STABLE STRONTIUM ISOTOPIC BEHAVIOR OF CALCIUM-ALUMINUM RICH INCLUSIONS FROM ALLENDE

Carbonaceous chondrites are generally considered to be the building blocks of Earth. They consist of three major components: chondrules, Calcium-Aluminum Rich Inclusions, (CAIs) and matrix. Understanding both the origins of these components and any interaction between them is essential for assessing the hypothesis that they were major contributors to the solid portion of the Earth. Isotope ratios provide a mechanism to both trace sources and assess processes through the fractionation of stable isotopes. Here we report on the first steps we are taking to analyze stable Sr isotope ratio variations in meteorites from all scales, ranging from whole rock powders down to individual mineral grains.

The CV3 group of carbonaceous chondrites bears around 40% matrix by volume and contains mm-sized chondrules and CAIs. These meteorites have low levels of aqueous alteration and are considered the least metamorphosed out of all CV chondrites. Interestingly, CV chondrites have been identified as bearing very light stable Sr isotope ratios, (high Sr⁸⁴/Sr⁸⁸ or hereafter low δ⁸⁸Sr) especially in their CAIs. This behavior is particularly evident in the Allende meteorite, where CAIs are especially light. For several decades, the origin of these light signatures has puzzled cosmochemists. We are undertaking a study to assess mass balance of Sr isotopes in Allende to evaluate if a complementary heavy (high δ⁸⁸Sr) enriched source in the matrix exists to balance isotopically light CAIs and chondrules. If so, some of these signatures could reflect petrologic processes happening after Allende was assembled.

This study will use mass spectrometry (MC-ICP-MS) to determine stable Sr isotope ratios of homogenous bulk samples of Allende components as well as microdrilled minerals. Work thus far has optimized separation of Sr during column chemistry using USGS rock standards. We will perform SEM X-ray mapping on multiple Allende cross-sections in order to determine the major mineral phases present in this meteorite, which will then be compared with our bulk data from the MC-ICP-MS. LA-ICP-MS will also be used to assess concentration variations in minerals, enabling us to determine the level of alteration and fluid diffusion-related transport of isotopes from the Allende matrix into the CAIs and chondrules.