18O-enriched Fluids Research Articles

Biomarker and Isotopic Composition of Seep Carbonates Record Environmental Conditions in Two Arctic Methane Seeps

Present-day activity of cold seeps in the ocean is evident from direct observations of methane emanating from the seafloor, the presence of chemosynthetic organisms, or the quantification of high gas concentrations in sediment pore waters and the water column. Verifying past cold seep activity and biogeochemical characteristics is more challenging but may be reconstructed from proxy records of authigenic seep carbonates. Here, we investigated the lipid-biomarker inventory, carbonate mineralogy, and stable carbon and oxygen isotope compositions of seep-associated carbonates from two active Arctic methane seeps, located to the northwest (Vestnesa Ridge; ∼1,200 m water depth) and south (Storfjordrenna; ∼380 m water depth) offshore Svalbard. The aragonite-dominated mineralogy of all but one carbonate sample indicate precipitation close to the seafloor in an environment characterized by high rates of sulfate-dependent anaerobic oxidation of methane (AOM). In contrast, Mg-calcite rich nodules sampled in sediments of Storfjordrenna appear to have formed at the sulfate-methane-transition zone deeper within the sediment at lower rates of AOM. AOM activity at the time of carbonate precipitation is indicated by the 13C-depleted isotope signature of the carbonates [−20 to −30‰ Vienna Pee Dee Belemnite (VPDB)], as well as high concentrations of 13C-depleted lipid biomarkers diagnostic for anaerobic methanotrophic archaea (archaeol and sn2-hydroxyarchaeol) and sulfate-reducing bacteria (iso and anteiso-C15:0 fatty acids) in the carbonates. We also found 13C-depleted lipid biomarkers (diploptene and a 4α-methyl sterol) that are diagnostic for bacteria mediating aerobic oxidation of methane (MOx). This suggests that the spatial separation between AOM and MOx zones was relatively narrow at the time of carbonate formation, as is typical for high methane-flux regimes. The seep-associated carbonates also displayed relatively high δ18O values (4.5–5‰ VPDB), indicating the presence of 18O-enriched fluids during precipitation, possibly derived from destabilized methane gas hydrates. Based on the combined isotopic evidence, we suggest that all the seep carbonates resulted from the anaerobic oxidation of methane during intense methane seepage. The seepage likely was associated to gas hydrates destabilization, which led to the methane ebullition from the seafloor into the water column.

Diagenetic Evolution of Selected Parasequences Across A Carbonate Platform: Late Paleozoic, Tengiz Reservoir, Kazakhstan

Abstract Predicting the diagenetic modification of reservoir quality in carbonate fields is poorly understood but of paramount importance to reservoir characterization. The diagenetic processes and current pore system are reported for two parasequences from the platform center to the margin of the Tengiz reservoir (∼ 16 km); the late Visean (Lvis13 cbs 900) and Bashkirian (Bash2.5 MSF 350) parasequences are separated by 8 million years. Syndepositional fibrous cement (> 10% rock volume) and aragonite skeletal molds developed in both parasequences. On subaerial exposure, brown soil-related calcite, and meniscus and pendant cements were emplaced in both parasequences. The application of CL petrography to these rocks for the first time has revealed the depositional components and early marine components are altered developing a mottled pattern implying that their original chemistry is not preserved. The principal calcite cement shows a complex growth pattern by CL that is unreported from any other calcite cement. CL is also used to divide the main cement into stage 1, nonluminescent, and stage 2, orange to brown luminescent, calcite. Stage 1 calcite shows vadose meniscus and pendant fabrics followed by uniform phreatic euhedral terminations. Stage 1 cement is patchily distributed at a millimeter scale; 28% to 40% in the Visean and 0.5% to 22% in Bashkirian grainstones. Stage 1 cement is more abundant at the center of the platform in the late Visean than at its margin. Abundant stage 1 calcite creates a framework that prevents later compaction preserving a high porosity at the center of the platform. This sweet spot is reinforced by the deposition of late-stage bitumen cement that occupies 8% rock volume at the margin of the platform but is almost absent at its center. Stage 1 δ13C values (−2.4‰ to +1.7‰) are derived by dissolution of host rock, and their δ18O values (−6.3‰ to −3.7‰) are compatible with precipitation from meteoric or mixed marine-meteoric water. Depositional micrite and micritic grains are transformed by dissolution–precipitation processes into porous microrhombic calcite not by Ostwald ripening. Microrhombic calcite is ubiquitous in the Tengiz icehouse, dispelling the myth that this fabric is restricted to greenhouse epochs. At ∼ 300 million years before significant burial the study grainstones were patchily cemented and dissolved to reach peak porosity; their transformed texture set up flow paths that controlled the distribution of later diagenetic products. Reservoir compartmentalization did not occur through extensive hardground formation at this time as occurs in some greenhouse sequences, but ash bands at parasequence boundaries were turned into tonsteins and wackestones were compacted perhaps sufficiently to influence vertical flow. CL examination of stage 2 calcite that adds 0.5% to 16% to rock volume reveals that its precipitation was episodic and switched sites as the pore network became blocked through precipitation and compaction and was opened by new fractures. A change from acute to equant habit occurred during stage 2 precipitation, causing the change from fibrous to equant crystal fabric. The δ18OPDB values of stage 2 calcite (−3‰ to −9‰) are linked to precipitation from an 18O-enriched fluid that evolved to ∼ +6‰ SMOW at temperatures of 60°C to 120°C, using the equation of Kim and O'Neil (1997). Many open macropores are lined with small cement crystals that CL shows have all stages of growth present and are terminated by euhedral surfaces. These macropores are only partly filled because of cement inhibition, not because they were dissolved during burial, as was previously claimed. Late-stage dissolution did create some macropores and rounded micrite crystals, but its effect on overall platform porosity is small. The CL examination of Tengiz platform calcite cement has recognized many diagenetic events that cannot be recognized without CL. The new Tengiz diagenetic textures recognized here are probably present in other carbonate platforms, especially those from icehouse epochs.

Past methane release events and environmental conditions at the upper continental slope of the South China Sea: constraints by seep carbonates

Authigenic carbonates and seep biota are archives of seepage history and record paleo-environmental conditions at seep sites. We obtained the timing of past methane release events at the northeastern slope of the South China Sea based on U/Th dating of seep carbonates and seep bivalve fragments from three sites located at 22°02′–22°09′N, 118°43′–118°52′E (water depths from 473 to 785 m). Also, we were able to reconstruct the paleo-bottom water temperatures by calculating the equilibrium temperature using the ages, the corresponding past δ18O of seawater (δ18Osw) and the δ18O of the selected samples formed in contact with bottom seawater with negligible deep fluid influence. A criterion consists of mineralogy, redox-sensitive trace elements and U/Th-isotope systematics is proposed to identify whether the samples were formed from pore water or have been influenced by deep fluid. Our results show that all methane release events occurred between 11.5 ± 0.2 and 144.5 ± 12.7 ka, when sea level was about 62–104 m lower than today. Enhanced methane release during low sea-level stands seems to be modulated by reduced hydrostatic pressure, increased incision of canyons and increased sediment loads. The calculated past bottom water temperature at one site (Site 3; water depth: 767–771 m) during low sea-level stands 11.5 and 65 ka ago ranges from 3.3 to 4.0 °C, i.e., 1.3 to 2.2 °C colder than at present. The reliability of δ18O of seep carbonates and bivalve shells as a proxy for bottom water temperatures is critically assessed in light of 18O-enriched fluids that might be emitted from gas hydrate and/or clay dehydration. Our approach provides for the first time an independent estimate of past bottom water temperatures of the upper continental slope of the South China Sea.

Authigenic carbonates from seeps on the northern continental slope of the South China Sea: New insights into fluid sources and geochronology

Seep carbonates represent excellent archives of seepage activities near seafloors. In order to provide insights on sources of seeping fluids, stable carbon and oxygen isotopes and 87Sr/86Sr ratios were applied to seep carbonates from Shenhu, SW Dongsha, and NE Dongsha on the northern continental slope of the South China Sea. U/Th dating was selected to constrain the time involved in carbonate formation and the potential driving force. The δ13C values of the Shenhu and NE Dongsha carbonates vary from −52.3‰ to −32.6‰ (V-PDB), indicating that biogenic methane is the primary carbon source at both locations. The δ13C values of the SW Dongsha carbonates are much higher, ranging from −18.8‰ to −11.4‰ (V-PDB), suggesting that thermogenic methane is the predominant carbon source in this region. A relatively deep origin of thermogenic methane for the SW Dongsha seeps is also supported by somewhat lower 87Sr/86Sr ratios of the samples (from 0.709025 to 0.709097). The δ18O values of the Shenhu and NE Dongsha carbonates vary from 1.4‰ to 5.1‰ (V-PDB), while the δ18O values of the SW Dongsha carbonates range from 2.5‰ to 3.4‰ (V-PDB). Many samples from Shenhu and NE Dongsha demonstrate an 18O-enriched fluid source that most likely originated from waters released from the dissociation of locally abundant gas hydrate. U/Th ages of the Shenhu carbonates show a large range from 330 ka to 152 ka BP and the NE Dongsha carbonates have ages from 77 ka to 63 ka BP. Interestingly, U/Th ages of most samples are either within times of sea-level lowstands or correspond to periods when the sea-level was falling. Taken together, our data suggest that in the South China Sea gas hydrate dissociation during sea-level lowstands or falling stages resulted from reduced hydrastastic pressures, which in turn enhanced the seep activities and promoted the formation of authigenic carbonates close to the seafloor.

Lithogeochemical and Stable Isotopic Insights into Submarine Genesis of Pyrophyllite-Altered Facies at the Boco Prospect, Western Tasmania

The Boco prospect is a large, fault dismembered, pipelike, hydrothermally altered zone in the Mount Read Volcanics of western Tasmania. It is a synvolcanic alteration zone hosted by felsic volcanic rocks formed in a subaqueous proximal intracaldera setting. Previous detailed geochemical and geophysical surveys and extensive drill testing have indicated it contains no economic metals. The strong to intense, pervasively quartz + phyllosilicate + pyrite-altered northern segment of the prospect is semiconcentrically zoned. Short wavelength infrared (SWIR) spectral analysis has revealed that phyllosilicate assemblages grade from phengitic white mica in the least altered peripheries, through normal potassic white mica, to central zones containing kaolinite, slightly sodic white mica, and pyrophyllite. Mass balance calculations indicate average net mass losses in the altered facies were about 10 to 30 g/100 g, mainly owing to loss of SiO2, which implies very high hydrothermal water-rock ratios. Whole-rock oxygen isotope compositions of the enclosing least altered felsic rocks (δ18O values 8.2–11.7‰) are indistinguishable from those of altered facies (9.6–11.8‰). We attribute the former to low-temperature diagenetic isotopic exchange with 0 per mil δ18O seawater in the peripheral least-altered zones, and the latter to exchange with 3 to 6 per mil δ18O hydrothermal fluids at high water/rock ratios and temperatures generally greater than 220°C, and locally greater than 270°C, in the intensely altered facies. Pyrite sulfur isotope compositions in the Boco altered facies (δ34S values 1.2–7.2‰) are distinctly lower than most Tasmanian massive sulfide deposits (6–15‰), compatible with a dominantly magmatic source of sulfur. The alteration mineral assemblages, estimated mass changes, and isotopic data show that the Boco alteration system was formed by a large volume of focused acidic hydrothermal fluid which had an oxygen isotope composition of 3 to 6 per mil δ18O at and temperature greater than 270°C. The slightly 18O-enriched fluid isotope composition suggests derivation from either mixed magmatic fluid and seawater or isotopically evolved sea water. Its advanced argillic altered facies place Boco among a newly recognized class of southeast Australian Cambrian volcanic-hosted prospects and deposits. These include Chester, Basin Lake, Western Tharsis, and North Lyell in Tasmania, and Rhyolite Creek, Hill 800, and Mike’s Bluff in eastern Victoria. SWIR spectral analyses with field-portable spectrometers allow early discrimination of this type of hydrothermally altered system, and can potentially assist subsequent exploration in mapping facies zonation.

Geology and geochemistry of jasperoids from the Gold Bar district, Nevada

Gold Bar is one of several Carlin-type gold mining districts located in the Battle Mountain–Eureka trend, Nevada. It is composed of one main deposit, Gold Bar; five satellite deposits; and four resources that contain 1.6 Moz (50 t) of gold. All of the deposits and resources occur at the intersection of north-northwest- and northeast-trending high-angle faults in slope facies limestones of the Devonian Nevada Group exposed in windows through Ordovician basin facies siliciclastic rocks of the Roberts Mountains allochthon. Igneous intrusions and magnetic anomalies are notably absent. The Gold Bar district contains a variety of discordant and stratabound jasperoid bodies, especially along the Wall Fault zone, that were mapped and studied in some detail to identify the attributes of those most closely associated with gold ore and to constrain genetic models. Four types of jasperoids, J0, J1, J2, and J3, were distinguished on the basis of their geologic and structural settings and appearance. Field relations suggest that J0 formed during an early event. Petrographic observations, geochemistry, and δ18O values of quartz suggest it was overprinted by the hydrothermal event that produced ore-related J1, J2, and J3 jasperoids and associated gold deposits. The greater amount of siliciclastic detritus present in J0 jasperoids caused them to have higher δ18O values than J1,2,3 jasperoids hosted in underlying limestones. Ore-related jasperoids are composed of main-ore-stage replacements and late-ore-stage open-space filling quartz with variable geochemistry and an enormous range of δ18O values (24.5 and −3.7‰). Jasperoids hosted in limestones with the most anomalous Au, Ag, Hg, ±(As, Sb, Tl) concentrations and the highest δ18O values are associated with the largest deposits. The 28‰ range of jasperoid δ18O values is best explained by mixing between an 18O-enriched fluid and an 18O-depleted fluid. The positive correlation between the sizes of gold deposits and the δ18O composition of jasperoids indicates that gold was introduced by the 18O-enriched fluid. The lowest calculated δ18O value for water in equilibrium with late-ore-stage quartz at 200°C (−15‰) and the measured δD value of fluid inclusion water extracted from late-ore-stage orpiment and realgar (−116‰) indicate that the 18O-depleted fluid was composed of relatively unexchanged meteoric water. The source of the 18O-enriched ore fluid is not constrained. The δ34S values of late-ore-stage realgar, orpiment, and stibnite (5.7–15.5‰) and barite (31.5–40.9‰) suggest that H2S and sulfate were derived from sedimentary sources. Likewise, the δ13C and δ18O values of late-stage calcite (−4.8 to 1.5‰ and 11.5 to 17.4‰, respectively) suggest that CO2 was derived from marine limestones. Based on these data and the apparent absence of any Eocene intrusions in the district, Gold Bar may be the product of a nonmagmatic hydrothermal system.

Origins of CO 2 in permian carbonate reservoir rocks (Zechstein, Ca2) of the NW-German Basin (Lower Saxony)

Permian carbonate reservoir rocks (Stassfurt-Carbonate) of the CO 2-province (Lower Saxony Basin, NW Germany) are locally characterised by high amounts of CO 2 leading to serious problems on exploration. In order to receive information on the major CO 2 sources and CO 2 migration and accumulation processes mineralogical, geochemical and stable isotope analyses ( 13C, 18O) were carried out on authigenic precipitations within the Stassfurt carbonate rocks (Ca2). These rocks contain early diagenetic (e.g., formed in soft sediment) carbonate minerals (calcite, dolomite) and anhydrite in concretions and small cracks. In addition, large (up to 2 cm wide) carbonate-cemented fractures formed in solid Ca2 rocks. Semi-quantitative estimates of the diagenetic mineral content showed variations between the investigated rock cores. Nine out of 12 Ca2 rock cores are derived from wells situated in the area of modern CH 4-rich reservoirs and have distinct fracture systems developed. In contrast, the other three rock cores are from wells characterised by CO 2-rich reservoirs and lack fractures of similar thickness. The assumed relationship between tectonically induced fractures and the gas content of the Ca2 reservoir rocks implies CO 2 degassing of the Ca2 rocks at approximately 102 °C to ∼ 150 °C (assumed δ 18O pore water variation from 0‰ to + 4‰ SMOW). REE distribution indicates at least three types of fracture- and crack-filling carbonates. Two carbonate generations formed from evolving diagenetic pore waters whereas one type reflects carbonate formation under hydrothermal influence. The ages of these cementations derived from Rb/Sr and U/Pb isotope analyses indicate their formation during late stage of/or after the inversion of the Lower Saxony Basin in Late Cretaceous times. The stable carbon isotope composition of the authigenic carbonate minerals reflects at least two carbon source endmembers available during diagenesis. Whereas many early-formed carbonates are related to the Permian 13C-rich carbon source, those carbonates which precipitated or recrystallised during early to late stage diagenesis and/or were induced by local tectonic events incorporated 12C-enriched CO 2 from organic degradation processes. The stable oxygen isotope values of authigenic carbonates probably still reflect low temperatures of mineral formation from Permian sea water (as it is assumed for “non” recrystallised carbonates) or late-stage high-temperature (up to ∼ 150 °C) precipitation from sea water-derived pore waters which may have been locally influenced by 18O-enriched fluids. However, most of the carbonate samples investigated are recrystallised (crystal enlargement of concretionary and fracture carbonate is observed) and hence, these carbonates are at least partly re-equilibrated (original 18O-content reduced) with surrounding pore waters at elevated temperature. Similar to the authigenic carbonate at least two sources may explain the concentration/carbon isotope relationship of modern CO 2 present in the Ca2 reservoir rocks. An organic-derived CO 2 endmember source ( δ 13C near − 20‰) is present in relatively low concentrations whereas large CO 2 concentrations derived from an endmember source with an isotope value near 0‰. Although the latter source is unknown such “heavy” CO 2 sources are most likely attributed to carbonate decomposition processes.

Pore water evolution in oilfield sandstones: constraints from oxygen isotope microanalyses of quartz cement

Oxygen isotope microanalyses of authigenic quartz, in combination with temperatures of quartz precipitation constrained by fluid inclusion microthermometry and burial history modelling, are employed to trace the origin and evolution of pore waters in three distinct reservoirs of the Brae Formation in the Miller and Kingfisher Fields (North Sea). Oxygen isotope ratios of quartz cements were measured in situ in nine sandstone thin sections with a Cameca ims-4f ion microprobe. In conjunction with quartz cement paragenesis in the reservoirs, constrained from textural and cathodoluminescence (CL) microscopy studies, pore water evolution was reconstructed from the time of deposition of the sandstones in the Upper Jurassic until the present. CL photomicrographs of quartz overgrowths in the Brae Formation sandstones show three cement zones (A, B and C) which can be related to different oxygen isotope compositions: (1) the earliest, and thinnest, zone A (homogeneous CL pattern with probable δ 18O values between +23‰ and +26‰—direct measurements were not possible) precipitated in the sandstones at temperatures <60 °C; (2) the second zone B (complex CL pattern and directly measured δ 18O values between +15‰ and +18‰) precipitated in the sandstones most likely between 70 and 90 °C; (3) the third zone C (homogeneous CL pattern and directly measured δ 18O values between +16‰ and +22‰) precipitated in the sandstones most likely at temperatures >90 °C. Calculated oxygen isotope compositions of pore waters show that zone A quartz cements, and enclosing concretionary calcite, precipitated from a meteoric-type fluid (∼−7‰) during shallow burial (<1.5 km). Zone B quartz cements precipitated from fluids which evolved in composition from a meteoric-type fluid ( δ 18O −7‰) to a more 18O-enriched fluid ( δ 18O −4‰) as burial continued to ∼3.0 km. Data from zone C quartz cements are consistent with further fluid evolution from δ 18O −4‰ to basinal-type fluids with δ 18O similar to the present-day formation water oxygen isotope composition (+0.6‰ at 4.0 km burial). A similar pore water evolution can be derived for all three reservoirs studied, indicating that hydrogeologic evolution was similar across sandstones of the whole Brae Formation. The quartz cement zones observed in the Brae Formation sandstones, and the pore water history derived for the area studied, is analogous to published petrographic and pore water evolution data from the nearby Brent Group reservoirs and from reservoirs located in the Haltenbanken area on the Atlantic margin offshore Norway. Considering quartz cement is a major porosity-occluding phase in many reservoir sandstones, and because pore waters both dissolve quartz and carry the dissolved silica to cementation sites, the data presented are valuable for improving the understanding and prediction of reservoir quality development in sandstones globally.

Oxygen and Carbon Isotope Study of Calcite and Dolomite in the Disseminated Au‐Ag Telluride Bulawan Deposit, Negros Island, Philippines

Abstract: The disseminated Au‐Ag telluride Bulawan deposit, Negros island, Philippines, is hosted by dacite porphyry breccia pipes which formed in a Middle Miocene dacite porphyry stock. Electrum and Au‐Ag tellurides occur mostly as grains intergrown with or filling voids between sphalerite, pyrite, chalcopyrite, galena and tennantite. Calcite, quartz and rare dolomite are the principal gangue minerals. Four types of alteration were recognized in the deposit, namely; propylitic, K‐feldspar‐sericitic, sericitic and carbonate alteration. Carbonate alteration is correlatable to the gold deposition stage and occurs mostly along fault zones.The δ18O and δ13C compositions of calcite and dolomite in propylite zone and ore‐stage dacite porphyry breccia were determined. The δ18O values of calcite in propylitized andesite range from +12.2 to +14.7%, and their δ13C values range from ‐6.1 to ‐1.0%. The δ18O values of calcite and dolomite in sericite‐ and carbonate‐altered, mineralized dacite porphyry breccia and dacite porphyry rocks range from +15.1 to +23.1%, and the δ13C values of calcite and dolomite range from ‐3.9 to +0.9%. The δ18O and δ13C values of the hydrothermal fluids were estimated from inferred temperatures of formation on the basis of fluid inclusion microthermometry. The δ18O values of hydrothermal fluid for the propylitic alteration were calculated to be +8.5 ‐ +9.5%, assuming 375°C. On the other hand, the δ18O values of ore solutions for base metal and Au mineralization were computed to be +13.6 ‐ +14.6%, assuming 270°C. The hydrothermal fluids that formed the Bulawan deposit are dilute and 18O‐enriched fluids which reacted with 18O‐ and 13C‐rich wallrocks such as limestone.

Hydrated sub-arc mantle: a source for the Kluchevskoy volcano, Kamchatka/Russia

Oxygen isotope ratios of olivine and clinopyroxene phenocrysts from the Kluchevskoy volcano in Kamchatka have been studied by CO2 and ArF laser techniques. Measured δ18O values of 5.8–7.1‰ for olivine and 6.2–7.5‰ for clinopyroxene are significantly heavier than typical mantle values and cannot be explained by crustal assimilation or a contribution of oceanic sediments. Positive correlations between δ18O and fluid-mobile elements (Cs, Li, Sr, Rb, Ba, Th, U, LREE, K) and a lack of correlation with fluid-immobile elements (HFSE, HREE) suggest that 18O was introduced into the mantle source by a fluid from subducted altered oceanic basalt. This conclusion is supported by radiogenic isotopes (Sr, Nd, Pb). Mass balance excludes simple fluid-induced mantle melting. Instead, our observations are consistent with melting a mantle wedge which has been hydrated by 18O-rich fluids percolating through the mantle wedge. 18O-enriched fluids are derived from the subducted oceanic crust and the Emperor seamount chain, which is responsible for a particularly high fluid flux. This hydrated mantle wedge was subsequently involved in arc magmatism beneath Kluchevskoy by active intra-arc rifting.

Clay minerals as records of temperature conditions and duration of thermal anomalies in the Paris Basin, France: discussion

AbstractA highly illitic phase containing &lt;5–10% interstratified smectite was described by Mossmann et al. (1992) and, more recently, by Clauer et al. (1995) from Upper Triassic sandstones of the Paris Basin (France). This phase is present in all size-fractions of all samples and was interpreted as authigenic. The K/Ar model ages of this illite range from 189–200 Ma and, combined with oxygen isotope data, were cited as evidence of high-temperature (220–250°C hydrothermal precipitation from highly 18O-enriched fluids at burial depths of only 500 m.We suggest that this type of illite is more likely to be diagenetically altered detrital illitic material and unlike authigenic illite-smectite which is also present in many of their samples. This reassessment of their petrographic observations leads to a much more realistic diagenetic interpretation consistent with previous studies of the basin and avoids the need to invoke basin-wide hydrothermal activity during shallow burial.

Petrologic constraints on the source of fluid during mylonitization in the Blue Ridge province, North Carolina and Virginia, U.S.A.

Constraints on the source of fluid during retrograde mylonitization in and adjacent to the Blue Ridge province of the southern Appalachians are provided by (1) the geologic setting, (2) chemical changes in the mylonites relative to the host gneisses, (3) fluid inclusion studies and (4) oxygen and hydrogen isotope analyses of gneisses and mylonites. The Blue Ridge province was emplaced onto Paleozoic carbonates and late Precambrian clastic rocks during middle to late Paleozoic thrusting. Major element chemical data for mylonites from three localities (Fries, Virginia; Hot Springs, North Carolina, and Rosman, North Carolina) in the Blue Ridge province indicate large bulk rock losses of silica (and alkalis). Fluid inclusion studies in the Blue Ridge province suggest the fluids were dominated by CaCl 2MgCl 2 brines of variable salinity and similar in composition to fluids in the adjacent Valley and Ridge province. The variations in δ 18O values for quartz and feldspar are consistent with an enrichment in feldspar due to interaction with a δ 18O enriched fluid. These data together with the δ 18O values of syntectonic quartz veins (11.8–14.4%.) indicate the mylonites exchanged with a fluid with a δ 18O value of +3.7 to +9.2%. under open system conditions. The oxygen and hydrogen isotope data argue against direct input from a meteroic source, and the large silica losses in the mylonites imply a silica undersaturated fluid was involved in fluid-rock interaction. Relatively low temperature formation fluids expelled from the underlying carbonate bank (Knox Group) between 80 and 160 °C would have the required isotopic composition. Dilute metamorphic dehydration fluids may also have been expelled from the underlying clastic sedimentary rocks (Snowbird Group) at higher temperatures and mixing of these fluids with brines from the carbonate bank cannot be ruled out.

Paleopedology and Stable Isotope Chemistry of Late Silurian Vertic Paleosols, Bloomsburg Formation, Central Pennsylvania

ABSTRACT Paleosols with vertic (Vertisol-like) features occur in repetitively stacked fining-upward sequences bounded by marine erosional surfaces in the Bloomsburg Formation (Ludlovian to Pridolian) of central Pennsylvania. Paleosol development was strongly influenced by repeated changes in relative sea level, which resulted in subaerial exposure and pedogenesis of marginal-marine sediment accumulating along a low-energy, mud-dominated coastline in a delta-plain environment. Bloomsburg paleosols are characterized by very weak horizonation, abundant pedogenic slickensides, and a micromorphology dominated by sepic fabrics and peds bounded by stress cutans. Pedogenic carbonate is sparse, occurring principally as micrite nodules. Stable carbon isotope compositions of stratigraphically-low pedogen c carbonate (mean 13C = -1.92 PDB) are attributed to pedogenesis proximal to a marine coastline. In stratigraphically-high paleosols showing evidence of rhizolith development, isotopic compositions of pedogenic carbonate suggest a significant contribution of carbon from C3 flora. Atmospheric P(CO2) levels estimated for these calcretes (4500-5500 ppmV) are significantly greater than present-day values. Low 18O values for pedogenic carbonate (mean 18O = -12.12 PDB) record isotopic exchange between paleosol carbonate and 18O-enriched fluids associated with the conversion of original soil smectites to illite and chlorite during burial diagenesis. Paleosols bounded by marine erosional surfaces show variable degrees of marine hydromorphism attributable to rising water table during transgression', evidence for hydromorphism includes extensive reduction of iron as well as relative increases in CaO, MnO, MgO, P2O5, Na2O, and Sr at the tops of paleosols, due to introduction of marine fluids and sediment into open soil fractures and burrows. The abundance of vertic features in all Bloomsburg paleosols, by analogy with Holocene Vertisols, indicates the recurrence of a seasonally wet-dry (dry 4-8 months/year) paleoclimate n the central Appalachian region at a paleolatitude of about 20° south.

A stable-isotope study of the hydrothermal alteration of the East Taiwan Ophiolite

The East Taiwan Ophiolite (ETO) occurs as allochthonous fragments in the Lichi Mélange and has been suggested to have formed at an active ridge-ridge transform fault in a marginal basin. The brecciated plutonic sequence has experienced a pre-brecciation ocean-ridge type high-temperature metamorphism as well as a post-brecciation off-axis type low-temperature metamorphism, whereas the extrusive sequence has been subjected only to an off-axis low-temperature hydrothermal alteration. Stable-isotopic studies of the ETO show a slightly decreasing trend in oxygen-isotope compositions from basalt through plagiogranite/gabbro to ultramafic rocks. The isotopic compositions of the ETO basalts indicate low-temperature interaction with seawater under variable water/rock ratios. The amphiboles from the ETO gabbro show low δ 18O- (i.e. +3.1 to + 3.8‰) and high δD- (i.e. −19 to −9‰) values. This is interpreted as having resulted from high-temperature interaction with seawater-derived hydrothermal fluid. The plagioclase in the gabbro is mostly albitic in composition and exhibits very high δ 18O-values (i.e. > + 12.0‰), which may have resulted from low-temperature reaction with 18O-enriched fluid. Such 18O-enriched fluid may have been derived from a high-temperature seawater-plutonic rock interaction system. The antigorite- and lizardite/chrysotile-serpentinites in the ETO formed in oceanic environments under pre- and post-brecciation metamorphic conditions, respectively. The isotopic compositions of the antigorite are compatible with its formation in an oceanic environment. Although the δ 18O-values of the lizardites/chrysotiles are comparable to those of oceanic serpentines, their δD-values, ranging from −91 to −57‰, are too low to be accounted for by reaction with seawater-derived fluid. Post-formation hydrogen-isotope exchange between the lizardite/chrysotile and local meteoric waters during/after on-land emplacement of the serpentinites is postulated as a probable explanation. Calcite veins are ubiquitous in the ETO. Three carbon sources are identified: inorganic seawater carbon, juvenile carbon and organic carbon. In the last case, it is shown that some calcite veins may contain a substantial amount of CO 2 derived from the CH 4 oxidation process. The organic carbon was most likely derived from organics in the surrounding Lichi mudstone. Therefore, the calcite veins with an organic carbon signature are suggested to have formed after the ETO emplacement within the Lichi Mélange.

Stable isotopic composition of atypical ophiolitic rocks from east Liguria, Italy

In east Liguria, Italy, pelagic sediments and discontinuous pillow lava formations (< 300 m thick) commonly rest with depositional contact upon serpentinite, gabbro or ophiolitic breccias that lack a sheeted dyke complex. Stable isotopic data on these unusual ophiolites indicate that basalts and, unexpectedly, gabbros are significantly enriched in 18O due to moderate- to low-temperature alteration, that veins cutting the basalts formed at ∼ 150–200°C (quartz-epidote) to ∼ 100–150°C (calcite), that serpentinization took place at ∼ 100–200°C, and that the ophicalcitic upper surfaces of the serpentinites formed in contact with normal seawater at ∼ 80–135°C. During uplift of deep-seated gabbro to a near-seafloor position, mafic phases were progressively transformed to lower-temperature phases such as tremolite then chlorite, whereas plagioclase underwent low- T retrograde isotopic exchange, probably with 18O-enriched fluids. Following the formation of a discontinuous volcanic carapace to these lithologies, moderate- to low-temperature hydrothermal activity produced vein systems and general 18O enrichment in the volcanics. If the 18O enrichment of the east Ligurian ophiolite is typical of other stratigraphically similar ophiolites in the western Tethys, then waters in this ocean basin may not have been buffered to a δ 18O-value of ∼0%.

Constraints on effective diffusivity during oxygen isotope exchange at a marble-schist contact, Sifnos (Cyclades), Greece

A smooth sigmoidal δ 18O profile is observed at a marble-schist contact within rocks affected by the Miocene greenschist overprint in the Alpine metamorphic complex of the Cyclades on Sifnos. The point of inflection of the profile occurs at the lithological contact, and indicates that advective movement of fluids into the marble was negligible despite previous evidence showing that 18O-enriched fluids infiltrated the schist sequence. Least squares model fits of the diffusion equation to the data give Dt products of 0.10 m 2 and 0.13 m 2 for marbles and schists, respectively ( D =effective diffusion coefficient, t =time). To reconcile these figures with quoted free fluid diffusivities one has to propose that either the effective porosities were extremely low and/or the diffusional exchange time was as low as 10 2–10 3 years. Low effective porosities in schists are consistent with a model of greenschist evolution in which hydration reactions induced by ( 18O-enriched) fluids substantially decreased porosity and reduced exchange to fluid diffusional control.

Transformation of blueschist to greenschist facies rocks as a consequence of fluid infiltration, Sifnos (Cyclades), Greece

The transformation from blueschist to greenschist facies forms a major part of the Alpine regional geodynamic evolution of the Cyclades. The transition in metabasic rocks on Sifnos involves the retrogression of eclogites, blueschists and actinolite-bearing rocks from high-pressure conditions which have been estimated at 460±30° C and 15±3 kb. Petrographically observed parageneses are interpreted by a sequence of hydration and carbonation reactions involving the breakdown of omphacite and glaucophane-bearing assemblages to albite+chlorite±actinolite±calcite assemblages. The retrograde processes are calculated to occur at pressures of 10 to 8 kb during the isothermal uplift of the Sifnos units. Oxygen isotope analyses of different rock types show that interlayered lithologies have experienced a large degree of isotopic equilibration during both metamorphic phases. However, chemically equivalent rocks show systematic increases in δ18O from lowest values (∼8 to 11‰ in metabasics) in the unaltered blueschists in the upper stratigraphic levels to higher values (>15‰ in metabasics) associated with greenschists in the deepest stratigraphic levels. Relict eclogites enclosed within greenschists have the lower δ18O values typical of unaltered blueschist facies rocks. These isotopic gradients and the δ13C and δ18O compositions of carbonates demonstrate an infiltration mechanism involving the upward movement of 18O-enriched fluids whose compositions were buffered by exchange with marble units. Calculated minimum fluid/rock ratios for the blueschist-to-greenschist transition decrease from ∼0.4 in the deepest studied level (Central Sifnos) and ∼0.2 in the intermediate level (Kamares Bay samples) to an assumed value of zero in unaltered blueschists. These ratios may be lower if recycling of fluids occurred between schists and marbles. Infiltration of fluids became inhibited as the transformation advanced as a consequence of “sealing effect” of the hydration and carbonation reactions. Although infiltration most probably was a pre-requisite for the regional occurrence of the blueschist-to-greenschist transformation in the Cyclades, the evidence on Sifnos does not suggest the introduction of large quantities of fluid.