The Importance of Diffusion in Understanding Mineral Reaction During Metamorphism

变质作用涉及地壳深处高压和温度升高的矿物之间的固态化学反应，通常与安第斯山脉和喜马拉雅山脉等山脉的发展有关。一般说来，这些反应对某些矿床的形成和地壳中元素的分布都很重要。一些变质反应释放出二氧化碳，最终被输送到大气层和水圈。人们对变质作用最不了解的方面是它的持续时间。传统观点认为，区域变质作用持续1000万年或更长时间。然而，新的间接证据表明，持续时间可能要短得多，10万年甚至更短。解决这一差异对于充分了解变质矿物反应如何通过二氧化碳的释放影响成矿、地壳中的元素运输和全球气候至关重要。拟议的研究是使用一种新开发的仪器，以高空间分辨率精确测量变质岩中方解石(碳酸钙)单个晶体内的碳同位素组成。方解石晶体中碳同位素组成的变化可以作为一个时钟，更准确地记录或设定变质时间的上限。拟议的研究将涉及加州、缅因州、佛蒙特州和苏格兰的接触和区域变质岩石。约翰霍普金斯大学将使用电子成像技术对样品进行筛选。碳同位素组成的分析将涉及几个机构和仪器。马里兰大学和新墨西哥州大学的微钻孔和激光烧蚀方法以及传统的气源质谱学方法将被用于针对单个方解石晶体进行详细研究。创新的分析技术将是使用威斯康星大学新安装的离子微探测器在10微米空间尺度上进行高精度的碳同位素分析。将利用标准的扩散数学处理和橡树岭国家实验室优秀的新扩散系数测量方法，对单个方解石晶体内测得的碳同位素组成剖面进行反演，以估计变质作用的持续时间。此外，结果还将涉及方解石的碳同位素组成在估计变质过程中的温度、流体组成和质量传输距离方面的应用。

The process of metamorphism involves solid-state chemical reactions among minerals at elevated pressure and temperature deep in Earth's crust, usually associated with the development of mountain belts like the Andes and the Himalayas. The reactions are important in the formation of certain ore deposits and in the distribution of elements in the crust, generally. Some metamorphic reactions release carbon dioxide that ultimately is transported to the atmosphere and hydrosphere. The least understood aspect of metamorphism is its duration. Conventional wisdom holds that regional metamorphism lasts ten million years or more. New, indirect evidence, however, has emerged that the duration could be much shorter, 100,000 years or less. Resolution of this difference is essential to fully understand how metamorphic mineral reactions affect ore formation, element transport in the crust, and global climate through the release of carbon dioxide. Proposed research is to use a newly developed instrument for precise measurement of carbon isotope composition at high spatial resolution within individual crystals of calcite (calcium carbonate) in metamorphic rocks. Variations in carbon isotope composition within calcite crystals can serve as a clock that will more precisely record or set an upper bound on the duration of metamorphism.Proposed research will involve contact and regionally metamorphosed rocks from California, Maine, Vermont, and Scotland. Samples will be screened using electron imaging techniques at Johns Hopkins University. Analysis of carbon isotope composition will involve several institutions and instruments. Micro-drilling and laser ablation methods with conventional gas source mass spectrometry at the Universities of Maryland and New Mexico will be used to target individual calcite crystals for detailed study. The innovative analytical technique will be high-precision carbon isotope analysis at the 10-micron spatial scale using the newly installed ion microprobe at the University of Wisconsin. Measured profiles in carbon isotope composition within individual calcite crystals will be inverted, using standard mathematical treatments of diffusion and excellent new measurements of diffusion coefficients from Oak Ridge National Laboratory, to estimate the duration of metamorphism. Results will additionally bear on application of the carbon isotope composition of calcite to estimate temperature, fluid composition, and mass transport distances during metamorphism.