Search results for " ROC"
showing 10 items of 951 documents
Geochemical modeling of evaporation processes on Mars: Insight from the sedimentary record at Meridiani Planum
2005
New data returned from the Mars Exploration Rover (MER) mission have revealed abundant evaporites in the sedimentary record at Meridiani Planum. A working hypothesis for Meridiani evaporite formation involves the evaporation of fluids derived from the weathering of martian basalt and subsequent diagenesis. On Earth, evaporite formation in exclusively basaltic settings is rare. However, models of the evaporation of fluids derived from experimentally weathering synthetic martian basalt provide insight into possible formation mechanisms. The thermodynamic database assembled for this investigation includes both Fe2+ and Fe3+ in Pitzer's ion interaction equations to evaluate Fe redox disequilibr…
Exploration of Victoria Crater by the Mars Rover Opportunity
2009
“Lake” Victoria? After having explored the Eagle and Endurance craters, which are separated by only 800 meters, the Mars Exploration Rover Opportunity spent 2 years at Victoria, a much larger impact crater located 6 kilometers south across Meridiani Planum. Sedimentary rocks previously analyzed at Eagle and Endurance point to local environmental conditions that included abundant liquid water in the ancient past. Now, an analysis of rocks in the walls of Victoria by Squyres et al. (p. 1058 ) reveals that the aqueous alteration processes that operated at Eagle and Endurance also acted at Victoria. In addition, sedimentary layering in the crater walls preserves evidence of ancient windblown du…
Chemistry and mineralogy of outcrops at Meridiani Planum
2005
Analyses of outcrops created by the impact craters Endurance, Fram and Eagle reveal the broad lateral continuity of chemical sediments at the Meridiani Planum exploration site on Mars. Approximately ten mineralogical components are implied in these salt-rich silicic sediments, from measurements by instruments on the Opportunity rover. Compositional trends in an apparently intact vertical stratigraphic sequence at the Karatepe West ingress point at Endurance crater are consistent with non-uniform deposition or with subsequent migration of mobile salt components, dominated by sulfates of magnesium. Striking variations in Cl and enrichments of Br, combined with diversity in sulfate species, pr…
Soils of Eagle crater and Meridiani Planum at the Opportunity Rover landing site.
2004
The soils at the Opportunity site are fine-grained basaltic sands mixed with dust and sulfate-rich outcrop debris. Hematite is concentrated in spherules eroded from the strata. Ongoing saltation exhumes the spherules and their fragments, concentrating them at the surface. Spherules emerge from soils coated, perhaps from subsurface cementation, by salts. Two types of vesicular clasts may represent basaltic sand sources. Eolian ripples, armored by well-sorted hematite-rich grains, pervade Meridiani Planum. The thickness of the soil on the plain is estimated to be about a meter. The flatness and thin cover suggest that the plain may represent the original sedimentary surface.
An astrobiological perspective on Meridiani Planum
2005
Sedimentary rocks exposed in the Meridiani Planum region of Mars record aqueous and eolian deposition in ancient dune and interdune playa-like environments that were arid, acidic, and oxidizing. On Earth, microbial populations have repeatedly adapted to low pH and both episodic and chronic water limitation, suggesting that, to a first approximation, the Meridiani plain may have been habitable during at least part of the interval when deposition and early diagenesis took place. On the other hand, the environmental conditions inferred for Meridiani deposition would have posed a challenge for prebiotic chemical reactions thought to have played a role in the origin of life on Earth. Orbital obs…
Single zircon ages of migmatitic gneisses and granulites in the Obudu Plateau: Timing of granulite-facies metamorphism in southeastern Nigeria
2006
Abstract A single zircon geochronological study of gneisses from the Obudu Plateau of southeastern Nigeria, using the evaporation technique, indicates that zircons recorded several Precambrian high-grade metamorphic events (Eburnean and Pan-African). Igneous and multifaceted metamorphic zircons yielded 207 Pb/ 206 Pb ages of 2062.4 ± 0.4 Ma, 1803.8 ± 0.4 Ma and 574 ± 10 Ma, respectively and confirm for the first time that granulite-facies metamorphism affected the basement of southeastern Nigeria, resulting in the formation of charnockites and granulitic gneisses. The Pan-African high-grade event was coeval with the formation of granulites in Cameroon, Togo and Ghana and resulted from colli…
Late Archaean to early Proterozoic granitoid magmatism and high-grade metamorphism in the central Limpopo belt, South Africa
1997
The Central Zone of the Limpopo belt in southern Africa has previously been interpreted as a segment of Archaean crust which experienced its main deformation and metamorphism around 2.7 Ga ago. We report new single zircon U/Pb and Pb/Pb ages for granitoid gneisses, supracrustal rocks and anatectically derived granitic melt patches in the area around Messina, South Africa. The Sand River Gneiss is a composite suite of tonalitic to trondhjemitic rocks with protolith ages between 2.6 and 3.2 Ga. The Singelele gneiss, a heterogeneous granodioritic to quartz monzonitic rock, has protolith ages between 2.55 and 2.58 Ga. Since both the Sand River and Singelele gneisses experienced polyphase high-s…
Polymetamorphism in the mainland Lewisian complex, NW Scotland - phase equilibria and geochronological constraints from the Cnoc an t’Sidhean suite
2012
The metamorphic evolution of rocks cropping out near Stoer, within the Assynt terrane of the central region of the mainland Lewisian complex of NW Scotland, is investigated using phase equilibria modelling in the NCKFMASHTO and MnNCKFMASHTO model systems. The focus is on the Cnoc an t’Sidhean suite, garnet-bearing biotite-rich rocks (brown gneiss) with rare layers of white mica gneiss, which have been interpreted as sedimentary in origin. The results show that these rocks are polymetamorphic and experienced granulite facies peak metamorphism (Badcallian) followed by retrograde fluid-driven metamorphism (Inverian) under amphibolite facies conditions. The brown gneisses are inferred to have c…
Clockwise, low- metamorphism of the Aus granulite terrain, southern Namibia, during the Mesoproterozoic Namaqua Orogeny
2013
Abstract The Aus granulite terrain forms part of the Mesoproterozoic Namaqua Metamorphic Complex (NMC) of southern Africa. The terrain consists of pre- to syn-tectonic granitoid gneisses containing subordinate aluminous metasediments, metapsammites, mafic granulite and calc-silicate, all of which have been metamorphosed and migmatised to varying degrees. Pseudosection modelling of garnet–sillimanite–cordierite–biotite aluminous metapelitic samples constrain peak metamorphic conditions at 5.5 kbar and 825 ° C. Pseudomorphs of sillimanite after andalusite constrain early prograde conditions of ∼ 550–600 ° C at below 4 kbar, whereas the replacement of sillimanite by garnet–cordierite and sub…
Exhumation of the lower crust during crustal shortening: an Alice Springs (380 Ma) age for a prograde amphibolite facies shear zone in the Strangways…
2000
Foliated garnet-bearing amphibolites occur within the West Bore Shear Zone, cutting through granulite facies gneisses of the Strangways Metamorphic Complex. In the amphibolites, large euhedral garnet (up to 3 cm) occurs within fine-grained recrystallized leucocratic diffusion haloes of plagioclase–quartz. The garnet and their haloes include a well-developed vertical foliation, also present in the matrix. This foliation is the same as that cutting through the unconformably overlying Neoproterozoic Heavitree Quartzite. The textures indicate syn- to late kinematic growth of the amphibolite facies mineral assemblages. All mineral assemblages record an arrested prograde reaction history. Notewor…