Isotopic and Elemental Compositions of Stardust and Protosolar Dust Grains in Primitive Meteorites
Date of Award
Doctor of Philosophy (PhD)
Chair and Committee
This dissertation presents the results and implications of the isotopic and elemental analyses of presolar silicate grains from the primitive chondrites, Acfer 094, SAH 97096, and ALHA77307. Oxygen-anomalous, C-anomalous, and N-anomalous grains were identified by O, C, and N isotopic imaging, respectively, using the NanoSIMS 50. Subsequently, the elemental compositions of the grains carrying the anomalous isotopic signatures were acquired in the PHI 700 Auger Nanoprobe. Some silicate grains with unique O isotopic compositions were measured for Si and Fe isotopes. The isotopic analyses indicate that a majority of the silicate and oxide grains are 17O-rich with solar to sub-solar 18O/16O ratios and come from less than 2.2M├» Red Giant or Asymptotic Giant Branch stars. The second most abundant fraction of grains show large 18O excesses and come from core collapse supernovae. The next most abundant fraction of grains comes from high metallicity AGB stars of approximately solar mass. A minor fraction of the grains exhibit large excesses in 16O and formed in core collapse supernova ejecta. Grains with extreme 17O excesses are the latest addition to the presolar grain inventory. These grains may come from binary star systems where one star goes nova. Numerous presolar SiC and N-anomalous carbonaceous grains were identified in the matrix of ALHA77307. The SiC grains are predominantly mainstream grains and may have condensed in 1-3M├» AGB stars. The carbonaceous grains may have formed by ionmolecule reactions in the protosolar nebula or interstellar medium. A few carbonaceous grains exhibit 13C-rich compositions; grains with such compositions are rare, which implies that either the fractionation effects that produce C anomalies in opposite directions cancel them out or secondary processing destroyed grains with C anomalies. The elemental compositions of the silicate grains are predominantly nonstoichiometric: 61%), with some grains exhibiting olivine- or pyroxene-like compositions. About 25% of the silicate grains contain Ca and/or Al. Furthermore, most of the presolar silicates are Fe-rich with Fe contents reaching up to about 45 at.% in contrast to equilibrium condensation models, which predict Mg-rich phases such as forsterite and enstatite to form. Although secondary alteration processes have probably modified the compositions of some presolar silicate grains in Acfer 094, the source of the Fe enrichments in most silicate grains is arguably primary. This work has led to the identification of presolar Si-oxide and Mg-oxide grains that had not been observed before. The formation of SiO2 grains requires non-equilibrium conditions in the outflows of stars. The abundance of SiC and silicate grains in ALHA77307 is high indicating its pristine characteristics, similar to the CR chondrites QUE 99177 and MET 00426. Although, the number of stardust grains identified in the enstatite chondrite SAH 97096 is low, their identification indicates that such grains were not all destroyed during the high temperature phase experienced by enstatite chondrites. Finally, silicate grain abundances are much higher than oxide grain abundances in all the three meteorites.
Bose, Maitrayee, "Isotopic and Elemental Compositions of Stardust and Protosolar Dust Grains in Primitive Meteorites" (2011). All Theses and Dissertations (ETDs). 41.
Permanent URL: http://dx.doi.org/10.7936/K7NP22JS