The magnesium isotopic composition of Earth is not yet well constrained despite significant advances in methods for measuring Mg isotope ratios in rocks. One impediment to establishing 25Mg/24Mg and 26Mg/24Mg of Earth is the lack of constraints on inter-mineral Mg isotope fractionations at high temperatures. Advances in computational chemistry afford the capacity to predict quantitatively Mg isotope fractionations among high-temperature minerals. High-precision MC-ICPMS measurements in turn provide the opportunity to test these predictions in well-characterized samples. Toward this end, we present new high-precision 25Mg/24Mg and 26Mg/24Mg measurements of mantle minerals and compare these ratios with predictions for temperature-dependent inter-mineral fractionations. Our results for two San Carlos volcanic field xenoliths show that there is measurable and systematic fractionation in Mg isotope ratios between constituent minerals that are consistent with theoretical predictions. The observed order from highest to lowest 25Mg/24Mg is spinel > clinopyroxene > orthopyroxene > olivine. The fractionation between spinel and olivine suggests an equilibration temperature of 814° +/− 60 °C based on the temperature dependence obtained from ab initio calculations. This temperature is consistent with independent T indicators involving spinel, suggesting that spinel and olivine are in Mg isotopic equilibrium in these mantle rocks, and lending credence to the accuracy of the results. Pyroxene, on the other hand, is apparently not in Mg isotopic equilibrium with spinel and olivine if the predicted temperature-dependent fractionations are correct. Consideration of the influences of modal abundances and inter-mineral fractionations on the Mg isotopic compositions of mantle minerals, and comparisons to new meteorite data reported herein, strengthen previous suggestions that the Earth may be different from carbonaceous chondrites in 25Mg/24Mg.

Far from being the relatively unprocessed materials they were once believed to be, we now know that a significant number of carbonaceous chondrites were thermally metamorphosed on their parent asteroid(s). Numerous studies indicate that 7 and 2 chondrites have been naturally heated, variously, at from 400 to over 700 C on their parent asteroid(s). Petrographic textures reveal that this thermal metamorphism occurred after the dominant aqueous alteration phase, although some meteorites show evidence of a heating event between two aqueous alteration episodes, i.e. pro- and retrograde aqueous alteration. Aside from the issues of the identification of the transient heat source, timing of metamorphism, and the relation of these materials (if any) to conventional CM and CI chondrites, there is also a mystery related to their recovery. All of these meteorites have been recovered from the Antarctic; none are falls or finds from anyplace else. Indeed, the majority have been collected by the Japanese NIPR field parties in the Yamato Mountains. In fact, one estimate is that these meteorites account for approx. 64 wt% of the CM carbonaceous chondrites at the NIPR. The reasons for this are unclear and might be due in part to simple sampling bias. However we suggest that this recovery difference is related to the particular age of the Yamato Mountains meteorite recovery surfaces, and differences in meteoroid fluxes between the Yamato meteorites and recent falls and substantially older Antarctic meteorites. Additional information is included in the original extended abstract.

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The canonical initial 26Al/27Al ratio of 4.5 × 10-5 has been a fiducial marker for the beginning of the solar system. Laser ablation and whole-rock multiple-collector inductively coupled plasma-source mass spectrometry magnesium isotope analyses of calcium- and aluminum-rich inclusions (CAIs) from CV3 meteorites demonstrate that some CAIs had initial 26Al/27Al values at least 25% greater than canonical and that the canonical initial 26Al/27Al cannot mark the beginning of solar system formation. Using rates of Mg diffusion in minerals, we find that the canonical initial 26Al/27Al is instead the culmination of thousands of brief high-temperature events incurred by CAIs during a 105-year residence time in the solar protoplanetary disk.

Some phyllosilicates in CM carbonaceous chondrites formed by aqueous alteration of anhydrous precursor phases. Although broad trends in the compositions of hydrous phyllosilicates are recognized and believed to be related to trends in degree of aqueous alteration, details of the reactions that formed specific secondary minerals remain obscure. This paper reports compositional relationships between remnants of partially pseudomorphically replaced silicates and their alteration products (serpentine) in the CM2 chondrites QUE93005 and Nogoya and compares both with previously published results for Allan Hills 81002. By focusing on serpentine formed from known reactants (olivines), and on only those instances in which some of the reactant silicate remains, direct compositional relationships between reactants and products, and the elemental mobility required by the reactions, can be established.

Introduction: D'Orbigny, is a relatively a new an-grite find [1]. Angrites are achondrites that show unique mineralogy and typically ancient crystallization ages. D'Orbigny has been described extensively by [2]. We have initiated a study of D'Orbigny. A Pb-Pb model age of 4559 Ga has been reported [3]. The presence of 244 Pu fission Xe in D'Orbigny has also been reported [4]. We present Sm-Nd and Rb-Sr results and a 53 Mn-53 Cr study is in progress. We had expected to find a relatively well-behaved Sm-Nd system, and distinct evidence for the