地球化学

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WiscSIMS IMS 1280 has been used to study oxygen isotope systematics and Al–Mg chronology of early solar system solids, such as meteoritic chondrules and cometary particles. Analytical methods and technical developments are reviewed. Individual chondrules are internally homogeneous in oxygen isotopes except for relict olivine grains, supporting the open-system formation of chondrules in dust-rich protoplanetary disk through evaporation and recondensation of major oxides at high temperatures. Oxygen isotope ratios in chondrules from carbonaceous chondrites and cometary particles show mass-independent fractionation that correlates with Mg# of mafic minerals. They might have formed under the existence of ¹⁶O-poor H₂O ice that mixed with ¹⁶O-rich anhydrous silicates. Chondrules in ordinary chondrites show variable degrees of mass-dependent fractionations, suggesting that their precursors might have condensed from homogeneously ¹⁶O-poor nebula gas located at inner disk regions. High resolution Al–Mg chronology of chondrules show discrete formation ages of 1.8–2.2 Ma for ordinary chondrites, 2.2–2.8 Ma for major carbonaceous chondrites, after the birth of the solar system, while majority of chondrules in CR and cometary particles formed later than 3–4 Ma. These results suggest that chondrule formation events were related to the growth of planetesimal and protoplanets, which started early for inner disk than outer disk regions.

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