Stable isotopes in high temperature geological processes: by J.W. Valley, H.P. Taylor, Jr. and J.R. O'Neil (Editors). Reviews in Mineralogy, Volume 16, Mineralogical Society of America, Washington, D.C., 1986, 570 pp., $18 (US)     

Eion M. Cameron 《Journal of Geochemical Exploration》1988,30(1-3)

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Derivation of gold by oxidative metamorphism of a deep ductile shear zone: Part 1. Conceptual model   总被引：3，自引：0，他引：3  

Eion M. Cameron   《Journal of Geochemical Exploration》1989,31(2)

Quartz-carbonate gold deposits were emplaced in shear zones at or above the brittle-ductile transition. Some of the largest deposits are known to have formed along major, long-lived, transcurrent shears. Shears of this type widened downwards in the ductile regime, as a result of decreasing rock viscosity with depth; some were as wide as 40 km at depths of granulite facies metamorphism. Ductile shears are permeable and, since the permeability is along microfractures, fluid flow was pervasive, providing the opportunity for extensive chemical reaction. Reaction rates were enhanced by shear heating and by deformation-induced stress gradients in minerals, and reductions in grain size. Fluid flow tended to be upwards, because of pressure drop into the brittle portion of the shear. Given the wedge-shaped profile of ductile shears, fluids that had passed through a large volume of lower crust would have been focused at the brittle-ductile transition. Thus, if processes existed to selectively remove elements during fluid movement through the lower crust, these elements would also have been focused at this transition.One of the most constant features of quartz-carbonate lodes is carbonate alteration, which may extend kilometers out from major deposits. The ¹³C signature of this is consistent with a mantle source for the CO₂. Upward-moving CO₂ vapour of probable mantle origin has been implicated in the dehydration of amphibolite facies rocks to granulites and the concomitant depletion of large ion lithophile elements (LILE). The best documented cases of modification of the lower crust by CO₂ are from major shear zones. CO₂ streaming at depth could only have occurred under conditions more oxidizing than that required for graphite stability. These conditions favour solubility of gold by (a) oxidizing Au⁰ to Au⁺; (b) by dissolving sulphide from the rocks to complex with Au⁺. Recent work has shown that some major Archean gold deposits were derived from relatively oxidized fluids.A conceptual model is outlined for the genesis of at least some quartz-carbonate gold deposits. CO₂ permeating deep ductile shear zones dehydrated amphibolite facies rocks. A relatively oxidized CO₂-H₂O fluid was produced, which dissolved sulphide and gold from large volumes of lower crust. Gold was carried upwards in the narrowing shear, to be focused and precipitated at or above the brittle-ductile transition.  相似文献   

Fluids in the lithosphere, 2. Experimental constraints on CO₂ transport in dunite and quartzite at elevated P-T conditions with implications for mantle and crustal decarbonation processes     

James M. Brenan  E. Bruce Watson 《Earth and Planetary Science Letters》1988,91(1-2)

In a series of experiments at 0.5–1.3 GPa and 1050–1200°C we have monitored the transport, via crack propagation, of CO₂ into well-annealed olivine and quartz aggregates. The objectives were to determine (1) the extent and rate of fluid penetration; (2) the effect of varying both P-T conditions and microstructure; and (3) the fluid penetration pathways. Experiments on CO₂ penetration into dunite annealed in the absence of MgO indicate rapid and pervasive fluid transport on a grain-dimension scale, but a limited penetration distance ( 1 mm). Additional experiments on dunite annealed in the presence of MgO (either dispersed or present at both ends), however, resulted in CO₂ penetration that was both pervasive on the scale of individual grains and almost always completely through the 5 mm long samples. The abundance of fine (10 μm) grains in the MgO-free dunite, in contrast to the much larger grain sizes of the samples annealed with MgO present, suggests the difference in fluid penetration behavior may arise because the strength variation in dunite scales with the grain size. Effects arising from changes in olivine point defect chemistry, however, are an additional possibility. The response of synthetic quartzite to CO₂ overpressure is distinct from that of dunite: Quartzite experiences rapid and complete penetration of CO₂, via a macroscopically visible system of transgranular fractures, over the range of P-T conditions investigated.The small amount of porosity ( 2–3%) present in most rock samples fabricated for this study, lacks three-dimensional connectivity, thus precluding any enhanced fluid penetration via porous flow. Pores could possibly enhance fluid penetration as the result of a small reduction in resistance to fracture, but the probable abundance of strength-controlling flaws in natural rocks is likely to produce similar behavior.The results of our experiments on olivine and olivine + MgO suggest that the transport of pressurized CO₂ in very olivine-rich mantle environments will be pervasive on the scale of individual grains and its extent may be dependent on rock microstructure and/or crystal chemical effects. Such pervasive fluid transport, perhaps associated with magma decarbonation, may have interesting implications for both magma transport and local LREE enrichment of adjacent mantle wall-rock. The ease with which quartzite is penetrated by CO₂ at the conditions of our experiments underscores the possible role of decarbonation reactions in crustal permeability-enhancement processes.  相似文献   

NOAA WP-3D instrumentation and flight operations on AGASP-II     

R. C. Schnell  T. B. Watson  B. A. Bodhaine 《Journal of Atmospheric Chemistry》1989,9(1-3):3-16

The second Arctic Gas and Aerosol Sampling Program (AGASP-II) was conducted across the non-Soviet Arctic in March and April 1986, to study the aerosol, gaseous, chemical, and optical properties of Arctic haze. One component of the program was supported with an instrumented NOAA WP-3D atmospheric research aircraft. Measurements of wind, temperature, ozone, water vapor, condensation nucleus concentration, and aerosol scattering extinction coefficient were used to determine the locations and properties of haze layers. The first three NOAA WP-3D research flights were conducted north of Barrow, Alaska, and over the Beaufort Sea northeast of Barter Island, Alaska. The next three sampled conditions in the high Arctic near Alert, Northwest Territories, Canada. All basic meteorological, gas, and aerosol systems are described. The WP-3D flight tracks and operations are presented.  相似文献   

Evaporation from the understorey in the Jarrah (Eucalyptus marginata Don ex Sm.) forest, southwestern Australia     

E.A.N. Greenwood  L. Klein  J.D. Beresford  G.D. Watson  K.D. Wright 《Journal of Hydrology》1985,80(3-4):337-349

Annual evaporation from groundflora, litter and soil of the jarrah forest was estimated from measurements of daily evaporation by ventilated chambers on several days over two separate 12-month periods. In the first year, when sampling ranged over 0.1 ha of forest, annual evaporation during daylight hours was estimated as 410 mm (0.32 rainfall). In the second year, sampling was more frequent, on a larger scale, and included the night hours. Annual evaporation was estimated at 360 mm (0.36 rainfall).

Similarly, in the second year, annual evaporation from two trees of the dominant middle storey species, Banksia grandis, was estimated at 7500 and 18,9001 respectively. The leaf area of these two trees was 9.6 and 22.4 m², respectively, so that annual evaporation, when expressed as mm³ per mm² leaf area, was similar for both trees (mean = 820 ± 30 mm). Applying that value to all Banksia trees in a hectare of forest, and using a measured estimate of leaf area index of 0.19, the estimated annual evaporation from the Banksia component was 155 mm (0.16 rainfall). For the upland part of the forest sampled, the combined annual evaporation from the lower and middle storeys accounted for about half (0.51) of the annual rainfall.

We conclude that reduced evaporation from the upper storey following clearing or thinning may be strongly counteracted by increased evaporation from the understorey due to increased availability of energy and water.  相似文献   

Rare earth element, 87Sr/86Sr,and 143Nd/144Nd compositions of Cenozoic orogenic dacites from Baja California,northwestern Mexico,and adjacent west Texas: evidence for the predominance of a subcrustal component     

Kenneth L. Cameron  Maryellen Cameron 《Contributions to Mineralogy and Petrology》1985,91(1):1-11

Dacitic lavas and ignimbrites were examined from seven localities that span the entire 700 km width of the mid- to late Cenozoic magmatic arc of northwestern Mexico and adjacent west Texas. These rocks have remarkably similar REE patterns that are parallel in the heavy REE and have modest negative Eu anomalies. Samples from three localities including Baja California, the Sierra Madre Occidental, and the Chihuahuan Basin and Range have initial ⁸⁷Sr/⁸⁶Sr between 0.7044 and 0.7050 and _Nd near 0.0±1.0. These dacites are isotopically similar to associated basalts, and they show no systematic isotopic variation that is correlated with age or composition of the basement. There is no evidence that magmas parental to these dacites interacted significantly with continental crust. Samples form three other localites in the Basin and Range vary in initial ⁸⁷Sr/⁸⁶Sr from 0.7051 to 0.7070 and _Nd from about -1 to –2. The composition of these rocks reflects contamination of the parental magmas by relatively small amounts of Precambrian crust. Collectively, the dacites of this study show much less isotopic variation than do Mesozoic granitoids (Farmer and DePaolo 1983) and late Cenozoic olivine tholeiites (Hart 1985) from similar transects of the western United States. The distinctive source region for the magmas parental to the Mexican dacites was relatively uniform isotopically, but it was enriched in LIL and HFS elements beneath the eastern Basin and Range.  相似文献   

The Hemlo gold deposit,Ontario: A geochemical and isotopic study     

Eion M. Cameron  Keiko Hattori 《Geochimica et cosmochimica acta》1985,49(10):2041-2050

The Hemlo deposit, near Marathon, Ontario, is one of the largest gold deposits in North America. It is stratiform within Archean metamorphosed volcano-sedimentary rocks. The main ore zone is composed of pyritic, sericitic schist, and massive barite. This is the first report of stratiform barite in the Archean of North America, but other occurrences have since been found west of Hemlo. The mineralization is substantially enriched in Au, Mo, Sb, Hg, Tl and V and lacks carbonate. Because of metamorphism and deformation of the body its genesis is uncertain.${}^{87}\text{Sr}{}^{86}\text{Sr}$ of .7017 for barite from the deposit is similar to that of the sedimentary barite west of Hemlo and to initial ratios of contemporaneous volcanic rocks. At the base of the main ore zone, barite with δ^34S of +8 to +12%. was deposited with ~0%. pyrite. Upward, both barite and pyrite get isotopically lighter, with minimum values for pyrite, to ?17.5%, in non-baritic schist forming the upper part of the ore zone. In drill section, Au grades correlate with the isotopic composition of pyrite. This, and the association of fractionated sulphide with sulphate, suggests that Au, pyrite and barite were deposited contemporaneously. The linked, asymmetric distributions of S minerals and isotopic distributions, which are continuous from section to section, and the isotopic similarity of the Hemlo and western barites are consistent with a syngenetic depositional model.Two sources for the S minerals are considered. In the first, exogenous sulphate from a restricted basin were partially reduced in a geothermal system to form ³⁴S-depleted sulphide. In the second, the sulphate and sulphide are of magmatic-hydrothermal origin. Sulphate and fractionated sulphide are uncommon in Archean rocks, but one or both occur with unusual frequency in major Archean gold deposits. Hydrothermal fluids of moderately high , containing sulphate and permitting isotopic fractionation between oxidized and reduced S species, may have favoured the dissolution, transport and precipitation of Au.  相似文献   

The behavior of apatite during crustal anatexis: Equilibrium and kinetic considerations   总被引：5，自引：0，他引：5  

T.Mark Harrison  E.Bruce Watson 《Geochimica et cosmochimica acta》1984,48(7):1467-1477

The solubility and dissolution kinetics of apatite in felsic melts at 850°–1500°C have been examined experimentally by allowing apatite crystals to partially dissolve into apatite-undersaturated melts containing 0–10 wt% water. Analysis of P and Ca gradients in the crystal/melt interfacial region enables determination of both the diffusivities and the saturation levels of these components in the melt. Phosphorus diffusion was identified as the rate-limiting factor in apatite dissolution. Results of four experiments at 8 kbar run in the virtual absence of water yield an activation energy (E) for P diffusion of 143.6 ± 2.8 kcal-mol^?1 and frequency factor (D₀) of 2.23^+2.88_?1.26 × 10⁹cm²-sec^?1. The addition of water causes dramatic and systematic reduction of both E and D₀ such that at 6 wt% H₂O the values are ~25 kcal-mol^?1 and 10^?5 cm²-sec^?1, respectively. At 1300°C, the diffusivity of P increases by a factor of 50 over the first 2% of water added to the melt, but rises by a factor of only two between 2 and 6%, perhaps reflecting the effect of a concentration-dependent mechanism of H₂O solution. Calcium diffusion gradients do not conform well to simple diffusion theory because the release of calcium at the dissolving crystal surface is linked to the transport rate of phosphorus in the melt, which is typically two orders of magnitude slower than Ca. Calcium chemical diffusion rates calculated from the observed gradients are about 50 times slower than calcium tracer diffusion.Apatite solubilities obtained from these experiments, together with previous results, can be described as a function of absolute temperature (T) and melt composition by the expression: In D^apatite/melt_P = [(8400 + ((SiO₂ ? 0.5)2.64 × 10⁴))/T] ? [3.1 + (12.4(SiO₂ ? 0.5))] where SiO₂ is the weight fraction of silica in the melt. This model appears to be valid between 45% and 75% SiO₂, 0 and 10% water, and for the range of pressures expected in the crust.The diffusivity information extracted from the experiments can be directly applied to several problems of geochemical interest, including I) dissolution times for apatite during crustal anatexis, and 2) pileup of P, and consequent local saturation in apatite, at the surfaces of growing major-mineral phases.  相似文献   

Petrography of low rank coals with reference to liquefaction potential     

S. Parkash  M.P. Du Plessis  A.R. Cameron  W.D. Kalkreuth 《International Journal of Coal Geology》1984,4(3):209-234

Ten sub-bituminous coals were analyzed petrographically and chemically as part of a program by the Alberta Research Council to investigate the compositional properties of Alberta coals and relationship to liquefaction potential. In addition, four lignites, two from Saskatchewan and one each from Texas and North Dakota, were studied for comparative purposes. Liquefaction experiments were carried out on three of the Alberta coals and on density fractions of the two U.S. lignites. Petrographically the ten sub-bituminous coals could be divided into two populations; one with high huminite and liptinite contents (>90%) and one with substantially lower contents (70–75%). The two U.S. lignites showed huminite plus liptinite contents of less than 70% while the Saskatchewan coals were somewhat higher. Reflectance measurements on the sub-bituminous coals showed a range of 0.33 to 0.51 with a rough correlation to the ASTM rank designations of these coals. Reflectivities determined on the lignites were generally lower. Float-sink fractionation of the two U.S. lignites and two sub-bituminous coals showed enrichment of huminite and liptinite in the lighter fractions and inertinite and mineral matter in the heavier splits. Examination of the relationship between liquefaction yields and petrographic composition showed a rough positive correlation with huminite-liptinite content. Examination of liquefaction residues suggested total conversion of the liptinite, extensive conversion of huminite and possible partial conversion and reactivity of the semifusinite maceral of the inertinite group.  相似文献   

Petrological influence on differential weathering and inselberg development in the Kora area of central Kenya     

K. Pye  A. S. Goudie  A. Watson 《地球表面变化过程与地形》1986,11(1):41-52

In the Kora area of central Kenya domed inselbergs are well developed on outcrops of granitoid migmatite, while positive relief features are rare on the surrounding gneiss. Block-strewn, vegetated hills occur on restricted areas of granoblastite, gabbro, and metagabbro. Schmidt Hammer measurements have shown that the apparent differences in resistance to weathering and erosion are not due to variations in rock hardness, since all the rock types have similar ‘R’ values. The results of geochemical analyses have shown that the migmatites are significantly more potassic than the surrounding gneiss. Samples of migmatite from the inselbergs were also found to be slightly richer in potassium than migmatite samples from the inter-inselberg areas. The variations in potassium content probably reflect differences in protolith composition, chemical fractionation during partial melting, and the effects of metasomatism. These findings support earlier suggestions that, other things being equal, potassium-rich granitoid rocks weather more slowly than less potassic rocks.  相似文献