Refractory element fractionation in the Allende meteorite: Implications for solar nebula condensation and the chondritic composition of planetary bodies
Refractory element fractionation in the Allende meteorite: Implications for solar nebula condensation and the chondritic composition of planetary bodies
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DOI：
10.1016/j.gca.2012.02.006
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发表时间：
2012-05
影响因子：
5
通讯作者：
A. Stracke;H. Palme;M. Gellissen;C. Münker;T. Kleine;Karin Birbaum;D. Günther;B. Bourdon;J. Zipfel
中科院分区：
文献类型：
--
作者：
A. Stracke;H. Palme;M. Gellissen;C. Münker;T. Kleine;Karin Birbaum;D. Günther;B. Bourdon;J. Zipfel
球粒陨石代表着原始的未分化的太阳系物质，在成分上类似于太阳的不挥发部分。然而，球粒陨石的矿物学和结构是复杂的，因为它们是在太阳星云中不同条件下形成的几种成分的混合物，并在其母体上进一步加工：球粒陨石，一种挥发性丰富的细粒基质，包括各种矿物和岩屑碎屑、金属、硫化物和富钙铝包裹体(CaI)。因此，单一等分的球粒陨石的块体化学成分取决于其成分的大小和分布。本文采用直径22.5 cm~2、厚4 mm的30g切片，研究了样品不均质性对CV球粒陨石Allende主量元素和微量元素组成的影响。将39个平均样品重量约为0.6g(相当于一个边长为5~6 mm的立方体)的相同大小的样品粉碎，并对0.12g和0.02-0.03g的等量样品进行X射线荧光光谱分析和电感耦合等离子体质谱(ICP-MS)的微量元素分析。一个样本含有较大的CAI，另一个样本以暗包涵体(DI)为主。除去这两个样品，主要元素镁、硅和铁的浓度在毫米厘米级的分析不确定度范围内是恒定的(标准偏差分别为0.9、1.3和2.6%)。非难熔的次要元素和微量元素同样是恒定不变的，包括不同的元素，如锰、铬、镍、钴、磷、锌和铅。这反映了这些元素在吸积过程中各种主相的均匀混合，排除了母体上水蚀变和/或热变质作用在毫米级以上的元素重新分布。难熔元素Al、Ca、Ti等变化较大(分别为S.D.17%、10%和9%)，这主要是由于毫米级CaI的不同比例所致，其中许多元素具有强烈的II族稀土分馏模式，即较易挥发的难熔元素(Ta、U、Nb、Sr、Tm、Nd)比强难熔元素(Lu、Zr、Hf)具有不同的富集性。II组CaI的混合也可以解释CV球粒陨石中的亚软骨质Nb/Ta和Zr/Nb比值。所有37个样品的总平均值具有明显的II型稀土元素配分模式。如果这种分离的难熔元素模式代表了阿连德母体，那么这一观测结果表明，可能包括地球形成的行星胚胎在内的大宗行星体可能具有相对于CI球粒陨石的难熔元素模式。
Chondritic meteorites represent primitive undifferentiated solar system material that is compositionally similar to the non-volatile fraction of the Sun. The mineralogy and texture of chondritic meteorites is complex, however, because they are mixtures of several components that formed under different conditions in the solar nebula and were further processed on their parent bodies: chondrules, a volatile rich, fine-grained matrix, including a variety of mineral and lithic clasts, metal, sulfides, and Ca, Al-rich inclusions (CAI). The bulk chemistry of a single aliquot of a chondritic meteorite consequently depends on the size and distribution of its constituents. Here, we investigate the effect of sample heterogeneity on the major and trace element composition of the CV chondrite Allende using a single 30g slice, which is 22.5cm2in dimension and 4mm thick. Thirty-nine equally sized pieces with an average sample weight of ca. 0.6g (corresponding to a cube with an edge length of 5 to 6mm) were powdered and aliquots of 0.12g and 0.02–0.03g were analyzed by XRF for major and ICP-MS for trace elements. One sample contained a large CAI, another sample was dominated by a dark inclusion (DI). Excluding these two samples, the concentrations of the major elements Mg, Si and Fe are constant within analytical uncertainty at the millimeter-centimeter scale (S.D. 0.9, 1.3 and 2.6%, respectively). Non-refractory minor and trace elements are similarly constant, including geochemically very different elements such as Mn, Cr, Ni, Co, P, Zn and Pb. This reflects a uniform mixture of the various host phases of these elements during accretion, and excludes elemental redistribution above a millimeter-scale by aqueous alteration and/or thermal metamorphism on the parent body. The refractory elements Al, Ca, Ti etc. are more variable (S.D. 17, 10 and 9%, respectively), which is mainly the result of different proportions of millimeter-size CAI, many of them with strongly fractionated group II rare earth element patterns, i.e., variable enrichment of the more volatile refractory elements (Ta, U, Nb, Sr, Tm, Nd) over the strongly refractory elements (Lu, Zr, Hf). Admixture of group II CAI can also account for the sub-chondritic Nb/Ta and Zr/Nb ratios in CV chondrites. The total average of all 37 samples has a clear group II-type rare earth element pattern. If this fractionated refractory element pattern is representative of the Allende parent body, this observation suggests that bulk planetary bodies, possibly including the Earth-forming planetary embryos, may have refractory element patterns that are fractionated relative to those of CI chondrites.