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A one-dimensional model for coupled heat flow, fluid flow, and mineral reaction predicts the identity and spatial and temporal
distributions of mineral assemblages produced during contact metamorphism of siliceous dolomite. Results are compared to mineral
distributions predicted by transport models for fluid flow along steady-state temperature profiles and to mineral distributions
observed in contact aureoles to assess whether a detailed analysis of heat flow is required to extract meaningful information
of fluid flow history from field data. Results identify several mineral assemblages whose interpretation in terms of fluid
flow is dependent on an understanding of their thermal history. Certain key mineral assemblages, however, are sensitive records
of the geometry and amount of fluid flow but are insensitive to thermal history. The presence or absence of these mineral
assemblages constrains the history of fluid flow during contact metamorphism of siliceous dolomite regardless of the details
of heat flow. Occurrences of the key assemblages record both magmatic fluid flow in the direction of decreasing temperature
(Alta, Elkhorn aureoles) and metamorphic fluid flow in the direction of increasing temperature (Beinn an Dubhaich, Kasuga-mura
aureoles) during contact metamorphism. Time-integrated input fluid flux averages on the order of 100 mol/cm2 with a range of ±1 order of magnitude.
Received: 13 October 1995 / Accepted: 20 March 1996 相似文献
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Ian M. Coulson Mike E. Villeneuve Gregory M. Dipple Robert A. Duncan James K. Russell James K. Mortensen 《Journal of Volcanology and Geothermal Research》2002,114(3-4)
Knowledge of the time-scales of emplacement and thermal history during assembly of composite felsic plutons in the shallow crust are critical to deciphering the processes of crustal growth and magma chamber development. Detailed petrological and chemical study of the mid-Cretaceous, composite Emerald Lake pluton, from the northern Canadian Cordillera, Yukon Territory, coupled with U–Pb and 40Ar/39Ar geochronology, indicates that this pluton was intruded as a series of magmatic pulses. Intrusion of these pulses produced a strong petrological zonation from augite syenite, hornblende quartz syenite and monzonite, to biotite granite. Our data further indicate that multiple phases were emplaced and cooled to below the mineral closure temperatures over a time-scale on the order of the resolution of the 40Ar/39Ar technique (1 Myr), and that emplacement occurred at 94.3 Ma. Simple thermal modelling and heat conduction calculations were used to further constrain the temporal relationships within the intrusion. These calculations are consistent with the geochronology and show that emplacement and cooling were complete in less than 100 kyr and probably 70±5 kyr. These results demonstrate that production, transport and emplacement of the different phases of the Emerald Lake pluton occurred essentially simultaneously, and that these processes must also have been closely related in time and space. By analogy, these results provide insights into the assembly and petrogenesis of other complex intrusions and ultimately lead to an understanding of the processes involved in crustal development. 相似文献
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Biologically induced mineralization of dypingite by cyanobacteria from an alkaline wetland near Atlin,British Columbia,Canada 总被引：1，自引：0，他引：1
Ian M Power Siobhan A Wilson James M Thom Gregory M Dipple Gordon Southam 《Geochemical transactions》2007,8(1):13
Background
This study provides experimental evidence for biologically induced precipitation of magnesium carbonates, specifically dypingite (Mg5(CO3)4(OH)2·5H2O), by cyanobacteria from an alkaline wetland near Atlin, British Columbia. This wetland is part of a larger hydromagnesite (Mg5(CO3)4(OH)2·4H2O) playa. Abiotic and biotic processes for magnesium carbonate precipitation in this environment are compared. 相似文献7.
Ian M. Power Siobhan A. Wilson Anna L. Harrison Gregory M. Dipple Jenine McCutcheon Gordon Southam Paul A. Kenward 《Sedimentology》2014,61(6):1701-1733
This study formulates a comprehensive depositional model for hydromagnesite–magnesite playas. Mineralogical, isotopic and hydrogeochemical data are coupled with electron microscopy and field observations of the hydromagnesite–magnesite playas near Atlin, British Columbia, Canada. Four surface environments are recognized: wetlands, grasslands, localized mounds (metre‐scale) and amalgamated mounds composed primarily of hydromagnesite [Mg5(CO3)4(OH)2·4H2O], which are interpreted to represent stages in playa genesis. Water chemistry, precipitation kinetics and depositional environment are primary controls on sediment mineralogy. At depth (average ≈ 2 m), Ca–Mg‐carbonate sediments overlay early Holocene glaciolacustrine sediments indicating deposition within a lake post‐deglaciation. This mineralogical change corresponds to a shift from siliciclastic to chemical carbonate deposition as the supply of fresh surface water (for example, glacier meltwater) ceased and was replaced by alkaline groundwater. Weathering of ultramafic bedrock in the region produces Mg–HCO3 groundwater that concentrates by evaporation upon discharging into closed basins, occupied by the playas. An uppermost unit of Mg‐carbonate sediments (hydromagnesite mounds) overlies the Ca–Mg‐carbonate sediments. This second mineralogical shift corresponds to a change in the depositional environment from subaqueous to subaerial, occurring once sediments ‘emerged’ from the water surface. Capillary action and evaporation draw Mg–HCO3 water up towards the ground surface, precipitating Mg‐carbonate minerals. Evaporation at the water table causes precipitation of lansfordite [MgCO3·5H2O] which partially cements pre‐existing sediments forming a hardpan. As carbonate deposition continues, the weight of the overlying sediments causes compaction and minor lateral movement of the mounds leading to amalgamation of localized mounds. Radiocarbon dating of buried vegetation at the Ca–Mg‐carbonate boundary indicates that there has been ca 8000 years of continuous Mg‐carbonate deposition at a rate of 0·4 mm yr?1. The depositional model accounts for the many sedimentological, mineralogical and geochemical processes that occur in the four surface environments; elucidating past and present carbonate deposition. 相似文献
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Ian M. Power Siobhan A. Wilson James M. Thom Gregory M. Dipple Janet E. Gabites Gordon Southam 《Chemical Geology》2009,260(3-4):286-300
Anthropogenic greenhouse gas emissions may be offset by sequestering carbon dioxide (CO2) through the carbonation of magnesium silicate minerals to form magnesium carbonate minerals. The hydromagnesite [Mg5(CO3)4(OH)2·4H2O] playas of Atlin, British Columbia, Canada provide a natural model to examine mineral carbonation on a watershed scale. At near surface conditions, CO2 is biogeochemically sequestered by microorganisms that are involved in weathering of bedrock and precipitation of carbonate minerals. The purpose of this study was to characterize the weathering regime in a groundwater recharge zone and the depositional environments in the playas in the context of a biogeochemical model for CO2 sequestration with emphasis on microbial processes that accelerate mineral carbonation.Regions with ultramafic bedrock, such as Atlin, represent the best potential sources of feedstocks for mineral carbonation. Elemental compositions of a soil profile show significant depletion of MgO and enrichment of SiO2 in comparison to underlying ultramafic parent material. Polished serpentinite cubes were placed in the organic horizon of a coniferous forest soil in a groundwater recharge zone for three years. Upon retrieval, the cube surfaces, as seen using scanning electron microscopy, had been colonized by bacteria that were associated with surface pitting. Degradation of organic matter in the soil produced chelating agents and acids that contributed to the chemical weathering of the serpentinite and would be expected to have a similar effect on the magnesium-rich bedrock at Atlin. Stable carbon isotopes of groundwater from a well, situated near a wetland in the southeastern playa, indicate that 12% of the dissolved inorganic carbon has a modern origin from soil CO2.The mineralogy and isotope geochemistry of the hydromagnesite playas suggest that there are three distinct depositional environments: (1) the wetland, characterized by biologically-aided precipitation of carbonate minerals from waters concentrated by evaporation, (2) isolated wetland sections that lead to the formation of consolidated aragonite sediments, and (3) the emerged grassland environment where evaporation produces mounds of hydromagnesite. Examination of sediments within the southeastern playa–wetland suggests that cyanobacteria, sulphate reducing bacteria, and diatoms aid in producing favourable geochemical conditions for precipitation of carbonate minerals.The Atlin site, as a biogeochemical model, has implications for creating carbon sinks that utilize passive microbial, geochemical and physical processes that aid in mineral carbonation of magnesium silicates. These processes could be exploited for the purposes of CO2 sequestration by creating conditions similar to those of the Atlin site in environments, artificial or natural, where the precipitation of magnesium carbonates would be suitable. Given the vast quantities of Mg-rich bedrock that exist throughout the world, this study has significant implications for reducing atmospheric CO2 concentrations and combating global climate change. 相似文献
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Metasomatism and fluid flow in ductile fault zones 总被引：8，自引：0，他引：8
Observed major element metasomatism in 5 amphibolite facies ductile fault zones can be explained as the inevitable consequence of aqueous fluid flow along normal temperature gradients under conditions of local chemical equilibrium. The metasomatism does not require the infiltration of chemically exotic fluids. Calculations suggest that metasomatized ductile fault zones are typically infiltrated by 105 moles H2O/cm2, fluid flow is in the direction of decreasing temperature, and fluids contain about 1.0 molal total chloride. Where available, stable isotopic alteration data confirm both flow direction and fluid fluxes calculated from major element metasomatism. The fluid fluxes inferred from metasomatism do not require large-scale fluid recirculation or mantle sources if significant lateral fluid flow occurs in the deep crust. Time-integrated fluid fluxes are combined with estimates of flow duration to constrain average flow rates and average permeabilities. Rocks in ductile fault zones are probably much more permeable during metasomatism (average permeabilities of 10-17 to 10-15 m2) than rocks normally are during regional metamorphism (10-21 to 10-18 m2). Estimated average fluid flow rates (3.5×10-3 to 0.35 m/yr) are insufficient, however, to significantly elevate ambient temperatures within ductile faults. Fluid flow in the direction of decreasing temperature may increase the ductility of silicate rocks by adding K to the rocks and thereby driving mica-forming reactions. 相似文献
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The capacity of mine waste to trap CO2 is, in some cases, much larger than the greenhouse gas production of a mining operation. In mine tailings, the presence of secondary carbonate minerals that trap CO2 can therefore represent substantial fixation of this greenhouse gas. The abilities of three methods of quantitative phase analysis to measure trace nesquehonite (MgCO3·3H2O) in samples of processed kimberlite have been assessed: the method of reference intensity ratios (RIR), the internal standard method, and the Rietveld method with X-ray powder diffraction data. Tests on synthetic mixtures made to resemble processed kimberlite indicate that both the RIR and Rietveld methods can be used accurately to quantify nesquehonite to a lower limit of approximately 0.5 wt.% for conditions used in the laboratory. Below this value, estimates can be made to a limit of approximately 0.1 wt.% using a calibration curve according to the internal standard method. The RIR method becomes increasingly unreliable with decreasing abundance of nesquehonite, primarily as a result of an unpredictable decline in preferred orientation of crystallites. For Rietveld refinements, structureless pattern fitting was used to account for planar disorder in lizardite by considering it as an amorphous phase. Rietveld refinement of data collected from specimens that were serrated to minimize preferred orientation of crystallites gives rise to systematic overestimates of refined abundances for lizardite and underestimates for other phases. The resulting pattern of misestimates may be mistaken for the effect of amorphous and/or nanocrystalline material in samples. This effect is mitigated by collecting data from non-serrated specimens, which typically give relative errors on refined abundances for major and minor phases in the range of 5–20%. However, relative error can increase rapidly for abundances less than 5 wt.%. Nonetheless, absolute errors are sufficiently small that estimates can be made for the amount of CO2 stored in secondary nesquehonite using the RIR method or the Rietveld method for abundances ?0.5 wt.% and a calibration curve for abundances <0.5 wt.%. The extent to which C is being mineralized in an active mine setting at the Diavik Diamond Mine, Northwest Territories, Canada, has been investigated. Rietveld refinement results and calibrated abundances for trace nesquehonite are used to estimate the amount of CO2 trapped in Diavik tailings. Results of quantitative phase analysis are also used to calculate neutralization potentials for the kimberlite mine tailings and to estimate the contribution made by secondary nesquehonite. 相似文献