DETRITAL GARNET GEOCHRONOLOGY: CHALLENGES AND OPPORTUNITIES (Invited Presentation)

Garnet is an abundant, rock-forming mineral in many metamorphic assemblages and consequently a common detrital mineral in sediment and sedimentary rocks derived from orogenic settings. Garnet’s ubiquity and chemical variability between multiple end-members has already made it a widely-used, valuable provenance tool. However, technological and methodological hurdles meant that adding a time constraint from this proven provenance tool through detrital garnet geochronology was not possible until the late-2000s.

Detrital garnet geochronology is now possible, though continues to be accompanied by both challenges and opportunities. All commonly utilized parent-daughter pairs for garnet geochronology (Sm-Nd, Lu-HF, and U-Pb) occur at low concentration within garnet, which is further magnified by the need to date single detrital grains – resulting in sample size challenges. Additionally, isochron methods require a second point on the isochron or some assumption of the initial daughter isotope composition to anchor the garnet. Using the sediment or sedimentary whole rock or pooling multiple detrital garnet grains to obtain an age both require the assumption that all sediment and garnet was derived from a single source [1,2]. We utilize an alternative approach of leaching the inclusions out of a single garnet grain and using the Sm-Nd leachate as a proxy for the whole rock isochron point.

Single-grain, Sm-Nd detrital garnet geochronology utilizing TIMS has been demonstrated as useful and sufficiently precise and accurate, returning ages with 1-10% age uncertainty for Paleozoic samples [2,3]. However, it is slow, exacting work and building a large dataset for detrital study is challenging. Recent developments in LA-ICPMS in situ garnet geochronology using either U-Pb or Lu-Hf offer the exciting opportunity of faster throughput and large datasets, though precision and accuracy of the resulting geochronological data currently suffer from sample size limitations, assumptions about initial daughter isotope composition, and calibration issues [4-6].

References: 1. Oliver et al., 2000, Geol.; 2. Maneiro, Jordan, & Baxter, 2022, Ch. 9 in Isotopic Constraints on Earth System Processes; 3. Maneiro et al., 2019, Geol.; 4. Simpson et al., 2021, Chem. Geol.; 5. Mark et al., 2022, EGU; 6. Verhaert et al., 2023, Goldschmidt