The effect of pressure on DSr (plag/melt) and DCr (opx/melt): implications for anorthosite petrogenesis

The crystal–liquid partition coefficients for Sr and Cr, DSr and DCr, have been determined from electron microprobe analyses of plagioclase–liquid and orthopyroxene–liquid pairs produced in melting experiments run at pressures from 1 bar to 27 kbar on two compositions relevant to anorthosite petrogenesis. One is a primitive jotunite (hypersthene monzodiorite: TJ); the other is a sample of an anorthositic dyke (500B). Results indicate that DSr (plag/liq) remains nearly constant with increasing pressure (TJ: 1.7 to 2.6; 500B: 0.9 to 1.4). This modest variation apparently results from the combined and opposing effects of crystal chemistry and pressure: DSr increases with the albite content of plagioclase, which itself increases with pressure along a composition’s liquidus, so pressure must have an intrinsic negative effect. The two models for DSr [J. Blundy, B. Wood, Geochim. Cosmochim. Acta 55 (1991) 193–209; I. Bindeman, A. Davis, M. Drake, Geochim. Cosmochim. Acta 62 (1998) 1175–1193] that take into account the strong correlation between DSr and plagioclase composition overestimate DSr at high pressure; whereas the two models that ignore plagioclase composition [S. Morse, Geochim. Cosmochim. Acta 56 (1992) 1735–1738; R. Nielsen, Comput. Geosci. 18 (1992) 773–788] underestimate it. Moreover, since none of these models takes into account any pressure effect, the discrepancies between predicted and observed DSr increase with pressure for all models. The new results also show that DCr (opx/liq) increases significantly with pressure: DCr=2 at 1 atm and 14.2 at 10 kbar. These new data confirm earlier, less precise determinations of DCr that were used to infer a high-pressure origin for Al- and Cr-rich orthopyroxene megacrysts. The calculated Sr and Cr concentrations of liquids in equilibrium at 10 kbar with plagioclase and orthopyroxene megacrysts from anorthosite massifs (Sr=370 to 610 ppm and Cr=20 to 130 ppm) are in the range of what is observed in high-Al gabbros and primitive jotunites, the inferred parent magmas of massive anorthosites.