Systematic iron isotope variations in mantle rocks and minerals: The effects of partial melting and oxygen fugacity
Systematic iron isotope variations in mantle rocks and minerals: The effects of partial melting and oxygen fugacity
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DOI：
10.1016/j.epsl.2005.04.020
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发表时间：
2005-06
影响因子：
5.3
通讯作者：
H. Williams;A. Peslier;C. McCammon;A. Halliday;S. Levasseur;N. Teutsch;J. Burg
中科院分区：
文献类型：
--
作者：
H. Williams;A. Peslier;C. McCammon;A. Halliday;S. Levasseur;N. Teutsch;J. Burg
铁同位素组成可能提供了一个强大的新的示踪剂的行星形成和分化过程和长期和空间变化的地幔氧化状态。然而，在火成岩中控制铁同位素分馏的过程仍然知之甚少。地幔岩石和矿物（橄榄石0.6‰、单斜辉石0.9 ‰和正斜辉石0.8‰）的铁同位素组成（δ57/54 Fe）变化显著，其中尖晶石的变化最大，为1.7‰。共存的斜方辉石、单斜辉石和橄榄石的δ57/54 Fe值之间存在正线性函数关系，斜率在误差范围内等于1，强烈表明这些矿物的δ57/54 Fe值反映了样品内矿物-矿物同位素平衡。硅酸盐矿物和尖晶石的δ57/54 Fe值之间也存在正相关关系，但分布较分散，这可能是矿物-尖晶石同位素平衡的后期扰动所致。块状岩石、单斜辉石和尖晶石δ57/54 Fe值与熔体萃取和氧化的化学指标相关。利用简单的模型研究了尖晶石相部分熔融过程中的铁同位素分馏，结果表明，熔融物和残渣之间的最大预期分馏将为10.5 ‰，残渣相对于熔融物和初始源区变得同位素轻。因此，熔体萃取，结合地幔氧化态的显着变化，可能是一个解释地幔橄榄岩的Fe同位素变化。来自俯冲板片的富铁硅酸盐熔体对弧下地幔的交代作用也可以解释某些弧橄榄岩的轻δ57/54 Fe值（−0.2‰至−0.6‰），但质量平衡计算要求这些交代剂具有极端的δ57/54 Fe值（例如−3.0‰）。石榴子石相和尖晶石相岩石δ57/54 Fe值的巨大差异可能是由于尖晶石相和石榴子石相熔融过程中Fe 3+的对比行为造成的。然而，尽管根据不相容元素丰度，认为OIB主要是由石榴石稳定场中的熔融产生的，但MORB和OIB的δ57/54 Fe值差别不大。由于铁是一种相对相容的元素，MORB和OIB的δ57/54 Fe值的相似性提供了强有力的证据，表明MORB和OIB都是以尖晶石相熔融为主。
Iron isotopic compositions potentially provide a powerful new tracer of planetary formation and differentiation processes and of secular and spatial changes in mantle oxidation state. However, the processes governing iron isotope fractionation in igneous rocks remain poorly understood. Here we show that there are significant variations in the iron isotope compositions (δ57/54Fe) of mantle rocks (0.9‰) and minerals (olivines 0.6‰, clinopyroxenes 0.9‰ and orthopyroxenes 0.8‰), with spinels showing the greatest total variation of 1.7‰. Positive linear functional relationships with slopes that are, within error, equal to unity are found between the δ57/54Fe values of coexisting orthopyroxene, clinopyroxene and olivine, strongly suggesting that the δ57/54Fe values of these minerals reflect intra-sample mineral–mineral isotopic equilibrium. Positive correlations between the δ57/54Fe values of silicate minerals and spinels also exist, although they are more scattered, which could be caused by late disturbance of mineral-spinel isotopic equilibrium. Bulk-rock, clinopyroxene and spinel δ57/54Fe values correlate with chemical indices of both melt extraction and oxidation. Iron isotope fractionation during spinel-facies partial melting is investigated using simple models, which demonstrate that the maximum expected fractionation between melt and residue will be ∼0.5‰, with the residue becoming isotopically light relative to the melt and to the initial source region. Hence melt extraction, in combination with significant changes in mantle oxidation state, may be an explanation for Fe isotopic variations in mantle peridotites. Metasomatism of the sub-arc mantle by iron-rich silicate melts originating from the subducting slab may also explain the light bulk-sample δ57/54Fe values of some arc peridotites (−0.2‰ to −0.6‰), but mass-balance calculations require these metasomatic agents to have extreme δ57/54Fe values (e.g. −3.0‰). The large differences in the δ57/54Fe values of garnet and spinel facies rocks are likely to be caused by the contrasting behaviour of Fe3+during melting in the spinel and garnet facies. However, there is little difference in the δ57/54Fe values of MORB and OIB, despite the fact that OIB are considered, on the basis of incompatible element abundances, to arise dominantly by melting in the garnet stability field. Given that iron is a relatively compatible element, the similarities in the δ57/54Fe values of MORB and OIB provide strong evidence that MORB and OIB are both dominated by melting in the spinel facies.