Melting phase relations of a fertile lherzolite KLB-1 have been studied in the pressure range from 1 atm to 14 GPa (140 kbar). Olivine is the liquidus phase at all pressures studied. The second mineral to crystallize changes with increasing pressure; chromian spinel (1 atm), Ca-poor orthopyroxene (up to 3 GPa), pigeonitic clinopyroxene (up to 7 GPa), pyrope-rich garnet (above 7 GPa). The melting temperature interval of the peridotite is more than 600°C wide at 1 atm but narrows to about 150°C at 14 GPa. The partial melts along the peridotite solidus become increasingly more MgO-rich as pressure increases throughout the pressure range studied. At 5–7 GPa, the partial melts formed within 50°C of the solidus contain more than 30 wt % MgO and are very similar to Al-undepleted-type peridotitic komatiite which is common in Archean volcanic terrains. Due to the increase of enstatite component in clinopyroxene solid solution at high pressure and temperature, the orthopyroxene liquidus field narrows as pressure increases and disappears at 3.5 GPa. Harzburgites which are common in the basal peridotite in ophiolite suites may have been produced as residues by partial melting at relatively shallower depths ( 100 km). A diapiric model is consistent with the genesis of komatiite magma by partial melting of mantle peridotite at 150–200 km depth. Based on the following observations, (1) convergence of the liquidus and solidus of the peridotite at pressures > 14 GPa, (2) the near solidus partial melt composition very close to the bulk rock at 14 GPa, and (3) change in liquidus mineral from olivine to majorite garnet at pressures between 16 and 20 GPa in preliminary experiments, it is proposed that the upper mantle peridotite was generated originally as a magma (or magmas) by partial melting of the primitive earth at 400–500 km depth.

Melting of a dry peridotite KLB‐1 up to 14 GPa: Implications on the Origin of peridotitic upper mantle

Os, Sr, Nd, and Pb isotope systematics of southern African peridotite xenoliths: Implications for the chemical evolution of subcontinental mantle

In search of a bulk-Earth composition

Phase transformations and their bearing on the constitution and dynamics of the mantle

Melting experiments have been performed on anhydrous mantle peridotite KLB-1 at 5 to 22.5 GPa using the multianvil press, with special attention paid to precision and accuracy of temperature and pressure measurement, oxygen fugacity, equilibrium, temperature gradient, and the effects of temperature gradient on the phase diagrams. The new phase diagram reveals complexities in the liquidus and solidus phase relations that were not reported by Takahashi (1986). At no pressure do the liquidus and solidus converge to a common temperature or to a narrow range of temperatures, a result that refutes the conjecture that mantle peridotite formed on the solidus as a partial melt (Herzberg and O'Kara, 1985) or a residual liquid. However, mantle peridotite could have formed as a cotectic liquid because KLB-1 exhibits nearly cotectic liquidus crystallization behavior in garnet plus magnesiowustite [L + Gt + Mw] from 18 to 22 GPa. The implication is that mantle peridotite may be the product of a large-scale differentiation event and that MgO/SiO2 for mantle peridotite may have been phase equilibrium controlled by cotectic crystallization in a magma ocean. Fractionation of magnesiowustite or majorite garnet is inferred at 18 to 22 GPa for a bulk Earth MgO/SiO2 that is higher or lower than mantle peridotite, respectively. Alternatively, if the Earth formed from materials that were the same in MgO/SiO2 as mantle peridotite, then the liquidus phase equilibria reported here may be irrelevant or incidental in understanding Earth structure.

Melting experiments on anhydrous peridotite KLB-1 from 5.0 to 22.5 GPa

Theoretical calculations and some preliminary experimental observations show that the liquidus and solidus temperatures for each of a wide range of upper mantle peridotite and komatiite compositions become coherent at high pressures. This suggests that these materials may have been liquids in eutectic-like equilibrium (rather than peritectic-like) with mantle assemblages. It is shown that the major element geochemistry of 83 mantle peridotites and 61 komatiites define a trend which is not primarily due to an olivine control process; rather it is interpreted to represent the pressure-induced compositional trace of eutectic liquids in equilibrium with mantle assemblages from 4 to 15 GPa. If the bulk Earth is chondritic in composition these phase equilibrium constraints would imply that the upper mantle had formed from the whole mantle as an ultrabasic partial melt, the transition zone and lower mantle being the complementary eclogite and pyroxenite residua. This is consistent with mineralogical interpretations of seismic data for the present-day Earth which call for a peridotite upper mantle and pyroxene-like transition zone and lower mantle.

C. T. Herzberg

Papers

Origin of mantle peridotite and komatiite by partial melting