Variable refractory lithophile element compositions of planetary building blocks: Insights from components of enstatite chondrites
Variable refractory lithophile element compositions of planetary building blocks: Insights from components of enstatite chondrites
复制标题
行星构建块的可变难熔亲石元素组成：来自顽火辉石球粒陨石成分的见解
DOI：
10.1016/j.gca.2021.05.057
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发表时间：
2021
影响因子：
5
通讯作者：
McDonough, William F.
中科院分区：
文献类型：
--
作者：
Yoshizaki, Takashi;Ash, Richard D.;Lipella, Marc D.;Yokoyama, Tetsuya;McDonough, William F.
球粒陨石是太阳系历史最早阶段遗留下来的物质沉积物。基于其未分化的性质和较少分离的化学成分，陨石被广泛认为是类地行星及其胚胎的未加工构件。类地行星的化学成分模型通常发现大块天体中难熔亲石元素（RLE）的相对丰度（“恒定RLE比规则”），基于陨石和太阳光球之间RLE比的有限变化。在这里，我们表明，RLE比，如Nb/Ta，Zr/Hf，Sm/Nd和Al/Ti，分馏从太阳值的球粒顽火辉石（EC）。个别EC球粒的分馏RLE比文件不同的亲硫亲和性的RLE高度还原环境和分离的RLE轴承硫化物硅酸盐球粒形成之前和/或期间。与此相反，散装EC有太阳一样的RLE比，表明物理分选的硅酸盐和硫化物是可以忽略不计的EC母体的吸积之前和期间。同样地，如果地球的吸积是由EC类物质主导的，正如多同位素系统学所支持的那样，吸积盘中的硅酸盐和硫化物的物理分选并没有发生。或者，地球的前身是高温星云冷凝物，形成之前，沉淀的RLE轴承硫化物。块状硅酸盐地球中缺乏Ti耗尽，再加上Nb和Ti类似的硅酸盐-硫化物和金红石-熔体分配行为，倾向于Nb的中度亲铁行为作为地球Nb耗尽的起源。经历了选择性的硅酸盐或金属/硫化物相的去除或吸积的高度还原的行星，如水星，可能会产生分馏，非太阳的整体RLE比。
Chondrites are sediments of materials left over from the earliest stage of the solar system history. Based on their undifferentiated nature and less fractionated chemical compositions, chondrites are widely considered to represent the unprocessed building blocks of the terrestrial planets and their embryos. Models of chemical composition of the terrestrial planets generally find chondritic relative abundances of refractory lithophile elements (RLE) in the bulk bodies (“constant RLE ratio rule”), based on limited variations of RLE ratios among chondritic meteorites and the solar photosphere. Here, we show that ratios of RLE, such as Nb/Ta, Zr/Hf, Sm/Nd and Al/Ti, are fractionated from the solar value in chondrules from enstatite chondrites (EC). The fractionated RLE ratios of individual EC chondrules document different chalcophile affinities of RLE under highly reducing environments and a separation of RLE-bearing sulfides from silicates before and/or during chondrule formation. In contrast, the bulk EC have solar-like RLE ratios, indicating that a physical sorting of silicates and sulfides was negligible before and during the accretion of EC parent bodies. Likewise, if the Earth’s accretion was dominated by EC-like materials, as supported by multiple isotope systematics, physical sorting of silicates and sulfides in the accretionary disk did not occur. Alternatively, the Earth’s precursors were high-temperature nebular condensates that formed prior to the precipitation of RLE-bearing sulfides. A lack of Ti depletion in the bulk silicate Earth, combined with similar silicate-sulfide and rutile-melt partitioning behaviors of Nb and Ti, prefers a moderately siderophile behavior of Nb as the origin of the accessible Earth’s Nb depletion. Highly reduced planets that have experienced selective removal or accretion of silicates or metal/sulfide phases, such as Mercury, possibly yield fractionated, non-solar bulk RLE ratios.