Jean-Louis Bodinier

Orogenic, ophiolitic and abyssal peridotites

Orogenic, ophiolitic, and abyssal peridotites represent subcontinental, suboceanic, and subarc mantle rocks that were exhumed to the surface in various tectonic settings. These rocks provide coverage of vast regions of the Earths upper mantle that are sparsely sampled by mantle xenoliths. They notably allow the observation of a wide range of lithospheric mantle compositions, including cratonic roots and subduction mantle wedges (high-pressure orogenic garnet lherzolites), variably rejuvenated subcontinental lithosphere (lower-pressure orogenic spinel and plagioclase lherzolites), and newly accreted oceanic lithosphere (ophiolitic mantle and abyssal peridotites). It is shown here that most of geochemical variability recorded by these mantle rocks is attributable to melt processes associated with partial melting and asthenosphere–lithosphere interactions. Rather than remnants of pristine mantle, the fertile orogenic lherzolites are now widely considered as former refractory lithospheric mantle refertilized by upwelling partial melt.

Relationships between geochemistry and structure beneath a palaeo-spreading centre: a study of the mantle section in the Oman ophiolite

The Oman ophiolite exposes a large and well-preserved mantle section beneath a palaeo-spreading centre. The mantle section is mainly composed of extremely refractory harzburgites with relatively homogeneous modal and major element compositions. Nevertheless, our trace element data exhibit variations connected with the main mantle structures, which allow us to define three geochemical and structural domains. The main harzburgitic mantle section, mainly constituted of strongly refractory harzburgites characterised by chondrite-normalised REE patterns that are steadily depleted from HREE to LREE. These rocks are interpreted as mantle residues after s 15% melt extraction. Their REE signature can be explained by melt transport associated with partial melting. The diapir areas (mainly the Maqsad diapir), defined by plunging lineations. They are constituted of harzburgites with roughly the same modal composition as the main mantle section but distinct, concave-upward REE patterns. The regions of most active upwelling (characterised by sub-vertical lineations) are further distinguished by higher Al2O3/CaO ratios and TiO2 contents. This character is ascribed to focused partial melt upwelling. The diapirs are interpreted as local instabilities in upwelling mantle, possibly triggered by feedback mechanisms between deformation and melt percolation. The Maqsad diapir is topped by a thick, dunitic, mantle^crust transition zone (MTZ) that displays the same trace-element signature as the diapir. However, the dunites are distinguished by low Mg# values and Ni contents. Together with structural evidence, this allows us to interpret the MTZ dunites as diapir harzburgites that were strongly modified by olivine-forming melt^rock reactions at high melt/ rock ratios. The MTZ is thought to act as a major collecting zone for mantle melts. The cpx-harzburgites from the base of the mantle section. These rocks are distinguished by high clinopyroxene contents (s 5%), low AL2O3/CaO and ‘spoon-shaped’ REE patterns. They were individualised from the rest of the harzburgite mantle section by a cpxforming melt^rock reaction at decreasing malt mass. This reaction probably occurred at near-solidus conditions along the lithosphere^asthenosphere boundary. The formation of these three domains may be integrated in a geodynamic scenario involving the reactivation of an oceanic lithosphere, a process that would be related to the ridge propagator identified in the Oman ophiolite. fl 2000 Elsevier Science B.V. All rights reserved.

A plate model for the simulation of trace element fractionation during partial melting and magma transport in the Earth's upper mantle

We propose a new plate model for the simulation of trace element transfer during magmatic and metasomatic processes taking place in the Earths upper mantle. As in previously published plate models, porous flow is accounted for by propagation of fluid batches through macrovolumes of mantle rocks. Being released from spatiotemporal constraints, the plate model allows much more freedom than the one-dimensional porous-flow models for the simulation of fluid-rock interactions. Hence this approach may account for a wide range of mantle processes, including melt extraction during compaction of molten peridotites, porous flow associated with Chromatographic effects, or fluid-rock reactions occurring upon melt infiltration at the base of the conductive mantle. The applications presented in this study show several results consistent with published trace element data for mantle rocks and basaltic volcanism. In particular, the proposed models may provide simple explanations for (1) the ultra-rare-earth-element-depleted composition of peridotites and interstitial melts residual after mid-ocean ridge basalt extraction, (2) the negative correlation between light rare earth element / heavy rare earth element (LREE/HREE) ratio and refractory character of peridotites, as observed in several suites of mantle rocks, and (3) the origin of ultra-LREE-enriched meta-somatic fluids infiltrated in the lithospheric mantle.

Geochemistry and petrogenesis of Eastern Pyrenean peridotites

Abstract The high-temperature peridotite bodies of the Eastern Pyrenees (France), which are composed of spinel peridotites containing bands of pyroxenites and veins of amphibole-bearing ultrabasic rocks, have gone through a multi-stage evolution. The peridotites underwent partial melting in the stability field of garnet resulting in major variations of Mg, Al, Ca, Na, Ti, Sc, V, Ni and HREE. Then the peridotite residue was invaded by basaltic melts. The pyroxenite bands in the peridotites are high-pressure crystal segregates from these melts. Subsequently, after cooling in subcontinental lithospheric conditions, the peridotites interacted with alkali magma which was probably associated with the Cretaceous alkali magmatism of the Pyrenees. In addition to the crystallization of amphibole-rich ultrabasic rocks in vein-conduits and the re-equilibration of the wall-rock peridotites leading to LREE, Ti and Fe enrichments, this event was accompanied by extensive metasomatic processes. The metasomatism locally affected lherzolites, producing an increase of the modal proportions of clinopyroxene (± amphibole) (Caussou massif). The metasomatism was more widespread in the harzburgites where it produced an enrichment of LREE relative to HREE without a significant change in the modal composition.

Petrogenesis of layered pyroxenites from the Lherz, Freychinéde and Prades ultramafic bodies (Ariége, French Pyrénées)

The pyroxenite bands in spinel Iherzolites from three high-temperature peridotite massifs (Lherz, Freychinede and Prades) in Ariege (French Pyrenees) have geochemical features including distributions of Mg, Ca, K, Ti, Zr and REE typical of cumulates derived by high P−T crystal segregation. However, these features of the whole-rocks are not reflected in the element distribution among the minerals which was partially modified during subsequent recrystallization. In addition, samples from the margins of the pyroxenite bands were affected by secondary processes such as metasomatism or subsolidus re-equilibration. Unlike pyroxenite xenoliths from basalts which are usually assumed to be derived from alkalic basaltic magmas, the pyroxenite layers are related to tholeiitic magmas similar to the parental liquids of the Triassic dolentes encountered throughout the Pyrenees. The data show that high P−T fractionation significantly affected the ascending tholeiitic magma and suggest that the continental tholeiites could have been derived from an upper mantle source with a chondrite shaped REE pattern.

Contrasting lithospheric mantle domains beneath the Massif Central (France) revealed by geochemistry of peridotite xenoliths

We report major and trace element analyses for 82 coarse-grained peridotite xenoliths from 25 Cenozoic volcanic centres throughout the Massif Central (France). These data cover a region of about 150×150 km, allowing an investigation of large scale compositional variations in the subcontinental lithospheric mantle (SCLM). In agreement with textural variations, geochemical data define two contrasting lithospheric domains, situated north and south of latitude 45°30′. Peridotites of the northern domain show protogranular textures, characterised by clustered pyroxene–spinel distributions. They are rather refractory and depleted in MREE relative to HREE, but pervasively enriched in LREE and other highly incompatible elements. The samples show mantle-normalised patterns with negative anomalies of Nb, Ta, Zr and Hf, similar to enriched mantle xenoliths ascribed to carbonatitic metasomatism. In contrast, the peridotites of the southern domain are devoid of pyroxene–spinel clusters and are therefore referred to as coarse-granular. They are distinguished from the northern suite by more fertile compositions and relatively flat MREE–HREE patterns. In addition, only the harzburgites and a few lherzolites are enriched in LREE. Most southern domain lherzolites are depleted in these elements and the average composition of the southern suite is comparable to that of depleted MORB-source mantle (DMM). The main compositional differences between the two domains cannot be accounted for by a secular evolution of the Massif Central SCLM caused by Cenozoic plume upwelling. Instead, these differences record the existence of distinct lithospheric blocks assembled during the Variscan orogeny. To some degree, the northern and southern domains are reminiscent of cratonic and circumcratonic SCLM domains. Being relatively refractory and pervasively enriched in LREE, the northern domain displays similarities with cratonic SCLM. It is interpreted as a relatively ancient (pre-Variscan) lithospheric block involved in the Variscan belt. Conversely, the fertile composition and the DMM signature of the southern domain evoke more juvenile lithospheric mantle, possibly accreted or rejuvenated during the Variscan orogeny. Geophysical data indicate that asthenospheric upwelling beneath Massif Central is focused beneath the southern domain and follows a NW–SE trend, roughly parallel to Variscan structures in the crust. Though poorly constrained in direction, the limit between the two SCLM domains recognised in this study is consistent with this trend. This may suggest a link between the inherited architecture of the SCLM and channelling of asthenospheric upwelling. Secular variations in xenolith geochemistry, as well as correlations between trace element data and geophysical anomalies, suggest that the geochemical imprint of Cenozoic plume upwelling on SCLM xenoliths is limited to selective enrichments in U, Sr and Pb relative to Th and REE.

Effects of mineralogical reactions on trace element redistributions in mantle rocks during percolation processes: A chromatographic approach

Mantle rock studies provide evidence of interaction with upwelling magmas. In erogenic lherzolites, one of the most conspicuous effects of these interactions is the development of harzburgite and dunite bands. Recent studies have suggested that these bands were formed at the expense of the host lherzolites by melt-rock reactions associated with magma percolation. In order to better understand the geochemical effects associated with percolation-reaction processes, we propose a numerical model of melt infiltration that takes into account modal variations in time and space resulting from melt-rock reactions. Melt volume variations are considered by means of porosity variations, and a local equilibrium approach is used for trace element exchange between melt and minerals. The transport of trace elements by the interstitial melt is described by a mass balance equation while the modal variations are constrained by the mineralogical trends observed in refractory peridotites massifs. The model is applied to RBE, Cr and Ni in percolated peridotites affected by an olivine-forming reaction, with the aim of reproducing the evolution of these elements in refractory peridotites from the Ronda massif. Our modelling can explain the negative correlation between the LREE/HREE ratio and the HREE content and between Cr and Ni in the Ronda refractory peridotites. Our results validate the hypothesis that, in the Ronda, the bands of refractory peridotites represent porous-flow channels formed by olivine-forming melt-rock reaction, at increasing melt volume. Because similar geochemical features are observed in ophiolitic peridotites and in mantle xenoliths, it is likely that melt-rock reactions associated with magmatic infiltration are widespread and represent important mantle processes.

Incompatible trace element partitioning and residence in anhydrous spinel peridotites and websterites from the Ronda orogenic peridotite

We report solution-ICPMS analyses of Rb, Ba, Th, U, Nb, Ta, REE, Sr, Zr and Hf for acid-leached minerals of anhydrous spinel peridotites and websterites from the Ronda peridotite (S. Spain). The same elements were also analyzed by LA-ICPMS in the silicates of three peridotites. The results obtained by solution-ICPMS and LA-ICPMS are similar for the less (HREE) and the most incompatible (Rb–Ba) elements, and provide comparable inter-element distribution coefficients (Dxt/cpx) for these elements. However, moderately incompatible elements (typically LREE) show significant discrepancies between solution and in situ analyses. Dopx/cpx and Dol/cpx for these elements are generally lower for solution than for in situ analyses. Dxt/cpx for MREE, HREE, Zr and Hf are consistent with experimental values. In contrast, Dxt/cpx for highly incompatible elements and LREE are higher than expected from available experimental data and/or crystal-chemical considerations. The observed Dxt/cpx for the most incompatible trace elements may be explained by very small amounts of melt/fluid, or solid, inclusions trapped in these minerals. Inclusions would affect both solution- and LA-ICPMS data, but their proportion would be less important for LA-ICPMS analyses. We show with a mixing model that an extremely small amount of equilibrium partial melt (typically 0.01–0.1%) trapped in minerals is sufficient to increase the Dopx/cpx for HIE and LREE by a factor of 5–20 and the Dol/cpx by two or three orders of magnitude. Similar effects may be produced by sub-percent amounts of HIE-rich fluids of solid microphases. Such very small volumes of inclusions may pass unnoticed during mineral handpicking and LA-ICPMS analysis. Hence, Dxt/cpx for HIE and LREE should be considered cautiously when mineral analyses are used to constrain melt processes and mantle composition. Mass balance calculations were performed for a nominally anhydrous spinel harzburgite sample. Similar to previous studies, the mass balance indicates important discrepancies for HIE between peridotite composition reconstructed from mineral analyses (bulk and in situ) and whole rock composition. The major silicate minerals are the main repositories for REE, Zr and Hf (>75% of the whole rock budget), and also host ≥65% of Th and U. In contrast, more than 80% of the budget of Rb, Ba and Nb, and about 60% of Ta and Sr, is hosted by micro-components in grain boundaries (GBC) or trapped in minerals (inclusions). Alone, the GBC accounts for 50% of the budget of Nb and Ta. The inclusions are an important repository for Rb (39%), Nb (40%) and Sr (49%). The GBC and inclusion repositories display very similar trace element signatures, suggesting that they were once a single repository (<1 wt%) now re-distributed in different textural components. This repository could be a combination of hydrous phases and/or Ti oxides, and/or melt/fluid inclusions of mantle origin.

Origin of the island arc Moho transition zone via melt-rock reaction and its implications for intracrustal differentiation of island arcs: Evidence from the Jijal complex (Kohistan complex, northern Pakistan)

If the net fl ux to the island arc crust is primitive arc basalt, the evolved composition of most arc magmas entails the formation of complementary thick ultramafi c keels at the root of the island arc crust. Dunite, wehrlite, and Cr-rich pyroxenite from the Jijal complex, constituting the Moho transition zone of the Kohistan paleo‐island arc (northern Pakistan), are often mentioned as an example of high-pressure cumulates formed by intracrustal fractionation of mantle-derived melts, which were later extracted to form the overlying mafi c crust. Here we show that calculated liquids for Jijal pyroxenites-wehrlites are strongly rare earth element (REE) depleted and display fl at or convex-upward REE patterns. These patterns are typical of boninites and are therefore unlike those of the overlying mafi c crust that have higher REE concentrations and are derived from light rare earth element (LREE)‐enriched melts similar to island arc basalt. This observation, along with the lower 208 Pb/ 204 Pb and 206 Pb/ 204 Pb ratios of Jijal pyroxenites-wehrlites relative to gabbros, rejects the hypothesis that gabbros and ultramafi c rocks derive from a common melt via crystal fractionation. In the 208 Pb/ 204 Pb versus 206 Pb/ 204 Pb diagram, ultramafi c rocks and gabbros lie on the same positive correlation, suggesting that their sources share a common enriched mantle 2 (EM2) signature but with a major depleted component contribution for the ultramafi c rocks. These data are consistent with a scenario whereby the Jijal ultramafi c section represents a Moho transition zone formed via melt-rock reaction between subarc mantle and incoming melt isotopically akin to Jijal gabbroic rocks. The lack in the Kohistan arc of cogenetic ultramafi c cumulates complementary to the evolved mafi c plutonic rocks implies either (1) that a substantial volume of such ultramafi c cumulates was delaminated or torn out by subcrustal mantle fl ow from the base of the arc crust in extraordinarily short time scales (0.10‐0.35 cm/yr), or (2) that the net fl ux to the Kohistan arc crust was more evolved than primitive arc basalt.

Geochemistry of Precambrian ophiolites from Bou Azzer, Morocco

The Upper Proterozoic ophiolite complex of Bou Azzer, Morocco, includes ultramafic rocks, cumulate gabbros, sheeted dykes, pillow lavas and diorite-quartz diorite intrusions and an overlying volcano-sedimentary sequence. The gabbroic cumulates, basaltic flows and dykes have compositions similar to recent ocean-floor rocks (N- and/or T-type). Among other features, they have comparable light REE-depleted patterns and relations of Ti-Zr and La-Nb. Although fractional crystallization played an important role in the evolution of these rocks, the large variations in their chemical compositions require generation from a heterogeneous upper mantle source and/or by a dynamic partial melting process. Diorites, quartz diorites and the volcanic rocks of the overlying sequence are calc-alkaline, genetically unrelated to the tholeiitic suite and indicative of an island arc setting. A possible tectonic model for the ophiolite complex is a marginal basin just behind a still active island arc.