Metasomatic processes revealed by trace element and redox signatures of the lithospheric mantle beneath the Massif Central, France

Abstract

Trace element compositions of clinopyroxene and oxidation state measurements are both usefulparameters for constraining metasomatic interactions within the subcontinental lithospheric mantle(SCLM). Laser ablation inductively coupled plasma mass spectrometry analyses of trace elementsin clinopyroxene in combination with oxidation state data from the same samples provide afuller picture of the metasomatic processes that occurred within the SCLM below the FrenchMassif Central (FMC). A total of 89 spinel peridotite xenoliths from 39 localities across the FMCdocument variable degrees of partial melting and metasomatism at both regional and local scales.According to the degree of enrichment in light rare earth elements (LREE) and middle rare earthelements (MREE) in clinopyroxene, four main groups (A, B, C and D) are identified. Variations inother trace element concentrations are consistent with these groups. LREE enrichment correlateswith a tendency to higher oxygen fugacity (fO2). These characteristics, along with their Ti/Eu-Zr/Hfsystematics, are useful in assessing the different metasomatic agents that have interacted with theFMC SCLM. The behaviour of Lu and Hf additionally provides a means for distinguishing secondaryclinopyroxene grains from those that have been overprinted. Most samples have been affected bycryptic metasomatism and different signatures indicate interaction with different types of metasomaticagent. Modal metasomatism is apparent by the presence of amphibole or secondary clinopyroxene.The lack of any notable trace element or redox signature related to the occurrence ofamphibole suggests that it formed from small amounts of dilute H2O-rich fluids. Most samplesfrom the northern sector of the FMC are fairly refractory and clinopyroxene exhibits a 'tick-shaped'REE signature (group D), consistent with metasomatism by subduction-related fluids duringVariscan times. Such fluids were oxidizing (Δlog fO2 values>FMQ+1, where FMQ is the fayalite-magnetite-quartz buffer) and responsible for LREE enrichment in clinopyroxene and low concentrationsof the less mobile elements such as Ti, Eu, Zr and Hf. Clinopyroxene in several sampleshas very low Ti/Eu, elevated (La/Yb)N and suprachondritic Zr/Hf ratios, reflecting local carbonatitemetasomatism. These samples also record oxidizing conditions, with Δlog fO2 values>FMQ+1.Many other samples have clinopyroxenes with higher Ti/Eu and approximately chondritic Zr/Hfratios, consistent with having undergone silicate melt metasomatism. Their chondrite-normalizedREE element patterns vary from LREE-depleted to relatively flat (group A), indicating small but variablemetasomatic enrichments in the more mobile elements, which can be attributed to chromatographiceffects during reactive flow. The oxidation states of these samples record ΔlogfO2∼FMQ+0·4±0·5. Some samples have clinopyroxene with negative Zr and Hf anomalies anddistinctly low Ti/Eu (<2000), but not as low as expected for carbonatite metasomatism. These may have interacted with carbonated-silicate melts, which also imposed somewhat elevated oxidationstates (Δlog fO2 values up to ∼FMQ+0.9). Some samples exhibit evidence for having been overprintedby more than one metasomatic event, in part indicated by two populations of geochemicallydistinct clinopyroxene. These samples, among others, also demonstrate that immobility ofheavy REE (HREE) cannot be assumed when modeling the degree of melt extraction of stronglymetasomatized peridotites. Addition of HREE by refertilization lowers the apparent degree of meltingcompared with calculations based upon Cr/(Cr+Al) in spinel. In other cases, a lowering ofHREE concentrations via interaction with a low-HREE metasomatic agent produces higher apparentdegrees of melting.