Iron isotopes in ancient and modern komatiites: Evidence in support of an oxidised mantle from Archean to present
Iron isotopes in ancient and modern komatiites: Evidence in support of an oxidised mantle from Archean to present
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DOI：
10.1016/j.epsl.2012.01.011
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发表时间：
2012-03-01
影响因子：
5.3
通讯作者：
Puchtel, I. S.
中科院分区：
文献类型：
--
作者：
Hibbert, K. E. J.;Williams, H. M.;Puchtel, I. S.
现代地球的地幔是相对氧化相比，最初推断为核心形成的还原条件。地幔氧化的时间并没有最终解决，但对水圈和大气层的发展时间有重要的影响。为了研究这一氧化事件的时间，我们提出了铁同位素数据，从三个异常保存完好的科马提岩的地方，Belingwe（2.7 Ga），Vetreny（2.4 Ga）和戈尔戈纳（0.089 Ga）。全岩样品的铁同位素组成的测量补充了橄榄石，尖晶石和辉石分离的分析。块状岩石和橄榄石铁同位素组成（δ Fe-57）定义明确的线性相关性与岩浆分异的指标（镁#，铬#）。2.7- 2.4Ga和0.089Ga样品的平均Fe同位素组成在统计学上是不同的，这种差异可以解释为更大程度的部分熔融所代表的老样品和更高的地幔环境温度在太古代和早元古代相对于今天。值得注意的是，所有年龄的样品定义连续的正线性关系之间的散装岩石三角洲Fe-57和V/Sc和三角洲Fe-57和V，和V/Sc和V与TiO 2之间的相关性，提供证据的V（相对于Sc）和同位素重铁的不相容行为。部分熔融模型计算使用分配系数为0和+1的还原氧逸度（fO（2）s）相对于铁橄榄石磁铁矿石英缓冲区（FMQ）的最佳匹配的数据阵列，这是由所有样本定义，从太古代晚期到第三纪。因此，这些数据为晚太古代以来在中等氧化条件下科马提岩的生成提供了证据，并反对地幔fO（2）的变化伴随着大气氧化作用在2.4 Ga左右。(C)2012爱思唯尔有限公司版权所有。
The mantle of the modem Earth is relatively oxidised compared to the initially reducing conditions inferred for core formation. The timing of the oxidation of the mantle is not conclusively resolved but has important implications for the timing of the development of the hydrosphere and atmosphere. In order to examine the timing of this oxidation event, we present iron isotope data from three exceptionally well preserved komatiite localities, Belingwe (2.7 Ga), Vetreny (2.4 Ga) and Gorgona (0.089 Ga). Measurements of Fe isotope compositions of whole-rock samples are complemented by the analysis of olivine, spinel and pyroxene separates. Bulk-rock and olivine Fe isotope compositions (delta Fe-57) define clear linear correlations with indicators of magmatic differentiation (Mg#, Cr#). The mean Fe isotope compositions of the 2.7-2.4 Ga and 0.089 Ga samples are statistically distinct and this difference can be explained by greater extent of partial melting represented by the older samples and higher mantle ambient temperatures in the Archean and early Proterozoic relative to the present day. Significantly, samples of all ages define continuous positive linear correlations between bulk rock delta Fe-57 and V/Sc and delta Fe-57 and V, and between V/Sc and V with TiO2, providing evidence for the incompatible behaviour of V (relative to Sc) and of isotopically heavy Fe. Partial melting models calculated using partition coefficients for Vat oxygen fugacities (fO(2)s) of 0 and +1 relative to the fayalite-magnetite-quartz buffer (FMQ) best match the data arrays, which are defined by all samples, from late Archean to Tertiary. These data, therefore, provide evidence for komatiite generation under moderately oxidising conditions since the late Archean, and argue against a change in mantle fO(2) concomitant with atmospheric oxygenation at similar to 2.4 Ga. (C) 2012 Elsevier B.V. All rights reserved.