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801.
802.
John A. Grant Matthew P. Golombek John P. Grotzinger Sharon A. Wilson Michael M. Watkins Ashwin R. Vasavada Jennifer L. Griffes Timothy J. Parker 《Planetary and Space Science》2011,59(11-12):1114-1127
The process of identifying the landing site for NASA’s 2011 Mars Science Laboratory (MSL) began in 2005 by defining science objectives, related to evaluating the potential habitability of a location on Mars, and engineering parameters, such as elevation, latitude, winds, and rock abundance, to determine acceptable surface and atmospheric characteristics. Nearly 60 candidate sites were considered at a series of open workshops in the years leading up to the launch. During that period, iteration between evolving engineering constraints and the relative science potential of candidate sites led to consensus on four final sites. The final site will be selected in the Spring of 2011 by NASA’s Associate Administrator for the Science Mission Directorate. This paper serves as a record of landing site selection activities related primarily to science, an inventory of the number and variety of sites proposed, and a summary of the science potential of the highest ranking sites. 相似文献
803.
Channel evolution of Des Moines Lobe till drainage ditches in southern Minnesota (USA) 总被引:1,自引:1,他引:0
Joe Magner Brad Hansen Tim Sundby Geoff Kramer Bruce Wilson John Nieber 《Environmental Earth Sciences》2012,67(8):2359-2369
Some drainage ditches in the intensively managed row-crop agricultural region of southern Minnesota evolved from a trapezoidal form to multi-staged channel forms similar to natural streams. Older ditches constructed in cohesive sediment of the Des Moines Lobe till tend to follow a channel evolution model developed by Simon and Hupp. Site cross sections, longitudinal water and bed profiles and bed material particle size were determined according to Harrelson and others at 24 older ditch reaches, 5 newly constructed ditch reaches and 13 natural stream reaches. Morphological features were hypothesized to change from trapezoidal form to flat bench banks, similar to benches found in natural stream channels. All data were statistically analyzed with respect to drainage area using regression, because channel form is directly related to drainage area for a given climate, geology and land use. Results show similar regression slope and intercept for bankfull channel width and bankfull cross-sectional area (CSA) of older ditches and natural streams compared to typical trapezoidal designed ditches. Evolved ditches developed a small floodplain bench above the ditch bed and adjusted their bankfull widths similar to natural stream channels with respect to drainage area. Old ditches showed a relatively strong R 2 (0.82, 0.68) for channel CSA and width, a weaker R 2 (0.45) for water surface slope, and little to no correlation with bed particle size. Channel form appears to have adjusted more quickly than bed facets and/or bed particle size distribution. However, stepwise regression determined that D84, width/depth ratio and mean bankfull depth explained 83?% of the variability of channel features across varying drainage areas. Findings suggest a possible reduction of long-term maintenance costs if older ditches are allowed to evolve over time. A stable ditch form similar to natural streams is typically self-sustaining, suggesting that prior to a scheduled clean-out, the ditch should be examined for hydraulic capacity, sediment transport and bank stability. 相似文献
804.
Extraction of groundwater for onion and other cash crop production has been increasing rapidly during the last two decades in the dry zone areas of Sri Lanka. As a result of overuse, the quantity of available groundwater is gradually declining, while water quality is deteriorating. The deteriorating water quality has a negative impact on agricultural production, especially for crops (such as onions) that are sensitive to increases in salinity levels. This issue is examined with respect to onion production in Sri Lanka. A stochastic frontier production function (SFPF) is used, in which technical efficiency and the determinants of inefficiencies are estimated simultaneously. The results show that farmers are overusing groundwater in their onion cultivation, which has resulted in decreasing yields. Factors contributing to inefficiency in production are also identified. The results have important policy implications. 相似文献
805.
Bin?FuEmail author Mark?A.?Kendrick Alison?M.?Fairmaid David?Phillips Christopher?J.?L.?Wilson Terrence?P.?Mernagh 《Contributions to Mineralogy and Petrology》2012,163(3):427-447
Fluid inclusion microthermometry, Raman spectroscopy and noble gas plus halogen geochemistry, complemented by published stable
isotope data, have been used to assess the origin of gold-rich fluids in the Lachlan Fold Belt of central Victoria, south-eastern
Australia. Victorian gold deposits vary from large turbidite-hosted ‘orogenic’ lode and disseminated-stockwork gold-only deposits,
formed close to the metamorphic peak, to smaller polymetallic gold deposits, temporally associated with later post-orogenic
granite intrusions. Despite the differences in relative timing, metal association and the size of these deposits, fluid inclusion
microthermometry indicates that all deposits are genetically associated with similar low-salinity aqueous, CO2-bearing fluids. The majority of these fluid inclusions also have similar 40Ar/36Ar values of less than 1500 and 36Ar concentrations of 2.6–58 ppb (by mass) that are equal to or much greater than air-saturation levels (1.3–2.7 ppb). Limited
amounts of nitrogen-rich fluids are present at a local scale and have the highest measured 40Ar/36Ar values of up to 5,700, suggesting an external or distinct source compared to the aqueous fluids. The predominance of low-salinity
aqueous–carbonic fluids with low 40Ar/36Ar values, in both ‘orogenic’ and ‘intrusion-related’ gold deposits, is attributed to fluid production from common basement
volcano-sedimentary sequences and fluid interaction with sedimentary cover rocks (turbidites). Aqueous fluid inclusions in
the Stawell–Magdala deposit of western Victoria (including those associated with N2) preserve mantle-like Br/Cl and I/Cl values. In contrast, fluid inclusions in deposits in the eastern structural zones, which
contain more abundant shales, have elevated molar I/Cl ratios with maximum values of 5,170 × 10−6 in the Melbourne Zone. Br/I ratios in this zone range from 0.5 to 3.0 that are characteristic of fluid interaction with organic-rich
sediments. The maximum I/Cl and characteristic Br/I ratios provide evidence for organic Br and I released during metamorphism
of the shales. Therefore, the regional data provide strong evidence for the involvement of sedimentary components in gold
mineralisation, but are consistent with deeper metamorphic fluid sources from basement volcano-sedimentary rocks. The overlying
sediments are probably involved in gold mineralisation via fluid–rock interaction. 相似文献
806.
Colin J. N. Wilson Terry M. Seward Aidan S. R. Allan Bruce L. A. Charlier Léa Bello 《Contributions to Mineralogy and Petrology》2012,164(2):359-368
Trace concentrations of Ti in quartz are used to indicate the pressure and temperature conditions of crystallization in the ‘TitaniQ’ geothermobarometer of Thomas et al. (Contrib Miner Petrol 160:743–759, 2010). It utilises the partitioning of Ti into quartz as an indicator of the pressures and/or temperatures of crystal growth. For a given value of TiO2 activity in the system, if temperatures are inferred to ±20 °C, pressure is constrained to ±1 kbar and vice versa. There are significant contrasts, however, between the conclusions from TitaniQ and those for natural quartz (as well as other mineral phases) in volcanic rocks. Application of the TitaniQ model to quartz from the 27 ka Oruanui and 760 ka Bishop high-silica rhyolites, where the values of T, P and TiO2 activity are constrained by other means (Fe–Ti oxide equilibria, melt inclusion entrapment pressures in gas-saturated melts, melt and amphibole compositions), yields inconsistent results. If realistic values are given to any two of these three parameters, then the value of the third is wholly unrealistic. The model yields growth temperatures at or below the granite solidus, pressures in the lower crust or upper mantle, or TiO2 activities inconsistent with the mineralogical and chemical compositions of the magmas. CL imagery and measurements of Ti (and other elements) in quartz are of great value in showing the growth histories and changes in conditions experienced by crystals, but direct linkages to P, T conditions during crystal growth cannot be achieved. 相似文献
807.
M. A. Etheridge D. M. Ransom P. F. Williams C. J. L. Wilson 《Australian Journal of Earth Sciences》2013,60(4):465-470
The pre‐Devonian sedimentary and volcanic sequence exposed along the south coast of New South Wales has previously been divided into three stratigraphic groups: (1) Upper Ordovician graptolite‐bearing slate which is conformable with (2) interlayered thinly‐bedded greywacke and pelite of undifferentiated Ordovician age and (3) Cambrian successions of interlayered chert, pelite and volcanic rock at Bate‐mans Bay and Narooma. The main bases for this subdivision are a change in rock types between (1) and (2), and the unconformity between (2) and (3) formerly proposed on the basis of changes in rock type and fold style across the boundary. New structural data are presented which refute the presence of the unconformity, and conformity of (2) and (3) with the fossiliferous slates is established. 相似文献
808.
Allan F. Wilson 《Australian Journal of Earth Sciences》2013,60(1-2):39-44
Three samples from the Mud Tank Carbonatite have very similar isotopic ratios, averaging δ13C=‐4.3 and δ18O=+7.5(SMOW). These isotope values are distinct from those of nearby highly metamorphosed Carpentarian marbles, which have marine limestone values of δ13C=‐1.3±0.5, and δ18O=+17.6+0.7 with n=11. Minor variations in the values for the normal marbles show no correlation with stratigraphy or geographic location; however, somewhat lighter oxygen is found in some other marbles known to be affected by low‐temperature fluids within the Woolanga Lineament. Isotopes of C and O, if discretely used in conjunction with other geochemical features, not only may discriminate between deep‐seated carbona‐tites and marbles, but may also help to identify zones of carbonate metasomatism and define the isotopic characters of the fluids. 相似文献
809.
The molybdenite and scheelite mineralisation in the Native Dog Pluton at Monkey Gully near Yea is hosted within an I-type (post-orogenic) pluton, which shows extensive fractionation and magma mixing, and was emplaced in an extensional environment. The pluton comprises four principal rock types: tonalite, granodiorite, dacite and leucogranite. Emplacement of the pluton was in an extensional northwest to southeast paleostress field. Early extensional quartz veins, related to cooling, are overprinted by both dacitic dykes and late-stage quartz sheeted veins. The late-stage veins host the molybdenum and tungsten mineralisation in the deposit. 206Pb/238U zircon ages of 356 ±14 Ma and 375 ± 22 Ma place pluton formation and mineralisation at the onset of the Kanimblan Orogeny and later than other major molybdenum deposits in Victoria. Key factors governing the source for the granite and its associated mineralisation are: (1) the presence of a highly fractionated and sulfur-rich leucogranite; and (2) the pluton's location in a regional jog overlying the Selwyn basement block. Finally, a model is developed to explain the differences between this Melbourne Zone molybdenum and tungsten deposit compared with other metallogenic porphyry deposits. 相似文献
810.
Sigismond Ayodele Wilson 《GeoJournal》2013,78(6):997-1012
This paper uses the resource curse hypothesis to explore diamond exploitation in Sierra Leone during the period 1930–2010. Focusing on national and local level analysis, it examines whether the net impact of diamond exploitation was a ‘resource curse’ or ‘blessing’ during four time periods: colonial and early post-independence era, the APC era, the civil war period, and post-war era. The paper argues that the net impact of diamond exploitation in Sierra Leone has not been constant; rather it has changed between resource blessing and curse over different major periods of Sierra Leone’s history and at local and national scales since inception of diamond exploitation. This paper illustrates that during the period 1968–1992 patrimonial politics undermined official diamond exploitation and significantly contributed to a pendulum shift in the net effects of diamond exploitation from resource blessing to curse. The study shows that the net effect of diamond exploitation was: a resource blessing (especially at the national level) prior to 1968; more of a resource curse during the APC era; a full blown manifestation of the curse during the civil war period; and that governance of the diamond sector has improved sufficiently in post-civil war Sierra Leone to start the gradual transformation of diamonds to resource blessing, at national and local levels. 相似文献