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Schists from two mylonitic localities in the footwall of a low-angle normal fault in the eastern Alps record different degrees
of embrittlement during exhumation, depending on the extent to which fluid–rock reactions proceeded. At one site, graphitic
schists preserve textural evidence for two metamorphic reactions that modified
and/or fluid volume: (1) reaction between graphite and aqueous fluid that increased without changing the molar amount of fluid, and (2) replacement of titanite by rutile, calcite, and quartz. The latter reaction
involved net consumption of increasingly CO2-rich fluid. Areas where the first reaction proceeded are associated with abundant Mode I microcracks. Fluid inclusion arrays
within the microcracks show that increased from ∼0.1 to 0.6 during decompression from 4.75 to 2 kbar at a reference temperature of 500°C. Titanite consumption
is most pronounced within transgranular Mode I microcracks, but microcracks do not crosscut products of this reaction; fluid
consumption during reaction was coeval with the end of microcracking, at least on a local scale. At the other site, graphitic
schists lack small-scale Mode I cracks as well as evidence for graphite consumption during decompression. SEM imaging shows
that graphite is anhedral and pitted at the first site, but occurs in clusters of euhedral grains at the second site. Mass
balance calculations demonstrate that rocks with partially consumed graphite were infiltrated by an externally derived, H2O-rich fluid that drove subsequent graphite-fluid reaction. Evidence for similar fluid infiltration is absent at the second
site. Variations in the degree of reaction progress indicate that fluid pathways and deformation style were heterogeneous
on the scale of millimeters to kilometers during exhumation from mid-crustal depths. 相似文献
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Gabriel A. Vargo Cynthia A. Heil Kent A. Fanning L. Kellie Dixon Merrie Beth Neely Kristen Lester Danylle Ault Susan Murasko Julie Havens John Walsh Steven Bell 《Continental Shelf Research》2008
Identifying nutrient sources, primarily nitrogen (N) and phosphorus (P), sufficient to support high biomass blooms of the red tide dinoflagellate, Karenia brevis, has remained problematic. The West Florida Shelf is oligotrophic, yet populations >106 cells L−1 frequently occur and blooms can persist for months. Here we examine the magnitude and variety of sources for N and P that are available to support blooms. Annual average in situ or background concentrations of inorganic N in the region where blooms occur range 0.02–0.2 μM while inorganic P ranges 0.025–0.24 μM. Such concentrations would be sufficient to support the growth of populations up to ∼3×104 cells L−1 with at least a 1 d turnover rate. Organic N concentrations average 1–2 orders of magnitude greater than inorganic N, 8–14 μM while organic P concentrations average 0.2–0.5 μM. Concentrations of organic N are sufficient to support blooms >105 cells L−1 but the extent to which this complex mixture of N species is utilizable is unknown. Other sources of nutrients included in our analysis are aerial deposition, estuarine flux, benthic flux, zooplankton excretion, N2-fixation, and subsequent release of organic and inorganic N by Trichodesmium spp., and release of N and P from dead and decaying fish killed by the blooms. Inputs based on atmospheric deposition, benthic flux, and N2-fixation, were minor contributors to the flux required to support growth of populations >2.6×104 cells L−1. N and P from decaying fish could theoretically maintain populations at moderate concentrations but insufficient data on the flux and subsequent mixing rates does not allow us to calculate average values. Zooplankton excretion rates, based on measured zooplankton population estimates and excretion rates could also supply all of the N and P required to support populations of 105 and 106 cells L−1, respectively, but excretion is considered as “regenerated” nutrient input and can only maintain biomass rather than contribute to “new” biomass. The combined estuarine flux from Tampa Bay, Charlotte Harbor, and the Caloosahatchee River can supply a varying, but at times significant level of N and P to meet growth and photosynthesis requirements for populations of approximately 105 cells L−1 or below. Estimates of remineralization of dead fish could supply a significant proportion of bloom maintenance requirements but the rate of supply must still be determined. Overall, a combination of sources is required to maintain populations >106 cells L−1. 相似文献
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Kristen M. Lester Cynthia A. Heil Merry B. Neely Danylle N. Spence Susan Murasko Thomas L. Hopkins Tracey T. Sutton Scott E. Burghart Richard N. Bohrer Andrew W. Remsen Gabriel A. Vargo John J. Walsh 《Continental Shelf Research》2008
Blooms of the toxic dinoflagellate Karenia brevis are common in the Gulf of Mexico, yet no in situ studies of zooplankton and K. brevis have been conducted there. Zooplankton abundance and taxonomic composition at non-bloom and K. brevis bloom stations within the Ecology of Harmful Algal Blooms (ECOHAB) study area were compared. At non-bloom stations, the most abundant species of zooplankton were Parvocalanus crassirostris, Oithona colcarva, and Paracalanus quasimodo at the 5-m isobath and P. quasimodo, O. colcarva, and Oikopleura dioica at the 25-m isobath. There was considerable overlap in dominance of zooplankton species between the 5 and 25-m isobaths, with nine species contributing to 90% of abundance at both isobaths. At stations within K. brevis blooms however, Acartia tonsa, Centropages velificatus, Temora turbinata, Evadne tergestina, O. colcarva, O. dioica, and P. crassirostris were dominant. Variations in abundance between non-bloom and bloom assemblages were evident, including the reduction in abundance of three key species within K. brevis blooms. 相似文献
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The water-rich condensates of fumarolic gases, obtained from degassing lavas of the 1959–60 eruption of Kilauea volcano, contain unexpectedly high concentrations of hydrofluoric and hydrochloric acids, and thereby suggest that halogens are significant constituents of basaltic magmas. Vents on the pumice hill of Kilauea Iki yielded one sample that contained, in parts per million, 21,000 HF (1.1 N) with 2,920 HCl, and another sample, 20 HF with 70,500 HCl (2.0 N). Samples from vents elsewhere on the volcano had from one-fortieth to one-thousandth as much of the two acids. A rough correlation exists between the temperature of the fumaroles (range 110 to 820°C) and the total concentration of the halogen acids. This correlation is mainly due to progressive dilution of the magmatic halogen acids by water of probable meteoric origin in fumaroles of lower temperatures. Variations in the HF/HCl ratio (range 0.0003 to 7.2) may be explained by means of two different processes whose relative importance cannot be assessed with the data at hand (1). In their migration to the surface, the acid gases may have reacted with the lava to a variable extent owing to the widely different configurations of the several vents (steaming areas in glassy pumice, glowing cracks, and drillhole in lava lake). In the reaction, relatively more HF could have reacted at temperatures around 300°C with the glassy pumice (2). There is some indication that the HF/HCl ratio increases with time,suggesting that the crystallizing lava may have released HCl early, with HF concentrated in the later exhalations. The Br/Cl ratio ranges from 0.0036 down to 0.0014, as compared to 0.0034 of seawater. 相似文献
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