Earth's moderately volatile element composition may not be chondritic: Evidence from In, Cd and Zn
Earth's moderately volatile element composition may not be chondritic: Evidence from In, Cd and Zn
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DOI：
10.1016/j.epsl.2015.12.012
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发表时间：
2016-02-01
影响因子：
5.3
通讯作者：
Becker, Harry
中科院分区：
文献类型：
--
作者：
Wang, Zaicong;Laurenz, Vera;Becker, Harry
目前的模型假设，亲铁挥发性元素（WE）在散装地球耗尽到相同程度的亲石元素的类似的挥发性。所观察到的许多SVE相对于散装硅酸盐地球（BSE）中的亲石元素的额外损耗归因于分配到地球的核心WE。然而，地球形成过程中中等挥发性元素的挥发性与太阳气体相似的假设是相当不确定的。在这里，这些假设将进行测试，通过评估丰度和比值的铟和镉在BSE使用新的地幔岩石数据，并应用高，低压力温度的金属硅酸盐分区数据。通过同位素稀释法获得的橄榄岩构造岩和捕虏体的大块岩石中In和Cd的新的大块岩石丰度数据完善了以前从玄武岩和橄榄岩捕虏体中硅酸盐矿物的原位分析推断的结果。BSE中In的Cl-辉铜矿标准化丰度与锌相似，并且比Cd高3-4倍。新的和已发表的低和高P-T金属硅酸盐分区数据表明，在一系列条件下的核心形成过程中，在总是更多的亲铁体比锌和镉。添加这些元素在地球的核心的BSE的结果在散装地球组合物，产生更高的CI球粒陨石归一化丰度在散装地球相比，锌和镉的分数。由于In在太阳组成的气体中比Zn和Cd更易挥发，因此大块地球中的粒晶上In/Zn和In/Cd表明，在地球或其建筑材料的形成过程中，这些元素的挥发性以及其他挥发性元素可能发生了显着变化（即In变得不那么挥发）。结果还表明，已知的碳质碳酸盐可能没有提供地球的主要挥发性元素丰富的部分。各种论点认为，在行星吸积过程中，中等挥发性元素的损失应该是有限的，因此，它们在地球主体中的丰度可能反映了地球建筑材料的平均组成。结合来自核合成同位素异常的证据，这些数据表明，地球的主要建筑材料来自太阳系内部的隔间，其中挥发性元素丰度的演变与已知陨石的形成区域不同。具有非磁性挥发性元素组成的物质可能已经被用于建造类地行星。(C)2015 Elsevier B. V.版权所有。
Current models assume that siderophile volatile elements (WE) are depleted in bulk Earth to the same extent as lithophile elements of similar volatility. The observed additional depletion of many SVE relative to lithophile elements in the bulk silicate Earth (BSE) is ascribed to partitioning of WE into Earth's core. However, the assumption of similar volatility of moderately volatile elements during Earth formation processes as in solar gas is quite uncertain. Here, these assumptions will be tested by assessing abundances and ratios of indium and cadmium in the BSE using new data on mantle rocks, and the application of high- and low-pressure-temperature metal-silicate partitioning data. New bulk rock abundance data of In and Cd obtained on bulk rocks of peridotite tectonites and xenoliths by isotope dilution refine previous results inferred from basalts and in-situ analyses of silicate minerals in peridotite xenoliths. The CI chondrite-normalized abundance of In in the BSE is similar to zinc and is 3-4 times higher than Cd. New and published low- and high-P-T metal-silicate partitioning data indicate that, during core formation at a range of conditions, In is always more siderophile than Zn and Cd. Adding the fraction of these elements in Earth's core to the BSE results in bulk Earth compositions that yield higher CI chondrite normalized abundances of In in the bulk Earth compared to Zn and Cd. Because In is more volatile than Zn and Cd in gas of solar composition, suprachondritic In/Zn and In/Cd in the bulk Earth suggest that during formation of Earth or its building materials, the volatilities of these elements and perhaps other volatile elements likely have changed significantly (i.e. In became less volatile). The results also suggest that known carbonaceous chondrites likely did not deliver the main volatile element rich fraction of the Earth. Various arguments suggest that the loss of moderately volatile elements during planetary accretion should be limited, thus, their abundances in the bulk Earth likely reflect the average composition of Earth's building materials. Combined with evidence from nucleosynthetic isotope anomalies, the data suggest that Earth's main building materials originated from compartments of the inner solar system where volatile element abundances evolved differently from the formation area of known chondrites. The materials with nonchondritic volatile element composition may have been used up for building the terrestrial planets. (C) 2015 Elsevier B.V. All rights reserved.