Collaborative Research: Geochronology of Carbonate Mineralization in the Lithosphere

固体地球在长期的地质碳循环中起着重要作用。来自大气、海洋和地幔的二氧化碳或富二氧化碳流体在各种地质环境(区域变质作用、接触变质作用、俯冲带变质作用、大陆和洋壳的热液和成矿系统、沉积盆地和风化作用)中与岩石圈中的硅酸盐矿物和/或溶解的阳离子反应，形成次生碳酸盐矿物。通过这些碳化反应进入次生碳酸盐的二氧化碳的净速率、时间尺度和通量因此对全球碳循环平衡施加了一级控制，并在这些不同的岩石圈背景下通过流体流动和相关的构造过程监测更广泛的化学运移。为了在地质时间尺度(即1MyRS)上询问和量化岩石圈内二氧化碳的速率、时间和通量(以及一般的热液流体流动)，需要一台准确和精确的碳酸盐地质计时仪。由于自然的复杂性和分析的局限性，碳酸盐地质年代学已被证明是一个重大的挑战。这项研究的重点是通过改进和验证U/Pb和Sm/ND碳酸盐地质年代学来提高我们直接测量碳酸盐成矿时代的能力。其发展重点将放在试验频率较低的碳酸盐Sm/ND系统上，以及随后的Sm/ND和U/Pb数据的整合和交叉核对上。这一进展将利用波士顿大学和其他地方已经开发的新的分析和样品制备技术。初步资料表明，Sm/Nd可测年的碳酸盐矿物确实存在，但可测年矿物S的确切背景和身份尚不清楚。该小组将寻求通过以下方式完善碳酸盐地质年代学：1)仔细的样品表征，以确定最终确定年代的确切矿物及其地质赋存状态；2)完善样品制备方法，以分离和提取可测年的共生相，进行精确的Sm/Nd和U/Pb地质年代学分析；3)建立测试碳酸盐地质年代学准确性的方案。该项目将探索碳酸盐成矿的三种野外环境，包括1)区域变质碳酸盐，2)与硫化物/硫酸盐或成矿系统有关的热液碳酸盐，3)形成于温泉和海底的现代碳酸盐。这个项目将提供新的工具，固体地球地球科学家可以用来1)探索、量化和阐明固体地球在全球地质碳循环中的作用，2)探索总体上岩石圈中流体流动和相关的化学运输和构造过程的速度、时间和流量。通过波士顿大学和斯坦福大学的本科课程和高中推广计划，学生们将被教育到固体地球与更广泛的地球科学问题的相关性，包括碳管理、气候和地球进化。该项目将汇集两名拥有互补工具和兴趣的地球化学家，并将有助于在斯坦福大学建立新的实验室基础设施，用于职业生涯早期的PI。将推动这项研究的波士顿大学研究生将为波士顿大学和斯坦福实验室的工作做出贡献。

The solid earth plays a major role in the long-term geologic carbon cycle. Atmospheric, oceanic, and mantle derived CO2 or CO2-rich fluids reacts with silicate minerals and/or dissolved cations in the lithosphere to form secondary carbonate minerals in a variety of geological environments (regional metamorphism, contact metamorphism, subduction zone metamorphism, hydrothermal and ore-forming systems in the continental and oceanic crust, sedimentary basins, and weathering). The net rate, timescales, and fluxes of CO2 into secondary carbonates via these carbonation reactions thus exerts a first order control on the global carbon cycle balance, and serves as a monitor of broader chemical transport via fluid flow and related tectonic processes within these diverse lithospheric contexts. In order to interrogate and quantify these matters of rate, timing, and flux of CO2 (and hydrothermal fluid flow in general) within the lithosphere over geologic (i.e. 1 Myrs) timescales, an accurate and precise carbonate geochronometer is required. Carbonate geochronology has proven to be a significant challenge due to natural complexities and analytical limitations. This study is focused on improving our ability to directly measure the timing of carbonate mineralization by refining and validating both the U/Pb and Sm/Nd carbonate geochronometers. Its developmental emphasis will be on the less-frequently tested Sm/Nd system for carbonates, and on the subsequent integration and cross-checking of Sm/Nd and U/Pb data. This development will take advantage of new analytical and sample preparation techniques that have already been developed at BU and elsewhere. Preliminary data suggest that carbonate minerals datable by Sm/Nd do exist, though the exact context and identity of the datable mineral?s occurrence is not clear. The team will seek to refine carbonate geochronology by, 1) careful sample characterization to identify the exact minerals that are ultimately being dated as well as their geological occurrence, 2) refining sample preparation methods to separate and extract datable co-genetic phases for precise Sm/Nd and U/Pb geochronological analysis, 3) establish protocols for testing the accuracy of carbonate geochronology. Three field contexts of carbonate mineralization will be explored including 1) regional metamorphic carbonate, 2) hydrothermal carbonate associated with sulfide/sulfate or ore forming systems, and 3) modern carbonates forming at hot springs and on the sea floor.This project will provide new tools that solid-earth geoscientists can use to 1) explore, quantify, and illuminate the role of the solid-earth in the global geological carbon cycle, and 2) explore the rate, timing, and flux of fluid flow and associated chemical transport and tectonic processes in the lithosphere in general. Through undergraduate coursework and high school outreach programs in place at BU and Stanford, students will be educated as to the relevance of the solid earth in broader geoscience issues including carbon management, climate, and earth evolution. The project will bring together two geochemists with complementary tools and interests and will contribute to the establishment of new lab infrastructure at Stanford for an early career PI. The BU graduate student who will drive this research will contribute to work in both the BU and Stanford labs.