ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00016.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTThe Chicago mineral‐carbonate oxygen isotope fractionation curves have been combined with mineral‐water fractionation data for jadeite, zoisite and rutile and new data for grossular‐water to provide a set of self‐consistent mineral‐pair calibrations. TheAcoefficients in the equation 1000 In α=A× 106T‐2of the new mineral‐pair fractionations areJadeite Zoisite Grossular RutileQuartz 1.69 2.00 3.03 5.02Jadeite 0.31 1.34 3.33Zoisite 1.03 3.02Grossular 1.99The isotopic fractionation properties of natural pyralspite garnet [(Ca, Fe, Mg, Mn)3Al2Si3O12] can be approximated by those of the grossular end‐member. Appropriate substitutions also yield coefficients for the solid‐solution minerals: sodic pyroxene and epidote, e.g.Aquartz‐sodic pyroxene= 2.75 ‐ 1.06Xjd,Aquartz‐epidote= 2.00 + 0.75XpswhereXJdandXPsare the mole fractions of the jadeite and pistacite components, respectively.The new data set is particularly suitable for the geothermometry of metamorphic rocks. δ18O data from minerals of the high‐pressure metamorphic rocks of the Sesia Zone of Italy and Cyclades Complex of Greece yield well‐constrained mean temperatures of 572 and 478d̀ C, respectively. Type III blueschist metabasalts of the Franciscan Formation of California give mean quartz

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00017.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTRetrograde exchange of oxygen isotopes between minerals in igneous and metamorphic rocks by means of diffusion is explored using a finite difference computer model, which predicts both the zonation profile of δ18O within grains, and the bulk δ18O value of each mineral in the rock. Apparent oxygen isotope equilibrium temperatures that would be observed in these rocks are calculated from the δ18O values of each mineral pair within the rock. In systems which cool linearly from a sufficiently high temperature or at a low enough cooling rate, such that the final oxygen isotope values are not dependent upon the initial oxygen isotope values (‘slow cooling’), the apparent oxygen isotope temperature derived for a rock composed of a single mineral pair can be shown to be simply related to the Dodson closure temperatures (Tc) for the two phases and the mode of the rock. Adding a third phase into a system which undergoes ‘slow’ cooling will cause the apparent temperature derived for the two minerals already present to differ from the simple relationship for a two‐phase system. In some systems oxygen isotope reversals can be developed. If cooling is not ‘slow’, then the mineral δ18O values resulting from cooling will be partly dependent upon the initial temperature of the system concerned. The model successfully simulates the mineral δ18O values that are often observed in granitic rocks. Application of the model will help in assessing the validity of oxygen isotope thermometry in different geological settings, and allows quantitative prediction of the oxygen isotope fractionations that are developed in cool

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00018.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTIncipient metamorphism accompanying thrusting, folding and cleavage development has been investigated in a varied sequence of Palaeozoic sediments near the Variscan front in SW Dyfed, Wales. The aim was to evaluate a critical stage in the progression from heterogeneous sediment, whose detrital phases are neither in equilibrium with one another, nor with pore fluids, through indurated sedimentary rock to metamorphic rock comprising newly formed crystals that equilibrated with one another as they grew.Quartz veins are widely developed in the area, especially in the more psammitic lithologies, while finer grained rocks became cleaved during tectonic deformation. Mineralogical constraints and fluid inclusion measurements suggest maximum temperatures around 200‐310d̀ C (slightly higher in the Marloes‐Musselwick Thrust Sheet than in other parts of the structural succession) at depths of the order of 6‐13 km.Quartz veins yield distinctly heavier oxygen isotopic compositions than detrital quartz grains in the adjacent wall rocks, although care must be taken in interpreting the data because slivers of detrital grains may become incorporated into veins, while matrix detrital grains may incorporate veinlets or rims of newly formed quartz. It is concluded that vein quartz grew in isotopic equilibrium with a fluid phase whose isotopic composition was primarily controlled by exchange with phyllosilicates, not detrital quartz grains. Vein and matrix quartzes from the Marloes‐Musselwick Thrust Sheet are distinctly lighter (δ18Oveins=+14 to +18% and δ18Omatrix=+11 to +14%) than those from other thrust sheets (δ18O =+17 to +20% and +14 to +17%, respectively).We conclude that vein quartz and phyllosilicate grains in cleavage domains probably attained equilibrium with a locally buffered pore fluid at the peak of metamorphism, but many relict grains of different chemical and isotopic composition remained elsewhere in the rock. Local fluid migration along veins and through cleavage lamellae facilitated the attainment of equilibrium, but there is little evidence for large‐scale infiltration of externally derived fluids. With further metamorphism the quartz in these rocks would attain an isotopic composition intermediate between that of the heavy vein material and light detritus which

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00019.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTVein‐controlled retrograde infiltration of H2O‐CO2fluids into Dalradian epidote amphibolite facies rocks of the SW Scottish Highlands under greenschist facies conditions resulted in alteration of calcite‐rich marble bands to dolomite and spatially associated18O enrichment of about 10%. on a scale of metres. Fluid inclusion data indicate that the retrograde fluid was an H2O‐salt mixture with a low CO2content, and that the temperature of the fluid was about 400d̀ C. Detailed petrographic and textural (backscattered electron imaging) studies at one garnet‐grade locality show that advection of fluid into marbles proceeded by a calcite‐calcite grain edge flow mechanism, while alteration of non‐carbonate wall‐rock is associated with veinlets and microcracks.Stable isotopic analysis of carbonates from marble bands provides evidence for advection of isotopic fronts through carbonate wall‐rocks perpendicular to dolomite veins, and fluid fluxes in the range 2.4–28.6 m3/m2have been computed from measured advection distances. Coincidence of isotope and reaction fronts is considered to result from reaction‐enhanced kinetics of isotope exchange at the reaction front. Front advection distances are related to the proportion of calcite to quartz in each marble band, with the largest advection distance occurring in nearly pure calcite matrix. This relationship indicates that fluid flow in carbonates is only possible along fluid‐calcite‐calcite grain edges. However, experimental constraints on dihedral angles in calcite‐fluid systems require that pervasive infiltration occurred in response to calcite dissolution initiated at calcite‐calcite grain junctions rather than to an open calcite pore geometry.The regional extent of the retrograde infiltration event has been documented from the high δ18O of dolomite‐ankerite carbonates from veins and host‐rocks over an area of least 50 × 50 km in the SW Scottish Highlands. Isotopically exotic18O‐rich retrograde fluids have moved rapidly upwards through the crust, inducing isotopic exchange and mineral reaction in wall‐rocks only where lithology, pore geometry or mineral solubilities, pressure and temperature have been appropr

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00020.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTQuantitative modelling of oxygen exchange by diffusion during slow cooling has been compared to the observed oxygen isotope distributions from high‐grade metamorphic and granitic rocks of the High Himalayan Crystallines, Langtang Valley, central Nepal, in order to investigate the effect of retrograde diffusional exchange on the preservation of high‐temperature, oxygen isotope systematics.Modelled fractionations, using water‐present diffusion data reported in the literature, predict quartz‐mica fractionations to be much larger than those at peak metamorphic and igneous conditions due to low closure temperatures for micas. Quartz‐feldspar fractionations may be less than those at peak conditions, and in some samples may even be slightly negative.The observed oxygen isotope fractionations in the metamorphic rocks are small and largely appear to record equilibrations close to peak conditions determined by other methods. Hence these rocks clearly do not conform to predictions of fluid‐present diffusional retrograde exchange. It is suggested that their retrograde history was therefore within an anhydrous closed system in which diffusion was slow and hence mineral closure temperatures were high. The granitic rocks record rather larger quartz‐biotite fractionations, approaching those predicted by the diffusion modelling. However, quartz‐feldspar fractionations are large and hence, although significant retrograde exchange has clearly occurred, simple diffusion alone is not sufficient to explain the observed data and open‐system exchange may be required. The presence of fluids during the retrograde history of this part of the section is supported by petrographic evidence.The different retrograde oxygen exchange histories recorded between the regional metamorphic and magmatic regimes of the Langtang section would appear to support the importance of water on the kinetics of such exchange, and suggests that in its absence, diffusional exchange may become insignificant, allowing oxygen isotope thermometry to record meaningful high

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00021.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTThe 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 anin‐situpartial 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 ofc.3.0%. The greater homogenization in the chaotic zone is attributed to the increased degree of melting and infiltration of H2O‐rich flu

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00022.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTThe role of volatiles in the stabilization of the lower (granulite facies) crust is contentious. Opposing models invoke infiltration of CO2‐rich fluids or generally vapour‐absent conditions during granulite facies metamorphism. Stable isotope and petrological studies of granulite facies metacarbonates can provide constraints on these models. In this study data are presented from metre‐scale forsteritic marble boudins within Archaean intermediate to felsic orthogneisses from the Rauer Group, East Antarctica.Forsteritic marble layers and associated calcsilicates preserve a range of13C‐ and18O‐depleted calcite isotope values (δ13C= ‐9.9 to ‐3.0% PDB, δ18O = 4.0 to 12.1% SMOW). A coupled trend of13C and18O depletion (∼2%, ∼5%, respectively) from core to rim across one marble layer is inconsistent with pervasive CO2infiltration during granulite facies metamorphism, but does indicate localized fluid‐rock interaction. At another locality, more pervasive fluid infiltration has resulted in calcite having uniformly low, carbonatite‐like δ18O and δ13C values. A favoured mechanism for the low δ18O and δ13C values of the marbles is infiltration by fluids that were derived from, or equilibrated with, a magmatic source. It is likely that this fluid‐rock interaction occurred prior to high‐grade metamorphism; other fluid‐rock histories are not, however, ruled out by the available data. Coupled trends of13C and18O depletion are modified to even lower values by the superposed development of small‐scale metasomatic reaction zones between marbles and internally folded mafic (?) interlayers. The timing of development of these layers is uncertain, but may be related to Archaean high‐temperature (>1

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00023.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTThe enderbites from Tromøy in the central, granulite facies part of the Proterozoic Bamble sector of southern Norway contain dominantly CO2and N2fluid inclusions. CO2from fluid inclusions in quartz segregations in enderbites was extracted by mechanical (crushing) and thermal decrepitation and the δ13C measured. Measurement was also made on samples washed in 10% HCl, oxidized with CuO at high temperatures, and step‐wise extracted with progressive heating. Results between the different techniques are systematic. The main results show δ13C of ‐4.5±1.5% for crushing and ‐7±2% for thermal decrepitation. δ13C is about constant for CO2extracted at different temperatures and points to a homogeneous isotopic composition. Due to the presence of carbonate particles and/or induced contaminations for the extraction by thermal decrepitation, the results for the crushing experiments are assumed the most reliable for fluid‐inclusion CO2. Very low values of δ13C have not been found in enderbite samples and δ13C combined with δ18O of the host quartzes (8‐11%) indicates juvenile values. In addition, the fluid inclusions were examined by microthermometry and Raman analysis and host quartz by acoustic emission and cathodoluminescence. CO2fluid inclusions have varying densities with a frequency maximum of 0.92 g cm‐3and generally do not concur with trapping densities at granulite conditions. Textures show that CO2must have been trapped in fluid inclusions in one early event, but transformed to different extents during late isothermal uplift without important fractionation of isotope compositions. The present data support a model of intrusion and crystallization of a CO2‐rich enderbitic magma a

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00024.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTGranulite facies anorthosites on Holsenøy Island in the Bergen Arcs region of western Norway are transected by shear zones 0.1–100 m wide characterized by eclogite facies assemblages. Eclogite formation is related to influx of fluid along the shears at temperatures ofc.700d̀C and pressures in excess of 1.7 GPa. Combined carbon and nitrogen stable isotope,40Ar/36Ar, trace‐element and petrological data have been used to determine the nature and distribution of fluids across the anorthosite‐eclogite transition.A metre‐wide drilled section traverses the eclogitic centre of the shear into undeformed granulite facies garnet‐clinopyroxene anorthosite. Clinozoisite occurs along grain boundaries and microcracks in undeformed anorthosite up to 1 m from the centre of the shear and clinozoisite increases in abundance as the edge of the shear zone is approached. The eclogite‐granulite transition, marked by the appearance of sodic pyroxene and loss of albite, occurs within the most highly sheared section of the traverse. The jadeite‐in reaction coincides with increased paragonite activity in mica. The separation between paragonite and clinozoisite reaction fronts can be semiquantitatively modelled assuming advective fluid flow perpendicular to the shear zone. The inner section of the traverse (0.25 m wide) is marked by retrogressive replacement of omphacite by plagioclase + paragonite accompanied by veins of quartz‐phengite‐plagioclase.C‐N‐Ar characteristics of fluid inclusions in garnet show that fluids associated with precursor granulite, eclogite and retrogressed eclogite are isotopically distinct. The granulite‐eclogite transition coincides with a marked change in CO2abundance and δ13C (<36ppm, δ13C=‐2% in the granulite;25 times 103) and pre‐existing Ar in the granulite (40Ar/36Ar<8 times 103). δ15N values decrease from +6% in the anorthosite to +3% within the eclogite shear. The central zone of retrogressed eclogite post‐dates shearing and is characterised by substantial enrichment of Si, K, Ba and Rb. Fluids are CO2‐rich (δ13C ∼ ‐5%) with variable N2and Ar abundances and isotopic compositions.Both Ar and H2O have penetrated the underformed granulite fabric more than 0.5m beyond the granulite/eclogite transition during eclogite formation. Argon isotopes show a mixing profile consistent with diffusion through an interconnecting H2O‐rich fluid network. In contrast, a carbon‐isotope front coincides with the deformation boundary layer, indicating that the underformed anorthosite was impervious to CO2‐rich fluids. This is consistent with the high dihedral angle of carbonic fluids, and may be interpreted in terms of

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00025.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY