The Effect of Variable CO2/H2O Ratio on Melting in the Mantle Wedge

Abstract:

The release and the subsequent interaction of slab-derived fluids with the mantle wedge significantly reduces the peridotite solidus and influences the composition of primary arc magmas. H₂O and CO₂ are the two most abundant volatile species contributing to melting in the mantle wedge as inferred from experimental determination of slab-derived fluid and melt compositions ^1,2, primitive melt inclusions hosted in arc lava phenocrysts ³, and sub-arc mantle xenoliths containing hydrous minerals and carbonates ^4,5. Most previous experimental studies on peridotite-H₂O-CO₂ systems have investigated melting under CO₂ dominated conditions^{6, 7}. However, melting in the mantle wedge likely occurs under H₂O dominated conditions.

Here we present the melting phase relations of a depleted peridotite + C-O-H fluids with a bulk H₂O content of 3.5 wt.% and CO₂/H₂O mass ratio between 0 and 0.5. The goal was to assess how the presence of small amounts of dissolved CO₂ in aqueous fluid affect melting systematics in the mantle wedge. Experiments were performed at P = 2-3 GPa and T = 1200 °C in a piston cylinder using Au75Pd25 capsules. Our experiments show that the addition of even small amounts of CO₂ has a large effect on the melt fraction and composition of mantle-derived melts relative to CO₂-free systems. In particular, at a constant P-T, the melt fraction systematically decreases from 14 to 2 wt.% at 2 GPa and 16 to 6 wt.% at 3 GPa with increasing CO₂/H₂O of the bulk composition from 0 to 0.5. The SiO₂ and CaO contents of experimental melts generated at a given pressure and temperature are also strongly affected by the addition of CO₂ where the SiO₂/CaO weight ratio of melts produced decreases from 5.3 to 0.6 at 2 GPa and from 6 to 1.6 at 3 GPa with increasing CO₂/H₂O of the bulk composition from 0 to 0.5. Comparing our experimental melts to fractionation corrected primitive arc magmas, whose SiO₂/CaO weight ratios rarely drop below 3, suggests that melting in the mantle wedge is dominated by H₂O and more specifically occurs at a CO₂/H₂O mass ratio < 0.3.

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