Sedimentary Mineralogy and Geochemistry on Mars
Chemical and spectroscopic studies of surface exposures by landed missions, coupled with orbital images and spectroscopy, have revealed a diverse sedimentary record on Mars that spans most of the planet's geological history. Observations have been supplemented with experimental programs that further constrain the character of Martian sedimentary processes. A surprising result is the high abundance of chemical sedimentary constituents. Included in the global record are a variety of Mg-, Fe(III)-, and Ca-sulfates of varying hydration state and of likely evaporative and diagenetic origins, chlorides, amorphous silica, iron oxides of varying crystallinity, and minor occurrences of Mg- and Ca-carbonates. For example, in the Burns formation at Meridiani Planum, chemical constituents make up at least 50% of the sedimentary rocks and, on average, surface soil deposits contain 2.5% sulfur and 0.7% chlorine. At the Phoenix high-latitude landing site, significant amounts of perchlorates were detected in surface soils but their distribution beyond this site is unknown. Basaltic rock debris, in some cases inferred to have been weathered prior to deposition, dominate clastic constituents in Martian sedimentary rocks. However, in some Noachian terrains, a variety of clay minerals have been observed from orbit. Clay mineralogy appears dominated by Fe/Mg-smectites, but locally also include more aluminous varieties. Related phases, including serpentine and zeolites also have been suggested. The overall mineralogy and geochemistry of Martian sedimentary deposits are consistent with a weathering regime dominated by the sulfur cycle, rather than the carbon cycle, in which basaltic rocks were altered under low fluid/rock ratio and low pH conditions. On a global scale, dissolution and alteration of olivine, pyroxenes and iron-titanium oxides dominate over alteration of plagioclase during Martian weathering processes. However, identification of smectite clay minerals in ancient layered terrains suggests more modest pH conditions at certain times and/or locations. Accordingly, the relative distributions of sulfate- dominated and clay-dominated sedimentary rocks, in both space and time, are matters of ongoing study and controversy.
The APXS for the Mars Science Laboratory Mission: Extended Calibrations
The Alpha Particle X-ray Spectrometer (APXS) is part of the payload for the Mars Science Laboratory (MSL) mission. It will analyze the bulk chemistry of martian materials using a 244Cm source and an x-ray detector that are housed in a head that is brought into contact with the sample. The incident x-rays and alpha particles cause x-ray excitation in the sample of the elements Na to Sr. For MSL, the APXS has been modified to have a Peltier-cooled detector for day use; a shorter sample-detector distance for higher count rates and shorter integration times; no alpha detector; and a calibration target. The flight APXS has passed qualification tests and we are now using a laboratory MSL APXS to test aspects of the instrument capabilities. 1) Rocks versus powdered samples are being analyzed with APXS. Our results for the basaltic calibration target rock vs. powder produced concentrations that agreed within 5% rel. for all elements, except Cr which was ∼30% higher in the powder (contamination from the steel mill will be examined). This preliminary data suggests that the effects of bulk sample density on APXS results are minor. 2) To ensure that the calibration standards are relevant to martian compositions, we created a database of certified geostandards including igneous (109) and sedimentary (233) rocks and minerals (129). We chose 28 additional geostandards for extended calibrations and are synthesizing additional calibration materials using USGS BHVO-2 mixed with known masses of dried, reagent grade NiO, ZnO, S, FeS2, FeS, Fe, Fe2O3, NaCl, CuCl, KBr and HgBr2. 3) Martian analog rocks are being synthesized using fired, reagent grade oxides, silicates, phosphates, and decarbonated carbonates. We will compare the APXS analyses of these analog rocks with those made using appropriate minerals. Candidate minerals were chosen using a database of 174 minerals (from literature and spectral databases) then candidate minerals were hand-picked and analyzed. The final minerals include: olivine- San Carlos AZ; augite- Kilbourne Hole NM; labradorite- Sonora Mexico; apatite- Durango Mexico; magnetite- Pena Colorado Mexico; hematite- Minas Gerais Brazil; goethite- Oreland, PA; pyrophyllite- Hillsboro NC; Ca-montmorillonite- Gonzales Co. TX; natroalunite- Quartzite AZ; opal- Virgin Valley NV; and reagent NaCl. Metastable hydrous sulfates (reagent MgSO4.XH2O, CaSO4 and Fe3+SO4.5H2O) will be heated (<∼180 deg.C, 24 hr) and the absence of H2O verified using infrared spectroscopy so that we can put stable, dry sulfates in the APXS vacuum. Matrix effects on APXS spectra will be evaluated with pure minerals, and crystallographic orientation effects are being measured using muscovite, biotite, phlogopite, and lepidolite. 4) Campbell et al. (2008 JGR 113) developed a new method to determine light element contents by modeling the Pu-scatter peaks in the x-ray spectrum. We tested this method using well-characterized Fe-H-C-S-O minerals, a standard XRF sealed 244Cm source (E>∼10keV), and controlled relative humidity conditions. The samples were kept sealed with thin Kapton foil to maintain stable mineralogy. APXS-deduced OH and H2O contents are in reasonable agreement with actual values, in spite of the large uncertainties imposed by the column of air and Kapton foil between the sample and detector. Now, we plan to analyze some of those samples, phyllosilicates, carbonate rocks, and olivine-salicylic acid mixtures using the MSL lab instrument in vacuum.
Complementing APXS Data with the Textural and Mineralogical Appraisal of Martian Materials: Value-Added Fusion of Compositional and Vision Systems
The alpha-particle X-day spectrometer (APXS) for NASA's Mars Science Laboratory (MSL) rover mission has a proven record of providing quality in-situ chemical analyses of martian materials. This instrument has been previously deployed successfully on the Pathfinder and MER missions, the latter with the twin rovers Opportunity and Spirit. The MSL APXS will have a number of refinements that will enhance its performance, including improved sensitivity by a factor of 3 (with full chemical analysis in 3 hours), better sensitivity for high atomic number elements, operable with good X-ray FWHM at temperatures up to -5C, on-board basaltic calibration standard, no alpha channel and compressed short-duration X-ray spectra (10 seconds) to support the deployment on sample. In order to complement the chemical data obtained by APXS, we are investigating the addition of textural and mineralogical interpretation via imaging. This will primarily be achieved by integrating Mars Hand Lens Imager (MAHLI) data, as well as other camera systems. The goal is to facilitate further interpretation of the chemical data and derived normative mineralogy by combining image analysis software with the APXS results. Two specific areas of application are being developed: (1) grain size shape and size determination, and (2) inferred mineralogy based on grain colour, reflectivity, shape and relief. For example, the chemical composition of a basalt, diabase and gabbro can be the same, but the genetic implications of each of these different rock types can be different (i.e., basalt is extrusive, diabase is high-level intrusive and gabbro is plutonic). Volcanic sands can also be derived by the dry (e.g., post-Noachian) weathering of basalt. Grain size and shape can distinguish these variants. The short-term goal is to combine image and analytical data via science team interpretation of information transmitted back to Earth during the mission. The long-term objective is to develop autonomous vision systems that are integrated with APXS and other analytical data in order to increase the efficiency of exploration methodology via on-board decision- making protocols. An autonomous system would permit on-site triage, with the rover being able to evaluate martian materials within a pre-determined hierarchy (e.g., during a given sol of operations, do not analyze basalts, but do analyze any diabases encountered).
Spectral Reflectance Properties of Carbonates from Diverse Environments and Related Phases: Implications for Mars
Carbonates are known to be present on Mars from their direct detection in Martian meteorites and from spectroscopic observations of the planet's surface, and are dominated by Ca- and Mg-bearing species. We have conducted laboratory spectral reflectance studies of a wide variety of Mg-, Ca-, and MgCa-bearing species. We have included carbonate-bearing samples subject to tectonic forces, such as landslides, dynamiting, and impacts (shatter cones, and brecciated and melted/recrystallized carbonates), eroded and iron-stained samples, as well as hydrated species and possible decomposition products (Ca- and Mg- hydroxides and oxalates), and carbonates mixed with common associated minerals, such as iron oxides/hydroxides and sulfates. The goal is to determine what factors may affect carbonate detectability on Mars. Focusing on the 0.3-5 micron region, we have found that different anhydrous MgCa carbonates can be spectrally distinguished on the basis of differences in CO-associated absorption bands in the 2.3, 2.5, 3.4, and 3.9 micron regions. Such absorption bands dominate this spectral region. For hydrated Ca and Mg species (monohydrocalcite, whewellite, Ca-hydroxide, artinite, hydromagnesite), the spectra are dominated by O+H absorption bands; CO-associated absorption bands are weaker than in their anhydrous counterparts. In the 3- 4 micron region, CO absorption bands are at different positions than for the anhydrous species spectra, and are also very weak due to the dominance of the major 3 micron region O+H absorption feature. For carbonates derived from various environments, we have found that the presence of pervasive iron oxide/hydroxide staining strongly affects carbonate spectra below approximately 1 micron; longer wavelength carbonate absorption bands are unaffected. Tectonic processes, such as landslides (slickensides), blast hole surfaces, shatter cone surfaces, and impact brecciation, do not appear to significantly affect the spectral properties of carbonates. In samples showing evidence of impact melting and recrystallization, there is variable evidence for changes in carbonate spectral properties. Carbonate detectability in samples containing other minerals is strongly affected by the spectral properties of the accessory phases. These results suggest that: (1) a wide variety of MgCa carbonates can be spectrally distinguished; (2) the CO absorption bands in the spectra of hydrated carbonates are weaker than the equivalent bands in anhydrous carbonate spectra; (3) iron oxide staining does not affect carbonate spectral detectability; (4) high shock levels are required to noticeably affect carbonate spectra; (5) carbonate decomposition products (hydroxides) are spectrally different from carbonates and exhibit diagnostic absorption bands; (6) spectral detectability is strongly affected by the spectral properties of accessory phases; and (7) hydrated carbonate species will be more difficult to detect spectrally on Mars than anhydrous carbonates.
Determining the Water Content on the Martian Surface Using Reflectance Infrared Spectroscopy of Fe-H-sulfates
Hydrogen has been detected in the martian near-surface and it is likely that it is hosted by minerals that contain hydroxyl and molecular water (H2Omol). Such minerals may include assemblages of Fe-H-sulfates that have been found in martian meteorites (e.g., Wentworth and Gooding 1996 LPSC XXVII 1421) and on the martian surface (Lane et al. 2008 Am Min 93 728; Johnson et al. Geophys Res Lett, 34). To better quantify the total water content of areas of the martian surface rich in Fe-H-sulfates we have calibrated reflectance IR to quantify the water and hydroxyl contents in eight Fe-H-sulfates. We synthesized ferricopiapite, paracoquimbite, szomolnokite, schwertmannite, melanterite and rhomboclase following Hyde et al. 2008 (LPI Contrib 1401 4042) and obtained natural samples of jarosite, ferricopiapite, and butlerite. For each sample, the mineralogy was confirmed using X-ray diffraction and the major element composition was confirmed using Energy Dispersive Spectrometry. H, C and O were determined using mass spectrometry (Hyde et al. 2008) except for the last two samples. Biconical reflectance IR spectra were obtained using a Nicolet Nexus 670 FT-IR with a diffuse rflectance accessory at UNM. A Globar source, KBr beamsplitter and DTGS detector were used to collect spectra at 4 cm-1 resolution, 100 scans, from 700 to 5400 cm-1. Samples were sieved to <45 microns and diluted with KBr. An Al-coated alignment mirror for background spectra. We confirmed that there were no changes in the spectral features over time. Calibration curves for H2Omol (at 1635-45 cm-1) and H2Ototal (at 3186-3384 cm-1) derived using Kubelka-Munk (KM) and effective single- particle absorption thickness (ESPAT) theories are linear, while the normalized optical pathlength approach gives calibration factors that are fit following Milliken and Mustard (2005, JGR 110). Our results show that Fe- sulfate mineralogy affects the calibration factors for H2Omol and H2Ototal. For H2Omol, KM calibration factor=-0.189(1640 band position)+312.57 and the ESPAT calibration factor=-0.00421(1640 band position)+6.95. Using these factors, we predict water contents for natural ferricopiapite and butlerite within 5% rel. of the theoretical values.
Hydrothermal Deposits in the Isachsen Formation, Axel Heiberg Island, Nunavut, Canada: a Terrestrial Analogue of Ancient Springs on Mars
Evidence for past life on Mars is most likely to be found within the preserved mineralized remains of subaerial, subaqueous, and shallow subsurface hydrothermal systems. A case in point is the interpretation of elliptical structures as evidence for the presence of ancient spring deposits in the Vernal Crater, Arabia Terra. The analysis of high-resolution images, and detailed comparison with similar structures on Earth, provide convincing evidence for past spring activity and the potential for life in this region of Mars. Cold, perennial springs rising through permafrost are found at the periphery of evaporite domes in several locations on Axel Heiberg Island, in the Canadian Arctic. The springs are well-documented in their modern setting, and provide terrestrial analogues of potential groundwater flow conditions in polar regions of Mars. Field evidence for hydrothermal activity in the Isachsen Formation at two localities now suggest that some evaporite domes have focused groundwater flow since the Early Cretaceous. At East Fiord, remnant blocks and rafts of basaltic rock in the evaporite host rock were discovered in July 2004 (Geological Survey of Canada). The basalts are pervasively altered and contain Fe sulfides. Blocks of lava at the Junction Diapir, south of Strand Fiord, also contain chalcopyrite suggesting the possibility of smoker wall assemblages precipitated from hydrothermal vents, and of prevailing P-T conditions favorable to chemosynthesis. Detailed investigations of the East Fiord evaporite domes (August 2008, Canadian Space Agency) were carried out using a portable X-Ray diffractometer. On site mineralogical analysis, later repeated in the laboratory, revealed the presence of the secondary sulfates copiapite, fibroferrite, and jarosite, that have formed through oxidation of sulfide minerals associated with the basalts. Sedimentary units consisting of interbedded sandstone and pebble conglomerate locally contain abundant pyrite, and are spatially associated with basaltic rocks (dykes and lava flows) and evaporites. Gossan zones are associated with intermittent springs that flow from the poorly consolidated, quartz-rich sandstones of the Isachsen Formation. These gossan zones within the sandstone consist of thin layers containing goethite, hematite, illite and jarosite. The gossan does not extend into the permafrost, locally measured to start at one meter below the surface. This is an important observation that requires further investigation. The Early Cretaceous Isachsen Formation is a clastic succession dominated by quartz-rich sandstone that contains abundant recycled detrital quartz with intriguing compositional and textural characteristics. A collaborative study of volcanism and sediment provenance in this area is underway to reconstruct Cretaceous to early Tertiary paleoenvironments. The new, compelling evidence for ancient hydrothermal activity in this complex setting of salt diapirism, basaltic volcanism, sandstone deposition, and intense faulting, leads us to conclude that the East Fiord deposits constitute a natural laboratory to study the mineralogy of ancient spring deposits, and the potential for hydrothermal systems, in Arabia Terra on Mars. Results from the 2008 arctic expedition are also integrated in the geomatic analysis of chaotic terrain at Apollinaris Patera volcano, located to the south-east of Elysium Planitia and to the north of Gusev Crater.
Extreme shock effects in relatively enriched shergottite Northwest Africa 4797
This study reports new data on strongly shocked shergottite Northwest Africa (NWA) 4797, found in 2001 near Missour, Morocco as a single stone weighing 15.0 grams. Microtextures were characterized using a JEOL 6301F Field Emission SEM at the University of Alberta. NWA 4797 exhibits two igneous lithologies: poikilitic and non-poikilitic. The stone is composed principally of mm-size grains of olivine (40.3%), pyroxene (33.9%), and glass or recrystallized glass (after plagioclase; 9.1%). Minor phases include chromite (3.5%), merrillite (0.8%), and melt inclusions (0.4%). NWA 4797 is texturally complex, with strong shock overprinting primary igneous features. A thick (1 mm wide; 10.8 vol% abundance) vein of melt material transects the sample. Within both lithologies olivine exhibits strong mosaicism and pervasive fracturing; partial melting and local recrystallization is restricted to poikilitic regions. Pyroxene shows deformed, recrystallized and partially melted margins. The large pigeonite oikocrysts exhibit polysynthetic twinning associated with shock, with reduced birefringence. Precursor igneous plagioclase has been completely melted, vesiculated, and mobilized throughout the host rock resulting in complex recrystallization textures. The bulk composition has been calculated using the mineral mode combined with electron microprobe analyses to be (wt%): SiO2 (41.6), TiO2 (0.4), Al2O3 (3.7), Cr2O3 (2.0), FeO (19.9), MnO (0.5), MgO (25.6), CaO (4.7), Na2O (0.7), K2O (0.1) and P2O5 (0.4). We note that our calculated FeO and Na2O values are within error to INAA values reported previously. Compositions of the earliest formed pyroxene, olivine and chromite yield fO2 = 0.8 log units above the Iron- Wustite (IW) buffer (QFM -2.5). A relative increase in fO2 during crystallization is indicated from spinel-ilmenite pairs in the non-poikilitic areas, which yield fO2 ∼ IW + 3 (QFM -0.5). NWA 4797 is nearly identical in terms of the petrography of igneous lithologies, mineral compositions, modal abundance, oxygen fugacity and bulk composition to some "lherzolitic" shergottites; however, REE abundances indicate that this meteorite may be derived from a distinct mantle source region. NWA 4797 is also distinguished by its strong degree of shock damage, representing a growing group of Martian meteorites shocked within the range of 59 -75 GPa, previously represented only by Dhofar 378.
Igneous Crustal Stratigraphy of Martian Volcanic Provinces
Experimental investigations of a magma of Humphrey composition have delineated the changes in composition of residual liquids and cumulus minerals if the magma ponded at the base of the primary crust and the secondary crust thickened over time due to input of residual liquids. These data were used to model igneous stratigraphy in major volcanic provinces (e.g., the Tharsis rise) and indicate that low density silica-rich, Fe-poor lavas would be overlain by denser Fe-rich, alumina- and silica-poor lavas and finally, by more aluminum-rich, alkali-rich, and Ca-poor ne-normative lavas with low silica and high Fe characteristics. The cumulus layers evolve from early olivine- and pigeonite-dominated assemblages to olivine-free, pyroxene assemblages. Crustal density profiles are being developed based on the compositional data to evaluate the possibility of dense layers in these regions that could contribute to the observed large positive free air gravity anomalies [1-3].  McGovern et al. (2002) JGR 107 (E12), 5136, doi:10.1029/2002JE001854.  McGovern et al. (2004) JGR 109, E07007, doi:10.1029/2004JE002286.  Neumann et al. (2004) JGR 109, E08002.