PHASE EQUILIBRIA OF PLAGIOCLASE ULTRAPHYRIC BASALTS: CONSTRAINTS FROM HIGH-PRESSURE EXPERIMENTS

The focus of these experiments is to understand the liquidus phase relations of Plagioclase Ultraphyric MORB (PUBs). These basalts contain up to 40 volume % anorthitic (>An₈₅) plagioclase phenocrysts. However, in no case are the host lavas in equilibrium with such anorthitic compositions. What we know about such compositions is based on experiments as diverse as those on synthetic compositions (e.g. Osborn and Tait, 1952) or natural MORB glasses (Kohut and Nielsen, 2003). Such PUB host liquids are all olivine and plagioclase saturated at 1-atm pressure. Our experimental approach is to understand the more primitive precursors by pushing the liquids back up the olivine -plagioclase cotectic, using added anorthitic plagioclase and forsteritic olivine. Previous results at 1 atmosphere documented the existence of an olivine + plagioclase - spinel + liquid reaction point. New CO₂ contents from plagioclase hosted melt inclusions (Drignon et al., 2017) indicate a depth of crystallization of ~8-20 km (3-7 kbar). Our new experiments are designed to extend the conditions to those upper mantle conditions (5 and 10 kbar). The results confirm the presence of Al-rich spinel at or near the liquidus (together with olivine). Those spinel compositions are similar to those produced by Kohut and Nielsen, 2003 and are more extreme than observed in PUBs, extending the trend predicted by Allan et al., 1988. When present, plagioclase is found to be less anorthitic (An_86-87) than the observed phenocrysts compositions (An_87-93). Therefore, these experiments confirm the presence of spinel near the liquidus (consistent with the presence of spinel as inclusions in PUB phenocrysts), and may suggest that the plagioclase loop is flattened by the existence of the spinel reaction point.

Osborn, E.F. and Tait, D.B., 1952, Am. J. Sci. 38, 413. Kohut, E.J. and Nielsen, R.L., 2003. G-cubed 4,7. Drignon et al., 2017, Goldschmidt abstract. Allan et al., 1988, Am. Min. 73, 741