Iron isotope fractionation during planetary differentiation
Iron isotope fractionation during planetary differentiation
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DOI：
10.1016/j.epsl.2005.09.023
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发表时间：
2005-12
影响因子：
5.3
通讯作者：
S. Weyer;A. Anbar;G. Brey;C. Münker;K. Mezger;A. Woodland
中科院分区：
文献类型：
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作者：
S. Weyer;A. Anbar;G. Brey;C. Münker;K. Mezger;A. Woodland
采用高质量分辨率MC-ICP-MS测量了月球、火星（SNC陨石）、HED母体（长绿长岩）、pallasites（金属和硅酸盐）和地幔样品的铁同位素组成。这些高精度测量（δ56Fe≈±0.04‰，2 S.D.）对行星分异过程中Fe同位素分馏有严格的限制。在行星核形成过程中，钯石体金属（包括硫化物和磷化物）和橄榄石分离物的铁同位素组成难以区分，δ56Fe的分馏限制在<0.1‰。然而，在钯岩金属分离物中，同位素变化较大（≈0.5‰），并随三黄石、晶伯石、卡玛长石和带长石含量的变化而系统变化。Troilite通常具有最轻（δ56Fe≈−0.25‰）和最重（δ56Fe≈+0.2‰）的铁同位素组成。带长石比镁长石重。因此，这些变化可能反映了富S和富p金属熔体演化和分异后期，以及低温卡玛石溶出过程中铁同位素的分异，而不是硅酸盐-金属分离过程中的分异。行星硅酸盐部分的分化似乎也使铁同位素分馏。值得注意的是，岩浆岩（部分熔体）在系统同位素上比它们的地幔原岩重。不同构造背景下11个陆相橄榄岩样品的δ56Fe=+0.015±0.018‰，明显轻于陆相玄武岩样品的δ56Fe=+0.076±0.029‰。我们认为橄榄岩平均值是硅酸盐大地的铁同位素组成的最佳估计，也可能是大地的铁同位素组成的最佳估计。除高钛玄武岩外，陆相玄武岩平均值与月球样品的平均值（δ56Fe=+0.073±0.019‰）吻合较好。高钛玄武岩的铁同位素组成最重（δ56Fe≈+0.2‰）。这被解释为月球岩浆海洋的指纹，月球岩浆海洋产生了一个非常不均匀的地幔，包括这些玄武岩富含钛铁矿的来源区域。在不确定度范围内，来自火星（SNC陨石）、HED（长绿长石）和pallasites（平均橄榄石+金属）的样品具有与地球地幔相同的铁同位素组成。这表明太阳系的铁同位素是非常均匀的。它的平均δ56Fe非常接近IRMM-014的标准。
The Fe isotope composition of samples from the Moon, Mars (SNC meteorites), HED parent body (eucrites), pallasites (metal and silicate) and the Earth's mantle were measured using high mass resolution MC-ICP-MS. These high precision measurements (δ56Fe≈±0.04‰, 2 S.D.) place tight constraints on Fe isotope fractionation during planetary differentiation. Fractionation during planetary core formation is confined to <0.1‰ for δ56Fe by the indistinguishable Fe isotope composition of pallasite bulk metal (including sulfides and phosphides) and olivine separates. However, large isotopic variations (≈0.5‰) were observed among pallasite metal separates, varying systematically with the amounts of troilite, schreibersite, kamacite and taenite. Troilite generally has the lightest (δ56Fe≈−0.25‰) and schreibersite the heaviest (δ56Fe≈+0.2‰) Fe isotope composition. Taenite is heavier then kamacite. Therefore, these variations probably reflect Fe isotope fractionation during the late stage evolution and differentiation of the S- and P-rich metal melts, and during low-temperature kamacite exsolution, rather than fractionation during silicate–metal separation. Differentiation of the silicate portion of planets also seems to fractionate Fe isotopes. Notably, magmatic rocks (partial melts) are systematically isotopically heavier than their mantle protoliths. This is indicated by the mean of 11 terrestrial peridotite samples from different tectonic settings (δ56Fe=+0.015±0.018‰), which is significantly lighter than the mean of terrestrial basalts (δ56Fe=+0.076±0.029‰). We consider the peridotite mean to be the best estimate for the Fe isotope composition of the bulk silicate Earth, and probably also of bulk Earth. The terrestrial basaltic mean is in good agreement with the mean of the lunar samples (δ56Fe=+0.073±0.019‰), excluding the high-Ti basalts. The high-Ti basalts display the heaviest Fe isotope composition of all rocks measured here (δ56Fe≈+0.2‰). This is interpreted as a fingerprint of the lunar magma ocean, which produced a very heterogeneous mantle, including the ilmenite-rich source regions of these basalts. Within uncertainties, samples from Mars (SNC meteorites), HED (eucrites) and the pallasites (average olivine+metal) have the same Fe isotope compositions as the Earth's mantle. This indicates that the solar system is very homogeneous in Fe isotopes. Its average δ56Fe is very close to that of the IRMM-014 standard.