Search

Potassium metasomatism is a common geologic process occurring in a range of environments on Earth. K‐metasomatism can dramatically change the chemistry of large volumes of crust resulting in formation of abundant K‐rich feldspar while preserving original rock textures. Remote sensing of Mars has revealed the presence of rare but striking felspar‐rich terrains as well as K‐rich phyllosilicates such as illite or muscovite. Furthermore, the Curiosity Rover has detected evidence for K‐enrichment in rocks at Gale Crater. In this study, we use spectroscopic and remote sensing analyses of K‐metasomatized rocks on Earth as a comparison to K‐rich and/or possible granitic rocks on Mars. The results suggest that evidence for K‐feldspar rich, quartz‐bearing, and illite‐bearing rocks on Mars is consistent with K‐metasomatism. K‐rich rocks in Gale Crater have themselves likely not been metasomatized, but the abundant feldspar within them might have been derived from metasomatized crust. Plain Language Summary: Mars is a volcanic planet with a surface dominated by basalt and sedimentary rocks derived from basalt, similar to oceanic crust on Earth (though Mars' crust is thicker). For decades, Mars has been considered a "one plate planet" where plate tectonics never occurred and where continental type rocks never formed. Recent observations have, however, revealed enticing evidence that felsic "continental type" rocks containing K‐feldspar and quartz are present. Such rock units were recognized in Gale Crater based on their high potassium abundances. Granite‐type rocks detected from orbit using infrared data would also presumably be rich in potassium. However, rocks of the same composition can be made through aqueous alteration rather than from magma in the crust. This study shows a striking example of potassium enrichment and mineralogical changes in basaltic rocks on Earth caused entirely by low‐temperature hydrothermal brine in the crust. We use analogous remote sensing measurements to study these potassic rocks on Earth and compare the results to Mars. It appears that at least some of the felsic rock compositions on Mars were caused by metasomatism. Key Points: Subsurface brines can deposit significant potassium in volcanic rocks while preserving original texturesSpectral signatures of K‐metasomatism include K‐feldspar and illiteFelsic compositions on Mars might occur because of low‐temperate alteration by subsurface brines [ABSTRACT FROM AUTHOR]

Abstract A range of phyllosilicate compositions have been detected spectroscopically on Mars, but the largest fraction by far corresponds to clay minerals rich in Fe and Mg. Given that most of our understanding of Martian clays comes from remote sensing data, it is critically important to explore the details of how compositional variation affects spectral features of phyllosilicates. The greatest efforts have focused so far on near-infrared (NIR) spectroscopy. Recently, ambiguities have been detected in the NIR spectra of 2:1 phyllosilicates with intermediate Fe Mg content that preclude mineral and chemical discrimination. Such ambiguities highlight the relevance of exploring the visible spectral range as a complementary tool to characterize Martian phyllosilicates precisely. This article reports the investigation of laboratory reflectance spectra (330–800 nm) from 34 Earth analogues of Martian phyllosilicates with a wide range of Mg Fe composition, including nontronite, celadonite and saponite end-members, as well as interstratified glauconite-nontronite, talc-nontronite, and talc-saponite. The spectra indicated the presence of Fe(III) by absorption modulations and a decrease in total reflectance, especially in samples with tetrahedral Fe(III). Absorption bands at 370 and 420 nm were diagnostic of octahedrically and tetrahedrally coordinated Fe(III), respectively. Band amplitudes in the second derivative of the Kubelka-Munk function correlated positively with Fe(III) content (R 2 > 0.8). Standard color analyses of the visible reflectance spectra under the CIE illuminant D65 indicated that the CIELAB color parameter a * 10 was positively correlated with tetrahedral Fe(III), b * 10 was positively correlated with octahedral Fe(III), and L * 10 was negatively correlated with Fe(III) in both structural sites. Because Fe(II) was in relatively low amount, it did not provide clear spectral evidence. Multiple regression models using the amplitude of the diagnostic absorption bands predicted well absolute Fe content in the phyllosilicates (R 2 = 0.89) and the ratio Fe/(Fe + Mg + Al) (R 2 = 0.84). CIELAB color parameters improved the prediction of total Fe (R 2 = 0.92) and the ratio Fe/(Fe + Mg + Al) (R 2 = 0.93). Application of these analyses to Martian data has challenges set by Fe oxide dust coating and spatial and spectral resolution. However, these results mark an avenue to develop testable tools using visible-wavelength spectral data from both satellite and lander probes to help establishing Fe content and mineral identification of Martian phyllosilicates. Highlights • Visible-range spectra of phyllosilicates shaped by Fe content and site occupancy. • Phyllosilicate Fe content is predicted well by 2nd-derivative and color analyses. • Vis spectra from Martian rocks with phyllosilicates comparable to lab spectra. • 2nd-derivative and color analyses hopeful tool to estimate Fe in Martian clays. [ABSTRACT FROM AUTHOR]

Earth analogs are indispensable to investigate mineral assemblages on Mars because they enable detailed analysis of spectroscopic data from Mars and aid environmental interpretation. Samples from four sites in the Iberian Pyrite Belt (El Villar, Calañas, Quebrantahuesos, and Tharsis) were investigated using mineralogical, chemical, and spectroscopic techniques, with a focus on clay minerals and alteration environments. They represent Earth analogs of areas on Mars that underwent acidic alteration. X‑ray diffraction and transmittance mid-infrared data indicate that the rocks were subjected to several degrees of acid alteration corresponding to assemblages characterized by the following mixtures: (1) illite, chlorite, interstratified chlorite-vermiculite, kaolinite-smectite, and kaolinite; (2) illite, kaolinite, and alunite; and (3) jarosite and goethite. According to mineral stability data, these three assemblages correspond to pH values 7–5, 5–3, and <3, respectively. The lack of goethite in the illite-kaolinitealunite assemblage suggests an alteration in reducing conditions. Illite was progressively dissolved by acidic alteration but is sufficiently resilient not to be diagnostic of the intensity of the alteration. Illite and kaolinite were the two most abundant phyllosilicate minerals observed, and the main reaction involving phyllosilicates was the alteration of illite to kaolinite. Mixed-layer phases appeared mainly in the mildest degree of acid alteration, with few exceptions. This suggests a transition from a mechanism dominated by transformation to a mechanism dominated by dissolution-precipitation as the intensity of the acid alteration increases. Our results highlight the sparse kaolinite–alunite occurrences on Mars as worthy of specific investigation. Acid alteration on Mars is expected to be patchy and/or consisting of fine alteration rims. Alunite occurrences on Mars in the absence of goethite may indicate an acid alteration in reducing conditions. Kaolinite produced through acid alteration on Mars is expected to exist mainly as an end-member phase of low crystallinity, which would enhance IR absorption and increase its visibility. [ABSTRACT FROM AUTHOR]

On Earth, trioctahedral (Mg-rich) vermiculite occurs as a transient alteration product of Mg/Fe2+-mica and chlorite, pyroxene, olivine, amphibole and even plagioclase where there is a nearby Mg source, indicating mild/short-lived weathering or hydrothermal conditions. On Mars, the abundant mafic rocks and low water availability suggest the likelihood of widespread vermiculite formation from the Noachian to the early Hesperian. Although it can be challenging to identify vermiculite by remote sensing or in situ infrared spectroscopy alone, we show that numerous previous remote sensing results suggest that vermiculite is widespread on the martian surface and perhaps a more common mineral on that planet than on Earth. Investigation of rocks from Riotinto (SW Spain) provides an example of how chlorite alters to vermiculite in an acidic environment. Similar incomplete vermiculite replacements of Mg/Fe2+-mica, chlorite and mafic minerals could have resulted on Mars from acidic to neutral weathering at low water regimes. While possible bias exists towards remote sensing detection of vermiculite on Mars in sites (1) with advanced alteration and (2) where vermiculite is not mixed (at pixel level) with other phyllosilicates, we find that vermiculite occurs in Noachian terrains, in sites (wider than pixel level) where other phyllosilicates are detected, and that vermiculite has more Fe3+ than is typical on Earth. [Display omitted] • Vermiculite is a transient alteration product of mafic minerals, mica and chlorite. • Vermiculite is widespread on Mars and found in Noachian altered terrains. • The mineral might be more commonplace on Mars than on Earth. • Most identified vermiculite on Mars so far is Fe3+-rich. • Existing biases suggest more occurrences and wider composition range. [ABSTRACT FROM AUTHOR]

With the arrival of Curiosity on Mars, the MSL has started its ground validation of some of the phyllosilicate characterization carried out with remote sensing near-IR spectroscopy from orbital instruments. However, given the limited range of action of the rover, phyllosilicate identification and characterization will have to rely mainly on orbital near-IR data. Investigation of Earth analogs can greatly assist interpretation of martian spectra and enable more robust analyses. In this contribution, Mg/Fe-rich clays from submarine hydrothermal origin that had been thoroughly characterized previously were investigated with near-IR reflectance spectroscopy. The clays are mixed-layer glauconite-nontronite, talc-nontronite, talc-saponite, and nontronite samples. The hydroxyl bands in the range 2.1-2.35 μm were decomposed into their several individual components to investigate correlations between the octahedral chemistry of the samples and the normalized intensity of several bands. Good correlations were found for the samples of exclusive dioctahedral character (glauconite-nontronite and nontronite), whereas poor or no correlations emerged for the samples with one (talc-nontronite) or two (talc-saponite) trioctahedral layer components, indicating a more complex spectral response. Because these bands analyzed are a combination of the fundamental OH stretching and OH bending vibrations, the response of these fundamental bands to octahedral chemistry was considered. For 2:1 dioctahedral phyllosilicates, Fe and Mg substitution for Al displaces both fundamental bands to lower wavenumbers (longer wavelengths), so that their effect on the position of the combination band is coherent. In contrast, for trioctahedral clays, Al and Fe3+ substitution of octahedral Mg displaces the OH stretching band to lower wavenumber values, and the OH bending band to higher wavenumber values, resulting in partial or total mutual cancelation of their effects. As a result, clays with near-IR spectra indicating Mg-dominated octahedral compositions may in fact contain abundant Fe and some Al substitution. Thus, remote-sensing near-IR mineralogical and chemical identification of clays on Mars appears relatively straightforward for dioctahedral clay minerals but more problematic for trioctahedral clays, for which it may require a more detailed investigation of their near-IR spectra. [ABSTRACT FROM AUTHOR]

Near-infrared remote sensing data of Mars have revealed thousands of ancient deposits of Fe/Mg-rich smectitic clay minerals within the crust with relevance to past habitability. Diagnostic metal–OH infrared spectroscopic absorptions used to interpret the mineralogy of these phyllosilicates occur at wavelengths of 2.27–2.32 μm, indicating variable Fe/Mg ratios in the clay structures. The objective of this work is to use these near infrared absorptions to constrain the mineralogy of smectites on Mars. Using Fe/Mg-rich seafloor clay minerals as mineralogical and spectroscopic analogs for Martian clay minerals, we show how crystal–chemical substitution and mixed layering affect the position of the diagnostic metal–OH spectral feature in smectitic clay minerals. Crystal-chemistry of smectites detected on Mars were quantitatively constrained with infrared data and categorized into four mineralogical groups. Possible alteration processes are constrained by comparisons of clay chemistry detected by remote sensing techniques to the chemistry of candidate protoliths. Of the four groups identified, three of them indicate significant segregation of Fe from Mg, suggestive of alteration under water-rich and/or oxidizing conditions on Mars. The fourth group (with low Fe/Mg ratios) may result from alteration in reducing or water-limited conditions, potentially in subsurface environments. Some samples are interstratified di–trioctahedral clay minerals that have characteristics of dioctahedral clay minerals but clear chemical evidence for trioctahedral sheets. Approximately 70% of smectite deposits previously detected on Mars are classified as Fe-rich (FeO/MgO > 10). Only 22% of detections are trioctahedral and relatively Mg-rich. An additional ∼8% are difficult to characterize, but might be very Fe-rich. The segregation of Fe from Mg in Martian clay minerals suggests that Mg should be enriched in other contemporaneous deposits such as chlorides and carbonates. [ABSTRACT FROM AUTHOR]

The cameras on the Pathfinder probe (Imager for Pathfinder) and the rovers Spirit and Opportunity (Panoramic Camera), Curiosity (Mast Camera) and Perseverance (Mast Camera-Z) produce visible-range spectra of limited wavelength resolution but of great target resolution which allows mineralogical analysis of selected areas within martian rocks. Laboratory spectra of relevant specimens were transformed into the spectra corresponding to each of the above cameras to increase our capability to interpret martian spectral data collected in-situ. The focus was on finding spectral features that can be detected by the cameras on Mars and are diagnostic of specific minerals. The samples are a collection of (1) Fe/Mg-phyllosilicates from submarine hydrothermal sites and (2) of rocks from acid alteration environments containing goethite, hematite, jarosite and Fe-bearing chlorite as these minerals are detectable in the extended visible range. Among all the samples, interstratified glauconite-nontronite has the most unique spectral features and should be easily detectable with the rover cameras. The spectral features of talc from Fe-bearing interstratified specimens are described. These data are especially relevant as glauconite and talc have been proposed to be fairly abundant on Mars and their detections are suggested from remote-sensing near-IR data. Several indices are proposed to assess Fe content on the investigated samples as well as mineral concentration of goethite and hematite. Among these indices, the normalized spectral slope in the range 420–600 nm increases with total Fe content for all samples, whether phyllosilicates, oxides or sulphates (R2 = 0.7–0.8). For pure phyllosilicates, the slope from 600 to 1010 nm decreases with increasing octahedral Fe (R2 = 0.75). An index for goethite produced excellent results assessing goethite concentration (R2 = 0.84). Of all cameras, Mast Camera reproduces the spectra with lowest fidelity and generates the poorest correlations between indices and tested variables. The other three cameras perform similarly. [Display omitted] • Lab spectra of Earth analogues shown with the resolution of martian rover cameras. • Spectral fidelity of Fe-clays and Fe oxides is: IMP, PanCam, MastCam-Z > MastCam. • Interstratified glauconite-nontronite easily recognizable on Mars by rover cameras. • Spectral features of Fe-rich talc and interstratified talc layers are shown. • Spectral indexes correlate well with Fe content and goethite content in samples. [ABSTRACT FROM AUTHOR]

The Curiosity rover on Mars, landed in 2012, is capable of mineralogical investigation using X-ray diffraction, complementing the abundant infrared remote sensing data already available on clay minerals. We can, however, expect that the in situ X-ray diffraction information will convey a more complex picture than that inferred from infrared spectroscopy alone. CheMin has landed. [ABSTRACT FROM AUTHOR]