Magnesium isotopic compositions of olivine, spinel, and hibonite from the Murchison carbonaceous chondrite

The Mg isotopic compositions of olivine, spinel, and hihonite from the Murchison carbonaceous chondrite have been determined by secondary ion mass spectrometry. The olivines exhibit only marginal effects. They have no detectable intrinsic mass fractionation; however, after correcting for mass fractionation, the mean residual 26Mg/24Mg for the five grams is slightly in excess of the terrestrial value. Blue spine], which often contains inclusions of hihonite and perovskite, is character&d by positive fractionation (enriched in heavier isotopes) ranging from + 1 to +46 permil/amu, and with a roughly constant residual of + 1.04 f 0.25 (2~~) perrnil. These characteristics can he derived from an initial terrestrial Mg isotopic composition through varying degrees of mass fractionation, and the addition of 26Mg from 26Al decay. The hibonites show both positive and negative fractionation and residuals. Three deep-blue granular hibonites show large fractionation from + 10 to + 17 permil/amu with residuals tanging from 1.8 to +9.7 permii. The remaining five colourless hibonites are closer to the terrestrial composition. The fractionation for these grains varies from -5 to +3 nermil/amu and the residual from -7.0 to +2.5 nermil. There is a hint of a correlation between fractionation and residual effects.