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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >The duration of ordinary chondrite metamorphism inferred from tungsten microdistribution in metal
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The duration of ordinary chondrite metamorphism inferred from tungsten microdistribution in metal

机译：从钨在金属中的微观分布推断普通球粒陨石变质的持续时间

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摘要

Precise ratios of W and Re relative to Ir determined by laser ablation ICP-MS reveal that matrix metal in equilibrated ordinary chondrites (ECO) exhibits a correlated variation of these ratios. In unequilibrated ordinary chondrites (UOC), the W/Ir ratio varies by over two orders of magnitude (W/Ir = 0.03-0.6), and shows no correlation with any other siderophile element. most matrix metal in UOC is depleted in W relative to metal metal in EOC. Thus, W must enter metal during metamorphism. The correlation between W/Ir and Re/Ir is evident in type 4 chondrites. THis implies that the abundance (and isotopic composition) of W is set in ordinary chondrite metal by petrologic type 4, and that the reductant must be exhausted to limit further isotopic exchange, as indicated by the ~(182)Hf-~(182)W ages of ordinary chondrite metal [Lee and Halliday, Science 274 (1996) 1876-1879]. This offers a means of dating the onset of metamorphic heating of OC parent bodies. A discrepancy in ages between ~(182)Hf-~(182)W [Lee and Halliday, Science 274 (1996) 1876-1879] and ~(207)Pb-~(206)Pb [Gopel et al., Earth Planet. Sci. Lett. 121 (1994) 153-171] (or ~(129)I-~(129)Xe [Brazzle et al., Geochim. Cosmochim. Acta 63 (1999) 739-760]) dating is real, and measure the interval of metamorphism: the time difference between reduction of W during the onset of heating and isotopic closure of Pb (or Xe) by cooling, e.g., ΔT_m = 2-12 Myr for H chondrites. The ~(187)Re-~(187)Os and ~(182)Hf-~(182)W ages are set by the same process, metamorphic equilibration, establishing an important link between an absolute chronometer (t_(1/2)[~(187)Re] = 41.6 billion years) and a short-lived radioisotope system (t_(1/2)[~(182)Hf] = 9 Myr).
机译：通过激光烧蚀ICP-MS确定的相对于Ir的W和Re的精确比值表明，平衡普通球粒晶体（ECO）中的基质金属表现出这些比值的相关变化。在未平衡的普通球粒陨石（UOC）中，W / Ir之比变化超过两个数量级（W / Ir = 0.03-0.6），并且与任何其他嗜铁元素无关。相对于EOC中的金属金属，UOC中的大多数基质金属的W都被耗尽。因此，W在变质过程中必须进入金属。 W / Ir和Re / Ir之间的相关性在4型球粒陨石中很明显。这表示岩石类型4在普通球粒陨石金属中设定了W的丰度（和同位素组成），并且必须用尽还原剂以限制进一步的同位素交换，如〜（182）Hf-〜（182）所示。普通球粒陨石的年龄[Lee and Halliday，Science 274（1996）1876-1879]。这提供了一种方法，可以确定OC母体变质加热的发生。 〜（182）Hf-〜（182）W [Lee and Halliday，Science 274（1996）1876-1879]和〜（207）Pb-〜（206）Pb [Gopel等人，地球行星]之间的年龄差异。科学来吧121（1994）153-171]（或〜（129）I-〜（129）Xe [Brazzle et al。，Geochim。Cosmochim。Acta 63（1999）739-760]）约会是真实的，并测量变质作用：加热开始时W的减少与冷却导致Pb（或Xe）的同位素封闭之间的时间差，例如H球粒陨石的ΔT_m= 2-12 Myr。 〜（187）Re-〜（187）Os和〜（182）Hf-〜（182）W年龄是通过相同的过程，变态平衡设定的，从而在绝对计时码表（t_（1/2） [〜（187）Re] = 416亿年）和寿命短的放射性同位素系统（t_（1/2）[〜（182）Hf] = 9 Myr）。

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