δ18O Values Of Rocks Research Articles

Weathering as a control on the triple oxygen isotopes of groundwater-associated ferromanganese deposits: lessons from the Grimlock Ni–Co–Mn prospect, Northern Territory, Australia

At the Grimlock laterite deposit (Northern Territory, Australia), Co and Ni mineralization occurs mainly in the Mn-oxide rich layers of the ferromanganese (Fe-Mn) crust overlying ultramafic bedrock. Groundwater-associated Fe-Mn crusts consist of mineral (e.g. Mn-oxide, Fe-oxyhydroxide and silicate) groups suitable for studying triple oxygen isotopes and present unique interpretative challenges (e.g. small Mn-oxide fractions relative to Fe-Mn precipitates from other formation environments and extensive weathering). We evaluate triple oxygen isotopes within the context of changes to properties (i.e. mineralogy, major and trace element geochemistry, and degree of weathering) of a lateritic profile. We use pre-existing mineral-water fractionation factors, meteoric water δ 18 O and temperature data to calculate δ 18 O values of fully altered mineralogical endmembers, then, using mass balance, discuss scenarios to elucidate measured whole-rock δ 18 O values. The δ ′ 18 O (′ denotes linearized notation) and Δ ′ 17 O of near-surface samples (0–8 m) are generally lower (mean of 8.892‰) and higher (mean of −0.141‰), respectively, than the δ ′ 18 O (mean of 12.767‰) and Δ ′ 17 O (mean of −0.176‰) of samples from greater depths (19–22 m). At 16–17 m depth, δ ′ 18 O and Δ ′ 17 O are relatively high (means of 17.509 and −0.118‰, respectively). The measured whole-rock δ 18 O values are explainable by substituting lower δ ′ 18 O values for the Mn-oxide and Fe-oxyhydroxide fractions, and higher δ ′ 18 O values for the aluminosilicate fraction, changes coinciding with greater alteration. These results suggest that mineral weathering is primarily responsible for observed variations in the triple oxygen isotopes of groundwater-associated Fe-Mn crusts, rather than variation in the initial source of oxygen incorporated into Mn-oxide. Supplementary material: Grimlock sample photographs, XRD patterns and supplementary figures and tables are available at https://doi.org/10.6084/m9.figshare.c.7071907

A re-assessment of the nitrogen geochemical behavior in upper oceanic crust from Hole 504B: Implications for subduction budget in Central America

The geochemical behavior of N during seawater-oceanic crust alteration remains poorly constrained. Yet, it is a central parameter to assess the flux of N to subduction zones. Most studies proposed that hydrothermally altered basaltic rocks are enriched in N relative to fresh basalts. However, published data from DSDP/ODP Hole 504B, a reference site for the composition of the oceanic crust, suggest that seawater alteration leads to the N depletion of the upper ocean crust. To better address this issue, we analyzed N concentration and isotope composition of 21 altered basalts from the lavas and sheeted dikes sampled by Hole 504B. These new analyses show significant N enrichment (up to 14.1 ppm) relative to fresh degassed MORB (∼1 ppm). The differences observed between earlier and modern data are interpreted as resulting from analytical artifact due to the earlier use of a molybdenum crucible for N extraction. Furthermore, our new data show a progressive decrease of N concentration with depth, from 14.1 to 1.4 ppm. Nitrogen isotope compositions display a large range, with δ15N values from −0.9 to +7.3‰, and most likely reflect multiple stages of alteration with fluids of various compositions. In contrast to N concentration, δ15N values do not show a global depth trend but oscillate around a mean value of 3.0 ± 2.2‰ (1SD). The N concentration shows a positive correlation with bulk rock δ18O values, suggesting that N behavior during alteration process is mainly controlled by temperature. We propose that N speciation in the hydrothermal fluid is dominated by NH3/NH4 at low temperature (<200°C) but is transformed to N2, associated with H2, at higher temperature (>200°C). These new data are used to re-evaluate the global flux of N input into Central American subduction zone, showing that the upper basaltic crust represent about 20% of the total N buried in subduction zone. A comparison with previous results obtained on N degassed in volcanic arc illustrates that, in “warm” subduction zone like Central America, up to 50% of the subducted N may be transferred to the deep mantle. This contrasts with “cold” subduction environments, where >80% of the N inputs escape sub-arc slab devolatilization and supports that the geothermal gradient plays a major role in determining the N fate in subduction zones.

Isotopic constraints on fluid evolution and ore precipitation in a sediment-hosted Pb-Ag-Ba-Zn-Cu-Au deposit in the Capricorn Orogen, Western Australia

The Abra Pb-Ag-Ba-Zn-Cu-Au deposit in the Capricorn Orogen, Western Australia is primarily a lead and silver resource currently estimated at 47.8 Mt (indicated and inferred) of 7.3–10.1% Pb and 18–28 gt-1 Ag, although significant Cu-Au zones are also identified. The deposit is unique within sediment-hosted Pb-Zn deposits for its low Zn content, significant Cu-Au zone and high Fe content, providing a case study where the source of fluid and ore-forming processes are contentious. The combination of whole-rock hydrogen and oxygen isotope data, in situ oxygen isotope data in quartz, and in situ sulphur isotope data of pyrite and chalcopyrite, has been used to reconstruct a complex history of overprinting, involving stages of sedimentation, diagenesis and hydrothermal activity. The host sedimentary rocks consist of detrital quartz (δ18O ∼11–18‰) and whole rock δ18O values (∼9–16‰) reflecting the combined composition of detrital and authigenic minerals, diagenetic-metamorphic exchange, chlorite and iron content. Quartz in recrystallised chemical sedimentation, quartz cementation, and quartz-barite veins at low temperatures (∼100–250 °C) involved predominantly surface and formation fluids with a wide range of fluid δ18O values between ∼ -5‰ and 2.6‰. Quartz in chloritized host rock with disseminated pyrite and chalcopyrite-galena veins at 250–320 °C reflect exchange with fluids(s) having a narrow range of δ18O values (∼5–9‰), most likely formation fluids. The fluid responsible for iron oxide, pyrite and polymetallic carbonate veins appears to be a mixture of formation and lighter surface fluids, with a range of fluid δ18O values (∼0.8–5.5‰). In situ sulphur isotopes are consistent with reduced seawater sulphate source in all samples, therefore it is likely that metal-rich formation fluids have interacted with reduced sulphate in the host sediments to precipitate as sulphide. Mineralisation and associated alteration at Abra has caused whole rock δ18O values to decrease in the deposit which may be useful as a tool for exploration in similar sediment-hosted base-metal deposits. We have shown the combination of different isotopic systems, and utilisation of in situ techniques, can constrain the sources and evolution of fluid and sulphur involved in basin formation, hydrothermal alteration and base metal mineralisation. Isotopic values can be directly related to different mineral populations within a relative temporal framework and can be used to distinguish fluids between multiple events.

Petrogenesis of ultramafic rocks of komatiitic composition from the Central Zone of the Limpopo Belt, South Africa: Evidence from O and H isotopes

Komatiites are MgO-rich ultramafic volcanic rocks, mostly interpreted to be essentially anhydrous at the time of extrusion, and are often characterized by spinifex texture. They mainly occur in Archean or Proterozoic greenstone belts within cratons. However, in the Central Zone of the Limpopo Belt in South Africa, ultramafic rocks of komatiitic composition occur within a high-grade metamorphic terrane. Their chemical composition overlaps with that of Al-undepleted (Munro-type) komatiites from elsewhere, but unlike komatiites, they are mainly composed of pargasitic amphibole, along with olivine, orthopyroxene, Cr-rich spinel and magnetite. The studied rocks have a narrow range of whole rock δ18O values (+5.4 to +6.3‰), unlike δ18O values from komatiites, which are generally altered and have δ18O values from ∼+2 to +9‰. However, they are within the range estimated for unaltered komatiites (+5.4 to +6.0‰). Amphibole separates from the studied rocks have δ18O values between +5.5 and + 6.1‰, similar to that of the bulk rock. The δD values of amphiboles range between −78 and −83‰, similar to primary amphiboles from other komatiites and many other mantle-derived mafic igneous rocks. The δ18O values, along with textural and mineralogical evidence, are consistent with the ultramafic rocks of komatiitic composition from the Limpopo Belt being intrusive and unaffected by low-temperature alteration. The O and H isotope composition of the amphiboles are consistent with them being primary magmatic in origin. It cannot be distinguished with certainty whether the water was introduced to the magma through partial melting of hydrated mantle or during ascent through hydrous mantle subsequent to their formation. The minerals observed at present most likely crystallized from komatiite magma that intruded into the lower crust and subsequently re-equilibrated at the ambient P-T conditions.

High-resolution Campanian–Maastrichtian carbon and oxygen stable isotopes of bulk-rock and skeletal components: palaeoceanographic and palaeoenvironmental implications for the Boreal shelf sea

AbstractA high-resolution latest Early Campanian to Early Maastrichtian carbon and oxygen stable isotope record from the northern German Boreal shelf sea based on 537 analyses of co-occurring belemnites, brachiopods, inoceramids, oysters, and bulk rock samples is presented. All samples are precisely related to their stratigraphic, systematic and facies backgrounds and form an integrated, nearly 10-myr-long dataset with considerable palaeoenvironmental and palaeoceanographical implications. Petrographic studies indicate that low-magnesium calcitic coccoliths and calcispheres (i.e., planktic carbonate) predominate the bulk-rock data (marl-limestone rhythmites and chalks), thus representing a sea-surface water signal, and that only minor diagenetic alteration of the carbonate muds took place. Based on TL and CL microscopy, the investigated belemnites are extraordinarily well preserved, which may in part be explained by their early diagenetic surficial silicification (container effect), while the other macroinvertebrate groups are all less well preserved. The (plankton-dominated) δ13C values of the marl-limestone rhythmites and chalks (+1.1 to +2.5 ‰), recording a surface water signal, compare well with the δ18C data of inoceramids while δ13Cbrach. values (+1.5 to +3.0 ‰) are heavier than the bulk rock data. The large variation in the δ13Cbel. (-0.1 to +3.6 ‰) is attributed to isotopic disequilibrium of the biogenic carbonate formed by the belemnite animal. The bulk rock δ18O values show a remarkable low scatter, supporting petrographic observation of only minor diagenetic stabilisation/cementation, and can be approximated with northern German shelf sea-surface temperatures of ca. 20°C for the Late Campanian (ca. -2 ‰ δ18O), being slightly cooler during the Early Maastrichtian. The δ18O values of the belemnite rostra are even less variable and quite rich in heavier18O (-0.7 to +0.6 with a mean of -0.1 ‰ δ18Obel.) in comparison to bulk rock and other skeletal components. Based on their excellent microstructural preservation and non-luminescence, we conclude that the belemnite rostra are diagenetically unaltered and have preserved the primary δ18O signal of ambient seawater (12 ± 2°C). In the absence of any indication for migration from cooler water masses and evidence for authochtonous populations we assume that the belemnites of the genera Belemnitella and Belemnella lived as nektobenthos near the sea-floor and thus record the temperature of the bottom mixed layer of the seasonally weakly stratified north German shelf sea at water depths of 100 to 150 m; the temperature gradient was thus 12.5-18.75 m/1°C. A conspicuous latest Campanian cooling event is evident in both sea-surface and bottom-water temperatures. The δ18O values of nearly all investigated benthic fossils lie between the isotope values of pristine belemnites and bulk rock, and, therefore, should be used for palaeotemperature reconstructions only with great care.

Cretaceous ongonites (topaz-bearing albite-rich microleucogranites) from Ongon Khairkhan, Central Mongolia: Products of extreme magmatic fractionation and pervasive metasomatic fluid: rock interaction

Ongonites were defined at their type locality at Ongon Khairkhan, central Mongolia, as pristine magmatic topaz-bearing albite–quartz-keratophyres with up to 4wt. % F and containing phenocrysts of albite, K-feldspar, quartz and rare mica and topaz hosted in a groundmass composed of the same minerals. However, detailed petrographic and SEM-EDS studies indicate that these rocks underwent considerable subsolidus exchange with deuteric fluids, as evidenced by the presence of albitic plagioclase (Ab~100) and end-member orthoclase (Or~100), secondary Li–Fe-rich mica (zinnwaldite) enriched in rare metals (Sn, W, Ta, Nb), pitted feldspars containing fluid inclusions, and disseminated fluorite. The ~120Ma old dyke rocks, emplaced at a high structural level in the crust, are strongly peraluminous leucogranites characterized by high Al and alkalis that are also enriched in Rb, Cs, Ga and Ta, depleted in Mg, Ca, Zr, Ba, Sr and Eu, and have anomalous K/Rb, Rb/Sr, Zr/Hf and Nb/Ta ratios compared to the average continental crust. However, the suite has Nd isotopic ratios (ɛNd(120) ~−1) similar to those of contemporaneous A-type granites of the Mongolian–Transbaikalian igneous province of the Central Asian Orogenic Belt. The inferred primary δ18O (~+6 to +7‰) and Pb isotopic values are consistent with a granitic parent magma and interaction with orthomagmatic fluids. The ongonites and constituent minerals record (1) an extensive and protracted crystal fractionation history, in part due to the presence of volatiles (particularly F) which depressed the solidus temperature of the felsic rocks and extended its duration of crystallization and (2) subsolidus exchange with fluids which includes late flux of heated meteoric water as indicated by modified whole rock δ18O values (+0.5 to +2.7‰). The interaction of the ongonites with internally derived orthomagmatic fluids is considered to result in enrichment and/or redistribution of several incompatible elements, but not to have greatly modified their original major element chemistry which indicates that this suite represents the last stages of fractionation of a highly differentiated, F-rich granitic magma. The escape of these evolved melts from the apical part of the underlying pluton is now represented by the ongonite dykes. Late-stage magmatic, water-rich fluids enriched in incompatible elements including Nb, Ta, Sn and W were responsible for the late- to post-magmatic alteration and associated W mineralization.

Downhole variation of lithium and oxygen isotopic compositions of oceanic crust at East Pacific Rise, ODP Site 1256

Bulk rock lithium and oxygen isotope compositions from ODP Site 1256 were analyzed to investigate the seawater circulation in the upper oceanic crust formed at the East Pacific Rise (EPR). The upper extrusive basalts have δ18O values from +6.1‰ to +9.2‰, reflecting alteration of oceanic crust by seawater at low temperatures (&lt;200–250°C). Bulk rocks from the sheeted dike complex and plutonic section have overall lower δ18O values (+3.0‰–+5.5‰). In the sheeted dike complex bulk rock δ18O values gradually decrease with depth, and then increase toward the fresh MORB δ18O value after reaching a minimum of +3.0‰ at ∼1350 m below seafloor (mbsf). The entire sampled crust is dominated by rocks with low lithium contents relative to fresh MORBs except for a few localized Li enrichment. The upper volcanic zone is characterized by a spread of δ7Li from low to high values relative to average unaltered MORB values (δ7Li = +3.4 ± 1.4‰). The presence of rocks with low δ7Li values in the upper crust most likely indicates zones of upwelling of relatively hot (∼200–250°C) hydrothermal fluids. In the sheeted dike complex, bulk rock δ7Li values show wide range of variation, but exhibit a general trend from enriched to depleted values at ∼1280 mbsf and then return to that for fresh MORB within the upper tens of meters of the plutonic section at the bottom of the after reaching a minimum at ∼1350 mbsf (δ7Li = −1.6‰). The downhole pattern of δ7Li principally reflects variations in water‐rock ratio (w/r) together with a downhole increase of temperature. Seawater flow in the upper volcanic zone is likely to be channeled with generally small but variable w/r ratios. The w/r ratios increase rapidly with depth in the lower volcanic section into the sheeted dike complex indicating water dominated pervasive hydrothermal flow due to intensive upwelling of hydrothermal fluids.

Hydrothermal alteration geochemistry of the Betam gold deposit, south Eastern Desert, Egypt: mass-volume-mineralogical changes and stable isotope systematics

Although there is relatively minor gold production from the Arabian–Nubian shield at present, extensive alluvial and lode fields along the western side of the Red Sea in Upper Egypt and northern Sudan were worked out by the ancient Egyptians for thousands of years. In the Eastern Desert of Egypt, numerous but small gold deposits are generally related to auriferous quartz veins commonly associated with brittle-ductile shear zones, generally cutting through the Neoproterozoic crystalline basement rocks and trending in different directions. Gold mineralisation at the Betam mine area, south Eastern Desert, is related to a series of milky quartz veins along a NNW- trending brittle-ductile shear zone cutting through successions of pelitic schists, next to a small granite intrusion. Gold-sulphide mineralisation (pyrite, arsenopyrite, galena, subordinate chalcopyrite and gold) is closely associated with a conspicuous hydrothermal alteration halo. Textural relationships, including replacement and crosscutting of mineral phases and quartz veins record a post-foliation alteration assemblage of quartz+sericite+chlorite+calcite±albite±epidote. The hydrothermal alteration halo alongside the auriferous quartz veins comprises three distinct zones, namely distal chlorite-calcite zone (chlorite+calcite±biotite±pyrite±sericite±epidote), an intermediate sericite-chlorite zone (sericite+chlorite+pyrite±biotite), and a proximal pyrite-sericite zone (quartz+pyrite+sericite±albite). These zones merge to each other gradually, ending outwards into the unaltered metasediments. The pyrite-sericite zone contains the highest gold grades, especially in zones thickly seamed with sulphide-rich quartz veinlets. Mass balance calculations have revealed that the pyrite-sericite zone experienced significant metasomatic changes relative to limited mass and volume changes for the chlorite-calcite zone. The overall picture of chemical gains and losses with increasing the intensity of hydrothermal alteration is indicative of addition of SiO2, K2O, Na2O, and volatile elements (S, CO2), removal of MgO, and relatively inert behaviour of Al2O3, TiO2, MnO, Fe2O3. CaO is variably mobile; slightly enriched in the chlorite-calcite and sericite-chlorite zones but depleted in the pyrite-sericite zone. Concentrations of the trace elements are variable in the different alteration types, but a notable increase in Au, As, Ba, Sr, Rb, V, and Ni in the intensively altered rocks is apparent. Investigation of the rare earth element (REE) behaviour reveals a little modification; heavy REE are more or less unchanged, whereas light REE are significantly mobile in all alteration types. New geochemical data provide evidence for progressive silicification, sericitisation, and sulphidation as a function of gold mineralisation. A proposed model for the hydrothermal alteration system for the Betam deposit includes fluctuation in pH and redox state (fO2), mainly during the wallrock sulphidation. This might have destabilised gold complexes and lowered gold solubility in the ore solution, and hence contributed at least partly in gold deposition in the study area. A decrease in the whole rock δ18O values from un-altered country metasediments to the distal, intermediate and proximal alteration zones, respectively, might be attributed to rock interaction with an isotopically lighter fluid. In addition, sulphur isotope data of pyrite-arsenopyrite pairs in the mineralised quartz veins and adjacent wallrock along with the calculated δ34S∑s values for the ore fluids suggest derivation from non-homogenous (mixed magmatic and metamorphic) fluids.

Oxygen isotope composition of a section of lower oceanic crust, ODP Hole 735B

The oxygen and hydrogen isotope compositions of vein minerals and δ18O values of 64 whole rocks from the deeper kilometer of the 1.5 km lower crustal section of ODP Hole 735B in the SW Indian Ocean were analyzed in order to investigate seawater penetration into and reaction with the lower oceanic crust. Vein amphiboles have δD values of −35 to −65‰, indicating formation from seawater fluids and hydrothermal fluids containing a component of low‐δD magmatic fluid. Crystal‐plastic shear zones reacted with 18O‐enriched fluids at high temperatures and have δ18O values of 6.0–7.0‰, indistinguishable from surrounding rocks. Reaction with seawater‐dominated fluids over a range of temperatures (∼350–700°C) led to local 18O‐depleted rocks, but much of the section exhibits 18O enrichment (δ18O up to 7.6‰) as the result of static alteration of plagioclase over a range of temperatures (&lt;250 to ∼600°C) and fluid compositions. Chloritic veins and associated alteration by 18O‐enriched fluids occurred at temperatures of 150–250°C, and late carbonates and Fe‐oxyhydroxides formed from cold (∼10°) seawater associated with open fault‐related fractures. Bulk rock δ18O values of the lower 1 km range from 3.7 to 10.2‰ but are mostly 5.6–7.6‰, and the section has a mean bulk δ18O of 6.5 ± 1.0‰. In contrast, rocks from the shallower 500 m exhibit net 18O depletion (5.1 ± 1.0‰) reflecting high‐temperature deformation and seawater hydrothermal alteration (Stakes, 1991). The weighted average δ18O for the entire gabbro section of Hole 735B is 6.0‰, exhibiting little or no change compared to unaltered crust. Although tectonism at slow spreading rates may allow exposure of lower crust to greater amounts of low‐temperature alteration and 18O enrichment, there is no evidence for a consistent difference in the bulk δ18O of the plutonic sections of oceanic crust formed at fast versus slow spreading rates.

The redox state of granitoids relative to tectonic setting and earth history: The magnetite–ilmenite series 30 years later

ABSTRACTThe redox state variation of orogenic granitoids along convergent plate margins is examined in the Phanerozoic Circum-Pacific Belt and in some Cryptozoic terranes. The Phanerozoic granitoids of the NW and NE Pacific Rims can be divided into reduced ilmenite series occurring in the accretionary terranes with compressional tectonic setting, and oxidised magnetite series intruding crystalline basements under extensional to intermediate regional stress regime. The ilmenite-series granitoids have negative but the magnetite series have positive δ34S values, which show a positive correlation with magnetic susceptibility of the granitoids. The negative δ34S sulphur originated in biogenic sulphur from accreted pelitic sediments and positive δ34S values show that sulphate sulphur migrated from seawater through subduction processes. The whole rock δ18O values are higher than 8 permil in the ilmenite series, but lower than 8 permil in the magnetite series, and as a whole show negative correlation with the magnetic susceptibility of the granitoids. The higher δ18O values reflect those of accreted sediments, whilst the lower δ18O values represent magmatic values of an oxidised mafic protolith at depth.The predominance of ilmenite-series granitoids of the NW Pacific rim can be explained by well-developed accretionary terranes in which mafic magmas from depth mingled with felsic magmas from the accretionary complex to form granodioritic magmas, whilst that of magnetite-series granitoids is postulated to be oxidised igneous sources for the magma generation and an extensional and/or intermediate tectonic setting for the magma ascent. The absence of the accretionary wedges by tectonic erosion and/or no fore-arc sedimentation also helped to form magnetite-series granitoids. Potassic granitoids are generally of oxidised type. A-type granites in late orogenic environments also belong to the magnetite series. Adakitic high-Sr/Y granitoids are oxidised in the Mesozoic–Cenozoic but are reduced in the Archaean TTG, reflecting the redox state of the then-current sea-floor environment. The oldest magnetite-series granite so far known is the 3105 Ma-old biotite granite of the Nelspruit batholith, South Africa.

Fluid flow patterns in fast spreading East Pacific Rise crust exposed at Hess Deep

Tectonic exposures of a volcanic sequence and sheeted dike complex over a 4‐km‐wide region at Hess Deep (equatorial Pacific) reveal significant spatial heterogeneity (10–103 m) in the extent and nature of hydrothermal alteration in young, fast spreading East Pacific Rise crust. The volcanic sequence is fairly uniformly altered, with only minor oxidation and alteration to clay minerals. Sheeted dikes in the eastern part of the field area are highly fractured with narrow intervals of intact dikes that dip up to 60°. Their alteration characteristics show a simple depth trend such that with increasing depth the dominant secondary mafic mineral changes from chlorite to amphibole, clinopyroxene replacement increases (&lt;20% to &gt;40%), whole rock δ18O values decreases (4.4–5.5‰ to 3.5–4.5‰), and calculated peak metamorphic temperatures increase (∼250°C to 450°–700°C). Within the deepest dikes, localized zones up to 400‐m‐wide are chlorite‐rich and have low‐δ18O (2.9–4.1‰) and low peak metamorphic temperatures (∼345°C). These alteration patterns likely formed within broad recharge zones whereby the low‐δ18O zones developed in the regions with the highest fluid flux. In the west, massive, slightly rotated sheeted dikes near the volcanic‐sheeted dike transition are δ18O and Cu depleted and display higher peak temperatures (≥345°C) than elsewhere in the shallow dikes. These characteristics are consistent with formation within a high temperature, hydrothermal discharge zone. We propose that the spreading history of a fast spreading ridge segment can create significant spatial heterogeneity in fluid flow and alteration patterns within sheeted dike complexes, similar to those preserved in many ophiolites.

Stable Isotope Study of the Langshan Polymetallic Mineral District, Inner Mongolia, China

Abstract: A comprehensive stable isotope investigation was carried out to clarify the geneses of the ore deposits in the Langshan Pb‐Zn mineral district. The lead isotope study shows that these deposits were probably formed from 2. 0 to 1. 5 Ga, and were deformed and metamorphosed 1. 45 Ga. Ore lead could be a mixture of mantle lead and crustal lead. The C and S isotope results indicate that these deposits were precipitated in closed or semi‐closed rift basins, and the source of sulfur might be Proterozoic ocean sulfate. The H and O isotope results indicate that the δD and δ18O values of rocks were changed by water‐rock interaction during metamorphism and hydrothermal alteration. The scale of δD and δ18O shift of rocks reflects the grade of metamorphism and alteration as well as the water‐rock ratios. However, the water‐rock ratios in the metamorphic processes of Langshan mineral district were relatively low, and the source of water during metamorphism is suggested to be ancient meteoric water. Based on isotopic results and the geological background, it is concluded that these deposits may belong to Proterozoic sedimentary exhalative (SEDEX) type.

Meteoric water‐rock interaction in the lower plate of the Whipple Mountain metamorphic core complex, California

ABSTRACTOxygen 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 (&gt;c.600d̀C) episode involving a metamorphic or magmatic fluid with δ18O values ∼ 7‰ and a low‐temperature (c.350d̀C) 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. 350d̀ 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.

The role of magmatic and hydrothermal processes in the chemical evolution of the Strange Lake plutonic complex, Qu�bec-Labrador

The Strange Lake plutonic complex consists of three annular Mid-Proterozoic arfvedsonite-aegirine-bearing alkali granites emplaced in the Rae province of the Canadian Shield. The mineralogy, chemistry and structural setting of the complex are very similar to that of many peralkaline central salic complexes associated with the development of the Gardar rift in southern Greeland. The Strange Lake granites are highly fractionated (Rb/Sr=5 to 160 and K/Rb=27 to 120) and carry unusually high abundances of HFSE and REE-bearing exotic minerals (e.g. pyrochlore, gittinsite, elpidite, gadolinite and kainosite) which are reflected in the elevated HFSE (e.g.Zr=307 to 16800 ppm) and REE (e.g. La=84 to 1337 ppm) contents of the granites. HFSE and REE increase from the oldest intrusive unit, which is hypersolvus and unaltered, to the youngest, which is subsolvus and metasomatized. The unaltered granites display a restricted range of δ18O values (+8.2 to+9.6‰) and low δ18O signatures for fresh arfvedsonite/aegirine (+4.8 to+5.2‰). Anomalously high CaO (0.7 to 3.2 wt%) and MgO (0.1 to 0.6wt%) concentrations characterize the altered subsolvus granites. These rocks also have elevated whole rock δ18O values (+9.6 to +11.9‰), negative Δδ18Oquartz-alk.feld. (-0.1 to-1.6), and high δ18O values of altered arfvedsonite (i.e.+6.5 to 13.75‰) that correlates positively with whole rock δ18O values. The chemical and isotopic data are consistent with a model in which the least evolved alkali granites are formed through differentiation from trachytic (syenitic) parents. Extreme HFSE and REE-enrichment may have been accomplished by differentiation through fractional crystallization and heterogenous distribution of F-rich silicic residual melts in which the REE and HFSE are transported as fluorocomplexes. The O-isotopic values are consistent with the circulation of low temperature (lt;200°C) hydrothermal fluids in the youngest subsolvus intrusive unit which caused extensive Ca (Mg and Sr) metasomatism and fluorine leaching, widespread hematization, and remobilization of the HFSE and REE.

Trace-element and Sr, Nd, Pb, and O isotopic composition of Pliocene and Quaternary alkali basalts of the Patagonian Plateau lavas of southernmost South America

The Pliocene and Quaternary Patagonian alkali basalts of southernmost South America can be divided into two groups. The “cratonic” basalts erupted in areas of Cenozoic plateau volcanism and continental sedimentation and show considerable variation in 87Sr/86Sr (0.70316 to 0.70512), 143Nd/144Nd (ɛNd) and 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios (18.26 to 19.38, 15.53 to 15.68, and 38.30 to 39.23, respectively). These isotopic values are within the range of oceanic island basalts, as are the Ba/La, Ba/Nb, La/Nb, K/Rb, and Cs/Rb ratios of the “cratonic” basalts. In contrast, the “transitional” basalts, erupted along the western edge of the outcrop belt of the Pliocene and Quaternary plateau lavas in areas that were the locus of earlier Cenozoic Andean orogenic arc colcanism, have a much more restricted range of isotopic composition which can be approximated by 87Sr/86Sr=0.7039±0.0004, ɛNd, 206Pb/204Pb=18.60±0.08, 207Pb/204Pb=15.60±0.01, and 208Pb/204Pb=38.50±0.10. These isotopic values are similar to those of Andean orogenic are basalts and, compared to the “cratonic” basalts, are displaced to higher 87Sr/86Sr at a given 143Nd/144Nd and to higher 207Pb/204Pb at a given 208Pb/204Pb. The “transitional” basalts also have Ba/La, Ba/Nb, La/Nb, and Cs/Rb ratios higher than the “cratonic” and oceanic island basalts, although not as high as Andean orogenic are basalts. In contrast to the radiogenic isotopes, δ18O values for both groups of the Patagonian alkali basalts are indistinguishable and are more restricted than the range reported for Andean orogenic are basalts. Whole rock δ18O values calculated from mineral separates for both groups range from 5.3 to 6.5, while measured whole rock δ18O values range from 5.1 to 7.8. The trace element and isotopic data suggest that decreasing degrees of partial melting in association with lessened significance of subducted slabderived components are fundamental factors in the west to east transition from arc to back-arc volcanism in southern South America. The “cratonic” basalts do not contain the slab-derived components that impart the higher Ba/La, Ba/Nb, La/Nb, Cs/Rb, 87Sr/86Sr at a given 143Nd/144Nd, 207Pb/204Pb at a given 208Pb/204Pb, and δ18O to Andean orogenic arc basalts. Instead, these basalts are formed by relatively low degrees of partial melting of heterogeneous lower continental lithosphere and/or asthenosphere, probably due to thermal and mechanical pertubation of the mantle in response to subduction of oceanic lithosphere below the western margin of the continent. The “transitional” basalts do contain components added to their source region by either (1) active input of slab-derived components in amounts smaller than the contribution to the mantle below the arc and/or with lower Ba/La, Ba/Nb, La/Nb, and Cs/Rb ratios than below the arc due to progressive downdip dehydration of the subducted slab; or (2) subarc source region contamination processes which affected the mantle source of the “transitional” basalts earlier in the Cenozoic.

Patterns and processes of oxygen isotope exchange in a fossil meteoric hydrothermal system, Cuillins Gabbro Complex, Isle of Skye, Scotland

Oxygen isotope compositions of whole rock specimens and mineral separates from the Cuillins Gabbro Complex, Isle of Skye, Scotland, are employed to determine the patterns and processes of18O depletion in the Outer Unlayered Gabbro (OUG) and associated dikes. Whole rockδ18O values range from +4.8 to −1.1‰ (SMOW) and dikeδ18O values range from +4.7 to −2.8‰ Mineral separates from three OUG samples yieldδ18O values from +5.3 to +4.8‰ for augite and +4.1 to +0.8‰ for plagioclase. An early, small-scale hydrothermal circulation system was initiated by the OUG prior to the large-scale hydrothermal convection established by the later Layered Cuillins Complex (LCC). Dikes were emplaced in the OUG after intrusion of the LCC and had only a minor effect on hydrothermal circulation in the OUG. There is evidence of enhanced fluid flow along dike/gabbro contacts. Isotopic compositions of augite separates demonstrate a normalδ18O value for the OUG magma with all18O depletion in the OUG due to subsolidus exchange processes including diffusion and surface reaction. The mineral separates yield a pattern of18O depletion consistent with a diffusion mechanism, the bulk of the exchange having occurred in the plagioclase. Secondary mineral formation played a subordinate role in the18O depletion of the OUG. The calculated water to rock mass ratio necessary to effect the observed18O depletion in the OUG is on the order of 0.2, although a much greater amount of water circulation probably occurred. The cooling duration required to explain the measured18O depletion in the OUG by diffusion is very short (140 years at 750° C, 2400 years at 550° C) compared to the duration necessary for pure conductive cooling (105 to 106 years). Rapid local cooling rates in the OUG due to meteoric water convection are consistent with the observed18O depletion in OUG samples.

Isotope geochemistry of minerals and fluids from Newberry volcano, Oregon

Isotopic compositions were determined for hydrothermal quartz, calcite, and siderite from core samples of the Newberry 2 drill hole, Oregon. The δ15O values for these minerals decrease with increasing temperatures. The values indicate that these hydrothermal minerals precipitated in isotopic equilibrium with water currently present in the reservoirs. The δ18O values of quartz and calcite from the andesite and basalt flows (700–932 m) have isotopic values which require that the equilibrated water δ18O values increase slightly (− 11.3 to −9.2‰) with increasing measured temperatures (150–265°C). The lithic tuffs and brecciated lava flows (300–700 m) contain widespread siderite. Calculated oxygen isotopic compositions of waters in equilibrium with siderite generally increase with increasing temperatures (76–100°C). The δ18O values of siderite probably result from precipitation in water produced by mixing various amounts of the deep hydrothermal water (− 10.5 ‰) with meteoric water (− 15.5 ‰) recharged within the caldera. The δ13C values of calcite and siderite decrease with increasing temperatures and show that these minerals precipitated in isotopic equilibrium with CO2 of about −8 ‰. The δ18O values of weakly altered (<5% alteration of plagioclase) whole-rock samples decrease with increasing temperatures above 100°C, indicating that exchange between water and rock is kinetically controlled. The water/rock mass ratios decrease with decreasing temperatures. The δ18O values of rocks from the bottom of Newberry 2 show about 40% isotopic exchange with the reservoir water. The calculated δ18O and δD values of bottom hole water determined from the fluid produced during the 20 hour flow test are −10.2 and −109‰, respectively. The δD value of the hydrothermal water indicates recharge from outside the caldera.

Oxygen isotope compositions of selected laramide-tertiary granitoid stocks in the Colorado Mineral Belt and their bearing on the origin of climax-type granite-molybdenum systems

Quartz phenocrysts from 31 granitoid stocks in the Colorado Mineral Belt yield δ 18O values less than 10.4‰, with most values between 9.3 and 10.4‰. An average magmatic value of about 8.5‰ is suggested. The stocks resemble A-type granites; these data support magma genesis by partial melting of previously depleted, fluorine-enriched, lower crustal granulites, followed by extreme differentiation and volatile evolution in the upper crust. Subsolidus interaction of isotopically light water with stocks has reduced most feldspar and whole rock δ 18O values. Unaltered samples from Climax-type molybdenumbearing granites, however, show no greater isotopic disturbance than samples from unmineralized stocks. Although meteoric water certainly played a role in post-mineralization alteration, particularly in feldspars, it is not required during high-temperature mineralization processes. We suggest that slightly low δ 18O values in some vein and replacement minerals associated with molybdenum mineralization may have resulted from equilibration with isotopically light magmatic water and/or heavy isotope depletion of the ore fluid by precipitation of earlier phases. Accumulation of sufficient quantities of isotopically light magmatic water to produce measured depletions of 18O requires extreme chemical stratification in a large magma reservoir. Upward migration of a highly fractionated, volatile-rich magma into a small apical Climax-type diapir, including large scale transport of silica, alkalis, molybdenum, and other vapor soluble elements, may occur with depression of the solidus temperature and reduction of magma viscosity by fluorine. Climax-type granites may provide examples of 18O depletion in magmatic systems without meteoric water influx.

Contrasting fluid/rock interaction between the Notch Peak granitic intrusion and argillites and limestones in western Utah: evidence from stable isotopes and phase assemblages

The Jurassic Notch Peak granitic stock, western Utah, discordantly intrudes Cambrian interbedded pure limestones and calcareous argillites. Contact metamorphosed argillite and limestone samples, collected along traverses away from the intrusion, were analyzed for δ18O, δ13C, and δD. The δ13C and δ18O values for the limestones remain constant at about 0.5 (PDB) and 20 (SMOW), respectively, with increasing metamorphic grade. The whole rock δ18O values of the argillites systematically decrease from 19 to as low as 8.1, and the δ13C values of the carbonate fraction from 0.5 to −11.8. The change in δ13C values can be explained by Rayleigh decarbonation during calcsilicate reactions, where calculated \(\Delta ^{13} {\text{C}}_{\left( {{\text{CO}}_{\text{2}} - {\text{cc}}} \right)}\) is about 4.5 permil for the high-grade samples and less for medium and low-grade samples suggesting a range in temperatures at which most decarbonation occurred. However, the amount of CO2 released was not anough to decrease the whole rock δ18O to the values observed in the argillites. The low δ18O values close to the intrusion suggest interaction with magmatic water that had a δ18O value of 8.5. The extreme lowering of δ13C by fractional devolatilization and the lowering of δ18O in argillites close to the intrusion indicates oxgen-equivalent fluid/rock ratios in excess of 1.0 and X(CO2)F of the fluid less than 0.2. Mineral assemblages in conjunction with the isotopic data indicate a strong influence of water infiltration on the reaction relations in the argillites and separate fluid and thermal fronts moving thru the argillites. The different stable isotope relations in limestones and argillites attest to the importance of decarbonation in the enhancement of permeability. The flow of fluids was confined to the argillite beds (argillite aquifers) whereas the limestones prevented vertical fluid flow and convective cooling of the stock.