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首页> 外文期刊>Geochimica et Cosmochimica Acta: Journal of the Geochemical Society and the Meteoritical Society >A general moment NRIXS approach to the determination of equilibrium Fe isotopic fractionation factors: Application to goethite and jarosite
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A general moment NRIXS approach to the determination of equilibrium Fe isotopic fractionation factors: Application to goethite and jarosite

机译：确定矩铁同位素分馏因子的通用矩NRIXS方法：在针铁矿和黄钾铁矾中的应用

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摘要

The equilibrium Fe isotopic fractionation factors of goethite and jarosite have considerable importance for interpreting Fe isotope variations in low temperature aqueous systems on Earth and possibly Mars in the context of future sample return missions. We measured the β-factors of goethite FeO(OH), potassium-jarosite KFe _3(SO _4) _2(OH) _6, and hydronium-jarosite (H _3O)Fe _3(SO _4) _2(OH) _6, by Nuclear Resonant Inelastic X-ray Scattering (NRIXS, also known as Nuclear Resonance Vibrational Spectroscopy - NRVS or Nuclear Inelastic Scattering - NIS) at the Advanced Photon Source. These measurements were made on synthetic minerals enriched in ~(57)Fe. A new method (i.e., the general moment approach) is presented to calculate β-factors from the moments of the NRIXS spectrum S(E). The first term in the moment expansion controls iron isotopic fractionation at high temperature and corresponds to the mean force constant of the iron bonds, a quantity that is readily measured and often reported in NRIXS studies. The mean force constants of goethite, potassium-jarosite, and hydronium-jarosite are 314±14, 264±12, and 310±14N/m, respectively (uncertainties include statistical and systematic errors). The general moment approach gives ~(56)Fe/ ~(54)Fe β-factors of 9.7, 8.3, and 9.5‰ at 22°C for these minerals. The β-factor of goethite measured by NRIXS is larger than that estimated by combining results from laboratory exchange experiments and calculations based on electronic structure theory. Similar issues have been identified previously for other pairs of mineral-aqueous species, which could reflect inadequacies of approaches based on electronic structure theory to calculate absolute β-factors (differences in β-factors between aqueous species may be more accurate) or failure of laboratory experiments to measure mineral-fluid equilibrium isotopic fractionation at low temperature. We apply the force constant approach to published NRIXS data and report 1000×lnβ for important Fe-bearing phases of geological and biochemical relevance such as myoglobin, cytochrome f, pyroxene, metal, troilite, chalcopyrite, hematite, and magnetite.
机译：针铁矿和黄钾铁矾的平衡Fe同位素分馏因子对于解释未来样品返回任务中地球低温水系统（可能还有火星）中的Fe同位素变化具有重要意义。我们通过核能法测量了针铁矿FeO（OH），钾铁矾KFe _3（SO _4）_2（OH）_6和水合锂铁矾（H _3O）Fe _3（SO _4）_2（OH）_6的β因子。高级光子源处的共振非弹性X射线散射（NRIXS，也称为核共振振动光谱-NRVS或核非弹性散射-NIS）。这些测量是对富含〜（57）Fe的合成矿物进行的。提出了一种新方法（即一般矩方法），以根据NRIXS频谱S（E）的矩计算β因子。瞬间膨胀中的第一项控制高温下的铁同位素分馏，并且对应于铁键的平均力常数，该量很容易测量，并且经常在NRIXS研究中报道。针铁矿，钾铁矾和钾铁矾的平均力常数分别为314±14、264±12和310±14N / m（不确定性包括统计误差和系统误差）。对于这些矿物，一般矩量法在22°C下得出〜（56）Fe /〜（54）Feβ因子为9.7、8.3和9.5‰。通过NRIXS测量的针铁矿的β因子大于通过结合实验室交换实验和基于电子结构理论的计算得出的估计值。对于其他成对的矿物质，以前也已经发现了类似的问题，这可能反映了基于电子结构理论来计算绝对β因子的方法的不足（含水物种之间β因子的差异可能更准确）或实验室的失败。测量低温下矿物流体平衡同位素分馏的实验。我们将力常数方法应用于已发布的NRIXS数据，并报告了1000×lnβ的地质和生化相关的重要含铁阶段，如肌红蛋白，细胞色素f，辉石，金属，三叶草，黄铜矿，赤铁矿和磁铁矿。

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