Experimental Calibration of Inter-mineral Magnesium and Iron Isotope Fractionation at High Temperatures

常见的造岩元素的同位素对于了解地质过程中物质的再分配具有很大的潜力。为了利用这些同位素，人们必须知道它们是如何在岩石中不同的矿物质之间分配的。地质学界最感兴趣的两种重要元素是镁（Mg）和铁（Fe）。这个小组建议在他们之前的实验项目的基础上，测试和增强高温下矿物相之间铁和镁同位素分配的理论预测。该项目是实验岩石学和同位素地球化学的罕见结合。近年来，研究人员培养了三名研究生，其中两名是女性，这一群体在地质科学领域的代表性不足。铁和镁是这项跨学科研究的目标，因为它们作为岩石形成元素的重要性，以及最近的迹象表明，这些元素在地幔中的流动性可能比以前认为的要大。事实上，对这些同位素在自然样品中的分布进行了大量的研究。本研究的重点是尖晶石与其他相之间的铁和镁同位素分馏。尖晶石之所以引起人们的兴趣，是因为这些阳离子在尖晶石结构中有不止一个配位位点，从而提供了稳定同位素分配原理的测试。实验将在活塞缸装置中进行。研究小组将利用三同位素方法建立这两种同位素体系的平衡分馏因子。这种方法涉及到在其中一个相中加入过量的丰富同位素，以干扰实验中三种同位素之间依赖质量的分馏的通常状态。这三种同位素之间的质量分馏关系是平衡的必要条件。在实验运行过程中，向重建该条件的努力被用作实验中同位素交换的监测。该团队将研究Mg(Al,Cr)2O4尖晶石和橄榄石之间的平衡Mg同位素分馏，以及铝石榴石和磁铁矿（一种反向尖晶石）之间的平衡Fe同位素分馏。后一种实验的结果可以与先前的实验相结合，得出与温度相关的石榴石-橄榄石铁同位素分馏因子。这两个系统都是解释天然镁和铁同位素数据的关键。

Isotopes of common rock-forming elements have great potential for understanding the redistribution of mass during geological processes. In order to make use of these isotopes one must know how they partition between different minerals in the rocks. Two of the most important elements of interest to the geological community are magnesium (Mg) and iron (Fe). This group proposes to build on their previous experimental program to test and augment theoretical predictions for iron and magnesium isotope partitioning between mineral phases at high temperatures. The project represents an uncommon integration of experimental petrology and isotope geochemistry. The investigators have trained three graduate students in recent years, two of whom are women, an underrepresented demographic in the geological sciences.Iron and magnesium are targeted for this interdisciplinary study because of their importance as rock-forming elements and recent indications that these elements may be more mobile in Earth's mantle than previously thought. Indeed, there has been an explosion of studies on the distributions of these isotopes in natural samples. The focus of this study is on Fe and Mg isotope fractionation between spinels and other phases. Spinels are of interest because there is more than one coordination site for these cations in the spinel structure, affording tests of the principles of stable isotope partitioning. Experiments will be performed in a piston cylinder apparatus. The group will use the three-isotope method to establish equilibrium fractionation factors in these two isotope systems. This method involves spiking one of the phases with an excess of the abundant isotope in order to disturb the usual state of mass-dependent fractionation among three isotopes in the experiments. This mass fractionation relationship among the three isotopes is a necessary condition for equilibrium. The drive towards reestablishment of that condition during the experimental run is used as a monitor for isotope exchange in the experiments. The team will investigate equilibrium Mg isotope fractionation between Mg(Al,Cr)2O4 spinels and forsterite olivine and equilibrium Fe isotope fractionation between almandine garnet and magnetite (an inverse spinel). Results of the latter experiments can be combined with previous experiments to arrive at temperature-dependent garnet-olivine iron isotope fractionation factors. Both systems are pivotal in the interpretation of natural Mg and Fe isotope data.