Isotopic Heterogeneity in Calcium-Aluminum-Rich Meteoritic Inclusions

Citation

Brigham, Cheryl A. (1990) Isotopic Heterogeneity in Calcium-Aluminum-Rich Meteoritic Inclusions. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/TYS3-HQ41. https://resolver.caltech.edu/CaltechTHESIS:04282010-113920823

Abstract

Calcium-Aluminum-rich inclusions (CAI), found in meteorites, are among the oldest known solids identified in the solar system. Analyses of CAI have provided constraints on physical and chemical conditions that existed just prior to, and during planetary formation. A few rare inclusions, called FUN (for Fractionation and Unknown Nuclear effects), exhibit large isotopic anomalies and have provided insight into nucleosynthetic and nebular processes.

In this thesis, data obtained on the petrography, chemistry and isotopic compositions of CAI, identified in the carbonaceous chondrite Allende, are used as tracers to address several specific questions: 1) What are the relationships between fine and coarse-grained CAI? 2) What are the differences, in composition and origin, between FUN inclusions and isotopically normal CAI? 3) What was the role of volatility-controlled processes, such as distillation and condensation, in the evolution of CAI? 4) What was the role of chemical alteration and isotopic reequilibration in the evolution of CAI?

Isotopic data were obtained by thermal ionization and ion microprobe mass spectrometry for individual grains within both fine and coarse-grained CAI, and correlated with petrographic and chemical observations. Evidence is presented for the enrichment of fine-grained inclusions in the lighter isotopes of Mg, in contrast to coarse-grained CAI, which are enriched in the heavier isotopes. Isotopic heterogeneity was observed within both fine and coarse-grained inclusions. Heterogeneity is discussed in the context of primary and secondary phases, mineral alteration processes, and isotopic reequilibration.

A new class of coarse-grained inclusion, characterized by a distinct purple color and high spinel contents (≥50%), were identified and found to exhibit a high frequency (20%) of FUN isotopic anomalies. Four new FUN inclusions were identified and studied in detail. The correlated isotopic fractionation for Mg, Si, and Cr in these inclusions, suggests the importance of volatility-controlled formation processes. A model is presented for the evolution of FUN inclusions, involving distillation of ordinary chondritic material, with a mass loss of around 70%, followed by exchange with isotopically normal reservoirs. The high spinel contents and large isotopic fractionation of these inclusions, may indicate that they formed at higher temperatures than isotopically normal CAI.

Thesis Files