Impact-generated Hydrothermal Systems in Mafic to Ultramafic Noachian Crust on Mars

Introduction: Frequent impacts [e.g., 1-3] reworked the crust of Mars during the Noachian [4]. The energy of those impacts created thousands of prominent heat sources that supported long-lived, deep-reaching hydrothermal systems [5,6]. In this paper, we investigate how those systems altered the chemistry and mineralogy of Noachian crust. Previous work and motivation for this study: We previously examined secondary minerals formed by the alteration of the plutonic lherzolithic shergottite LEW 88516 [8-9]. As the Martian crust contains diverse lithologies, we extended our calculations to compositions of more feldspathic shergottites (Dhofar 378) and olivine-rich dunites (Chassigny) [10]. Here we present the missing piece: pyroxenitic compositions that are represented by Nakhla, whose chemistry is taken from [11] with FeO/Fe 2 O 3 partitioning as suggested by [12]. The initial fluid composition is a Ca-Mg-Fe-brine that is in equilibrium with Nakhla. We also considered a brine with sufficient K + to charge balance SO 4 2-at a water/rock (W/R) value of unity. Furthermore, we considered a brine with ~5 times this amount of K + (and SO 4 2-) to evaluate its affect on alteration assemblages. We calculated results for five temperatures: 13 °C (which is the minimum temperature at 1 km depth according to the geothermal gradient of Mars after [13]) and 50 °C for the low temperature part of the system, i.e., the radially most-distant parts of the system beneath the crater at any time and also the last stage of the system before it returns to ambient thermal conditions. The high-T steps are 150, 200, and 300 °C. Figure 1 displays the 150 °C results. Results: From the pyroxene-rich Nakhla protolith, pyroxene and/or amphibole (re-)precipitates at the lowest W/R values and from all fluid compositions. Nontronite is an abundant product at intermediate W/R values (Fig. 1). Nakhla alteration vs. that of other protoliths. Alteration at 150 °C (110 bar) and W/R of 1 creates secondary pyroxene (Dio 31.9 Hed 68.1) and amphibole (Rie 20.4 Trem 32.7 Act 46.9) accompanied by serpentine, chlorite, and garnet (andradite). At an intermediate W/R of 1000 the dominating phases are hematite and clay (Mg-nontronite), which are accompanied by quartz and pyrite. If the W/R is very high (100000), hematite dominates the precipitate with pyrite and traces of diaspore. This pyroxene-rich alteration is unlike that produced from other mafic and ultramafic protoliths (Fig. 1). At W/R of unity, 28 wt.-% of the alteration assemblage is iron-dominated …