Hydration state of calcium sulfates in Gale crater, Mars: Identification of bassanite veins

Highlights

• Experiments on calcium sulfate pellets were performed to calibrate the LIBS hydrogen signal.

• Bright veins analyzed in situ at Gale crater are made predominantly of bassanite.

• Bassanite could have resulted from the desiccation of gypsum.

Abstract

In-situ analyses reveal the presence of hydrogen within calcium sulfate veins crosscutting the sediments found in Gale crater. Laboratory experiments were performed to calibrate the hydrogen signal measured by laser induced breakdown spectroscopy (LIBS) in a range applicable to martian data. The analyses indicate that all veins targeted so far at Gale consist predominantly of bassanite which most likely formed by dehydration of gypsum. This scenario suggests that the percolating water produced gypsum, possibly by hydration of anhydrite in aqueous solution, and remained at temperatures below ∼60 °C at that time. Desiccating conditions followed, consistent with a hyperarid climate and favored by burial or impacts. Additionally, anhydrite with lesser bassanite has been found by XRD in samples of sediments hosting the veins. Our result suggests bassanite is likely found in the veins and anhydrite may be more common as a fine-grained component within the sediments.

Introduction

Calcium sulfates are relatively common evaporitic minerals which are found to hold structural water in three different phases: gypsum (${\text{CaSO}}_4\times 2{\mathrm{H}}_2\mathrm{O}$) with 20.9 wt.% water, bassanite (${\text{CaSO}}_4\times 0.5{\mathrm{H}}_2\mathrm{O}$) 6.2 wt.% (but variable), and anhydrite (anhydrous CaSO₄). Because these sulfates mostly form in the presence of water, they can hold information about past aqueous activity on Mars in terms of temperature, humidity and salinity.

Calcium sulfates have been identified on Mars from orbit in different states of hydration. In near-infrared reflectance spectroscopy, gypsum presents strong but similar spectral features to bassanite making the distinction difficult (Bishop et al., 2014). The anhydrous form is even more difficult to detect because it lacks diagnostic water absorption bands. Using data from orbital instruments, in particular Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) and Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité (OMEGA), gypsum dunes were identified in Olympia Planitia in the north polar region (Langevin et al., 2005) and light toned deposits with spectral signatures consistent with gypsum were also found at lower latitudes (Gendrin et al., 2005) in Iani Chaos (Gilmore and Greenwood, 2009) and Noctis Labyrinthus (Weitz et al., 2013). However, a horizontal unit in Mawrth Vallis presents signatures more consistent with bassanite (Wray et al., 2010) and a thin alteration layer described in Mangold et al. (2010) also has spectral features most similar to bassanite. CaO vs. SO₃ correlations were seen by the Alpha Particle X-Ray Spectrometer in Endurance crater on the Homestake white vein (Squyres, 2012). The APXS is not sensitive to hydrogen but PanCam filter images suggested a hydration band near ∼950 nm in the spectra acquired on the vein (Squyres, 2012). In most places where gypsum or bassanite deposits were identified in-situ or from the orbit, the spectral signatures were not strong enough to distinguish unambiguously the two hydrated species.

More recently, numerous calcium-sulfate veins were found in-situ at Gale crater using the ChemCam instrument (Nachon et al., 2014) onboard Curiosity. The veins cross-cut the stratigraphic column from the oldest to the most recent units, including various types of sediments (e.g., mudstones and sandstones). This fracture filling suggests that the aqueous activity occurred after sedimentation. Some vein material in the mudstone named John Klein was sampled by the rover drill in the Yellowknife Bay area and analyzed by the XRD instrument; no gypsum was found in the drill powders but small amounts of anhydrite and bassanite (respectively ∼1 and 0.7 wt.%) were present (Vaniman et al., 2014). The rover mast color cameras (MastCams) have a series of spectral filters which cover the band depth at 900–1000 μm (Rice et al., 2013). Most of these multispectral images show no evidence for gypsum either, except a possible detection in some of the thick veins or nodules in the Yellowknife Bay area (Vaniman et al., 2014).

Given the ChemCam instrument's ability to detect hydrogen (Schröder et al., 2015), this study focuses on a calibration method to constrain the water content of the calcium sulfate veins targeted on Mars. We first present the experimental method and the signal processing used, then the calibrated result for the vein dataset at Gale crater. Finally the implications of the findings on the environment at the time of formation and during subsequent evolution are discussed based on the stability of the different calcium sulfate phases and kinetics of their formation and alteration.