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

常见成岩元素的同位素在了解地质过程中物质的再分布方面具有很大的潜力。为了利用这些同位素，必须知道它们是如何在岩石中的不同矿物之间分配的。地质界最感兴趣的两种元素是镁(Mg)和铁(Fe)。该小组建议在他们之前的实验计划的基础上，测试和加强高温下铁和镁同位素在矿物相之间分配的理论预测。该项目代表了实验岩石学和同位素地球化学的不同寻常的结合。近年来，研究人员培养了三名研究生，其中两名是女性，这是地质学中代表性较低的人群。铁和镁被选为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之前认为的更大。事实上，关于这些同位素在天然样品中分布的研究呈爆炸式增长。本研究的重点是尖晶石与其他相之间的铁、镁同位素分馏。尖晶石之所以令人感兴趣，是因为在尖晶石结构中，这些阳离子有不止一个配位位置，这为稳定同位素分配的原理提供了测试。实验将在活塞式气缸装置中进行。该小组将使用三同位素方法在这两个同位素系统中建立平衡分馏系数。这种方法包括在其中一个相中加入过量的丰度同位素，以扰乱实验中三个同位素之间依赖质量的分馏的通常状态。三种同位素之间的这种质量分馏关系是达到平衡的必要条件。在实验运行期间，重新建立这种条件的驱动力被用作实验中的同位素交换的监测。该团队将研究镁(Al，Cr)2O4尖晶石和镁橄榄石之间的平衡镁同位素分馏，以及铁铝榴石和磁铁矿(反尖晶石)之间的平衡铁同位素分馏。后一次实验的结果可以与前一次实验相结合，得出与温度有关的石榴石-橄榄石铁同位素分馏系数。这两个系统在解释天然镁和铁同位素数据方面都起着关键作用。

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.