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1.
Berm breaching of intermittently closed estuaries, either naturally or due to management practices, results in a physicochemical disturbance due to the flushing of water, material and biota into the ocean and the subsequent tidal influx. In 2007 and 2008 comparative and controlled studies were employed in three New Zealand estuaries: Sawmill Creek (46°04′ S 170°12′ E), Otokia Creek (45°57′ S 179°20′ E) and Kaikorai Lagoon (45°56′ S 170°23′ E), to investigate the impact of berm breaching on the hyperbenthic macroinvertebrate community in intermittently closed estuaries. Significant changes in community structure occurred in both the weekly comparative and the controlled studies (ANOSIM P < 0.01). Furthermore, the catch per unit effort of both total and key invertebrate taxa significantly increased after breaching (ANOVA P < 0.01). However, substantial numbers of individuals were expelled into the ocean (5,800 individuals, 20-min sample) while new taxa immigrated into the estuaries. Along with migration-related changes to community structure, berm breaches also resulted in the loss of ∼80% of inundated habitat and the concentration of existing populations. Consequently, the management of intermittent estuary connections to the ocean has implications for the ecology of individual, managed estuaries and also for regional coastal populations of epibenthic invertebrates.  相似文献   

2.
A case study for the ion–aerosol interactions is presented from the simultaneous measurements of mobility spectra of atmospheric ions in the mobility range of 2.29 to 2.98 × 10 − 4 cm2 V − 1 s-1^{-1}(diameter range 0.41–109 nm) and of size distribution of atmospheric aerosol particles in the size ranges of 4.4–700 nm and 500–20,000 nm diameters made at Maitri (70°45′52′ ′S, 11°44′2.7′ ′E; 130 m above mean sea level), Antarctica, on two days January 17 and February 18, 2005, with contrasting meteorological conditions. In contrast to January 17, on February 18, winds were stronger from the morning to noon and lower from the noon to evening, atmospheric pressure was lower, cloudiness was more, the land surface remained snow-covered after a blizzard on February 16 and 17 and the airmass over Maitri, descended from an altitude of ~3 km after an excursion over ocean. On these days mobility spectra showed two modes, corresponding to intermediate ions and light large ions and an indication of additional one/two maxima for small/cluster ions and heavy large ions. The small ions generated by cosmic rays, and the nucleation mode particles generated probably by photochemical reactions grew in size by condensation of volatile trace gases on them and produced the cluster and intermediate ion modes and the Aitken particle mode in ion/particle spectra. Particles in the size range of 9–26 nm have been estimated to grow at the rate of 1.9 nm h − 1 on February 18, 2005. Both, ions and aerosol particles show bimodal size distributions in the 16–107 nm size range, and comparison of the two size distributions suggests the formation of multiple charged ions. Attachment of small ions to particles in this bimodal distribution of Aitken particles together with the formation of multiple charged ions are proposed to result in the light and heavy large ion modes. Growth of the nucleation mode particles on February 18, 2005 is associated with the passage of the airmass over ocean. In contrast, though the ion size distributions were not much different, the aerosol size distributions did not show a dominant peak for the formation and growth of nucleation mode particles on January 17. More measurements are needed before the conclusion of this case study is generalized.  相似文献   

3.
 Unit-cell dimensions of a natural phlogopite from Pargas, Finland, have been determined in the temperature interval of 27–1050 °C by X-ray powder diffraction technique. Expansion rates vary discontinuously with temperature with a break at 412 °C. Below this temperature, the linear expansions (α) for a, b and c axis lengths are 3.74 × 10−5 K−1, 1.09 × 10−5 K−1, and 1.19 × 10−5 K−1, respectively, and above that they are 0.86 × 10−5 K−1, 0.80 × 10−5 K−1, and 1.93 × 10−5 K−1. The volume thermal expansion coefficients are 6.26 × 10−5 K−1 and 3.71 × 10−5 K−1 for low-temperature and high-temperature intervals, respectively. The observed kink in the rate of thermal expansions with temperature could be due to the different mode of structural changes. Thermogravimetric analysis of the sample indicates the oxidation of iron in the temperature range of 500–600 °C and dehydroxylation as well as decomposition of phlogopite in the temperature range of 900–1200 °C. Received: 8 September 1998 / Accepted: 28 February 2000  相似文献   

4.
 Using the high-pressure differential thermal analysis (HP-DTA) system in a cubic multianvil high-pressure apparatus, we measured the melting points of portlandite, Ca(OH)2, up to 6 GPa and 1000 °C. We detected endothermic behavior at the temperature and pressure conditions of 800 °C and 2.5 GPa, 769 °C and 3.5 GPa, 752 °C and 4.0 GPa, 686 °C and 5.0 GPa, and 596 °C and 6.0 GPa, respectively, due to melting of portlandite. By in situ X-ray studies under pressure, the melting of portlandite was observed at 730 °C and 4.32 GPa and at 640 °C and 5.81 GPa, respectively. Results of both HP-DTA and X-ray studies were consistent within experimental error. The melting is congruent and has a negative Clapeyron slope, indicating that liquid Ca(OH)2 has higher densities than crystalline portlandite in this pressure range. Received: 19 June 1999 / Revised, accepted: 11 September 1999  相似文献   

5.
The San José district is located in the northwest part of the Deseado massif and hosts a number of epithermal Ag–Au quartz veins of intermediate sulfidation style, including the Huevos Verdes vein system. Veins are hosted by andesitic rocks of the Bajo Pobre Formation and locally by rhyodacitic pyroclastic rocks of the Chon Aike Formation. New 40Ar/39Ar constraints on the age of host rocks and mineralization define Late Jurassic ages of 151.3 ± 0.7 Ma to 144.7 ± 0.1 Ma for volcanic rocks of the Bajo Pobre Formation and of 147.6 ± 1.1 Ma for the Chon Aike Formation. Illite ages of the Huevos Verdes vein system of 140.8 ± 0.2 and 140.5 ± 0.3 Ma are 4 m.y. younger than the volcanic host rock unit. These age dates are among the youngest reported for Jurassic volcanism in the Deseado massif and correlate well with the regional context of magmatic and hydrothermal activity. The Huevos Verdes vein system has a strike length of 2,000 m, with several ore shoots along strike. The vein consists of a pre-ore stage and three main ore stages. Early barren quartz and chalcedony are followed by a mottled quartz stage of coarse saccharoidal quartz with irregular streaks and discontinuous bands of sulfide-rich material. The banded quartz–sulfide stage consists of sulfide-rich bands alternating with bands of quartz and bands of chlorite ± illite. Late-stage sulfide-rich veinlets are associated with kaolinite gangue. Ore minerals are argentite and electrum, together with pyrite, sphalerite, galena, chalcopyrite, minor bornite, covellite, and ruby silver. Wall rock alteration is characterized by narrow (< 3 m) halos of illite and illite/smectite next to veins, grading outward into propylitic alteration. Gangue minerals are dominantly massive quartz intergrown with minor to accessory adularia. Epidote, illite, illite/smectite, and, preferentially at deeper levels, Fe-chlorite gangue indicate near-neutral pH hydrothermal fluids at temperatures of >220°C. Kaolinite occurring with the late sulfide-rich veinlet stage indicates pH < 4 and a temperature of <200°C. The Huevos Verdes system has an overall strike of 325°, dipping on average 65° NE. The orientations of individual ore shoots are controlled by vein strike and intersecting north-northwest-striking faults. We propose a structural model for the time of mineralization of the San José district, consisting of a conjugate shear pair of sinistral north-northwest- and dextral west-northwest-striking faults that correspond to R and R′ in the Riedel shear model and that are related to master faults (M) of north-northeast-strike. Veins of 315° strike can be interpreted as nearly pure extensional fractures (T). Variations in vein strike predict an induced sinistral shear component for strike directions of >315°, whereas strike directions of <315° are predicted with an induced dextral strike–slip movement. The components of the structural model appear to be present on a regional scale and are not restricted to the San José district.  相似文献   

6.
Life spans and thermal evolution of hydrothermal systems are of fundamental metallogenic importance. We were able to establish the chronology and cooling history of the Zaldívar porphyry copper deposit (Northern Chile) by applying a combination of different isotopic dating methods in minerals with different closure temperatures, including 40Ar/39Ar geochronology and zircon fission track thermochronology, together with fluid inclusion thermometry and previous published U–Pb zircon geochronology. The hydrothermal mineralization in the Zaldívar deposit is genetically related to the Llamo Porphyry unit. Samples of igneous biotites from this intrusion yielded 40Ar/39Ar plateau ages between 35.5 ± 0.7 and 37.7 ± 0.4 Ma defining a weighted average of 36.6 ± 0.5 Ma (2σ). In contrast, one sample from the Zaldívar porphyry, one from the andesites, and two from the Llamo porphyry yielded considerably younger fission track ages of approximately 29 Ma with a weighted mean for all ages of 29.1 ± 1.7 Ma (2σ). Thermal and compositional constraints for the hydrothermal system in the Zaldívar deposit from fluid inclusions thermometry show that at least three fluid types broadly characterize two main hydrothermal episodes during the evolution of the deposit. The main mineralization and alteration event is characterized by high temperature (above 320°C) hypersaline fluids (salinity between 30 and 56 wt.% NaCl equivalents) coexisting with low-density gas-rich inclusions (salinity less than 17 wt.% NaCl equivalents) that homogenizing into the gas phase at temperatures above 350°C. The second episode corresponds to a low-temperature event which is characterized by liquid-rich fluid inclusions that homogenize into the liquid phase at temperatures ranging from 200°C to 300°C with salinities lower than 10 wt.% NaCl equivalents. The 40Ar/39Ar data (36.6 ± 0.5 Ma, weighted average) obtained from igneous biotites represent the minimum age for the last high-temperature (above 300°C) hydrothermal pulse. When compared with previously published U–Pb ages (38.7 ± 1.3 Ma) in zircons from the Llamo porphyry, a close temporal relationship between crystallization of the parental intrusion and the thermal collapse of the last high-temperature hydrothermal event is evident. Cooling took place from approximately 800°C (crystallization of the intrusive complex defined by zircon U–Pb ages) to below 300 ± 50°C (biotite 40Ar/39Ar closure temperature) within approximately 1.5 m.y. Because the thermal annealing of fission tracks in zircons occurs at temperatures of 240 ± 30°, the zircon fission track (ZFT) ages of 29.1 ± 1.7 Ma (2σ) mark the end of the thermal activity in the Zaldívar area, specifically the time when the whole area cooled below this temperature, well after the collapse of the main hydrothermal event in the Zaldívar porphyry copper deposit. This cooling age roughly coincides with the age defined for the emplacement of dacitic dikes at 31 ± 2.8 Ma (2σ) (published K–Ar whole rock), 5 km south of the Zaldívar deposit, in the Escondida area. This late magmatic pulse probably is responsible for high heat flow in the Zaldívar deposit as late as 29 Ma. There is no evidence that the low temperature hydrothermal pulse recognized by fluid inclusion studies is related to this thermal event. The zircon fission track cooling ages are interpreted to be related to the time lag required for complete relaxation of the perturbation of the isotherms in the geothermal field imposed by the intrusion of magmatic bodies, with or without any association with low temperature hydrothermal activity.  相似文献   

7.
 The lattice constants of paragonite-2M1, NaAl2(AlSi3)O10(OH)2, were determined to 800 °C by the single-crystal diffraction method. Mean thermal expansion coefficients, in the range 25–600 °C, were: αa = 1.51(8) × 10−5, αb = 1.94(6) × 10−5, αc = 2.15(7) ×  10−5 °C−1, and αV = 5.9(2) × 10−5 °C−1. At T higher than 600 °C, cell parameters showed a change in expansion rate due to a dehydroxylation process. The structural refinements of natural paragonite, carried out at 25, 210, 450 and 600 °C, before dehydroxylation, showed that the larger thermal expansion along the c parameter was mainly due to interlayer thickness dilatation. In the 25–600 °C range, Si,Al tetrahedra remained quite unchanged, whereas the other polyhedra expanded linearly with expansion rate proportional to their volume. The polyhedron around the interlayer cation Na became more regular with temperature. Tetrahedral rotation angle α changed from 16.2 to 12.9°. The structure of the new phase, nominally NaAl2 (AlSi3)O11, obtained as a consequence of dehydroxylation, had a cell volume 4.2% larger than that of paragonite. It was refined at room temperature and its expansion coefficients determined in the range 25–800 °C. The most significant structural difference from paragonite was the presence of Al in fivefold coordination, according to a distorted trigonal bipyramid. Results confirm the structural effects of the dehydration mechanism of micas and dioctahedral 2:1 layer silicates. By combining thermal expansion and compressibility data, the following approximate equation of state in the PTV space was obtained for paragonite: V/V 0 = 1 + 5.9(2) × 10−5 T(°C) − 0.00153(4) P(kbar). Received: 12 July 1999 / Revised, accepted: 7 December 1999  相似文献   

8.
 An instrumented trial-fill dike was constructed on soft, laminated soils of the Lisan Peninsula foreshore of the Dead Sea. The earthwork had base dimensions of 180 m by approximately 70 m wide and was raised in two stages to a maximum height of 12.5 m above original ground level. The geotechnical data of the dike were monitored in order to: assess the short- and long-term strength of the foundation, obtain and analyze the pore-pressure response of the foundation soils for potential use in construction control, obtain data on embankment settlement in order to refine end-of-construction and post-construction settlement assessments, and optimize the height of the dike to be constructed by providing information on the construction sequence for use in calculation of capital costs and alternative layouts and dike heights. The successful completion of the trial dike has demonstrated that steep-side dikes up to 12.5 m high can be constructed rapidly on soft soils. The construction of the trial dike has therefore proved a very substantial benefit to the evaluation of the likely performance of a dike constructed along the Lisan shore. The key factor which made this fast construction possible was the unexpected, very rapid consolidation of the majority of the foundation soil which has been shown to occur. The principal observations from the trial dike were: (1) end of construction settlements may be calculated using drained stiffnesses where E′/su initial has a value of around 65; (2) post-construction settlements can be calculated using a coefficient of secondary compression, Cα equal to 0.015; (3) a rapid increase in undrained shear strength occurred when loading the soil up to a value of su equal to around 30 kPa. The value of su/σv′ was as high as 0.5 at this stage. With further loading the strength increase was more modest and su/σv fell to around 0.25 for a vertical effective stress of 160 kPa; (4) for the undrained stability analyses of the trial dike, the mean vane shear-strength profile provided an appropriate assessment of the short-term factor of safety against failure, 5) For the drained stability analyses of the trial dike, lower bound effective strength parameters for the foundation and embankment fill (c′=0, φ′=30° and c′=2 kN/m2, φ′=33°, respectively), combined with field measurements of end of construction pore water pressures provided an analysis which was broadly compatible with the undrained analysis; and (6) the trial dike has been stable, pre- and post-construction, because of the well drained nature of its foundation which prevented the build up of high pore water pressures and led to rapid consolidation. Received: 22 June 1998 · Accepted: 30 October 1998  相似文献   

9.
 Grain boundary diffusion rates of oxygen, potassium and calcium in fine-grained feldspar aggregates were determined experimentally. The starting materials were a natural albite rock from the Tanco pegmatite and aggregates hot-pressed from fragments of Amelia albite or Ab, Or and An composition glasses. The technique employed isotopic tracers (18O, 41K, 42Ca) either evaporated onto the surface or in an aqueous solution surrounding the sample, and depth profiling using an ion microprobe (SIMS). From the depth profiles, the product of the grain boundary diffusion coefficient (D′) and effective boundary width (δ) was calculated using numerical solutions to the appropriate diffusion equation. The experimental reproducibility of D′δ is a factor of 3. A separate determination of D′ independent of δ yields an effective grain boundary width of ∼3 nm, consistent with high resolution TEM observations of a physical grain boundary width <5 nm. Oxygen (as molecular water) grain boundary diffusion rates were determined in the Ab and Or aggregates at 450°–800° C and 100 MPa (hydrothermal), potassium rates in Or aggregates at 450°–700° C both at 0.1 MPa (in air) and at 100 MPa (hydrothermal), and calcium rates in An aggregates at 700°–1100° C and 0.1 MPa (in air). Oxygen grain boundary diffusion rates are similar in all three of the Ab aggregates and in the Or aggregate. Potassium and oxygen depth profiles measured in the same samples yield different D′δ values, confirming a diffusional transport mechanism. Potassium diffusion in the Or aggregate has a greater activation energy (216 vs 78 kJ/mol) than oxygen, and the Arrhenius relations cross at ∼625° C. Potassium D′δ values in Or aggregates are about a factor of five greater in hydrothermal experiments at 100 MPa than in experiments at 0.1 MPa in air. Calcium grain boundary diffusion rates in An aggregates are 4 to 5 orders of magnitude slower than potassium in Or and have a greater (291 kJ/mol) activation energy. This suggests that differences in formal charge and/or size of diffusing species may play an important role in their relative grain boundary diffusion rates. Received: 24 December 1993 / Accepted: 16 June 1994  相似文献   

10.
Sample cylinders of two galena ore hand specimens from Braubach, Germany were axially shortened in the strain rate range 5 × 10−5 s−1–5 × 10−7 s−1 at a confining pressure of 200 (300) MPa and at temperatures of 20 °C–600 °C. Neutron diffraction analyses of the crystallographic preferred orientation (texture) were carried out before and after experimental deformation on the same sample cylinder. Up to a deformation temperature of 300 °C and a strain rate of 5 × 10−6 s−1 a more or less complete <110> fiber texture develops, the strength of the fiber texture only depending on strain and the strength of the original preferred orientation. At slower strain rate and higher temperature, there is a distinct decrease of the fiber texture development. Diffusional mass transfer starts to become a significant deformation mechanism. Deformation at 500 °C changes the original texture only slightly, which indicates a rapid increase of importance of diffusional flow processes. The alteration of the accompanying sulfosalts indicates that the temperature is high enough for the movement of atoms. The microstructure only reveals remarkable deformation structures at higher strains and in areas of locally higher stresses. Received: 10 June 1997 / Accepted: 14 May 1998  相似文献   

11.
We measured seasonal variations in microzooplankton grazing in Long Island Sound (LIS) and San Francisco Bay (SFB). There was consistent evidence of nutrient limitation in LIS, but not SFB. We found higher chlorophyll a concentrations in LIS compared with SFB. In spite of differences in phytoplankton, there were no differences in microzooplankton abundance (summer: LIS, 12.4 ± 1.8 × 103 indiv. L−1; SFB, 14.1 ± 3.0 × 103 indiv. L−1), biomass (summer: LIS, 30.4 ± 5.0 μg C L−1; SFB, 26.3 ± 5.9 μg C L−1), or grazing rates (summer: LIS, 0.66 ± 0.19 day−1; SFB, 0.65 ± 0.18 day−1) between the two estuaries. In common with many other investigators, we found many instances of saturated as well as insignificant grazing. We suggest that saturation in some cases may result from high particle loads in turbid estuarine systems and that insignificant grazing may result from extreme saturation of the grazing response due to the need to process non-food particles.  相似文献   

12.
The unit cell parameters, extracted from Rietveld analysis of neutron powder diffraction data collected between 4.2 K and 320 K, have been used to calculate the temperature evolution of the thermal expansion tensor for gypsum for 50 ≤ T ≤ 320 K. At 300 K the magnitudes of the principal axes are α 11  = 1.2(6) × 10−6 K−1, α 22  = 36.82(1) × 10−6 K−1 and α 33  = 25.1(5) × 10−6 K−1. The maximum axis, α 22 , is parallel to b, and using Institution of Radio Engineers (IRE) convention for the tensor orthonormal basis, the axes α 11 and α 33 have directions equal to (−0.979, 0, 0.201) and (0.201, 0, 0.979) respectively. The orientation and temperature dependent behaviour of the thermal expansion tensor is related to the crystal structure in the I2/a setting. Received 12 February 1998 / Revised, accepted 19 October 1998  相似文献   

13.
Cation diffusion rates at 690 ± 30 °C have been calculated by inverse modelling of observed manganese (Mn) zonation profiles in 40 garnets from two kyanite-bearing metapelite samples from the High Himalayan Crystalline Series, Zanskar, northwest India. Knowledge of the initial growth profile of Mn in garnet is a pre-requisite for this technique. Following previous workers we model Mn partitioning into growing garnet in terms of a Rayleigh fractionation process, and demonstrate that the Mngarnet:whole rock partition coefficient is 60–100. Three-dimensional zonation profiles were obtained by successively grinding and polishing ∼1 cm slabs of each sample at 0.1–0.2 mm intervals and analysing the garnets at each stage, thus ensuring that core sections were measured. The diffusion model assumes that garnet has a spherical geometry and behaves as a closed system, and simulates diffusive modification of the hypothetical Mn Rayleigh growth profile for each garnet. The derived measure of the time-integrated diffusion history for each garnet is then combined with radiometric and field-relation constraints for the duration of the Himalayan metamorphic event to calculate cation diffusion rates. The average cation interdiffusion rate calculated for garnets in the two samples examined is (6 ± 3.2) × 10−23 m2s−1. This interdiffusion rate pertains to a temperature of 690 ± 30 °C, which is 0.97 × T PEAK, the peak temperature conditions experienced by the samples estimated using standard thermobarometric techniques. Garnet compositions are Py2–17Alm65–77Gro6–16Sp1–17. These new diffusion data are consistent with, and more precise than, existing high-temperature (>1000 °C) experimentally determined diffusion data, although some uncertainties remain difficult to constrain. Qualitative comparison between diffusively modified Mn growth profiles in garnets from the Scottish Dalradian and the Himalayan garnets suggests that the duration of metamorphism affecting the Dalradian garnets was 10–20 times longer than that endured by the Himalayan garnets. Received: 5 June 1996 / Accepted: 29 January 1997  相似文献   

14.
 Calcium self-diffusion rates in natural calcite single crystals were experimentally determined at 700 to 900° C and 0.1 MPa in a stream of CO2. Diffusion coefficients (D) were determined from 42Ca concentration profiles measured with an ion microprobe. The Arrhenius parameters yield an activation energy (Q)=382±37 kJ/mol and pre-exponential factor (D0)=0.13 m2/s, and there is no measurable anisotropy. Calcium grain boundary diffusion rates were experimentally determined in natural (Solnhofen) limestone and hot-pressed calcite aggregates at 650° to 850° C and 0.1 to 100 MPa pressure. The Solnhofen limestone was first pre-annealed for 24 h at 700° C and 100 MPa confining pressure under anhydrous conditions to produce an equilibrium microstructure for the diffusion experiments. Values for the product of the grain boundary diffusion coefficient (D′) and the effective grain boundary diffusion width (δ) were determined from 42Ca concentration profiles measured with an ion microprobe. The results show that there is no measurable difference between D′δ values obtained for pre-annealed Solnhofen samples at 0.1 and 100 MPa or between hot-pressed calcite aggregates and pre-annealed Solnhofen samples. The temperature dependence for calcium grain boundary diffusion in Solnhofen samples annealed at 0.1 MPa is described by the Arrhenius parameters D′ 0δ=1.5×10−9 m3/s and Q=267±47 kJ/mol. Comparison of the results of this study with previously published data show that calcium is the slowest volume diffusing species in calcite. The calcium diffusivities measured in this study place constraints on several geological processes that involve diffusive mass transfer including diffusion-accommodated mechanisms in the deformation of calcite rocks. Received: 19 December 1994/Accepted: 30 June 1995  相似文献   

15.
Fluid inclusions have been studied in three pegmatite fields in Galicia, NW Iberian Peninsula. Based on microthermometry and Raman spectroscopy, eight fluid systems have been recognized. The first fluid may be considered to be a pegmatitic fluid which is represented by daughter mineral (silicates)-rich aqueous inclusions. These inclusions are primary and formed above 500 °C (dissolution of daughter minerals). During pegmatite crystallization, this fluid evolved to a low-density, volatile-rich aqueous fluid with low salinity (93% H2O; 5% CO2; 0.5% CH4; 0.2% N2; 1.3% NaCl) at minimum P–T conditions around 3 ± 0.5 kbar and 420 °C. This fluid is related to rare-metal mineralization. The volatile enrichment may be due to mixing of magmatic fluids and fluids equilibrated with the host rock. A drop in pressure from 3 ± 0.5 to 1 kbar at a temperature above 420 °C, which may be due to the transition from predominantly lithostatic to hydrostatic pressure, is recorded by two-phase, water-rich inclusions with a low-density vapour phase (CO2, CH4 and N2). Another inclusion type is represented by two-phase, vapour-rich inclusions with a low-density vapour phase (CO2, CH4 and N2), indicating a last stage of decreasing temperature (360 °C) and pressure (around 0.5 kbar), probably due to progressive exhumation. Finally, volatile (CO2)-rich aqueous inclusions, aqueous inclusions (H2O-NaCl) and mixed-salt aqueous inclusions with low Th, are secondary in charac- ter and represent independent episodes of hydrothermal fluid circulation below 310 °C and 0.5 kbar. Received: 14 October 1999 / Accepted: 5 October 1999  相似文献   

16.
The Pueblo Viejo deposit (production to 1996: 166 t Au, 760 t Ag) is located in the Dominican Republic on the Caribbean island of Hispaniola and ranks as one of the largest high-sulfidation/acid-sulfate epithermal deposits (reserves in 2007: 635 t Au, 3,648 t Ag). One of the advanced argillic ore bodies is cut by an inter-mineral andesite porphyry dike, which is altered to a retrograde chlorite–illite assemblage but overprinted by late-stage quartz–pyrite–sphalerite veins and associated low-grade Au, Ag, Zn, Cd, Hg, In, As, Se, and Te mineralization. The precise TIMS U–Pb age (109.6 ± 0.6 Ma) of the youngest zircon population in this dike confirms that the deposit is part of the Early Cretaceous Los Ranchos intra-oceanic island arc. Intrusion-related gold–sulfide mineralization took place during late andesite–dacite volcanism within a thick pile (>200 m) of carbonaceous sand- and siltstones deposited in a restricted marine basin. The high-level deposit was shielded from erosion after burial under a late Albian (109–100 Ma) ophiolite complex (8 km thick), which was in turn covered by the volcano-sedimentary successions (>4 km) of a Late Cretaceous–Early Tertiary calc-akaline magmatic arc. Estimates of stratigraphic thickness and published alunite, illite, and feldspar K-Ar ages and closure temperatures (alunite 270 ± 20°C, illite 260 ± 30°C, K-feldspar 150°C) indicate a burial depth of about 12 km at 80 Ma. During peak burial metamorphism (300°C and 300 MPa), the alteration assemblage kaolinite + quartz in the deposit dehydrated to pyrophyllite. Temperature–time relations imply that the Los Ranchos terrane then cooled at a rate of 3–4°C/Ma during slow uplift and erosion.  相似文献   

17.
Summary Augite and diopside crystals from the Oligo-Miocene high-Mg and high-Al basalts, basaltic andesites and andesites of Montresta (Sardinia, Italy) have been analysed by means of X-ray single crystal diffraction and electron microprobe. It has been found that crystallization or equilibration processes took place within the crust at pressures below 5 kbar. In particular, samples from high-Al basalts, basaltic andesites and andesites crystallised at pressures below one kbar while those from high-Mg basalts crystallised in a range from about 1.5 to 4.5 kbar. The intracrystalline temperatures range between about 550 °C for high-Mg basalts and 750 °C for andesites. The M2′ site is occupied in each of the studied crystals, which implies slow cooling rates.  相似文献   

18.
The Mesoarchean Nuasahi chromite deposits of the Singhbhum Craton in eastern India consist of a lower chromite-bearing ultramafic unit and an upper magnetite-bearing gabbroic unit. The ultramafic unit is a ∼5 km long and ∼400 m wide linear belt trending NNW-SSE with a general north-easterly dip. The chromitite ore bodies are hosted in the dunite that is flanked by the orthopyroxenite. The rocks of the ultramafic unit including the chromitite crystallized from a primitive boninitic magma, whereas the gabbro unit formed from an evolved boninitic magma. A shear zone (10–75 m wide) is present at the upper contact of the ultramafic unit. This shear zone consists of a breccia comprising millimeter- to meter-sized fragments of chromitite and serpentinized rocks of the ultramafic unit enclosed in a pegmatitic and hybridized gabbroic matrix. The shear zone was formed late synkinematically with respect to the main gabbroic intrusion and intruded by a hydrous mafic magma comagmatic with the evolved boninitic magma that formed the gabbro unit. Both sulfide-free and sulfide-bearing zones with platinum group element (PGE) enrichment are present in the breccia zone. The PGE mineralogy in sulfide-rich assemblages is dominated by minerals containing Pd, Pt, Sb, Bi, Te, S, and/or As. Samples from the gabbro unit and the breccia zone have total PGE concentrations ranging from 3 to 116 ppb and 258 to 24,100 ppb, respectively. The sulfide-rich assemblages of the breccia zone are Pd-rich and have Pd/Ir ratios of 13–1,750 and Pd/Pt ratios of 1–73. The PGE-enriched sulfide-bearing assemblages of the breccia zone are characterized by (1) extensive development of secondary hydrous minerals in the altered parts of fragments and in the matrix of the breccia, (2) coarsening of grain size in the altered parts of the chromitite fragments, and (3) extensive alteration of primary chromite to more Fe-rich chromite with inclusions of chlorite, rutile, ilmenite, magnetite, chalcopyrite, and PGE-bearing chalcogenides. Unaltered parts of the massive chromitite fragments from the breccia zone show PGE ratios (Pd/Ir = 2.5) similar to massive chromitite (Pd/Ir = 0.4–6.6) of the ultramafic unit. The Ir-group PGE (IPGE: Ir, Os, Ru) of the sulfide-rich breccia assemblages were contributed from the ultramafic–chromitite breccia. Samples of the gabbro unit have fractionated primitive mantle-normalized patterns, IPGE depletion (Pd/Ir = 24–1,227) and Ni-depletion due to early removal of olivine and chromite from the primitive boninitic magma that formed the ultramafic unit. Samples of the gabbro and the breccia zone have negative Nb, Th, Zr, and Hf anomalies, indicating derivation from a depleted mantle source. The Cu/Pd ratios of the PGE-mineralized samples of the breccia zone (2.0 × 103–3.2 × 103) are lower than mantle (6.2 × 103) suggesting that the parental boninitic magma (Archean high-Mg lava: Cu/Pd ratio ∼1.3 × 103; komatiite: Cu/Pd ratio ∼8 × 103) was sulfur-undersaturated. Samples of the ultramafic unit, gabbro and the mineralized breccia zone, have a narrow range of incompatible trace element ratios indicating a cogenetic relationship. The ultramafic rocks and the gabbros have relatively constant subchondritic Nb/Ta ratios (ultramafic rocks: Nb/Ta = 4.1–8.8; gabbro unit: Nb/Ta = 11.5–13.2), whereas samples of the breccia zone are characterized by highly variable Nb/Ta ratios (Nb/Ta = 2.5–16.6) and show evidence of metasomatism. The enrichment of light rare earth element and mobile incompatible elements in the mineralized samples provides supporting evidence for metasomatism. The interaction of the ultramafic fragments with the evolved fluid-rich mafic magma was key to the formation of the PGE mineralization in the Nuasahi massif.  相似文献   

19.
A detailed seismicity map of the Central Indian Ridge for the period 1912–1993 is presented, and the earthquakes pertaining to four major transforms offsetting the ridge are utilized to study the moment release pattern. The scalar moment release for the period 1912–1993, and the summed moment rate tensors for both short period (1977–1993) and long period (1912–1993) bring out a unified picture of moment release pattern. The fraction of seismic slip calculated based on depths of 100°C and 400°C limiting temperatures suggests that the Marie-Celeste transform requires a slip almost to a depth of 400°C isotherm to account for the observed moment, and the Argo transform requires depth of faulting much above the 400°C isotherm. A very small fraction of slip is accounted seismically for Vema (53%) and 12° 12′S (23%) even to depths of 100°C isotherm, suggesting a very low order of moment release along these transforms. The horizontal plate velocities and the corresponding strain rates obtained from moment tensor summation of long period data (82 years) give rise to (V y y ; V y x mm. yr−1) of 6.0 and 6.1 along Marie-Celeste, 1.3 and 0.50 along Argo, 0.06 and 0.06 along 12° 12′S, 1.6 and 0.25 along Vema transforms. The corresponding strain rates (ε y y ;ε y x × 10−15 S−1) are 12.7 and 6.8 along MarieCeleste, 6.9 and 1.4 along Argo, 0.27 and 0.14 along 12° 12′S, 7.3 and 0.58 along Vema transforms. These results suggest that the strain rates were highest and almost all predicted motion is taken up seismically along the Marie-Celeste transform. The strain rates are lower along Argo transform and the observed moment release require shallower depth of faulting in order to slip to be accounted seismically. The Vema and 12° 12′S transforms are characterized by low strain rates and less than 15 per cent of motion is accommodated seismically within the seismogenic layer. It is proposed that the deficiency of moment release along the Vema and 12° 12′S multiple transform system may be due to most of the plate motion occurring aseismically.  相似文献   

20.
The thermal stabilities and decay kinetics of three peroxy radicals (Centers #1, B and B′) and three other radiation-induced defects (#3, C′ and E1′) in natural quartz from the high-grade McArthur River uranium deposit (Athabasca basin, Canada) have been investigated by isochronal and isothermal annealing experiments and electron paramagnetic resonance (EPR) spectroscopy. Single-crystal EPR spectra of isochronally (2 h) annealed quartz show that these centers all grow in intensity to 280°C and then decay with further increase in temperature, but their disappearance temperatures differ markedly and depend on the initial concentrations (e.g., Center #1 in a dark smoky quartz is annealed out at 380°C, B and B′ at 420°C and #3 and C′ at 580°C). The isothermal decay processes of these centers are all of the second order type. The calculated activation energies for the peroxy radicals [#1 and B + B′ at 0.36 (9) and 0.83 (8) eV, respectively] are smaller than those of Centers #3, C′ and E1′ [1.09 (8), 1.24 (8) and 1.45 (7) eV, respectively]. Gamma-ray irradiations of thermally bleached quartz restore a fraction of the peroxy radicals, suggesting that their diamagnetic precursors are stable up to at least 800°C. The unusual decay characteristics of “peroxy radicals” in quartz reported in the literature are shown to most likely arise from multiple radiation-induced defects. These results have implications for not only applications of peroxy radicals in quartz for EPR dating but also better understanding of thermoluminescence and cathodoluminescence spectra of this mineral.  相似文献   

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