Research Opportunity Description
Lithium is a critical mineral commodity important for clean energy technology such as Li-ion batteries for electric vehicles. After brine deposits, Li-rich pegmatites represent the largest resource of lithium in the United States and are of interest because they occur in multiple locations and in turn are less susceptible to supply chain disruptions (Kesler et al., 2012). Li-rich pegmatites are enriched in elements such as cesium and tantalum that are similarly important for energy-efficient and strategic technologies (Bradley et al., 2017). First mined in the late 1800’s, Li-rich pegmatites are coarse-grained igneous rocks typically found as small bodies with notable occurrences in Maine, North Carolina, and South Dakota (Karl et al., 2019; Bradley et al., 2017).
Despite decades of study, primarily focusing on mineralogy and geochemistry, the geologic evolution leading to the generation of Li-rich pegmatites remains uncertain due to challenges in dating their formation and incomplete knowledge of their precursor petrologic history. The origin of lithium-rich pegmatites remains debated and centers on two genetic models. A longstanding explanation is that Li-pegmatites form at the very final stage of solidification, after >95% crystallization, of granitic magma. The sequential crystallization excludes elements, for example lithium, cesium, and tantalum, that are incompatible in common minerals which leads to enrichment in the “last dregs” of the solidifying magma. These Li-enriched magmas are segregated or injected into surrounding rocks to form discrete pegmatites. In this case, the granitic and pegmatitic magmas share close temporal and spatial relations. Other studies implicate partial melting (anatexis) during metamorphism and crustal evolution and note significant age differences between spatially related granites and pegmatites (Simmons et al., 2016). Geochemical modeling indicates that the extreme enrichments of Li in pegmatites are unlikely to be produced by distillation of a single granitic magma, and instead are derived from multi-stage anatexis of pre-existing granite or crustal rocks (Koopmans et al., 2024). However, the petrologic evolution that leads to one or more episodes of anatexis and ultimately Li-rich pegmatites is unclear because chronologies of thermal and compositional evolution for associated crustal rocks are poorly delimited.
The goals of this Mendenhall Opportunity are to apply geochronology and thermochronology to determine the crystallization and crustal evolution leading to the generation of Li-rich pegmatites. The project will take advantage of the SHRIMP-RG ion microprobe jointly operated by USGS and Stanford University, as well as the USGS Noble Gas Geochronology Lab at Moffett Field, CA. The Mendenhall Fellow can employ a variety of techniques, for example U-Pb dating and trace element geochemistry of columbite, zircon, and other accessory minerals, Li and B isotope analyses of spodumene and tourmaline, K-Ca and 40Ar/39Ar dating of K-feldspar and muscovite, as well as trace element geothermometers to resolve the thermal and petrologic history responsible for the generation Li-rich pegmatites in one or more localities in the United States.
Interested applicants are strongly encouraged to contact the Research Advisor(s) early in the application process to discuss project ideas.
References
Bradley, D.C., et al., 2017, Lithium: U.S. Geological Survey Professional Paper 1802-K.
Karl, N.A., et al., 2019, Lithium Deposits in the United States: U.S. Geological Survey data release, https://doi.org/10.5066/P9ZKRWQF.
Kesler, S.E, et al., 2012, Global lithium resources: Relative importance of pegmatite, brine and other deposits: Ore Geology Reviews, v. 48, p. 55–69.
Koopmans, L., et al., 2024, The formation of lithium-rich pegmatites through multi-stage melting: Geology, v. 52, p. 7–11.
Simmons, W., et al., 2016, Bulk composition of Mt. Mica pegmatite, Maine, USA: Implications for the origin of an LCT type pegmatite by anatexis: The Canadian Mineralogist, v. 54, p. 1053–1070.
Proposed Duty Station
Moffett Field, California
Areas of PhD
Geology, geochemistry, or related fields (candidates holding a Ph.D. in other disciplines, but with extensive knowledge and skills relevant to the Research Opportunity may be considered).
Qualifications
Applicants must meet the following qualifications: Research Geologist.
(This type of research is performed by those who have backgrounds for the occupations stated above. However, other titles may be applicable depending on the applicant's background, education, and research proposal. The final classification of the position will be made by the Human Resources specialist.)
