Did chondrules interact with a silicon-rich gas? Evidence from silicon and magnesium isotopes

Primitive chondritic meteorites contain submillimeter-size, spherical, silicate objects that were molten for a brief time, called chondrules.  Chondrules formed early in the accretion disk and were eventually incorporated into the asteroids and planets that formed from the disk.  The astrophysical environment from which the planets formed can be elucidated by the formation environment of chondrules.  A recent oxygen-isotope study proposed that upon formation chondrules interacted with a silicon-rich gas that converted existing olivine to pyroxene (by reaction with SiO₂).  The goal of this study is to measure silicon and magnesium isotopes in order to search for additional isotopic evidence that chondrules interacted with a silicon-rich gas. 

Meteorite samples from the CV and CR carbonaceous chondrite groups containing Type 1AB chondrules with pyroxene rims were selected for the study.  Silicon isotopes of chondrules in the Allende CV chondrite were measured in-situ using UV laser ablation multi-collector inductively coupled plasma mass spectroscopy (LA-MC-ICPMS).  Preliminary results indicate that silicon (magnesium data will be collected in future samples) in pyroxene rims may be isotopically lighter than interior olivine grains, but additional data will be collected to verify this result.  This is the expected result if a silicon-rich gas was produced by the evaporation of nebular dust due to the higher vapor pressure of the lightest isotope of silicon.  This project has the potential to enhance our understanding of the conditions in the early solar nebula prior to the accretion of our solar system.