Crystal-liquid relationships in some ultrabasic dykes and their petrological significance

Abstract

Phase relations have been determined at one atmosphere and an oxygen fugacity of 10−8 atmospheres for a number of rocks from two differentiated ultrabasic dykes. It is demonstrated that these are consistent with other petrological evidence and that they support the view that the dykes were differentiated mechanically during intrusion. Crystal-liquid relations during the equilibrium crystallization of one of these rocks were determined by electron probe microanalysis. Over part of the temperature range where the three phase assemblage olivine+spinel+liquid exists an “anomalous” relationship was found. Immediately below the liquidus crystallization is normal with the olivine and liquid both becoming relatively enriched in iron as temperature decreases. When spinel starts to crystallize the iron enrichment in the liquid is suppressed and the forsterite content of the olivine actually increases until at lower temperatures a normal trend of iron enrichment is resumed. This phenomenon is tentatively attributed to the crystallization of the spinel under conditions of constant oxygen fugacity.