Compositions of partial melts formed in mantle peridotites at high pressures and their relation to those of primitive MORB

Abstract

The compositions of partial melts formed from five different mantle peridotites (Mg# ranging from 85.3 to 90.5) have been determined using the diamond aggregate method. When melt compositions were compared with those of primitive mid-ocean ridge basalt (MORB), it was found that partial melts formed from relatively fertile peridotites at 10–15 kbar were close to, but distinctly lower in CaO than, primitive MORB. However, fractionation of 3–10 wt.% olivine can produce primitive MORB compositions. Partial melts from peridotites with Mg# between about 89 and 85 have FeO and MgO abundances close to those of primitive MORB at pressures of 10–15 kbar, and again, primitive MORB compositions can be reproduced with small amounts of olivine fractionation. However, peridotite with Mg# greater than about 90 cannot produce primitive MORB melts at pressures less than 15 kbar. From such peridotite, primitive MORB melts can be produced by partial melting at pressures greater than 15 kbar, but only after extensive olivine fractionation. Experimental melts produced by polybasic, stepwise fractional melting of relatively fertile mantle peridotite along a model adiabat indicate that the compositions of accumulated, incremental melts produced from 20 kbar to 10 and 5 kbar are close to, but slightly more olivine-rich than, those of primitive MORB. Primitive MORB magmas can be generated by fractional melting over a wide depth range; however, magmas formed over a wide depth range may re-equilibrate with mantle peridotite at shallow levels during their ascent. The fact that the major element abundances of the primitive MORB are rather close to those of partial melts formed by batch melting at about 10 kbar suggests that re-equilibration in major elements may occur at depths of about 30 km or the mean pressure of polybasic melting is close to 10 kbar.