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

Abstract

The crystal–liquid partition coefficients for Sr and Cr, D Sr and D Cr, 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 D Sr (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: D Sr 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 D Sr [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 D Sr and plagioclase composition overestimate D Sr 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 D Sr increase with pressure for all models. The new results also show that D Cr (opx/liq) increases significantly with pressure: D Cr=2 at 1 atm and 14.2 at 10 kbar. These new data confirm earlier, less precise determinations of D Cr 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.