Carbonates in mantle xenoliths from the French Massif Central: inference for carbonatite-related metasomatism Christiane WAGNER1 Etienne DELOULE2 1 Sorbonne Universités, UPMC, Univ Paris 06, CNRS, ISTeP, F-75005, Paris, France. christiane.wagner_raffin@upmc.fr 2 CRPG, UMR-CNRS 5873 – Univ de Lorraine, F-54501 Vandoeuvre-lès-Nancy, France deloule@crpg.cnrs-nancy.fr Vienne – 2016 The interpretation of carbonate - and particularly - carbonate “globules” in mantle xenoliths has been much debated since the 80’/90’s in relation to the origin of carbonate melt and mantle metasomatism. In regard to this debate, we discuss here two representative types of carbonated lherzolites from Mont Coupet (Devès, French Massif Central). Both samples show no infiltration of the host magma. Sample MC2 locally shows traces of late oxidation. Mt Coupet Cc Carbonate occurrences Veins and Vesicles in reaction zones BSE image pg2 + gl ol spongy cpx OPX 1 Reaction zones with calcite (MgO <1 wt%) vesicles and Al- and Ti-rich cpx2, Ca-rich ol2, Cr-rich sp2, plagio2 and glass. Sometimes the CC veins cut former spongy cpx. Pockets Sample MC9 Sample MC2 Less developed reaction zones commonly located around resorbed sp grains. ol2 SP 1 pg2 + gl sp2 CPX 1 Mg-rich fibrous Cc Mg-poor Cc Mg-poor and Mg-rich Cc The carbonate vesicles differentiated into Mg-rich (3.3-5 wt% MgO) and Mg-poor (1.8-2.7 wt% MgO) calcite due to pressure decrease during the magmatic ascent and eruption. Carbonate composition In both samples the calcite have low REE abundances. In sample MC9, the REE abundances are mostly below the detection limit except for La and Ce. The calcite (Mg-poor) from MC9 contains higher P2O5 and lower Sr and Ba than the two calcite types from MC2. CPX composition core rim cpx 2 MC2 MC9 reaction OPX 1 in contact with host bas spongy cpx host basalt - cpx2 show variable and higher Al and Ti than CPX 1. They differ from cpx in the host, or in reaction at OPX 1 / host contact, or spongy cpx. - They equilibrated at higher pressure than igneous cpx - They are thus unrelated to any influence of the host magma Glass composition In both samples, glasses vary from andesite to trachyte. They are silica-oversaturated, and differ from the host basanite. Glasses straddle the boundary for mantle glasses related to carbonatite/Na-silicate metasomatism. Na-silicate* Carbonatite* K-silicate* *Coltorti et al 2000 host Spongy cpx (in MC9) have lost REE compared to CPX 1 and the glass is enriched in REE. Introduction C Spongy cpx are formed by reaction of the primary peridotite with an infiltrating melt. This first metasomatic event occurs prior to the formation of the cross-cutting carbonate veins. This points to a metasomatic co-precipitation of carbonate and associated secondary minerals. As inferred from pressure estimated for cpx2, this occurred near the base of the crust. This suggests the carbonate being related with magmatic fractionation of an alkali and /or alkali carbonate mantle-derived melt, that was injected shortly before the eruption to preserve the calcite crystals. Interpretation The carbonate is always associated with secondary minerals and silicate glasses sometimes reduced to a thin rim at the margin of veins or pockets Secondary ol2 and cpx2 that indent the carbonate (red arrows in C) and may be included the carbonate crystals. sharp meniscus A Although apparent immiscibility features (A), the tie-line glass-carbonate do not intersect experimentally determined immiscibility (B). The carbonate represents thus crystal cumulates and not quenched carbonate melts or immiscible liquids. The carbonate have high XCa, low alkalis and low REE. CaO+MgO+FeO SiO2+Al2O3 Na2O+K2O 0.5 GPa 1.5 GPa B MC9 - MC2 (Brooker & Kjarsgaard, J Petrology, 52, 2011) 1 2 3 MC southern domain sp facies pl facies gar facies 90 mWm-2 60 mWm-2 400 800 1200 0.5 1.0 1.5 2.0 2.5 P GPa T °C CPX 1 cpx2 Moho D