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1.
 Petrological and chemical variations, as well as oxygen and strontium isotopic data are presented for metagabbros from the Romanche and Vema fracture zones. These rocks were affected by several types and degrees of alterations ranging from slight hydrothermal alteration to complete amphibolitization. Five major kinds of alteration processes ranging from late-magmatic deuteric alteration (stage I) to low temperature (<150 °C) alteration (stage V) were identified. Water-rock interactions between 300 and 650 °C are the most dominant interactions resulting in the most prevailing secondary mineralogical assemblages which characterize the amphibolite and/or greenschist facies (amphibole ± plagioclase ± epidote ± titanite ± chlorite ± prehnite). Hydrothermal alteration of these gabbroic rocks results in isotopic exchanges between rocks and seawater-derived fluids. These exchanges lead to decrease of gabbroic δ18O toward values as low as +3.9‰, and larger Sr isotopic variations than other oceanic gabbroic rocks (87Sr/86Sr ratios shift to 0.7029–0.7051). Calculation of a chemical budget indicates that metagabbros are hydrated and enriched in Fe and probably in Mg and Cl, while Si, Ca and Ti are released to the hydrothermal fluids. In addition to metamorphic recrystallization and geochemical transformation, hydrothermal alteration of oceanic gabbros contributes to the control of the global ocean geochemistry. Received: 8 March 1999 / Accepted: 12 July 1999  相似文献   

2.
 Hydrogen and oxygen isotope analyses have been made of hydrous minerals in gabbros and basaltic xenoliths from the Eocene Kap Edvard Holm intrusive complex of East Greenland. The analyzed samples are of three types: (1) primary igneous hornblendes and phlogopites that crystallized from partial melts of hydrothermally altered basaltic xenoliths, (2) primary igneous hornblendes that formed during late–magmatic recrystallization of layered gabbroic cumulates, and (3) secondary actinolite, epidote and chlorite that formed during subsolidus alteration of both xenoliths and gabbros. Secondary actinolite has a δ18O value of −5.8‰ and a δD value of −158‰. These low values reflect subsolidus alteration by low–δ18O, low–δD hydrothermal fluids of meteoric origin. The δD value is lower than the −146 to −112‰ values previously reported for amphiboles from other early Tertiary meteoric–hydrothermal systems in East Greenland and Scotland, indicating that the meteoric waters at Kap Edvard Holm were isotopically lighter than typical early Tertiary meteoric waters in the North Atlantic region. This probably reflects local climatic variations caused by formation of a major topographic dome at about the time of plutonism and hydrothermal activity. The calculated isotopic composition of the meteoric water is δD=−110 ± 10‰, δ18O ≈−15‰. Igneous hornblendes and phlogopites from pegmatitic pods in hornfelsed basaltic xenoliths have δ18O values between −6.0 and −3.8‰ and δD values between −155 and −140‰. These are both much lower than typical values of fresh basalts. The oxygen isotope fractionations between pegmatitic hornblendes and surrounding hornfelsic minerals are close to equilibrium fractionations for magmatic temperatures, indicating that the pegmatites crystallized from low–δ18O partial melts of xenoliths that had been hydrothermally altered and depleted in 18O prior to stoping. The pegmatitic minerals may have crystallized with low primary δD values inherited from the altered country rocks, but these values were probably overprinted extensively by subsolidus isotopic exchange with low–δD meteoric–hydrothermal fluids. This exchange was facilitated by rapid self–diffusion of hydrogen through the crystal structures. Primary igneous hornblendes from the plutonic rocks have δ18O values between +2.0 and +3.2‰ and δD values between −166 and −146‰. The 18O fractionations between hornblendes and coexisting augites are close to equilibrium fractionations for magmatic temperatures, indicating that the hornblendes crystallized directly from the magma and subsequently underwent little or no oxygen exchange. The hornblendes may have crystallized with low primary δD values, due to contamination of the magma with altered xenolithic material, but the final δD values were probably controlled largely by subsolidus isotopic exchange. This inference is based partly on the observation that coexisting plagioclase has been extensively depleted in 18O via a mineral–fluid exchange reaction that is much slower than the hydrogen exchange reaction in hornblende. It is concluded that all hydrous minerals in the study area, whether igneous or secondary, have δD values that reflect extensive subsolidus isotopic equilibration with meteoric–hydrothermal fluids. Received: 22 March 1994 / Accepted: 26 January 1995  相似文献   

3.
A generalized approach for retrieving equilibrium isotope fractionations from natural rocks is proposed in which models of prograde reaction histories and retrograde diffusional exchange are used to identify coexisting minerals with similar isotope closure temperatures. Examples using literature data and new analyses from 32 natural amphibolite-facies schists demonstrate both the feasibility and limitations of obtaining equilibrium oxygen isotope fractionations from minerals in natural rocks. By screening samples according to the theoretical models, natural data are shown to have highly consistent mineral fractionations (±2σ reproducibilities of ±0.16 to 0.54‰) that within uncertainty reproduce experimental determinations among the minerals quartz, biotite, muscovite, and calcic amphibole. This correspondence indicates that the proposed theoretically-based selection criteria improve the likelihood of measuring equilibrium fractionations. The new data further corroborate the expected progressive enrichment of δ18O in the orthosilicates with increasing Al+Si relative to Fe+Mg: Δ(Ky-Grt) ∼1.05‰, Δ(St-Grt) ∼0.6‰, and Δ(St-Cld) ∼0.3‰ at 525–575 °C. In contrast, typical samples that fail to satisfy screening criteria exhibit fractionations involving quartz, biotite, and amphibole that are strongly disequilibrium because of exchange during cooling. Theoretical screening of samples prior to isotope analysis allows robust, independent assessment of theoretical and experimental determinations of equilibrium isotope fractionations. Received: 14 January 1997 / Accepted: 9 March 1998  相似文献   

4.
The Cretaceous blueschist belt, Tavşanlı Zone, representing the subducted and exhumed northern continental margin of the Anatolide–Tauride platform is exposed in Western Anatolia. The Sivrihisar area east of Tavşanlı is made up of tectonic units consisting of i) metaclastics and conformably overlying massive marbles (coherent blueschist unit), ii) blueschist-eclogite unit, iii) marble–calcschist intercalation and iv) metaperidotite slab. The metaclastics are composed of jadeite–lawsonite–glaucophane and jadeite–glaucophane–chloritoid schists, phengite phyllites, and calcschists with glaucophane–lawsonite metabasite layers. The blueschist-eclogite unit representing strongly sheared, deeply buried and imbricated tectonic slices of accreted uppermost levels of the oceanic crust with minor metamorphosed serpentinite bodies consists of lawsonite-bearing eclogitic metabasites (approximately 90% of the field), lawsonite eclogites, metagabbros, serpentinites, pelagic marbles, omphacite–glaucophane–lawsonite metapelites and metacherts. The mineral assemblage of the lawsonite eclogite (garnet + omphacite > 70%) is omphacite, garnet, lawsonite, glaucophane, phengite and rutile. Lawsonite eclogite lenses are enclosed by garnet–lawsonite blueschist envelopes.Textural evidence from lawsonite eclogites and country rocks reveals that they did not leave the stability field of lawsonite during subduction and exhumation. The widespread preservation of lawsonite in eclogitic metabasites and eclogites can be attributed to rapid subduction and subsequent exhumation in a low geothermal gradient of the oceanic crust material without experiencing a thermal relaxation. Peak P–T conditions of lawsonite eclogites are estimated at 24 ± 1 kbar and 460 ± 25 °C. These P–T conditions indicate a remarkably low geotherm of 6.2 °C/km corresponding to a burial depth of 74 km.  相似文献   

5.
Oxygen isotopic composition of emerald from 62 occurrences and deposits in the world reveals a wide range in δ18O (SMOW) between +6.2 and +24.7‰. The δ18O-values for each deposit are restricted and can be used to determine the origin of emerald from the world's most important producers. The δ18O-value of emerald appears to be a fingerprint of its origin, especially for gems of exceptional quality from Colombia (eastern emerald zone, δ18O = +16.8 ± 0.1‰; western emerald zone, δ18O = +21.2 ± 0.5‰), Afghanistan (δ18O = +13.5 ± 0.1‰), Pakistan (Swat-Mingora districts, δ18O = +15.7 ± 0.1‰), Brazil (Santa Terezinha de Goiás, δ18O = +12.2 ± 0.1‰; Quadrilatero Ferrifero, δ18O = +6.9 ± 0.4‰) and Zimbabwe (Sandawana, δ18O = +7.5 ± 0.5‰). Furthermore, the 18O-composition of emerald appears to be a good marker of its geological environment because the data suggest that host-rock-buffering of fluid δ18O is considerable during fluid-rock interaction. Received: 29 January 1998 / Accepted: 25 March 1998  相似文献   

6.
Natural gases of shallow reservoirs with the carbon isotopic compositions of methane ranging from -50‰ to -60‰ (PDB) were considered as mixed gases of biogenic and thermogenic origins previously and some of them were considered as low-mature (or low temperature thermogenic) gases lately. In this paper natural gases with the carbon isotopic compositions of methane in the above range were identified using the molecular and stable carbon isotopic compositions of methane, ethane and propane. The mixed gases of biogenic and mature thermogenic origins display the characteristics of δ 13 C1 ranging from -50‰to -60‰,δ13C2 ＞ -35‰,Δvalues (δ13C3 -δ13C2) ＜ 5‰ and C1/∑C2 ratios ＜ 40. Immature to low-mature gases display the characteristics of δ 13 C1 ranging from - 50‰ to - 60‰, δ13 C2 ＜- 40‰,Δ values (δ13C3 -δ13C2) ＞7‰, and C1/∑C 2 ratios ＞60.  相似文献   

7.
In order to shed light on upper crustal differentiation of mantle-derived basaltic magmas in a subduction zone setting, we have determined the mineral chemistry and oxygen and hydrogen isotope composition of individual cumulus minerals in plutonic blocks from St. Vincent, Lesser Antilles. Plutonic rock types display great variation in mineralogy, from olivine–gabbros to troctolites and hornblendites, with a corresponding variety of cumulate textures. Mineral compositions differ from those in erupted basaltic lavas from St. Vincent and in published high-pressure (4–10 kb) experimental run products of a St. Vincent high-Mg basalt in having higher An plagioclase coexisting with lower Fo olivine. The oxygen isotope compositions (δ18O) of cumulus olivine (4.89–5.18‰), plagioclase (5.84–6.28‰), clinopyroxene (5.17–5.47‰) and hornblende (5.48–5.61‰) and hydrogen isotope composition of hornblende (δD = −35.5 to −49.9‰) are all consistent with closed system magmatic differentiation of a mantle-derived basaltic melt. We employed a number of modelling exercises to constrain the origin of the chemical and isotopic compositions reported. δ18OOlivine is up to 0.2‰ higher than modelled values for closed system fractional crystallisation of a primary melt. We attribute this to isotopic disequilibria between cumulus minerals crystallising at different temperatures, with equilibration retarded by slow oxygen diffusion in olivine during prolonged crustal storage. We used melt inclusion and plagioclase compositions to determine parental magmatic water contents (water saturated, 4.6 ± 0.5 wt% H2O) and crystallisation pressures (173 ± 50 MPa). Applying these values to previously reported basaltic and basaltic andesite lava compositions, we can reproduce the cumulus plagioclase and olivine compositions and their associated trend. We conclude that differentiation of primitive hydrous basalts on St. Vincent involves crystallisation of olivine and Cr-rich spinel at depth within the crust, lowering MgO and Cr2O3 and raising Al2O3 and CaO of residual melt due to suppression of plagioclase. Low density, hydrous basaltic and basaltic andesite melts then ascend rapidly through the crust, stalling at shallow depth upon water saturation where crystallisation of the chemically distinct cumulus phases observed in this study can occur. Deposited crystals armour the shallow magma chamber where oxygen isotope equilibration between minerals is slowly approached, before remobilisation and entrainment by later injections of magma.  相似文献   

8.
 The thermoelastic parameters of natural andradite and grossular have been investigated by high-pressure and -temperature synchrotron X-ray powder diffraction, at ESRF, on the ID30 beamline. The P–V–T data have been fitted by Birch-Murnaghan-like EOSs, using both the approximated and the general form. We have obtained for andradite K 0=158.0(±1.5) GPa, (dK/dT )0=−0.020(3) GPa K−1 and α0=31.6(2) 10−6 K−1, and for grossular K 0=168.2(±1.7) GPa, (dK/dT)0=−0.016(3) GPa K−1 and α0=27.8(2) 10−6 K−1. Comparisons between the present issues and thermoelastic properties of garnets earlier determined are carried out. Received: 7 July 2000 / Accepted: 20 October 2000  相似文献   

9.
The potential of teleost scales as indicator tissues of recent dietary history was investigated in the estuarine fish, Notolabrus celidotus. Treatment fish were fed a 15N-enriched bivalve diet (δ15N = 108.8‰) for up to 90 days to examine integration of the dietary source signature in scale margin, regenerated scale, and dorsal muscle tissues. Regenerated scales exhibited the greatest and most rapid enrichment in δ15N (to 53.4 ± 5.0‰) of the tissues examined. Scale margin δ15N signatures also increased significantly from 14.5‰ to 30.9 ± 6.0‰, while muscle tissue only increased by 2.8‰ after 90 days. Dietary contribution models suggested that isotopic equilibration of dietary nitrogen would be most rapid in regenerated scales, followed by scale marginal increments and then muscle. In conclusion, teleost scales hold considerable potential as novel, nondestructive indicators of recent dietary history, yet currently face constraints from uncertainties in isotopic integration mechanisms.  相似文献   

10.
Alluvial and colluvial gem sapphires are common in the basaltic fields of the French Massif Central (FMC) but sapphire-bearing xenoliths are very rare, found only in the Menet trachytic cone in Cantal. The O-isotope composition of the sapphires ranges between 4.4 and 13.9‰. Two distinct groups have been defined: the first with a restricted isotopic range between 4.4 and 6.8‰ (n = 22; mean δ18O = 5.6 ± 0.7‰), falls within the worldwide range defined for blue-green-yellow sapphires related to basaltic gem fields (3.0 < δ18O < 8.2‰, n = 150), and overlaps the ranges defined for magmatic sapphires in syenite (4.4 < δ18O < 8.3‰, n = 29). A second group, with an isotopic range between 7.6 and 13.9‰ (n = 9), suggests a metamorphic sapphire source such as biotite schist in gneisses or skarns. The δ18O values of 4.4–4.5‰ for the blue sapphire-bearing anorthoclasite xenolith from Menet is lower than the δ18O values obtained for anorthoclase (7.7–7.9‰), but suggest that these sapphires were derived from an igneous reservoir in the subcontinental spinel lherzolitic mantle of the FMC. The presence of inclusions of columbite-group minerals, pyrochlore, Nb-bearing rutile, and thorite in these sapphires provides an additional argument for a magmatic origin. In the FMC lithospheric mantle, felsic melts crystallized to form anorthoclasites, the most evolved peraluminous variant of the alkaline basaltic melt. The O-isotopic compositions of the first group suggests that these sapphires crystallized from felsic magmas under upper mantle conditions. The second group of isotopic values, typified for example by the Le Bras sapphire with a δ18O of 13.9‰, indicates that metamorphic sapphires from granulites were transported to the surface by basaltic magma.  相似文献   

11.
The Shasta gold-silver deposit, British Columbia, Canada, is an adularia-sericite-type epithermal deposit in which deposition of precious metals coincided with the transition of quartz- to calcite-dominant gangue. Mineralization is associated with stockwork-breccia zones in potassically altered dacitic lapilli tuffs and flows, and consists of pyrite, sphalerite, chalcopyrite, galena, acanthite, electrum and native silver. Pre- and post-ore veins consist solely of quartz and calcite, respectively. Fluid inclusion microthermometry indicates that ore minerals were deposited between 280 ° and 225 °C, from a relatively dilute hydrothermal fluid (˜1.5 wt.% NaCl equivalent). Abundant vapor-rich inclusions in ore-stage calcite are consistent with boiling. Oxygen and hydrogen isotopic data (δ18Ofluid = −1.5 to −4.1‰; δDfluid = −148 to −171‰) suggest that the fluid had a meteoric origin, but was 18O-enriched by interaction with volcanic wallrocks. Initial (˜280 °C) fluid pH and log f O2 conditions are estimated at 5.3 to 6.0, and −32.5 to −33 bar, respectively; during ore deposition, the fluid became more alkaline and oxidizing. Ore deposition at Shasta is attributed to localization of meteoric hydrothermal fluids by extensional faults; mineralization was controlled by boiling in response to hydraulic brecciation. Calcite and base metal sulfides precipitated due to the increase in pH that accompanied boiling, and the associated decrease in H2S concentration led to precipitation of gold and silver. Received: 23 February 1995 / Accepted: 16 April 1996  相似文献   

12.
Crustal xenoliths from basanitic dikes and necks that intruded into continental sediments of the Cretaceous Salta Rift at Quebrada de Las Conchas, Provincia Salta, Argentina were investigated to get information about the age and the chemical composition of the lower crust. Most of the xenoliths have a granitoid composition with quartz-plagioclase-garnet-rutile ± K-feldspar as major minerals. The exceedingly rare mafic xenoliths consist of plagioclase-clinopyroxene-garnet ± hornblende. All xenoliths show a well equilibrated granoblastic fabric and the minerals are compositionally unzoned. Thermobarometric calculations indicate equilibration of the mafic xenoliths in the granulite facies at temperatures of ca. 900 °C and pressures of ca. 10 kbar. The Sm-Nd mineral isochron ages are 95.1 ± 10.4 Ma, 91.5 ± 13.0 Ma, 89.0 ± 4.2 Ma (granitoid xenoliths), and 110.7 ± 23.6 Ma (mafic xenolith). These ages are in agreement with the age of basanitic volcanism (ca. 130–100 and 80–75 Ma) and are interpreted as minimum ages of metamorphism. Lower crustal temperature at the time given by the isochrons was above the closure temperature of the Sm-Nd system (>600–700 °C). The Sm-Nd and Rb-Sr isotopic signatures (147Sm/144Nd = 0.1225–0.1608; 143Nd/144Ndt 0 = 0.512000–0.512324; 87Rb/86Sr = 0.099–0.172; 87Sr/86Srt 0 = 0.708188–0.7143161) and common lead isotopic signatures (206Pb/204Pb = 18.43–18.48; 207Pb/204Pb = 15.62–15.70; 208Pb/204Pb = 38.22 –38.97) of the granitoid xenoliths are indistinguishable from the isotopic composition of the Early Paleozoic metamorphic basement from NW Argentina, apart from the lower 208Pb/204Pb ratio of the basement. The Sm-Nd depleted mantle model ages of ca. 1.8 Ga from granitoid xenoliths and Early Paleozoic basement point to a similar Proterozoic protolith. Time constraints, the well equilibrated granulite fabric, P-T conditions and lack of chemical zoning of minerals point to a high temperature in a crust of nearly normal thickness at ca. 90 Ma and to a prominent thermal anomaly in the lithosphere. The composition of the xenoliths is similar to the composition of the Early Paleozoic basement in the Andes of NW Argentina and northern Chile. A thick mafic lower crust seems unlikely considering low abundance of mafic xenoliths and the predominance of granitoid xenoliths. Received: 21 July 1998 / Accepted: 27 October 1998  相似文献   

13.
The isotopic composition of dissolved boron, in combination with the elemental concentrations of B, Cl and salinities in freshwater-seawater mixed samples taken from the estuary of the Changjiang River, the largest one in China, was investigated in detail in this study. Brackish water and seawater samples from the estuary of the Changjiang River were collected during low water season in November, 1998. Boron isotopic compositions were determined by the Cs2BO^＋2-graphite technique with a analytical uncertainty of 0.2‰ for NIST SRM 951 and an average analytical uncertainty of 0.8‰ for the samples. The isotopic compositions of boron, expressed in δ^11B, and boron concentrations in the Changjiang River at Nanjing and seawater from the open marine East Sea, China, are characterized by δ^11B values of -5.4‰ and 40.0‰, as well as 0.0272 and 4.43 mg B/L, respectively. Well-defined correlations between δ^11B values, B concentrations and Cl concentrations are interpreted in terms of binary mixing between fiver input water and East Sea seawater by a process of straightforward dilution. The offsets of δ^11B values are not related to the contents of clastic sediment and to the addition of boron. These relationships favor a conservative behavior of boron at the estuarine of the Changjiang River.  相似文献   

14.
The eastern Alaska Beaufort Sea coast is characterized by numerous shallow (2–5 m) estuarine lagoons, fed by streams and small rivers that drain northward from the Brooks Range through the arctic coastal plain, and bounded seaward by barrier islands and shoals. Millions of birds from six continents nest and forage during the summer period in this region using the river deltas, lagoons, and shoreline along with several species of anadromous and marine fish. We examined biogeochemical processes linking the benthic community to the overall food web structure of these poorly studied but pristine estuaries, which are largely covered by 1.8 m of ice for 10 months annually. In summer, these lagoons are relatively warm with brackish salinities (5–10°C, S = 10–25) compared to more open coastal waters (0–5°C, S > 27). The stable isotopic composition of organic materials in sediments (i.e., benthic particulate organic matter) and water column suspended particulate organic matter from both streams and lagoons are largely indistinguishable and reflect strong terrestrial contributions, based upon δ13C and δ15N values (−25.6‰ to −27.4‰ and 1.4‰ to 3.3‰, respectively). By comparison, shifts toward more heavy isotope-enriched organic materials reflecting marine influence are observed on the adjacent coastal shelf (−24.8‰ to −25.4‰ and 3.4‰ to 5.3‰, respectively). The isotopic composition of lagoon fauna is consistent with a food web dominated by omnivorous detritovores strongly dependent on microbial processing of terrestrial sources of carbon. Biomagnification of 15N in benthic organisms indicate that the benthic food web in lagoons support up to four trophic levels, with carnivorous gastropod predators and benthic fishes (δ15N values up to 14.4‰) at the apex.  相似文献   

15.
Gossan Hill is an Archean (∼3.0 Ga) Cu–Zn–magnetite-rich volcanic-hosted massive sulfide (VHMS) deposit in the Yilgarn Craton of Western Australia. Massive sulfide and magnetite occur within a layered succession of tuffaceous, felsic volcaniclastic rocks of the Golden Grove Formation. The Gossan Hill deposit consists of two stratigraphically separate ore zones that are stratabound and interconnected by sulfide veins. Thickly developed massive sulfide and stockwork zones in the north of the deposit are interpreted to represent a feeder zone. The deposit is broadly zoned from a Cu–Fe-rich lower ore zone, upwards through Cu–Zn to Zn–Ag–Au–Pb enrichment in the upper ore zone. New sulfur isotope studies at the Gossan Hill deposit indicate that the variation is wider than previously reported, with sulfide δ34S values varying between −1.6 and 7.8‰ with an average of 2.1 ± 1.4‰ (1σ error). Sulfur isotope values have a broad systematic stratigraphic increase of approximately 1.2‰ from the base to the top of the deposit. This variation in sulfur isotope values is significant in view of typical narrow ranges for Archean VHMS deposits. Copper-rich sulfides in the lower ore zone have a narrower range (δ34S values of −1.6 to 3.4‰, average ∼1.6 ± 0.9‰) than sulfides in the upper ore zone. The lower ore zone is interpreted to have formed from a relatively uniform reduced sulfur source dominated by leached igneous rock sulfur and minor magmatic sulfur. Towards the upper Zn-rich ore zone, an overall increase in δ34S values is accompanied by a wider range of δ34S values, with the greatest variation occurring in massive pyrite at the southern margin of the upper ore zone (−1.0 to 7.8‰). The higher average δ34S values (2.8 ± 2.1‰) and their wider range are explained by mixing of hydrothermal fluids containing leached igneous rock sulfur with Archean seawater (δ34S values of 2 to 3‰) near the paleoseafloor. The widest range of δ34S values at the southern margin of the deposit occurs away from the feeder zone and is attributed to greater seawater mixing away from the central upflow zone. Received: 10 June 1999 / Accepted: 28 December 1999  相似文献   

16.
 The monoclinic titanite-like high-pressure form of calcium disilicate has been synthesized and quenched to ambient conditions to form the triclinic low-pressure phase containing silicon in four-, five- and sixfold coordination. The enthalpy of formation of the quench product has been measured by high-temperature oxide melt calorimetry. The value obtained from samples from a series of several synthesis experiments is ΔH f = (−26.32 ± 4.27) kJ mol−1 for the formation from the component oxides, or ΔH f  = (−2482.81 ± 4.59) kJ mol−1 for the formation from the elements. The result is identical within experimental error to available estimates, although the previously predicted energy difference between the monoclinic and triclinic phases could not be verified. Received: 16 February 2000 / Accepted: 14 July 2000  相似文献   

17.
The low-pressure stability of clinohumite has been investigated in phase-equilibrium experiments on the reaction forsterite + brucite = clinohumite. The reaction was bracketed between 2.45 and 2.84 GPa at 650 °C, extending to between 1.37 and 1.57 GPa at 850 °C. At temperatures above the reaction brucite = periclase + vapour, the reaction clinohumite = forsterite + vapour was bracketed between 1.27 and 1.52 GPa at 900 °C, rising to between 1.90 and 2.00 GPa at 1000 °C. The position of the reaction forsterite + brucite = clinohumite is ∼0.5 GPa below the position determined in previous work, the difference arising either from pressure uncertainties in both studies, from enhanced reaction to clinohumite in this study due to the presence of excess brucite in the starting material, or from different concentrations of defects in the two samples. The brackets on the reaction were combined with other measured and estimated thermodynamic data for clinohumite to determine its enthalpy of formation and entropy, in a revised version of the thermodynamic dataset of Holland and Powell (1998). The values obtained were ΔH f =−9607.29±3.05 kJ mol−1, S=445 J mol−1 K−1. These data were used to calculate positions of other reactions involving clinohumite. The calculations suggest a larger stability field for clinohumite than implied by the results of previous experimental studies, indicating a need for more high-pressure phase-equilibrium studies to provide better thermodynamic data. Received: 30 April 1999 / Accepted: 8 November 1999  相似文献   

18.
The Tono sandstone-type uranium mine area, middle Honsyu, Japan is composed of Miocene lacustrine sedimentary rocks in the lower part (18–22 Ma) and marine facies in the upper part (15–16 Ma). Calcite and pyrite occur as dominant diagenetic alteration products in these Neogene sedimentary rocks. The characteristics of calcite and pyrite differ significantly between lacustrine and marine facies. Abundant pyrite, calcite, organic matter, and small amounts of marcasite or pyrrhotite occur in the lacustrine facies, whereas small amounts of calcite and framboidal pyrite, organic matter and no marcasite or pyrrhotite are found within the marine units. The δ13C values of calcite in the lacustrine deposits are low (−19 to −6‰ PDB) but those in marine formation are high (−11 to +3‰). This implies that the contribution of marine carbonate is larger in upper marine sedimentary rocks, and carbon in calcite in the lower lacustrine formation was derived both from oxidation of organic matter and from dissolved marine inorganic carbon. The δ34S values of framboidal pyrite in the upper marine formation are low (−14 to −8‰ CDT), indicating a small extent of bacterial seawater sulfate reduction, whereas those of euhedral-subhedral pyrite in the lower lignite-bearing arkose sandstone are high (+10 to +43‰), implying a large extent of closed-system bacterial seawater sulfate reduction. The δ34S and δ13C data which deviate from a negative correlation line toward higher δ13C values suggest methanogenic CO2 production. During diagenesis of the lacustrine unit, large amounts of euhedral-subhedral pyrite were formed, facilitated by extensive bacterial reduction of seawater sulfate with concomitant oxidation of organic matter, and by hydrolysis reactions of organic matter, producing CH4 and CO2. Uranium minerals (coffinite and uraninite) were also formed at this stage by the reduction of U6+ to U4+. The conditions of diagenetic alteration within the lacustrine deposits and uranium mineralization is characterized by low Eh in which nearly equal concentrations of CH4 and HCO3 − existed and reduced sulfur species (H2S, HS−) are predominant among aqueous sulfur species, whereas diagenetic alteration of the marine formations was characterized by a predominance of SO4 2− among dissolved sulfur species. Modern groundwater in the lacustrine formation has a low Eh value (−335 mV). Estimated and measured low Eh values of modern and ancient interstitial waters in lacustrine environments indicate that a reducing environment in which U4+ is stable has been maintained since precipitation of uranium minerals. Received: 9 February 1996 / Accepted: 11 April 1997  相似文献   

19.
Geochemical mixing models were used to decipher the dominant source of freshwater (rainfall, canal discharge, or groundwater discharge) to Biscayne Bay, an estuary in south Florida. Discrete samples of precipitation, canal water, groundwater, and bay surface water were collected monthly for 2 years and analyzed for salinity, stable isotopes of oxygen and hydrogen, and Sr2+/Ca2+ concentrations. These geochemical tracers were used in three separate mixing models and then combined to trace the magnitude and timing of the freshwater inputs to the estuary. Fresh groundwater had an isotopic signature (δ 18O = −2.66‰, δD −7.60‰) similar to rainfall (δ 18O = −2.86‰, δD = −4.78‰). Canal water had a heavy isotopic signature (δ 18O = −0.46‰, δD = −2.48‰) due to evaporation. This made it possible to use stable isotopes of oxygen and hydrogen to separate canal water from precipitation and groundwater as a source of freshwater into the bay. A second model using Sr2+/Ca2+ ratios was developed to discern fresh groundwater inputs from precipitation inputs. Groundwater had a Sr2+/Ca2+ ratio of 0.07, while precipitation had a dissimilar ratio of 0.89. When combined, these models showed a freshwater input ratio of canal/precipitation/groundwater of 37%:53%:10% in the wet season and 40%:55%:5% in the dry season with an error of ±25%. For a bay-wide water budget that includes saltwater and freshwater mixing, fresh groundwater accounts for 1–2% of the total fresh and saline water input.  相似文献   

20.
The origin of monzonitic intrusions that are associated with Proterozoic massif-type anorthosite complexes is controversial. A detailed oxygen isotope study of the Sybille intrusion, a monzonitic intrusion of the Laramie Anorthosite Complex (Wyoming), indicates that either derivation from a basaltic magma of mantle origin with a metasedimentary component (∼20%) incorporated early in its magmatic history, or a partial melt of lower crustal rocks is consistent with the data. The oxygen isotope compositions of plagioclase, pyroxene and zircon from the Sybille monzosyenite, the dominant rock type in the Sybille intrusion, were analyzed in order to establish the isotopic composition of the source of the magma. Plagioclase δ18O values range from 6.77 to 9.17‰. We interpret the higher plagioclase δ18O values (average 8.69 ± 0.30‰, n = 19) to be magmatic in origin, lower plagioclase δ18O values (average 7.51 ± 0.44‰, n = 22) to be the result of variable subsolidus alteration, and pyroxene δ18O values (average 6.34 ± 0.38‰, n = 19) to be the result of closed-system diffusional exchange during cooling. Low magnetic zircons, which have been shown to retain magmatic oxygen isotope values despite high grade metamorphism and extensive subsolidus hydrothermal alteration, have δ18O values (7.40 ± 0.24‰, n = 11) which are consistent with our interpretation of the plagioclase and pyroxene results. Oxygen isotope data from all three minerals indicate that the magmatic oxygen isotope composition of the Sybille intrusion is enriched in 18O relative to the composition of average or “normal” mantle-derived magmas. This enrichment is approximately twice the oxygen isotope enrichment that could result from closed-system fractionation, rendering a closed-system, comag- matic petrogenetic model between the Sybille intrusion and the mantle-derived anorthositic lithologies of the Laramie Anorthosite Complex improbable. Received: 7 April 1998 / Accepted: 19 January 1999  相似文献   

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