Trace element composition of anorthosite plagioclase

Abstract Plagioclases from massif anorthosites are strongly depleted in Rb and Cs relative to those from anorthosites associated with volcanic rocks, but are enriched in K and REE. Anorthosite plagioclases generally show strong enrichment of the lighter REE. The inclusions in Icelandic basalts are unique in having low K and K/Rb and relatively less-fractionated REE patterns; they may be the closest terrestrial analogues to the lunar highlands anorthosites. Regular variations in the trace element concentrations of terrestrial plagioclases are produced by the systematic relations between bulk composition and trace element composition of magma, on the one hand, and between plagioclase composition and element partitioning on the other. The failure of lunar plagioclases to follow these regular trends reflects differences in the relative abundances of K, Ca, Ba and Sr between terrestrial and lunar magmas of similar bulk composition.