Geochronology of the Martian meteorite Zagami revealed by U-Pb ion probe dating of accessory minerals
Geochronology of the Martian meteorite Zagami revealed by U-Pb ion probe dating of accessory minerals
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DOI：
10.1016/j.epsl.2013.05.035
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发表时间：
2013-07
影响因子：
5.3
通讯作者：
Qin Zhou;C. Herd;Q. Yin;Xian‐Hua Li;Fu-Yuan Wu;Qiu-li Li;Yu Liu;G. Tang;T. Mccoy
中科院分区：
文献类型：
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作者：
Qin Zhou;C. Herd;Q. Yin;Xian‐Hua Li;Fu-Yuan Wu;Qiu-li Li;Yu Liu;G. Tang;T. Mccoy
由于缺乏火星时间标度的绝对年龄，以及将月球时间标度外推到火星上所产生的重大不确定性，阻碍了火星地质事件的精确年表测定（Hartmann和Neukum， 2001）。火星陨石是目前唯一可获得的火星样本。迄今为止，确定陨石来源陨石坑的尝试被证明是不确定的（Hamilton等人，2003年，Lang等人，2009年，Mouginis-Mark和Boyce， 2012年），排除了它们在限制火星绝对时间尺度方面的应用。大多数已知的火星陨石是玄武岩（“shergotites”）；所有确定年代的菱辉石均具有<600 Ma的矿物分离年龄（Rb-Sr或Sm-Nd） （Borg et al., 2005）。本文报道了Zagami辉长岩中坏辉长岩（ZrO2）晶粒的离子探针年龄为182.7±6.9 Ma的238u /206Pb。不协调性与激波变质作用下的变质岩颗粒位置之间没有相关性。矿物岩相学表明，坏辉石是晚期火成岩结晶的结果，拉曼光谱分析表明，坏辉石没有经过冲击转化为可保存的高压多晶岩。根据同样方法测得的磷酸盐颗粒年龄（153±81 Ma），我们得出Zagami在~180 Ma结晶的结论，与前人矿物分离年代学的结果一致。因此，这些辉长岩代表了在少数喷射事件中优先从火星年轻地形中喷射出来的火成岩。
The precise chronology of geological events on Mars is hampered by the lack of absolute ages for the Martian timescale, and the significant uncertainties that result from the extrapolation of the lunar timescale to Mars (Hartmann and Neukum, 2001). Martian meteorites represent the only samples of Mars currently available. Attempts to identify source craters for the meteorites have thus far proven inconclusive (Hamilton et al., 2003, Lang et al., 2009, Mouginis-Mark and Boyce, 2012), precluding their use in constraining the absolute Martian timescale. The majority of the known Martian meteorites are basalts (“shergottites”); all dated shergottites have mineral separate (Rb–Sr or Sm–Nd) ages of <600 Ma (Borg et al., 2005). Here we report a238U/206Pb age of 182.7±6.9 Ma by ion microprobe analysis of baddeleyite (ZrO2) grains in the Zagami shergottite. There is no correlation between discordancy and baddeleyite grain location relative to shock metamorphism. Mineral petrography demonstrates that baddeleyite is the result of late-stage igneous crystallization, and Raman spectroscopy shows that baddeleyite has not been transformed by shock into preservable high-pressure polymorphs. Supported by an age of 153±81 Ma for phosphate grains, obtained using the same method, we conclude that Zagami crystallized at ~180 Ma, in agreement with previous results from mineral separate geochronology. Therefore, the shergottites represent igneous rocks preferentially ejected from young terrains on Mars in a small number of ejection events.