Aqueous Alteration and Silicon and Magnesium Isotope Measurements in CR and CV Chondrites
Primitive chondritic meteorites formed at the same time the planets in our solar system were forming and can offer information about the conditions present in the early solar system, provided that they have not been significantly altered by heat and water. The CR carbonaceous chondrite group contains some of the least-altered meteorites known. The members of the group have undergone minimal thermal alteration, but while on the parent asteroid CR chondrites have interacted with water to varying degrees. A numerical aqueous alteration scale was developed to rank the CR chondrites based on their interactions with water. The majority of the CR carbonaceous chondrites have had minimal interaction with water and this scale is used to select appropriate samples for the study of the early solar system.
Silicon and magnesium isotope ratios were measured in selected CR and CV chondrules using multi-collector inductively-coupled plasma mass spectroscopy to test the hypothesis that a silicon-bearing gas was present in the solar nebula and condensed into fully or partially molten chondrules. The SiO gas could react with olivine in the chondrules to form pyroxene. These data were interpreted using a condensation model for silicon. Some of the chondrules studied did show a non-equilibrium fractionation in δ29Si between olivine and pyroxene that was calculated to have been caused by a supersaturated SiO gas that had condensed into the chondrule. If this SiO gas did condense into the chondrules the process happened at or near equilibrium with a undercooling of 3 K or less below the equilibrium temperature. These results provide additional evidence that a silicon-bearing gas was present in the early solar system during the time of planet formation, but not all chondrules studied show clear evidence of having incorporated this gas.