Collaborative Research: Geochronology of Carbonate Mineralization in the Lithosphere

固体地球在长期地质碳循环中起着重要作用。大气、海洋和地幔衍生的CO2或富CO2流体与岩石圈中的硅酸盐矿物和/或溶解阳离子发生反应，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、大陆和海洋地壳中的热液和成矿系统、沉积盆地和风化作用）中形成次生碳酸盐矿物。因此，二氧化碳通过这些碳酸化反应进入次生碳酸盐的净速率、时间尺度和通量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构造过程监测更广泛的化学运输。为了在地质（即1 Myrs）的时间尺度上询问和量化岩石圈内二氧化碳（以及一般的热液流动）的速率、时间和通量等问题，需要一台精确的碳酸盐地球计时器。由于自然的复杂性和分析的局限性，碳酸盐岩地质年代学已被证明是一项重大挑战。本研究的重点是通过改进和验证U/Pb和Sm/Nd碳酸盐岩地质时计，提高我们直接测量碳酸盐岩成矿时间的能力。其发展重点将放在较少测试的碳酸盐岩Sm/Nd体系，以及随后对Sm/Nd和U/Pb数据的整合和交叉核对。这一发展将利用已经在波士顿大学和其他地方开发的新的分析和样品制备技术。初步数据表明，可通过Sm/Nd测定的碳酸盐矿物确实存在，尽管可测定矿物的确切背景和身份？S的发生情况尚不清楚。该团队将通过以下方法来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分布；2)改进样品制备方法，以分离和提取可用于精确Sm/Nd和U/Pb地质年代学分析的数据共成因阶段；3)建立测试碳酸盐地质年代学准确性的协议。本文将探讨碳酸盐岩成矿的三个领域背景：1)区域变质碳酸盐岩；2)与硫化物/硫酸盐或成矿系统相关的热液碳酸盐岩；3)在温泉和海底形成的现代碳酸盐岩。该项目将为固体地球科学家提供新的工具，用于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)探索岩石圈中流体流动的速率、时间和通量，以及相关的化学运输和构造过程。通过波士顿大学和斯坦福大学的本科课程和高中拓展项目，学生们将了解固体地球在更广泛的地球科学问题中的相关性，包括碳管理、气候和地球进化。该项目将汇集两位具有互补工具和兴趣的地球化学家，并将为斯坦福大学早期职业PI建立新的实验室基础设施做出贡献。推动这项研究的波士顿大学研究生将在波士顿大学和斯坦福大学的实验室工作。

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.