Near-solidus melting of the Shallow Upper Mantle: Partial melting experiments on depleted peridotite

We present the results of melting experiments on a moderately depleted peridotite composition (DMMI) at 10 kbar and 1250-1390°C. Specially designed experiments demonstrate that liquids extracted into aggregates of vitreous carbon spheres maintained chemical contact with the bulk charge down to melt fractions of ∼0·02-0·04 and approached equilibrium closely. With increasing melt fraction, SiO₂, FeO^*, and MgO contents of the partial melts increase, Al₂O₃ and Na₂O contents decrease, and CaO contents first increase up to clinopyroxeneout at a melt fraction of 0·09-0·10, then decrease with further melting. A linear fit to melt fraction vs temperature data for Iherzolite-bearing experiments yields a solidus of 1272 ± 11°C. The melting reaction is 0·56 orthopyroxene + 0·72 clinopyroxene + 0·04 spinel = 0·34 olivine + 1 liquid. Above clinopyroxene-out, the reaction is 1·24 orthopyroxene = 0.24 olivine + 1 liquid. Near the solidus, DMMI glass compositions have lower SiO₂, TiO₂, Na₂O, and K₂O contents, higher FeO^*, MgO, and CaO contents, and higher CaO/Al₂O₃ ratios compared with glasses from low-degree melting of fertile peridotite compositions. Recent computational models predict partial melting trends generally parallel to our experimental results. We present a parameterization of 10kbar peridotite solidus temperatures suggesting that K₂O and P₂O₅ have greater effects on solidus depression than Na₂O, consistent with theoretical expectations. Our parameterization also suggests that abyssal peridotites have 10 kbar solidi of ∼1278-1295°C.