Crystallisation of an alkali-olivine basalt under controlled PO2, PH2O conditions

Nesbitt, R.W. and Hamilton, D.L. (1970) Crystallisation of an alkali-olivine basalt under controlled PO2, PH2O conditions. Physics of the Earth and Planetary Interiors, 3, 309-315. (doi:10.1016/0031-9201(70)90067-1).

Record type: Article

Abstract

An alkali-basalt from Hualalai, Hawaii, previously examined by Yoder and Tilley (1962) has been crystallised under controlled PO2 at 2 kb total pressure. Significant differences, in the order of appearance of the phases result from lowering the PO2 from 10?0.7 to about 10?10 atm. In particular magnetite, present at 1100 °C in the unbuffered runs does not crystallise until below 900 °C in the oxygen controlled runs. There is a drop of up to 50 °C on the liquidus of the system if the oxygen is controlled at the quartz-fayalite-magnetite buffer. Calcium amphiboles crystallised under such conditions are nepheline normative and rich in titanium and aluminium. Decreasing the water content but maintaining total pressure at 2 kb results in an increase in the melting point of the amphibole. There is no data in the runs to support the concept of a primary amphibole gabbro crystallising from a basaltic melt of this composition. Rather, the amphibole would form by reaction of the pyroxene with the liquid, or direct growth at lower temperatures from a basaltic mineralogy.

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