Diagenesis of Clay Minerals on Mars and Implications for the Mars Science Laboratory Rover

Introduction: The discovery of clay minerals in ancient Martian terrains [1-3] has had a profound effect on our understanding of the timing, duration, and extent of aqueous processes on that planet. Unlike Earth, the lack of plate tectonics and crustal recycling on Mars has resulted in a well-preserved ancient rock record. Therefore, the presence of clay minerals and other hydrous phases in strata regarded to have formed >3.5 Ga provides a unique opportunity to study the stability of such minerals over timescales not accessible in the terrestrial rock record. Indeed, recent studies have suggested that some of the oldest documented smectites on Earth may be on the order of ~600 Ma [4], billions of years younger than any of the terrains with clay-bearing deposits identified on Mars. The majority of clay minerals on Earth are transported by wind or water, deposited in basins, buried, and ultimately either subducted or uplifted, commonly experiencing various degrees of diagenesis or metamorphism during this cycle. During burial diagenesis, elevated temperatures and interaction with fluids commonly convert smectites to mixed-layered clays and later illite or chlorite depending on the initial smectite composition and fluid chemistry. In contrast, clay minerals in many of the martian deposits have been identified as smectites [1-3,5-6], suggesting they have not been significantly affected by diagenesis. Here we examine the identification of martian clay minerals in greater detail to show that many occurrences are consistent with the presence of chlorite and that some previously reported smectite deposits are consistent with mixed-layered chlorite-smectite (C/S), suggesting diagenesis of smectite deposits may have been common on Mars. Methods: Laboratory visible-near infrared reflectance spectra of well-characterized clay samples, including smectites, mixed-layered C/S, and chlorite, that were previously acquired under purged and/or heated conditions [7] were examined to devise parameters to distinguish between these different clay structures. The majority of martian clay deposits are consistent with Mg/Fe-rich varieties [6], thus our study focused on samples with similar compositions. It was found that a minimum of two parameters are necessary to distinguish fully hydrated smectites, partially hydrated/dehydrated smectites, mixed-layered C/S, and chlorite. A characteristic of mixed-layered C/S when compared with smectites is a strong asymmetry between the short and long wavelength edge of the Mg/Fe-OH band centered near 2.28-2.32 μm after a straight-line continuum removal from 2.2-2.4 μm. The first parameter takes advantage of this by defining two separate continuum lines, one from ~2.2-2.36 μm and one from 2.36-2.42 μm, and calculates the angle between the absorptions defined by these lines. This angle decreases with increasing chlorite content (Figure 1). However, dehydration of some smectites can also result in a decrease in this absorption angle, requiring an additional parameter to distinguish such clays from mixed-layered C/S. The second parameter consists of a ratio of the band depth of a ‘chlorite’ absorption (band depth at ~2.34 um) to the band depth of a ‘smectite’ absorption (band depth at ~2.29 um). This ratio was found to increase as smectites gain more chloritic layers during the transition from S→C/S→C (Figure 2). Utilizing both of these parameters effectively stretches the data points over a larger parameter space to help distinguish the effects of composition (S vs. C/S) and hydration state (which is dependent on the percentage of smectite layers for pure clay samples).