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1.
Boron isotope variations in nature: a synthesis   总被引：9，自引：0，他引：9  
The large relative mass difference between the two stable isotopes of boron, 10B and 11B, and the high geochemical reactivity of boron lead to significant isotope fractionation by natural processes. Published 11B values (relative to the NBS SRM-951 standard) span a wide range of 90. The lowest 11B values around — 30 are reported for non-marine evaporite minerals and certain tourmalines. The most 11B-enriched reservoir known to date are brines from Australian salt lakes and the Dead Sea of Israel with 11B values up to +59. Dissolved boron in present-day seawater has a constant world-wide 11B value of + 39.5. In this paper, available 11B data of a variety of natural fluid and solid samples from different geological environments are compiled and some of the most relevant aspects, including possible tracer applications of boron-isotope geochemistry, are summarized.
Résumé La grande différence relative de masse entre les isotopes stables du bore, 10B et 11B, et la grande réactivité geochimique du bore ont pour conséquence un fractionnement isotopique naturel important. Les valeurs de 11B publiées (par rapport au standard NBS SRM-951) varient de 90. Les valeurs de 11B les plus basses (–30) correspondent aux evaporites non-marines et à certaines tourmalines. Le réservoir le plus enrichi en 11B est représenté par les saumures des lacs salés d' Australie et par la Mer Morte en Israël, qui ont des valuers de 11B allent jusqu'à + 59. L'eau de mer a une valeur de 11B mondialement constante de + 39.5. Des valeurs de 11B des solutions naturelles ainsi que des roches et minéraux de différentes origines, publiées jusqu'à présent, sont présentées ici. En outre quelques aspects importants concernant la géochimie des isotopes du bore y compris quelques applications sont exposés.
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2.
In closed magma systems SiO2 approximately measures differentiation progress and oxygen isotopes can seem to obey Rayleigh fractionation only as a consequence of the behaviour of SiO2. The main role of 18O is as a sensitive indicator of contamination, either at the start of differentiation ( 18Oinit) or as a proportion of fractionation in AFC. Plots of 18O vs SiO2-allow to determine initial 18O values for different sequences for source comparison. For NBS-28=9.60, the 18O at 48% SiO2-varies between a high 6.4 for Kiglapait (Kalamarides 1984), 5.9 for Transhimalaya, 5.8 for Hachijo-Jima (Matsuhisa 1979), 5.6 for Koloula (Chivas et al. 1982) and a low 5.3 for the Darran Complex, New Zealand. The Transhimalayan batholiths (Gangdese belt) were emplaced in the Ladakh-Lhasa terrane, between the present-day Banggong-Nujiang, and Indus-Yarlung Tsangbo suture zones, after its accretion to Eurasia. The gradient of the least contaminated continuous ( 18O vs SiO2-igneous trend line is similar to that of Koloula, and AFC calculations suggest a low secondary assimilation rate of less than 0.05 times the rate of crystallisation. Outliers enriched in 18O are frequent in the Lhasa, and apparently rare in the Ladakh transsect. Low- 18O (5.0–0) granitoids and andesites on the Lhasa-Yangbajain axis are the result of present day or recent near-surface geothermal activity; their quartzes still trace the granitoids to the Transhimalaya 18O trend line, but the distribution of low total rock or feldspar 18O values could be a guide to more recent heat flow and thermally marked tectonic lineaments. Two ignimbrites from Maqiang show hardly any 18O-contamination by crustal material.  相似文献   

3.
The carbon and oxygen isotopic composition of Fe-carbonate ore and its calcitic to dolomitic Devonian host rocks at the Steirischer Erzberg siderite deposit (Greywacke zone, Upper Austroalpine Unit) were determined in order to constrain the source and nature of the Fe-rich mineralizing fluid. The 18O-values obtained for various Fe-carbonate generations and the carbonate host lie within a similar range between + 14.6 and + 21.6 (V-SMOW). No good correlation exists between the relative ages of the carbonate phases and their O isotopic composition. The variation in 18O-values is due to metamorphic recrystallization with locally variable fluid/rock ratios. The average 13C-value of the carbonate host is +0.5 ± 1.2 (PDB) which corresponds well to worldwide Phanerozoic marine carbonate values. The first Fecarbonate generation has slightly lower 13C-values, on average -1.4 ± 0.8 (PDB). Recrystallization of both the carbonate host minerals and the ankerite/siderite led to significantly lower 13C-values of -4.2 ± 0.6 and-4.7 ± 0.7, respectively. Within the basal breccia of the post-Hercynian transgression series matrix calcite/ dolomite shows an average 13C-value of -2.9 ± 0.7, and matrix siderite/ankerite an average value of-4.1 ± 0.4. These data, together with Sr isotope data published previously, strongly support a late-diagenetic or epigenetic first Fe-mineralization from convecting formation waters. They ascended along extension faults and were driven by an increased heat flow caused by crustal thinning during a Devonian rifting phase that initiated the separation of the Noric terrane from Africa. A potential source of the Fe could have been the underlying Ordovician acid volcanics. Regional metamorphism related to collision tectonics in the Late Carboniferous (Hercynian) and later during the Alpine orogeny, caused intensive recrystallization and partial mobilization of the various carbonate phases.  相似文献   

4.
Zusammenfassung Die S-Isotopenverteilung wurde an 67 Sulfid- und 17 Barytproben aus der Blei-Zink-Erzlagerstätte Grund untersucht. Die 34S-Werte der Zinkblende der Mineralisationsphase II liegen im Westfeld-Erzmittel I und in den östlich anschließenden Erzmitteln zwischen +4 und +6, in dem am weitesten westlich liegenden Westfeld-Erzmittel II zwischen +6 und +10. Die Werte für Bleiglanz der Mineralisationsphase II sind +2 bis +4 bzw. +4 bis +7. Die Sulfide der Mineralisationsphase III haben allgemein niedrigere -Werte. Koexistierende Sulfide zeigen eine deutliche Fraktionierung, wobei stets ZnS > PbS ist; die Differenz beträgt in der Mineralisationsphase II im Mittel 1,8, in der Phase III 3. Dies deutet auf niedrigere Bildungstemperatur der Minerale der Phase III hin. Zur genetischen Deutung der beobachteten -Abnahme beim Übergang zur Mineralisationsphase II werden vier Modelle diskutiert. Baryte zeigen innerhalb der Lagerstätte recht einheitliche 34S-Werte zwischen +11 und +14,5%. Diese Einheitlichkeit wird durch den Einfluß deszendenter Zechstein-Lösungen erklärt.
34S-values are given for 67 sulfide and 16 barite specimens from the Pb-Zn-deposit Grund (Harz mountains, W-Germany). In the central part of the deposit the sulfide 's of the first major mineralization (phase II) range from: ZnS +4 to +6 and PbS +2 to +4. The sulfides of the second major mineralization (phase III) are depleted in 34S and range from: ZnS +2 to +4, PbS –1,4 to 3. The sulfides at the western end of the vein system are heavier; the phase II minerals ranging from: ZnS +6 to 10 and PbS +4 to 7. The mean -difference between co-existing ZnS and PbS in phase II is 1,8, in phase III 3. This indicates lowering of temperature of formation for the phase III ore. Four models have been set up in order to explain the observed -variation. Barites with rather uniform 's from +11 to +14,5 are probably affected by descendent solutions from overlying sulfate sediments of Permian age.
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5.
Dalradian metamorphic rocks, Lower Ordovician meta-igneous rocks (MGS) and Caledonian granites of the Connemara complex in SW Connemara all show intense retrograde alteration. Alteration primarily involves sericitization and saussuritization of plagioclase, the alteration of biotite and hornblende to chlorite and the formation of secondary epidote. The alteration is associated with sealed microcracks in all rocks and planes of secondary fluid inclusions in quartz where it occurs, and was the result of a phase of fluid influx into these rocks. In hand specimen K-feldspar becomes progressively reddened with increasing alteration. Mineralogical alteration in the MGS and Caledonian granites took place at temperatures 275±15°C and in the MGS Pfluid is estimated to be 1.5 kbar during alteration. The °D values of alteration phases are:-18 to-29 (fluid inclusions),-47 to-61 (chlorites) and-11 to-31 (epidotes). Chlorite 18O values are +0.2 to +4.3, while 18O values for quartz-K-feldspar pairs show both positively sloped (MGS) and highly unusual negatively sloped (Caledonian granites) arrays, diverging from the normal magmatic field on a - plot. The stable isotope data show that the fluid that caused retrogression continued to be present in most rocks until temperatures fell to 200–140°C. The retrograde fluid had D -20 to-30 in all lithologies, but the fluid 18O varied both spatially and temporally within the range-4 to +7. The fO2 of the fluid that deposited the epidotes in the MGS varied with its 18O value, with the most 18O-depleted fluid being the most oxidizing. The D values, together with low (<0) 18O values for the retrograde fluid in some lithologies indicate that this fluid was of meteoric origin. This meteoric fluid was probably responsible for the alteration in all lithologies during a single phase of fluid infiltration. The variation in retrograde fluid 18O values is attributed to the effects of variable oxygen isotope shifting of this meteoric fluid by fluid-rock interaction. Infiltration of meteoric fluid into this area was most likely accomplished by convection of pore fluids around the heat anomaly of the Galway granite soon after intrusion at 400 Ma. However convective circulation of meteoric water and mineralogical alteration could possible have occurred considerably later.  相似文献   

6.
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   

7.
The world-class Idrija mercury deposit (western Slovenia) is hosted by highly deformed Permocarboniferous to Middle Triassic sedimentary rocks within a complex tectonic structure at the transition between the External Dinarides and the Southern Alps. Concordant and discordant mineralization formed concomitant with Middle Triassic bimodal volcanism in an aborted rift. A multiple isotopic (C, O, S) investigation of host rocks and ore minerals was performed to put constraints on the source and composition of the fluid, and the hydrothermal alteration. The distributions of the 13C and 18O values of host and gangue carbonates are indicative of a fracture-controlled hydrothermal system, with locally high fluid-rock ratios. Quantitative modeling of the 13C and 18O covariation for host carbonates during temperature dependent fluid-rock interaction, and concomitant precipitation of void-filling dolomites points to a slightly acidic hydrothermal fluid (13C–4 and 18O+10), which most likely evolved during isotopic exchange with carbonates under low fluid/rock ratios. The 34S values of hydrothermal and sedimentary sulfur minerals were used to re-evaluate the previously proposed magmatic and evaporitic sulfur sources for the mineralization, and to assess the importance of other possible sulfur sources such as the contemporaneous seawater sulfate, sedimentary pyrite, and organic sulfur compounds. The 34S values of the sulfides show a large variation at deposit down to hand-specimen scale. They range for cinnabar and pyrite from –19.1 to +22.8, and from –22.4 to +59.6, respectively, suggesting mixing of sulfur from different sources. The peak of 34S values of cinnabar and pyrite close to 0 is compatible with ore sulfur derived dominantly from a magmatic fluid and/or from hydrothermal leaching of basement rocks. The similar stratigraphic trends of the 34S values of both cinnabar and pyrite suggest a minor contribution of sedimentary sulfur (pyrite and organic sulfur) to the ore formation. Some of the positive 34S values are probably derived from thermochemical reduction of evaporitic and contemporaneous seawater sulfates.Editorial handling: P. Lattanzi  相似文献   

8.
The Precipitation of carbonate cements in the Pobitite Kamani area (Lower Eocene) began during early diagenesis of sediments. There is evidence, however, that calcite is still forming today.The negative 13C values to –29.2 suggest that the carbonate formed during degradation of 12C-enriched organic matter (perhaps partly from oxidation of methane). The 18O values of –0.9 to –1.6 reflect the marine origin of the early diagenetic carbonate cements. Most of the carbonates, however, formed during late diagenesis (at approximately 1300 m burial depth) and/or recently (after uplift) from percolating groundwaters. These carbonates have an isotopic composition characteristic of carbonates which precipitated from meteoric waters under normal sedimentary temperatures in isotopic equilibrium with 12C-enriched soil carbon dioxide.  相似文献   

9.
The 18O and D values in mud-volcanic waters of the Taman Peninsula and Kakhetia vary from +0.7 to +10.0 and from –37 to –13 , respectively. These values increase as the Greater Caucasus is approached. The increase in 18O and D also positively correlates with fluid generation temperatures based on hydrochemical geothermometers. This is accompanied by changes in the chemical composition of waters, in which contents of alkali metals, HCO– 3 ion, and boron increase, while the content of halogen ions (Cl, Br, J) decreases. Changes in the isotopic composition of water are also accompanied by the increase of 13 in methane and decrease of 11 B in clays. Analysis of formal models of the evolution of isotopic composition of mud-volcanic waters showed that mud volcanoes are recharged by freshened water from the Maikop paleobasin with an inferred isotopic composition of D –40 and 18 O –6. Based on this assumption, the 18O and D values observed in mud-volcanic waters can be explained not only by processes of distillation and condensation in a closed system, but also by combined processes of isotopic reequilibration in the water-illite-methane system.Translated from Litologiya i Poleznye Iskopaemye, No. 2, 2005, pp. 143–158.Original Russian Text Copyright © 2005 by Lavrushin, Dubinina, Avdeenko.  相似文献   

10.
Sea water basalt interaction in spilites from the Iberian Pyrite Belt   总被引：2，自引：0，他引：2  
Low grade hydrothermally metamorphosed mafic rocks from the Iberian Pyrite Belt are enriched in 18O relative to the oxygen isotopic ratio of fresh basalt (+6.5±1). The observed 18O whole rock values range from +0.87 to +15.71 corresponding to positive isotopic shifts of +5 to +10, thus requiring isotopic exchange with fluids under conditions of high water:rock ratios at low temperatures. The lowest 18O observed corresponds to an albitized dolerite still and is compatible with independent geochemical data suggesting lower water: rock ratios for the alteration of these rocks.The isotope data are consistent with the hypothesis that the spilites from the Pyrite Belt were produced by interaction of basaltic material with sea water.Significant leaching of transition metals from the mafic rocks during alteration coupled with available sulphur isotopic data for the sulphide ores also suggest that sea water may have played an important role in the formation of ore deposits in the Iberian Pyrite Belt.  相似文献   

11.
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   

12.
The S-isotopic compositions of sulfide deposits from Steinmann, granitoid and felsic volcanic associations have been examined. Ores of Steinmann association have 34S values close to zero per mil (34S=+0.3±3.1) it appears they are of mantle origin. Isotopically, ores of granitoid association regularly show a variable enrichment in 32S relative to meteoritic (34S=–2.7±3.3). The composition is in accord with an upper mantle/lower crustal source. Two stratiform accumulations of felsic volcanic association show a narrow spread of 34S values (+0.2 to 2.4); a mantle origin for the sulfur in these deposits is favored. In contrast, vein, stockwork and cement ores are moderately enriched in 32S relative to meteoritic (34S=–4.0±6.4). These ores are polygenetic; sulfur and metals appear to have been leached from local country rocks where volcanogenic and biogenic sulfur predominate.  相似文献   

13.
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   

14.
Summary A set of 354 sulphur isotope data from the Bleiberg deposit, the type deposit of Alpine low temperature carbonate hosted Pb–Zn deposits (APT deposits), is critically evaluated applying statistical methods. The sulphur isotope patterns vary significantly among the ore horizons. This suggests a long lasting and polyphase mineralisation system. The sulphur isotope composition of barite corresponds to that of Carnian seawater (i.e. 16 34S). The 34S values of the iron sulphides correspond to data from sedimentary iron sulphides. Pb and Zn sulphides are characterized by three normally distributed 34S populations with mean values of –6 to –8, –13 to –18, and –25 to –29. Heavy sulphur (>–10 34S) indicates contribution of sulphide sulphur from epigenetic-hydrothermal fluids, whereas light sulphide sulphur (<–21 34S) was produced by sulphur – reducing bacteria. The intermediate population is explained by mixing of sulphur derived from these two sources. Other sources of local importance, however, can not be excluded. The isotope populations correspond only partly to the paragenetic ore stages. The sulphur isotope patterns in the APT deposits are regionally different. Data from other low-temperature sediment-hosted Pb–Zn deposits support the proposed interpretation. A comparison demonstrates that the sulphur isotope patterns of APT deposits correspond to patterns of the Irish type deposits, but are different to those of Mississippi Valley type deposits.Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1007/s00710-004-0071-3  相似文献   

15.
Isotopic compositions of carbon and oxygen are studied in different (rhodochrosite, calcareous-rhodochrosite, and chlorite–rhodochrosite) types of manganese carbonate ores from the Usa deposit (Kuznetskii Alatau). The 13C value varies from –18.4 to –0.7, while the 18O value ranges between 18.4 and 23.0. Host rocks are characterized by higher values of 13C (–1.9 to 1.0) and 18O (21.2 to 24.3). The obtained isotope data suggest an active participation of oxidized organic carbon in the formation of manganese carbonates. Manganese carbonate ores of the deposit are probably related to metasomatic processes.  相似文献   

16.
Sulfur isotope ratios have been determined in 27 selected volcanic rocks from Iceland together with their whole rock chemistry. The 34S of analyzed basalts ranges from –2.0 to +0.4 with an average value of –0.8 Tholeiitic and alkaline rocks exhibit little difference in 34S values but the intermediate and acid rocks analyzed have higher 34S values up to +4.2 It is suggested that the overall variation in sulfur isotope composition of the basalts is caused by degassing. The small range of the 34S values and its similarity to other oceanic and continental basalts, suggest that the depleted mantle is homogeneous in its sulfur isotope composition. The 34S of the depleted mantle is estimated to be within the range for undegassed oceanic basalts, –0.5 to +1.0  相似文献   

17.
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   

18.
The Ordovician volcano-sedimentary succession of Erquy (northern Brittany) is made of immature sediments thermally metamorphosed at the contact of intruding basic sills. Pillow lavas constitute the upper part of the sequence. The trace element geochemistry of sills and pillow lavas suggests that they were derived from a tholeiitic source located beneath a passive margin. This volcanic sequence was metamorphosed under low to moderate greenschist facies conditions. In this study the direction and amplitude of chemical and isotopic fluxes in the basalt-sediment-water system were established and the oxygen and hydrogen isotope compositions of the aqueous fluid that reacted with the volcanic rocks were characterized. Cationic thermometry on chlorites and isotopic thermometry on plagioclase-chlorite pairs indicate closure metamorphic temperatures in the range 200–250°C for the basaltic sills. Stable isotope compositions of iron-rich chlorites (18O-5.5; D from-60 to-50) and plagioclases (18O from +9 to +10) reveal that the source of the fluid was certainly seawater. The 18O variations within the sills are strongly correlated with the rate of progress of the main metamorphic reaction:clinopyroxene+plagioclase+ilmenite chlorite+albite+epidote+quartz+sphene that produced major element mobility at the scale of the volcanosedimentary sequence. Calculation of elemental fluxes by mass balance combined with oxygen isotopic compositions of basalts shows that the highest water-rock ratios (1) were at sill-sediment boundaries and within pillow lavas at the top of the pile. The volcanosedimentary sequence of Erquy was a net sink for Na and a source for Ca. No Mg uptake could be detected whereas the hydrothermal alteration of the sediments released Fe, Si, and K trapped by the volcanic rocks. The 18O value of the fluid reacting with sills appears to have shifted no more than +4 after percolation at low temperature through immature sediments (18O12). The Erquy volcano-sedimentary sequence represents a marine hydrothermal system dominated by low-temperature exchange which allowed a general 18O-enrichment of the volcanic rocks and a 18O-depletion of sediments.  相似文献   

19.
The Changba Pb-Zn SEDEX deposit occurs in the Middle Devonian sequence of the Anjiaca Formation of the Western Qinling Hercynian Orogen in the Gansu Province, China. The Changba-II orebody is hosted in biotite quartz schist and is the largest of 143 stratiform orebodies that are hosted either in biotite quartz schist or marble. The Changba-II comprises two types of mineralization: a bedded facies and an underlying breccia lens. The bedded section exhibits three sulfide sub-facies zoned from bottom to top: 1) banded sphalerite intercalated with quartz albitite; 2) interbedded massive pyrite and sphalerite ore; and 3) banded sphalerite ore intercalated with banded baritite. Major metallic minerals are sphalerite, pyrite, galena, with minor arsenopyrite, pyrrhotite, boulangerite, and rare chalcopyrite. The bedded sulfides are underlain by a lens of brecciated and albitized biotite-quartz schists cemented by sulfides and tourmaline.Massive and bedded sulfide 34S values range from 8.1 to 29.3, whereas barite 34S values range from 20.8 to 31.5. Disseminated pyrite in footwall schists has 34S values ranging from 8.1 to 10.6, and increase to values ranging from 11.1 to 14.7 in the hangingwall. The lower 34S values for massive and bedded sulfides are interpreted to be derived from progressive bacterial sulfate reduction (BSR) of Devonian seawater in a sulfate-restricted sub-basin. The higher 34S values for massive and bedded sulfides could be a product of quantitative BSR but this is incompatible with barite being more abundant above the bedded sulfides. Instead, it is more likely that thermochemical sulfate reduction of seawater sulfate or of evaporite was the source of heavy hydrothermal sulfur. Heavy hydrothermal sulfur was injected into a sulfate-restricted sub-basin where it mixed with low 34S BSR sulfide to form the massive and bedded sulfides. The REE patterns of sulfide layers and associated quartz albitite and baritite are similar to those of the host biotite quartz schists, suggesting that the hydrothermal fluids leached REE from the underlying rocks. Pb isotope ratios in galena form an array between the Upper Crust and the Mantle reservoir curves, which indicates that the lead is derived from upper crustal rocks comprising mafic igneous units. The Sr87/Sr86 ratio of 0.7101 for carbonate within the sulfide layers also suggests that Sr is derived from the mixing of Sr leached from upper crustal rocks with Middle Devonian seawater Sr. A Rb-Sr isochron age of 389.4 ± 6.4 Ma for sulfide layers and the interbedded hydrothermal sediments is consistent with the age of host Mid-Devonian strata. Ar39/Ar40 plateau age at 352.8 ± 3.5 Ma and Ar39-Ar40 isochron age of 346.6 ± 6.4 Ma for albite in the quartz albitite intercalated with sulfide layers indicate either albite formation after the sulfides or thermal resetting of the Rb-Sr system at about 350 Ma, the age of collision between the North China and Yangtze cratons.Editorial handling: E. Frimmel  相似文献   

20.
Ion microprobe analysis of magnetites from the Adirondack Mountains, NY, yields oxygen isotope ratios with spatial resolution of 2–8 m and precision in the range of 1 (1 sigma). These analyses represent 11 orders of magnitude reduction in sample size compared to conventional analyses on this material and they are the first report of routinely reproducible precision in the 1 per mil range for analysis of 18O at this scale. High precision micro-analyses of this sort will permit wide-ranging new applications in stable isotope geochemistry. The analyzed magnetites form nearly spherical grains in a calcite matrix with diopside and monticellite. Textures are characteristic of granulite facies marbles and show no evidence for retrograde recrystallization of magnetite. Magnetites are near to Fe3O4 in composition, and optically and chemically homogeneous. A combination of ion probe plus conventional BrF5 analysis shows that individual grains are homogeneous with 18O=8.9±1 SMOW from the core to near the rim of 0.1–1.2 mm diameter grains. Depth profiling into crystal growth faces of magnetites shows that rims are 9 depleted in 18O. These low 18O values increase in smooth gradients across the outer 10 m of magnetite rims in contact with calcite. These are the sharpest intracrystalline gradients measured to date in geological materials. This discovery is confirmed by bulk analysis of 150–350 m diameter magnetites which average 1.2 lower in 18O than coarse magnetites due to low 18O rims. Conventional analysis of coexisting calcite yields °18O=18.19, suggesting that bulk 18O (Cc-Mt)=9.3 and yielding an apparent equilibration temperature of 525° C, over 200° C below the temperature of regional metamorphism. Consideration of experimental diffusion data and grain size distribution for magnetite and calcite suggests two contrasting cooling histories. The data for oxygen in calcite under hydrothermal conditions at high P(H2O) indicates that diffusion is faster in magnetite and modelling of the low 18O rims on magnetite would suggest that the Adirondacks experienced slow cooling after Grenville metamorphism, followed by a brief period of rapid cooling, possibly related to uplift. Conversely, the data for calcite at low P(H2O) show slower oxygen diffusion than in magnetite. Modelling based on these data is consistent with geochronology that shows slow cooling through the blocking temperature of both minerals, suggesting that the low 18O rims form by exchange with late, low temperature fluids similar to those that infiltrated the rock to serpentinize monticellite and which infiltrated adjacent anorthosite to form late calcite veinlets. In either case, the ion microprobe results indicate that two distinct events are recorded in the post-metamorphic exchange history of these magnetites. Recognition of these events is only possible through microanalysis and has important implications for geothermometry.  相似文献   

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