along ( 100) and are similar in composition to relict igneous clinopyroxenes from the blueschist Fe-Ti-gabbros. Igneous diopsides are commonly topotactically replaced by aegirine-augite; abundant fine inclusions of rutile outline the cleavages of the original clino- pyroxene. Some of the aegirine-augite shows undulose extintion and fractures healed by inclusion-free neoblastic clinopyroxene. Recrys- tallization of aegirine-augite into neoblastic clinopyroxene is also common along contacts with garnet coronas. As a whole, the clino- pyroxenes pseudomorphing the igneous diopside have lower jadeite contents than the clinopyroxenes in the fine grained aggregates of the plagioclase domain (by up to 20 mol%; Fig. 3). Where igneous diopsides are preserved (rock sample MS15/6), their rims are re- placed by unzoned aegirine-augites with high aegirine contents (a35 mol%).

Igneous Fe-Ti-oxides are replaced by aggregates of rutile and mi- nor aegirine-augite. These aggregates preserve the original interstitial texture of the igneous Fe-Ti-oxides and, occasionally, ilmenite rel- icts. Clinopyroxenes in the oxide domains have generally higher ae- girine contents than those growing in the plagioclase or diopside domains.

Garnets are almandine-rich with 7-21 mol% and 2-14 mol% of grossular and pyrope, respectively; spessartine and andradite are commonly subordinate. Garnet grains are generally zoned, with slight outward decrease of grossular and spessartine, accompanied by a slight increase in the Mg/Fe*+ ratio. As a rule, garnet coronas are characterized by asymmetric zoning patterns, whereas garnet idio- blasts are symmetrically zoned (for further details see Messiga and Scambelluri, 1991, and references therein).

FIG. 2. Relevant microtextural features of blueschist and eclogitized Fe-Ti-gabbros. The igneous mineral on which the metamorphic minerals develop is reported in brackets. Scale bar is 1 mm; mineral abbreviations here and in all tables and figures according to Kretz (1983). (A) blueschist; (B) undeformed eclogite; (C) Baser eclogite; (D) mylonitic eclogite.

62 B. Messiga et al.

QUAD ?? igneous Di

. (Di/PI)

?? (Di)

A V)

o (ox)

+ porphyroclasts

0 matrix

JO AEG FIG. 3. Composition of clinopyroxenes. The areas enclosed by

dashed and dotted lines pertain to metamorphic clinopyroxenes from eclogites and blueschists, respectively. The microtextural domain in which the metamorphic clinopyroxenes develop is reported in paren- theses. The parent minerals for corona development are separated by a slash. Nomenclature of chnopyroxenes after Morimoto (1988).

Accessory alhmite (40 pm) generally occurs in plagioclase do- mains or as inclusion within garnets. In the rock samples CM1 / 1 and CM9, the effects of the decompressional evolution are minor, with limited development of glaucophane or barroisitic amphibole along the garnet coronas (Table 2). On the other hand, MS15/6 is char- acterized by the pervasive development of barroisitic amphibole, ep- idote, albite, and magnetite in the plagioclase domain.

Deformed eclogites

On textural grounds, deformed eclogites have been subdivided in flaser and mylonitic eclogites. Flaser eclogites consist of aegirine- augite porphyroclasts, probably pseudomorphs after the igneous cli- nopyroxene, in a fine grained matrix of omphacite, garnet, and rutile (Fig. 2~). The porphyroclasts commonly show undulose extinction, twins, kink bands, and a mantle of recrystallized grains; they are in places bounded by garnet coronas. Matrix omphacites are aligned along the foliation plane. They are richer in jadeite than the porphy- roclasts by about 10 mol% and compositionally resemble the om- phacites in the plagioclase domain of undeformed eclogites. Garnets frequently display optical zoning, with dark inclusion-rich cores and light rims. The chemical zoning is slight (Table 3) and closely re- sembles that of garnet idioblasts in undeformed eclogites. Mylonitic eclogites (Fig. 2d) differ from flaser eclogites in the absence of cli- nopyroxene porphyroclasts and in the local occurrence of atoll- shaped garnets. Both flaser and mylonitic eclogites contain significant amounts of apatite, generally associated with elongated Wile and minor ilmenite. Accessory phases are quartz, Fe-sulphides, and al- lanite.

The Reaction Behaviour

Blueschists are characterized by commonly preserved metastable mineral relicts, strong chemical heterogeneities in metamorphic cli- nopyroxenes related to the microtextural domains, and coronitic min- erals with large compositional gradients. In contrast, undeformed eclogites rarely display igneous relicts, show small variations in cli- nopyroxene compositions, and have mineral coronas with subtle zon- ing patterns. The eclogite facies metamorphic reactions were, there- fore, characterized by longer-range element diffusion than those per- taining to the blueschist facies metamorphism. Likely, in both blueschists and eclogites, metamorphic reactions were controlled by intergranular diffusion in the presence of a fluid phase, whereas in- tracrystalline diffusion was subordinate (for further details see Mes- siga, 1987 and Pognante and Kienast, 1987).

Table 2. Visually estimated modes of minerals (vol. %) in the investigated rock samples.

blueschists Sample IGC21/1 MNll/:

Di IO

Ilm 5 5

*eg 20 30

Na-Amp 25 20

LWS 10 15

Id 10 10

Sph 5 5

Chl 10 12

Ab 5 3

undeformed eclogites deformed eclogites

MSIY6 CMI/l CM9 CM6 RMu4 flaw mylon.

Di 20 - - Ilm 10 _ -

A# 15 25 35 20 - Omp - 38 25 39 59

Grt 5 30 30 35 35

Ap - - - 3 3

RI 5 5 3 3 Amp 25 2 5 _ -

EP 10 Ab 15 _ -

Deformed eclogites are characterized by obliteration of igneous microdomains and by relatively homogeneous metamorphic clino- pyroxenes. Synmetamorphic deformation provides the continuous production of crystal defects and dislocations necessary to accelerate element diffusion (Brodie and Rutter, 1985). Ductile shear zones moreover represent the preferred channelways for metamorphic flu- ids, thus increasing the element mobility (Dipple and Ferry, 1992, and references therein). Even in deformed eclogites, however, the

Table 3. Representative major element mineral analyses. The microtextural domain in which the metamorphic minerals developed is reported in Parentheses. The Parent minerals for corona development are separated by a stash. Hyphens represent concentrations under the detection limit of the electron microprobe (C 0.01 wt 96). Fe?* in clinopyroxenes and amphiboles has been estimated according to the method of Lindsley (1983) and Laird and Albee (1981), respectively. (a) Blueschists.

SiOl TiOz

AL4 FeW MnO

MgO CaO

Na,O

K,O

52.12 53.85 57.16 54.02 0.74 0.11 0.14 0.19 1.90 3.54 8.31 4.46 9.29 22.67 16.96 21.42

0.38 0.05 0.19 - 14.27 3.37 7.85 2.61 19.83 4.90 0.66 5.06 0.59 10.61 7.03 11.53

0.02 -

16.92 8.58 0.19 1.23 2.59

13.46

Tot. 99.12 99.10 98.32 99.29 100.00

OX. 6 6 23 6 6

Si Ti Al Fe’+ Fe” Mn

Mg Ca Na K

1.96 2.00 7.98 1.98 1.98 0.02 - 0.02 - 0.08 0.15 1.37 0.19 0.69 0.01 0.61 0.54 0.65 0.25 0.28 0.09 1.44 0.01 - 0.01 - 0.02 - 0.01 0.80 0.19 1.63 0.14 0.06 0.80 0.20 0.10 0.20 0.10 0.04 0.76 1.90 0.82 0.91

Trace-element geochemistry of cpx 63

Table 3. (b) Undeformed eclogites.

MS;;/6 MSlY6 CMlll C~Iklll C;:/l CM&/l CMl&l CM9

CL%) (“Dg:, (PIP (8x) (Di/PI) (Pl) (“Dp:,

SiO,

TiO,

A&Q FeO’rn

MnO MgO CaO

Na,O

52.38 54.15 54.18 55.31 54.15 31.34 31.66 54.36 55.14 31.21 31.54

0.65 0.01 - 0.06 0.21 0.10 0.02 - 0.13 0.10 - 1.73 5.34 3.17 8.02 3.83 21.17 21.43 4.86 8.83 21.12 21.41

9.43 15.27 10.88 9.39 12.64 32.61 33.96 12.97 10.17 33.12 34.89

0.47 0.21 0.02 0.02 - 1.08 0.91 0.06 0.03 1.10 0.30 12.75 5.61 10.07 7.64 6.91 1.39 1.88 7.71 6.69 1.43 2.17

21.31 10.16 16.45 12.47 12.48 6.52 4.66 13.45 10.75 6.20 4.43

0.71 8.31 4.53 7.20 7.97 - - 6.60 1.95 -

Tot. 99.43 99.12 99.31 100.04 98.22 100.27 100.52 100.01 100.29 100.19 100.74

OX. 6 6 6 6 6 12 12 6 6 12 12

Si 1.97 1.99 2.00 1.99 2.00 3.00 3.01 1.98 2.00 Ti 0.02 - - - 0.01 0.01 - - -

Al 0.08 0.23 0.14 0.34 0.17 2.00 2.02 0.21 0.37

Fe’+ - 0.38 0.19 0.19 0.39 - - 0.29 0.18

Fe2+ 0.30 0.09 0.15 0.10 - 2.19 2.27 0.10 0.13 Mll 0.02 0.01 - - - 0.07 0.06 - -

Mg 0.72 0.31 0.55 0.41 0.38 0.17 0.22 0.42 0.36 Ca 0.86 0.40 0.65 0.48 0.49 0.56 0.40 0.53 0.41

Na 0.05 0.59 0.32 0.50 0.57 - - 0.47 0.55

2.99 2.99 0.01 -

2.00 2.01 0.01 - 2.22 2.32

0.07 0.02

0.17 0.26

0.53 0.38 _ _

presence of compositional zoning in garnets indicates that within sample equilibration was not achieved.

The K. values for the Fe-Mg exchange between garnet and om- phacite are constant in undeformed and deformed eclogites, if fine- grained recrystallized omphacites and the outer portions of adjacent garnets are used (Ernst, 1976; Messiga and Scambelluri, 1991). In fine grained domains, equilibrium was, therefore, locally attained, probably favoured by the higher rate of surface reactions (Rubie, 1986, 1990). In undeformed eclogites, equilibrium microdomains are commonly found in the pseudomorphs after plagioclase (Ernst, 1976; Messiga and Scambelluri, 199 1)

TRACE ELEMENT MINERAL CHEMISTRY

Blueschists

Relict igneous diopside grains display relatively high REE contents (Table 4; Fig. 4), with flat HREE patterns at about 40 X Cl chondrites, LREE-depletion (CeN/SmN = 0.4) and slight negative Eu anomalies. The abundances of Y, Zr, SC, Ti, and Sr are generally higher than in the metamorphic cli- nopyroxenes (Fig. 5 )

Metamorphic clinopyroxenes have low REE contents, with differing C 1 -normalized patterns in different microtextural domains (Fig. 4). The highest REE abundances are found in the pseudomorphic aegirines after diopsides, which display a constant decrease from heavy to light REEs ( CeN/YbN = 0.2, for yb, = 3) and a slight positive Eu anomaly (Eu/Eu* a 1.7). Aegirines rimming diopside have extremely low REE concentrations, with positive Eu anomalies (Eu/Eu* = 3). Relative to diopside, aegirines are markedly depleted in Y and Sr, and extremely enriched in V (Fig. 5). Jadeitic clinopy- roxenes have extremely low REE abundances, with strong positive Eu anomalies (Eu/Eu* = 4- 11). Jadeites have

much lower concentrations of Zr, SC, Y, Ti, and V than ae- girines, either pseudomorphic or coronitic (Table 4).

Undeformed Eclogites

The trace element concentrations of relict diopsides are close to those of relict diopsides from blueschists. Only Ti and V are significantly different; they are lower in the diop- sides from the eclogite than in those from the blueschist (Ta- ble 4).

Metamorphic clinopyroxenes have lower total REE con- tents and stronger LREE-depletion (CeN/SmN < 0.1) than igneous clinopyroxenes (Fig. 6). Aegirine-augite pseudo- morphs after the original diopside generally have C 1 -normal- ized REE pattern characterized by a slight HREE-depletion relative to the middle REE (Gd,/Er, = 2). Aegirine-augites from the rock sample CM 1/ 1 have a maximum value at Eu, whereas those from CM9 display a slight negative Eu anom- aly (Eu/Eu* = 0.8). The coronitic aegirine-augites replacing diopside in rock sample MS1516 display a steady increase of over two orders of magnitude from La to Yb. Relative to the original diopside, the pseudomorphic clinopyroxenes are markedly depleted in Ti, Zr, and Y, whereas V, SC, and Sr abundances overlap the range for diopsides (Fig. 7). Al- though coronitic aegirine-augites from rock sample MS15/6 display significant trace element inhomogeneity, they invari- ably have higher Y and V, and lower Sr abundances than the aegirine-augite pseudomorphs after diopside.

Omphacites that replace the original plagioclase display a strong HREE-depletion (GdN/ErN = 5). The REE patterns are bell-shaped, with the maximum value corresponding to

64 B. Messiga et al.

Table 3. (c) Deformed eclogites.

CM6 CM6 %t6

CM6 RhW4 RM1/4 @I) (matrix) (matrix) (m%x) &X) (m%x) A@ Omp

SiQ

TiO,

*l,Q

FeO”*

MnO

MgG

CaO

Na,O

54.20 54.86 37.28 37.48 54.67 37.26

0.04 0.10 - 0.05 0.10

5.95 7.66 20.94 21.13 8.56 20.94

17.68 14.35 35.22 36.28 11.92 35.32

0.57 0.24 - 0.62

3.86 4.72 0.72 0.91 5.40 1.64

10.40 9.76 5.35 4.13 11.42 4.51

8.25 8.81 - - 7.58 -

Tot. 100.34 100.20 100.18 100.18 99.60 100.39

Ox. 6 6 12 12 6 12

Si 1.99 1.99 3.02 3.04 1.99 3.00

Ti 0.01 - - 0.01

Al 0.26 0.33 2.00 2.02 0.37 1.99

Fe” 0.35 0.32 - - 0.18 0.01

Fe2’ 0.20 0.12 2.38 2.46 0.18 2.37

Mn - 0.02 0.04 0.02 - 0.04

Mg 0.21 0.26 0.09 0.11 0.29 0.20

Ca 0.41 0.38 0.46 0.36 0.45 0.39

Na 0.59 0.62 - - 0.54 -

Eu (Fig. 6). The abundances of Y, Zr, and SC are generally lower than in aegirine-augites that replaced diopside (Fig. 7). In sample CM9, omphacites have higher Sr contents than aegirine-augites after the igneous diopside. The contents of V

and Ti in the omphacites do not differ significantly from the clinopyroxenes pseudomorphing original diopsides.

In the rock sample CM1 / 1, we have analyzed the aegirine- augites in the aggregates with rutile, i.e., replacing the original oxides. The REEs, Y, and Sr of these clinopyroxenes are close to those of omphacites replacing plagioclase, but V abun- dances are higher (by a factor of 5; Fig. 7). The aegirine- augites have a wide range in Ti, Zr, and SC concentrations, which partially overlap those of the omphacites replacing pla- gioclase.

Garnet grains in the pseudomorphs after plagioclase and in the coronas between the plagioclase and diopside domains have been analyzed. Their Cl-normalized patterns are char- acterized by a strong enrichment in HREEs relative to LREEs (CeN/YbN = lo-?, for YbN = lo-lOO), generally with a maximum corresponding to Dy (Fig. 6). Strontium and zir- conium contents are low, whereas SC and V cover a wide compositional range (Table 5).

Deformed Eclogites

The C 1 -normalized REE patterns of clinopyroxenes from deformed eclogites are invariably bell-shaped (Fig. 8), closely resembling those of the omphacites from undeformed eclogites. Apart from SC, matrix omphacites and porphyro- elastic aegirine-augites from the flaser eclogite do not display significant variations in trace element composition (Fig. 9). Scandium is higher in the porphyroclasts than in matrix om- phacites (45-80 ppm and 8-25 ppm, respectively). Scan- dium data on matrix omphacites from the mylonitic eclogite indicate, however, a large grain-to-grain inhomogeneity (20- 90 ppm) . Clinopyroxenes from the flaser eclogite exhibit low V abundances; in omphacites from RMll4, the V concentra-

Table 4. Trace element clinopyroxene compositions. Textural domain indicated as on Table 3. Hyphens represent concentrations under the detection limit of the ion microprobe, n.a. = not analyzed. (a) Blueschists.

IGC21/1 IGC21/1 IGCZl/l IGC21/1 IGC21/1 MN11/5 MNIU5 MNl1/5 MN1115 MN1115 Di Di Di *eg *eg *eg *eg

(Dim) (Dim) (yi) (Di) $) {) (z) 1 2 3 1 2 2 1 2 3

La 1.49

Ce 8.82

Nd 14.3

Sm 6.34

Eu 1.81

Gd 8.13

DY 10.2

Er 5.70

Yb 5.39

Y 53.4

SC 143

V 593

Ti 3880

Zr 54.2

Sr 11.5

1.55 n.a.

8.83 9.70

14.1 14.8

6.01 6.15

1.70 1.77

8.01 n.a.

9.86 10.8

5.55 5.46

5.29 5.93

0.03

0.08

0.03

0.02

0.02

0.04

-

0.02

0.06

0.02

0.02

0.01

0.01

0.17 0.16

0.36 0.34

0.43 0.41

0.21 0.20

0.14 0.13

0.30 0.34

0.43 0.52

0.33 0.33

0.54 0.54

0.08 0.02 0.11

0.24 0.05 0.22

0.12 0.04 0.06

0.05 - 0.04

0.17 0.03 0.13

0.05 0.03 0.07

0.05 0.02 0.08

56.0 58.5 0.9 n.a. 4.3 6.3 3.3 0.3 1.0

135 138 62.0 64.2 37.6 21.3 <2 <2 <2

666 771 2930 4640 6320 8230 47.8 123 50.8

3470 3960 889 1990 712 887 190 181 76.7

66.4 58.4 26.3 24.2 13.7 7.8 <O.l <O.l <O.l

12.8 12.5 2.6 3.9 2.0 2.1 1.2 1.2 1.4

Trace-element geochemistry of cpx 65

Table 4. (II) Undeformed eclogites. Due to the frequent rutile microinclusions, Ti analyses on aegirine-augites which pseudomorph the igneous diopside were often precluded.

MS15/6 MS15/6 MSW6 MS15/6 MS15/6 MS15/6 MS15/6 CMlll CMlll Di Di Di Agt Agt Agt Agt Agt Agl

(DiIPl) (DUPI) (DUPI) (Di) 1 2 3 1 2 3 (?) I (?)

La n.a. n.a. n.a. 0.03 0.07 n.a.

Ce 9.70 9.13 10.7 0.12 0.28 0.11

Nd 13.8 13.6 15.3 0.28 0.47 0.36

Sm 6.21 5.86 6.65 0.28 0.40 0.18

Eu 1.75 1.79 2.00 0.14 0.19 0.08

Gd n.a. n.a. n.a. 0.62 0.85 n.a.

DY 11.3 11.3 12.0 2.09 3.05 1.05

Er 6.12 5.91 6.51 1.63 2.93 0.73

Yb 6.16 5.86 6.53 1.74 3.51 0.88

Y 60.1 65.5 67.9 11.0 23.4 11.5

SC 129 138 126 108 128 104

V 339 363 325 1120 963 540

Ti 3050 2980 3150 334 n.a. 327

Zr 56.5 45.7 46.6 1.4 2.5 3.3

Sr 14.3 14.6 16.5 8.3 11.5 8.3

n.a. n.a. n.a.

0.20 0.07 0.18

0.77 0.36 0.51

0.42 0.28 0.39

0.22 0.16 0.19

n.a. n.a. n.a.

1.97 0.20 0.22

1.40 0.07 0.14

1.62 0.07 0.11

20.8 0.8 0.5

140 93.1 104

507 499 403

n.a. n.a. n.a.

10.5 0.9 0.7

9.6 31.7 31.8

tions are similar to those observed for the omphacites from zoning to that of the HREEs, varying from 690 ppm at the undeformed eclogites. core to 320 ppm at the rim (Table Sb) . Garnets from the flaser

Garnet idioblasts (Fig. 8) from the flaser eclogite display eclogite exhibit low V concentrations, with a slight rimward a strong chemical zoning in the HREEs, which gradually de- increase (e.g., from 5 ppm to 8 ppm), whereas Zr abundances crease from the core to rim. As predictable on the basis of on the same garnet grain decrease from 5 ppm to 2 ppm. charge and ionic radius (Shannon, 1976), Y shows similar Significant grain-to-grain compositional inhomogeneity is ev-

Table 4. @) Undeformed eclogites (continued).

CM111 CMl/l CMl/l CMl/l CMl/l CMlll CM9 CM9 CM9 CM9 Gmp Omp Gmp Agt Agt Agt Agt Agt Omp Omp

(Pl) (PI) (Pl) (W (Ox) (Ox) @i) (Di) (PI) (Pl) 1 2 3 1 2 3 1 2 1 2

La n.a.

Ce 0.07

Nd 0.32

Sm 0.17

Eu 0.09

Gd n.a.

DY 0.10

Er 0.03

Yb -

Y 0.1

SC 7.1

V 241

Ti 256

Zf 0.6

Sr 21.0

n.a. n.a.

0.08 0.06

0.27 0.37

0.19 0.21

0.12 0.14

n.a. “.a.

0.07 0.10

0.02 0.02

0.1 n.a.

6.3 12.5

399 236

265 309

0.9 0.6

27.4 21.3

0.01

0.08

0.51

0.49

0.23

0.40

0.04

0.1

16.2

0.01

0.06

0.19

0.17

0.08

0.19

0.06

0.2

7.7

0.05 0.07

0.02 0.17 0.34

0.66 0.63

0.07 0.75 0.65

0.06 0.24 0.18

0.10 0.94 1.19

0.04 0.81 1.44

0.35 0.76

0.26 0.54

0.1 5.0 7.7 1.0

36.4 105 150 12.4

0.01 n.a.

0.12 0.14

0.57 0.47

0.57 0.49

0.22 0.23

0.57 n.a.

0.24 0.20

0.10 0.05

0.4

5.2

1720 3270 2240 566 686 448 560

322 678 276 324 390 320 294

0.6 1.7 0.4 5.3 3.7 0.6 0.8

47.1 22.6 18.6 21.7 14.9 30.1 65.2

66 B. Messiga et al.

Table 4. (c) Deformed eclogites.

CM6 CM6 CM6 CM6 CM6 RM1/4 RM1/4 RM1/4 Agt &t Omp Omp Omp Omp Omp Omp @i) (Di) (matrix) (matrix) (matrix) (matrix) (matrix) (matrix)

1 2 1 2 3 1 2 3

La 0.03 0.04

Ce 0.21 0.26

Nd 0.64 0.90

Sm 0.50 1.06

Eu 0.23 0.52

Gd 0.71 1.50

DY 0.43 0.30

Er 0.11 0.08

Y 1.0 0.7

SC 45.2 81.5

V 8.4 39.4

Ti 523 463

Zr 0.8 0.9

Sr 15.3 34.4

0.05 n.a. n.a. 0.04

0.21 0.20 0.21 0.34

0.88 0.97 0.79 1.12

0.94 1.15 0.68 1.09

0.47 0.51 0.26 0.44

1.11 “.a. n.a. 1.15

0.39 0.31 0.31 0.62

0.15 0.10 0.07 0.19

1.2 1.2 0.6 3.0

8.1 9.9 25.1 19.4

8.7 27.6 14.0 200

453 433 436 596

0.9 0.9 0.7 1.2

10.1 21.3 15.7 31.0

n.a. n.a.

0.24 0.23

0.93 1.19

0.90 0.88

0.38 0.38

“.a. n.a.

0.44 0.54

0.08 0.17

2.6 “.a.

87.8 n.a.

143 n.a.

424 n.a.

1.3 n.a.

22.2 n.a.

ident from SC contents: despite being rather constant in indi- vidual garnet grains, SC ranges from 6-42 ppm in the flaser eclogite.

TILE PROTOLITHS

The diopside relicts in both blueschist and eclogites display similar compositions for major elements and for REEs, Y, Zr, SC, and Sr. This indicates that they were formed from similar igneous liquids, implying that blueschists and eclogites were derived from similar igneous protoliths. The data indicate, however, that the abundances of V and Ti are lower in the diopsides from the eclogite than in those from the blueschist. Such differences may be ascribed either to primary igneous differences, or to contamination by metamorphic minerals which replace the thin lamellae of exsolved Ca-poor pyrox-

REE/Cl 100

I0 -b/F

0 igneous DI

+Aeg(DI/PI)

+Aeg(DI)

* Jd(PI)

FIG. 4. Cl (Anders and Ebihara, 1982)-normalized REE abun- dances in selected clinopyroxenes from blueschists.

ene. If the different V and Ti concentrations are a primary igneous character, they could be related to small variations in the modal fractionation of Fe-Ti oxides (i.e., the ilmenite/ magnetite ratio) during the differentiation process.

EVIDENCE FOR IGNEOUS SIGNATURES IN METAMORPHIC CLINOPYROXENRS

Major Element Controls on the Incorporation of Trace Elements in Clinopyroxene

Theoretical and experimental investigations indicate that trace elements reside in regular crystallographic sites of cli- nopyroxene. The main crystal-chemical constraints are charge and ionic radius of the trace element, in relation to the nominal

Cpx/Cl

FIG. 5. Trace element abundances in the clinopyroxenes from blueschists, normalized to Cl-chondrites. The elements are plotted in order of increasing abundance in relict diopside.

Trace-element geochemistry of cpx

B. Messiga et al. 68

Cpx/Cl

100 c undeformed eclogites 0

031

OsOl I V zr SC Y

FIG. 7. Trace element abundances, normalized t( the clinopyroxenes from undeformed eclogites.

O igneous 0

= Agt(Di/PI)

?? Agt(Di)

A Omp(PI)

v Agt(Ox)

Zl-chondrite,

charge and volume of the incorporating polyhedron (e.g., Philpotts, 1978; McKay, 1989). Therefore, the compatibility of a trace element with clinopyroxene could significantly vary in relation to the wide range in jadeite and aegirine substitu- tions observed for the clinopyroxenes studied here.

REEs and Y enter the eightfold M2 site (McKay, 1989; Shearer et al., 1989; Caporuscio and Smyth, 1990). In partic- ular, Caporuscio and Smyth (1990) found a negative corre- lation between total REE and jadeite component for clino- pyroxenes from eclogite-xenoliths in kimberlites from South Africa. Based on charge balance considerations, Caporuscio and Smyth ( 1990) concluded that jadeite substitution exerts a negative effect on lanthanide incorporation. Zirconium, ti- tanium, vanadium, and scandium are commonly regarded as octahedral cations. The ionic radii of SC and Zr are relatively large (Shannon, 1976) and, therefore, their incorporation into the sixfold Ml site of clinopyroxene could be favoured by larger Ml volumes. Increasing octahedral Al through jadeite substitution could hence reduce the compatibility of SC and Zr with clinopyroxene. The incorporation mechanisms of V and Ti have not been completely established. Vanadium in- corporation is dependent on its oxidation state (V 3+, V 4+, or V 5+), whereas the presence of Ti into the tetrahedral site cannot definitely be excluded (see Carbonin et al., 1989). On the basis of charge and ionic radius, Sr is inferred to enter the M2 site of clinopyroxene. Due to local charge balance re- quirements, Sr compatibility with clinopyroxene could be minimized by increasing Na contents.

The clinopyroxenes studied here show some rough corre- lations between trace and major elements, which support the existence of a major element control on trace element incor- poration. Blueschist clinopyroxenes have the highest Na/Na + Ca ratio and the lowest lanthanide contents, whereas ig- neous clinopyroxenes have the lowest Na/Na + Ca ratio and the highest lanthanide contents. This agrees with the negative effect of jadeite substitution on REE incorporation reported by Caporuscio and Smyth ( 1990). Since REE substitution for Na requires a more complex heterovalent mechanism than for Ca, we suggest that the incorporation of REEs into clinopy- roxene when the M2 site is occupied by Na is precluded by

local charge balance requirements. The analyzed metamor- phic clinopyroxenes moreover show a rough positive corre- lation between V and Fe3+ (cf. Tables 1, 2). Since V 3+ and Fe3+ display close ionic radii (Shannon, 1976), this correlation suggests that V is dominantly trivalent and that the aegirine substitution strongly favours the incorporation of v3+.

Trace Element Distribution in the Original Igneous Assemblage

On the basis of the published distribution coefficients between minerals and magmatic liquids (e.g., Drake and Weill, 1975; Pearce, 1978; Kelemen et al., 1990), and of analyses on natural assemblages (e.g., Shearer et al., 1989; Mazzucchelli et al., 1992a), we have inferred that SC, Zr, Ti, V, and trivalent REEs were strongly partitioned into diopside relative to plagioclase. Plagioclase could concen- trate significant amounts of LREEs and was likely the ma- jor sink for Sr and Eu2+ (Mazzucchelli et al., 1992a). Fe-Ti oxides probably played a primary role for V and, obviously, Ti. Zirconium partition coefficients between minerals and igneous liquids (McCallum and Charette, 1978) suggest that original igneous Fe-Ti-oxides also con- tained significant Zr amounts.

Blueschists

Jadeitic clinopyroxenes in plagioclase domains retain the geochemical characteristics of igneous plagioclases (Shearer et al., 1989; Mazzucchelli et al., 1992a), such as low REE abundances with Cl-normalized pattern characterized by a marked positive Eu anomaly, and extremely low contents of SC, Y, and Zr (Figs. 4, 5). Unlike igneous plagioclases,

Table 5. Trace element compositions of garnets. Hyphens represent concentrations under the detection limit of the ion microprobe: n.a. = not analyzed. (a) Undeformed eclogites.

CMl/l CMl/l CMlll CMlll CM9 CM9 (Pl) (Pl) (PlIDi) (Pl/Di) (PI) (Pl/Di)

1 2 1 2 1 1

Ce 0.04 0.02 0.05 0.05 0.02 -

Nd 0.16 0.13 0.19 0.12 0.26 0.08

Sm 0.26 0.29 0.67 0.31 1.63 1.44

EU 0.50 0.61 1.17 0.75 1.92 1.95

Gd n.a. n.a. n.a. “.a. n.a. 11.7

DY 9.97 8.46 5.97 12.2 27.4 11.4

Er 4.01 2.36 1.79 5.33 13.74 5.27

Yb 2.86 1.57 1.63 3.89 13.40 5.02

Y 33.0 18.2 n.a. n.a. “.a. 60.4

SC 25.4 12.5 33.9 54.1 26.7 56.9

V 43.5 53.6 23.0 18.9 92.0 55.4

Ti 435 317 302 409 232 344

zr 0.6 0.7 0.6 2.9 0.4 0.4

Sr 0.1 0.2 0.1 0.1 0.3 <O.l

Trace-element geochemistry of cpx 69

Table 5. @) Deformed eclogites.

CM6 CM6 CM6 CM6 CM6 CM6 RM1/4 RM1/4 RM114 1 1 1 1 2 3 1 2 3

core interm. intenn. rim rim

Ce 0.02 0.02 0.02 0.02 0.09 0.03 - 0.02 0.02

Nd 0.18 0.28 0.28 0.29 0.56 0.22 0.31 0.24 0.32

Sm 1.77 2.02 2.49 2.12 3.34 1.49 2.55 2.67 3.09

EU 3.43 3.60 4.01 2.71 3.81 2.62 3.72 5.03 4.98

Gd 24.8 24.5 26.8 23.1 n.a. 17.5 20.3 n.a. n.a.

Dy 141 103 68.8 56.4 33.3 88.5 31.4 60.6 41.7

Er 73.9 42.6 22.4 17.9 13.1 52.7 14.3 50.9 24.5

Yb 55.9 26.7 14.2 10.5 11.2 39.4 9.59 55.4 20.8

Y 691 663 483 321 114 318 210 n.a. n.a.

SC 36.2 41.6 35.4 31.1 5.6 20.0 8.6 16.0 n.a.

V 5.2 5.6 6.6 8.4 4.3 6.0 24.7 22.8 n.a.

Ti 717 807 883 839 482 617 996 711 n.a.

zr 4.6 2.4 2.0 1.6 0.7 4.2 1.3 0.5 n.a.

Sr 0.1 0.1 0.1 0.1 <O.l 0.2 0.1 <O.l n.a.

however, jadeites exhibit extremely low Sr abundances. This is probably related to: ( 1) the high Na/Ca ratios in the jadeites which, due to charge balance requirements, inhibit the Sr in- corporation; and (2) buffering by the coexisting lawsonite. Lawsonite contains high Sr contents (x2500 ppm; Tribuzio, 1992) and its crystal structure has large cavities occupied by Ca and Hz0 (Baur, 1978) in which large divalent cations such as Sr may be easily incorporated.

,ooo REE/Cl c t 1 sample CM6

100 =

0.01' I ’ I I ’ ’ I a I L I ’ I I ’ LaGa Wd SmEu Qd Dy Er Yb

The positive Eu anomaly of plagioclases is generally re- lated to the larger ionic radius of Eu2+ , which is highly com- patible with the plagioclase structure, with respect to trivalent REEs (e.g., McKay, 1989; Shearer et al., 1989). Therefore, due to the similarity in ionic radius between Eu2+ and Sr (Shannon, 1976), we would expect that during blueschist metamorphism Eu2+ would follow Sr into the lawsonite. The presence of positive Eu anomaly in the Cl-normalized REE

REEK1 1000 k

- sample RAW4

43 Omp(matrlx) + Grt

0.01’ I I ’ I I I ’ I I I I I ’ La Ce Nd Sm Eu Qd Dy Er Yb

FIG. 8. Cl-normalized REE abundances in selected clinopyroxenes and garnets from deformed eclogites (CM6 = flaser eclogite; RMl/4 = mylonitic eclogite). The garnet compositions from rock sample CM6 refer to a single grain, in which the distance between ion microprobe spots is 45 pm.

70 B. Messiga et al.

CpxlC1

deformed eciogites

?? Agt(porphyr.)

o Omp(matrix)

FIG. 9. Trace element abundances, normalized to Cl-chondrite, in the clinopyroxenes from deformed eclogites.

pattern of jadeites, coupled with low Sr contents, thus sug- gests that at least part of the Eu2+ once in the original plagi- oclase was oxidized to Eu 3+ . Europium oxidation due to post- magmatic alterations, i.e., in a metamorphic environment with high oxygen fugacity, is consistent with the high Fe3+ amounts observed in blueschists.

Coronitic and pseudomorphing aegirines after igneous di- opside display different REE compositions (Fig. 4). The co- ronitic aegirines display extremely low REE contents and pos- itive Eu anomalies, thus suggesting that the REEs were mainly derived from the igneous plagioclase, i.e., the REE contribution from the diopside was subordinate. The aegirine pseudomorphs have higher REE contents than both coronitic aegirines and jadeites, despite the similar Na/Na + Ca ratios. This probably reflects the higher REE contents of their pre- cursor igneous diopsides relative to the other igneous miner- als. Both coronitic and pseudomotphing aegirines are char- acterized by extremely large V contents (up to one order of magnitude higher than in the igneous diopside, Fig. 5)) which indicate an important contribution from the breakdown of the Fe-Ti-oxides.

Undeformed Eclogites

The metamorphic clinopyroxenes from undeformed eclo- gites are LREE-depleted, but display variable HREE patterns (Fig. 6). The aegirine-augites growing as rims after the relicts of diopside (rock sample MS 15/6) are HREE-enriched. Ae- girine-augite pseudomorphs after igneous diopside display a slight HREE-depletion. A stronger HREE-depletion is exhib- ited by fine grained omphacites in plagioclase domains and by aegirine-augites in oxide domains. Likewise, Y contents gradually decrease from the coronitic aegirine-augite to the clinopyroxenes in plagioclase and oxide domains. The vari- ations in the shape of REE pattern are probably not related to major element variations, since the size of M2 site, in which both LREE and HREE are incorporated, is nearly constant (single-crystal X-ray diffraction data, Tribuzio, 1992). Since garnet has strong affinity for HREE and Y (e.g., Irving and Frey, 1978; Caporuscio and Smyth, 1990), we thus conclude

that the different HREE and Y concentrations in the eclogitic clinopyroxenes result from different degrees of re- equilibration with the garnet.

To our knowledge, published data on REE compositions of clinopyroxenes from eclogites (Philpotts et al., 1972; Griffin and Brueckner, 1985; Shervais et al., 1988; Caporuscio and Smyth, 1990; Thoni and Jagoutz, 1992; Jerde et al., 1993a,b) have never shown as strong a LREE-depletion as that ob- served here. This geochemical feature does not reflect the composition of the host rock, since the C 1 -normalized pattern of the eclogites is only slightly LREE-depleted. Probably, the LREE once contained in the igneous diopsides and plagio- clases are almost entirely incorporated into accessory allanite, which is absent in the blueschists and developed during the eclogite metamorphism. The bell-shaped REE pattern of the clinopyroxenes in plagioclase and oxide domains is likely due to coexistence with allanite and garnet, and presumably rep- resents complete reequilibration under eclogite facies condi- tions.

The metamorphic clinopyroxenes from undeformed eclo- gites display different trace element compositions in relation to the microtextural domain. Aegirine-augites which replace igneous diopside invariably have higher amounts of SC, Y, and HREEs (Figs. 6, 7), probably inherited from their pre- cursor, than clinopyroxenes which develop in plagioclase and oxide domains. The relatively high contents of HREEs, Y, and SC in clinopyroxenes after igneous diopsides, therefore, agree with the Sm-Nd isotopic evidence for igneous signa- tures in pseudomorphic clinopyroxenes from gabbro-derived eclogites of western Norway and southeastern Austria (Mork and Mearns, 1986, and Thoni and Jagoutz, 1992, respec- tively). Likewise, aegirine-augites in aggregates with rutile (oxide domain) have the highest V contents, which are clearly related to the breakdown of the original Fe-Ti-oxides.

Clinopyroxenes in plagioclase and diopside domains from rock-sample CM9 display also wide variations for Zr and Sr (Table 4), which probably reflect the original trace element partitioning in the igneous protolith. In particular, despite the higher Na/Na + Ca ratio, the clinopyroxenes growing in the plagioclase domain have higher Sr contents than those after igneous diopsides. Probably, the trace element heterogeneities of clinopyroxenes in different textural domains, although par- tially controlled by major element variations, are largely due to limited trace element mobility during metamorphism.

The differences in trace element compositions of metamor- phic clinopyroxenes in different microdomains are smaller for eclogites than for blueschists. Eclogite facies metamorphism is, therefore, characterized by longer-range trace element mo- bility than blueschist facies metamorphism. This is probably related to the greater major element equilibration of eclogites relative to blueschists.

Deformed Eclogites

The clinopyroxenes from deformed eclogites are relatively homogeneous in composition. In particular, the Cl-normal- ized REE patterns of clinopyroxenes from both flaser and my- lonitic eclogites are invariably characterized by strong HREE- depletion (Fig. 8), similar to that observed for the clinopy- roxenes in the plagioclase and oxide domains in undeformed

Trace-element geochemistry of cpx 71

eclogites. In flaser eclogite, the ratios of trace element con- tents between clinopyroxene porphyroclasts and fine grained matrix are close to unity. The enlargement of reaction do- mains related to the synmetamorphic deformation, therefore, obliterated the igneous geochemical signatures.

Scandium represents an exception. In deformed eclogites, SC data show a large grain-to-grain variations for both cli- nopyroxenes and garnets. However, in flaser eclogites, the porphyroclastic clinopyroxenes have higher SC abundances than matrix omphacites (Fig. 9). In undeformed eclogites, the largest heterogeneities between clinopyroxenes in the plagi- oclase and diopside domains were indeed found for SC. Among the trace elements analyzed, SC is thus inferred to have the lowest diffusivity.

TRACE ELEMENT PARTITIONING BETWEEN OMPHACITE AND GARNET

Approach to Local Equilibrium in Eclogites

Despite evidence for chemical disequilibrium at the hand sample scale, there is some evidence for local attainment of equilibrium between garnet and clinopyroxene in eclogites. The ratios of trace element abundances in the fine grained omphacites to idioblastic garnets in the plagioclase domain are similar in the two undeformed samples (Table 6). These ratios almost coincide with the ratios between matrix om- phacites and adjacent garnet rims from deformed eclogites. This consistency, which is independent of texture and of mo- dal compositions, suggests that chemical equilibrium was lo- cally attained and that these ratios may be regarded as parti- tion coefficients.

The different omphacite/gamet pairs indeed highlight a significant scatter of Cpx’Grf D values for several elements. This scatter partly results from analytical uncertainty, due to poor counting statistics (e.g., the small amounts of Ce and Sr in the garnets, and of HREEs and Y in the omphacites) and zoning at the limit of spatial resolution for ion microprobe analysis (e.g., HREEs, Y, and Zr in the garnets from flaser eclogite). The variations may also be due to grain-to-grain inhomogeneity (e.g., SC in both clinopyroxenes and garnets from deformed eclogites).

The LREEs display greater affinity for omphacite than for garnet (Fig. 10). The lanthanide contraction results in a grad- ual tendency of REEs to concentrate into the garnet: Cpx'GrtDEr is three orders of magnitude lower than Cpx'GnDCe. Vanadium preferably partitions into the omphacite, whereas the Cpx'GnD values for Ti and Zr are close to unity (Table 6). Strontium exhibits a strong affinity for the omphacite ( Cpx'GnDSr > 100)) whereas Y is almost entirely incorporated into the garnet. The Cpx'GnDSc values resulting from omphacite/gamet pairs in un- deformed eclogites indicate a marked preference for the gar- net. In deformed eclogites, due to the large grain-to-grain in- homogeneity, the ratios between SC abundances in omphacite and garnet highlight a large scatter and cannot be considered as partition coefficients. Even between fine grained ompha- cites and garnet rims, at least in deformed eclogites, chemical equilibrium was thus not completely attained.

Comparison with Published REE Partition Coefficients

Most of the published Cpx'GltDREE data referring to ompha- cite/garnet pairs are on eclogitic xenoliths included in kim-

Table 6. Mean trace element ratios between omphacite and garnet. Undeformed eclogitex omphacWgamet pairs from the plagioclaae textural domain. Deformed eclogites: matrix omphaciWgamet rim pairs.

undeformed deformed eclogites eclogites

sample CMl/l CM9 CM6 RM114

Ce 2.3 6.5 3.8 13.5

Nd 2.1 2.0 2.1 3.1

Sm 0.68 0.33 0.34 0.35 EtI 0.21 0.12 0.13 0.09

Gd 0.05 0.06

DY 0.010 0.008 0.008 0.012

Er o.cNI9 0.005 0.007 0.005

Y 0.005 - 0.006 0.013

SC 0.5 0.3 - -

V 6.0 5.5 3.3 7.2

Ti 0.7 1.3 0.7 0.6 Zr 1.0 1.8 0.7 1.8

berlites from South Africa (Caporuscio and Smyth, 1990). These data, obtained from analyses on mineral separates, are generally higher than those obtained here (Fig. 10). More- over, Caporuscio and Smyth (1990) determined DLEEID~~~ ratios about one order of magnitude lower than those calcu- lated here. To our knowledge, Cpx’Gn DREE patterns as steep as those observed here for eclogitized Fe-Ti-gabbros have pre- viously been determined only by ion microprobe on diopside/ garnet pairs from mafic granulites (Mazzucchelli et al., 1992b). The Cpx'GnDREE values referring to the mafic granu- lites are, however, up to one order of magnitude higher than those reported here.

This study shows that: ( 1) clinopyroxenes have different REE compositions related to the textural domain in which they are found and (2) garnets with slight major element zon- ing exhibit abrupt rimward decreases in HREEs. Hence, the Cpx'GnDREE determined by bulk methods would yield mean- ingless results. Literature data determined by bulk methods on high temperature eclogites, which are presumably char- acterized by equilibrium trace element distributions, could be affected by contamination because of the low LREE concen- trations in garnet (see also Sisson and Bacon, 1992; Schwandt et al., 1993). For instance, in the eclogites studied here, ac- cessory allanite could affect the LREE analyses on garnet sep- arates. Caporuscio and Smyth ( 1990), however, included a few ion microprobe measurements which tend to confirm the bulk mineral separates data.

In both pyroxene and garnet, the REEs are incorporated into the eightfold sites (e.g., Irving and Frey, 1978; McKay, 1989; Caporuscio and Smyth, 1990). In garnets, cation po- lyedra have a high number of shared edges (e.g., Novak and Gibbs, 1971). Consequently, the volume of the eightfold X site of garnet is significantly smaller than that of clinopyrox-

12 B. Messiga et al.

Cpx/Grt 1OOE I.

Fe- Ti-gabbros 0.001 ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’

Ce Nd Sm Eu Gd Dy Er Yb

Cpx/Grt lOOk

eclogites in kimberlites

0.001: Ce Nd Sm Eu Tb Yb

FIG. 10. (a) Omphacite/garnet partition ratios for REEs relative to eclogitized Fe-Ti-gabbros from Ligurian Alps. The REE partition ratios obtained from ion microprobe analyses on diopside/gamet pairs from mafic granulites (Mazzucchelli et al., 1992b) are reported for comparative purposes. (b) Omphacite-garnet partition ratios for REEs, obtained from analyses on mineral separates, relative to the bimineralic eclogite xenoliths included in kimberlites from South Africa (Caporuscio and Smyth, 1990).

ene (i.e., Smyth and Bish, 1989). The smaller volume of the X site of garnet relative to the M2 site of omphacite, therefore, explains the relation between lanthanide contraction and abrupt decrease in cpx’ortDREE values. In different clinopyrox- ene-garnet pairs, the slope of the Cpx’GnDm pattern could be correlated to the difference between the sizes of the M2 and X sites. Therefore, apart from the uncertainty related to garnet LREE analyses by bulk methods, the different slopes of the cPx’cnDWE pattern could be due to compositional variations.

blueschist and eclogite facies Fe-Ti-gabbros from Ligurian Alps (north western Italy) have allowed us to draw the fol- lowing conclusions:

1) Trace and major element data on relict igneous diopsides indicate that eclogites and blueschists were derived from similar protoliths.

Compared with diopside/gamet pairs from granulites (Mazzucchelli et al., 1992b), the lower Cpx’GrtDREE for the eclogites could be due to: ( 1) the higher Na contents of the omphacites (-0.5 apfu), which inhibit the incorporation of trivalent REEs due to local charge balance requirements and/ or (2) the lower temperature of equilibration in the eclogite facies (=5OO”C), compared with the granulites ( =8OO”C; Mazzucchelli et al., 1992b). Literature data on Na-clinopy- roxenelgarnet pairs from high temperature eclogites (Griffin and Brueckner, 1985; Caporuscio and Smyth, 1990) invari- ably show higher cPx’GRDHREE values than those obtained here, thus suggesting that higher temperature favours the partition- ing of REEs into the clinopyroxene. This temperature depen- dence could be explained by the greater thermal expansion of the clinopyroxene structure relative to that of the garnet, re- lated to the lower number of shared edges, which would fa- vour the incorporation of large cations at higher temperatures.

3)

4)

Detailed microtextural investigations coupled with ion mi- croprobe analyses on clinopyroxenes and garnets from

2) In undeformed blueschists and eclogites, the metamorphic clinopyroxenes have different compositions dependent on the different microdomains in which they were formed. In particular, the clinopyroxenes replacing the igneous di- opside commonly retain geochemical signatures of the precursor mineral. The trace element variations among metamorphic clino- pyroxenes in the different textural domains are larger in the blueschists than in the eclogites. This reflects the more limited major element equilibration of blueschists relative to eclogites. Clinopyroxenes from mylonitic eclogites lack the geo- chemical signatures of the original igneous minerals. Syn- metamorphic deformation, therefore, enlarges the reaction domains, thus resulting in greater homogenization of cli- nopyroxenes. In both undeformed and deformed eclogites, chemical equilibrium is only locally approached. The trace element contents of clinopyroxene are con- trolled by the coexisting mineral assemblage. For in- stance, clinopyroxenes in eclogites are strongly HREE- and LREE-depleted (i.e., with bell-shaped pattern) due to the equilibration with garnet and allanite, respec- tively.

5)

6)

CONCLUSIONS

Trace-element geochemistry of cpx 13

7) The partition coefficients for REEs between omphacite and garnet are markedly lower than those reported in the literature. On the basis of crystal-chemical considerations, we suggest that this is probably due to lower temperature conditions during formation.

The trace element composition of metamorphic minerals in high pressure/low temperature terranes is controlled by a large number of variables, such as: ( 1) trace element com- position of precursor mineral(s); (2) major element compo- sition of metamorphic mineral (i.e., crystal-chemical con- straints); (3) buffering by other coexisting minerals; (4) synmetamorphic deformation; and (5) temperature of crys- tallization. The reliability of data produced on mineral sepa- rates from rocks which did not attain chemical equilibrium should be critically reviewed, since they could be affected by uncertainties related to compositional zoning, to contamina- tion by microinclusions, and to recrystallization in textural domains inherited from the protolith.

Acknowledgments-This study has been carried out as part of the Ph.D. research of R.T., who gratefully acknowledges L. Ungaretti and R. Vannucci for constant support and stimulating discussions. F. A. Frey, R. I. Grauch, and two anonymous referees reviewed an early version of the manuscript. W. L. Griffin, R. W. Hinton, D. W. Mittlefehldt, and M. Thoni are thanked for the final revision of the paper. Electron microprobe facilities are due to the Centro Grandi Strumenti, Universim di Pavia. Funding was provided by CNR and MURST of Italy.

Editorial handling: D. Mittlefehldt

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APPENDIX

Major Element Analyses

Mineral analyses have been performed on the JEOL JXA-840A electron probe microanalyzer at Centro Grandi Strumenti (Universitlt di Pavia-Italy), employing three JEOL wavelength-dispersive spectrometers (WDS) and one TRACOR energy-dispersive spec- trometer (EDS) The data collected by the WDS and EDS spectrom- eters are simultaneously processed by the matrix correction program. Operating conditions were 20 kV accelerating voltage and 20 nA sample current. Natural standards were used. Representative major element analyses of minerals are reported in Table 3.

Trace Element Analyses

REEs and other trace elements (Y, SC, V, Zr, Ti, Sr) in clinopy- roxenes and garnets (Tables 4 and 5, respectively) have been ana- lyzed by using secondary ion mass spectrometry (SIMS) with the Cameca IMS 4F ion microprobe located at C.N.R.-CSCC, Pavia. The energy filtering technique (Shimizu et al., 1978) has been applied to remove molecular ion interferences. Analyses have been performed on polished thin sections with a spot size of about 20 pm. The se- lected areas were previously investigated under electron microscope in order to exclude the presence of solid or fluid microinclusions.

Under the experimental set up described by Bottazzi et al. (1990), clinopyroxenes and garnets have been analyzed for REEs by mea- suring the signals from one isotope of each investigated REE, i.e. lmCe, ‘&Nd, ‘%m, ls3Eu, 15*Gd, ‘63Dy, 16’Er, I’%. The presence of Cs and Ba, as possible sources of interferences, was monitored at masses 133 and 138, respectively; the concentrations of both these elements were less than 0.1 ppm. The determination of Gd on a single isotope (158) was sufficiently accurate only for compositions with strong LREE-depletion with respect to HREE; otherwise, we have adopted a deconvolution procedure on masses 156,158, 161, and 162 in order to discriminate the Gd (and Dy) signal from CeO and NdO interferences (see Zinner and Crozaz, 1986; Bottazzi et al., 1991). The low ion signals for HREEs of most metamorphic clinopyroxenes, with respect to the interfering oxides of the middle REEs, introduced some uncertainty in the evaluation of Er and Yb concentrations. In the case of Er, we have improved the measurement reliability by evaluating the 15’Eu I60 contribution at mass 167 from the Eu + signal (at mass 153, corrected for the isotopic abundance) and from the Eu oxidation ratio (i.e., EuO’/Eu+) as determined on suitable standards under the adopted analytical conditions. In the case of Yb, the highly unfavourable Yb+/GdO+ ratio did not allow in many cases any re- liable measurement. Concerning the other trace elements, more strin- gent energy filtering (sample voltage offset by - 100 V) has been used in measuring signals from the following isotopes: 45Sc, 47Ti,

Trace-element geochemistry of cpx 75

5’V, “Sr, @Y, 90Zr. As a matrix reference element, Si has been mea- sured as 3”Si.

Concentrations have been calculated by the empirical approach of relative sensitivity factors, which have been obtained from well determined mineral standards. Precision and accuracy of the method are believed to be = 10% above 1 ppm concentration. Be- low 1 ppm concentration, precision is constrained by counting sta- tistics in the range IO-30%. The varying Fe contents of clinopy- roxenes and garnets could represent the major concern in the quan- tification of trace elements. Sisson ( 199 1 ), however, has shown that in silicate matrices the Fe content has no influence on the ionization efficiency of trace elements. Moreover, on the basis of previous

investigations (Bottazzi et al., 199 1, 1992). we have inferred that the influence of different matrix composition and structure on REE ion yield in silicates is less than 25%. Reproducibility has been monitored over a span of three years, and is better than 10%. The instrumental background is l-3 ions counted over 10 minutes, which correspond to <4 ppb for REEs, depending on the abundance of the chosen isotope (Bottazzi et al., 1994). Detection limits are, therefore, determined by counting statistics. Under the adopted measurement conditions, precision of 30% ( lo,) is achieved at few tens of ppb for the LREEs. When significant interferences from light and medium REE oxides occur, detection limits for the HREEs are on the order of few hundreds of ppb.