Manganese-Iron Phosphate Nodules at the Groken Site, Gale Crater, Mars

Abstract

The MSL Curiosity rover investigated dark, Mn-P-enriched nodules in shallow lacustrine/fluvial sediments at the Groken site in Glen Torridon, Gale Crater, Mars. Applying all relevant information from the rover, the nodules are interpreted as pseudomorphs after original crystals of vivianite, (Fe2+,Mn2+)3(PO4)2·8H2O, that cemented the sediment soon after deposition. The nodules appear to have flat faces and linear boundaries and stand above the surrounding siltstone. ChemCam LIBS (laser-induced breakdown spectrometry) shows that the nodules have MnO abundances approximately twenty times those of the surrounding siltstone matrix, contain little CaO, and have SiO2 and Al2O3 abundances similar to those of the siltstone. A deconvolution of APXS analyses of nodule-bearing targets, interpreted here as representing the nodules’ non-silicate components, shows high concentrations of MnO, P2O5, and FeO and a molar ratio P/Mn = 2. Visible to near-infrared reflectance of the nodules (by ChemCam passive and Mastcam multispectral) is dark and relatively flat, consistent with a mixture of host siltstone, hematite, and a dark spectrally bland material (like pyrolusite, MnO2). A drill sample at the site is shown to contain minimal nodule material, implying that analyses by the CheMin and SAM instruments do not constrain the nodules’ mineralogy or composition. The fact that the nodules contain P and Mn in a small molar integer ratio, P/Mn = 2, suggests that the nodules contained a stoichiometric Mn-phosphate mineral, in which Fe did (i.e., could) not substitute for Mn. The most likely such minerals are laueite and strunzite, Mn2+Fe3+2(PO4)2(OH)2·8H2O and –6H2O, respectively, which occur on Earth as alteration products of other Mn-bearing phosphates including vivianite. Vivianite is a common primary and diagenetic precipitate from low-oxygen, P-enriched waters. Calculated phase equilibria show Mn-bearing vivianite could be replaced by laueite or strunzite and then by hematite plus pyrolusite as the system became more oxidizing and acidic. These data suggest that the nodules originated as vivianite, forming as euhedral crystals in the sediment, enclosing sediment grains as they grew. After formation, the nodules were oxidized—first to laueite/strunzite yielding the diagnostic P/Mn ratio, and then to hematite plus an undefined Mn oxy-hydroxide (like pyrolusite). The limited occurrence of these Mn-Fe-P nodules, both in space and time (i.e., stratigraphic position), suggests a local control on their origin. By terrestrial analogies, it is possible that the nodules precipitated near a spring or seep of Mn-rich water, generated during alteration of olivine in the underlying sediments.

Manganese‐Rich Sandstones as an Indicator of Ancient Oxic Lake Water Conditions in Gale Crater, Mars

“…These concretions may be similar to those observed by Treiman et al. (2023) later in the mission. The chemistry of the diagenetic features tends to have a greater abundance of trace elements, including P (e.g., Lasue et al., 2016; L’Haridon et al., 2018; Meslin et al., 2018; Sun et al., 2019; Treiman et al., 2023), and importantly are not always Mn‐rich.…”

“…(2023) later in the mission. The chemistry of the diagenetic features tends to have a greater abundance of trace elements, including P (e.g., Lasue et al., 2016; L’Haridon et al., 2018; Meslin et al., 2018; Sun et al., 2019; Treiman et al., 2023), and importantly are not always Mn‐rich. In addition, the morphological differences between the diagenetic features described by Sun et al.…”

“…The timing and geologic context for manganese precipitation in the Gale crater is complex and appears to be the result of multiple processes. Manganese abundance along the Curiosity rover's traverse has shown considerable variation within sedimentary rocks and veins, ranging from below ChemCam's detection limit of 27 ppm up to 22 wt% MnO in localized (mm to cm) areas (Frydenvang et al., 2020; Gasda et al., 2021, 2022; Lanza et al., 2014, 2016; Treiman et al., 2023). Note that a recent ChemCam calibration for MnO (Gasda et al., 2021) has revised, and generally decreased the compositions reported earlier.…”

“…Some instances of Mn precipitation clearly post‐date the lithification of the sediments—including coatings (Lanza et al., 2015) and fracture fills (Lanza et al., 2016). Other instances of elevated MnO precipitation may have accumulated during early diagenesis of the lake sediments within concretions (Gasda et al., 2022; Lanza et al., 2022; L’Haridon et al., 2018; Meslin et al., 2018; Sun et al., 2019; Treiman et al., 2023). On the other hand, many high MnO observations present in sedimentary rocks do not fit either model.…”

“…Recent observations of manganese precipitation in rocks on Mars by the NASA Mars Science Laboratory Curiosity and Mars Exploration Rover Opportunity rovers (Arvidson et al, 2016;Berger et al, 2020Berger et al, , 2022Gasda et al, 2022;Lanza et al, 2014Lanza et al, , 2016Lanza et al, , 2022Meslin et al, 2018;Thompson et al, 2020;Treiman et al, 2023) and in martian meteorites (Cao et al, 2018;Liu et al, 2017Liu et al, , 2021Nakamura et al, 2023) raise important questions about the range of redox conditions that have existed within martian environments and how they have changed over time. Since manganese-rich materials were first observed in Gale crater (Lanza et al, 2014), sediments containing abundant MnO have been observed periodically throughout the Curiosity rover's traverse in a range of geologic contexts (Frydenvang et al, 2020;Gasda et al, 2022;Lanza et al, 2016Lanza et al, , 2022L'Haridon et al, 2018L'Haridon et al, , 2020Meslin et al, 2018;Sun et al, 2019).…”