Determination of rare earth and refractory trace element abundances in early solar system objects by ion microprobe

Abstract

Experimental and analytical procedures devised for measurement of rare earth element (REE) abundances using a secondary ion mass spectrometer (ion microprobe) are described. This approach is more versatile than the conventional techniques such as neutron activation analysis and isotope dilution mass spectrometry by virtue of its high spatial resolution that allows determination of REE abundances in small domains (10-20 micron) within individual mineral phases. The ion microprobe measurements are performed at a low mass-resolving power adopting the energy-filtering technique (Zinner and Crozaz 1986) for removal and suppression of unresolved complex molecular interferences in the REE masses of interest. Synthetic standards are used for determining various instrument specific parameters needed in the data deconvolution procedure adopted for obtaining REE abundances. Results obtained from analysis of standards show that our ion microprobe may be used for determining REE abundances down to ppm range with uncertainties of ∼ 10 to 15%. Abundances of rare earth and several other refractory trace elements in a set of early solar system objects isolated from two primitive carbonaceous chondrites were determined using the procedures devised by us. The results suggest that some of these objects could be high temperature nebular condensates, while others are products of melting and recrystallization of precursor nebular solids in a high temperature environment.