Impact-generated Hydrothermal Alteration on Mars: Clay Minerals, Oxides, Zeolites, and More

Introduction: In the Noachian, Mars’ crust was deeply affected by impact crater gardening [1], and subsequent hydrothermal alteration. While water was present in the Martian crust [2,3], frequent impacts [4,5] provided prominent heat sources that drove hydrothermal systems. Those systems were scattered randomly over the planet’s surface and variable in size, but generally spanned the diameter of any ≥10 km complex crater (based on analyses by [6]). These systems were also deep reaching and long-lived [7] with central crater uplifts and crater modification zones affected by the most intense water flow [8]. Because circulating water driven by a temperature gradient changes the thermochemical status of a system, mineralogical reactions inevitably took place. Similar mineralogical consequences have been documented in terrestrial impact-generated hydothermal systems. Approximately half of all known impact structures on Earth show hydrous alteration phases produced by the interaction of water with a melt sheet and/or hydrothermal systems that penetrated the crust beneath the crater floor [9]. Immense systems have been documented in drill cores from Earth’s largest impact structures, such as the Chicxulub crater (Yucatán, Mexico) and the Sudbury structure (Ontario, Canada) [10,11]. As we have shown earlier for the basaltic crust of Mars [12–14], thermochemical changes caused by circulation of heated formation brines mainly result in the crystallization of sheet silicates (serpentine, chlorite, nontronite, and kaolinite), amphiboles, oxides and hydroxides (magnetite, hematite, diaspore), and additional minor phases. Our results compare favorably with those from sattelite-based spectroscopic measurements of the Martian surface by OMEGA and CRISM (aboard the European Mars Express and U.S. Mars Reconnaissance Orbiter, respectively). They report many of the same minerals, amongst them several smectites and micas – such as kaolinite, Fe/Mgsmectite, saponite, illite/muscovite, and chlorite – but also hydrous silica, zeolite, and carbonate [15–17]. Potential testimony of impact-generated hydrothermal systems has been found in three Martian craters so far: hydrous phases are located in central peaks and inner crater walls in a ~ 100 km diameter crater in Mawrth Vallis [18], a crater of similar size in Terra Meridiani [19], and an ~20 km diameter crater in the Western Isidis region [15]. In addition to producing mineral assemblages similar to those being observed on Mars, our calculations constrain formation conditions. Our results suggest that alteration assemblages are largely controlled by the water to rock ratio (W/R) and temperature, while the pH is bufferd by the precipitating and dissolved species. Our previous studies showed that three sheet silicates, which also have been found on Mars (chlorite, nontronite, kaolinite), point to specific formation conditions: Chlorite is indicative of intermediate to low W/R and occurs over the whole temperature range [12]. The clay minerals nontronite and kaolinite indicate intermediate and high W/R, respectively. Moreover, nontronite forms below 250 °C only. While our previous studies focussed on the parameters pressure, temperature, and W/R, in this study we explore the influence of host rock variability on the resulting alteration assemblages. Method and host rock chemistry: To understand the thermochemical conditions of impact-generated hydrothermal systems, we performed a series of ΔGminimization calculations using the computer code CHILLER [23]. For details of assumptions, the chemical input data of earlier calculations, as well as the water composition used in all of our studies, see [12]. The chemical proxy for these calculations was the plutonic shergottite LEW 88516. We chose this rock composition, because impact-generated hydrothermal systems are deep reaching [7] with the largest part of the affected volume being at depths where plutonic rocks are found. However, rock variability is to