| NSF Org: |
EAR Division Of Earth Sciences |
| Recipient: |
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| Initial Amendment Date: | December 6, 2021 |
| Latest Amendment Date: | October 18, 2022 |
| Award Number: | 2203532 |
| Award Instrument: | Standard Grant |
| Program Manager: |
Jonathan G Wynn
jwynn@nsf.gov (703)292-4725 EAR Division Of Earth Sciences GEO Directorate for Geosciences |
| Start Date: | December 15, 2021 |
| End Date: | November 30, 2023 (Estimated) |
| Total Intended Award Amount: | $80,155.00 |
| Total Awarded Amount to Date: | $80,155.00 |
| Funds Obligated to Date: |
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| History of Investigator: |
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| Recipient Sponsored Research Office: |
3100 MARINE ST Boulder CO US 80309-0001 (303)492-6221 |
| Sponsor Congressional District: |
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| Primary Place of Performance: |
3100 Marine St, Room 481, 572 UC Boulder CO US 80303-1058 |
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Performance Congressional District: |
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| Unique Entity Identifier (UEI): |
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| Parent UEI: |
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| NSF Program(s): | Geobiology & Low-Temp Geochem |
| Primary Program Source: |
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| Program Element Code(s): |
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| Award Agency Code: | 4900 |
| Fund Agency Code: | 4900 |
| Assistance Listing Number(s): | 47.050 |
ABSTRACT
This project is focused on improving methods that Earth scientists can use to determine the formation age of the mineral hematite. Hematite is a wide-spread mineral in rocks and soils and often forms due to oxidation on Earth?s surface as well as on other planets ? notably Mars. Hematite can incorporate uranium when it forms. Given that uranium undergoes radioactive decay it is possible to conduct both uranium-lead dating (U-Pb) and uranium-thorium-helium dating ((U-Th)/He) on hematite crystals. Investigators will apply methods that can make these measurements simultaneously at a small scale on hematite crystals by blasting them with a laser and measuring the isotopes through mass spectrometry. An additional property of hematite is that it records the magnetic field at the time it crystallizes. This ability to record the ancient magnetic field enables Earth scientists to reconstruct the past position of Earth?s continents and can also be used to gain insight into the timing of hematite formation. They will make magnetic measurements of hematite including measurements that use new capabilities to make magnetic maps at the microscopic scale using an instrument called a quantum diamond microscope. The study will focus on hematite within ancient sedimentary rocks known as iron formations. In the United States, iron formations in the Lake Superior region are the major source of domestic iron production. By applying these dating methods to iron formation, they will constrain the timing of hematite formation in these units, where the timing of oxidation (that is, hematite formation) is debated. If this project is successful, these combined methods of measuring the age of hematite will enable a multitude of future studies. For example, it would be possible to pursue more advanced studies on hematite formation within iron formation, to study the processes and timescales of deep soil leaching (also known as laterization), and determine the timing of ancient surface exposure and chemical alteration in far greater detail using these combined methods. The project will support the research of an early-career scientist, advance development of new analytical capabilities at both University of Carlifornia Berkeley and University of Colorado Boulder, and support both undergraduate and graduate student research for first-generation students.
These investigators seek to develop simultaneous in situ laser ablation (U-Th)/He and U-Pb dating of hematite, which is termed LA-(U-Th)/(He-Pb). Although bulk (U-Th)/He and in situ (U-Pb) methods have been used previously on hematite, the coupled laser-ablation technique has never been applied. This method can provide a powerful tool for assessing the timing of oxidation and weathering in a wide range of hematite-bearing environments. Researchers will investigate the timing of hematite crystallization in Lake Superior region iron formation through this method development in conjunction with paleomagnetic data, which can provide complementary chronologic insight. They will focus on a carefully selected set of samples that will enable method development and give new insights into the origin of iron formations. All samples will be characterized prior to geochronologic measurements via electron backscatter diffraction and electron microprobe to understand the chemical heterogeneity of the samples and the distribution of crystallites within hematite aggregates. This characterization will permit targeting of individual crystallites through LA-(U-Th)/(He-Pb) and provide context relative to potential polycrystalline diffusion behavior. This study has two main sample targets: (1) large, high-purity hematite samples from iron formation that will be used for method development; (2) typical iron formation from the Menominee Group. Menominee Group samples will be analyzed through both LA-(U-Th)/(He-Pb) and paleomagnetism. Paleomagnetic analyses will be conducted at both the centimeter and micrometer scale to constrain hematite formation relative to folding and through comparison to Laurentia?s apparent polar wander path. The well-constrained regional history of deposition, tectonism, and near-surface weathering provides testable hypotheses for the timing of hematite formation in these samples. Successful radiometric dating of these materials will provide confidence in the utility of the LA-(U-Th)/(He-Pb) method in natural samples beyond museum-quality hematite specimens.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
PROJECT OUTCOMES REPORT
Disclaimer
This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.
Overview
Limited tools are available to constrain the formation age of oxidation and weathering products. Advancing geochronologic techniques that can date these materials can lead to constraints on a variety of Earth science problems related to oxidation in the near-surface environment, including interpretations of primary vs. secondary origins of hematite within hematite-bearing sedimentary rocks, the timing and depth of paleosupergene weathering, and the mechanisms of ferruginization and laterization. Hematite is widespread, has been dated by U-Pb geochronology, has been analyzed by whole crystal (U-Th)/He thermochronology, and is a robust paleomagnetic recorder, making it a promising target for additional geochronologic development. The goal of this project was to refine hematite as a geo- and thermochronometer through high-spatial resolution, laser-ablation U-Pb and (U-Th)/He measurements interpreted in conjunction with paleomagnetic data and ancillary textural and geochemical data.
Intellectual Merit
During this project, we established new hematite U-Pb and (U-Th)/He dating capabilities in the CU Thermochronology Research and Instrumentation Lab. We successfully acquired in situ laser-ablation U-Pb and (U-Th)/He data at <100 µm spatial resolution in multiple samples with heterogenous grains and texturally complex relationships, providing important proof-of-concept of the methods.
In the Lake Superior region, in situ U-Pb data for detrital hematite, banded iron ore, and hematite ore samples show the promise of laser-ablation U-Pb geochronology for addressing the timescales and processes of iron oxide formation. In particular, the U-Pb data for the sedimentary samples suggest a detrital rather than authigenic source for hematite, bearing on models for the origin of hematite in such units and how paleomagnetic data should be interpreted. In addition, U-Pb data for hematite in an iron formation postdates the unit’s deposition, indicating hematite enrichment during post-depositional events.
In Colorado, hematite U-Pb work on hematite-quartz veins associated with the Tavakaiv sand injectites and sandstone ridges firmly document a Cryogenian age for this unit. Together with field observations, our findings indicate that the Tava represents a rare terrestial sediment record of the Cryogenian Snowball ice sheets in the cratonic interior of North America. Early Tava sand injection episodes are attributed to basal melting associated with rifting and geothermal heating, and later injections to meltwater generation during ~661 Ma Sturtian deglaciation.
We additionally acquired the first in situ laser-ablation (U-Th)/He data on hematite, with outcomes that compare well with previously published whole crystal (U-Th)/He dates on the same samples. We found that the largest obstacle in constraining meaningful laser-ablation (U-Th)/He dates is accurately estimating the volumes of He-degassed material, such that our future work will focus on ways to remove or reduce redeposition of ablated material in a vacuum to allow for more accurate volume measurements.
Broader Impacts
This project supported, and provided scientific training and professional development for, two CU postdoctoral scholars and one CU undergraduate student. One of the postdoctoral scientists was hired by Kobold Metals, where he is focused on using machine learning and other scientific computing techniques to increase the ethical supply of critical minerals for battery development. Thus far, four abstracts have been produced from this project, with one paper in review. This project is setting the foundation for broad application of hematite U-Pb and (U-Th)/He dating at high spatial resolution in heterogeneous and texturally complex samples in order to understand processes and timescales of iron oxide formation at Earth's surface and near surface, with potential applications to planetary materials such as those associated with Martian sample return.
Last Modified: 03/22/2024
Modified by: Rebecca M Flowers
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