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101.
102.
Frances Westall Frédéric Foucher Sjoukje T. de Vries Victoria Pearson Alexander Verchovsky Jean-Noel Rouzaud Severine Anne 《Planetary and Space Science》2011,59(10):1093-479
Within the context of present and future in situ missions to Mars to investigate its habitability and to search for traces of life, we studied the habitability and traces of past life in ∼3.5 Ga-old volcanic sands deposited in littoral environments an analogue to Noachian environments on Mars. The environmental conditions on Noachian Mars (4.1-3.7 Ga) and the Early Archaean (4.0-3.3 Ga) Earth were, in many respects, similar: presence of liquid water, dense CO2 atmosphere, availability of carbon and bio-essential elements, and availability of energy. For this reason, information contained in Early Archaean terrestrial rocks concerning habitable conditions (on a microbial scale) and traces of past life are of relevance in defining strategies to be used to identify past habitats and past life on Mars.One such example is the 3.446 Ga-old Kitty’s Gap Chert in the Pilbara Craton, NW. Australia. This formation consists of volcanic sediments deposited in a coastal mudflat environment and is thus a relevant analogue for sediments deposited in shallow water environments on Noachian Mars. Two main types of habitat are represented, a volcanic (lithic) habitat and planar stabilized sediment surfaces in sunlit shallow waters. The sediments hosted small (<1 μm in size) microorganisms that formed colonies on volcanic particle surfaces and in pore waters within the volcanic sediments, as well as biofilms on stabilised sediment surfaces. The microorganisms included coccoids, filaments and rare rod-shaped organisms associated with microbial polymer (EPS). The preserved microbial community was apparently dominated by chemotrophic organisms but some locally transported filaments and filamentous mat fragments indicate that possibly photosynthetic mats formed nearby. Both microorganisms and sediments were silicified during very early diagenesis.There are no macroscopic traces of fossilised life in these volcanic sediments and sophisticated instrumentation and specialized sample preparation techniques are required to establish the biogenicity and syngenicity of the traces of past life. The fact that the traces of life are cryptic, and the necessity of using sophisticated instrumentation, reinforces the challenges and difficulties of in situ robotic missions to identify past life on Mars. We therefore recommend the return of samples from Mars to Earth for a definitive search for traces of life. 相似文献
103.
Tauranga low-temperature geothermal system (New Zealand) has been used for the last 40 years for direct uses including space heating, bathing and greenhouses. Warm-water springs in the area are between 22 and 39 °C, with well temperatures up to 67 °C at 750 m depth. A heat and fluid flow model of the system is used to determine reservoir properties and assess thermal potential. The model covers 130 km by 70 km to 2 km depth, and was calibrated against temperatures measured in 17 wells. Modelling shows that to maintain the observed primarily conductive heat flow regime, bulk permeability is ≤2.5?×?10?14 m2 in sedimentary cover and ≤1?×?10?16 m2 in the underlying volcanic rocks. The preferred model (R 2?=?0.9) corresponds to thermal conductivities of 1.25 and 1.8 W/m2 for sedimentary and volcanic rocks, respectively, and maximum heat flux of 350 mW/m2. The total surface heat flow is 258 MW over 2,200 km2. Heat flux is highest under Tauranga City, which may be related to inferred geology. Model simulations give insights into rock properties and the dynamics of heat flow in this low-temperature geothermal system, and provide a basis to estimate the effects of extracting hot fluid. 相似文献
104.
Kreshimir Nenadovitch Malitch Elena A. Belousova William L. Griffin Inna Yu Badanina Norman J. Pearson Sergey L. Presnyakov Evgeniya V. Tuganova 《Contributions to Mineralogy and Petrology》2010,159(6):753-768
The ultramafic–mafic Kharaelakh intrusion in the northwestern part of the Siberian Craton (Russia) hosts major economic platinum-group-element
(PGE)–Cu–Ni sulphide deposits. In situ U–Pb, REE and Hf-isotope analyses of zircon from these rocks, combined with detailed
study of crystal morphology and internal structure, identify four zircon populations. U–Pb ages of these populations cover
a significant time span (from 347 ± 16 to 235.7 ± 6.1 Ma) suggesting multiple magmatic events that cluster around 350 and
250 Ma, being consistent with two recognised stages of active tectonism in the development of the Siberian Craton. The oldest
zircon population, however, represents previously unknown stage of magmatic activity in the Noril’sk area. Epsilon-Hf values
of +2.3 to +16.3 in the analysed zircons reflect a dominant role of mantle-derived magmas and suggest that juvenile mantle
material was the main source for the ultramafic–mafic Kharaelakh intrusion. A significant range in initial 176Hf/177Hf values, found in zircons that cluster around 250 Ma, indicate mixing between mantle and crustal magma sources. Our findings
imply that economic intrusions hosting PGE–Cu–Ni deposits of the Noril’sk area have a far more complex geological history
than is commonly assumed. 相似文献
105.
Stratigraphy and sedimentology of the Upper Cretaceous to Paleogene Kilwa Group, southern coastal Tanzania 总被引:2,自引:0,他引:2
Christopher J. Nicholas Paul N. Pearson Paul R. Bown Tom Dunkley Jones Brian T. Huber Amina Karega Jackie A. Lees Ian K. McMillan Aoife OHalloran Joyce M. Singano Bridget S. Wade 《Journal of African Earth Sciences》2006,45(4-5):431-466
The geology of southern coastal Tanzania has remained poorly understood since the first comments on its stratigraphy were made over 100 years ago. However, new field surveys combined with shallow drilling along the coast between Kilwa and Lindi are beginning to resolve the depositional history and structural evolution of this region over the past 85 Ma. Here we present the first attempt to synthesize the results of these studies to provide a coherent sedimentological, litho- and sequence stratigraphic framework, including new geological maps, for the Upper Cretaceous and Paleogene of the coastal zone.Santonian to Oligocene sediments crop out along a broad coastal belt south of the Rufiji River from the Kilwa peninsula to Lindi Creek in southern Tanzania. During 55 Ma, over 1 km of a broadly homogeneous, mid to outer shelf clay-dominated succession was deposited across the passive margin, which we define here as the Kilwa Group. This lies disconformably across the shelf on Albian marls and is itself unconformably overlain by shallow water Miocene clays and more recent limestones, sands and gravels. Four formations can be identified within the Kilwa Group on the basis of characteristic secondary lithologies and facies, described here for the first time; the Nangurukuru, Kivinje, Masoko and Pande Formations. These formations include conformable stratigraphic intervals through both the Paleocene–Eocene and Eocene–Oligocene boundaries. Within the Kilwa Group, 12 sequence stratigraphic cycles can be identified at present, demonstrating relatively uniform and continual subsidence across the margin from Santonian to Early Oligocene time. A further major bounding surface is present between the Upper Cretaceous and Paleogene, but this may become partly conformable in the Lindi area. Although the principal lithology in all formations is clay or claystone, there are more permeable intervals containing pervasive coarser siliciclastic sediments and these have yielded traces of crude oil which is likely to have migrated from lower in the succession. The Kilwa Group thus also provides important new evidence for petroleum play development in the southern coastal zone. 相似文献
106.
107.
V. K. Pearson M. A. Sephton I. A. Franchi J. M. Gibson I. Gilmour 《Meteoritics & planetary science》2006,41(12):1899-1918
Abstract— We have undertaken a comprehensive study of carbon and nitrogen elemental abundances and isotopic compositions of bulk carbonaceous chondrites. A strategy of multiple analyses has enabled the investigation of hitherto unconstrained small‐scale heterogeneities. No systematic differences are observed between meteorite falls and finds, suggesting that terrestrial processing has a minimal effect on bulk carbon and nitrogen chemistry. The changes in elemental abundance and isotopic composition over the petrologic range may reflect variations in primary accreted materials, but strong evidence exists of the alteration of components during secondary thermal and aqueous processing. These changes are reflected within the CM2 and CO3 groups and follow the published alteration scales for those groups. The nitrogen isotope system appears to be controlled by an organic host, which loses a 15N‐rich component with progressive alteration. This study recommends caution, however, over the use of bulk carbon and nitrogen information for classification purposes; variance in relative abundance of different components in carbonaceous chondrites is significant and reflects intrameteorite heterogeneities. 相似文献
108.
I. W. A. Browne P. N. Wilkinson N. J. F. Jackson S. T. Myers C. D. Fassnacht L. V. E. Koopmans D. R. Marlow M. Norbury D. Rusin C. M. Sykes A. D. Biggs R. D. Blandford A. G. de Bruyn K.-H. Chae P. Helbig L. J. King J. P. McKean T. J. Pearson P. M. Phillips A. C. S. Readhead E. Xanthopoulos T. York 《Monthly notices of the Royal Astronomical Society》2003,341(1):13-32
We report the final results of the search for gravitationally lensed flat-spectrum radio sources found in the combination of CLASS (Cosmic Lens All-Sky Survey) and JVAS (Jodrell Bank VLA Astrometric Survey). VLA (Very Large Array) observations of 16 503 sources have been made, resulting in the largest sample of arcsec-scale lens systems available. Contained within the 16 503 sources is a complete sample of 11 685 sources which have two-point spectral indices between 1.4 and 5 GHz flatter than −0.5, and 5-GHz flux densities ≥30 mJy . A subset of 8958 sources form a well-defined statistical sample suitable for analysis of the lens statistics. We describe the systematic process by which 149 candidate lensed sources were picked from the statistical sample on the basis of possessing multiple compact components in the 0.2-arcsec resolution VLA maps. Candidates were followed up with 0.05-arcsec resolution MERLIN and 0.003-arcsec VLBA observations at 5 GHz and rejected as lens systems if they failed well-defined surface brightness and/or morphological tests. To illustrate the candidate elimination process, we show examples of sources representative of particular morphologies that have been ruled out by the follow-up observations. 194 additional candidates, not in the well-defined sample, were also followed up. Maps for all the candidates can be found on the World Wide Web at http://www.jb.man.ac.uk/research/gravlens/index.html . We summarize the properties of each of the 22 gravitational lens systems in JVAS/CLASS. 12 are double-image systems, nine are four-image systems and one is a six-image system. 13 constitute a statistically well-defined sample giving a point-source lensing rate of 1:690 ± 190 . The interpretation of the results in terms of the properties of the lensing galaxy population and cosmological parameters will be published elsewhere. 相似文献
109.
M. K. Argo N. J. Jackson I. W. A. Browne T. York J. P. McKean A. D. Biggs R. D. Blandford A. G. de Bruyn K. H. Chae C. D. Fassnacht L. V. E. Koopmans S. T. Myers M. Norbury T. J. Pearson P. M. Phillips A. C. S. Readhead D. Rusin P. N. Wilkinson 《Monthly notices of the Royal Astronomical Society》2003,338(4):957-961
110.