Trace element composition and U-Pb age of zircons from Estherville: Constraints on the timing of the metal-silicate mixing event on the mesosiderite parent body
Trace element composition and U-Pb age of zircons from Estherville: Constraints on the timing of the metal-silicate mixing event on the mesosiderite parent body
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埃斯特维尔锆石的微量元素组成和 U-Pb 年龄：对中菱铁矿母体金属硅酸盐混合事件时间的限制
DOI：
10.1016/j.gca.2017.07.028
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发表时间：
2017
影响因子：
5
通讯作者：
Hidaka Hiroshi
中科院分区：
文献类型：
--
作者：
Haba Makiko K.;Yamaguchi Akira;Kagi Hiroyuki;Nagao Keisuke;Hidaka Hiroshi
中辉石陨石是一组石铁陨石，被认为是硅酸盐与铁镍金属混合的结果。在这项研究中，我们将Estherville中黄铁矿中发现的两颗锆石颗粒的结构观测与地球化学和年代学研究相结合。其中锆石（Zrc1）与辉石、斜长石、三硝石、二氧化硅共生，锆石（Zrc2）位于铁镍金属与辉石、斜长石为主的硅酸盐部分交界处。织构观察表明，Zrc1是相对均匀的，而Zrc2则由至少两个不同的化学结构域组成。Zrc2的微量元素分析表明，在这一单一颗粒中，稀土元素（REE）的浓度梯度很大，变化幅度为一个数量级，U和Th的变化幅度为两个数量级。Zrc2中微量元素的最低浓度比月球锆石和欧陆锆石低一个数量级以上。然而，它与Zrc1和Vaca Muerta中黄铁矿中的锆石相似。与Zrc2平衡熔体的稀土元素组成表明，Zrc2和Zrc1并不是由原生岩浆矿物组合分馏结晶产生的熔体结晶。低REE、低U、低Th的锆石可以解释为在金属-硅酸盐混合过程中，大量REE、U、Th混入次生磷矿物后形成的残余熔体。在Zrc2中观察到的大浓度梯度表明锆英结晶的熔体具有明显的非均质性。或者，残留锆石和新形成的锆石之间的混合或扩散可以解释观察到的浓度梯度。然而，Zrc2的REE模式不能用两代不同锆石的混合或扩散来解释。综上所述，Zrc1和Zrc2是在高温再加热事件中形成的，可能与金属硅酸盐混合事件有关。SIMS加权平均207pb - 206pb年龄为4521±26 Ma （2σ）。该年龄小于Muerta Vaca原生岩浆锆石的年龄（4563±15 Ma），可能对应于金属硅酸盐混合事件或较晚的撞击事件。
Mesosiderites are a group of stony-iron meteorites, which are thought to be the result of mixing of silicates with Fe-Ni metal. In this study, we combined textural observations with geochemical and chronological studies of two zircon grains found in the Estherville mesosiderite. One of the zircons (Zrc1) occurs with pyroxene, plagioclase, troilite, and silica, and the other (Zrc2) is located at a boundary between Fe-Ni metal and a silicate part mainly composed of pyroxene and plagioclase. The textural observations demonstrate that Zrc1 is relatively homogenous, whereas Zrc2 is composed of at least two chemically distinct domains. Trace element analyses of Zrc2 resolve large concentration gradients within this single grain with variations that are an order of magnitude for rare earth elements (REE) and two orders of magnitude for U and Th. The lowest trace element concentration in Zrc2 is more than an order of magnitude lower than those of lunar and eucritic zircons. However, it is similar to those of Zrc1 and a zircon from the Vaca Muerta mesosiderite. The calculated REE composition of the melt in equilibrium with Zrc2 shows that Zrc2 and perhaps also Zrc1 did not crystallize from a melt that was produced by fractional crystallization of the primary magmatic mineral assemblages. The zircons with low REE, U, and Th concentrations can be interpreted to have formed in a residual melt after incorporation of large amounts of REE, U, and Th into secondary phosphate minerals, which formed during the metal-silicate mixing event. The large concentration gradients observed in Zrc2 suggest significant heterogeneities in the melt from which the zircon crystallized. Alternatively, either mixing or diffusion between a relict zircon and a newly formed zircon could explain the observed concentration gradients. However, the REE patterns of Zrc2 cannot be explained by mixing or diffusion between the two distinct generations of zircons. These considerations suggest that Zrc1 and Zrc2 formed during a high-temperature reheating event, which is probably related to the metal-silicate mixing event. The weighted average207Pb-206Pb age obtained by SIMS from both zircons is 4521 ± 26 Ma (2σ). This age is younger than that of a primary magmatic zircon from Vaca Muerta (4563 ± 15 Ma) and probably corresponds to the timing of the metal-silicate mixing event or a later impact event.
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