Geochemical constraints on the formation of chondrules: Implication from Os and Fe isotopes and HSE abundances in metals from CR chondrites
Geochemical constraints on the formation of chondrules: Implication from Os and Fe isotopes and HSE abundances in metals from CR chondrites
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球粒形成的地球化学限制：来自 CR 球粒陨石金属中 Os 和 Fe 同位素以及 HSE 丰度的影响
DOI：
10.1016/j.gca.2021.11.009
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发表时间：
2022
影响因子：
5
通讯作者：
Hirata Takafumi
中科院分区：
文献类型：
--
作者：
Nakanishi Nao;Yokoyama Tetsuya;Okabayashi Satoki;Iwamori Hikaru;Hirata Takafumi
由于与球粒共存的金属相的独特特征，CR陨石适合于理解金属和球粒之间的遗传联系。金属颗粒存在于CR球粒的三个不同位置：球粒内部（“内部颗粒”）、球粒表面壳（“边缘颗粒”）和基质（“孤立颗粒”）。利用飞秒LA-ICP-MS（fs LA-ICP-MS）和EPMA分析了3种铬铁矿（NWA 801、NWA 7184和Dhofar 1432）中3种金属（内部、边缘和孤立颗粒）的高亲铁元素（HSEs）和常量元素的丰度。此外，我们还利用微磨取样结合N-TIMS和MC-ICP-MS技术，对这些金属中的Os和Fe同位素组成进行了研究。CR金属的187 Os/188 Os和δ 57 Fe值分别在0.1193 ~ 0.1314和-1.05 ~+0.25之间变化。除Pd和Au外，三种金属元素的HSE丰度均随Ir丰度的增加而增加。颗粒内和颗粒间HSE丰度、颗粒内187 Os/188 Os值和颗粒间δ 57 Fe值的变化可能是液态金属从气态储层中冷凝后分离结晶的结果。大多数CR金属具有负的δ 57 Fe值，这表明金属相中的Fe可能是在硅酸盐相中的Fe凝聚之前通过凝聚形成的。铬榴石中的球粒和三种类型的金属颗粒被认为是在同一地区同时形成的。基质和复合球粒中存在的孤立金属可能是金属和硅酸盐液滴碰撞合并的结果。
CR chondrites are suitable for understanding the genetic linkage between metals and chondrules due to the unique characteristics of the coexisting metal phases with chondrules. Metal grains are found in three different locations of CR chondrites; chondrule interior (“interior grain”), chondrule surficial shells (“margin grain”), and the matrix (“isolated grain”). Here we report the abundances of highly siderophile elements (HSEs) and major elements in three types of metals (interior, margin, and isolated grains) from three CR chondrites (NWA 801, NWA 7184, and Dhofar 1432) by using femtosecond LA-ICP-MS (fs LA-ICP-MS) and EPMA. Additionally, we report the isotopic compositions of Os and Fe in the metals by using micro-milling sampling coupled with N-TIMS and MC-ICP-MS. The CR metals have variations in187Os/188Os and δ57Fe values ranging from 0.1193 to 0.1314 and from −1.05 to +0.25, respectively. HSE abundances, except for Pd and Au, in the three types of metals increase as the abundance of Ir increases. A possible explanation for the variations of HSE abundances within and among grains,187Os/188Os values within each grain, and δ57Fe values among grains, is the condensation of liquid metal from a gaseous reservoir followed by fractional crystallization. Most of the CR metals have negative δ57Fe values, suggesting that Fe in metal phases might have formed by condensation prior to Fe condensation in silicate phases. The chondrules and three types of metal grains in CR chondrites are believed to have formed contemporaneously in the same region. The existence of large isolated metals in matrix and compound chondrules might be the result of collision and merging of the metal and silicate droplets.