150-7 New dating apps: 40Ar/39Ar dating of K-free host minerals

Session: Geochronology of Critical Mineral Deposits with Special Reference to U-Th-Pb Dating of Common-Pb-Rich Minerals (Posters)

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Abstract:

For several decades, geochronologists have raced to harness the newest mass spectrometry and geochemical technologies to achieve ultra-high precision ages for a variety of rock types and geologic processes. However, each radioisotopic geochronometer is limited, in part, by the concentration of parent and daughter in the host mineral. Despite its vast versatility as a geo- and thermochronometer, ⁴⁰Ar/³⁹Ar dating has been restricted to phases which incorporate structural quantities of Potassium (K). We demonstrate that precise ⁴⁰Ar/³⁹Ar ages can be produced on K-free accessory minerals such as zircon, apatite, titanite, as well as on quartz and olivine, all of which are the cornerstones of other geo-petro-thermochronological techniques. The structure of these minerals lack K, but they can be ⁴⁰Ar/³⁹Ar dated due to the presence of tiny (<25 μm) K-bearing, melt or polymineralic melt/microgranite inclusions (“nanogranites”) entrapped at the time of crystal growth. Data will be presented from well-studied igneous localities with independent, existing age constraints. Our results show that it is possible to obtain accurate ⁴⁰Ar/³⁹Ar plateau ages from K-free minerals that are unrestricted in application by petrogenesis or age. These new mineral chronometers serve as bridges between the higher T chronometers (U-Pb in zircon, Ar/Ar in pyroxene, olivine, and amphibole) and lower T chronometers (U-Th/(He) and fission track), thereby allowing the most complete thermotemporal view of the rock record to date. The exceptionally low K/Ca values (in some cases < 0.001) of the accessory phases + inclusions suggest that lower limit for K content for the Ar chronometer has not yet been realized. Moreover, these findings offer new potential applications to detrital mineral geochronology in a variety of terranes and can provide new types of data pairs for high-precision geochronometer optimization.

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

doi: 10.1130/abs/2025AM-8405

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