Plagioclase-wehrlites and peridotites on the East Pacific Rise (Hess Deep) and the Mid-Atlantic Ridge (DSDP Site 334): evidence for magma percolation in the oceanic upper mantle

Earth and Planetary Science Letters, 115 (1993) 137-149 Elsevier Science Publishers B.V., Amsterdam

137

[PT]

Plagioclase-wehrlites and peridotites on the East Pacific Rise (Hess Deep) and the Mid-Atlantic Ridge (DSDP Site 334): evidence for magma percolation in the oceanic upper mantle Jacques G i r a r d e a u a and Jean F r a n c h e t e a u b a Laboratoire de Pdtrologie Structurale, UniL'ersitd de Nantes, 2 rue de la Houssini~re, 44072 Nantes Cedex 03, France b Laboratoire de Gdophysique Marine, IPGP, 4place Jussieu, 75252 Paris Cedex 05, France Received January 6, 1992; revision accepted December 10, 1992

ABSTRACT Textural and petrological data are presented that record the formation of plagioclase-bearing wehrlites and peridotites at slow-spreading and fast-spreading ridges that are comparable to the wehrlites and peridotites formed in ophiolitic complexes. Evidence is provided for locally pervasive magma percolation in the oceanic residual upper mantle. The samples studied come from the East Pacific Rise (EPR) (Hess Deep) and the Mid-Atlantic Ridge (MAR) (DSDP Site 334). At both sites, parts of the peridotites display poikilitic textures with oikocrysts of clinopyroxene and interstitial plagioclase that include xenocrysts of strained and partly annealed olivine and subidiomorphic spinel crystals. Petrofabric data for olivine suggest a magmatic origin for the Hess Deep wehrlite. Its phase chemistry is comparable to intrusive plagioclase-wehrlites and lherzolite from ophiolites that are thought to have crystallized from a crystal mush. It differs drastically from the associated Hess Deep diopside-bearing harzburgites and dunites, which are quite similar to oceanic and ophiolitic residual peridotites. The EPR wehrlite is therefore considered as an intrusive rock. The Leg 37 plagioclase-peridotites display a weak lattice fabric of olivive which may have resulted from high-temperature plastic flow; these rocks have a phase chemistry that is typical of ophiolitic and oceanic residual rocks. These rocks are hence interpreted as residual dunite or harzburgite which has been pervasively impregnated by a melt from which the clinopyroxene and plagioclase crystallized. These peridotites could have an intrusive origin or may have formed in situ at the crust-mantle transition in association with intrusions of large gabbro bodies.

1. Introduction

Impregnation in mantle rocks is now well known from studies of ophiolites [1]. This process, which reflects magmatism at oceanic ridges, is generally considered to have led to the local resorption of orthopyroxenes [1,2] and to the crystallization of 'late' clinopyroxene a n d / o r plagioclase within depleted peridotites. In these rocks, the 'secondary' crystals display either poikilitic shapes or magmatic twins [1]. In some samples, the impregnated plagioclase and clinopyroxene modal content is so large that the original depleted dunites or harzburgites are transformed into true plagioclase wehrlites and lherzolites. These rocks may occur either as thin

bands interbedded within layered gabbros at the crust-mantle transition or as large bodies cutting through the lowermost crustal rocks or the uppermost upper mantle rocks. As such, they may represent a significant constituent of the oceanic lithosphere. However, evidence for melt impregnation is rare in peridotites collected in the oceanic domains. Indeed, plagioclase-wehrlites and lherzolites seem to be absent from the peridotites collected from the present-day oceanic ridge axes (mainly peridotites cored during the DSDP and ODP legs). The peridotites from Leg 45 are residual harzburgites and lherzolites [3,4] with weak crystallographic preferred orientations that have been interpreted as cumulate fabrics [5].

0012-821X/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved

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Those cored during Leg 82 are residual spinel harzburgites [6] and those from Leg 109 are residual dunites, harzburgites and lherzolites [7] with a clear tectonite fabric [8]. The peridotites cored in the Tyrrhenian Sea during Leg 107 are strongly depleted residual harzburgites [9] with tectonite fabrics. The peridotites sampled during Leg 37 have also been described as residual harzburgites and lherzolites [10,11]. Some of them, however (the plagioclase-bearing varieties), were considered to be cumulate rocks [12,13]. Peridotites dredged at transform faults range from diopside-free spinel-harzburgites to diopside-rich harzburgites and lherzolites, and all of them are interpreted as residual mantle rocks [6,14-16]. Some of these peridotites, however, contain various amounts of plagioclase thought to have crystallized at a late stage from trapped melts [17-19]. Even though some general agreement now exists concerning the impregnated nature of some phases in mantle rocks, the significance of plagioclase- and clinopyroxene-rich peridotites referred to as plagioclase- wehrlite and lherzolite is still a matter of debate. Wehrlites are considered to be either (1) cumulates crystallized from Al-poor magmas formed by remelting of a depleted oceanic mantle [20,21], (2) to have formed through direct crystallization in the early stages of differentiation of picritic magmas [22-25], or (3) to have formed from a crystal mush consisting of magma and mantle xenocrysts [26-30]. From the study of the Oman wehrlites and Troodos plagiclase-wehrlites and lherzolites it has been recently suggested that such a crystal mush could have formed at relatively low pressure by resorption of the orthopyroxene in the uppermost mantle by the ascending tholeiitic melt [26]. This crystal mush would have been later extracted from the partly melted upper mantle by a filter press process related to compression from intraoceanic thrusting at the ridge itself [25-28] or to mantle rock expulsion at the edge of a mantle diapir [26,27,29,30]. This paper describes the texture and petrology of plagioclase-bearing wehrlites and peridotites in the oceanic crust from present oceanic ridges. The plagioclase-wehrlites studied came from the Hess Deep in the Equatorial Pacific [31], and they are presented together with data from asso-

J. G 1 R A R D E A U A N D J. F R A N C H E T E A U

ciated peridotites with which they are compared. The plagioclase-peridotites come from Site 334 (DSDP Leg 37] at the Mid-Atlantic Ridge (MAR) [32]. To discuss the process by which the plagioclase-bearing wehrlites and lherzolites have been formed, they are compared to similar rocks from the ophiolites of Oman (new data) and Troodos [27-33].

2. Sample locations Hess Deep is located at the triple junction between the E P R and the C o c o s - N a z c a (Galapagos) Ridge, to the west-northwest of the Galapagos Islands (Fig. 1A). It is a deep (5300 m) depression located at the western tip of the eastward propagating Cocos-Nazca Ridge, about 40 km east of the EPR. At its western tip, this propagating ridge has developed a rift valley marked by normal fault escarpments, along which some of the upper part of the E P R oceanic lithosphere outcrops. The rocks recovered at that site are thought to represent the oceanic lithosphere created at the adjacent fast-spreading (13 c m / y r ) EPR axis [31]. This oceanic lithosphere, which was first sampled by dredging [34], was studied in 1988 using the submersible Nautile [31]. This allowed good sampling of the abundant rocks that represent the E P R oceanic lithosphere. The samples include pillow lavas, dolerites, isotropic coarsegrained gabbros, layered gabbros and peridotites (Fig. 1A). These peridotites comprise mainly depleted harzburgites and dunites, some of them with clinopyroxene and plagioclase [31]. Site 334 was drilled during DSDP Leg 37 of the Glomar Challenger in 1974 [32], on the western flank of the median valley of the MAR, southwest of the Azores (Fig. 1B). More specifically, it was drilled about 100 km west of the Famous area in a small deep basin that was selected to avoid fracture zone effects [35]. The samples recovered were therefore considered to be representative of the oceanic lithosphere formed at the slow-spreading (1 c m / y r ) MAR. The drilling yielded 50 m of basalts overlying 67 m of a layered sequence comprising coarsegrained two-pyroxene gabbros, olivine gabbros, peridotites and tectonic breccias (Fig, 1B). The peridotites comprise residual spinel-harzburgites

MAGMA PERCOLATION

and lherzolites [10,11], and some plagioclasebearing rocks considered to be cumulate rocks [12,13]. It was proposed that the latter crystallized in small magma pockets that would have acted as temporary reservoirs for upwelling magmas [12]. 3. Textural data

3.1 Hess Deep peridotites The Hess Deep peridotites range from dunites to diopside-bearing harzburgites, and most of the samples recovered are diopside-bearing harzbur-

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140

also the case for plagioclase, which is interstitial. Spinel is idio- to subidiomorphic. The olivine constitutes large crystals or small rounded ones when included in the clinopyroxenes and shows widely spaced kinkbands (0.5-0.8 mm). Other phases, however, and particularly the clinopyrox-

Fig. 2. (A) Hess Deep plagioclase-wehrlite (sample NZ-17-9). (B) Hess Deep diopside-bearing spinel-harzburgite (sample NZ-17-2b). (c) Leg 37 plagioclase-peridotite (sample 22-2, 51-54). O/=olivine; Cpx = diopside; O p x = enstatite; Sp = spinel; Pla = plagioclase.

J. GIRARDEAUAND J. FRANCHETEAU

enes, are quite unstrained. The latter do show abundant exsolution lameUae though, and the largest crystals show some undulatory extinction. Using U-stage analysis of decorated dislocations and kinkband geometry, it can be shown that the olivine plastically deformed by the (010) [100] high-temperature slip system. To reveal plastic deformation microstructures, we have decorated the olivine crystal dislocations by heat treatment. The plastic deformation history is indeed well recorded by the dislocation structures, which define well-organized, straight, subgrain boundaries and [001] tilt walls (Fig. 3A). The olivine shows evidence of recovery: some of the subgrains display low densities of free dislocations. Moreover, the smallest olivine crystals included in the clinopyroxene oikocrysts (Fig. 2A) are almost totally free of dislocations (Fig. 3B), a feature which is also ascribed to late recovery during annealing of the olivine crystals. Alternatively, one may consider that these crystals represent phenocrysts directly crystallized from the ascending magma. To enable a discussion of the deformation conditions we have carried out detailed petrofabric analyses of the olivine. However, because of the large degree of serpentinization of all the rocks the diagrams show only < 25 data points, and this does not permit definitive conclusions. Nevertheless, we do believe that the fabric patterns are representative of the strain regime, with each point plotted that corresponds to a distinct crystal, subgrain or neoblast that was formed by SGR (subgrain rotation recrystallization) having been systematically removed from the computed diagrams. The olivine crystals in the plagioclase-wehrlite show no good lattice preferred orientation, particularly for the [100] olivine axis, the maxima of which is located at a high angle to the lineation (Fig. 4A). Such a weak preferred orientation of olivine could correspond to a magmatic or flattening fabric rather than to a plastic "d'~formation fabric [36]. If this is the case, the, plastic deformation of the olivine exemplified ~gy the kinkbands must have formed prior to the crystallization of the plagioclase-wehrlite, which would accord with the absence of deformation in the poikilitic clinopyroxene. The fine crystals would hence represent xenocrysts rather than phenocrysts. From a

MAGMA PERCOLATION IN THE OCEANIC UPPER MANTLE

textural point of view it can be therefore suggested that the Hess Deep plagioclase-wehrlite represents a magmatic rock. The porphyroclastic diopside-bearing harzburgites associated with the plagioclase-wehrlites are well-foliated rocks (Fig. 2B) that show better evidence of plastic deformation in a rotational regime (Fig. 4B; sample NZ17-2b). This is shown by the orientations of the [001] and [100] axes, which each define fairly good maxima, although the [010] axes are somewhat scattered. Such an olivine fabric would have resulted from slip on the (010) [100] system active at high temperature. Olivine from an associated dunite displays a weak lattice fabric (Fig. 4C, sample NZ17-12b), similar to that of the wehrlites. Hence, with the excep-

141

tion of the porphyroclastic diopside-bearing harzburgites which can be considered as tectonites, all other rocks from the Hess Deep area display relatively weak crystallographic fabrics that can be interpreted as magmatic.

3.2 Leg 37 plagioclase-peridotites The Leg 37 peridotites are serpentinized rocks that contain abundant relicts (often up than 40%) of their primary phases (olivine, orthopyroxene, clinopyroxene and spinel). Primary plagioclase remnants only occur in minor amounts [13]. These rocks have very heterogeneous textures, with plagioclase and pyroxene locally forming large patches with no shape fabric. Hence, the modal

Fig. 3. Photomicrographs of decorated dislocation microstructures in olivine. (A) Straight subgrain (010) walls (Hess Deep plagioclase-wehrlite). (B) Dislocation-free annealed crystal of olivine included in a large clinopyroxene oikocryst (Hess Deep plagioclase-wehrlite). (C) Straight and closely spaced subgrains with (010) walls (Leg 37 plagioclase-peridotite). (D) High-angle tilt walls corresponding to optical kinkbands; note the near-absence of free dislocations and (010) walls in some of the subgrains due to recovery (Leg 37 plagioclase-peridotite).

142

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Deep plagioclase-wehrlite (sample NZ-17-9, 25 points). (B) Hess Deep diopside-bearing spinel-harzburgite (sample NZ17-2b, 25 points). (C) Hess Deep dunite (sample NZ-17-12b, 25 points). (D) Leg 37 (sample 22-2-51, 23 points).

composition of the peridotites varies significantly from one sample to the other. Some samples (e.g,, 22-2 and 51-53) with 58-66% of olivine (modal composition) contain relatively large amounts of clinopyroxene (19%) and plagioclase (18-10%), about 1% chrome spinel, and minor quantities of orthopyroxene ( < 4%). Others (22-2, 21-33 and 43-46), with similar olivine contents, have less clinopyroxene (2-5%), much more orthopyroxene (5-10%) and less plagioclase (515%). Accordingly, their compositions vary from orhopyroxene- and plagioclase-bearing wehrlite to plagioclase- and clinopyroxene-bearing harzburgites. These peridotites are coarse grained (average

crystal sizes between 2 and 5 mm) and display poikilitic textures similar to heteradcumulate textures (Fig. 2C). The clinopyroxene forms large poikilitic crystals with locally magmatic twins. It includes crystals of olivine and spinel and, more rarely, orthopyroxene. These clinopyroxene crystals are unstrained. The orthopyroxene forms relatively large crystals (around 4 mm), with irregular and locally lobed outlines that suggest some resorption; this is in good agreement with the abnormally low content of this mineral seen in some of the samples. Locally, orthopyroxene shows well-developed kinks, indicating some plastic deformation of the rock. Plagioclase is always interstitial. Spinel is idiomorphic to subidiomorphic and included in all other phases. Olivine is generally coarse grained (5-7 mm), but may also form small (2 ram) rounded crystals included within the clinopyroxene. It often shows large kinkbands (0.2 mm), a feature which was mentioned in previous studies but which was ascribed to a late tectonic event [13]. Here again, microstructures and kinkband geometry reflects a high-temperature plastic deformation for these olivine crystals, which result from gliding along the (010) [100] slip system. Dislocations also show closely spaced, straight, subgrain boundaries with only limited free dislocations (Fig. 3C). This feature is also observed in some crystals displaying high-angle tilt walls corresponding to optical kinkband boundaries (Fig. 3D). Moreover, the heterogeneity of the free dislocation density within crystals, with a near-absence of dislocations in some of the subgrains, is reminiscent of similar features in xenoliths. Hence, these microstructures probably formed through recovery postdating the plastic deformation of the olivine crystals. The olivine crystals display well-oriented (010) crystallographic planes that are nearly parallel to the foliation/lamination plane defined in the peridotite by the spinel crystal flattening (Fig. 4D). The corresponding [001] and [100] axes tend to define girdles subparallel to the foliation plane, but with maxima, particularly for the [100] axis, close to the lineation. Although such a fabric is difficult to interpret due to the limited number of measurements [23] it does suggest that the olivine preferred orientation results from slip along the (010) [100] system at high temperature.

143

M A G M A P E R C O L A T I O N IN T H E OCEANI C U P P E R M A N T L E

These rocks are therefore ambiguous due to the unstrained and poikilitic nature of the clinopyroxenes and the presence of an olivine fabric that is possibly due to plastic deformation. This bivalent character can, however, be reconciled if one considers clinopyroxene and plagioclase to represent impregnation products in plastically deformed harzburgite. 4. Phase compositions The Hess Deep and Leg 37 peridotites are strongly altered rocks, the serpentinization averaging 60-70% in most of the samples studied. Nevertheless, the primary phases, i.e. olivine, clinopyroxene, spinel, plagioclase and the occasional orthopyroxene, are present and indeed locally abundant, allowing good estimates of the primary phase chemical composition. Olivine and spinel have been analyzed in their cores and peripheries using point analysis techniques. Analyses of pyroxenes integrate the exsolution lamellae composition to obtain an estimate of the bulk mineral composition. Olivine, spinel and pyroxene formula units were calculated on the basis of 4, 4 and 6 oxygens respectively. The analyses were made using a Camebax microprobe (Camparis, Universit~ Paris V/). All analyses may be obtained on request.

4.1 Hess Deep peridotites The olivines from the Hess Deep harzburgites are unzoned and have compositions quite similar to those of other oceanic ridge peridotites in terms of forsterite or NiO contents (Fig. 5A). Internal comparison of the Hess Deep rock types shows that the forsterite content of olivine ranges between 90 and 91% for the diopside-harzburgites (NZ17-1, 2b, 8 and 12), declining to 89.3 for a dunite (NZ17-12b) and a diopside-bearing troctolite (NZ9-7b). It reaches even lower values in the plagioclase-wehrlites, ranging from 87.8 to 88.4 in sample NZ9-1 and declining to 84.9-85.7 in samples NZ17-9 and -9B. Hence, the two latter rock types, which are both plagioclase rich, have forsterite contents below that of xenoliths of the peridotites from San Carlos and Kilbourne Hole, the latter being believed to represent less residual mantle (LRM; M g # LRM = 89.6 + 0.4 [37]). The

NiO content of the olivine shows large variations from 0.2 wt% to 0.45, and with the exception of wehrlites NZ17-9 and -9B this is broadly correlated with the forsterite content. Indeed, the average value for this NiO (0.3 wt%) is similar to that of some diopside-harzburgites and is above Ni0 LRM (0.26). The spinel is chromium rich (Fig. 5B; C r # 0.5-0.6; C r # L R M = 0.1). Its Ti content discriminates the diopside-harzburgites and the dunites, which are Ti poor (Ti < 0.002), from the plagioclase-wehrlites, which are Ti rich (0.022-0.031); i.e., the groups of rock types with L R M values above and below 0.002 are distinguished. The spinels are not zoned, implying rapid cooling. The clinopyroxenes in all the rock types are chrome-rich diopsides, similar to those from highly depleted ophiolitic and oceanic peridotites (Fig. 6). On the basis of Cr partitioning, the clinopyroxenes are in equilibrium with spinel and, when it is present, orthopyroxene (k d o p x / s p between 1.7 and 2;, k d o p x / c p x between 1.4 and 1.7). Average reequilibration temperatures of about 1050 +_ 30°C can be estimated for pyroxene pairs using Bertrand and Merciers's thermometer

[381. The Na and Ti contents of the clinopyroxene (Fig. 7) also discriminate the two groups mentioned above: the diopside-harzburgites with very low contents ( < 0.005 and 0.001, respectively; similar to the values from depleted residual peridotites in ophiolites) and the plagioclase-rich rocks which are enriched in Na and Ti (0.027 and 0.01, respectively), and especially in Ti, but still without reaching the LRM values (Na = 0.1170.126, Ti = 0.013-0.015). The plagioclase is strongly anorthitic (86%).

4.2 Leg 37 plagioclase-peridotites Olivine is unzoned and iron rich compared to depleted residual upper mantle rocks as its M g # varies from 86.7 to 88.2 (Fig. 5A). This relatively low Mg content, however, contrasts with the NiO content, which is high in these olivines (0.29-0.38 wt%) and comparable to that in many residual oceanic and ophiolitic peridotites. Such an iron enrichment is tentatively ascribed to reequilibration between olivine and an Fe-rich melt. Spinel (Fig. 5B) is chromium rich ( C r # varies from 0.55

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145

MAGMA PERCOLATION IN THE OCEANIC UPPER MANTLE

The clinopyroxene is also chromium rich, as is the orthopyroxene whenever it is present (Fig. 6). Here again, both phases appear to be in equilibrium, with a partitioning similar to that of ophiolitic harzburgites. Both phases last reequilibrated at a temperature of around 880 _+ 45°C.

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I 0 0 No (cpx ) Fig. 7. T i / N a correlation diagram for clinopyroxene. The residual peridotite trend is that defined in Fig. 5. Field for Oarrett transform fault (tf) from [18] and corresponding to plagioclase-bearing peridotites, other associated spinel-peridotites plotting on the partial melting trend (same legend as in Fig. 5).

Most of the Ti and Na contents of the clinopyroxene (0.018 and 0.004, respectively) are similar to those measured in diopside from depleted ophiolitic peridotites (Fig. 7). However, in one clinopyroxene-rich sample, this mineral is enriched in Ti by a factor of two (0.008), although this is still much lower than the mantle value (0.015). The plagioclase is particularly anorthitic (91%). 5. Discussion

To discuss the origin of the Hess Deep and Leg 37 rocks, the compositions of some intrusive wehrlites and plagioclase-lherzolites from the Troodos [27] and the Oman ophiolites [new data] have also been reported (Figs. 5-7). The samples from Oman include both plagioclase-wehrlites from large intrusions cutting through the gabbroic sequence (samples SO7, 8, 9, 9b, 9c) and plagioclase-lherzolites from a thin intrusive layer within the gabbros (samples S012, 12a, 12b, 12c). These data are also compared with some layered plagioclase-wehrlites from the Donqiao-Xainxa ophiolite [39].

The diagrams show that the intrusive wehrlites and lherzolites from Oman and Troodos have distinct compositions and are very different from the layered plagioclase wehrlites from the Donqiao-Xainxa ophiolite. The olivine in the intrusive wehrlites is very NiO rich compared to that of the layered wehrlites (Fig. 5A), the spinel bears higher Ti contents (Fig. 5B), and the pyroxenes have lower Ti and Na contents (Fig. 7). Figure 5A also shows large variations in the Mg# of olivine for the Oman wehrlite and lherzolite, depending on their location within the gabbroic sequence. The olivines from the plagioclase-lherzolites from the thin intrusive layers are much more iron rich (Mg# 84%) relative to those from the large intrusions (Mg# 88.5%, plagioclase-wehrlites), a feature which probably reflects some reequilibration between the melt and the surrounding gabbroic rock; Ti and Na enrichments in clinopyroxenes also indicate reequilibrium. A temperature of 720°C is estimated for the F e / M g exchange using the olivine-spinel thermometer [40]. The chemical differences between the Oman plagioclase-wehrlites and lherzolites are also well illustrated using their spinel and clinopyroxene compositions (Fig.

MAGMA

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6B and Fig. 7, respectively), the clinopyroxenes from the wehrlites being slightly enriched in Ti compared to those formed through partial melting of the lesser depleted peridotites. On the basis of phase compositions the Hess Deep wehrlites more closely ressemble the Oman plagioclase-wehrlite and Troodos plagioclaselherzolite than depleted harzburgites from ophiolites and oceanic domains. On the other hand, from textural evidence, it can be suggested that the Hess Deep plagioclase-wehrlite represents a cumulate crystallized as a crystal mush including deformed and partly annealed xenocrysts of olivine. This could have resulted from the impregnation by a large melt fraction of a residual rock that would have lost its original cohesion and would have later behaved as a suspension. Such an interpretation, which has been proposed to explain the crystallization of the intrusive Troodos [27] and Oman [28-30] plagioclase-peridotites, seems adequate for the formation of the Hess Deep wehrlite, for which important F e - M g reequilibration between the xenocrysts and the melt occurred. Magmatic processes in the oceanic crust sampled in the Hess Deep area would therefore be similar to those inferred from studies of the Samail ophiolite [30,31,41]. However, the intrusive nature of the rock is still impossible to demonstrate owing to the lack of in-situ submersible observations. The nature of the Leg 37 plagioclase-peridotite is slightly different as it chemically bears much more affinity to residual rocks than to magmatic ones, although the olivine M g # of the Leg 37 plagioclase-peridotite is relatively low, a feature which again can be ascribed to late lowtemperature (700-750°C) F e - M g reequilibration. On textural grounds, the origin of the Leg 37 plagioclase-lherzolites appears ambiguous due to the unstrained and poikilitic nature of the clinopyroxene and the presence of a weak olivine fabric thatb was possibly caused by plastic deformation. This ambivalent character can be reconciled if one considers that clinopyroxene and plagioclase represent impregnation products of plastically deformed dunites and harzburgites, an interpretation which is in good agreement with the observation of deformed and subsequently annealed olivine crystals in the rock. Nevertheless, this scenario may still seem unlikely given the

possible layered nature of the peridotite-gabbro association cored at Leg 37, which has led to the interpretation of these rocks as cumulates [12,13]. To reconcile these observations, it can be suggested that the Leg 37 plagioclase-lherzolite indeed represents an impregnated residual peridotite, but one which has locally been so lubricated by interstitial melt that it behaved as a suspension that was capable of intruding the highest levels of the M A R crust. The lattice fabric would then have been partly preserved in the less impregnated zones, which would have locally kept their internal grain cohesion during the crystal mush flow. An alternative interpretation is to consider that the section sampled at Site 334 represents a crust-mantle boundary composed of gabbro bodies with peridotite screens [42]. In this latter case, the mantle rocks would therefore represent impregnated peridotites formed in situ and related to the gabbroic intrusions. In both cases the peridotites must be carried up to the surface at a late stage by displacement along normal faults (a tectonic denudation process) or by late serpentinization-related diapirism, two processes which can explain the presence of peridotites at the seafioor of many oceanic ridges [31,43-47]. Whichever detailed genetic model is accepted for the origin of these rocks, this study shows that plagioclase-peridotites comparable to those found in ophiolitic complexes do exist in oceanic lithosphere formed at the present-day oceanic ridges. Their formation reflects localized pervasive magma percolation in the uppermost residual upper mantle.

Acknowledgements We wish to thank J. Karson and A. Nicolas for their constructive remarks. This work was supported by DBT G~odynamique.

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44

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48

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