Geochemistry of the main facies

MSL/ChemCam at Glen Torridon: geochemistry ofthe orbitally-identified clay-bearing unit of Gale crater

Abstract #6125

Erwin Dehouck1, A. Cousin2, N. Mangold3, J. Frydenvang4, J. Lasue2, P.-Y. Meslin2, O. Gasnault2, V. K. Fox5, K. A. Bennett6, S. Maurice2, R. C. Wiens7

1LGL-TPE, Université de Lyon, France; 2IRAP, Toulouse, France; 3LPG, Université de Nantes, France; 4University of Copenhagen, Denmark; 5Caltech, Pasadena, California, USA; 6USGS Astrogeology Science Center, Flagstaff, Arizona, USA; 7LANL, New Mexico, USA

@ErwinDehouckerwin.dehouck@univ-lyon1.fr

Introduction

ChemCam dataset

One of the main goals of the Mars Science Laboratory (MSL) mission is to investigate the clay-to-sulfate mineralogical transition observed from orbit within the lower strata of Mt Sharp (Aeolis Mons), the sedimentary mound at the center of Gale crater [1,2]

Curiosity reached the “clay-bearing unit” in January 2019 – this area is now referred to as “Glen Torridon” (GT) by the MSL science team [3]

Clay-bearing rocks have already been encountered along the rover traverse, in both the Yellowknife Bay and Murray formations [4-6], but the stronger and more continuous spectral signatures in GT may indicate that the clay minerals are more abundant or better exposed

The ongoing scientific campaign aims at determining the nature and abundance of the GT clay minerals, as well as their detailed geologic and geochemical settings

Here, we present the findings of the ChemCam instrument during the first few months of the GT campaign

Laser-induced breakdown spectroscopy (LIBS); 300-500 µm spot size

Routine quantification of eight major rock-forming oxides: SiO2, TiO2, Al2O3, FeOt, MgO, CaO, Na2O and K2O [7]

1 ChemCam analysis = a raster of several points

1 raster point = a series of 30 laser shots

Possible contaminations from Ca-sulfate veins and soil cover removed by discarding the corresponding points [e.g., 6]

As of sol 2457 (~July 4th, 2019), ChemCam has analyzed 202 targets at Glen Torridon, corresponding to >1500 raster points

References: [1] Milliken R. E. et al. (2010) GRL, 37. [2] Grotzinger J. P. et al. (2012) Space Sci. Rev., 170. [3] Fox V. K. et al. (2019) this conf. [4] Vaniman D. T. et al. (2014) Science, 343. [5] Bristow T. F. et al. (2018) Science Adv., 4. [6] Mangold N. et al. (2019) Icarus, 321. [7] Clegg S. M. et al. (2017) Spectrochim. Acta B, 129. [8] Bristow T. F. et al. (2019) this conf.[9] McAdam A. C. et al. (2019) this conf. [10] Fraeman A. A. et al. (2016) JGR-Planets, 121. [11] McLennan S. M. et al. (2014) Science, 343.

9th Int. Conf. on Mars

In April 2019, two drill samples were collected within the coherent/high-Mg bedrock

X-ray diffraction analyses performed by the CheMin instrument revealed an abundance of clay minerals slightly higher than found in previous samples of the Murray formation [8]

This is consistent with the CIA values measured by ChemCam, on the high end (~55-60; fig. above) of the range observed so far [6,11]

The more widespread rubbly/high-K mate-rial could not be drilled, but the equally-high CIA values suggest that it also contributes to the orbital signature of clay minerals

Context of the Aberlady/Kilmarie drill holes

Curiosity on May 31

▲ Average MgO content of ChemCam targets around the Aberlady/Kilmarie drill holes. The two drill samples werecollected within the coherent/high-Mg bedrock, but the rubbly/high-K material dominates the surrounding terrains.

Coherent bedrock

Rubbly bedrock

Pebbles▲ Main facies encountered during the firstphase of the campaign (RMI mosaics).

▲ Major-oxide compositions of GT coherent bedrock,rubbly bedrock, pebbles and soils. Each symbol corres-ponds to a single analysis point.

▲ A-CN-K ternary diagram and CIA scale. Each symbol correspondsto a single target (average of four points or more). Data from othermembers of the Murray formation [6] are shown for comparison.

Two compositionally-distinct types of bedrock exist at GT: coherent/Mg-rich and rubbly/K-rich

Mg enrichment in coherent bedrock likely indicative of Mg-rich phyllosilicates, including saponitic smectite [8,9]

K and Si enrichments in rubbly bedrock could be consistent with other clay minerals such as illite, although the orbital signatures are more consistent with smectites [3,10]

Most pebbles show a composition very similar to the rubbly bedrock, suggesting a genetic link

HiRISE image – Credit: NASA/JPL-Caltech

Aberlady/Kilmariedrill site

N

50 m

Chemical Index of Alteration (CIA)