Melting of Garnet Peridotite and the Origin of Komatiite and Depleted Lithosphere

Abstract

Melting experiments on fertile peridotite KR4003, a ‘pyrolitic’ composition, were made from 3 to 7 GPa in piston-cylinder and multi-anvil apparatus. Temperature gradients across the sample were minimized (<25°C), and the compositions of all phases were determined. Modal abundances of coexisting phases were calculated by mass balance, and the results were used to determine phase relations. Orthopyroxene is not stable at the solidus of garnet peridotite above ∼3.3 GPa, but crystallizes above the solidus by incongruent melting of cpx. Melt compositions from 3 to 7 GPa (gt;10% melting) are picritic, komatiitic, and peridotitic. The Al2O3 content of partial melts decreases with increase in pressure because of an increase in garnet stability, providing a barometer for melting. The Al2O3 contents of komatiites indicate secular variation in the average pressure of melt segregation from residues, with early Archean komatiites and Cretaceous komatiites generated at the highest and lowest average pressures, respectively. The high CaO/Al2O3 ratios of Archean alumina undepleted komatiites ( 0.9–1.5) require residual garnet if their sources were pyrolitic. Paradoxically, chondrite-normalized Gd/Yb of about unity in these komatiites precludes garnet involvement. Archean komatiite source regions may have had CaO/Al2O3 values of about 1.4 and 1.0 in the early and late Archean, respectively, significantly greater than the pyrolitic ratio of 0.8, whereas the source of Cretaceous komatiites may have had pyrolitic CaO/Al2O3. Thus, secular variations in this ratio are indicated. Chemical differences between coeval alumina undepleted and alumina depleted komatiites can be explained by melting at similar pressures, with alumina undepleted komatiites segregating from a garnet-free residue, and alumina depleted komatiites segregating from a garnet-bearing residue. Depleted, high-temperature peridotites from cratons, and oceanic peridotites, can be melting residues of pyrolitic mantle at low pressures (<3 GPa). Average low-temperature peridotite from the Siberian craton can be generated as a residue of komatiite melt extraction from a near-pyrolitic mantle at ∼6 GPa and 40% melting. Average southern African low-temperature peridotite cannot be a melting residue of pyrolitic mantle. However it can be a residue of komatiite melt extraction at >7 GPa from a mantle enriched in SiO2 relative to pyrolite.