EAGER: Collaborative Research: Developing new laser ablation (U-Th)/(He-Pb) hematite double dating techniques to date ancient oxidation

University Of California-Berkeley, Berkeley CA

Investigators

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.