WERE KOMATIITES WET

The main arguments used to support the concept that komatiites form by melting of hydrous mantle are as follows: (1) Water reduces liquidus temperatures from extreme values to loner, more ''normal'' temperature s. (2) Some komatiites are pyroclastic and some contain vesicles, feat ures that have been attributed to magmatic volatiles. (3) It is claime d from experimental studies of peridotite melting that the chemical co mposition of komatiite requires the presence of water, as does their c haracteristic spinifex textures. Counterarguments are the following: ( 1) Loss of volatiles as hydrous komatiite approaches the surface shoul d produce degassing textures and structures, which, though not unknown , are rare in komatiites, Degassing should produce a highly supercoole d liquid that partially crystallizes to porphyritic magma; komatiites commonly erupt as phenocryst-poor, highly magnesian lavas. (2) Chemica l and isotopic compositions of most komatiites indicate that their man tle source became depleted in incompatible elements soon before magma formation. Such depletion removes mater, leaving a dry source. (3) The experimental data are at best ambiguous; neither the chemical composi tion of komatiites, nor the crystallization of spinifex, requires the presence of water We conclude that although some rare komatiites may b e hydrous, most are dry.