The Fe and Zn isotope composition of deep mantle source regions: Insights from Baffin Island picrites
The Fe and Zn isotope composition of deep mantle source regions: Insights from Baffin Island picrites
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DOI：
10.1016/j.gca.2018.07.021
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发表时间：
2018-10
影响因子：
5
通讯作者：
A. McCoy-West;J. Fitton;M. Pons;E. Inglis;H. Williams
中科院分区：
文献类型：
--
作者：
A. McCoy-West;J. Fitton;M. Pons;E. Inglis;H. Williams
来自加拿大东北部巴芬岛（Baffin Island）的年轻（61 Ma）未改变的苦橄岩拥有地球上最高的3 He/4 He（高达50 Ra），并提供了一个独特的机会来研究逃脱了随后化学修饰的原始地幔。这些高度部分熔体也记录了异常高的182 W/184 W比值，但它们的Sr-Nd-Hf-Pb同位素组成（包括142 Nd）与北大西洋洋中脊玄武岩没有区别。巴芬岛苦橄岩新的高精度Fe和Zn稳定同位素分析显示，δ 56 Fe变化范围为−0.03‰至0.13‰，δ 66 Zn变化范围为0.18‰至0.28‰。然而，在两组同位素数据中，在母体熔体的组成（MgO = 21重量%; δ 56 Fe = 0.08 ± 0.04‰;和δ 66 Zn = 0.24 ± 0.03‰）周围看到了明显的拐点，两个不同的趋势被解释为反映了低MgO样品中橄榄石和尖晶石的结晶和高MgO样品中橄榄石的积累。橄榄石矿物分离物的同位素明显轻于其对应的全岩（δ 56 Fe ≥ −0.62‰和δ 66 Zn ≥ −0.22‰），单个橄榄石斑晶的分析具有极其可变的Fe同位素组成（δ 56 Fe = −0.01‰至−0.80‰）。通过在三同位素空间进行模拟，我们表明，非常负的橄榄石斑晶的Fe同位素组成的动力学同位素分馏的结果，从不平衡扩散过程。δ 56 Fe和δ 66 Zn之间具有良好的相关性，表明在以岩浆橄榄石为主的简单体系中，Zn同位素的分馏过程与Fe相同。在岩浆演化过程中的铜的不兼容行为是一致的硫化物不饱和的性质，这些熔体。因此，锌的行为作为一个纯粹的亲石元素，估计散装地球锌同位素组成的基础上巴芬岛，因此应该是强大的。巴芬岛古未脱气下地幔的δ 56 Fe值与前人估算的整体地幔δ 56 Fe值误差不大，但我们估算的巴芬岛地幔δ 66 Zn值（0.20 ± 0.03‰）明显低于前人的估算值。我们的新数据与太古代和元古宙科马提岩的比较是一致的地幔的Fe和Zn同位素组成保持不变，从至少3 Ga到今天。通过关注大程度的部分熔融（如科马提岩和苦橄岩），我们可能会使我们的记录偏向于在上升过程中不可避免地与周围浅地幔相互作用、夹带和熔融的样品。对于一个主要元素，如铁，将继续参与熔融，因为它通过地幔上升，岩浆的最终同位素组成将是一个完整的熔融柱的加权平均值。因此，毫不奇怪，最小的铁同位素变化之间的地方。相反，巴芬岛苦橄岩显示的独特的地球化学特征（如He和W）被推断为仅源自最低地幔，并将在岩浆上升时不断稀释。
Young (61 Ma) unaltered picrites from Baffin Island, northeast Canada, possess some of the highest3He/4He (up to 50 Ra) seen on Earth, and provide a unique opportunity to study primordial mantle that has escaped subsequent chemical modification. These high-degree partial melts also record anomalously high182W/184W ratios, but their Sr-Nd-Hf-Pb isotopic compositions (including142Nd) are indistinguishable from those of North Atlantic mid-ocean ridge basalts. New high precision Fe and Zn stable isotope analyses of Baffin Island picrites show limited variability with δ56Fe ranging from −0.03‰ to 0.13‰ and δ66Zn varying from 0.18‰ to 0.28‰. However, a clear inflection is seen in both sets of isotope data around the composition of the parental melt (MgO = 21 wt%; δ56Fe = 0.08 ± 0.04‰; and δ66Zn = 0.24 ± 0.03‰), with two diverging trends interpreted to reflect the crystallisation of olivine and spinel in low-MgO samples and the accumulation of olivine at higher MgO. Olivine mineral separates are significantly isotopically lighter than their corresponding whole rocks (δ56Fe ≥ −0.62‰ and δ66Zn ≥ −0.22‰), with analyses of individual olivine phenocrysts having extremely variable Fe isotope compositions (δ56Fe = −0.01‰ to −0.80‰). By carrying out modelling in three-isotope space, we show that the very negative Fe isotope compositions of olivine phenocryst are the result of kinetic isotope fractionation from disequilibrium diffusional processes. An excellent correlation is observed between δ56Fe and δ66Zn, demonstrating that Zn isotopes are fractionated by the same processes as Fe in simple systems dominated by magmatic olivine. The incompatible behaviour of Cu during magmatic evolution is consistent with the sulfide-undersaturated nature of these melts. Consequently Zn behaves as a purely lithophile element, and estimates of the bulk Earth Zn isotope composition based on Baffin Island should therefore be robust. The ancient undegassed lower mantle sampled at Baffin Island possesses a δ56Fe value that is within error of previous estimates of bulk mantle δ56Fe, however, our estimate of the Baffin mantle δ66Zn (0.20 ± 0.03‰) is significantly lower than some previous estimates. Comparison of our new data with those for Archean and Proterozoic komatiites is consistent with the Fe and Zn isotope composition of the mantle remaining constant from at least 3 Ga to the present day. By focusing on large-degree partial melts (e.g. komatiites and picrites) we are potenitally biasing our record to samples that will inevitably have interacted with, entrained and melted the ambient shallow mantle during ascent. For a major element such as Fe, that will continuosly participate in melting as it rises through the mantle, the final isotopic compositon of the magama will be a weighted average of the complete melting column. Thus it is unsuprising that minimal Fe isotope variations are seen between localities. In contrast, the unique geochemical signatures (e.g. He and W) displayed by the Baffin Island picrites are inferred to solely originate from the lowermost mantle and will be continuously diluted upon magma ascent.