Mineral chemistry of submarine lavas from Hilo Ridge, Hawaii: implicationsfor magmatic processes within Hawaiian rift zones

The crustal history of volcanic rocks can be inferred from the mineralogy a nd compositions of their phenocrysts which record episodes of magma mixing as well as the pressures and temperatures when magmas cooled. Submarine lav as erupted on the Hilo Ridge, a rift zone directly east of Mauna Kea volcan o, contain olivine, plagioclase, augite +/- orthopyroxene phenocrysts. The compositions of these phenocryst phases provide constraints on the magmatic processes beneath Hawaiian rift zones. In these samples, olivine phenocrys ts are normally zoned with homogeneous cores ranging from similar to Fo(81) to Fo(91). In contrast, plagioclase, augite and orthopyroxene phenocrysts display more than one episode of reverse zoning, Within each sample, plagio clase, augite and orthopyroxene phenocrysts have similar zoning profiles. H owever, there are significant differences between samples. In three samples these phases exhibit large compositional contrasts, e.g., Mg# [100 x Mg/(M g+Fe+2)] of augite varies from 71 in cores to 82 in rims. Some submarine la vas from the Puna Ridge (Kilauea volcano) contain phenocrysts with similar reverse zonation. The compositional variations of these phenocrysts can be explained by mixin g of a multiphase (plagioclase, augite and orthopyroxene) saturated, evolve d magma with more mafic magma saturated only with olivine. The differences in the compositional ranges of plagioclase, augite and orthopyroxene crysta ls between samples indicate that these samples were derived from isolated m agma chambers which had undergone distinct fractionation and mixing histori es. The samples containing plagioclase and pyroxene with small compositiona l variations reflect magmas that were buffered near the olivine + melt doub le right arrow Low-Ca pyroxene + augite + plagioclase reaction point by fre quent intrusions of mafic olivine-bearing magmas. Samples containing plagio clase and pyroxene phenocrysts with large compositional ranges reflect magm as that evolved beyond this reaction point when there was no replenishment with olivine-saturated magma. Two of these samples contain augite cores wit h Mg# of similar to 71, corresponding to Mg# of 36-40 in equilibrium melts, and augite in another sample has Mg# of 63-65 which is in equilibrium with a very evolved melt with a Mg# of similar to 30. Such highly evolved magma s also exist beneath the Puna Ridge of Kilauea volcano. They are rarely eru pted during the shield building stage, but may commonly form in ephemeral m agma pockets in the rift zones. The compositions of clinopyroxene phenocryst rims and associated glass rind s indicate that most of the samples were last equilibrated at 2-3 kbar and 1130-1160 degrees C. However, in one sample, augite and glass rind composit ions reflect crystallization at higher pressures (4-5 kbar). This sample pr ovides evidence for magma mixing at relatively high pressures and perhaps t ransport of magma from the summit conduits to the rift zone along the ocean ic crust-mantle boundary.