OFF-PLANET GEOCHRONOLOGY: ONGOING RESULTS OF RADIOMETRIC AND COSMOGENIC DATING ON MARS

Scientific instruments on board the Curiosity Rover exploring the fluvio-lacustrine sediments of Gale Crater, Mars measure rock elemental concentrations (using APXS), and noble gas isotopes thermally released at ~900^oC from drilled rock samples (SAM investigation). By combining these methods, it is possible to perform both radiometric dating using the K-Ar method, and cosmic ray surface exposure dating using spallogenic ³He and ²¹Ne, and neutron-capture produced ³⁶Ar. Our first dating attempt was undertaken on the Cumberland mudstone (Farley et al., 2013), yielding a K-Ar age of 4.21 ± 0.35 (1σ) Ga, in good agreement with crater-density estimates of the age of the mudstone's likely source terranes of 3.5 to 4.1 Ga. A much younger (<2 Ga) and non-repeatable K-Ar age was obtained on the Windjana sandstone. We attribute this result to incomplete thermal extraction of ⁴⁰Ar from the very Ar retentive phase sanidine, which dominates the K budget of this rock (Vasconcelos et al., 2016, in press). Both rock samples yielded surface exposure ages of 30-85 Ma from multiple isotopic systems. Coupled with geomorphic observations, these ages suggest moderately rapid wind-driven scarp retreat and inform a strategy for locating outcrops least-subjected to cosmic ray degradation of organic molecules. Key methodological lessons learned are: a) avoid coarse-grained rocks, and especially those with a significant proportion of sanidine, b) build-up of sample-derived HCl contamination in SAM steadily degrades the ability to measure the isobaric species ³⁶Ar. This build-up may be mitigated by a future instrumental bake-out, though such a step is not without risk to the instrument.

The team is presently readying to deploy a two-step heating protocol that is designed to obtain a K-Ar date on jarosite in a mudstone called Mojave. Jarosite-derived Ar should be released exclusively at low T (<500^oC) with higher temperature ⁴⁰Ar derived from plagioclase feldspar.