Phase relations of basalts in their melting range at PH2O = 5 kb as a function of oxygen fugacity: Part I. Mafic phases

The phase relations of three basalts, the Picture Gorge tholeiite, the 1921 Kilauea olivine tholeiite, and the 1801 Hualalai alkali basalt, were studied at 5 kb water pressure, 680–1000°C, at the oxygen fugacities of the quartz-fayalite-magnetite (QFM) and hematite-magnetite (HM) buffers.

In the range 680–850 °C, the crystalline assemblage on the QFM buffer is dominantly hornblende+ plagioclase, ± ilmenite, magnetite, sphene, fayalitic olivine, and phlogopitic mica. From 875 to 1000 °C the crystalline assemblage is hornblende+ olivine± augite+ ilmenite± magnetite. A melt phase is present from 700 to 1000 °C; a vapor phase was present in all charges.

The hornblendes formed on the QFM buffer range in composition from common green hornblendes at low temperatures to kaersutitic hornblendes at 1000 °C. A1(IV) and Ti increase temperature. AI(VI) passes through a maximum near 825 °C, decreasing both above and below this temperature. AI(IV) is proportional to the sum A1(VI)+2Ti. There is a positive linear correlation of approximately 3 : 1 between AI(IV) and the number of cations in the A-site. The most likely explanation for this correlation at present is that the substitution of AI(VI) or Ti⁺⁴for a divalent cation creates local charge imbalances in the amphibole structure which can be compensated only by further A-site substitution. There also appears to be a correlation between the a-cell dimension of hornblende and the A-site occupancy. Above a thresh hold value of approxmately 0.5 cations in A, a increases as A-site occupancy increases.

Phase relations on the hematite-magnetite buffer are considerably simpler. The hornblendes show relatively little change in composition as temperature increases, and in the tholelitic compositions break down at or below 970 °C 35–60 °C above the first appearance of augite±olivine. The melting of hornblende is incongruent in all cases. The Fe-Ti oxides are pseudo-brookite and titanohematite; at 1000 °C these oxides make up 10 per cent by weight of the assemblage and contain most of the Tio₂ and FeO in the charge.

The patterns of hornblende variation observed in this study compare closely with those reported in a wide range of experimental and field data. The appearance of high-TiO₂ kaersutitic hornblendes in the tholeities at 1000° C, P_H2O= 5 kb on the QFM buffer implies that the restricted occurence of kaersutite in nature (where it is associated only with mafic to intermediate alkalic rocks) is controlled by volatile content (H₂O_,F₂)rather than by differences in condensed bulk composition.