EXPLORING THE PARTIAL LOSS OF 40AR* AND POTENTIAL RECOIL EFFECTS IN TWO APOLLO 16 IMPACT MELT BRECCIAS: COMPARISON OF LASER MICROPROBE AND INCREMENTAL HEATING 40AR/39AR RESULTS

The ⁴⁰Ar/³⁹Ar dating method is a valuable tool in studying the ages and thermal histories of planetary materials. In particular, it has been used extensively to constrain the bombardment history of the Moon by dating impact-affected samples, most commonly impact melt breccias (IMBs; e.g., [1-3]). However, IMBs are often complex mixtures of now-solid melt components and inherited mineral and/or lithic clasts from the pre-impact target rocks. For some samples, this can make it difficult to infer age significance unambiguously from incremental heating ⁴⁰Ar/³⁹Ar data (e.g., [4]), though this can be alleviated in part by employing high-spatial resolution dating techniques (e.g., [5, 6]). Additional difficulties may arise if an IMB lost some (but not all) radiogenic ⁴⁰Ar (⁴⁰Ar*) through one or more reheating events following the breccia-forming impact, or if ³⁹Ar_K was redistributed or lost through recoil during neutron irradiation of analyzed samples [7]. In this contribution, we present both laser microprobe and incremental heating ⁴⁰Ar/³⁹Ar data obtained for the Apollo 16 IMBs 61015 and 63355. The high-spatial resolution and stepwise heating approaches provide complementary datasets for constraining the timing of initial IMB melt formation and subsequent reheating. In some instances, the low-temperature steps of incremental release spectra may better constrain the maximum age of reheating events since gas can be extracted differentially from fine-grained domains that are significantly smaller than the laser beam footprint. In addition, stepwise release spectra can provide evidence for ³⁹Ar_K recoil at high experimental temperatures. However, the laser microprobe excels at dating inherited (incompletely reset) or mixed-generation materials in isolation from one another in IMBs [6], and can provide valuable additional insights that enhance science return when studying lunar samples.

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