GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 144-15
Presentation Time: 5:15 PM


FARLEY, Kenneth A.1, COHEN, Barbara2, MAHAFFY, Paul R.3, MALESPIN, Charles3, MARTIN, Peter1, SCHWENZER, Susanne4 and VASCONCELOS, Paulo5, (1)Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, (2)NASA Marshall Space Flight Center, 320 Sparkman Dr, Huntsville, AL 35812, (3)NASA Goddard Space Flight Center, Code 699.0, Greenbelt, MD 20771, (4)Department of Environment, Earth and Ecosystems, The Open University, Walton Hall, Milton Keynes, MK7 6AA, United Kingdom, (5)School of Earth Sciences, University of Queensland, Brisbane, QC, Australia,

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 ~900oC 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 (<500oC) with higher temperature 40Ar derived from plagioclase feldspar.