Metasomatic Processes Revealed by Trace Element and Redox Signatures of the Lithospheric Mantle Beneath the Massif Central, France

Trace element compositions of clinopyroxene and oxidation state measurements are both useful parameters for constraining metasomatic interactions within the subcontinental lithospheric mantle (SCLM). Laser ablation inductively coupled plasma mass spectrometry analyses of trace elements in clinopyroxene in combination with oxidation state data from the same samples provide a fuller picture of the metasomatic processes that occurred within the SCLM below the French Massif Central (FMC). A total of 89 spinel peridotite xenoliths from 39 localities across the FMC document 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 earth elements (MREE) in clinopyroxene, four main groups (A, B, C and D) are identified. Variations in other trace element concentrations are consistent with these groups. LREE enrichment correlates with a tendency to higher oxygen fugacity (fO2). These characteristics, along with their Ti/Eu–Zr/Hf systematics, are useful in assessing the different metasomatic agents that have interacted with the FMC SCLM. The behaviour of Lu and Hf additionally provides a means for distinguishing secondary clinopyroxene grains from those that have been overprinted. Most samples have been affected by cryptic metasomatism and different signatures indicate interaction with different types of metasomatic agent. 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 of amphibole suggests that it formed from small amounts of dilute H2O-rich fluids. Most samples from 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 during Variscan 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 concentrations of the less mobile elements such as Ti, Eu, Zr and Hf. Clinopyroxene in several samples has very low Ti/Eu, elevated (La/Yb)N and suprachondritic Zr/Hf ratios, reflecting local carbonatite metasomatism. 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/Hf ratios, consistent with having undergone silicate melt metasomatism. Their chondrite-normalized REE element patterns vary from LREE-depleted to relatively flat (group A), indicating small but variable metasomatic enrichments in the more mobile elements, which can be attributed to chromatographic effects during reactive flow. The oxidation states of these samples record Δlog fO2 ∼ FMQ + 0·4 ± 0·5. Some samples have clinopyroxene with negative Zr and Hf anomalies and distinctly 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 oxidation states (Δlog fO2 values up to ∼FMQ + 0·9). Some samples exhibit evidence for having been overprinted by more than one metasomatic event, in part indicated by two populations of geochemically distinct clinopyroxene. These samples, among others, also demonstrate that immobility of heavy REE (HREE) cannot be assumed when modeling the degree of melt extraction of strongly metasomatized peridotites. Addition of HREE by refertilization lowers the apparent degree of melting compared with calculations based upon Cr/(Cr + Al) in spinel. In other cases, a lowering of HREE concentrations via interaction with a low-HREE metasomatic agent produces higher apparent degrees of melting.