CLAY MINERALS ON MARS AND EARTH: INSIGHTS INTO LONG-TERM CLAY MINERAL STABILITY?

Clay minerals (e.g., smectite, illite, mixed-layer phases) are common low-temperature alteration and diagenetic products on Earth. They are important in many geological and potentially biological processes, making their presence or absence significant. Recent Mars X-ray diffraction (XRD) data obtained with the CheMin instrument on the Curiosity rover show that clay minerals reside in sedimentary rocks with a provenance age of ~4.2 Ga (Farley et al., 2014), though any authigenic phases, including clays, may be 3.2 Ga or younger. Clay minerals of this age are rare to absent on Earth, often assumed to result from their thermodynamic instability or alteration to higher-temperature assemblages as a result of tectonic activity in the presence of liquid water. In contrast, clays at the surface of Mars may not have experienced similar levels of alteration or diagenesis since their time of formation, although many have been buried and since exhumed.

Clay mineral identifications on Mars are less definitive than on Earth, primarily due to the lack of treatments in the CheMin instrument, such as ethylene glycol solvation. Thus, it is difficult to determine unequivocally, based solely on Mars XRD data, whether mixed-layer clay minerals such as chlorite/smectite or illite/smectite exist. However, orbital spectroscopic data shed considerable light on the nature and distribution of martian clays. Recent study of central peaks on Mars with MRO data suggests that for settings in which clays appear to be uplifted there is a relative increase in chlorite with depth (at the expense of smectite); there is some evidence for chloritization, which is not complete even at 2-6 km depth (Sun and Milliken, 2015). Other spectral observations suggest the presence of mixed-layer chlorite/smectite (Milliken and Bish, 2010), which is consistent with the observation of a 14Å reflection in the Cumberland sample from the Sheepbed mudstone (Bristow et al., 2015). Whether or not the 10 Å and ~14 Å peaks seen in Sheepbed XRD patterns represent mixed-layer clay minerals, the comparative immaturity of martian clay minerals likely reflects the lack of tectonic activity, elevated temperatures and pressures, and limited interaction with water over the past several billion years, providing insight into the long-term stability of clay minerals.