T259 Frontiers in Geochronology: Ancient Systems and Planetary Environments (Posters)

Tuesday, 27 September 2016
9:00 AM - 6:30 PM
Exhibit Hall E/F

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

Paper number: T259-9
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Abstract

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 39ArK 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 39ArK 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.

References: [1] Bogard D. (1995) Meteoritics, 30, 244-268. [2] Norman M. et al. (2006) GCA, 70, 6032-6049. [3] Fernandes V. et al. (2013) Meteoritics & Planet. Sci., 48, 241-269. [4] Jourdan F. (2012) Aust. J. Earth Sci., 59, 199–224. [5] Eichhorn G. et al., (1979) Proc. Lunar Planet. Sci. Conf. 10, 763–788. [6] Mercer C. M. et al. (2015) Sci. Adv., 1, e1400050. [7] Turner G. and Cadogan P. H. (1975) GCA Supp 5 (5th LPSC), 2, 1601–1615.

Geological Society of America Abstracts with Programs. Vol. 48, No. 7

doi: 10.1130/abs/2016AM-287070

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