Identifying FUN inclusions with LA-MC-ICPMS and NanoSIMS

Abstract

A small group of CAIs exhibit some fractionated and unknown nuclear (FUN) isotopic effects, e.g., O, Mg, Si, Ca, and Ti, and are called FUN inclusions. These FUN CAIs are characterized by large mass-dependent fractionation effects in Mg, Si, and O, and a few ‰ of mass-independent isotopic anomalies in various elements, such as Ca, Ti, Cr, Sr, Ba, Nd, and Sm. Despite their discovery for more than 40 years, the origin and formation of the FUN CAIs remain poorly constrained. In order to better identify and, subsequently, constrain the nature and origin of FUN CAIs, we have set up in situ LA-MC-ICPMS Mg isotopic analysis for CAI in thick meteorite slab, to quickly search for CAIs deficient in 26Mg, a common characteristic of FUN CAIs. Subsequently, CAIs with abnormal Mg isotopes have been extracted and prepared for NanoSIMS study, e.g., O, Mg, and/or Si, and also being prepared for further solution-based MC-ICPMS study. Approximately ~1mm thick sections of Allende and DAG192 (CO3) were prepared and subjected to in situ laser ablation MC-ICPMS, a 193nm Analyte G2 excimer laser coupled with a Nu-Plasma II, Mg isotopic analysis at IES. The spot size was 50 microns with 50 sec ablation time. San Calos ol, opx, and cpx and two glass standards BCR-2G and BIR-1G were studied and the results were comparable to published data, while San Calos opx was used as the main standard during the experiments. Preliminary results showed that while the Mg isotopes for majority of the CAIs from both Allende and DAG192 exhibited normal to positive 26Mg, due to the decay of 26Al, two CAIs from Allende showed resolvable 26Mg deficits of -1 to -1.5‰, and 0 to -0.5‰, respectively. With a typical 2-sigma SD of 0.33‰ for our Mg isotopic analysis, the first CAI showed clearly resolvable 26Mg deficit from both sides of the thick sections, while questionable for the second CAI. The potential FUN CAIs, consisting of fine-grained spinel and pyroxenes, have been extracted from the thick sections and prepared for NanoSIMS study. The preliminary O isotopic analysis of spinel grains show typical FUN O isotopic fractionations while intercepting the CCAM line at ~ -50‰ for both delta17O and delta18O. While more NanoSIMS isotopic analyses are currently undergoing, the preliminary results are quite encouraging about the future searches and understanding of FUN CAIs.