Minerals as key tools for characterizing multi-stage processes in the lithospheric mantle.

Minerals are key markers of the multi-stage processes that occurred in their environment of formation and evolution. We present three micro textural and chemical studies of minerals from lherzolites in the French Massif Central.

 

-1- Carbonate-bearing mantle xenoliths are interesting as highly mobile carbonate melts are prominent metasomatic agents of the mantle. Here, carbonate fills globular vesicles in composite reaction zones that contain secondary clinopyroxene, olivine, spinel, ± plagioclase and glass. The secondary clinopyroxene and olivine indent or are included in the carbonate crystals. The carbonate is a REE- and alkali-poor calcite with low MgO <1 wt.%.

The presence of rounded vesicles of carbonate is usually interpreted as an evidence for silicate-carbonate liquid immiscibility, but alkali-free immiscible carbonates cannot be almost pure calcite. Here the textural and composition characteristics of carbonates rule out their origin as quenched carbonatitic melts or immiscible carbonate liquids and favor an origin as crystal cumulates from mantle-derived alkali-carbonate melts.

The co-precipitation of carbonate and secondary minerals occurs near the base of the crust. The injection of small amounts of a carbonate-rich melt occurred at mantle level shortly before the eruption to preserve the calcite crystals.

 

-2- Here we present Li abundances and isotopes data of co-existing silicates in anhydrous and amphibole-bearing lherzolites. Li abundances increase in all phases from the amphibole-bearing lherzolites, but deviate from the trend of equilibrium partitioning between phases with a preferential Li uptake in clinopyroxene. The correlation between Li and REE elements in clinopyroxene suggests that Li and REE were carried by the same silicate melt.

 In the amphibole-bearing lherzolites the  cpx d⁷Li (‰) values show large intra-grain variations. These variations do not provide evidence for different sources but likely result from high temperature diffusion-related Li fractionation during metasomatism by the silicate melt undergoing compositional changes as it percolates through the lherzolites.

The preservation of both the Li isotope kinetic fractionation in minerals and isotopic heterogeneities implies that the Li exchange event occurs just before the extraction of the xenoliths from the mantle.

 

-3- Glass-bearing pockets in peridotite xenoliths are usually studied for elucidating the origin of the infiltrating agent. We present here a chemical study of glass developed around spinels. We do not discuss the origin of the metasomatic agent but show that 1) the modification of the structure of the percolating melt is due to the accommodation of elements produced by the dissolution of minerals and (2) how this process could modify the oxidation state of the melt.

Spinel-derived Al³⁺ ions are first accommodated to the melt network as network-modifiers and secondly as network-formers using K⁺ ions as stabilizators within the tetrahedral site. The transfer of K⁺ ions (extracted from the aqueous fluid upon melt dehydration) from the aqueous fluid to the melt network is counterbalanced by an inverse transfer of CaO molecules that form crystalline phases exsolved upon eruption.

Spinel corrosion generates melt oxidation through dehydrogenation reactions resulting in the formation of Al³⁺ and Fe³⁺ anionic complexes within the melt network.