A highly oxidized ferrian salite-, kennedyite-, forsterite-, and rhöenite-bearing alkali gabbro from Kauai, Hawaii and its mantle xenoliths

Three types of rock, each with unusually oxidized mineral assemblages, are found in close association as part of a probable feeder pipe to the alkalic lavas on Kauai, Hawaii. The first type is an alkali gabbro consisting of potassic oligoclase (66% by volume), ferrian salite (27%), kennedyite (3%), magnesioferrite (3%) and trace amounts of rhönite, kaersutite, apatite, hematite and nearly pure forsterite. Electron microprobe analyses of these phases show them to be exceptionally enriched in Mg and ferric iron. These data combined with textural observations demonstrate that the alkali gabbro melt existed in a highly oxidized state prior to the onset of crystallization and that this condition prevailed throughout the course of crystallization.

The second lithologic type consists of an assortment of mantle xenoliths, mostly spinel lherzolites, that were entrained by the alkali gabbro at depth prior to its crystallization. There was extensive development of oxidation symplectites within the primary grains in the outer two cm of the xenoliths. Original mantle olivine (Fo₈₈–₉₁) has oxidized to form bleb-like intergrowths of orthopyroxene (En₉₄) and magnesioferrite (Mt₅₈) in a more magnesian (Fo₉₅) olivine host. Comparison of these data with relevant 1 atm experimental data in the system MgO–FeO–Fe₂O₃ÅSiO₂ indicates conditions of 10^-3fO₂ and 1165°C for the formation of these symplectites.

The third lithologic type is referred to here as the fine-grained groundmass. Like the alkali gabbro, the fine-grained groundmass has a high Fe³⁺/Fe²⁺ but consists only of ferrian salite (62%), potassic oligoclase (17%), forsterite (14%), magnesioferrite (7%) and trace amounts of apatite. The gradational contacts of the fine-grained groundmass with the xenoliths and comparative microprobe analyses suggest that the groundmass liquid was derived in part from the xenoliths by partial melting, and the subsequent crystallization of that liquid was complete prior to the introduction of the alkali gabbro. The formation of the oxidation symplectites is closely associated with the fine-grained groundmass and hence the melting event, and appears to have been prior to alkali gabbro intrusion.