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821.
Hydrothermal origin for the 2 billion year old Mount Tom Price giant iron ore deposit, Hamersley Province, Western Australia 总被引:5,自引:1,他引:4
Giant iron-ore deposits, such as those in the Hamersley Province of northwestern Australia, may contain more than a billion
tonnes of almost pure iron oxides and are the world's major source of iron. It is generally accepted that these deposits result
from supergene oxidation of host banded iron formation (BIF), accompanied by leaching of silicate and carbonate minerals.
New textural evidence however, shows that formation of iron ore at one of those deposits, Mount Tom Price, involved initial
high temperature crystallisation of magnetite-siderite-iron silicate assemblages. This was followed by development of hematite-
and ferroan dolomite-bearing assemblages with subsequent oxidation of magnetite, leaching of carbonates and silicates and
crystallisation of further hematite. Preliminary fluid inclusion studies indicate both low and high salinity aqueous fluids
as well as complex salt-rich inclusions with the range of fluid types most likely reflecting interaction of hydrothermal brines
with descending meteoric fluids. Initial hematite crystallisation occurred at about 250 °C and high fluid pressures and continued
as temperatures decreased. Although the largely hydrothermal origin for mineralisation at Mount Tom Price is in conflict with
previously proposed supergene models, it remains consistent with interpretations that the biosphere contained significant
oxygen at the time of mineralisation.
Received: 16 February 1999 / Accepted: 14 May 1999 相似文献
822.
Jaap S. Sinninghe Damst Stefan Schouten W. Irene C. Rijpstra Ellen C. Hopmans Harry Peletier Winfried W. C. Gieskes Jan A. J. Geenevasen 《Organic Geochemistry》1999,30(12):918
2,6,10,14-tetramethyl-7-(3-methylpent-4-enyl)-pentadeca-2,5E,9E,13-tetraene I possessing a C25 highly branched isoprenoid skeleton has been isolated from the marine diatom Rhizosolenia setigera and identified by 1H and 13C NMR spectroscopy. 相似文献
823.
Marcel T. J. van der Meer Stefan Schouten David M. Ward Jan A. J. Geenevasen Jaap S. Sinninghe Damst 《Organic Geochemistry》1999,30(12):3850
All-cis hentriaconta-9,15,22-triene (I) has been isolated from Chloroflexus mats, Yellowstone National Park (USA), and identified by GC–(HR)MS analysis of I and its hydrogenated and DMDS-derivatized products and by 1H and 13C NMR spectroscopy. 相似文献
824.
Long-Term Global Warming Scenarios Computed with an Efficient Coupled Climate Model 总被引:13,自引:0,他引:13
We present global warming scenarios computed with an intermediate-complexity atmosphere-ocean-sea ice model which has been extensively validated for a range of past climates (e.g., the Last Glacial Maximum). Our simulations extend to the year 3000, beyond the expected peak of CO2 concentrations. The thermohaline ocean circulation declines strongly in all our scenarios over the next 50 years due to a thermal effect. Changes in the hydrological cycle determine whether the circulation recovers or collapses in the long run. Both outcomes are possible within present uncertainty limits. In case of a collapse, a substantial long-lasting cooling over the North Atlantic and a drying of Europe is simulated. 相似文献
825.
To simulate effects of projected climate change on water temperature characteristics of small lakes in the contiguous U.S., a deterministic, one-dimensional year-round water temperature model is applied. In cold regions the model simulates ice and snow cover on a lake. The lake parameters required as model input are surface area, maximum depth, and Secchi depth as a measure of radiation attenuation and trophic state. The model is driven by daily weather data. Weather records from 209 stations in the contiguous U.S. for the period 1961–1979 were used to represent present climate conditions. The projected climate change owing to a doubling of atmospheric CO2 was obtained from the output of the Canadian Climate Center General Circulation Model. The simulated water temperature and ice characteristics are related to the geometric and trophic state lake characteristics and to geographic location. By interpolation, the sensitivity of lake water temperature characteristics to latitude, longitude, lake geometry and trophic status can therefore be quantified for small lakes in the contiguous U.S. The 2× CO2 climate scenario is projected to increase maximum and minimum lake surface temperatures by up to 5.2°C. (Maximum surface water temperatures in lakes near the northern and the southern border of the contiguous U.S. currently differ by up to 13°C.) Maximum temperature differences between lake surface and lake bottom are projected to increase in average by only 1 to 2°C after climate warming. The duration of seasonal summer stratification is projected to be up to 66 days longer under a 2×CO2 climate scenario. Water temperatures of less than 8°C are projected to occur on lake bottoms during a period which is on the order of 50 days shorter under a 2×CO2 climate scenario. With water temperature change projected to be as high as 5.2°C, ecological impacts such as shifts in species distributions and in fish habitat are most likely. Ice covers on lakes of northern regions would also be changed strongly. 相似文献
826.
827.
Gradual Fault Weakening with Seismic Slip: Inferences from the Seismic Sequences of L’Aquila, 2009, and Northridge, 1994 总被引:1,自引:0,他引:1
Luca Malagnini Irene Munafo’ Massimo Cocco Stefan Nielsen Kevin Mayeda Enzo Boschi 《Pure and Applied Geophysics》2014,171(10):2709-2730
We estimate seismological fracture energies from two subsets of events selected from the seismic sequences of L’Aquila (2009), and Northridge (1994): 57 and 16 selected events, respectively, including the main shocks. Following Abercrombie and Rice (Geophys J Int 162: 406–424, 2005), we postulate that fracture energy (G) represents the post-failure integral of the dynamic weakening curve, which is described by the evolution of shear traction as a function of slip. Following a direct-wave approach, we compute mainshock-/aftershock-source spectral ratios, and analyze them using the approach proposed by Malagnini et al. (Pure Appl. Geophys., this issue, 2014) to infer corner frequencies and seismic moment. Our estimates of source parameters (including fracture energies) are based on best-fit grid-searches performed over empirical source spectral ratios. We quantify the source scaling of spectra from small and large earthquakes by using the MDAC formulation of Walter and Taylor (A revised Magnitude and Distance Amplitude Correction (MDAC2) procedure for regional seismic discriminants, 2001). The source parameters presented in this paper must be considered as point-source estimates representing averages calculated over specific ruptured portions of the fault area. In order to constrain the scaling of fracture energy with coseismic slip, we investigate two different slip-weakening functions to model the shear traction as a function of slip: (i) a power law, as suggested by Abercrombie and Rice (Geophys J Int 162: 406–424, 2005), and (ii) an exponential decay. Our results show that the exponential decay of stress on the fault allows a good fit between measured and predicted fracture energies, both for the main events and for their aftershocks, regardless of the significant differences in the energy budgets between the large (main) and small earthquakes (aftershocks). Using the power-law slip-weakening function would lead us to a very different situation: in our two investigated sequences, if the aftershock scaling is extrapolated to events with large slips, a power law (a la Abercrombie and Rice) would predict unrealistically large stress drops for large, main earthquakes. We conclude that the exponential stress evolution law has the advantage of avoiding unrealistic stress drops and unbounded fracture energies at large slip values, while still describing the abrupt shear-stress degradation observed in high-velocity laboratory experiments (e.g., Di Toro et al., Fault lubrication during earthquakes, Nature 2011). 相似文献
828.
Liquidus phase relations in the system diopside–kalsilite–forsterite–quartz with 3 wt% F were examined at 1 bar and the locations
of important invariant points were determined at 18 kbar. At all pressures within this range a large liquidus field for fluorphlogopite
(Phl) exists, and has a large influence on both melting and fractionation processes. One eutectic point was found to the silica-rich
side of the plane Lc–Fo–Di at Di1Ks30Fo2Qz67, where a melt coexists with San, Qz, Phl and Di at 840 °C and 1 bar. Another eutectic point must exist in the silica-poor
part of the system because the phase topology determines that thermal barriers must exist. At this point a feldspathoid, either
Lc or Ks, must coexist with Fo, Phl and a Ca-bearing phase such as Di. The exact location and phase assemblage were not determined,
but the equilibrium melt must have a composition rich in Di (>29 wt%) and extremely poor in Qz (<8 wt%). The composition of
the first eutectic moves towards lower SiO2 contents with increasing pressure (Di3Ks40Fo1Qz56 at 18 kbar), whereas the second does not exist at 18 kbar due to the disappearance of Lc as a stable liquidus phase. Liquids
which coexist with mafic minerals such as En, Fo, Phl and Di are important for the genesis of potassium-rich mafic rocks by
partial melting in the mantle and for the early stages of fractional crystallisation. The equilibrium melt at the invariant
point Fo + En + Phl + Di + L at 1125 °C is very poor in Fo and Di components at atmospheric pressure (Di5Ks37Fo5Qz53), whereas at 18 kbar the melt contains large amounts of Fo and Di (Di19Ks31- Fo28Qz21), and has a composition close to that of natural lamproites. Kamafugites do not correspond to melts in this system under
any of the studied conditions, and appear to require CO2 in the source. Fractionation processes from primitive potassic basanite melts are controlled principally by the size (and
not the mere presence) of the liquidus phase field for phlogopite: at high pressures where the Phl field is large, olivine
is eliminated early from the fractionating assemblage and Cpx + Phl fractionation may lead to relatively silica-rich rock
differentiates such as trachytes. At low pressures, extensive olivine and restricted Phl crystallisation prevents silica enrichment
in the melt, resulting in phonolitic differentiates. Later crystallisation of alkali feldspar accentuates the trends laid
down in the early stages of fractionation.
Received: 2 February 1999 / Accepted: 14 October 1999 相似文献
829.
The Jupiter gold deposit in the northeastern Eastern Goldfields Province of the Yilgarn Craton of Western Australia is hosted
in greenschist facies metamorphosed tholeiitic basalt, quartz–alkali-feldspar syenite, and quartz–feldspar porphyry. Syenite
intrudes basalt as irregularly shaped dykes which radiate from a larger stock, whereas at least three E–W and NE–SW striking
quartz–feldspar porphyries intrude both syenite and basalt. Brittle–ductile shear zones are shallow-dipping, NW to NE striking,
or are steep-dipping to the south and west. Quartz ± carbonate veins that host gold at Jupiter occur in all lithologies and
are divided into: (1) veins that are restricted to the shear zones, (2) discrete veins that are subparallel to shear zone-hosted
veins, and (3) stockwork veins that form a network of randomly oriented microfractures in syenite wallrock proximal to shallow-dipping
shear zones. The gold-bearing veins comprise mainly quartz, calcite, ankerite, and albite, with minor sericite, pyrite, chalcopyrite,
galena, sphalerite, molybdenite, telluride minerals, and gold. Proximal hydrothermal alteration zones to the mineralised veins
comprise quartz, calcite, ankerite, albite, and sericite. High gold grades (>2 g/t Au) occur mainly in syenite and in the
hanging walls to shallow-dipping shear zones in syenite where there is a greater density of mineralised stockwork veins. The
Jupiter deposit has structural and hydrothermal alteration styles that are similar to both granitoid-hosted, but post-magmatic
Archaean lode-gold deposits in the Yilgarn Craton and intrusion-related, syn-magmatic, syenite-hosted gold deposits in the
Superior Province of Canada. Based on field observations and petrologic data, the Jupiter deposit is considered to be a post-magmatic
Archaean lode-gold deposit rather than a syn-intrusion deposit.
Received: 5 January 1999 / Accepted: 24 December 1999 相似文献
830.