High δD Values Research Articles

English

A deuterium tagged rainfall event with a δD of +213 was applied to the top surface of an experimental covered waste rock pile in May of 2003. The tracer experiment was designed to resolve the spatial variability of infiltrating water and to estimate the magnitude and rate of flow within covered waste rock. The five meter high pile was deconstructed one year later and waste rock was sampled along vertical profiles at 10cm increments. Pore waters were extracted from the waste rock using the centrifugal method. The measured δD values range from -90 to background levels of approximately -130 with a few zones of locally high δD values deeper than three meters in the pile. Variability between individual vertical δD profiles are observed within the 8x8 meter area. The combination of the spatial variability and the deep δD values yield strong evidence for intermediate scale (~15 cm) preferential flow.

Hydrogen Isotopic Compositions of Pyroxenes in Nushan Peridotite Xenoliths, SE China

Abstract Hydrogen isotopic compositions of pyroxenes in peridotite xenoliths from the Nushan volcano, Anhui Province, SE China have been obtained using an ion probe (Cameca 1270). D/H ratios are constant within a single grain and among the different grains from the same sample. The lack of correlation between the D/H ratios and the hydrogen contents indicates that the hydrogen isotopic compositions are inherited from their mantle source. Combining with the δD values of coexisting amphiboles, it is inferred that the Nushan mantle experienced at least a two-stage metasomatic event. One was responsible for formation of amphiboles with extremely variable δD values, and the other was probably responsible for the high δD values (up to −20‰) of some clinopyroxenes from peridotites, clinopyroxene and mica megacrysts. High δD values point to a subduction-related fluid being involved in one metasomatic event. The primary δD values (−90‰ to −140‰) of the Nushan pyroxenes, together with data gleaned from the literature, suggest that the D/H ratios of the nominally anhydrous mineral reservoir might have differed from that of the other mantle hydrogen in being relatively depleted in D.

Compound specific D/ H ratios and molecular distributions of higher plant leaf waxes as novel paleoenvironmental indicators in the Chinese Loess Plateau

We measured D/ H ratios of individual long chain leaf wax n-alkanes isolated from a loess profile spanning the last 130 ka from Xifeng, China. The δD values show large variation (−140 to −200‰) for the past 130 ka and are highly correlated with changes in magnetic susceptibility (MS), an accepted proxy for past monsoon circulation in the Chinese Loess Plateau. Peaks in high δD values (−140 to −160‰) coincide with low MS values for loess sections formed under relatively dry conditions (marine isotope stages, MISs, 2 and 4), whereas peaks in low δD values (−190 to −200‰) coincide with high MS values for paleosol sections formed under relatively wet conditions (MISs 1, 3 and 5). We attribute the isotopic variations to the combined effects of varying aridity, which controls the soil and leaf water evaporation, and temperature and monsoon intensity, which alter the D/ H ratios of precipitation. We also discovered a strong negative correlation between the molecular distributions (ratios of C 31/C 29 and C 31/C 27 n-alkanes) and the δD values of n-alkanes, based on studies of modern plants and the Xifeng loess sequence. The complex relationship between MS and monsoon circulation has been difficult to understand mechanistically. However, the climatic controls on the D/ H ratios of individual long chain n-alkanes and C 31/C 29 n-alkane ratios can be readily demonstrated and calibrated from modern plants and surface soils. Therefore, these new molecular and isotopic proxies have potential as quantitative proxies for paleoenvironmental reconstruction in the Chinese Loess Plateau.

Hydrogen Isotopes of Glassy and Phyllosilicate Spherules in Al Rais (CR) and Orgueil (CI) Chondrites

Abstract The hydroxyl in phyllosilicate minerals is the most common occurrence of water in primitive meteorites. Direct hydrogen isotopic analysis of this water component using an ion microprobe has been made in some glassy or phyllosilicate spherules from the Al Rais (CR) and Orgueil (CI) chondrites. The spherules from Al Rais show large deuterium excesses (δD = +200 – +800‰) relative to terrestrial standards, whereas deuterium‐enrichments in the spherules from Orgueil are much smaller (δD = +40 – +130‰). The phyllosilicate spherules are products of aqueous alteration of glassy precursors. In Al Rais the phyllosilicate spherules have relatively higher δD values than the glassy ones, indicating that water introduced during aqueous alteration was deuterium‐enriched. The deuterium‐enrichments in the phyllosilicate spherules from Orgueil could result from isotopic exchange under thermodynamic conditions within the solar nebula. The much larger δD excesses of the Al Rais spherules, however, cannot be attributed to the similar process; instead, an interstellar origin needs to be invoked.

Anomalously high δD values in the mantle

Hydrogen isotopic compositions of four amphibole grains from three lherzolite xenoliths hosted in Cenozoic basanites of Nushan, eastern China have been analyzed by ion microprobe. δD values of all analyzed points range from −94 to +46‰. Large heterogeneities of D/H ratios within single grains have been observed with variations of δD up to 80‰ on the scale of less than 400 μm. Neither correlation between δD values and water contents of the analyzed points nor regular variation between δD values and positions of analyzed points within the same grain can be found, implying that the scatter of δD values could not result from a late shallow process such as hydrogen loss or hydrothermal alterations and should be considered as inherited from the source at depth. Chemical compositions of Nushan amphiboles are very homogeneous, excluding the possibility that the scatter of δD values could arise from variable fractionation factors between a single fluid source and minerals. Therefore, metasomatic fluids responsible for the formation of Nushan amphiboles should have variable and unusually high hydrogen isotopic compositions. We suggest that this variation in the metasomatic fluids could be related to melt degassing in the mantle source.

Antiphase hydrogen and oxygen isotope periodicity in chert nodules

Oxygen and hydrogen isotope analyses were made of Jurassic-age chert nodules from the Holy Cross Mountains, SE Poland, along radial transects at high spatial resolution. There is a radial “sigmoidal” periodicity for both isotope ratios, but the two are out of phase, with high δD values corresponding to low δ 18O values. Periodicity for a 100- to 120-mm diameter nodule is approximately 16 mm, increasing slightly toward the rim, with amplitudes approaching 20 and 3.0‰ for hydrogen and oxygen, respectively. The combined hydrogen-oxygen isotope data for one nodule fall on a published curve for chert forming in equilibrium with seawater (Knauth and Epstein, 1976); the range of delta values corresponds to temperature variations of ∼10°C. Data for a second chert fall on a subparallel δD-δ 18O line with δD values that are almost 50‰ lower. The δD-δ 18O patterns for the nodules cannot be explained by periodic mixing of meteoric and ocean water because the hydrogen and oxygen isotope data are out of phase. Two possible explanations for the antiphase periodicity are (a) cyclical temperature variations, perhaps related to an unstable convection system (e.g., Bolton et al., 1999), and (b) self-organizing catalytic precipitation (e.g., Wang and Merino, 1990). The systematic isotopic variations are difficult to explain by diagenesis and strongly suggest that primary isotopic compositions are preserved. The isotopic data provide important information on the thermal history of the sedimentary basin, if temperature variations are the cause of the isotopic periodicity.

Anomalously high δD and micro-scale hydrogen isotope heterogeneities in the mantle: Ion microprobe analysis of amiboles from peridotite xenoliths at Nushan, eastern China

Hydrogen isotopic compositions of four amphibole grains from three pieces of lherzolite xenoliths in Cenozoic basanites of Nushan, eastern China have been analyzed by ion microprobe. δD values of all analyzed points range from −94‰ to +46‰, some of which are much higher than the highest δD (+8‰) reported previously for mantle materials. The heterogeneities of D/H ratios within single grains have been observed, the variation of δD is up to 80‰ on the scale of less than 400 μm. No correlation between hydrogen isotopic ratios and hydrogen contents can be found, implying that the scatter of δD values could not result from a late shallow process such as hydrogen loss or hydrothermal alterations and should be considered as inherited from the source at depth. Chemical compositions of Nushan amphiboles are very homogeneous, excluding the fact that the scatter of δD values could arise from variable fractionation factors between a single fluid source and minerals. Therefore, metasomatic fluids responsible for the formation of Nushan amphiboles should be heterogeneous and result in the observed large variable and anomalously high δD values of amphiboles. We suggested that such metasomatic fluids could be related to magma degassing in the mantle source. Based on the D-H diffusion data and the scale of hydrogen isotope heterogeneities, it was inferred that the mantle metasomatism took place soon before the eruption of host magma.

Influence of precipitation seasonality on piñon pine cellulose δD values

SummaryThe influence of seasonal to interannual climate variations on cellulose hydrogen isotopic composition (δD) was assessed by analysing tree rings and needles of piñon pine (Pinus edulis and P. monophylla). Sites spanned a gradient of decreasing summer precipitation, from New Mexico to Arizona to Nevada. Tree rings were divided into earlywood, latewood and whole‐year increments, and annual cohorts of needles were collected. The study period (1989–96) included two La Niña events (1989, 1996) and a prolonged El Niño event (1991–95). Winter and spring moisture conditions were strongly related to October–March Southern Oscillation Index (SOI) in New Mexico and Arizona, with above‐average precipitation occurring in El Niño years. Wood δD values at these sites were correlated with winter and spring moisture conditions. Needle δD values were correlated with summer moisture conditions in New Mexico and with winter moisture and SOI in Arizona. Low cellulose δD values observed from 1991 to 1993 in both wood and needles occurred during wet El Niño years, whereas high δD values in needles were present during the dry, La Niña years of 1989 and 1996. North‐eastern Nevada does not receive precipitation anomalies related to ENSO, and thus cellulose δD values did not reflect the ENSO pattern observed at the other sites. Cellulose δD values were strongly, inversely correlated with relative humidity variations at all sites, as predicted by a mechanistic model. Contrary to predictions from the same model and observations from more mesic areas, time series of cellulose δD values were not directly correlated with interannual or seasonal variations in precipitation δD values or temperature at any of the sites. On a regional basis, however, mean δD values in needles and wood were correlated with mean annual temperature and δD values of precipitation. This suggests that temporal averaging may bias relationships between biological systems and climate.

Water contents and hydrogen isotopic ratios of rocks and minerals from the 1991 eruption of Unzen volcano, Japan

Water contents and hydrogen isotopic ratios were determined for blocks from pyroclastic flow deposits, and bread-crust bombs and blocks from the 1991 Vulcanian eruptions of Unzen volcano, Japan. Groundmass water contents and δD values of samples were calculated by subtracting the contribution of major hydrous minerals (hornblende and biotite) from the bulk rock analyses, and range from 0.1 to 0.5 wt.% and −83 to −49‰, respectively. The samples do not show a systematic H 2O– δD relationship, although the block samples tend to have lower δD values than the bomb samples. The non-systematic H 2O– δD relationship is likely a result of near surface, kinetically-controlled gas loss. High viscosity of this magma would hinder attainment of hydrogen isotopic equilibrium between exsolved vapor and melt in the final degassing stage. The near surface degassing, however, was accompanied by kinetic fractionation resulting in enrichment of deuterium in the final products as exemplified by bread-crust bombs with high H 2O–low δD margins and low H 2O–high δD cores. Relatively high δD values of the blocks and bombs as well as high temperature volcanic gas (−30 to −35‰) suggest a closed system degassing of an initial water-rich magma (H 2O=6 wt.%) until its water content was reduced to 0.5 wt.%. The pre-eruptive δD value (−46‰) was estimated from the volcanic gas data and D/H analysis of hornblende phenocrysts coupled with assumed isotopic equilibration in the initial hydrous magma.

Hydrogen isotope compositions of mantle-derived amphibole megacrysts from Qilin, Guangdong and its tectonic significance

Hydrogen isotope compositions of mantle-derived amphibole megacrysts from Qilin, Guangdong Province have been obtained by ion micro-probe. δD and H contents are constant both among different samples and within single sample, demonstrating that their formation condition is very stable. High δD values suggest the presence of a component recycled from crust which is possibly related to the subcluction of Pacific Plate beneath Eurasian Plate in Mesozoic.

Stable Isotope Evolution of Volcanic Ash Layers During Diagenesis of the Miocene Monterey Formation, California

AbstractThe oxygen and hydrogen isotope compositions of volcanic ash layers from coastal outcrops of the Miocene Monterey Formation elucidate the progressive burial diagenesis of glass to bentonite and metabentonite. Volcanic glass that texturally appears unaltered is found to have δl8O and δD values that are significantly higher than fresh volcanic glasses. The positive shift in δ18O and δD values is proportional to the amount of hydration and chemical alteration of the glass samples. Initial hydration of the glass results in the exchange of Na for H; later hydration results in the loss of K and Fe in exchange for H and Na. The δ18O values of the most hydrated glass, and clay minerals from bentonite and metabentonite layers are approximately in equilibrium with slightly modified seawater. The δD values of the hydrated glass, and clay minerals from bentonite and metabentonite layers, are significantly depleted in D relative to seawater and suggest meteoric exchange associated with tectonic uplift and erosion in the Pliocene and Pleistocene. Smectite from bentonite layers has δ18O and δD values similar to the most hydrated glass samples, suggesting similar glass-water and smectite-water fractionation factors. Kaolinite and mixed-layered illite-smectite (I-S) altered from smectite have lower δ18O and higher δD values than their precursor. The δ18O and δD of nonvolcanic siliceous mudstones from the Pt. Arguello oil field show an unusual decrease in δD with increasing burial depth that probably reflects the presence of organic hydrogen in the analyzed samples as well as possible D-depleted formation waters from detrital illite-water and hydrocarbon-water exchange.

Richterite-bearing peridotites and MARID-type inclusions in lavas from North Eastern Morocco: mineralogy and D/H isotopic studies

K-richterite/phlogopite-bearing peridotite xenoliths and MARID inclusions have been found in Late Cretaceous (67±0.2 million years) monchiquites and an olivine nephelinite from North Eastern Morocco. It is the first evidence of MARID rocks and K-richterite/phlogopite-bearing peridotites outside the kimberlitic context. In the hydrous xenoliths, textural features suggest that K-richterite, phlogopite and Al-poor diopside are replacement minerals. K-richterites contain 2–5 wt% FeO, 0.1–1.5 wt% TiO2 and <0.5 wt% Cr2O3. Micas contain 5.4–7.4 wt% FeO and 0.3–2.2 wt% TiO2, with Cr and Ni contents <0.2 wt%. Diopsides are Al-poor (<0.2 wt% Al2O3) and contain 0.1–0.2 wt% TiO2, 0.9–1.1 wt% Na2O and 1.3–1.7 wt% Cr2O3. Compared to known K-richterites and micas from metasomatised peridotite nodules (PKP types), the Moroccan minerals are more Fe rich, K-richterites have higher Ti and micas less Cr and Ni. They are thus closer to MARID than to PKP minerals. K-richterites and mica from the MARID inclusions show typical characteristics, e.g. high FeO (4.3–4.7 wt% in richterite and 7.2 wt% in mica), low NiO and/or Cr2O3 and the incomplete filling of the tetrahedral site by Si+Al. Ion probe D/H analyses of amphiboles and micas from both xenolith types give high δD values ranging from –8 to –73, with large variations within single grains (up to 50‰). Both the D-enrichment and the δD variations are inherited from the mantle. The similar chemical composition and δD values of K-richterite/phlogopite from the hydrous peridotites and MARID minerals suggest a genetic link between the two types of xenoliths. The conditions required for producing MARIDs and K-richterite/phlogopite-bearing peridotites may thus exist in contexts other than stable cratonic settings. MARID rocks and the associated metasomites may result from a hydrous fluid interaction with a peridotite, the metasomatic agent being characterised by a high K and low Al signature and a high δD value. A D-rich source is involved in the metasomatic event producing the hydrous minerals, and the scatter observed in the δD values suggests a mixing between this source and another one with typical upper mantle D/H composition. As indicated by the low δD (–74) values of micas from the host lava, metasomatism predated and is unrelated to the alkaline volcanism.

Hydrogen isotope geochemistry of SNC meteorites

We report the yields and hydrogen isotopic compositions of water extracted by stepwise heating of eight whole-rock SNC meteorites. D/H values are consistent with terrestrial water at low temperatures and increase with temperature to values that far exceed terrestrial. The data are consistent with the water in the samples originating from two sources: a terrestrial component/contaminant, released largely at low temperature, and an extraterrestrial component, released at high temperature. The variation in δD values of the high-temperature hydrogen (∼+250 to +900 for the nakhlites, ∼+1200 to +2100 for the shergottites, and ∼+800 for ALH84001) could represent true variation of the δD of the extra-terrestrial water in the samples, or may reflect varying contributions of the terrestrial endmember, even at high temperature. The high δD values are consistent with a martian origin for the meteorites since the current martian atmosphere contains water with a δD of ∼+4000. The presence of alteration products in at least some of the samples suggests the D-enriched water was probably incorporated into the rocks through interaction at low temperature with aqueous crustal fluids that had exchanged with the martian atmosphere. Chassigny contains water that has terrestrial δD values at all temperatures (with the possible exception of the highest temperature step, with δD up to ∼+50), suggesting contamination of this sample by terrestrial water. Carbon and oxygen isotopic results for CO 2 extracted by stepwise heating are also reported. These data are consistent with formation of carbonates in the SNC meteorites by secondary processes on Mars (from fluids that had exchanged C and O with the atmosphere), perhaps in the same alteration events that formed the D-enriched minerals.

Hydrogen isotope fractionation factors between hydrous minerals and ore fluids at low temperatures: evidence from the Granny Smith gold deposit, Western Australia

There are no reported experimental data on hydrogen isotope fractionation between muscovite and water at low temperatures (< 400 °C). A fractionation curve derived from extrapolation of the high temperature calibration of Suzuoki and Epstein (1976) yields 20 to 40%. higher δD values than the empirical graphical calibration of Bowers and Taylor (1985) at temperatures of about 300 °C. Data from natural hydrothermal systems formed at approximately 300 °C, where δD analyses are available both from fluid inclusions and alteration muscovite/sericite, support the Bowers and Taylor (1985) calibration, thus indicating smaller fractionation factors at these temperatures than suggested by extrapolations from high-temperature experimental results.

Stable isotope geochemistry of clay minerals from fossil and active hydrothermal systems, southwestern Hokkaido, Japan

Miocene submarine to Quaternary terrestrial volcanism in southwestern Hokkaido, Japan, is associated with hydrothermal clay alteration and mineralization, including Kuroko-type deposits at Kagenosawa (14.2 Ma, Cu > Zn, Pb > Au) and Minamishiraoi (12.5 Ma, Ba > Zn, Pb, Cu), vein-style mineralization at Date (5.2 Ma, Au-Ag-Cu-Pb-Zn) and Chitose (3.6 Ma, Au-Ag), and geothermal activity at Noboribetsu (≤1.8 Ma). The δD and δ 18O values of mica (sericite), mica-smectite, chlorite, chlorite-smectite, nacrite, dickite, kaolinite, and smectite were used to deduce the type(s) of hydrothermal fluid at each locality. Calculated compositions for Minamishiraoi and Kagenosawa fluids suggest that seawater was dominant, but some mixing with magmatic water is also indicated, particularly for the polymetallic Kagenosawa deposit. Hydrothermal fluids at Date, Chitose, and the Noboribetsu geothermal area were dominated by meteoric water. Minor involvement of magmatic water during mineralization at Date cannot be ruled out, but evolution of local meteoric water along an evaporation trend and/or an 18O-shift due to hydrothermal rock-meteoric water interaction also could have produced appropriate fluid compositions. The δD and δ 18O values of modern hot-spring waters at Noboribetsu closely parallel fluid compositions calculated for the clay alteration at Date, Chitose, and Noboribetsu. Because relatively poor reproducibility was obtained for the δD values of the swelling clays, additional tests were conducted. Stepwise heating showed that, for some smectitic clays, water evolved between 200 and 300°C had anomalously high δD values because of residual interlayer water. This error can be minimized by sufficiently long preheating (in vacuo) at ≤200°C. In vacuo TG patterns of other smectitic clays suggested gradual loss of hydroxyl-groups beginning near 200°C, rather than the more typical distinct separation between interlayer water at <200°C and hydroxyl-groups at > 400°C. This behaviour constrains the maximum temperature that can be used for in vacuo preheating. Furthermore, shifts to lower δD values (by as much as 19‰) were obtained when this smectite was dispersed in low-D water for three weeks, perhaps indicating isotopic exchange. However, with appropriate care, δD values obtained by conventional procedures (including preheating to ≤200°C) normally reproduced natural compositions of the smectitic clays with acceptable accuracy and precision.

Crustal geochemistry in the Wawa–Foleyet region, Ontario

Fifty-three rock samples from the Kapuskasing structural zone (KSZ) and 56 from the Wawa domal gneiss terrane (WGT), Ontario, have been analysed for major elements, 32 trace elements, and δ18O; δD was measured in a few samples.Average chemical compositions for the KSZ and WGT regions have been calculated from map unit averages weighted by regional abundance. Compared with estimates of the composition of the upper continental crust (UCC), the KSZ is enriched in Al, Fe, Mg, Ca, P, transition elements, Sc, and Sr; depleted in Si, B, most rare earth elements, Zr, Hf, Nb, Ta, Li, Na, K, U, Th, Ba, and Rb; but similar in composition to estimates of the lower continental crust. The WGT is closer in composition to the UCC. These data support the interpretation, on geophysical and petrological grounds, that the crust here is layered and has been uplifted, the WGT and KSZ regions representing progressively deeper levels.Igneous rock and orthogneiss δ18O values in the KSZ and WGT show good correlation with the weight percentages of SiO2. Paragneisses and clastic sediments and metavolcanics in the Michipicoten greenstone belt have higher ratios, as in other greenstone belts. Low δ18O and high δD values in most of the higher grade mafic gneisses show that they have never undergone low-temperature marine alteration. However, a few mafic gneisses with relict pillows show δ18O and δD values indicating low-temperature aqueous alteration. The δ18O and δD evidence throughout the two regions supports the view that no regional fluid homogenization took place.Errorchron ages of 2725 ± 130 Ma (Rb/Sr) and 2755 ± 110 Ma (Sm/Nd) were obtained for tonalite–granodiorite gneiss and agree within error with results from other workers. The intrusive Floranna Lake complex is 2580 ± 120 Ma (Rb/Sr), not significantly different from the age of the tonalite. Scatter in the data is to be attributed to localized late-stage alteration under low water–rock ratios by Sr-rich–Rb-poor brines. Model Nd ages suggest that there is no old crust [Formula: see text] in the region and that the 2700 Ma mantle was depleted relative to the bulk earth.

Boron content and isotopic composition of oceanic basalts: Geochemical and cosmochemical implications

Ion microprobe determination of boron content and δ 11B values has been performed for a set of 40 oceanic basalt glasses (N-MORB, E-MORB, BABB and OIB) whose chemical characteristics (major and trace elements and isotopic ratios) are well documented. Boron contents, determined at ±10% relative, range from 0.34 to 0.74 ppm in N-MORB, whereas E-MORB, BABB and OIB extend to higher concentrations (0.5–2.4 ppm). After correction for crystal fractionation, this range is reduced to 0.5–1.3 ppm. N-MORB and E-MORB also exhibit different B/K ratios, 1.0 ± 0.3 × 10 −3 and 0.2 to 1.4 × 10 −3 respectively. This can be interpreted as resulting from the incorporation into the upper mantle of a K-rich and B-poor component (e.g., subducted oceanic crust having lost most of its initial boron). δ 11B values range between −7.40 ± 2 and +0.6 ± 2‰, with no significant difference between N-MORB, E-MORB, OIB or BABB. The Hawaiian samples define a strong linear correlation between boron contents, δ 11B values, MgO and water contents and δD values. This is interpreted as resulting from assimilation-fractionation processes which occurred within a water-rich oceanic crust, and which produced high δ 11B values associated with high δD values. The low level of 11B enrichment in the upper mantle constrains the amount of boron reinjected by subduction to a maximum of about 2% of the boron present in the subducted slab. This is turn corresponds to a maximum net Boron transfer of about 3 x 10 10 g/a towards the surface reservoirs. Finally, a boron content of 0.25 ± 0.1 ppm is estimated for the bulk silicate Earth (i.e., primitive mantle), corresponding to a depletion factor relative to C1 chondrites of about 0.15 and suggesting that B was moderately volatile upon terrestrial accretion.

Isotopic variation of the hydrothermal system on Vulcano Island, Italy

The isotopic and chemical composition of thermal waters and the isotopic composition of high-temperature fumarolic steam at Vulcano Island indicate an evolution of the hydrothermal system from its magmatic origin through interaction with rocks and mixing with meteoric water. Seawater is not a component in this system. The isotopic values of the fumaroles indicate a local magmatic composition of +7 to +8%. δ 18 O and −5 to +15%. δD. The high δD values of this vapor may reflect contamination of the magma by connate water. The isotopic and chemical composition of water from well W2 indicates the existence of geothermal fluid formed by mixing of HCl and SO 2-bearing condensates of volcanic origin with groundwater, with subsequent evolution via water-rock interaction. The estimated isotopic composition of the geothermal water is δ 18 O = +1%. and δD = −5%. Local shallow seismic events result in a temporary increase in fracture permeability, enabling the mixing of magmatic vapor with groundwater or geothermal water. Tectonic activity associated with deep structures appears to increase the contribution of magmatic vapor to the fumaroles.

Stable isotope geochemistry and phase equilibria of coesite-bearing whiteschists, Dora Maira Massif, western Alps

Peak metamorphic temperatures for the coesite-pyrope-bearing whiteschists from the Dora Maira Massif, western Alps were determined with oxygen isotope thermometry. The δ18O(smow) values of the quartz (after coesite) (δ18O=8.1 to 8.6‰, n=6), phengite (6.2 to 6.4‰, n=3), kyanite (6.1‰, n=2), garnet (5.5 to 5.8‰, n=9), ellenbergerite (6.3‰, n=1) and rutile (3.3 to 3.6‰, n=3) reflect isotopic equilibrium. Temperature estimates based on quartz-garnet-rutile fractionation are 700–750 °C. Minimum pressures are 31–32 kb based on the pressure-sensitive reaction pyrope + coesite = kyanite + enstatite. In order to stabilize pyrope and coesite by the temperature-sensitive dehydration reaction talc+kyanite=pyrope+coesite+H2O, the a(H2O) must be reduced to 0.4–0.75 at 700–750 °C. The reduced a(H2O) cannot be due to dilution by CO2, as pyrope is not stable at X(CO2)>0.02 (T=750 °C; P=30 kb). In the absence of a more exotic fluid diluent (e.g. CH4 or N2), a melt phase is required. Granite solidus temperatures are ∼680 °C/30 kb at a(H2O)=1.0 and are calculated to be ∼70°C higher at a(H2O)=0.7, consistent with this hypothesis. Kyanite-jadeite-quartz bands may represent a relict melt phase. Peak P-T-f(H2O) estimates for the whiteschist are 34±2 kb, 700–750 °C and 0.4–0.75. The oxygen isotope fractionation between quartz (δ18O=11.6‰) and garnet (δ18O=8.7‰) in the surrounding orthognesiss is identical to that in the coesitebearing unit, suggesting that the two units shared a common, final metamorphic history. Hydrogen isotope measurements were made on primary talc and phengite (δD(SMOW)=-27 to-32‰), on secondary talc and chlorite rite after pyrope (δD=-39 to -44‰) and on the surrounding biotite (δD=-64‰) and phengite (δD=-44‰) gneiss. All phases appear to be in nearequilibrium. The very high δD values for the primary hydrous phases is consistent with an initial oceanicderived/connate fluid source. The fluid source for the retrograde talc+chlorite after pyrope may be fluids evolved locally during retrograde melt crystallization. The similar δD, but dissimilar δ18O values of the coesite bearing whiteschists and hosting orthogneiss suggest that the two were in hydrogen isotope equilibrium, but not oxygen isotope equilibrium. The unusual hydrogen and oxygen isotope compositions of the coesite-bearing unit can be explained as the result of metasomatism from slab-derived fluids at depth.

A carbon, oxygen, hydrogen and sulfur isotopic study of the gold mineralization at Loulo, Mali

The isotopic compositions of oxygen, hydrogen, carbon and sulfur have been determined on silicate, carbonate and sulfide samples from auriferous tourmalinized sandstone, its footwall and hanging wall, of Birimian age (2.1 Ga), at Loulo, in Mali. The sandstones have undergone two phases of deformation. The samples were taken from three different orebodies — Loulo 0, Loulo 3 and P 64. For Loulo 0, δ 18O-values for vein quartz are consistently between + 16.1 and + 16.6‰ (SMOW) for oxygen, with one exception at + 11.6‰. The δ 18O-values for detrital quartz grains from the tourmalinized sandstone are also 16.1‰. Tourmaline has a high δ 18 O of + 13.9‰ . The δ 18O-values for total rocks are also relatively homogeneous at + 13.2 to + 15.7‰, the lowest values (+ 13.2 and + 14‰) being those for the footwall samples. Dolomites from Loulo 0 and P 64 have δ 18O-values between 15.1 and + 19.5‰ (SMOW) for oxygen and δ 13C-values of − 13.5 to − 2.0‰ (PDB), with the exception of one sample from the hanging wall, which is of sedimentary origin ( δ 13 C + 0.5‰ , δ 18 O + 22.2‰ ). The sampled dolomites are in veinlets or veins or disseminated in rocks and were deposited by hydrothermal fluids. The δ 34S-values of samples from the three orebodies range between + 6.0 and + 12.9‰ (CDT). They fall into different groups according to location, but pre- and post-schistosity sulfides within each group are indistinguishable. The results are interpreted as indicating the existence of two hydrothermal systems enriched in 18O at different times. One of these gave rise to tourmalinization, silicification and dolomitization of the sandstone and the deposition of sulfides. The high δD-values of the tourmalines, in agreement with petrological data, indicate that the fluid of this system was of marine origin. The two phases of folding intervened between the activity of this hydrothermal system and the initiation of the second system, which gave rise to the stockwork. This fluid, rich in 18O, was possibly of metamorphic origin, with a low water/rock ratio. It remobilized the pre-schistosity sulfides, concentrating them into post-schistosity veins. The (partly remobilized?) silica and carbonates were deposited under temperature conditions such that their isotopic compositions were similar to those observed for the tourmalinized sandstone. The vertical extent of this hydrothermal system is as yet poorly known. It is very extensive horizontally and strongly pervasive in the tourmalinized sandstone, but in the vertical sense appears to be confined to fractures in the footwall.