The effect of pressure on D-Sr (plag/melt) and D-Cr (opx/melt): implications for anorthosite petrogenesis

The crystal-liquid partition coefficients for Sr and Cr, D-Sr and D-Cr, hav e been determined from electron microprobe analyses of plagioclase-liquid a nd orthopyroxene-liquid pairs produced in melting experiments run at pressu res from 1 bar to 27 kbar on two compositions relevant to anorthosite petro genesis. One is a primitive jotunite (hypersthene monzodiorite: TJ); the ot her 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 comb ined and opposing effects of crystal chemistry and pressure: D-Sr increases with the albite content of plagioclase, which itself increases with pressu re along a composition's liquidus, so pressure must have an intrinsic negat ive 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. Cosmoch im. Acta 62 (1998) 1175-1193] that take into account the strong correlation between D-Sr and plagioclase composition overestimate D-Sr at high pressur e whereas the two models that ignore plagioclase composition [S. Morse, Geo chim. 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 s how 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 det erminations of D-Cr that were used to infer a high-pressure origin for Al- and Cr-rich orthopyroxene megacrysts. The calculated Sr and Cr concentratio ns 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. (C) 2000 Pu blished by Elsevier Science B.V. All rights reserved.