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991.
Stable-isotope profiles show that flat-lying marble units acted as impermeable barriers to upward fluid flow in transitional amphibolite-granulite grade rocks of the Kigluaik Mountains, Seward Peninsula, Alaska. The degree of permeability is related to the composition of the marble. The margin of a thick pure dolomite marble chemically reacted with underlying metasyenite (aH2O=0.2) to form a 2 cm boundary layer of calcite + forsterite by introduction of SiO2. No fluid penetrated past this reaction front, although the high temperature of metamorphism (800°C) allowed transport of carbon and oxygen isotopes for an additional 2 cm by diffusion through the solid dolomite. A second marble with a higher silica content underwent more decarbonation, which enhanced porosity and lead to a greater extent of isotope transport (2–3 m) in contact with quartzo-feld-spathic gneiss below. An estimate of total fluid flux across the bottom of this marble layer based on the shape of the isotope profile is 1 cm3/cm2 directed down, out of the marble. At two other marble-gneiss contacts steep isotopic gradients coincide with lithologic contacts, indicating very little cross-lithology fluid flow. The extent of diffusional transport of isotopes in the marbles is limited and interpreted as indicating the transient presence of a pore fluid, generated by thermally driven devolatilization reactions. No wholesale pervasive advection of C-O-H fluid occurred across the thick, continuous, marble units near the exposed base of the Kigluaik Group section during the entire regional metamorphic cycle. Activities of pore-fluid species were controlled by internal processes. Movement of volatiles and stable-isotopes between contrasting rock-types was dominantly diffusive. Channelized fluid pathways through the marble units developed during uplift and cooling but were not present during peak metamorphism. Heating of the section occurred by conduction, probably from an underlying magma source, and not by advection of a C-O-H fluid. 相似文献
992.
Christian Mikutta Jan G. Wiederhold Olaf A. Cirpka Bernard Bourdon 《Geochimica et cosmochimica acta》2009,73(7):1795-1526
The application of stable Fe isotopes as a tracer of the biogeochemical Fe cycle necessitates a mechanistic knowledge of natural fractionation processes. We studied the equilibrium Fe isotope fractionation upon sorption of Fe(II) to aluminum oxide (γ-Al2O3), goethite (α-FeOOH), quartz (α-SiO2), and goethite-loaded quartz in batch experiments, and performed continuous-flow column experiments to study the extent of equilibrium and kinetic Fe isotope fractionation during reactive transport of Fe(II) through pure and goethite-loaded quartz sand. In addition, batch and column experiments were used to quantify the coupled electron transfer-atom exchange between dissolved Fe(II) (Fe(II)aq) and structural Fe(III) of goethite. All experiments were conducted under strictly anoxic conditions at pH 7.2 in 20 mM MOPS (3-(N-morpholino)-propanesulfonic acid) buffer and 23 °C. Iron isotope ratios were measured by high-resolution MC-ICP-MS. Isotope data were analyzed with isotope fractionation models. In batch systems, we observed significant Fe isotope fractionation upon equilibrium sorption of Fe(II) to all sorbents tested, except for aluminum oxide. The equilibrium enrichment factor, , of the Fe(II)sorb-Fe(II)aq couple was 0.85 ± 0.10‰ (±2σ) for quartz and 0.85 ± 0.08‰ (±2σ) for goethite-loaded quartz. In the goethite system, the sorption-induced isotope fractionation was superimposed by atom exchange, leading to a δ56/54Fe shift in solution towards the isotopic composition of the goethite. Without consideration of atom exchange, the equilibrium enrichment factor was 2.01 ± 0.08‰ (±2σ), but decreased to 0.73 ± 0.24‰ (±2σ) when atom exchange was taken into account. The amount of structural Fe in goethite that equilibrated isotopically with Fe(II)aq via atom exchange was equivalent to one atomic Fe layer of the mineral surface (∼3% of goethite-Fe). Column experiments showed significant Fe isotope fractionation with δ56/54Fe(II)aq spanning a range of 1.00‰ and 1.65‰ for pure and goethite-loaded quartz, respectively. Reactive transport of Fe(II) under non-steady state conditions led to complex, non-monotonous Fe isotope trends that could be explained by a combination of kinetic and equilibrium isotope enrichment factors. Our results demonstrate that in abiotic anoxic systems with near-neutral pH, sorption of Fe(II) to mineral surfaces, even to supposedly non-reactive minerals such as quartz, induces significant Fe isotope fractionation. Therefore we expect Fe isotope signatures in natural systems with changing concentration gradients of Fe(II)aq to be affected by sorption. 相似文献
993.
Accelerated sediment delivery to continental margins during post-orogenic rebound of mountain ranges
Changes in sediment flux to continental margins are commonly interpreted in terms of tectonic growth of topography or climatic change. Here, we show that variations in sediment yield from orogenic systems, previously considered as resulting from climate change, drainage reorganisation or mantle processes can be explained by intrinsic mechanisms of mountain belt/foreland basin systems naturally evolving during post-orogenic decay. Numerical modelling indicates an increase of sediment flux leaving the orogenic system synchronous with the cessation of deposition in the foreland basin and the transition from late syn- to post-orogenesis. Experiments highlight the importance of lithospheric flexure that causes the post-orogenic isostatic rebound of the foreland basin. Erosion of the rebounding foreland basin combined with continued sediment flux from the thrust wedge drives an acceleration in sediment outflux towards continental margins. Sediment budget records in natural settings such as the Northern Pyrenees or Western European Alps also indicate accelerated post-orogenic sediment delivery to the Bay of Biscay and Rhône Delta respectively. These intrinsic processes that determine sediment yield to continental margins must be accounted for prior to consideration of additional external tectonic or climatic controls. 相似文献
994.
995.