Rare Earth Element Measurements of Melilite and Fassaite in Allende Cai by Nanosims

Introduction: The rare earth elements (REEs) are concentrated in CAIs by ~20 times the chondritic average [e.g., 1]. The REEs in CAIs are important to understand processes of CAI formation including the role of volatilization, condensation, and fractional crystallization [1,2]. REE measurements are a well-established application of ion microprobes [e.g., 3]. However the spatial resolution of REE measurements by ion microprobe (~20µm) is not adequate to resolve heterogeneous distributions of REEs among/within minerals. We have developed methods for measuring REE with the NanoSIMS 50L at smaller spatial scales. Here we present our initial measurements of REEs in melilite and fassaite in an Allende Type-A CAI with the JSC NanoSIMS 50L. Experimental: We synthesized diopside glass standards doped with selected trace and REEs (100 and 500ppm) optimized for simultaneous measurement of 7 masses by NanoSIMS. These standards were used to accurately determine relative sensitivity factors and oxide-to-element ratios, as well as finding target REE peaks. A focused O-ion beam was rastered over 5×5 to 10x10 µm regions. Secondary [REE