Collaborative Research: Geochronology of Carbonate Mineralization in the Lithosphere

Stanford University, Stanford CA

Investigators

Abstract

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.