Mineralogy and morphology of geologic units at Libya Montes, Mars: Ancient aqueously derived outcrops, mafic flows, fluvial features, and impacts

Abstract

[1] There is ample evidence of both ancient and long-lasting fluvial activity and chemical alteration in the Libya Montes region south of Isidis Basin. The region hosts Noachian to Amazonian aged surface rocks with extensive outcrops of olivine-and pyroxene-bearing material. Libya Montes also features surface outcrops and/or deposits hosting Fe/Mgsmectite, Fe/Mg-smectite mixed with carbonate and/or other Fe/Mg-rich phyllosilicates, and Al-smectite. These units likely formed through chemical alteration connected with hydrothermal activity resulting from the formation of the Isidis Basin and/or the pervasive fluvial activity throughout this region. The morphology and stratigraphy of the aqueous and mafic minerals are described using High Resolution Imaging Science Experiment and High Resolution Stereo Camera derived digital terrain models. Analyses of the Compact Reconnaissance Imaging Spectrometer for Mars spectra show variations in the chemistry of the Fe/Mg-smectite from nontronite-like exposures with spectral features near 2.29 and 2.4 mm more consistent with Fe 3+ 2 OH groups in the mineral structure, and saponite-like outcrops with spectral features near 2.31 and 2.38 mm characteristic of Mg 2+ 3 OH groups. These Fe/Mg-smectite bearing materials also have bands near 1.9 mm due to H 2 O and near 2.5 mm that could be due to the smectite, other phyllosilicates, and carbonates. All regions exhibiting carbonate features near 3.4-3.5 mm also have features consistent with the presence of olivine and Fe/Mg-smectite, indicating that the carbonate signatures occur in rocks likely containing a mixture of these minerals. The Al-smectite-bearing rocks have bands near 1.41, 1.91, and 2.19 mm that are more consistent with beidellite than other Al-phyllosilicates, indicating a higher-temperature or diagenetically processed origin for this material. Our interpretation of the geologic history of this region is that ancient Noachian basaltic crustal materials experienced extensive aqueous alteration at the time of the Isidis impact, during which the montes were also formed, followed by emplacement of a rough olivine-rich lava or melt, and finally the smooth pyroxene-bearing caprock unit.All Supporting Information may be found in the online version of this article.

Deep alteration between Hellas and Isidis Basins

“…In terms of alteration products, studies report detections of phyllosilicates Fraeman et al, 2009;Loizeau et al, 2012) in impact craters using CRISM data. Carbonates have been detected at the northern border of our investigation area in Libya Montes (Bishop et al, 2012(Bishop et al, , 2013Ehlmann et al, 2008), in Leighton crater (Michalski and Niles, 2010) and around Huygens crater (Wray et al, 2011). Glotch and Rogers (2013) report detections of lime (CaO), periclase (MgO), portlandite (Ca(OH) 2 ), and brucite (MgO) near Syrtis Major.…”

Constraints on the crystal-chemistry of Fe/Mg-rich smectitic clays on Mars and links to global alteration trends

“…In fact, Mg-rich carbonates are often observed together with Ferich smectites and were in many cases difficult to completely isolate from the Fe-smectite signatures (e.g. Ehlmann et al, 2009;Bishop et al, 2013). Likewise, it would not be surprising if ancient chloride deposits (Osterloo et al, 2010) are rich in Mg.…”

“…Considering that our classification of clay minerals on Mars is focused on Fe-content, it makes sense to compare the geographic occurrence of different groups to the Fe-content of the Martian crust, as observed in Gamma Ray Spectrometer (GRS) data (Boynton et al, 2008) (Fig. 8b).…”

“…There are several observations to note: 1) the smectites occur almost entirely in Noachian terrain, considering the most recent global geologic surface mapping (Tanaka et al, 2014), 2) the most common group of smec- Carter et al (2013) to classify the composition of 1007 Fe/Mg-rich smectite detections on Mars according to our four groups described in the text. Smectite detections are overlaid on Noachian geologic surface ages (Tanaka et al, 2014) (a) and on Fe (converted to FeO) abundance detected by GRS (Boynton et al, 2008). Clay groups, which are categorized by Fe-content, are decoupled from crustal FeO-content, as derived by GRS.…”

Visible–near infrared spectra of hydrous carbonates, with implications for the detection of carbonates in hyperspectral data of Mars

“…Mg-Fe carbonate has been inferred from multiple instruments on the Spirit rover in the Comanche outcrop within Gusev crater (Morris et al, 2010), and by the Curiosity Rover in Gale Crater in low abundance (Leshin et al, 2013;Archer et al, 2014;Ming et al, 2014). Regional deposits of Mg-or Mgand Fe-carbonate have been observed in Nili Fossae and Huygens crater (Wray et al, 2011), Ca-or Fe-carbonates have been exhumed by a crater near Syrtis Major (Michalski and Niles, 2010) and in McLaughlin Crater (Michalski et al, 2013), and small amounts of dolomite has been identified in Libya Montes (Bishop et al, 2013a). With the exception of the global dust component and a potentially regional process germane to the carbonate discovered by the Phoenix lander, all of the carbonate deposits found on Mars to date are both rare and localized.…”