The δ53Cr isotope composition of komatiite flows and implications for the composition of the bulk silicate Earth
The δ53Cr isotope composition of komatiite flows and implications for the composition of the bulk silicate Earth
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DOI：
10.1016/j.chemgeo.2020.119761
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发表时间：
2020-09
期刊：
影响因子：
3.9
通讯作者：
M. Jerram;P. Bonnand;A. Kerr;E. Nisbet;I. Puchtel;A. Halliday
中科院分区：
文献类型：
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作者：
M. Jerram;P. Bonnand;A. Kerr;E. Nisbet;I. Puchtel;A. Halliday
关于行星储层铬稳定同位素组成的数据有可能提供有关核心形成、部分熔化和月球形成条件的信息。为了探测太阳系中不同储层之间微小的铬同位素差异，需要对其组成进行精确的限制。目前δ53Cr = - 0.11±0.06‰（Sossi et al., 2018）的BSE值不能解决无球粒陨石（灶神星δ53Cr = - 0.17±0.05‰）和球粒陨石（碳质δ53Cr = - 0.12±0.05‰，普通δ53Cr = - 0.11±0.04‰）之间的差异。块状硅酸盐土（BSE）的组成通常用作与其他行星储层比较的参考点。然而，过去估计疯牛病的Cr同位素组成的尝试无法提供一个良好的约束疯牛病值。利用地幔橄榄岩的传统方法受到交代过程中Cr同位素分馏敏感性的影响。最近，除了地幔橄榄岩外，还使用科马铁矿计算了BSE的Cr同位素组成，以产生更精确的值（Sossi等人，2018）。为了更精确地限定BSE的组成，我们详细研究了来自3个地区、年龄在2.7 Ga ~ 89 Ma之间的非常新鲜的科马地岩熔岩流的δ53Cr。其中包括津巴布韦贝林圭绿岩带的托尼流，芬诺斯坎迪亚的维多利亚熔岩湖，以及哥伦比亚戈尔戈纳岛的科马提岩。在科马长岩中，发现了δ53Cr = - 0.16±0.02 ~ - 0.01±0.02‰的Cr同位素组成范围。研究表明，高程度的部分熔融作用虽然产生了科马提岩，但并没有导致科马提岩熔体与地幔残留物之间的Cr同位素分馏。然而，发现有限的Cr同位素分馏是科马马岩熔岩分异的结果。对于高Mg含量和高Cr2+/ƩCrTOT（Cr2+/(Cr2++ Cr3+)摩尔比）的熔岩流，如Tony’s Flow和Gorgona，在橄榄石分选过程中，岩浆演化部分δ53Cr增加，这是由于轻Cr优先包裹入橄榄石。其他MgO含量较低的流体没有表现出这种行为，因为橄榄石中含有较小比例的Cr。因此，在计算科马提岩熔岩来源的铬同位素组成时，必须考虑到分数结晶的影响。分析的科马铁矿熔岩δ53Cr的加权平均值为- 0.12±0.04‰（n = 5），代表了我们对BSE Cr同位素组成的最佳估计。它与以前的估计一致，同时改善了不确定性。这一数值与球粒陨石的数值没有明显的差别。我们的数据还表明，至少从太古宙开始，地幔的δ53Cr就一直是恒定的。
Data on the chromium stable isotope composition of planetary reservoirs have the potential to provide information about core formation, partial melting and conditions of the Moon formation. In order to detect the small Cr isotopic differences between various reservoirs in the solar system, their compositions need to be precisely constrained. The current BSE value of δ53Cr = −0.11 ± 0.06‰ (Sossi et al., 2018) cannot resolve differences between achondrites, (Vesta δ53Cr = −0.17 ± 0.05‰) and chondrites (carbonaceous δ53Cr = −0.12 ± 0.05‰; ordinary δ53Cr = −0.11 ± 0.04‰). The composition of the bulk silicate Earth (BSE) is often used as a reference point for comparisons to other planetary reservoirs. However, past attempts to estimate the Cr isotopic composition of the BSE have been unable to provide a well-constrained BSE value. Traditional methods, using mantle peridotites, are affected by the susceptibility of Cr isotopes to fractionation during metasomatism. More recently, the Cr isotope composition of the BSE has been calculated using komatiites, in addition to mantle peridotites, to produce a more precise value (Sossi et al., 2018). In order to constrain the BSE composition to a higher precision, the δ53Cr of remarkably fresh komatiite lava flows from three localities, ranging in age from 2.7 Ga to 89 Ma, have been investigated in detail. These included the Tony's Flow in the Belingwe Greenstone Belt, Zimbabwe, the Victoria's Lava Lake in Fennoscandia, and komatiites from Gorgona Island in Colombia.In the komatiites studied, a range in Cr isotopic compositions was found, from δ53Cr = −0.16 ± 0.02 to −0.01 ± 0.02‰. We show that the high degrees of partial melting that produced the komatiites, did not result in Cr isotopic fractionation between the komatiitic melt and mantle residue. However, limited Cr isotopic fractionation is found to be a consequence of komatiite lava differentiation. For the lava flows with high Mg content and high Cr2+/ƩCrTOT(the molar ratio of Cr2+/(Cr2++ Cr3+)), such as Tony's Flow and Gorgona, δ53Cr increases in the evolved portion of the magma during olivine fractionation due to the preferential inclusion of light Cr into olivine. Other flows with lower MgO content do not show this behaviour because a smaller fraction of the Cr is contained in olivine. The effects of fractional crystallisation must, therefore, be taken into account when calculating the Cr isotopic composition of the source of komatiite lavas.The weighted average of δ53Cr for the komatiite lavas analysed is −0.12 ± 0.04‰ (n = 5) and represents our best estimate for the Cr isotopic composition of the BSE. It agrees with the previous estimates, while providing an improvement to the uncertainty. There is no resolvable difference between this value and that of chondritic meteorites. Our data also indicate that the δ53Cr of the mantle has been constant since at least the Archean.