P12A-01
Magmatic history of Martian highland volcanoes and Tharsis: clues from electron reflection magnetometry
Martian crust which has been demagnetized by magmatic intrusion in the last ~4 Ga cannot have subsequently acquired, upon cooling, any substantial thermoremanent magnetization, due to the lack of a global magnetic field since then. While some volcanoes may have been active earlier, most if not all have experienced significant magmatism in the absence of such a field, as likely has much of the Tharsis province. Therefore, unlike the terrestrial case, the magnetic signature of martian volcanoes should be dominated by thermal demagnetization. We examine, using the electron reflection (ER) map of crustal magnetic field magnitude at 185 km altitude, the degree and spatial extent of thermal demagnetization (assumed to be due to magmatic intrusion) at the highland volcanoes Tyrrhena Patera and Syrtis Major and in the southwestern region of Tharsis. Using topographic and photogeological estimates of extruded magmas and straightforward thermal modeling, we conclude it is likely that Syrtis has a substantially higher ratio of intruded to extruded magmas than Tyrrhena. This may be attributed to active extension due to Syrtis's proximity to the Isidis basin or possibly by more buoyant and/or differentiated magmas. We also observe that the sharp boundary between the moderately magnetized and completely demagnetized regions of southwestern Tharsis matches very well with the currently identifiable extent of fields of small volcanic vents between the major volcanoes. We investigate, again with simple thermal modeling, the possibility that these fields of small vents might represent significant volcanic events within the region, and when considered with major shield building events, may have contributed to the demagnetization of the entire province.
P12A-02
Spectral Evidence for Aqueous Alteration of the Plains Surrounding Valles Marineris, Mars
Data from the high-resolution CRISM visible-near infrared spectrometer onboard the MRO spacecraft reveal the presence of hydrous phases in the plains surrounding the Valles Marineris canyon system. These phases can be classified as either, phyllosilicates or, more generally, hydrated silicates. CRISM spectra of the former are consistent with Al and Fe/Mg-bearing smectites, whereas those of the latter may represent the presence of amorphous silica, hydrated glass (impact or volcanic), chalcedony, opal, or other hydrated Si-rich phases. The phyllosilicates occur in rough textured, light-toned, Noachian age outcrops exposed on the plains along the western portions of the canyon system, including the regions south of Eos and Coprates Chasma and in the walls of Ganges and Coprates Chasma. The presence of layering or total thickness of these deposits is currently unclear, but they are commonly covered by a thin, darker unit and it is likely that their lateral extent is greater than the size of the individual exposures. The spectra for these exposures exhibit H2O features near 1.4 and 1.9 um and Al-OH or Fe/Mg-OH combination bands near 2.2 or 2.3 um, respectively. The spectra are most consistent with smectites such as montmorillonite, nontronite, and/or saponite. In contrast, the hydrated silicates are found primarily in layered materials that exhibit parallel beds that vary in thickness and albedo in the Hesperian aged plains south of Melas and Ius Chasma and west of Juventae Chasma. The spectra and the smooth morphology and light-toned color of these units as observed in accompanying HiRISE and CTX images are inconsistent with lava flows and imply a pyroclastic, aeolian, or fluvial/lacustrine origin instead. Cross-bedding or other sedimentary structures indicative of aeolian or fluvial processes have not been observed to date, but this may be limited by the spatial resolution of the images or inadequate sampling of these units. Spectra of these materials exhibit H2O absorptions at 1.4, 1.9, and 3 um and an OH feature near 2.21 um, though sometimes this feature appears as a weaker band near 2.23 um. The OH feature is broader and sometimes more rounded than similarly positioned bands in Al-bearing phyllosilicates such as montmorillonite. The occurrences of these phases in the plains around V. Marineris do not appear to be associated with or directly adjacent to exposures of phyllosilicates or sulfates. The presence of phyllosilicates in the Noachian crust along V. Marineris, combined with observations of similar materials exposed throughout the ancient cratered highlands, suggests widespread aqueous alteration of the primary crust during the Noachian and is consistent with the model of Bibring et al. (2006). In contrast, the hydrated silicates in the Hesperian plains do not follow the convention of this model. Their composition may represent variations in parent lithology (e.g. volcanic ash instead of basalt), fluid chemistry, climatic changes, or a combination of these factors. Understanding the origin, age, relationship to other hydrous phases, and widespread of these phases may provide a better understanding of the transition from a phyllosilicate to sulfate dominated era on Mars.
P12A-03
Layering of the Phyllosilicates in the Mawrth Vallis Region of Mars, as Seen by OMEGA and HRSC Colors and DTMs
OMEGA/Mars Express has discovered large outcrops rich in phyllosilicates in the region of the outflow channel Mawrth Vallis, Mars (around 20°W, 25°N), through the detection of absorption bands at 1.4 and 1.9 \micron, and at 2.2 or 2.3 \micron. Comparison with laboratory spectra reveals similarities with Al-OH smectites (with the presence of 2.2 \micron band) and Mg- or Fe-OH smectites (with the 2.3 \micron band). Those hydrated minerals are located exclusively on strongly eroded bright outcrops, exhumed from the Noachian plateaus, and cut by the outflow channel, as seen on HRSC/MEx and MOC/MGS narrow angle images. Several MOC and HiRISE/MRO images also reveal that those bright Noachian terrains display meter-scale layers, over more than 100 meter depth as seen on some crater walls. The horizontal extension of more than 300 km x 400 km of this thick phyllosilicate-rich unit implies an important volume of altered rocks, formed during the "phyllosian era". HRSC color images reveal redder terrains for the Mg- or Fe-OH smectites-rich outcrops as detected by OMEGA, whereas Al-OH smectites-rich outcrops correspond to whiter terrains on HRSC color imagery, helping to a finer mapping of those hydrated terrains. These colors reveal a color thick layering of the phyllosilicate-rich units that corresponds to the thin layers seen with MOC and HiRISE. This color layering, along with the use of high resolution HRSC DTMs, help us in the understanding of the geometry of the phyllosilicate-rich unit. From this analysis we observe that stack of layers of a given composition can be followed over tens of kilometers. We also observe that Fe-OH smectite-rich and Al-OH smectite-rich layers are not only found in different geographical location but also over vertical sections, with the same alternation of layers separated by many kilometers, an important geometry to understand the sedimentary origin of the layered deposits. Future lander missions could help to understand further the processes of formation of the unit.
P12A-04
Mineralogic Diversity and Geomorphology of CRISM-detected Phyllosilicate Bearing Materials in Nili Fossae, Mars: Implications for Aqueous Alteration
The Noachian crust of Nili Fossae, northwest of the Isidis basin, is of substantial interest because of km-scale exposures of Fe,Mg-rich phyllosilicates (Bibring et al, 2005; Poulet et al, 2005), the largest spatial exposure of olivine on Mars (Hoefen et al, 2003; Hamilton and Christensen, 2005; Mustard et al, 2005), and stratigraphic relationships with the comparatively well-dated Isidis impact structure (early Noachian) and Syrtis Major lava flows (middle Hesperian) which provide temporal constraints on regional aqueous alteration. MRO-CRISM has identified diverse alteration minerals including nontronite, saponite, kaolinite, chlorite, illite/muscovite and hydrated silica-rich glass (Mustard et al, Nature, submitted). Zeolite is also a potential match to material in craters near 19 N, 65 E. We present the geologic setting of alteration minerals in this region through analyses of CRISM, HiRISE, and CTX data. Mineralogic and geomorphologic data are assessed relative to hydrothermal, near-surface pedogenic, deep crustal, and lacustrine modes of phyllosilicate formation. Based on the spatial distribution of assemblages of the alteration minerals, three distinct provinces are tentatively identified: (1) Western: Chlorite, smectite, hydrated glass, and potentially zeolite are found. In two cases, these minerals are associated with crater central peaks, identified by Bandfield et al (2004) as having quartzofeldspathic material. (2) Central: Phyllosilicates are associated with small eroded knobs and ridges in heavily cratered terrain. Chlorite dominates over smectites and illite/muscovite. (3) Eastern: Smectites and kaolinite underlie a cap of spectrally neutral mesa-forming material in erosional terrain. Fe,Mg-smectite, the dominant alteration mineral, usually occurs as a ridged or polygonally fractured bright unit. In Jezero crater (Fassett and Head, 2005), phyllosilicates have been transported from within a regional watershed and deposited in a delta fan during erosion by surface water flow. Distinct provinces suggest that the nature of aqueous activity varied in space and time across Nili Fossae. Smectites within the Eastern province are cut by the fossae and exposed as the lowermost stratigraphic unit in images sampling over 200,000 km2 of terrain. The great spatial extent of this unit seems to exclude hydrothermal or lacustrine processes as the primary mechanism for smectite formation, and instead favors near surface alteration or deep crustal processes. However, the existence of small-scale exposures of kaolinite within this province indicates that, locally, more intense weathering may have occurred due to enhanced throughflow of water or hydrothermal alteration. Further, a post-smectite formation episode of fluvial activity (e.g. Mangold et al, 2007) is indicated by transported phyllosilicates in Jezero crater. Greater mineral diversity in the Central and Western provinces may have been generated in small-scale hydrothermal systems related to impact events or may reflect changes in underlying regional crustal materials westward from the Isidis basin. Units underlying phyllosilicates (if exposed) are key in determining formation processes and are being sought in CRISM targeted observations.