Cosmochemists with the UCLA ion probe group

Dr. Mariana Cosarinsky is studying records of the high temperature processes in the solar nebular that resulted in the formation and modification of CAIs and chondrules; she also investigates the timescales of asteroid accretion and alteration processes. To do this, she uses the ion probe to analyze stable isotopes systems, such as oxygen, magnesium and silicon, as well as short-lived isotopes, such as ²⁶Al, to constrain processes such as condensation and evaporation in the solar nebula as well as isotopic exchange during interactions between different reservoirs, in nebular and asteroidal environments.

Al-Mg isotope records in select CAIs of primitive meteorites: by combining the unsurpassed spatial resolution of the NanoSIMS ion microprobe at the Lawrence Livermore National Laboratory with high precision in situ Mg isotope measurements performed on the same CAIs with the ims 1270 ion microprobe, we are able to address issues such as quantifying the primary ²⁶Al/²⁷Al at the time of CAI crystallization, unraveling the nature of possible redistribution of radiogenic ²⁶Mg* following CAI formation, and delineating the timing and mechanism of refractory Wark-Lovering rim (WLR) formation. We have also measured silicon isotope abundances in melilite as a function of distance from the rim. These measurements, combined with Mg fractionation analyses, help constrain the mechanisms of CAI formation and subsequent interactions or exchange with the nebular gas.

An important component of this work involves further development of techniques for measuring Mg isotope ratios by multicollector ion counting with the NanoSIMS. In addition to utilizing the NanoSIMS to determine Mg isotope fractionation, we will also analyze trace element contents within individual mineral layers of WLRs as well as in interior CAI phases. Such correlated analyses are key to establishing the formation mechanisms of WLR layers. The results of this study will help in developing a better understanding of the meaning of the classical ‘canonical’ ²⁶Al/²⁷Al value characteristic of Mg-poor minerals in CAIs, and will therefore have important implications for early solar system chronology and the origins of short-lived radioactivity in the solar nebula.

Left: BSE image of Allende TS25, a Type A CAI. Red ovals highlight areas analyzed for Al-Mg isotopes with the UCLA ims1270 ion microprobe, and red squares highlight areas analyzed with the NanoSIMS. Below: Magnesium isotopes fractionation in interior CAI phases (upper panel) and WLR phases (lower panel) obtained with the UCLA ims1270 probe and LLNL NanoSIMS. All results show consistent ranges in d^25Mg values. The interior phases show a large range in fractionation, whereas WLR phases are isotopically normal. Symbols: circles, melilite; triangles, hibonite; diamonds, spinel; squares, pyroxene.

Graduate student Dianne Taylor makes use of the long-lived ^176Lu-^176Hf isotope system to explore timescales of early planetary differentiation, in particular how long a global magma ocean such as was once present on the Moon would take to completely crystallize. She uses the ion probe to obtain U-Pb and ^207Pb-^206Pb ages, rare earth element compositions, Ti-calibrated crystallization temperatures and oxygen isotope compositions of extremely ancient lunar zircons, some of which date back to 4.4 billion years ago. The Lu-Hf isotopes of the lunar zircons are then measured using laser-ablation inductively coupled mass spectrometry. Other ion-microprobe projects have included ^26Al-^26Mg dating of the even-more ancient calcium-aluminum-rich inclusions found in type CV3 meteorites. Right: Dianne peers at a moon rock in the Lunar Receiving Lab at the Johnson Space Center, Houston, Texas.

Graduate student Ming-Chang Liu uses secondary ion mass spectrometry (SIMS) to study different isotope anomalies in refractory solids from primitive meteorites, which can be used as a probe to understand the origin and evolution of the early solar system in several aspects, such as chronologies, chemical evolution, irradiation processes and nucleosynthesis.