The Effect of CO2 on Partial Reactive Crystallization of MORB-Eclogite-derived Basaltic Andesite in Peridotite and Generation of Silica-Undersaturated Basalts

Recycled oceanic crust (MORB-eclogite) is considered to be the dominant heterogeneity in Earth's mantle. Because MORB-eclogite is more fusible than peridotite, siliceous partial melt derived from it must react with peridotite while the latter is still in the subsolidus state. Thus, studying such reactive process is important in understanding melting dynamics of the Earth's mantle. Reaction of MORB-eclogite-derived andesitic partial melt with peridotite can produce alkalic melts by partial reactive crystallization but these melts are not as silica-undersaturated as many natural basanites, nephelinites or melititites [1]. In this study, we constrain how dissolved CO₂ in a siliceous MORB-eclogite-derived partial melt affects the reaction phase equilibria involving peridotite and can produce nephelinitic melts. Here we compare experiments on CO₂-free [1] and 2.6 wt.% CO₂ bearing andesitic melt+lherzolite mixtures conducted at 1375 °C and 3 GPa with added melt fraction of 8-50 wt.%. In both CO₂-free and CO₂-bearing experiments, melt and olivine are consumed and opx and garnet are produced, with the extent of modal change for a given melt-rock ratio being greater for the CO₂-bearing experiments. While the residue evolves to a garnet websterite by adding 40% of CO₂-bearing melt, the residue becomes olivine-free by adding 50% of the CO₂-free melt. Opx mode increases from 12 to ~55 wt.% for 0 to 40% melt addition in CO₂-bearing system and 12 to ~43 wt.% for 0 to 50% melt addition in CO₂-free system. Garnet mode, for a similar range of melt-rock ratio, increases from ~10 to ~15 wt.% for CO₂ bearing system and to ~11 wt.% for CO₂-free system. Reacted melts from 25-33% of CO₂-bearing melt-added runs contain ~39 wt.% SiO₂ , ~11-13 wt.% TiO₂, ~9 wt.% Al₂O₃, ~11 wt.% FeO*, 16 wt.% MgO, 10-11 wt.% CaO, and 3 wt.% Na₂O whereas experiments with a similar melt-rock ratio in a CO₂-free system yield melts with 44-45 wt.% SiO₂, 6-7 wt.% TiO₂, 13-14 wt.% Al₂O₃, 10-11 wt.% FeO*, 12-13 wt.% MgO, ~8 wt.% CaO, and ~4 wt.% Na₂O. Our study shows that with only 2.6 wt.% CO₂, andesites, owing to partial reactive crystallization in a peridotite matrix, can evolve to nephelinites (as opposed to basanites for CO₂-free runs) that match with silica-undersaturated oceanic basalts better than reacted melts from CO₂-free conditions. The effects of CO₂ on the partial reactive crystallization of andesite in a fertile peridotite matrix thus are: a) lowered melt- SiO₂ owing to increased stability of opx at the liquidus of basalt, b) lowered Al₂O₃ content of basalts owing to increased crystallization of garnet. Experiments with 1 and 5 wt.% CO₂-bearing andesite-peridotite mixture are underway and will be presented. [1] Mallik and Dasgupta (2012), EPSL, 329-330, 97-108.