GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 86-11
Presentation Time: 10:45 AM


MARTIN, Peter E.1, FARLEY, Kenneth A.2, MALESPIN, Charles A.3, MAHAFFY, Paul R.4, VASCONCELOS, Paulo M.5, STACK, Kathryn M.6, GUPTA, Sanjeev7, MALIN, Michael C.8, EDGETT, Kenneth S.8 and DIETRICH, William E.9, (1)Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, (2)Division of Geological and Planetary Sciences, California Institute of Technology, MC170-25, Pasadena, CA 91125, (3)NASA Goddard Space Flight Center, Code 699.0, Greenbelt, MD 20771, (4)NASA Goddard Space Flight Center, Greenbelt, MD 20771, (5)School of Earth Sciences, University of Queensland, Brisbane, Australia, (6)Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, (7)Earth Science and Engineering, Imperial College London, South Kensington Campus, London, SW7 2AZ, United Kingdom, (8)Malin Space Science Systems, P.O. Box 90148, San Diego, CA 92191-0148, (9)Earth and Planetary Science, University of California, Berkeley, 307 McCone Hall, Berkeley, CA 94701

The cosmogenic nuclides 3He, 21Ne, and 36Ar in a drilled bedrock sample can be quantified using the SAM instrument aboard the Curiosity rover on Mars. By combining these observations with calculated production rates derived from sample chemistry measured by the APXS, the exposure age of the sample is obtained. Exposure ages for two samples from the floor of Gale crater (Aeolis Palus) have been reported: the Cumberland (CB) sample yielded 3He, 21Ne, and 36Ar ages of 72 ± 15, 84 ± 28, and 79 ± 24 Ma, respectively. Two aliquots of the Windjana sample gave error-weighted mean 3He, 21Ne, and 36Ar ages of 30 ± 27, 54 ± 19, and 63 ± 84 Ma. The relative youth of these exposure ages and the ~1:1 age ratio between neutron-capture 36Ar and spallation 3He and 21Ne were interpreted to suggest scarp retreat as the main mechanism of ongoing erosion within Gale crater.

We have now performed noble gas analyses on the samples Mojave 2 (MJ2) and Quela (QL) from the Murray formation (Mt. Sharp group) exposed on the lower slopes of Mt. Sharp. The MJ2 exposure ages are 1,320±240 Ma (3He), 910±420 Ma (21Ne), and 310±60 Ma (36Ar). Due to large uncertainties in isobar corrections, 21Ne and 36Ar could not be calculated for QL. A 3He age of 1,460±200 Ma in QL was successfully measured. These 3He and 21Ne ages (mainly held in detrital grains) cluster within uncertainty, giving an error-weighted mean age of 1300 ± 290 Ma. Interpretation of this result depends on the location of the erosional unconformity between the Murray and the overlying aeolian Stimson formation. Projected laterally from current exposures, it is uncertain whether MJ2 and/or QL are vertically >1 m below this undulatory contact, where cosmogenic nuclide production would have been inhibited during Murray paleoexposure. If within 1 m of the contact, these ages could represent the integrated paleoexposure prior to Stimson deposition and the modern exposure. If deeply buried, the older exposure ages for the Murray samples suggest the strata of lower Mt. Sharp are eroding more slowly than Aeolis Palus, implying a progressive deepening of the moat between Mt. Sharp and the walls of Gale crater with time. These results support the hypothesis that Mt. Sharp is an erosional remnant of sedimentary rock which previously extended to the crater walls and perhaps filled the crater.