194-2 Radiation Damage, Chemical Zoning, and Helium Diffusion in Monazite: Toward Routine (U-Th)/(He-Pb) Double Dating of Monazite

Session: Broad Applications of Thermochronology to Understanding Geologic Rates and Processes Through the Sedimentary Record (Posters)

Presenting Author:

Authors:

Abstract:

(U-Th)/(He-Pb) double dating of detrital minerals offers important insights into both provenance and post-depositional thermal history. Monazite has been targeted for (U-Th)/He dating, but a routine dating procedure is not yet established. Previous work has demonstrated variations in diffusion kinetics that result in different closure temperatures (Tc) among grains (~200-300°C) and some grains exhibit multiple diffusion domains. Importantly, the effects of varying radiation damage on helium diffusion in monazite remain poorly understood.

Raman spectroscopy has been successfully used to characterize radiation damage in zircon. As damage increases, Raman peaks broaden and shift to lower frequencies. Similar effects have been observed in monazite using point analyses, but full-grain Raman mapping has yet to be explored. Here we present new Raman spectroscopic analysis, U-Th-Pb geochronology, trace and rare-earth element geochemistry (T/REE), Electron Probe Microanalysis (EPMA) and energy-dispersive X-ray spectroscopy (EDS) of detrital monazite from the Upper Paleozoic Paganzo Basin, Argentina, to evaluate controls on diffusion kinetics for monazite (U-Th)/He thermochronology.

We generated Raman maps for the 554-monazite standard and 10 detrital monazite grains, focusing on Full Width at Half Maximum (FWHM) and peak positions around the ν1 (PO₄) band near 974 cm⁻¹. These same grains yielded 506-351 Ma U-Th-Pb ages and trace and rare earth element (T/REE) concentrations measured with LA-ICP-MS. EDS was used to map intragranular chemical variability and to guide site selection for EPMA analysis. Preliminary results from the combined Raman and EDS maps yield zoned patterns in FWHM and peak shift maps that do not fully correspond to the chemical zoning in U, Th and Ca. The broadening of the FWHM appears to result from both grain chemistry and radiation damage. Ongoing work includes correlative EPMA and Raman mapping to isolate the factors leading to Raman peak broadening (composition vs. radiation damage) using a previously proposed empirical formula. The same grains will then undergo (U-Th)/He cycled stepped-heating experiments to calculate diffusion parameters. We hypothesize that grains showing multiple domains in the radiation damage maps will exhibit similar multi-domain behavior in helium diffusion. Additionally, by comparing radiation damage among single-domain grains and their calculated Tc, we aim to better understand the relationship between radiation damage and (U-Th)/He Tc in monazite.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-6608

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