Geocheminar fall-2017

Until recently, all of the eucrites appeared to be simple products of anhydrous basaltic magmas, mostly annealed by dry thermal metamorphism. However, we now know that unannealed eucrites commonly feature secondary veining, typically dominated by an anomalous combination of fayalitic olivine and Na-poor plagioclase within pyroxene; from which secondary volatile-rich fluids have been inferred. The unannealed NWA 11040 eucrite shows new varieties of secondary alteration. Late, in-situ reduction is suggested from: a high Fe-metal abundance; pyroxene Mg/Fe and Fe/Mn zoning consistent with late diminution in [FeO]; and rims of many pyroxene grains, especially near Fe-metal, show a distinctive corrosion texture, with elongate silica-dominated inclusions oriented perpendicular to the rim, suggesting FeO reduction caused decomposition of rim pyroxene. Also, deposition from fluid at a far-subigneous T is implied by a peculiar texture where compositionally anomalous pyroxene fills cracks within cristobalite. But, you ask, then why pyroxene and not amphibole? Unearthly low pressures make an asteroidal crust ill-disposed to engender igneous or postigneous hydrous silicates.