The Importance of Diffusion in Understanding Mineral Reaction During Metamorphism

变质作用的过程涉及地壳深处在高压和高温下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带的形成有关。一般来说，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，最终被输送到大气和水圈。变质作用最不为人所知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，只有10万年或更短。解决这一差异对于充分了解变质矿物反应如何通过释放二氧化碳影响矿石形成、地壳中的元素运输和全球气候至关重要。建议的研究是使用一种新开发的仪器在高空间分辨率下精确测量变质岩中方解石（碳酸钙）单个晶体中的碳同位素组成。方解石晶体中碳同位素组成的变化可以作为一个时钟，更精确地记录或设定变质作用持续时间的上限。拟议的研究将涉及来自加利福尼亚、缅因州、佛蒙特州和苏格兰的接触岩和区域变质岩。样品将在约翰霍普金斯大学使用电子成像技术进行筛选。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大学将使用传统气源质谱的微钻孔和激光烧蚀方法来针对单个方解石晶体进行详细研究。创新的分析技术将是在10微米空间尺度上使用威斯康星大学新安装的离子微探针进行高精度碳同位素分析。在单个方解石晶体中，碳同位素组成的测量剖面将被反转，使用扩散的标准数学处理和橡树岭国家实验室的扩散系数的优秀新测量，以估计变质作用的持续时间。结果还将应用方解石的碳同位素组成来估计变质作用期间的温度、流体组成和质量运输距离。

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.