ID-TIMS zircon U?Pb geochronology of the Camel Donga eucrite
ID-TIMS zircon U?Pb geochronology of the Camel Donga eucrite
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骆驼东加锂辉石的 ID-TIMS 锆石 U?Pb 年代学
DOI：
10.1016/j.chemgeo.2021.120073
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发表时间：
2021
期刊：
影响因子：
3.9
通讯作者：
Wotzlaw Jorn-Frederik
中科院分区：
文献类型：
--
作者：
Haba Makiko K.;Wotzlaw Jorn-Frederik
Eucrite陨石被认为是来自小行星4-Vesta的上地壳。这些分异的玄武质陨石通常含有粒度小于20-30 μm的副锆石。二次离子质谱法（西姆斯）锆石U-Pb定年结果表明，在45.5 - 45.55亿年（Ma）前，维斯塔地壳发生了初始岩浆活动和全球热变质作用。为了提高时间分辨率，区分灶神星地壳演化早期不同热事件中生长的锆石，采用化学研磨法从块状陨石样品中提取小颗粒锆石晶体，获得了Camel Donga榴辉岩中锆石的高精度同位素稀释-热电离质谱（ID-TIMS）U-Pb年龄。一些骆驼栋加锆石发生在接触钛铁矿，这意味着他们已经形成的钛铁矿出溶，然后在高温变质作用与二氧化硅反应。拉曼测量表明，锆石受到了中等程度的辐射损伤，高温退火（900 °C，48 h）可能会改善它们的堆叠性。从氢氟酸-硝酸混合酸溶解其他矿物后的酸渣中分离出来的锆石在酸处理过程中经历了U-Pb和Pb同位素分馏。酸提前高温退火能有效抑制Pb同位素分馏，从而获得准确的207 Pb-206 Pb定年结果。退火锆石207 Pb-206 Pb年龄的加权平均值为4552.56 ± 0.69Ma（2σ），明显大于骆驼洞加锆石的西姆斯年龄（4531 ± 10 Ma），但与其它钙铝榴石的207 Pb-206 Pb年龄一致。因此，CamelDonga钙长榴辉岩锆石形成于初始岩浆作用或全球热变质作用的早期阶段的维斯坦地壳形成。
Eucrite meteorites are considered to originate from the upper crust of asteroid 4-Vesta. These differentiated basaltic meteorites commonly contain accessory zircons with grain sizes smaller than 20–30 μm in diameter. U–Pb zircon dating usingsecondaryion mass spectrometry (SIMS) revealed that initial magmatism and global thermal metamorphism occurred in the Vestan crust ~4550–4555 million year ago (Ma). To improve the temporal resolution in order to distinguish zircons grown during different thermal events in the early stage of crustal evolution of Vesta, we have applied a chemical abrasion-based method for extracting small zircon crystals from bulk meteorite samples and obtained high-precision isotope dilution-thermal ionization mass spectrometry (ID-TIMS) U–Pb dates for zircons from the Camel Donga eucrite. Some of the Camel Donga zircons occur in contact with ilmenite, implying that they have formed by exsolution of ilmenite followed by reaction with silica during high temperature metamorphism. Raman measurements indicate that the zircons are moderately radiation damaged and that high temperature annealing (900 °C for 48 h) is likely to improve their crystallinities. Zircons separated from acid residues after dissolving other minerals in a mixture of hydrofluoric and nitric acid experienced U–Pb and Pb isotope fractionation during the acid treatment. High temperature annealing prior to acid extraction efficiently suppresses Pb isotope fractionation resulting in accurate and precise207Pb–206Pb dates. The weighted mean207Pb–206Pb date of the annealed zircons is 4552.56 ± 0.69 Ma (2σ), which is significantly older than that of Camel Donga zircons previously obtained by SIMS (4531 ± 10 Ma) but is consistent with207Pb–206Pb dates of zircons in the other eucrites. Thus, the Camel Donga eucrite zircons formed during initial magmatism or global thermal metamorphism in the early stage of Vestan crustal formation.