Metasomatism and Melting in Carbonated Peridotite Xenoliths from the Mantle Wedge: The Gobernador Gregores Case (Southern Patagonia).

Spinel-facies mantle xenoliths occur in a diatreme, cutting through the Neogene Southern Patagonia Plateau at Gobernador Gregores (Santa Cruz Province, Argentina). This Plateau is in a back-arc position with respect to the Chile trench. Xenoliths differ in the whole-rock composition from other South-America occurrences for higher CaO/Al2O3 values and, in some samples, TiO2 enrichment, whereas the Na2O/Al2O3 variation range is similar. Three assemblages can be distinguished. Assemblage 1, in anhydrous protogranular lherzolites and harzburgites, contains clinopyroxene with depleted major and trace element composition, indicating pre-metasomatic depletion processes. This assemblage fully recrystallised to assemblage 2 (amphibole ± phlogopite ± Cl-apatite-bearing) during a metasomatic episode. This causes clinopyroxene to acquire geochemical characteristics often attributed to carbonate-melt metasomatism. Noticeably, amphibole is markedly enriched in Nb (up to 298 ppm), especially when depleted in Ti. A further event, related with decompression during xenolith uplift to the surface, induces closed-system (perhaps with the exception of CO2 addition) disequilibrium melting of assemblage 2, dominantly of amphibole. It is found in pockets (where amphibole is a residual phase) consisting of Na-Si-rich glass and carbonate (Mg-rich calcite) drops, and in veins originating from the pockets (assemblage 3). Euhedral olivine, clinopyroxene and spinel crystallise only in the silicate glass. So do new, euhedral apatite crystals, when glass is in contact with previous assemblage 2 apatite. Textural evidence and comparison with experimental work suggest that silicate glass and carbonates are the result of unmixing of a former homogeneous melt. Because of the different flow-rate of carbonate and silicate melt, xenoliths become enriched in carbonate during their migration, which is found in the veins. Thus, the high CaO/Al2O3 ratio of whole-rock provides inconclusive evidence of carbonatite metasomatism. This factor, and other minor deviations from the expected results of carbonatite metasomatism, lead us to hypothesise an aqueous, Cl-rich fluid, possibly slab-derived, as an alternative agent. Amphibole, resulting from reactive porous flow of this agent in the mantle, could fully explain the observed geochemical features, as indicated by estimates of its partition coefficients.