PETROGENESIS OF HAWAIIAN THOLEIITES - .1. PHASE-EQUILIBRIA CONSTRAINTS

The most magnesian olivine phenocrysts [Mg no. = 100 Mg/(Mg + Fe) = 90.5] in Hawaiian tholeiites provide evidence for the earliest stages of differentiation of Hawaiian magmas. Based on the correction of olivine fractionation effects, the primitive melt compositions which have crystallised these olivines are picritic with approximately 16 wt% MgO. They are excellent primary-melt candidates. An experimental study on a new Hawaiian picritic primary-melt estimate demonstrates multiple saturation with peridotite (harzburgite) at 2.0 GPa and 1450-degrees-C. Garnet is not a liquidus phase at pressures below approximately 3.5 GPa, and garnet peridotite is not a liquidus phase assemblage at any pressure or temperature. This result confirms previous experimental studies on Hawaiian primary-melt estimates and conflicts with trace-element-geochemistry-based interpretations, which claim that melt generation occurs in the presence of residual garnet. If Hawaiian tholeiite primary magmas are picritic and have equilibrated with garnet-absent peridotite residues, the geochemical and isotopic characteristics of Hawaiian tholeiites (i.e. Sm/Nd < chondrites and epsilon-Nd > 0) are consistent with their source recently having been enriched in incompatible elements. Previous modelling shows that such characteristics are consistent with source enrichment through the migration of small melt fractions generated at depth in the presence of garnet. This may be effected either at the time of Hawaiian magma genesis through dynamic melt segregation processes or, by melting of a previously enriched mantle source; possibly oceanic lithospheric mantle which has been infiltrated by melt fractions from the underlying asthenosphere prior to Hawaiian magmatism. Alternatively, if Hawaiian primary magmas are ultramafic in composition (greater-than-or-equal-to 20 wt% MgO) they may be generated in the presence of garnet peridotite at pressures greater-than-or-equal-to 3.0 GPa.