Oxides of low pressure origin from alkali basaltic rocks, southern highlands, n.s.w., and their bearing on the petrogenesis of alkali basaltic magmas

Alkali basaltic rocks from the Southern Highlands, N.S.W., contain oxide phases of both high and low pressure origin. The two phases are readily distinguished using chemical and textural criteria. Chemical data for low-pressure Fe-Ti oxides indicate that oxygen fugacities of the host lavas range from 10^12.8to 10⁸atm at 950° to 1110°C. In most cases, the oxygen fugacities of the individual lava flows appear to be principally a function of temperature and intrinsic chemical equilibria existing at the time of formation of the basaltic liquid. However, some relatively differentiated flows shows a high degree of oxidation due to volatile enrichment with fractionation. Rare glassy flows show dendritic crystallization of Fe-Ti oxides. Most flows in which abundant olivine was the first phase to be precipitated also contain Cr-rich spinels associated, and apparently coeval, with the earliest-crystallizing olivine. Copyright.