Partition coefficients for calcic plagioclase: Implications for Archean anorthosites

Abstract

By the use of a multiple aliquot technique combined with statistical calculations to overcome effects of alteration and contamination, we have determined partition coefficients ( D' s) for rare earth elements (REEs) between natural, high-Ca plagioclase megacrysts and their basaltic matrices. Some of the basalts that we investigated are shown experimentally to crystallize only high-Ca plagioclase over approximately a 20–30°C temperature interval below the liquidus. The REE D' s agree well with experimentally determined values and predict equilibrium melts for Archean anorthosites that agree well with coeval basaltic flows and dikes. Furthermore, the D' s appear to be valid for both tholeiitic and alkali basalts over a range of mg numbers (34–58) and REE concentrations (La from 3–50 ppm). Similarly calculated D' s for Co, Sc, Cr, Rb, Sr, Cs, Ba, Hf, Ta, and Th also predict equilibrium melts for anorthosites that agree reasonably well with coeval basaltic flows and dikes. The apparent values for Co, Sc, and Ta seem good to better than 50%, whereas the standard deviations for Rb and Th are equal to their means, making the D' s of questionable value. The others are useful probably to within a factor of 2. Experimental studies of the D for total Fe as FeO T indicate a value of about .04 at an oxygen fugacity slightly below that of the QFM buffer. A by-product of the FeO T experiments was the experimental D' s for MgO, TiO 2, and K 2O which could be compared with the D' s from natural assemblages. From the D' s that were determined, we suggest that plagioclase megacrysts in the Bad Vermilion Lake anorthosite were in equilibrium with melts having the composition of tholeiitic basalts at an oxygen fugacity slightly below the QFM buffer. The predicted melts have an FeO T content of about 13%, K 2O of about. 15%, TiO 2 of about 1.3%, a chondrite-normalized REE pattern that is nearly flat at 10 to 20 × chondrites with a slight depletion of LREEs, other incompatible trace elements at 10 to 20 × chondrites, and Cr and Co at 0.01 to 0.1 × chondrites. This range of compositions is essentially identical to the tholeiitic flows and dikes in Archean greenstone belts. We, therefore, suggest that the moderately Fe-rich tholeiites that are hosts to plagioclase megacrysts in greenstone belts form the parental melts for megacrysts which make up the Bad Vermilion Lake anorthositic complex and probably other similar Archean anorthositic complexes.