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 15 毫秒
1.
Oxygen isotope ratios, whole rock major and trace element compositions, and petrological characteristics of 52 samples from nine distinct igneous lithologies in the lower plate of the Whipple Mountain metamorphic core complex of south-eastern California indicate that both mylonitic and non-mylonitic lithologies underwent exchange with surface-derived meteoric waters. Broadly granodioritic lithologies are characterized by whole rock δ18O values that range from 10.6 to 2.6‰. Isotopic compositions of quartz and feldspar mineral separates indicate that quartz has largely retained original igneous compositions but that feldspar has undergone variable and often large 18O-depletions (up to 6.5‰). Over 4 km of structural relief is exposed in lower plate gneisses below the Whipple detachment fault including non-mylonitic lithologies at shallow structural levels above the mylonite front, and mylonitic gneisses at intermediate to deep levels below the mylonite front. Coupled δ18Oqtz - δ18OFsp systematics of non-mylonitic and mylonitic andesite to rhyolite dykes from shallow and intermediate structural levels of the lower plate document two episodes of hydrothermal alteration: a high-temperature (>c.600d?C) episode involving a metamorphic or magmatic fluid with δ18O values ～ 7‰ and a low-temperature (c.350d?C) episode involving low-δ18O meteoric fluids. All the dykes that document exchange with meteoric fluids are non-mylonitic. Coupled δ18OFsp systematics of non-mylonitic and mylonitic granodioritic gneisses from above and below the mylonite front also document low-temperature (c. 350d? C) exchange with meteoric fluids. The data indicate that infiltration of meteoric fluids occurred as lower plate lithologies were juxtaposed against the base of the faulted upper plate. High-angle normal faults in the upper plate served as the conduits for the downward circulation of surface-derived fluids. Meteoric fluids were able to penetrate across the detachment fault into the lower plate. Uplift rates coupled with independent cooling rates indicate that surface-derived fluids penetrated to a depth of c.4km and possibly as deep as c.8km. Penetration of surface-derived fluid into the ductile deformation regime is not required to explain the low δ18O values observed in lower plate lithologies of the Whipple Mountain metamorphic core complex.  相似文献   

2.
S. Viswanathan 《Lithos》1974,7(1):29-34
Oxygen isotope studies of granitic rocks from the 2.7 b.y.-old composite Giants Range batholith show that: (1) δ(O18)quartz values of 9 to 10 permil characterize relatively uncontaminated Lower Precambrian, magmatic granodiorites and granites; (2) granitic rocks thought to have formed by static granitization have δ(O18)quartz values that are 1 to 2 permil higher than magmatic granitic rocks; (3) satellite leucogranite bodies have values nearly identical to those of the main intrusive phases even where they transect O18-rich metasedimentary wall rocks; (4) oxygen isotopic interaction between the granitic melts and their O18-rich wall rocks was minimal; and (5) O18/O18 ratios of quartz grains in a metasomatic granite are largely inherited from the precursor rock, but during the progression — sedimentary parent → partially granitized parent → metasomatic granite → there is gradual decrease in δ(O18)quartz by 1 to 2 permil.  相似文献   

3.
Oxygen isotope analyses of five olivines from the Darrington peridotite, Washington, yield δO18 values of +7.3 to +8.9%. which are consistent with derivation of these rocks from a serpentinite precursor. The isotopic data are compatible with mineralogical, textural and chemical evidence that most of the Darrington peridotites have formed by deserpentinization. Olivine from a single, petro-graphically distinct peridotite sample has a δO18-value of +5.2%. which is within the field of high-temperature olivines. The isotopic and textural evidence indicate that this is a partially recrystallized peridotite tectonite.Oxygen and carbon isotope analyses of dolomites from olivine-carbonate rocks indicate that they could have originated by introduction of atmospheric CO2 via meteoric waters during the formation of ophidolomites or ophicalcites. Subsequent metamorphism and reequilibration have modified the δO18-values.  相似文献   

4.
The Middle Muschelkalk (Middle Triassic) of the Catalan Coastal Range (north-east Spain) comprises sandstone, mudstone, anhydrite and minor carbonate layers. Interbedded sandstones and mudstones which are dominant in the north-eastern parts of the basin are terminal alluvial fan deposits. South-westward in the basin, the rocks become dominated by interbedded evaporites and mudstones deposited in sabkha/mudflat environments. The diagenetic and pore water evolution patterns of the Middle Muschelkalk suggest a strong facies control. During eodiagenesis, formation of microdolomite, anhydrite, baryte, magnesite, K-feldspar and mixed-layer chlorite/smectite was favoured within and adjacent to the sabkha/mudflat facies, whereas calcite, haematite, mixed-layer illite/smectite and quartz formed mainly in the alluvial facies. Low δ18OSMOW values for microdolomite (+23.7 to +28.4%) and K-feldspar overgrowths (+17.3 to +17.7%) suggest either low-temperature, isotopic disequilibrium or precipitation from low-18O porewaters. Low-18O waters might have developed, at least in part, during low-temperature alteration of volcanic rock fragments. During mesodiagenesis, precipitation of quartz overgrowths and coarse dolomite occurred in the alluvial sandstones, whereas recrystallization of microdolomite was dominant in the sabkha/mudflat facies. The isotopic compositions of these mesogenetic phases reflect increasing temperature during burial. Upon uplift and erosion, telogenetic calcite and trace haematite precipitated in fractures and replaced dolomite. The isotopic composition of the calcite (δ18OSMOW=+21.5 to +25.6%o; δ13C= 7.7 to - 5.6%o) and presence of haematite indicate infiltration of meteoric waters.  相似文献   

5.
Polymetallic vein-type Zn-Pb deposits are located in the Xiangxi–Qiandong zinc-lead metallogenic belt (XQMB) of the northwestern margin of the Jiangnan Orogen, South China. Ores are mainly found in fault-bounded quartz veins hosted in the upper part of the Banxi Group that consists of low-grade metamorphic sandstone, siltstone with minor tuff interbeds. The Zn-Pb deposits primarily contain sphalerite, galena, chalcopyrite and pyrite, accompanied by quartz and minor calcite. Zinc, lead, copper, indium and gallium are enriched in these ores. Investigation of the ore fluid reveals low temperature (87–262 °C) with scattered salinity (range from 2.73 to 26.64 wt% NaCleqv.). Hydrogen and oxygen isotopic compositions of fluid inclusions in quartz indicate mixing of magmatic hydrothermal fluid and meteoric water (δ18OH2O SMOW = 0.2‰ to 4.2‰; δDH2O SMOW = −126‰ to −80‰). Carbon and oxygen isotopic composition of carbonate samples indicate the magmatic hydrothermal origin of CO32− or CO2 in ore-forming fluid (δ13CPDB = −6.9‰ to −5.7‰, δ18OSMOW = 11.3‰ to 12.7‰). Sulfur and lead isotopic compositions (δ34SVCDT = 8.8–14.2‰ and 206Pb/204Pb = 17.156–17.209, 207Pb/204Pb = 15.532–15.508, 208Pb/204Pb = 37.282–37.546) demonstrate that sulfur sources were relatively uniform, and low radiogenic lead isotopic compositions indicate that ore metals were derived from a relatively unradiogenic source, probably by mixing of mantle with crust. Therefore, polymetallic vein-type Zn-Pb mineralization in this area probably arose from a magmatic-related hydrothermal system, and the deposition of sulfides occurred in response to cooling and boiling of magmatic hydrothermal fluids (high salinity, high δ18OH2O and δDH2O and metal-bearing), and is mainly the result of emplacement into open space and mixing with meteoric water (low salinity, low δ18OH2O and δDH2O). This study provides direct evidence that magmatism was involved in the ore-forming processes of the low temperature metallogenic district, South China, and it raises awareness about the presence of polymetallic vein-type Zn-Pb deposits in the northwest margin of Jiangnan Orogen and their potential as a source of zinc, copper, indium and gallium.  相似文献   

6.
The oxygen isotope compositions of diagenetic carbonate minerals from the Lower Jurassic Inmar Formation, southern Israel, have been used to identify porewater types during diagenesis. Changes in porewater composition can be related to major geological events within southern Israel. In particular, saline brines played an important role in late (Pliocene-Pleistocene) dolomitization of these rocks. Diagenetic carbonates included early siderite (δ18OSMOW=+24.4 to +26.5‰δ13CPDB=?1.1 to +0.8‰), late dolomite, ferroan dolomite and ankerite (δ18OSMOW=+18.4 to +25.8‰; δ13CPDB=?2.1 to +0.2‰), and calcite (δ18OSMOW=+21.3 to +32.6‰; δ13CPDB=?4.2 to + 3.2‰). The petrographic and isotopic results suggest that siderite formed early in the diagenetic history at shallow depths. The dolomitic phases formed at greater depths late in diagenesis. Crystallization of secondary calcite spans early to late diagenesis, consistent with its large range in isotopic values. A strong negative correlation exists between burial depth (temperature) and the oxygen isotopic compositions of the dolomitic cements. In addition, the δ18O values of the dolomitic phases in the northern Negev and Judea Mountains are in isotopic equilibrium with present formation waters. This behaviour suggests that formation of secondary dolomite post-dates the tectonic activity responsible for the present relief of southern Israel (Upper Miocene to Pliocene) and that the dolomite crystallized from present formation waters. Such is not the case in the Central Negev. In that locality, present formation waters have much lower salinities and δ18O values, indicating invasion of freshwater, and are out of isotopic equilibrium with secondary dolomite. Recharge of the Inmar Formation by meteoric water in the Central Negev occurred in the Pleistocene, and halted formation of dolomite.  相似文献   

7.
The highest grade pelitic and semipelitic rocks of the Ballachulish aureole are dominantly potash feldspar + cordierite + biotite hornfelses with widely variable amounts of quartz, plagioclase, andalusite, sillimanite and corundum (together with accessory phases). On a microscopic scale these hornfelses show textural evidence of the presence of melt, whilst on a mesoscopic scale they contain a variety of leucosomes. Oxygen isotope studies have been carried out on both whole rocks and mineral separates in order to: (1) assess the sources of molten and volatile constituents and (2) determine the extents of isotopic homogenization and equilibration. Data from localities with both restricted and extensive evidence of leucosomes and melt development are compared, as well as one locality with petrographic evidence of melt incursion from the igneous complex. The whole-rock δ18O values of the leucosomes (10.5–14.9%.) are in general similar to the immediately adjacent mesosomes (9.9–14.5%.) which are typically cordierite- and feldspar-rich hornfelses. Isotopic evidence is thus consistent with an in-situ partial melt origin for the leucosomes, without the substantial addition of externally derived components. In the area of extensive melt development, the ‘chaotic zone’, it is possible there was addition of an H2O-rich fluid phase (6-13 wt%) from the igneous complex which resulted in a slight lowering of δ18O values by 0.5–1.0%. Quartz mineral separates were used to assess the degree of local isotopic homogenization. In the extensively molten area (chaotic zone) there is extensive homogenization between rock layers (quartz δ18O usually within 1.0%), whilst in less molten areas δ18O quartz has a range of c. 3.0%. The greater homogenization in the chaotic zone is attributed to the increased degree of melting and infiltration of H2O-rich fluid from the igneous complex.  相似文献   

8.
《Applied Geochemistry》1994,9(6):609-626
The Saint-Salvy vein-hosted Zn (+Ge) deposit occurs in an E–W fault system which flanks the southern margin of the late Variscan Sidobre granite, and cross-cuts Cambrian black shales of the Palaeozoic basement. Comprehensive mineralogical and geochemical studies of vein samples have revealed four mineralizing events (M1–M4) related to late and post-Variscan tectonic events. A further late-stage event may be related to weathering.M1 (=skarn deposits) and M2 (=patchily mineralized quartz veinlets) are associated with granite emplacement. Quartz contains low salinity, H2OCO2(NaCl)-dominated fluids(⩽6wt% NaCl equiv.) of relatively high temperature (300–580°C), trapped under moderate to high pressure. Estimated M1 fluid δD and calculated fluidδ18O plot within the metamorphic water field. There appears to be no involvement of magmatic fluids.By contrast, M3 (= barren quartz) and M4 (= zinciferous economic mineralization) stages have H2OCO2NaClCaCl2 fluid inclusions with high salinities (23–25 wt% NaCl equiv.) and low temperatures(∼ 80–140°C), which were trapped under low-pressure conditions. The high salinity and NaCl + CaCl2 content of both M3 and M4 indicates that their parent fluids leached evaporitic salts. M3 fluids are meteoric water dominated, falling close to the meteoric water line (δD andδ18O averaging −64 and −8‰, respectively). M4 fluids have highly distinctive δD averaging −101‰, and calculated fluidδ18O varying from−1.2to+7.1‰. The unusually low δD composition of M4 suggests the involvement of “organic” fluids, in which H is derived directly or indirectly from organic matter. The relatively highδ18O of M4 fluids indicates that considerable isotopic exchange with sedimentary material took place, displacing theδ18O from the meteoric water line. The data imply interaction of meteoric waters with evaporite and hydrocarbon-bearing sedimentary sequences, most probably the adjacent Aquitain Basin.The main economic mineralization (M4 stage) took place during a tensional event, probably coincident with the Lias-Dogger transition.Calculatedδ34SH2S of M4 sulphide(+5.4to+8.2‰) is almost identical toδ34S of local Cambrian sulphides(+4.7to+9.4‰) suggesting a genetic link. Abundant siderite associated with M4 sphalerite hasδ13C ranging from−2.6to−4.4‰ indicating that carbon was sourced from sedimentary carbonate mobilized by, or equilibrated with the hydrothermal fluid.Late-stage sulphides exhibit extraordinary and highly distinctiveδ34S. Sphalerite has extremely low δ34S(−42.5to−50.5‰), whereas pyrite has an extraordinary large range from−33.2‰to+74.3‰. Closed system sulphate reduction is held to be responsible for the extremely highδ34S: whereas more open system reduction produces the very low values. The coincidence of isotopically lowδ13C(−7.6to−11.9‰) for co-genetic calcite suggests the involvement of organic matter in the reduction process.  相似文献   

9.
The Bairendaba vein-type Ag–Pb–Zn deposit, hosted in a Carboniferous quartz diorite, is one of the largest polymetallic deposits in the southern Great Xing'an Range. Reserves exceeding 8000 tonnes of Ag and 3 million tonnes of Pb?+?Zn with grades of 30 g/t and 4.5% have been estimated. We identify three distinct mineralization stages in this deposit: a barren pre-ore stage (stage 1), a main-ore stage with economic Ag–Pb–Zn mineralization (stage 2), and a post-ore stage with barren mineralization (stage 3). Stage 1 is characterized by abundant arsenopyrite?+?quartz and minor pyrite. Stage 2 is represented by abundant Fe–Zn–Pb–Ag sulphides and is further subdivided into three substages comprising the calcite–polymetallic sulphide stage (substage 1), the fluorite–polymetallic sulphide stage (substage 2), and the quartz–polymetallic sulphide stage (substage 3). Stage 3 involves an assemblage dominated by calcite with variable pyrite, galena, quartz, fluorite, illite, and chlorite. Fluid inclusion analysis and mineral thermometry indicate that the three stages of mineralization were formed at temperatures of 320–350°C, 200–340°C, and 180–240°C, respectively. Stage 1 early mineralization is characterized by low-salinity fluids (5.86–8.81 wt.% NaCl equiv.) with an isotopic signature of magmatic origin (δ18Ofluid = 10.45–10.65‰). The main ore minerals of stage 2 precipitated from aqueous–carbonic fluids (4.34–8.81 wt.% NaCl equiv.). The calculated and measured oxygen and hydrogen isotopic compositions of the ore-forming aqueous fluids (δ18Ofluid = 3.31–8.59‰, δDfluid?=??132.00‰ to??104.00‰) indicate that they were derived from a magmatic source and mixed with meteoric water. Measured and calculated sulphur isotope compositions of hydrothermal fluids (δ34S∑S?=??1.2–3.8‰) indicate that the ore sulphur was derived mainly from a magmatic source. The calculated carbon isotope compositions of hydrothermal fluids (δ13Cfluid?=??26.52‰ to??25.82‰) suggest a possible contribution of carbon sourced from the basement gneisses. The stage 3 late mineralization is dominated (1.40–8.81 wt.% NaCl equiv.) by aqueous fluids. The fluids show lower δ18Ofluid (?16.06‰ to??0.70‰) and higher δDfluid (?90.10‰ to??74.50‰) values, indicating a heated meteoric water signature. The calculated carbon isotope compositions (δ13Cfluid?=??12.82‰ to??6.62‰) of the hydrothermal fluids in stage 3 also suggest a possible contribution of gneiss-sourced carbon. The isotopic compositions and fluid chemistry indicate that the ore mineralization in the Bairendaba deposit was related to Early Cretaceous magmatism.  相似文献   

10.
The volcano-sedimentary sequence at the Raul mine, central Peru, consists of andesitic volcanics, graywackes, and siltstones, and has been metamorphosed to the upper greenschist-lower amphibolite facies at temperatures of 400–500°C. Isotopic data (O and H) have been collected from: (a) quartz and magnetite from stratiform ores, (b) amphiboles from amphibolite units that host stratiform ores, (c) calcite from late veins, (d) detrital quartz from graywackes, and (e) whole rocks.Interunit differences in quartz and magnetite δ18O values suggest that these minerals have resisted isotopic exchange during metamorphism, and that quartz-magnetite isotopic temperatures (380–414°C) represent primary formational temperatures. Calculated δ18O values of water in equilibrium with quartz and magnetite range from 9.1 to 12.6%..Amphibole δ18O and δD values show no interunit differences and suggest that the amphiboles have exchanged isotopes with a large metamorphic fluid reservoir. Calculated δ18OH2O and δDH2O values range from 8 to 12%. and ?3 to +42%., respectively.δ18OH2O values calculated from δ18O calcite and fluid inclusion filling temperatures range from 7.5 to 10%.. Water extracted from fluid inclusions in calcite has a δD value of ?20%..δ18O values of metamorphosed graywackes and volcanic sediments are not atypical, but andesitic lavas are 18O-rich (8–10%.) compared to normal andesites.Waters involved in ore deposition, metamorphism, and late vein formation at Raul are all thought to have a common source, principally seawater. The δ18OH2O and δDH2O values could be produced by evaporation of seawater, shale ultrafiltration, and isotopic exchange with host rocks during deep circulation through the volcano-sedimentary pile.A model is proposed whereby coastal ocean water is restricted from the open sea by volcanic island arcs, and subsequently undergoes evaporation. Circulation of this water is initiated by heat associated with seafloor volcanism. 18O-enrichment in andesites may be produced by isotopic exchange with high 18O waters at elevated temperatures and sufficiently high water/rock ratios.  相似文献   

11.
Oxygen isotope compositions of biogenic phosphates from mammals are widely used as proxies of the isotopic compositions of meteoric waters that are roughly linearly related to the air temperature at high- and mid-latitudes. An oxygen isotope fractionation equation was determined by using present-day European arvicoline (rodents) tooth phosphate: δ18Op = 20.98(±0.59) + 0.572(±0.065) δ18Ow. This fractionation equation was applied to the Late Pleistocene karstic sequence of Gigny, French Jura. Comparison between the oxygen isotope compositions of arvicoline tooth phosphate and Greenland ice core records suggests to reconsider the previously established hypothetical chronology of the sequence. According to the δ18O value of meteoric water-mean air temperature relationships, the δ18O value of arvicoline teeth records variations in mean air temperatures that range from 0° to 15°C.  相似文献   

12.
The Lower Permian Aldebaran Sandstone is the principal hydrocarbon reservoir in the Denison Trough (Bowen Basin), east-central Queensland, Australia. It accumulated in a wide range of fluvio-deltaic and nearshore marine environments. Detailed petrological study of the unit by thin section, X-ray diffraction, scanning electron microscopy, electron microprobe and isotopic analysis reveals a complex diagenetic history which can be directly related to depositional environment, initial composition and burial-temperature history. Early diagenetic effects included the precipitation of pyrite, siderite and illite-smectite rims (δ18O (SMOW) =+8.9 to + 11.3‰). Deep burial effects included physico-chemical compaction and the formation of quartz overgrowths, siderite (δ13C(PDB) =?34.0 to + 11.5‰, δ18O =?0.7 to +22.7‰), illite/illite-smectite and ankerite (δ13C=?9.3 to ?4.9‰) δ18O=+ 7.6 to + 14.4‰). Involved fluids were in part ‘connate meteoric’ water derived from compaction of the underlying freshwater Reids Dome beds. Important post-maximum burial effects, controlled by deep meteoric influx from the surface, were ankerite and labile grain dissolution and formation of kaolinite (δ18O=+7.8 to +8.9‰, δD=?115 to ?99‰), calcite (δ13C=?9.5 to +0.9‰, δ18O=+9.0 to +20.0‰) and dawsonite (δ13C=?4.0 to +2.3‰, δ18O=+9.8 to +19.8‰), the formation of dawsonite reflecting eventual stagnation of the aquifer. Entrapment of contained hydrocarbons was a relatively recent event which may be continuing today. Reservoir quality varies from marginal to good in the west to poor in the east, with predictable trends being directly linked to depositional environment and diagenesis.  相似文献   

13.
Chert nodules of the Drunka Formation (Lower Eocene) are mostly spherical, have diameters from 40 to 120 cm, are quasi-uniformly spaced 2–3 m apart in the plane of bedding, have concentric internal structure and, except for rare small (<6 cm) solid chert nodules, are less than 85% chertified. Nodules formed after moderate alteration of limestone by meteoric water (δ18Ocalcite = –4 to –8‰) at shallow (<100 m) burial depths; more extensive alteration of limestone (δ18O = –10 to –16‰) by meteoric water followed nodule growth. Chertification was by low-temperature meteoric water (δ18Oquartz = +18‰ in margins to +22‰ in nuclei) at shallow burial depths. Meteoric water may have invaded the Drunka Formation in association with shelf progradation during the Early Eocene, or during the development of a Middle Eocene unconformity. Replacement of carbonate mud by microcrystalline quartz was the dominant chertification process, but fossils were replaced in part by fine-grained equant megaquartz, quartzine and chalcedony; the last of these occurs in places as beekite. Opal A-secreting marine organisms are the inferred source of silica, but none are preserved. There is no compelling evidence of an opal-CT precursor, so quartz may have formed by direct precipitation. Self-organization processes of enigmatic character established the spacing pattern of the nodules and also the Liesegang-banded internal structure of the chert nodules. Nodules grew chiefly by diffusive supply of silica, although one locality has elongate nodules that grew when there was some porewater advection. Chertification patterns and δ18O values of both calcite and quartz indicate that nodule growth was complex and variable. Some nodules probably grew from the centre outwards. Many nodules, however, initially grew simultaneously across the entire nodule, but late-stage growth was predominantly at the outer margins or at selective internal sites.  相似文献   

14.
The oxygen isotope geochemistry of igneous rocks   总被引：34，自引：0，他引：34  
Oxygen isotope analyses have been obtained for 443 igneous rock and mineral samples from various localities throughout the world. Detailed studies were made on the Medicine Lake, Newberry, Lassen, Clear Lake, S. E. Guatemala, Hawaii and Easter I. volcanic complexes and on the Bushveld, Muskox, Kiglapait, Guadalupe, Duluth, Nain, Egersund, Lac St. Jean, Laramie, Skaergaard, Mull, Skye, Ardnamurchan and Alta, Utah plutonic complexes, as well as upon several of the zoned ultramafic intrusions of S. E. Alaska. Basalts, gabbros, syenites and andesites are very uniform in O18/O16, commonly with δ-values of 5.5 to 7.0 per mil. Many rhyolite obsidians, particularly those from oceanic areas and the Pacific Coast of the United States, also lie in this range; this indicates that such obsidians are differentiates of basaltic or andesitic magma at high temperatures (about 1,000° C). They cannot represent melted sialic crust. The only plutonic granites with such low δ-values are some of the hypersolvus variety, suggesting that these also might form by fractional crystallization. Obsidians from the continental interior, east of the quartz-diorite line, have higher δ-values. This is compatible with their having assimilated O18-rich sialic crust. A correlation generally exists between the O18/O16 ratios of SiO2-rich differentiates and the chemical trends in volcanic complexes. High O18/O16 ratios accompany those trends having the lower Fe/Mg ratios, while ferrogabbro trends are associated with depletion in O18. Variations in oxygen fugacity may be responsible for these effects, as abundant early precipitation of magnetite should lead to both O18-enrichment and Fe-depletion in later differentiates. Plutonic granites have higher O18/O16 ratios than their volcanic equivalents, because (a) their differentiation occurred at much lower temperatures, or (b) they are in large part derived from O18-rich sialic crust by partial melting or assimilation. Also, the oxygen isotope fractionations among coexisting minerals are distinctly larger in plutonic rocks than in volcanic rocks. This is in keeping with their lower crystallization temperatures and their longer cooling history, which promotes post-crystallization oxygen isotope exchange. Hydrated obsidians and perlites have δO18-values that are much different from their primary, magmatic values. A correlation exists between D/H and O18/O16 ratios in hydrated volcanic glass from the western U.S.A., proving that the isotopic compositions are a result of exchange with meteoric waters. The O18 contents of the glasses appear to be about 25 per mil higher than their associated waters; hence, these hydrated glasses have not simply absorbed H2O, but they have exchanged with large quantities of it. The igneous rocks from Mull, Skye, Ardnamurchan and the Skaergaard intrusion are all abnormally depleted in O18 relative to “normal” igneous rocks. This is a result of their having exchanged at high temperatures with meteoric water that was apparently abundant in the highly jointed plateau lavas into which these igneous rocks were intruded. In part, this exchange occurred with liquid magma and in part with the crystalline rock; in the latter case the feldspar was more easily exchanged and has become much more depleted in O18 than has coexisting quartz or pyroxene. The later differentiates of the Muskox intrusion are markedly O18-rich, but this is not a result of fractional crystallization. It is in large part a result of deuteric exchange between feldspars and an oxygen-bearing fluid (H2O ?) that was either O18-rich or had a relatively low temperature. This phenomenon was also observed in a number of granophyres from other localities, particularly those containing brick-red alkali feldspar. The exchanged feldspars in all these examples are turbid or cloudy, and may be filled with hematite dust. It is concluded that most such feldspar in nature is the result of deuteric exchange and is probably drastically out of oxygen isotopic equilibrium with its coexisting quartz.  相似文献   

15.
碾子山晶洞碱性花岗岩矿物-水氧同位素交换反应动力学   总被引：4，自引：0，他引：4  
对黑龙江碾子山碱性花岗岩的全岩及其主要单矿物进行了氧同位素分析，结果表明，全岩和单矿物不仅δ^18O 值变化范围较大（全岩－2．4－2．0‰，石英0．0－5．8‰，碱性长石－3．8－0．1‰，磁铁矿－8．5－1．0‰），而且强烈亏损^18O。共生矿物之间表现出明显不平衡的氧同位素分馏特征，指示在花岗岩侵位之后与水之间发生了同位素交换，根据锆石和现代大气降水的氧同位素组成，对岩石与外来流体的δ^18O值进行了估计，多维矿水－岩反应时限约为0．3－3Ma，水／岩比（氧摩尔比）介于0．11－1．02之间。水－岩反应温度较高（约400度）和反应时间较长是导致石英δ^18O值降低的主要原因。  相似文献   

16.
δD and δ18O values have been determined for fluid inclusions in 45 samples of Permian halite. The inclusions are enriched in 18O relative to the meteoric water line but are depleted in D relative to ocean water. Inclusions with the more positive δ-values coincide with the isotopic composition expected for evaporating sea water which follows a hooked trajectory on a δD-δ18O diagram. Inclusions with more negative δ-values may represent more highly evaporated sea water but probably reflect synsedimentary or diagenetic mixing of meteoric water with evaporite brines. The isotope systematics in these inclusions are sufficiently similar to those of a modern evaporite pan to indicate that Permian sea water was isotopically similar to modern sea water.Connate evaporite brines can have negative δ-values because of the probable hooked isotope trajectory of evaporating sea water and/or synsedimentary mixing of evaporite brines with meteoric waters. Subsurface formation waters composed of mixtures of remnant primary evaporite brines and later meteoric waters may be more common than previous isotopic evidence has suggested.  相似文献   

17.
Analyses of 230 Franciscan rock and mineral samples, including the San Luis Obispo ophiolite, show that metamorphism produces no change in the δ18O of the graywackes (+11 to +14), but that igneous rocks become enriched in 18O by 2–6% and the cherts depleted by 5–10%. The shales are of two types, a high-18O type (+16 to +20) associated with chert and a low-18O type isotopically and mineralogically similar to the graywackes. The vein quartz (δ = + 15 to + 20) is invariably richer in 18O than the host rock quartz and in most of the rocks the δ18O of the clastic quartz is similar to the δ18O of the whole rock. Mineral assemblages are typically not in isotopic equilibrium. Although the δ18O values are very uniform (+13 to +16). the δ13C of vein aragonite and calcite is widely variable (0 to ? 14), implying that a major source of the carbon is oxidized organic material. The δD values of 83 igneous and sedimentary rocks are -45 to -80, exceptions are the Fe-rich minerals howieite and deerite, which have δD = ?100. All of these samples could have equilibrated with H2O having δD ≈ +10 to ?20 and δ18O ≈ ?3 to +8. assuming temperatures of 100–300°C. However, the serpentines (δD ≈ ?85 to ?110) and the vein minerals (δD = ?23 to ?55) are exceptions. The vein minerals are 10–20%, richer in deuterium than the adjacent wall rocks; they formed from a relatively D-rich metamorphic water, typically at lower temperatures than did their host rocks. The isotopic compositions of the other Franciscan rocks were affected by three distinct events: (1) hydrothermal alteration of the ophiolite complexes and volcanic rocks as a result of submarine igneous activity at a spreading center or in an island-arc environment; (2) low-temperature, high-pressure regional metamorphism and diagenesis; and (3) a late-stage, very low temperature (<100°C) alteration of the ultramafic bodies by meteoric ground waters, producing lizardite-chrysotile serpentine. In the first two cases, the pore fluid involved in the alteration of the Franciscan rocks was sea water. However, this water became somewhat depleted in D and enriched in 18O during blueschist metamorphism, evolving to values of δD ≈ ? 20 and δ18O ≈ + 6 to + 8 at the highest grades. Except for one graywacke sample, the meteoric waters that affected the serpentinites did not significantly change the DH ratios of the OH-bearing minerals in any other Franciscan rock.The δ18O values of orogenic andesites are too low for such magmas to have formed by direct partial melting of Franciscan-type materials in a subduction zone. Andesites either form in some other fashion, or the melts must undergo thorough isotopic exchange with the upper mantle. The great Cordilleran granodiorite-tonalite batholiths, however, are much richer in 18O and may well have formed by large-scale melting or assimilation of Franciscan-type rocks. The range of δD values of Franciscantype rocks is identical to the ?50 to ?80 range shown by most igneous rocks. This suggests that ‘primary magmatic H2O’ throughout the world may be derived mainly by partial melting of Franciscantype materials, or by dehydration of such rocks in the deeper parts of a Benioff zone.  相似文献   

18.
Abstract Coexisting quartz, feldspar and biotite vary widely in their δ18O values and display a remarkable 18O / 16O disequilibrium relation; especially, a quartz–feldspar reversal (Δ 18OQuartz?feldspar<0) exsists in the Aral granite pluton of the Altay Mountains, northern Xinjiang. The 18O / 16O exchange reaction definitely occurred between granite and water. Initial δ18O values of the granite and exotic fluid are evaluated by the mass balance consideration. The conventional method of discrimination between various magma derivations simply with δ18O values of either whole rock or separate minerals is misleading and unreliable. Experiments carried out by the authors show that the 18O / 16O exchange reaction is not accompanied by what geologists describe as petrological and mineralogical alteration effect. This decoupling relation implies that exchange reaction occurs at a relatively high temperature during subsolidus–postmagmatic cooling of magmas. The exchange mechanism is mainly diffusion–controlled. It is demonstrated through quantitative modelling that the hydrothermal system associated with the Aral pluton is long–lived (0.8—6 Ma), with a relatively high fluid flow rate (3 x 10?14 mol. s?1) and high W/ R ratio (0.79-6.14). This means that an intense flow and convection may exist at the midcrustal level of orogenic magmatic arcs.  相似文献   

19.
The Holder Formation (Pennsylvanian, Virgilian) of southern New Mexico, USA, consists of limestones interbedded with siliciclastics. It was deposited during times of glacio-eustatic sea-level change and was exposed subaerially during multiple sea-level lowstands. Microcomponents and whole-rock samples of limestones were analysed for δ13C and δ18O values to examine the method of whole-rock isotopic analysis for detecting subaerial exposure events and to determine the diagenetic processes acting during subaerial exposure. Whole-rock isotopic shifts are not consistently present across petrographically identified subaerial exposure surfaces. Apparently, whole-rock isotopic shifts do not result from wholesale replacement of the host sediment during soil formation. However, the isotopic shifts are present in calcareous, soil-precipitated microcomponents, such as rhizoliths, laminated crusts, and soil-precipitated cements. The components are heterogeneous in isotopic composition, but converge on a meteoric calcite line at about δ18O=?5.5‰. These microcomponents are heterogeneous in distribution and may either dominate or be a minor constituent of the whole rock at a single stratigraphic horizon. Without petrographic selection of palaeosol components, the detection of whole-rock isotopic shifts may depend on the selection or chance sampling of a rock containing abundant microcomponents precipitated in a soil environment. Only minor whole-rock isotopic shifts come from those rocks bearing no evidence of exposure and bearing lithological characteristics suggesting subaerial exposure was unlikely.  相似文献   

20.
We measured the δ18O of cellulose (δ18Ocel) extracted from fossil wood collected at 9 sites in the northern and southern hemispheres as a potential source of information about precipitation δ18O (δ18Oppt) in the past and paleotemperatures. The samples had been buried in fluvial sediments for periods of time ranging from ca. 45 million to 250 years. At the oldest localities (high latitude, Eocene- through Pliocene-age sites in Canada and Russia), mean annual temperature (MAT) estimates derived from the modern relationship between MAT and δ18Ocel are 6-16 °C lower than the MAT estimates derived from other biological proxies. Estimates of Pleistocene and Holocene mean annual temperatures are close to the modern values at those sites. These results are consistent with other recent findings that the MAT/δ18Oppt relationship across North America was not constant throughout the Cenozoic. Paleo-δ18Oppt estimates derived from fossil cellulose and the modern North American relationship between δ18Ocel and δ18Oppt are within the current annual range of δ18Oppt values at all locations. The middle Eocene δ18Oppt we determined from arctic cellulose samples (−21.9‰) is consistent with river water δ18O determined in two other studies (−19.1‰ to −22‰). These findings provide some evidence that a precipitation δ18O signal may be retained in wood cellulose during millions of years of burial, and that latitudinal patterns in δ18Oppt may not have changed much during the past 45 Ma. These interpretations depend, of course, on the assumption that the isotopic composition of the cellulose has not changed during burial, an assumption for which it is difficult to gather direct evidence. XRD analysis shows that the crystalline form of the fossil cellulose we used to estimate paleoprecipitation δ18O and paleo-MAT is the same as that of modern cellulose, and that the samples are free of quartz and iron oxide contaminants that result in negative errors in measured δ18Ocel.  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