Near-solidus Melting of the Shallow Upper Mantle: Partial Melting Experiments on Depleted Peridotite

Abstract

We present the results of melting experiments on a moderately depleted peridotite composition (DMM1) 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, SiO_2, FeO*, and MgO contents of the partial melts increase, Al_(2)O_3 and Na_(2)O contents decrease, and CaO contents first increase up to clinopyroxene-out at a melt fraction of 0·09–0·10, then decrease with further melting. A linear fit to melt fraction vs temperature data for lherzolite-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, DMM1 glass compositions have lower SiO_2, TiO_2, Na_(2)O, and K_(2)O contents, higher FeO*, MgO, and CaO contents, and higher CaO/Al_(2)O_3 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 10 kbar peridotite solidus temperatures suggesting that K_(2)O and P_(2)O_5 have greater effects on solidus depression than Na_(2)O, consistent with theoretical expectations. Our parameterization also suggests that abyssal peridotites have 10 kbar solidi of ~1278–1295°C.