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The H^sub 2^O -saturated solidus of a model mantle composition (Kilborne Hole peridotite nodule, KLB-1) was determined to be just above 1 000 deg C from 5 to 11 gigapascals. Given reasonable H^sub 2^O abundances in Earth's mantle, an H^sub 2^O -rich fluid could exist only in a region defined by the wet solidus and thermal stability limits of hydrous minerals, at depths between 90 and 330 kilometers. The experimental partial melts monotonously became more mafic with increasing pressure from andesitic composition at 1 gigapascal to more mafic than the starting peridotite at 10 gigapascals. Because the chemistry of the experimental partial melts is similar to that of kimberlites, it is suggested that kimberlites may be derived by low-temperature melting of an H^sub 2^O-rich mantle at depths of 150 to 300 kilometers.
H^sub2^Olowers melting temperatures of mantle peridotites and also affects the chemistry of their partial melts (1, 2). Earlier experiments demonstrated that partial melting of H^sub 2^0-saturated peridotite produces andesitic or dacitic melts at 1 GPa, in contrast to basaltic melts in dry peridotite (2). Therefore, it was surprising that partial melting of H^sub2^O-saturated peridotites at pressures greater than 5 GPa produced ultramafic melts (3, 4). Two ultramafic magma types are known on Earth: komatiites and kimberlites. Komatiites are believed to represent high-temperature melts in the early Earth. However, recent experiments (3, 4) led to suggestions that komatiites could be produced by wet peridotite melting (3-5), and hence they could shed light on the degassing history of Earth's mantle (6). Kimberlites contain diamonds and peridotite nodules that are the only samples from the mantle's deepest depths (~150 to 200 km). Understanding the origin of these ultramafic magmas would provide important clues to the evolution of Earth's mantle.
In an effort to understand the effect of H^sub 2^O on the generation of ultramafic magmas, we undertook a precise determination of solidus temperatures and partial melt chemistry in an H^sub 2^O-saturated peridotite. The starting material was a mixture of an Mg-free KLB-1 peridotite gel and brucite, resulting in molar ratios of 89 for Mg/ (Mg + Fe) x 100 (Mg#) and 1.46 for (Mg + Fe)/Si, with an H^sub 2^O content of 13.7% (by weight). The chemistry of KLB-1 peridotite is similar to that of...