Trace-element zonation in garnets from The Thumb: heating and melt infiltration below the Colorado Plateau

Abstract

Proton microprobe (PIXE) analysis of garnet, pyroxene, and olivine for Zr, Y, Ga, Ge, Sr, Ni, Mn and Zn has been combined with electron-probe and petrographic analysis to interpret the histories of garnetperidotite xenoliths from the minette neck at The Thumb on the Colorado Plateau. Garnet in seven rocks contains 10-110 ppm Zr and 25-95 ppm Ni. Substantial parts of these ranges are preserved in single, zoned garnets (Zr, 25-90 ppm; Ni, 25-60 ppm). Pyroxene and olivine are more homogeneous and equilibrated more quickly than granet to changing temperatures and metasomatic fluxes. The distribution coefficient of Ti between pyroxene and garnet rims may be sufficiently sensitive to pressure to b..