Compositions of isolated forsterites in Ornans (C3O)

Abstract

Luminescing forsterite with FeO between 0.25 and 2.0 wt% occurs as cores of isolated grains and within an Fe-rich porphyritic chondrule in the Ornans (C3O) carbonaceous chondrite. The color variation of the cathodoluminescence depends on the relative intensity of a red emission caused by Cr and a blue emission most intense when transition metal impurities are at the lowest levels. For Omans forsterite, the blue emission is quenched by Fe at 0.75 wt% and the red at 2.0 wt%. Compositional profiles from core to rim of five isolated forsterites show details of Al, Ca, Sc, Ti, V, Cr, Mn and Fe zoning with respect to position and cathodoluminescence color. FeO shows normal zoning within the core of the five grains, reaching values of 0.75 wt%, at which point rapid enrichment in FeO occurs, reaching FeO values of 20–30 wt% at the grain edge. Titanium and Al are linearly correlated but show erratic changes within each grain; TiO 2 ranges from 500 to 600 ppmw in the core to 100 ppmw in the rim; Al 2O 3 varies from 0.6 wt% in the core to below 0.1 wt% in the rim; CaO is near constant at 0.70 wt% within the blue luminescing core and below 0.4 wt% in the rim; Sc 2O 3 is present in two grains up to 130 ppmw; V 2O 3 shows 500 ppmw at the core and 100 ppmw at the rim. Evidence is presented for growth of euhedral forsterite either from a liquid or gas, subsequent fracturing, precipitation of Fe-metal, and growth of progressively more Fe-rich rims. Diffusion has affected these grains to give halos around metal and diffuse CL boundaries in some crystallographic directions. Some forsterite grains have been incorporated into an Fe-rich assemblage to form chondrules which retain evidence of their origin in the form of these relic grains.