Pressure Effects, Kinetics, and Rheology of Anorthositic and Related Magmas

Anhydrous experiments on natural and synthetic starting materials with basaltic to anorthositic bulk compositions show a systematic increase in the albite component of near-liquidus plagioclase and in the Al2O3 content of orthopyroxene with increasing pressure. These results are consistent with crystallization of the highly aluminous orthopyroxene megacrysts and most of the plagioclase in massif anorthosite complexes at lower crustal pressures. Comparison of plagioclase compositions from near-liquidus and subliquidus experiments conducted in this laboratory with plagioclase compositions predicted at 1 atm for the experimental temperature and liquid compositions by various empirical models indicates that the shift to more albitic plagioclase is predominantly a pressure effect on the partitioning of albite (Ab) and anorthite (An) components between plagioclase and liquid. However, even when pressure terms are added to the models for Ab and An partitioning, there remain statistically significant compositional dependencies that are most apparent when the liquid composition is nepheline normative. These compositional dependencies probably arise from the absence of highly aluminous and nepheline-normative liquids in the data from which the models were constructed. Accordingly, we present empirical adjustments to the plagioclase-liquid models of Drake (1976), Weaver and Langmuir (1990), and Ariskin and Barmina (1990).The positive pressure dependence of Al2O3 in orthopyroxene coexisting with plagioclase and liquid is almost entirely the result of changes in orthopyroxene-liquid partitioning and not related to increases in the Al2O3 concentration of the liquid. Data for Al2O3 partitioning from 46 orthopyroxene-liquid and 45 pigeonite-liquid pairs taken from the literature show that pressure is the most important control on the simple molar partition coefficient for Al2O3. Rapid crystal growth is rejected as an alternative explanation for the high Al2O3 contents of orthopyroxene megacrysts because rapid growth leads to low Cr2O3 concentrations in orthopyroxene, contrary to what is observed.