Plagioclase Compositions of Eucrites and Eucrite-Type Achondrites

Eucrites are basalts, diabases and gabbros composed of ferroan pigeonite and augite, calcic plagioclase, and a silica phase, with accessory amounts of ilmenite, chromite, Ca-phosphate, troilite, metal and olivine. Many of the gabbroic-textured eucrites are cumulates from a mafic magma, while most other eucrites are close to melt compositions. These are referred to as cumulate and basaltic eucrites. Petrologic, compositional and isotopic studies have identified several mafic achondrites with eucritic mineralogy that have anomalous properties, and some of these are considered to have been derived from different parent asteroids than normal eucrites. These are referred to as eucrite-type achondrites, and they include cumulate and basaltic types. Petrologic studies of eucrites often concentrate on pyroxene compositions to reveal the petrogenetic history of the rocks and plagioclase compositions are less utilized. Here I examine Na and K variations of a suite of basaltic and cumulate eucrites and eucrite-type achondrites.