Magnesium isotopic fractionation in chondrules from the Murchison and Murray CM2 carbonaceous chondrites

Abstract–We present high-precision measurements of the Mg isotopic compositions of a suite of types I and II chondrules separated from the Murchison and Murray CM2 carbonaceous chondrites. These chondrules are olivine- and pyroxene-rich and have low 27Al/24Mg ratios (0.012–0.316). The Mg isotopic compositions of Murray chondrules are on average lighter (d26Mg ranging from 0.95 & to 0.15 & relative to the DSM-3 standard) than those of Murchison (d26Mg ranging from 1.27 & to +0.77&). Taken together, the CM2 chondrules exhibit a narrower range of Mg isotopic compositions than those from CV and CB chondrites studied previously. The least-altered CM2 chondrules are on average lighter (average d26Mg = 0.39 0.30&, 2SE) than the moderately to heavily altered CM2 chondrules (average d26Mg = 0.11 0.21&, 2SE). The compositions of CM2 chondrules are consistent with isotopic fractionation toward heavy Mg being associated with the formation of secondary silicate phases on the CM2 parent body, but were also probably affected by volatilization and recondensation processes involved in their original formation. The low-Al CM2 chondrules analyzed here do not exhibit any mass-independent variations in 26Mg from the decay of 26Al, with the exception of two chondrules that show only small variations just outside of the analytical error. In the case of the chondrule with the highest Al/Mg ratio (a type IAB chondrule from Murchison), the lack of resolvable 26Mg excess suggests that it either formed>1 Ma after calcium-aluminum-rich inclusions, or that its Al-Mg isotope systematics were reset by secondary alteration processes on the CM2 chondrite parent body after the decay of 26Al