Genetic significance of fluid inclusions in the CSA Cu–Pb–Zn deposit, Cobar, Australia     

Alan D. Giles  Brian Marshall   《Ore Geology Reviews》2004,24(3-4):241

CSA mine exploits a ‘Cobar-type’ Cu–Pb–Zn±Au±Ag deposit within a cleaved and metamorphosed portion of the Cobar Supergroup, central New South Wales. The deposit comprises systems of ‘lenses’ that encompass veins, disseminations and semi-massive to massive Cu–Pb–Zn ores. The systems and contained lenses truncate bedding, are approximately coplanar with regional cleavage and similarly oriented shear zones and plunge parallel to the elongation lineation. Systems have extreme vertical continuity (>1000 m), short strike length (400 m) and narrow width (100 m), exhibit vertical and lateral ore-type variation and have alteration haloes. Models of ore formation include classical hydrothermalism, structurally controlled remobilisation and polymodal concepts; syntectonic emplacement now holds sway.Fluid inclusions were examined from quartz±sulphide veins adjacent to now-extracted ore, from coexisting quartz–sulphide within ore, and from vughs in barren quartz veins. Lack of early primary inclusions precluded direct determination of fluids associated with D₂–D₃ ore and vein emplacement. Similarly, decrepitation (by near-isobaric heating) of the two oldest secondary populations precluded direct determination of fluid phases immediately following D₂–D₃ ore and vein emplacement. Post-decrepitation outflow (late D₃ to early post-D₃) is recorded by monophase CH₄ inclusions. Entrained outflow of deeply circulated meteoric fluid modified the CH₄ system; modification is recorded by H₂O+CH₄ and H₂O+(trace CH₄) secondary populations and by an H₂O+(trace CH₄) primary population. The contractional tectonics (D₂–D₃) of ore emplacement was superseded by relaxational tectonics (D_4P) that facilitated meteoric water penetration and return flow.Under D₂ prograde metamorphism, entrapment temperatures (Tt) and pressures (Pt) for pre-decrepitation secondary inclusions are estimated as Tt300–330 °C and Pt1.5–2 kbar≈Plith (the lithostatic pressure). Decrepitation accompanied peak metamorphism (T350–380 °C) in mid- to late-D₃, while in late-D₃ to early post-D₃, essentially monophase CH₄ secondary inclusions were entrapped at Tt350 °C and Pt=1.5–2 kbar≈Plith. Subsequently, abundant CH₄ and entrained meteoric water were entrapped as H₂O+CH₄ secondaries under slowly decreasing temperature (Tt330–350 °C) and constant pressure (Pt1.5–2 kbar). Finally, with increasingly dominant meteoric outflow, H₂O+(trace CH₄) populations record decreasing temperatures (Tt>300 to <350 down to 275–300 °C) at pressures of Phydrostatic<Pt (1 kbar) <Plith (1.5 kbar).The populations of inclusions provide insight into fluid types, flow regimes and P–T conditions during parts of the deposit's evolution. They indirectly support the role of basin-derived CH₄ fluids in ore formation, but provide no insight into a basement-sourced ore-forming fluid. They fully support post-ore involvement of meteoric water. The poorly constrained entrapment history is believed to span 10 Ma from 395 to 385 Ma.  相似文献   

Bounded influence magnetotelluric response function estimation   总被引：3，自引：0，他引：3  

Alan D. Chave  David J. Thomson 《Geophysical Journal International》2004,157(3):988-1006

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From the editor     

Alan R. Gillespie 《Quaternary Research》2005,63(3):231

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Relationships among intrinsic properties of ordinary chondrites: Oxidation state, bulk chemistry, oxygen-isotopic composition, petrologic type, and chondrule size     

Alan E. Rubin 《Geochimica et cosmochimica acta》2005,69(20):4907-4918

The properties of ordinary chondrites (OC) reflect both nebular and asteroidal processes. OC are modeled here as having acquired nebular water, probably contained within phyllosilicates, during agglomeration. This component had high Δ¹⁷O and acted like an oxidizing agent during thermal metamorphism. The nebular origin of this component is consistent with negative correlations in H, L, and LL chondrites between oxidation state (represented by olivine Fa) and bulk concentration ratios of elements involved in the metal-silicate fractionation (e.g., Ni/Si, Ir/Si, Ir/Mn, Ir/Cr, Ir/Mg, Ni/Mg, As/Mg, Ga/Mg). LL chondrites acquired the greatest abundance of phyllosilicates with high Δ¹⁷O among OC (and thus became the most oxidized group and the one with the heaviest O isotopes); H chondrites acquired the lowest abundance, becoming the most reduced OC group with the lightest O isotopes.Chondrule precursors may have grown larger and more ferroan with time in each OC agglomeration zone. Nebular turbulence may have controlled the sizes of chondrule precursors. H-chondrite chondrules (which are the smallest among OC) formed from the smallest precursors. In each OC region, low-FeO chondrules formed before high-FeO chondrules during repeated episodes of chondrule formation.During thermal metamorphism, phyllosilicates were dehydrated; the liberated water oxidized metallic Fe-Ni. This caused correlated changes with petrologic type including decreases in the modal abundance of metal, increases in olivine Fa and low-Ca pyroxene Fs, increases in the olivine/pyroxene ratio, and increases in the kamacite Co and Ni contents. As water (with its heavy O isotopes) was lost during metamorphism, inverse correlations between bulk δ¹⁸O and bulk δ¹⁷O with petrologic type were produced.The H5 chondrites that were ejected from their parent body ∼7.5 Ma ago during a major impact event probably had been within a few kilometers of each other since they accreted ∼4.5 Ga ago. There are significant differences in the olivine compositional distributions among these rocks; these reflect stochastic nebular sampling of the oxidant (i.e., phyllosilicates with high Δ¹⁷O) on a 0.1-1 km scale during agglomeration.  相似文献   

Using monitoring and modeling to define the hazard posed by the reactivated Ferguson rock slide,Merced Canyon,California     

Jerome V. De Graff  Alan J. Gallegos  Mark E. Reid  Richard G. LaHusen  Roger P. Denlinger 《Natural Hazards》2015,76(2):769-789

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Elastoplastic model with loading memory surfaces (LMS) for monotonic and cyclic behaviour of geomaterials     

Hervé di Benedetto  Matthieu Blanc  Samir Tiouajni  Alan Ezaoui 《国际地质力学数值与分析法杂志》2014,38(14):1477-1502

A new elastoplastic model called loading memory surface based on the critical state concept and the multi‐surface framework is proposed for geomaterials. The model uses a hypoelastic formulation and two plastic mechanisms. The formulations of the model are made in three‐dimensional stress–strain space and work under both monotonic and cyclic loadings. A newly introduced formalism makes it possible to obtain the cyclic response directly from the monotonic loading one. This formalism gives a three‐dimensional generalization of the well‐known Masing rule. The model has been validated against test results of Hostun sand under several conditions: monotonic and cyclic, drained and undrained, tests in compression and in extension, and at different confining pressures and different densities. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

Intensification of Southern Hemisphere westerly winds 2000–1000 years ago: evidence from the subantarctic Campbell and Auckland Islands (52–50°S)          下载免费PDF全文

Chris S. M. Turney  Matt McGlone  Jonathan Palmer  Christopher Fogwill  Alan Hogg  Zoë A. Thomas  Mathew Lipson  Janet M. Wilmshurst  Pavla Fenwick  Richard T. Jones  Ben Hines  Graeme F. Clark 《第四纪科学杂志》2016,31(1):12-19

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A middle–late Ediacaran volcano‐sedimentary record from the eastern Arabian‐Nubian shield     

David Nettle  Galen P. Halverson  Grant M. Cox  Alan S. Collins  Mark Schmitz  James Gehling  Peter R. Johnson  Khalid Kadi 《地学学报》2014,26(2):120-129

The Ediacaran Jibalah Group comprises volcano‐sedimentary successions that filled small fault‐bound basins along the NW–SE‐trending Najd fault system in the eastern Arabian‐Nubian Shield. Like several other Jibalah basins, the Antaq basin contains exquisitely preserved sedimentary structures and felsic tuffs, and hence is an excellent candidate for calibrating late Ediacaran Earth history. Shallow‐marine strata from the upper Jibalah Group (Muraykhah Formation) contain a diversity of load structures and intimately related textured organic (microbial) surfaces, along with a fragment of a structure closely resembling an Ediacaran frond fossil and a possible specimen of Aspidella. Interspersed carbonate beds through the Muraykhah Formation record a positive δ¹³C shift from ?6 to 0‰. U‐Pb zircon geochronology indicates a maximum depositional age of ~570 Ma for the upper Jibalah Group, consistent with previous age estimates. Although this age overlaps with that of the upper Huqf Supergroup in nearby Oman, these sequences were deposited in contrasting tectonic settings on opposite sides of the final suture of the East African Orogen.  相似文献   

Bridging the climate information gap: a framework for engaging knowledge brokers and decision makers in state climate assessments     

Gillian L. Galford  Julie Nash  Alan K. Betts  Sam Carlson  Sarah Ford  Ann Hoogenboom  Deborah Markowitz  Andrew Nash  Elizabeth Palchak  Sarah Pears  Kristen L. Underwood 《Climatic change》2016,138(3-4):383-395

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Revised method to detect the onset and demise dates of the rainy season in the South American Monsoon areas     

Sâmia R. Garcia  Alan J. P. Calheiros  Mary T. Kayano 《Theoretical and Applied Climatology》2016,126(3-4):481-491

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