The upper-thermal stability of an iron-rich smectite: Implications for smectite formation on Mars

Abstract

Fe-rich smectite is a clay mineral commonly found on Mars, particularly in the lowest stratigraphic sequences. The presence of smectite is of interest because of its implications for the former presence of water in the near-surface environment. What is unclear are the range of bulk compositions and physical conditions under which this smectite formed. Here we investigated (i) the experimental synthesis of iron-rich tri-octahedral smectite (Fe-smectite) from a variety of bulk chemical compositions, primarily along the Fe-talc—Na-Fe-eastonite join, and (ii) the upper-thermal stability of Fe-smectite made from one of the investigated bulk compositions. It was found that Fe-smectite could be synthesized at 500 °C and 2 kbars in 7–10 days from all of the bulk compositions investigated, demonstrating some flexibility in the compositions of rock and soil on Mars that would yield Fe-smectite. The upper-thermal stability of the Fe-smectite synthesized from the bulk composition consisting of 87 mol% Fe-talc was investigated in the range of 1–3 kbar and 530–620 °C. Breakdown of this smectite (without interlayer water) was modeled thermodynamically by a dehydration reaction involving albite, fayalite, hercynite, quartz, magnetite, and water to permit extrapolation to lower pressures and temperatures. Results from this modeling indicate first that Fe-smectite is stable to depths of 30 km, for a geothermal gradient of 20 °C/km during the Noachian period, indicating that, even without any interlayer water, Fe-smectite has potential for storing considerable amounts of water in the crust of Mars. Second, there is no overlap in the upper-thermal stability of Fe-smectite and the water-saturated solidus of basalt, ruling out direct formation from a basaltic magma for this Mg-free smectite. Third, the upper-thermal stability of Fe-smectite is calculated to be 336 ± 25 °C at 100 bars, which is above the liquid-vapor boundary for H2O, suggesting that formation in the presence of a primordial dense vapor phase in the earliest history of Mars is possible.