An experimental investigation of the partitioning of iron isotopes between silicate melt and spinel as a function of oxygen fugacity

铁（Fe）是地幔中最常见的元素，以多种氧化态（如Fe2+和Fe3+）出现。它也有四种同位素（54Fe, 56Fe， 57Fe和58Fe）。直到最近才发现，这些同位素可能在高温地幔过程（如部分熔融）中被分馏。特别是地幔尖晶石表现出同位素分馏与氧逸度的相关性，它控制着Fe3+和Fe2+的相对含量。相对于地幔交代作用等其他过程，氧逸度在控制这种分馏中的重要性尚不清楚。目前还没有实验证据可以独立证明Fe3+/Fe2+对岩浆温度下尖晶石中Fe同位素分馏的影响。在这项工作中，我们建议从熔体中结晶一系列尖晶石，作为氧逸度的函数。样品将在大气压下，在受控的气体环境中，通过在液相中平衡尖晶石的熔体来制备。淬火后，将尖晶石和熔体分离，并通过Mössbauer光谱分析测定Fe3+/Fe2+，并用高分辨率多收集器电感耦合等离子体质谱仪测定Fe同位素比值。这将使氧逸度、Fe3+/Fe2+比值和同位素分馏直接相关。如果要正确解释自然样品铁同位素测量值的变化，这类实验是必不可少的，从而为跟踪地幔过程提供了一种新的工具。本项目将实验岩石学领域常用的技术应用于同位素地球化学这一新兴领域。

Iron (Fe) is the most common element in the Earth's mantle that occurs in more than one oxidation state (as Fe2+ and Fe3+). It also occurs as four isotopes (54Fe, 56Fe, 57Fe and 58Fe). It has only recently been discovered that these isotopes may be fractionated during high temperature mantle processes such as partial melting. In particular mantle spinels exhibit a correlation between isotope fractionation and oxygen fugacity, which controls the relative amounts of Fe3+ and Fe2+. The importance of oxygen fugacity in controlling this fractionation relative to other process such as mantle metasomatism is unclear. There is currently no experimental evidence to independently establish the effect of variable Fe3+/Fe2+ on the fractionation of Fe isotopes in spinel at magmatic temperatures. In this work we propose to crystallise a series of spinels from a melt as a function of oxygen fugacity. The samples will be prepared by equilibrating a melt, with spinel on the liquidus, at atmospheric pressure in a controlled gas environment. After quenching, the spinel and melt will be separated and analysed by Mössbauer spectroscopy to determine Fe3+/Fe2+, and with a high-resolution multi-collector inductively coupled plasma mass spectrometer to determine the Fe isotopic ratios. This will allow oxygen fugacity, the Fe3+/Fe2+ ratio, and isotope fractionation to be directly correlated. Experiments of this type are essential if variations in Fe isotopic measurements of natural samples are to be correctly interpreted, thus providing a new tool for tracking mantle processes. This project applies techniques commonly used in the area of experimental petrology to an emerging area of isotope geochemistry.