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.
Geochronology (age dating) is an important tool for assessing both natural resources, from the formation of ore deposits to the timing of petroleum migration, and natural hazards, from the timing of earthquakes to the reconstruction of long-term plate movement. As such, accurate and reliable techniques to determine the age of geologic deposits remains an outstanding challenge in the Earth Sciences. The purpose of this research project was thus to develop new methodologies to date carbonate-rich rocks in the Earth’s crust. Improved knowledge of the timing of important geologic events has the potential to provide key information to industry, decision makers and the general public. Carbonate minerals are some of the most common minerals in the Earth’s crust. In addition to the carbonate minerals associated with marine sediments (limestone), carbonate minerals such as calcite (CaCO3) are also found in direct association with ore deposits, lake shorelines, fault zones, metamorphic rocks, geothermal systems and low-temperature springs. However there are a very few methods for directly dating carbonate minerals that can be applied over timescales greater than the last 300,000 years. For assessing more ancient deposits, including mineral deposits and long-lived fault systems, there are very few dating methods applicable over the broad timespans required to assess geologic change. Thus, the goal of this collaborative project was to adapt and improve the samarium-neodymium (Sm/Nd) and uranium-lead (U/Pb) dating systems to directly date carbonate mineralization. We collected carbonate samples from a range of environments and analyzed the Sm/Nd, U/Pb and major element compositions. We found that high-temperature carbonates (such as those from ore deposits and metamorphic events) could be dated using Sm/Nd geochronology. However, these samples often did not contain enough U (the parent isotope of Pb) to allow for U/Pb geochronology. Nevertheless, the Pb isotope systematics were useful in evaluating the fluid source and the selection of samples used to determine the Sm/Nd age. We also found that low-temperature deposits, particularly those high in silica, tended to have unfavorable Sm/Nd but favorable U/Pb systematics. Thus, although we were not able to directly compare the two methods on the same samples, we were able to provide new information about what types of deposits can be dated. To determine the age of the low-temperature U/Pb deposits, we developed a set of new analytical approaches to measure U/Pb and used them to determine the age of fault deposits in southern California. Here we were able to show that the fault activity (based on the ages of the opal and calcite found in the fault planes) spanned a period from 10 million years ago to the present. This new approach for using U/Pb dating of secondary minerals to determine fault activity provides a new perspective on fault history that can be used to guide models for fault system evolution. The methods developed for this project were developed in an open user facility at Stanford University. By carefully documenting and archiving all procedures and materials, future scientists working in this area can have access to the methods to support their particular scientific questions.
