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

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Earth and Planetary Science Letters
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May 30
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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. (C) 2000 Published by Elsevier Science B.V. All rights reserved.


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