首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 718 毫秒
1.
The stable carbon isotopic composition of the planktonic foraminifera Globigerinoides sacculifer and G. ruber (white) and sedimentary organic matter from the northern Gulf of Aqaba have been investigated to estimate changes in 13CDIC in surface waters during the last 1,000 years. The high sedimentation rates at the core sites (about 54 cm/Kyear) provide high temporal resolution (~10 years). Recent sediments at the top of the cores reflect conditions younger than 1950. The 13C records of the planktonic foraminifera from three multicores display similar trends, showing a uniform and consistent pattern before the 1750s, and a gradual decrease of approximately 0.63 over the last two centuries. This decrease seems to track the decrease of 13CDIC in surface waters, which is mainly caused by the increase of anthropogenic input of 13C-depleted CO2 into the atmosphere. Similarly, a trend towards lighter values of the carbon isotopic composition of sedimentary organic matter (13Corg) during the last 200 years supports the interpretation obtained from the planktonic foraminiferal 13C. Furthermore, direct measurements of seawater show that 13C of the dissolved inorganic carbon (DIC) in the northern Gulf of Aqaba has decreased by about 0.44 during the period 1979–2000. The average annual decrease is 0.021, which is similar to that observed globally. The 13C values of planktonic foraminifera combined with organic matter 13C from marine sediments are good indicators for reconstructing past changes in atmospheric CO2 concentrations from the northern Gulf of Aqaba.  相似文献   

2.
This paper deals with barite from stratiform, karst, and vein deposits hosted within Lower Paleozoic rocks of the Iglesiente-Sulcis mining district in southwestern Sardinia. For comparison sulfates from mine waters are studied. Stratiform barite displays 34S=28.8–32.1, 18O=12.7–15.6, and 87Sr/86Sr=0.7087, in keeping with an essentially Cambrian marine origin of both sulfate and strontium. Epigenetic barite from post-Hercynian karst and vein deposits is indistinguishable for both sulfur and oxygen isotopes with 34S=15.3–26.4 and 18O=6.6–12.5; 87Sr/86Sr ratios vary 0.7094–0.7140. These results and the microthermometric and salinity data from fluid inclusions concur in suggesting that barite formed at the site of mineralization by oxidation of reduced sulfur from Cambrian-Ordovician sulfide ores in warm, sometimes hot solutions consisting of dilute water and saline brine with different 18O values. The relative proportion of the two types of water may have largely varied within a given deposit during the mineralization. In the karst barite Sr was essentially provided by carbonate host rocks, whereas both carbonate and Lower Paleozoic shale host rocks should have been important sources for Sr of the vein barite. Finally, 34S data of dissolved sulfate provide further support for the mixed seawater-meteoric water composition of mine waters from the Iglesiente area.  相似文献   

3.
Crystals of hydronium jarosite were synthesized by hydrothermal treatment of Fe(III)–SO4 solutions. Single-crystal XRD refinement with R1=0.0232 for the unique observed reflections (|Fo| > 4F) and wR2=0.0451 for all data gave a=7.3559(8) Å, c=17.019(3) Å, Vo=160.11(4) cm3, and fractional positions for all atoms except the H in the H3O groups. The chemical composition of this sample is described by the formula (H3O)0.91Fe2.91(SO4)2[(OH)5.64(H2O)0.18]. The enthalpy of formation (Hof) is –3694.5 ± 4.6 kJ mol–1, calculated from acid (5.0 N HCl) solution calorimetry data for hydronium jarosite, -FeOOH, MgO, H2O, and -MgSO4. The entropy at standard temperature and pressure (So) is 438.9±0.7 J mol–1 K–1, calculated from adiabatic and semi-adiabatic calorimetry data. The heat capacity (Cp) data between 273 and 400 K were fitted to a Maier-Kelley polynomial Cp(T in K)=280.6 + 0.6149T–3199700T–2. The Gibbs free energy of formation is –3162.2 ± 4.6 kJ mol–1. Speciation and activity calculations for Fe(III)–SO4 solutions show that these new thermodynamic data reproduce the results of solubility experiments with hydronium jarosite. A spin-glass freezing transition was manifested as a broad anomaly in the Cp data, and as a broad maximum in the zero-field-cooled magnetic susceptibility data at 16.5 K. Another anomaly in Cp, below 0.7 K, has been tentatively attributed to spin cluster tunneling. A set of thermodynamic values for an ideal composition end member (H3O)Fe3(SO4)2(OH)6 was estimated: Gof= –3226.4 ± 4.6 kJ mol–1, Hof=–3770.2 ± 4.6 kJ mol–1, So=448.2 ± 0.7 J mol–1 K–1, Cp (T in K)=287.2 + 0.6281T–3286000T–2 (between 273 and 400 K).  相似文献   

4.
Hydrolysis constants of dimethyltin(IV) cation, in different salt solutions (CaCl2: 0.15 I 0.90; MgCl2: 0.30 I 0.60; NaCl-–NaClO4, NaCl-–NaNO3 mixtures: I = 3; NaCl-–Na2SO4 mixtures: I = 1 mol dm-3) were determined by potentiometric ([H+]-glass electrode) measurements. These data, together with previous data (De Stefano et al., 1996b) were interpreted in terms of DHT (Debye–Hückel type) and Pitzer equations. The mixed electrolyte solution results also allowed us to obtain and parameters for the Pitzer equation. Calorimetric measurements were made at different ionic strengths in order to find the temperature dependence of hydrolysis constants and of the relative interaction parameters. The body of results allows us to determine the speciation of natural waters in a wide range of ionic strengths and temperatures.  相似文献   

5.
DC and AC electrical conductivities were measured on samples of two different crystals of the mineral aegirine (NaFeSi2O6) parallel () and perpendicular () to the [001] direction of the clinopyroxene structure between 200 and 600 K. Impedance spectroscopy was applied (20 Hz–1 MHz) and the bulk DC conductivity DC was determined by extrapolating AC data to zero frequency. In both directions, the log DC – 1/T curves bend slightly. In the high- and low-temperature limits, differential activation energies were derived for measurements [001] of EA 0.45 and 0.35 eV, respectively, and the numbers [001] are very similar. The value of DC [001] with DC(300 K) 2.0 × 10–6 –1cm–1 is by a factor of 2–10 above that measured [001], depending on temperature, which means anisotropic charge transport. Below 350 K, the AC conductivity () (/2=frequency) is enhanced relative to DC for both directions with an increasing difference for rising frequencies on lowering the temperature. An approximate power law for () is noted at higher frequencies and low temperatures with () s, which is frequently observed on amorphous and disordered semiconductors. Scaling of () data is possible with reference to DC, which results in a quasi-universal curve for different temperatures. An attempt was made to discuss DC and AC results in the light of theoretical models of hopping charge transport and of a possible Fe2+ Fe3+ electron hopping mechanism. The thermopower (Seebeck effect) in the temperature range 360 K < T <770 K is negative in both directions. There is a linear – 1/T relationship above 400 K with activation energy E 0.030 eV [001] and 0.070 eV [001]. 57Fe Mössbauer spectroscopy was applied to detect Fe2+ in addition to the dominating concentration of Fe3+.  相似文献   

6.
Peak metamorphic temperatures for the coesite-pyrope-bearing whiteschists from the Dora Maira Massif, western Alps were determined with oxygen isotope thermometry. The 18O(smow) values of the quartz (after coesite) (18O=8.1 to 8.6, n=6), phengite (6.2 to 6.4, n=3), kyanite (6.1, n=2), garnet (5.5 to 5.8, n=9), ellenbergerite (6.3, n=1) and rutile (3.3 to 3.6, n=3) reflect isotopic equilibrium. Temperature estimates based on quartz-garnet-rutile fractionation are 700–750 °C. Minimum pressures are 31–32 kb based on the pressure-sensitive reaction pyrope + coesite = kyanite + enstatite. In order to stabilize pyrope and coesite by the temperature-sensitive dehydration reaction talc+kyanite=pyrope+coesite+H2O, the a(H2O) must be reduced to 0.4–0.75 at 700–750 °C. The reduced a(H2O) cannot be due to dilution by CO2, as pyrope is not stable at X(CO2)>0.02 (T=750 °C; P=30 kb). In the absence of a more exotic fluid diluent (e.g. CH4 or N2), a melt phase is required. Granite solidus temperatures are 680 °C/30 kb at a(H2O)=1.0 and are calculated to be 70°C higher at a(H2O)=0.7, consistent with this hypothesis. Kyanite-jadeite-quartz bands may represent a relict melt phase. Peak P-T-f(H2O) estimates for the whiteschist are 34±2 kb, 700–750 °C and 0.4–0.75. The oxygen isotope fractionation between quartz (18O=11.6) and garnet (18O=8.7) in the surrounding orthognesiss is identical to that in the coesitebearing unit, suggesting that the two units shared a common, final metamorphic history. Hydrogen isotope measurements were made on primary talc and phengite (D(SMOW)=-27 to-32), on secondary talc and chlorite rite after pyrope (D=-39 to -44) and on the surrounding biotite (D=-64) and phengite (D=-44) gneiss. All phases appear to be in nearequilibrium. The very high D values for the primary hydrous phases is consistent with an initial oceanicderived/connate fluid source. The fluid source for the retrograde talc+chlorite after pyrope may be fluids evolved locally during retrograde melt crystallization. The similar D, but dissimilar 18O values of the coesite bearing whiteschists and hosting orthogneiss suggest that the two were in hydrogen isotope equilibrium, but not oxygen isotope equilibrium. The unusual hydrogen and oxygen isotope compositions of the coesite-bearing unit can be explained as the result of metasomatism from slab-derived fluids at depth.  相似文献   

7.
The structural behavior of -eucryptite (LiAlSiO4) has been investigated using infrared (IR) spectroscopy over a temperature range of 20 to 900 K and FT-Raman spectroscopy at room temperature. IR reflectance measurements show that -eucryptite possesses high reflectivity in the far-IR region, as is consistent with its reported superionic conductivity along the c-axis. On heating, the Li-related IR bands near 246 and 300 cm–1 (with A2 symmetry) broadened and weakened dramatically, presumably as a result of Li+ positional disordering along the structural channels parallel to c. The disordering process appears to induce a framework distortion, as is evidenced by the broadening of some vibrations of Si(Al)–O with increasing temperature. A change in slope in the temperature dependence of the phonon frequency near 300 cm–1 and the linewidth of the 760 cm–1 band at 715 K indicates that Li becomes completely disordered above this temperature. In addition, the temperature dependence of the linewidth for the 760 cm–1 band exhibits an additional change in slope at 780 K, implying the existence of an intermediate state within this temperature range. The detailed structure of this intermediate phase, however, needs further study. Our IR data provide no indication of structural changes between room temperature and 20 K.  相似文献   

8.
KAlSi3O8 sanidine dissociates into a mixture of K2Si4O9 wadeite, Al2SiO5 kyanite and SiO2 coesite, which further recombine into KAlSi3O8 hollandite with increasing pressure. Enthalpies of KAlSi3O8 sanidine and hollandite, K2Si4O9 wadeite and Al2SiO5 kyanite were measured by high-temperature solution calorimetry. Using the data, enthalpies of transitions at 298 K were obtained as 65.1 ± 7.4 kJ mol–1 for sanidine wadeite + kyanite + coesite and 99.3 ± 3.6 kJ mol–1 for wadeite + kyanite + coesite hollandite. The isobaric heat capacity of KAlSi3O8 hollandite was measured at 160–700 K by differential scanning calorimetry, and was also calculated using the Kieffer model. Combination of both the results yielded a heat-capacity equation of KAlSi3O8 hollandite above 298 K as Cp=3.896 × 102–1.823 × 103T–0.5–1.293 × 107T–2+1.631 × 109T–3 (Cp in J mol–1 K–1, T in K). The equilibrium transition boundaries were calculated using these new data on the transition enthalpies and heat capacity. The calculated transition boundaries are in general agreement with the phase relations experimentally determined previously. The calculated boundary for wadeite + kyanite + coesite hollandite intersects with the coesite–stishovite transition boundary, resulting in a stability field of the assemblage of wadeite + kyanite + stishovite below about 1273 K at about 8 GPa. Some phase–equilibrium experiments in the present study confirmed that sanidine transforms directly to wadeite + kyanite + coesite at 1373 K at about 6.3 GPa, without an intervening stability field of KAlSiO4 kalsilite + coesite which was previously suggested. The transition boundaries in KAlSi3O8 determined in this study put some constraints on the stability range of KAlSi3O8 hollandite in the mantle and that of sanidine inclusions in kimberlitic diamonds.  相似文献   

9.
The unit-cell and atomic parameters of perdeuterated brushite have been extracted from Rietveld analysis of neutron powder diffraction data within the temperature range 4.2 to 470 K. The thermal expansion of brushite is anisotropic, with the largest expansion along the b axis due principally to the effect of the O(1)···D(4) and O(3)···D(2) hydrogen bonds. Expansion along the c axis, influenced by the Ow1···D(5) interwater hydrogen bond, is also large. The high temperature limits for the expansion coefficients for the unit-cell edges a, b and c are 9.7(5) × 10–6, 3.82(9) × 10–5 and 5.54(5) × 10–5 K–1, respectively, and for the cell volume it is 9.7(1) × 10–5 K–1. The angle displays oscillatory variation, and empirical data analysis results in = 1.28(3) × 10–6sin(0.0105 T) K–1, within this temperature range. The evolution of the thermal expansion tensor of brushite has been calculated between 50 T 400 K. At 300 K the magnitudes of the principal axes are 11 = 50(6) × 10–6 K–1, 22 = 26.7(7) × 10–6 K–1 and 33 = 7.0(5) × 10–6 K–1. The intermediate axis, 22, is parallel to b, and using IRE convention for the tensor orthonormal basis, the axes 11 and 33 have directions equal to (–0.228, 0, –0.974) and (–0.974, 0, 0.228) respectively. Under the conditions of these experiments, the onset of dehydration occurred at temperatures above 400 K. Bond valence analysis combined with assessments of the thermal evolution of the bonding within brushite suggests that dehydration is precipitated through instabilities in the chemical environment of the second water molecule.  相似文献   

10.
Late Hercynian U-bearing carbonate veins within the metamorphic complex of La Lauzière are characterized by two parageneses. The first is dominated by dolomite or ankerite and the second by calcite and pitchblende. Fluids trapped in the dolomites and ankerites at 350–400° C are saline waters (20 to 15 wt % eq. NaCl) with D –34 to –49. In the calcite they are less saline (17 to 8 wt % eq. NaCl) and trapped at 300–350° C with D –50 to –65. All fluids contain trace N2, CO2 and probably CH4. The carbonates have 13C –8 to –14. and derived their carbon from organic matter. Evolution of the physico-chemical conditions from dolomite (ankerite) to calcite deposition was progressive.H and O-isotope studies indicate the involvement of two externally derived fluids during vein development. A D-rich ( –35) low fO2, saline fluid is interpreted to have come from underlying sediments and entered the hotter overlying metamorphic slab and mixed with more oxidizing and less saline U bearing meteoric waters during regional uplift. This evidence for a sedimentary formation water source for the deep fluid implies that the metamorphic complex overthrusted sedimentary formations during the Late-Hercynian.  相似文献   

11.
Hydrothermally-altered mesozonal synmetamorphic granitic rocks from Maine have whole-rock 18O (SMOW) values 10.7 to 13.8. Constituent quartz, feldspar, and muscovite have 18O in the range 12.4 to 15.2, 10.0 to 13.2, and 11.1 to 12.0, respectively. Mean values of Q–F ( 18Oquartz– 18Ofeldspar)=2.4 and Q–M ( 18Oquartz– 18Omuscovite)=3.3 are remarkably uniform (standard deviations of both are 0.2). Measured Q–F and Q–M values demonstrate that the isotopic compositions of the minerals are altered from primary magmatic 18O values but that the minerals closely approached oxygen isotope exchange equilibrium at subsolidus temperatures. Analyzed muscovites have D (SMOW) values in the range –65 to –82.Feldspars in the granitic rocks are mineralogically altered to either (a) muscovite+calcite, (b) muscovite+calcite+epidote, (c) muscovite+epidote, or (d) muscovite only. A consistent relation exists between the assemblage of secondary minerals and the oxygen isotope composition of whole rocks, quartz, and feldspar. Rocks with assemblage (a) have whole-rock 18O>12.1 and contain quartz and feldspar with 18O>13.8 and >11.4, respectively. Rocks with assemblages (b), (c), and (d) have whole-rock 18O<11.4 and contain quartz and feldspar with 18O< 13.1 and <11.0, respectively. The correlation suggests that the mineralogical alteration of the rocks was closely coupled to their isotopic alteration.Three mineral thermometers in altered granite suggest that the hydrothermal event occurred in the temperature range 400°–150° C, 100°–150° C below the peak metamorphic temperature inferred for country rocks immediately adjacent to the plutons. Calculations of mineral-fluid equilibria indicate that samples with assemblage (a) coexisted during the event with CO2-H2O fluids of and 18O=10.8 to 12.2 while samples with assemblages (b), (c), or (d) coexisted with fluids of and 18O=9.4 to 10.1. Compositional variations of the hydrothermal fluids were highly correlated: fluids enriched in CO2 were also enriched in 18O. Because CO2 was added to the granites during hydrothermal alteration and because fluids enriched in CO2 were enriched in 18O, some or all of the variation in 18O of altered granites may have been caused by addition of 18O to the rocks during the hydrothermal event. The source of both the CO2 and 18O could have been high-18O metasedimentary country rocks. The inferred change in isotopic composition of the granites is consistent with depletion of the metacarbonate rocks in 18O close to the plutons and with large volumes of fluid that were inferred from petrologic data to have infiltrated the metacarbonate rocks during metamorphism.A close approach of minerals to oxygen isotope exchange equilibrium in altered mesozonal rocks from Maine is in marked contrast to hydrothermally-altered epizonal granites whose mineral commonly show large departures from oxygen isotope exchange equilibrium. The difference in oxygen isotope systematics between altered epizonal granites and altered mesozonal granites closely parallels a differences between their mineralogical systematics. Both differences demonstrate the important control that depth exerts on the products of hydrothermal alteration. Deeper hydrothermal events occur at higher temperature and are longer-lived. Minerals and fluid have sufficient time to closely approach both isotope exchange and heterogeneous chemical equilibrium. Shallower hydrothermal events occur at lower temperatures and are shorter-lived. Generally there is insufficient time for fluid to closely approach equilibrium with all minerals.  相似文献   

12.
The Araripe Plateau in northeastern Brazil has an area of about 8,000 km2, confined by 39°05E and 40°55E, and 7°10S and 7°50S. Due to high permeability of soils, a surface drainage system is practically inexistent. Water is stored in excavations with clayey soil, the barreiros. Monthly samples were taken for 18O measurements, from September 1999 to August 2000, from four barreiros, three dug wells and five drilled wells. Results show that (1) groundwaters in the eastern part of the plateau are derived from present-day rainfall (18O–3.2), whereas groundwaters in the western portion are isotopically different (18O–5.0); (2) barreiros are strongly marked seasonally by elevated 18O during the dry period due to elevated evaporation; (3) a dug well at a distance of 30 m from a barreiro exhibits 18O similar to that of the reservoir, indicating a strong interaction between groundwater and surface water; and (4) a tubular well of 242-m depth, located in a fault, exhibits strong seasonal changes in 18O and electrical conductivity, revealing downward leakage between aquifers.  相似文献   

13.
The Archean Eye Dashwa Lakes pluton (2672±24 Ma) has domains of mineralogically fresh isotropic granite, domains that have undergone bulk hydrothermal alteration, and at least eleven sets of sequential fracture arrays, each with distinctive mineral assemblages. Fresh granite is characterized by whole rock 18O=8.1 to 8.6 and primary magmatic quartz-feldspar (+1.3), quartz-biotite (5.2 to 5.4) and quartz-magnetite (+9.8) fractionations. Magmatic fluids had a calculated isotopic composition of 18O=7.9±0.5, and D=–80±5. These isotropic volumes of the granite have not experienced significant incursion of external thermal waters. Pegmatites, quartz-molybdenite veins, and phlogopite-muscovite coated fractures are sporadically distributed in the granite, and were precipitated from high-temperature magmatic fluids where 18O=8.0 to 10.3 and D=–80±5.The most abundant variety of fracture filling assemblage is epidote-quartz-chlorite±muscovite: fractures are bounded by domains of mineralogically similar bulk hydrothermal alteration of the granite. These minerals formed at 160 to 280° C, in the presence of NaCl, and NaCl-MgCl2 brines (up to 25 wt% NaCl equivalent) of probable evolved marine water origin ( 18O=+0.4 to +3.8, D=–10 to –35) undergoing transient boiling. Upper plateau 40Ar/39Ar ages for the muscovite are 2650±15 Ma. Sequentially in the chronology of fracture-infiltration events, calcite-fluorite veins were deposited from boiling fluids at 340 to 390° C, isotopically characterized by 18O=4.7 and 13C=–5; and rare prehnite-chlorite lined fractures formed at 250 to 290° C. A generation of adularia-bearing veins precipitated at 140 to 230° C, from CaCl2-NaCl brines, where 18O=0 to –6.5 and D=–10 to –30. Incremental 40Ar/39Ar age spectra on the K-feldspar yield an upper plateau of 1100 Ma. Subsequently, hematite developed during reactivation of earlier fractures, at 140 to 210° C in the presence of fluids characterized by 18O=–0.4 to –5.4 and D=–15 to –25. Arrays of open fractures partially occupied by gypsum and goethite reflect a fluid infiltration event at temperatures <50° C. Many of the earlier generations of fracture minerals have transgranular fracture infillings which record the presence of low temperature (88–190° C), hypersaline CaCl2-NaCl brines. Narrow fractures lined with clays±calcite are sites for seepage of modern ground-waters. The isotopic signature of clay ( 18O=12 to 20, D=–80±5) plots near the line for modern kaolinites, confirming its formation in the presence of recent surface waters. Calcites coexisting with the clay minerals, and in fractured pegmatite show a common isotopic signature ( 18O=23±0.5, 13C=–13.6), indicating precipitation from modern groundwaters, where reactivated fractures have acted as conduits for infiltration of surface waters to depths of 200 m. Intermittent fracture-infiltration has occurred over 2.7 Ga. The early sequences of fracture-related fluid flow are interpreted in terms of devolatilization of the granite, followed by thermal contraction fracturing, incursion of marine water and convective cooling in the Archean. Hematite and adularia fracture fillings correspond to a stage when meteoric water infiltrated the volcanicplutonic terrain during Proterozoic and later times. Episodic fracture-fluid expulsion events may have been driven by seismic pumping, in response to magmatically and tectonically induced stresses within the Shield, with surface waters penetrating to depths of 15 km in the crust.  相似文献   

14.
Fluoride-hydroxyl exchange equilibria between phlogopite-pargasite and phlogopite-tremolite mineral pairs were experimentally determined at 1,173K, 500 bars and 1,073–1,173 K, 500 bars respectively. The distribution of fluorine between phlogopite and pargasite was found to favor phlogopite slightly, G ex . (1,173 K)=–1.71 kJ anion–1, while in the case of phlogopite-tremolite, fluorine was preferentially incorporated in the mica, G ex . (1,073)=– 5.67 kJ anion–1 and G ex . (1,173K)=–5.84 kJ anion–1. These results have yielded new values of entropy and Gibbs energy of formation for fluortremolite, S f =–2,293.4±16.0JK–1 mol–1 and G f = –11,779.3±25.0 kJ mol–1, respectively. In addition, F-OH mineral exchange equilibria support a recent molten oxide calorimetric value for the Gibbs energy of fluorphlogopite, G f =–6,014.0±7.0 kJ mol–1, which is approximately 40 kJ mol–1 more exothermic than the tabulated value.This work performed in part at Sandia National Laboratories supported by the U.S. Department of Energy, DOE, under contract number DE-AC04-76DP00789  相似文献   

15.
The Mount Lofty Ranges comprises interlayered marbles, metapsammites, and metapelites that underwent regional metamorphism during the Delamarian Orogeny at 470–515 Ma. Peak metamorphic conditions increased from lowermost biotite grade (350–400°C) to migmatite grade (700°C) over 50–55 km parallel to the lithological strike of the rocks. With increasing metamorphic grade, 18O values of normal metapelites decrease from 14–16 to as low as 9.0, while 18O values of calcite in normal marbles decrease from 22–24 to as low as 13.2 These isotopic changes are far greater than can be accounted for by devolatilisation, implying widespread fluid-rock interaction. Contact metamorphism appears not to have affected the terrain, suggesting that fluid flow occurred during regional metamorphism. Down-temperature fluid flow from synmetamorphic granite plutons (18O=8.4–8.6) that occur at the highest metamorphic grades is unlikely to explain the resetting of oxygen isotopes because: (a) there is a paucity of skarns at granite-metasediment contacts; (b) the marbles generally do not contain low-XCO2 mineral assemblages; (c) there is insufficient granite to provide the required volumes of water; (d) the marbles and metapelites retain a several permil difference in 18O values, even at high metamorphic grades. The oxygen isotope resetting may be accounted for by along-strike up-temperature fluid flow during regional metamorphism with time-integrated fluid fluxes of up to 5x109 moles/m2 (105 m3/m2). If fluid flow occurred over 105–106 years, estimated intrinsic permeabilities are 10-20 to 10-16m2. Variations in 18O at individual outcrops suggest that time-integrated fluid fluxes and intrinsic permeabilities may locally have varied by at least an order of magnitude. A general increase in XCO2 values of marble assemblages with metamorphic grade is also consistent with the up-temperature fluid-flow model. Fluids in the metapelites may have been derived from these rocks by devolatilisation at low metamorphic grades; however, fluids in the marbles were probably derived in part from the surrounding siliceous rocks. The marble-metapelite boundaries preserve steep gradients in both 18O and XCO2 values, suggesting that across-strike fluid fluxes were much lower than those parallel to strike. Up-temperature fluid flow may also have formed orthoamphibole rocks and caused melting of the metapelites at high grades.This paper is a contribution to IGCP Project 304 Lower Crustal Processes  相似文献   

16.
18O values of unaltered olivine and pyroxene phenocrysts in boninites from several areas range from 5.8 to 7.4 and indicate that the source for most boninites is more 18O-rich than MORBs and other oceanic basalts. The source for oxygen and other major elements is most likely a refractory portion of the mantle having a 18O value of up to 7.0 to which must be added a small amount of H2O-rich fluid to induce partial melting. This fluid, which is derived from subducted crust, is the vehicle for LREEs including Nd. The variable, normally low Nd values typical of boninites do not correlate with the 18O values.Post eruptive exchange of oxygen in the glass of boninites with that of sea water at low temperatures (<150° C) produces 18O values of >10 in optically fresh glass. Hydration of the glass has increased the water contents of most boninites from estimated magmatic values of 1–2 wt% to 2–4 wt% and produced D values of < –80, which may be lower than the original magmatic D values. In contrast to most submarine pillow basalts, the magmatic volatile composition of boninite lavas has been extensively modified as a result of post eruptive interaction with seawater.  相似文献   

17.
Emerald deposits in Swat, northwestern Pakistan, occurring in talc-magnesite and quartz-magnesite assemblages, have been investigated through stable isotope studies. Isotopic analyses were performed on a total of seven emeralds, associated quartz (seven samples), fuchsite (three samples) and tourmaline (two samples) from the Mingora emerald mines. The oxygen isotopic composition ( 18O SMOW) of emeralds shows a strong enrichment in18O and is remarkably uniform at + 15.6 ± 0.4 (1,n = 7). Each of the two components of water in emerald (channel and inclusion) has a different range of hydrogen isotopic composition: the channel waters being distinctly isotopically heavier (D = –51 to –32 SMOW) than the other inclusion waters (D = –96 to –70 SMOW). Similarly the oxygen isotopic compositions of tourmaline and fuchsite are relatively constant ( 18O = + 13 to + 14 SMOW) and show enrichment in18O. The 18O values of quartz, ranging from + 15.1 to + 19.1 SMOW, are also high (+ 16.9 ± 1.4 1, n = 7). The meanD of channel waters measured from emerald (–42 ± 6.6 SMOW) and that of fluid calculated from hydrous mineralsDcalculated (–47 ± 7.1 SMOW) are consistent with both metamorphic and magmatic origin. However, the close similarity between the measuredD values of the hydroxyl hydrogen in fuchsite (–74 to –6 SMOW) and tourmaline (–84 and –69 SMOW) with pegmatitic muscovite and tourmaline suggests that the mineralization was probably caused by modified (18O-enriched) hydrothermal solutions derived from an S-type granitic magma. The variation in the carbon and oxygen isotopic composition of magnesite, locally associated with emerald mineralization, is also very restricted ( 13 –3.2 ± 0.7%, PDB; 18O + 17.9 ± 1.27 SMOW). On the basis of the isotopic composition of fluid ( 13C –1.8 ± 0.7 PDB; 18O + 13.6 ± 1.2 SMOW calculated for the 250-550 °C temperature), it is proposed that the Swat magnesites formed due to the carbonation of previously serpentinized ultramafic rocks by a CO2-bearing fluid of metamorphic origin.  相似文献   

18.
The 1-bar structure and properties of the high-pressure SiO2 polymorph coesite have been simulated by lattice and molecular dynamics up to 1600 and 2100 K, respectively. In agreement with available experimental data, the monoclinic structure was found metastable (with respect to cristobalite or SiO2 liquid) up to the highest temperatures investigated. Thermal expansion of coesite is small because of restricted rotations of SiO4 tetrahedra. Above about 1000 K, the structure of coesite becomes dynamically disordered and similar to those reported for the -phases of quartz and cristobalite. Disorder sets smoothly, however, in contrast to its abrupt onset in quartz and cristobalite, which have – transitions. The radial distribution functions for all bond distances indicate that order then prevails only for the nearest neighbors whereas the angle distributions widen markedly so that the monoclinic form of coesite with an Si–O–Si angle of 180° is only a time-averaged structure.  相似文献   

19.
Mo mineralization within the Galway Granite at Mace Head and Murvey, Connemara, western Ireland, has many features of classic porphyry Mo deposits including a chemically evolved I-type granite host, associated K- and Si-rich alteration, quartz vein(Mace Head) and granite-hosted (Murvey) molybdenite, chalcopyrite, pyrite and magnetite mineralization and a gangue assemblage which includes quartz, muscovite and K-feldspar. Most fluid inclusions in quartz veins homogenize in the range 100–350°C and have a salinity of 1–13 eq. wt.% NaCl. They display Th-salinity covariation consistent with a hypothesis of dilution of magmatic water by influx of meteoric water. CO2-bearing inclusions in an intensely mineralized vein at Mace Head provide an estimated minimum trapping temperature and pressure for the mineralizing fluid of 355°C and 1.2 kb and are interpreted to represent a H2O-CO2 fluid, weakly enriched in Mo, produced in a magma chamber by decompression-activated unmixing from a dense Mo-bearing NaCl-H2O-CO2 fluid. 34S values of most sulphides range from c. 0 at Murvey to 3–4 at Mace Head and are consistent with a magmatic origin. Most quartz vein samples have 18O of 9–10.3 and were precipitated from a hydrothermal fluid with 18O of 4.6–6.7. Some have 18O of 6–7 and reflect introduction of meteoric water along vein margins. Quartz-muscovite oxygen isotope geothermometry combined with fluid inclusion data indicate precipitation of mineralized veins in the temperature range 360–450°C and between 1 and 2 kb. Whole rock granite samples display a clear 18O-D trend towards the composition of Connemara meteoric waters. The mineralization is interpreted as having been produced by highlyfractionated granite magma; meteoric water interaction postdates the main mineralizing event. The differences between the Mace Head and Murvey mineralizations reflect trapping of migrating mineralizing fluid in structural traps at Mace Head and precipitation of mineralization in the granite itself at Murvey.  相似文献   

20.
New sulphur and sulphate-oxygen isotope measurements for the main discordant and stratiform lead-zinc-barite orebodies at Silvermines Co. Tipperary, allow reappraisal of previously offered differing interpretations (Graham, 1970; Greig et al., 1971) of the bearing of sulphur isotopes on the genesis of this important Irish deposit. The following aspects of the data are confirmed: barite 34 S-values range from 17–21, similar to lower Carboniferous seawater sulphate: stratiform sulphide lens pyrites have 34 S-values ranging from –13 to –36; vein sulphide 34 S-values range from –8 to 4; sulphide 34 S-values increase upwards and outwards respectively in the related discordant and stratiform G orebodies; galena-sphalerite isotope palaeotemperatures are not too consistent, ranging from 40 to 430°C (using the calibration of Czamanske and Rye (1974). New facts are as follows: barite 18O-values range from –13 to –17, stratiform barites ranging from 13 to 14.5; sulphides separated from a single stratiform ore lens hand specimen usually have 34 Ssl > 34 Sga > 34 Spy; the outward decrease in 34 S-values in the stratiform G orebody is confined to the first few hundred feet only; pyrite 34 S-values progressively increase downwards through one stratiform sulphide orebody; yet variations of 13 occur within a single colloform pyrite structure from another stratiform orebody. It is concluded that there were at least two sources of sulphur, seawater sulphate and deep-seated sulphur. The former was the dominant source of all sulphate and, via biogenic reduction, of the sulphur in the bulk of the stratiform sulphide. The latter was the source of the sulphur in the vein sulphides. There was minimal isotopic interaction between the cool seawater sulphate and the warm unwelling ore fluid sulphur species, even though the latter precipitated under near isotopic equilibrium conditions when the temperature dropped and/or the pH and Eh increased. The lack of isotopic equilibrium between pyrite and ore sulphides in the stratiform ore lenses may result from the latter having precipitated slightly later than the former because of solubility relationships. Overall the present isotopic evidence supports considerable geological evidence favoring a syngenetic origin for the stratiform Silvermines orebodies.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号