Reaction Lengths and Reaction Rates in Metamorphic Rock Systems Using Isotopic and Age Zoning in Garnet Porphyroblasts

[805218 . DePaolo]这项拟议的研究旨在确定(1)在500至700摄氏度之间的温度下变质岩石中矿物-流体交换反应的速率(2)在这些条件下，变质作用期间维持化学“平衡”的长度尺度。该方法包括测量变质岩中石榴石卟啉母岩的Sr、Nd和O同位素组成，并结合整个岩石和基质样品的类似同位素测量。同位素测量将辅以对矿物主要元素组成的电子探针研究。研究将在层状变质岩的样品上进行，具体选择的位置是因为它们具有确定流体-岩石系统的“平衡长度”或“反应长度”的特征。(1)具有不同初始Sr同位素比和不同Rb/Sr比值的层间岩性；(2)丰富的大型石榴石卟卟母细胞适合于岩心、边缘和中间带的微采样进行同位素分析；(3)相对较近的变质作用时代。在阿尔卑斯山、佛蒙特州、加利福尼亚州和阿拉斯加州都发现了合适的露头。

9805218 DePaolo The proposed research is designed to determine (1) the rates of mineral-fluid exchange reactions in metamorphic rocks at temperatures between 500 and 700 C and (2) the length scales over which chemical "equilibrium" is maintained during metamorphism under these conditions. The approach involves measurements of Sr, Nd and O isotopic compositions of garnet porphyroblasts from metamorphic rocks, combined with similar isotopic measurements of whole rock and matrix samples. The isotopic measurements will be supplemented with electron microprobe studies of major element compositions of minerals. The study will be made on samples from layered metamorphic rocks, the localities chosen specifically because they have characteristics which allow determination of the "equilibration length" or "reaction length" of the fluid-rock system. The required characteristics of the field sites are: (1) interlayered lithologies with different initial Sr isotope ratios and different Rb/Sr rations; (2) abundant large garnet porphyroblasts suitable for microsampling of core, rim, and intermediate zones for isotopic analyses; and (3) a relatively recent age of metamorphism. Appropriate outcrops have been identified in the Alps, Vermont, California and Alaska.