The isotopic composition of oxygen and carbon in the hyaloclastites from the Mt. Iblei volcanic area,Eastern Sicily: A preliminary study

Oxygen and carbon isotopic analyses were carried out for some typical submarine volcanic products (a lava flow, a pillow fragment and four hyaloclastite breccias) from the northwestern zone of the Mt. Iblei volcanic complex, eastern Sicily. The δ¹⁸O value of the perental basaltic magma (6.0 ± 0.2‰), estimated from the analyses of some fresh unaltered glassy samples of various type, lies in the values range of primary basalts. Appreciably higher δ¹⁸O values, probably due to low-temperature exchanges with sea water, have been found for lava samples and the interior of the pillow fragment. The δ¹⁸O and δ¹³C of the calcites of the groundmass of the hyaloclastite samples, ranging from 30.59 to 33.65 and from ?2.99 to 0.46‰ respectively, are typical of low-temperature marine carbonates. Because calcite is one of the last minerals to form. these results suggest that the hyaloclastites studied formed entirely in a submarine environment. The¹⁸O/¹⁶O ratios recorded in the silicate portions of the matrices of the hyaloclasites (δ¹⁸O=13.99 to 16.61) are interpreted as the result of halmyrolytic processes occurring at temperatures somewhat higher than that of the sea floor.     

Hydrothermal alteration and tectonic evolution of an intermediate- to fast-spreading back-arc oceanic crust: Late Ordovician Solund-Stavfjord ophiolite, western Norway

Abstract The Solund‐Stavfjord ophiolite complex (SSOC) in western Norway represents a remnant of the Late Ordovician oceanic lithosphere, which developed in an intermediate‐ to fast‐spreading Caledonian back‐arc basin. The internal architecture and magmatic features of its crustal component suggest that the SSOC has a complex, multistage sea floor spreading history in a supra‐subduction zone environment. The youngest crustal section associated with the propagating rift tectonics consists of a relatively complete ophiolite pseudostratigraphy, including basaltic volcanic rocks, a transition zone between the sheeted dyke complex and the extrusive sequence, sheeted dykes, and high‐level isotropic gabbros. Large‐scale variations in major and trace element distributions indicate significant remobilization far beyond that which would result from magmatic processes, as a result of the hydrothermal alteration of crustal rocks. Whereas K₂O is strongly enriched in volcanic rocks of the extrusive sequence, Cu and Zn show the largest enrichment in the dyke complex near the dyke–volcanic transition zone or within this transition zone. The δ¹⁸O values of the whole‐rock samples show a general depletion structurally downwards in the ophiolite, with the largest and smallest variations observed in volcanic rocks and the transition zone, respectively. δ¹⁸O values of epidote–quartz mineral pairs indicate 260–290°C for volcanic rocks, 420°C for the transition zone, 280–345°C for the sheeted dyke complex and 290–475°C for the gabbros. The ⁸⁷Sr/⁸⁶Sr isotope ratios show the widest range and highest values in the extrusive rocks (0.70316–0.70495), and generally the lowest values and the narrowest range in the sheeted dyke complex (0.70338–0.70377). The minimum water/rock ratios calculated show the largest variations in volcanic rocks and gabbros (approximately 0–14), and generally the lowest values and range in the sheeted dyke complex (approximately 1–3). The δD values of epidote (?1 to ?12‰), together with the δ¹⁸O calculated for Ordovician seawater, are similar to those of present‐day seawater. Volcanic rocks experienced both cold and warm water circulation, resulting in the observed K₂O‐enrichment and the largest scatter in the δ¹⁸O values. As a result of metal leaching in the hot reaction zone above a magma chamber, Zn is strongly depleted in the gabbros but enriched in the sheeted dyke complex because of precipitation from upwelling of discharged hydrothermal fluids. The present study demonstrates that the near intact effect of ocean floor hydrothermal activity is preserved in the upper part of the SSOC crust, despite the influence of regional lower greenschist facies metamorphism.     

Oxygen isotope evidence for past and present hydrothermal regimes of Long Valley caldera, California

Whole-rock oxygen isotope compositions of cores and cuttings from Long Valley exploration wells show that the Bishop Tuff has been an important reservoir for both fossil and active geothermal systems within the caldera. The deep Clay Pit-1 and Mammoth-1 wells on the resurgent dome penetrate mildly to strongly altered Bishop Tuff with δ¹⁸O_WR values as low as −2.6% (vs V-SMOW). The idfu 44-16 well intercepts a thinner Bishop Tuff section with δ¹⁸O_WR values of +0.4 to +2.3%. in the western caldera moat, where milder and more sporadic ¹⁸O depletions occur in Tertiary volcanic rocks of the western caldera floor (δ¹⁸O_WR = +2.2 to +6.4‰). Bishop Tuff samples from deeper parts of the 715 m rdo-8 (Shady Rest) well in the SW moat are also strongly depleted in ¹⁸O (δ¹⁸O_WR = −1.5 to +0.6‰). Four shallow thermal gradient wells (469–715 m td drilled in the western moat did not penetrate Bishop Tuff, but Early Rhyolites from two of these holes are depleted in ¹⁸O (δ¹⁸O_WR = −1.2 to +6.0‰ inplv-1 +4.6 to +5.3%. inmlgrap-1), compared to lithologic equivalents from the other two holes (δ¹⁸O_WR = +6.3 to +8.0‰ inplv-2 andmlgrap-2).Whole-rock oxygen isotope profiles for the resurgent dome wells are unlike profiles calculated assuming alkali feldspar-H₂O fractionation behavior and total O-isotopic equilibration with −14.3‰ fluids at measured temperatures. The sense of this divergence implies an earlier hydrothermal episode within the central caldera driven by one or more shallow intrusions. Geochemical similarities between an intrusive granophyre at the bottom of the Clay Pit-1 well and a nearby Moat Rhyolite dome with a K/Ar cooling age of 0.5 Ma suggest that vigorous hydrothermal activity beneath the central resurgent dome may have occurred as much as 0.5 m.y. ago. Calculated and measured O-isotope profiles are similar for deep wells that penetrate the western moat of the caldera, where steep temperature gradients and low δ¹⁸O_WR values in Early Rhyolites from plv-1 are attributed to an active hydrothermal aquifer that has descended slightly from earlier, shallower elevations. Similarly, severe ¹⁸O depletions in Bishop Tuff samples from the idfu 44-16 and rdo-8 wells reflect active convection beneath the western moat, whereas milder ¹⁸O depletions in Early Rhyolites from mlgrap-1 were apparently caused by hydrothermal alteration at lower temperatures. The O-isotope profiles imply that surface discharge within and around the resurgent dome results from shallow, eastward-directed outflow from a zone of higher enthalpy hydrothermal upflow beneath the western caldera moat. Intrusive magmatic heat source(s) are inferred to exist beneath the western moat, perhaps beneath Mammoth Mountain.     

18O/16O studies of fossil fissure fumaroles from the Valley of Ten Thousand Smokes, Alaska

At three sample sites where there are good exposures of the upper 15 m of the 1912 ash-flow sheet in the Valley of Ten Thousand Smokes (VTTS), Alaska, ¹⁸O/¹⁶O studies indicate that fumarolic activity produced a very wide range of δ¹⁸O values (?0.1 to +12.6; n=32) in the groundmass adjacent to fossil fissure fumaroles. This contrasts sharply with the uniformity of δ¹⁸O in the groundmass away from fumarolic conduits (δ¹⁸O=+5.9 to +7.1; n=7) and in all of the feldspar phenocrysts (δ¹⁸O=+6.11 to +7.5 1 for 11 samples from this study and Hildreth 1987), independent of whether these were collected from fossil fumaroles or from unaltered tuff. Only one sample contained feldspars that were even slightly ¹⁸O-enriched relative to the others (cloudy plagioclase δ¹⁸O=+8.45). and this sample also contained the most ¹⁸0-enriched groundmass of any of those analyzed (δ¹⁸O=+12.6). This preservation of primary magmatic δ¹⁸O values in the VTTS feldspar phenocrysts is clearly a consequence of the extremely short time span (i.e., 1912 to ≈1923) of vigorous, high-temperature, fumarolic activity in the 1912 ash-flow sheet. These ¹⁸O/^l6O systematica are strikingly similar to those discovered in the 2.8-Ma intracaldera Chegem Tuff (Gazis et al. 1996) and in the fossil fumaroles in the outflow sheet of the 0.76 Ma Bishop Tuff (Holt and Taylor 1998), thus confirming that a similar type of fumarolic meteoric-hydro-thermal activity occurred above the zone of intense welding in all three of these ash-flow tuffs. This is particularly important, because it provides a direct linkage between the older tuffs and the actual observations at the VTTS of steam chemistry, water/rock interaction, circulation geometry, flow velocities, and fumarolic temperatures (up to 645°C). The ¹⁸O/^l6O effects in the VTTS can all be explained in terms of a two-stage history: (a) an early, 10- to 15-year-long, high-temperature (τ;450°C), fumarolic ¹⁸O-depletion event (groundmass δ¹⁸O=?0.1 to +4.8); and (b) a subsequent, much longer-lived, low-temperature (<150°C), ¹⁸O-enrichment episode (groundmass as high as δ¹⁸O=+12.6). Steam in these low-temperature fumaroles probably passed through various parts of the same hydrothermal system associated with the earlier, higher-temperature, fumarolic activity, and a weakened form of this low-temperature hydrothermal circulation continues to the present day (Keith et al. 1992; Lowell and Keith 1991). This low-temperature ¹⁸O/¹⁶O exchange probably occurred in combination with mineralogical alteration of both the groundmass and the calcium-rich portions of feldspar phenocrysts during the waning (<150°C) stages of fumarolic activity (Spilde et al. 1993). The slight ¹⁸O enrichment of apparently pristine, transparent feldspar phenocrysts (δ¹⁸O=+7.51) in one of the ¹⁸O-depleted, meteoric-hydrothermally altered fumarolic samples (whole-rock δ¹⁸O=+4.8) probably indicates that this sample was incipiently altered at low temperatures as fumarolic activity waned, and thus may have had a whole-rock δ¹⁸O value much lower than +4.87‰ prior to 1923.     

O/O mapping and hydrogeology of a short-lived (≈10 years) fumarolic (>500°C) meteoric–hydrothermal event in the upper part of the 0.76 Ma Bishop Tuff outflow sheet, California

¹⁸O/¹⁶O data from the 200-m-thick, 0.76 Ma Bishop Tuff outflow sheet provide evidence for a vigorous, short-lived (≈10 years), high-temperature, fumarolic meteoric–hydrothermal event. This is proved by: (1) the juxtaposition in the upper, partially welded Bishop Tuff of low-¹⁸O groundmass/glass (δ¹⁸O=−5 to +3) with coexisting quartz and feldspar phenocrysts having magmatic δ¹⁸O values (+8.7±0.3; +7.5±0.3); and (2) the fact that these kinds of ¹⁸O/¹⁶O signatures correlate very well with morphological features and mapped zones of fumarolic activity. Profiles of δ¹⁸O with depth in the Bishop Tuff within the fumarole area define a 40- to 50-m-thick, low-¹⁸O, stratigraphic zone that is sandwiched between the essentially unwelded near-surface portion of the tuff and an underlying, densely welded black tuff that displays magmatic ¹⁸O/¹⁶O values. Shallow-dipping columnar joints and other fumarolic features (i.e., subhorizontal tubular conduits and steep fissures) correlate very well with these pervasively devitrified, low-¹⁸O zones. The base of the low-¹⁸O zone is extremely sharp (3‰ per meter) and is located directly above the transition from partially welded tuff to densely welded black tuff. The observed average whole-rock ¹⁸O-depletions within this low-¹⁸O zone are about 6–7‰, requiring meteoric water/rock ratios in excess of 0.24 in mass units. Rainfall on the surface of the tuff would not have been high enough to supply this much H₂O in the short lifetime of fumarolic activity, suggesting that some recharge must have been from groundwater flow through the upper part of the tuff, above the sloping (1°–5°) top of the impermeable lower zone. This is compatible with the observation that the fumarolic areas roughly correlate with the preeruptive regional drainage pattern. Some of this recharge may in part have been from the lake that filled Long Valley caldera, which was dammed by the Bishop Tuff up to the level of this boundary between the partially and densely welded zones (≈7000 ft, the elevation of the highest Long Valley Lake shorelines). Gazis et al. had previously shown that the 2.8-Ma intracaldera Chegem Tuff from the Caucasus Mountains exhibits exactly the same kind of ¹⁸O-signature that we have correlated with fossil fumaroles in the Bishop Tuff outflow sheet. Although not recognized as such by McConnell et al.; ¹⁸O/¹⁶O data from drill-hole samples from the intracaldera Bishop Tuff in Long Valley also display this characteristic ¹⁸O signature (i.e., analogous δ¹⁸O-depth profiles, as well as low-¹⁸O groundmass coexisting with high-¹⁸O feldspar phenocrysts). This fumarolic ¹⁸O/¹⁶O signature is observed to much greater depths (≈650–750 m) in the intracaldera tuffs (≈1500 m thick) than it is in the ≈200-m-thick Bishop Tuff outflow sheet (≈80 m depth).     

Oxygen isotope variations in phosphate of biogenic apatites,II. Phosphorite rocks

Phosphorites from sedimentary sequences ranging in age from Archaean to Recent were analysed for δ¹⁸O in both the PO₄ (δ¹⁸O_p) and CO₃ (δ¹⁸O_c) in the apatite lattice. The oxygen isotope record is considerably better preserved in phosphates than in either carbonates or cherts. The use of the Longinelli and Nuti [8] temperature equation yields temperatures for Recent phosphorites that are in good agreement with those measured in the field. The δ¹⁸O_p values of ancient phosphorites decrease with increasing age. These changes with time are not likely to be due to post-depositional exchange. Changes in δ¹⁸O values of seawater and variations of temperature with time can account for the δ¹⁸O_p time trend, but the latter explanation is preferred. In Ancient phosphorites δ¹⁸O_c in structurally bound carbonate in apatite is not a reliable geochemical indicator.     

Oxygen isotopes and the origin of high-87Sr/86Sr andesites

Andesites from the Peruvian Andes and the Banda arc of Indonesia are characterized by unusually high and variable ⁸⁷Sr/⁸⁶Sr ratios. The Banda arc samples, including two cordierite-bearing lavas from Ambon, show a clear positive correlation between ⁸⁷Sr/⁸⁶Sr and δ¹⁸O. The andesitic rocks have δ¹⁸O values that range from 5.6 to 9.2‰. Over that range in δ¹⁸O, ⁸⁷Sr/⁸⁶Sr increases from 0.7044 to 0.7095. The cordierite-bearing lavas have δ¹⁸O values of approximately 15‰ and ⁸⁷Sr/⁸⁶Sr ratios of approximately 0.717. The similarity between δ¹⁸O values and ⁸⁷Sr/⁸⁶Sr ratios in total rocks and separated plagioclase phenocrysts of the Banda arc samples indicates that the measured isotope ratios are primary and have not been affected by secondary, low-temperature post-eruptive alteration. The observed variation between O and Sr isotopic ratios can be modeled by two-component mixing in which one component is of mantle isotopic composition. As the crust beneath the Banda arc is probably oceanic, contamination of the manle component may have resulted from the subduction of either continentally-derived sediments or continental crust. Mixing calculations indicate that the contaminant could have an isotopic composition similar to that observed in the cordierite-bearing lavas.The Andean samples, despite petrographic evidence of freshness, exhibit whole-rock δ¹⁸O values significantly higher than those of corresponding plagioclase phenocryst separates, indicating extensive low-temperature post-eruptive alteration. The plagioclase mineral separates show a range of δ¹⁸O values between 6.9 and 7.9‰. The ⁸⁷Sr/⁸⁶Sr ratios of these same samples are, in most instances, not significantly different from those measured for the whole rock, thus signifying that the phenocrysts and groundmass were in isotopic equilibrium at the time of eruption. Unlike the lavas of the Banda arc, the Andean lavas show no strong positive correlation between ⁸⁷Sr/⁸⁶Sr ratios and δ¹⁸O values, but instead lower ⁸⁷Sr/⁸⁶Sr ratios appear to be associated with higher δ¹⁸O values. The δ¹⁸O and ⁸⁷Sr/⁸⁶Sr values of the Peruvian samples are both slightly higher than those of “normal” island arc volcanics.The small proportions of contaminant implied by the O isotope results seem to preclude continental crustal contamination as a primary cause of high ⁸⁷Sr/⁸⁶Sr ratios. The most plausible process that can explain both O and Sr isotope results is one in which sediments of continental origin are partially melted in the subduction zone. These melts rise into overlying mantle material and subsequently participate in the formation of calc-alkaline magmas.If the involvement of a sialic component in the genesis of andesitic magma occurs in the subduction zone, melting of that sialic material signifies temperatures of at least 750–800°C at the top of the subducted lithospheric slab at depths of approximately 150 km. The fact that contamination has apparently occurred in the Banda arc samples without producing any simple widespread correlations between Sr and O isotopic compositions on the one hand and major or trace element abundances on the other, shows that isotopic correlations, possibly including pseudo-isochrons, can be produced by mixing without producing trace element mixing correlations. Because O versus Sr isotope correlations are little affected by processes of partial melting of differentiation, they provide a direct means of testing whether Sr isotopic variations in volcanic rocks are of mantle origin or are due instead to mixing with sialic material.     

18O/16O ratios in cherts associated with the saline lake deposits of East Africa

The cherts formed from sodium silicate precursors in East African saline, alkaline lakes have δ¹⁸O values ranging from 31.1 to 44.1. The δ¹⁸O values correlate in general with lake salinities as inferred from geologic evidence, indicating that most chert was formed from its precursor in contact with lake water trapped at the time of deposition. A few of the analyzed cherts probably formed in contact with dilute meteoric water. From the widely varying δ¹⁸O values we conclude that precursors were transformed to chert in fluids of widely varying salinity and a_Na⁺/a_H⁺ ratio.     

Biomass effects on stalagmite growth and isotope ratios: A 20th century analogue from Wiltshire, England

Increases in calcite deposition rates combined with decreases in δ¹³C and δ¹⁸O in three modern stalagmites from Brown's Folly Mine, Wiltshire, England, are correlative with a well-documented re-vegetation above the mine. Increased soil P_CO2 resulted in greater amounts of dissolved CaCO₃ in the drip waters, which consequently increased annual calcite deposition rates. The absence of deposition prior to 1916 (28 years after the mine was closed) indicates that vegetation had not yet sufficiently developed to allow higher P_CO2 values to form in the soil. Lower δ¹³C values through time may reflect the increased input of isotopically light biogenic carbon to the total dissolved inorganic carbon (DIC). δ¹⁸O decreased synchronously with δ¹³C, reflecting the increased importance of isotopically light winter recharge due to greater biomass-induced summer evapotranspiration. This is the first empirical demonstration that vegetation density can control stalagmite growth rates, δ¹³C, and δ¹⁸O, contributing critical insights into the interpretation of these climate proxies in ancient stalagmites.     

Spatial distribution and seasonal variation in 18O/16O of modern precipitation and river water across the conterminous USA

We report a quantitative analysis of regional differences in the the oxygen isotope composition of river water and precipitation across the USA because data are now available to undertake a more geographically and temporally extensive analysis than was formerly possible. Maps of modern, mean annual δ¹⁸O values for both precipitation (δ¹⁸O_PPT) and river water (δ¹⁸O_RIV) across the 48 contiguous states of the USA have been generated using latitude and elevation as the primary predictors of stable isotope composition while also incorporating regional and local deviations based on available isotopic data. The difference between these two maps was calculated to determine regions where δ¹⁸O_RIV is significantly offset from local δ¹⁸O_PPT. Additional maps depicting seasonal and extreme values for δ¹⁸O_RIV and δ¹⁸O_PPT were also constructed. This exercise confirms the presence of regions characterized by differences in δ¹⁸O_RIV and δ¹⁸O_PPT and specifically identifies the magnitude and regional extent of these offsets. In particular, the Great Plains has δ¹⁸O_RIV values that are more positive than precipitation, while much of the western USA is characterized by significantly lower δ¹⁸O_RIV values in comparison with local δ¹⁸O_PPT. The most salient feature that emerged from this comparison is the ‘catchment effect’ for the rivers. Because river water is largely derived from precipitation that fell upstream of the sample locality (i.e. at higher elevations) δ¹⁸O_RIV values are often lower than local δ¹⁸O_PPT values, particularly in catchments with high‐elevation gradients. Seasonal patterns in the isotopic data substantiate the generally accepted notion that amplitudes of δ¹⁸O variation are greatly dampened in river water relative to those of local precipitation. Copyright © 2005 John Wiley & Sons, Ltd.     

Using environmental isotopes to investigate the groundwater recharge mechanisms and dynamics in the North-eastern Basin,Palestine

ABSTRACT

This study aims to differentiate the potential recharge areas and flow mechanisms in the North-eastern Basin, Palestine. The results differentiate the recharge into three main groups. The first is related to springs and some of the deep wells close to the Anabta Anticline, through the Upper Aquifer (Turonian) formation, with depleted δ¹⁸O and δ²H. The second is through the Upper Cenomanian formation surrounding the Rujeib Monocline in the southeast, where the lineament of the Faria Fault plays an important role, with relatively enriched δ¹³C_DIC values of about ?4‰ (VPDB). The third is the Jenin Sub-series, which shows higher δ¹³C_DIC values, with enriched δ¹⁸O and δ²H and more saline content. The deep wells from the Nablus area in the south of the basin indicate low δ¹³C_DIC values due to their proximity to freshwater infiltrating faults. The deep wells located to the northwest of the basin have δ¹³C_DIC values from ?8 to ?9‰ (VPDB), with enriched δ¹⁸O signatures, indicating slow recharge through thick soil.     

Using stable isotopes of surface water and groundwater to quantify moisture sources across the Yellow River source region

Characterization of stable isotope compositions (δ²H and δ¹⁸O) of surface water and groundwater in a catchment is critical for refining moisture sources and establishing modern isotope–elevation relationships for paleoelevation reconstructions. There is no consensus on the moisture sources of precipitation in the Yellow River source region during summer season. This study presents δ²H and δ¹⁸O data from 111 water samples collected from tributaries, mainstream, lakes, and groundwater across the Yellow River source region during summertime. Measured δ¹⁸O values of the tributaries range from ?13.5‰ to ?5.8‰ with an average of ?11.0‰. Measured δ¹⁸O values of the groundwater samples range from ?12.7‰ to ?10.5‰ with an average of ?11.9‰. The δ¹⁸O data of tributary waters display a northward increase of 1.66‰ per degree latitude. The δ¹⁸O data and d‐excess values imply that moisture sources of the Yellow River source region during summertime are mainly from the mixing of the Indian Summer Monsoon and the Westerlies, local water recycling, and subcloud evaporation. Analysis of tributary δ¹⁸O data from the Yellow River source region and streamwater and precipitation δ¹⁸O data from its surrounding areas leads to a best‐fit second‐order polynomial relationship between δ¹⁸O and elevation over a 4,600 m elevation range. A δ¹⁸O elevation gradient of ?1.6‰/km is also established using these data, and the gradient is in consistence with the δ¹⁸O elevation gradient of north and eastern plateau. Such relationships can be used for paleoelevation reconstructions in the Yellow River source region.     

Oxygen and hydrogen isotope studies of plutonic granitic rocks

The primary δD values of the biotites and hornblendes in granitic batholiths are remarkably constant at about ?50 to ?85, identical to the values in regional metamorphic rocks, marine sediments and greenstones, and most weathering products in temperate climates. Therefore the primary water in these igneous rocks is probably not “juvenile”, but is ultimately derived by dehydration and/or partial melting of the lower crust or subducted lithosphere. Most granitic rocks have δ¹⁸O = +7.0 to +10.0, probably indicating significant involvement of high-¹⁸O metasedimentary or altered volcanic rocks in the melting process; such an origin is demanded for many other granodiorites and tonalites that have δ¹⁸O = +10 to +13. Gigantic meteoric-hydrothermal convective circulation systems were established in the epizonal portions of all batholiths, locally producing very low δ¹⁸O values (particularly in feldspars) during subsolidus exchange. Some granitic plutons in such environments also were emplaced as low-¹⁸O magmas probably formed by melting or assimilation of hydrothermally altered roof rocks. However, the water/rock ratios were typically low enough that over wide areas the only evidence for meteoric water exchange in the batholiths is given by low D/H ratios (δD as low as ?180); for example, because of latitudinal isotopic variations in meteoric waters, as one moves north through the Cordilleran batholiths of western North America an increasingly higher proportion of the granitic rocks have δD values lower than ?120. The lowering of δD values commonly correlates with re-setting of K-Ar ages, and in the Idaho batholith two broad zones (10,000 km²) can be defined where δD biotite <?100 and K-Ar “ages” have all been re-set to values less than 60 m.y., suggesting that the Ar loss was caused by the meteoric-hydrothermal circulation systems. In certain Precambrian batholiths, a much different type of very low-temperature, regional alteration by surface-derived waters took place over an extended period long after emplacement, producing “brick-red” feldspars and markedly discordant Rb-Sr isochron “ages”.     

Stable isotope ratios of volcanic steam from White Island,New Zealand

The D/H and¹⁸O/¹⁶O ratios of fumarole condensates from White Island, an andesite volcano in the Bay of Plenty, have been measured during the period 1965 to 1969 to determine the origin of the water and the changes which occur as the volcanic activity changes. The D/H and¹⁸O/¹⁶O ratios of all of the samples were correlated with a slope of ?2. The δ¹⁸O values were proportional to the logarithm of the chloride concentration but with distinctly different relationships between the period from 1965 to 1967, and the period of tephra eruptions in 1968. In the latter period the chloride contents were close to that of sea water while in the former quieter period the contents were lower. The δD values follow a similar pattern but with a poorer correlation, indicating that a more variable process is controlling the deuterium results. Possible hydrothermal models are: mixing of near surface water and magmatic water; progressive leaching of chloride from underground rocks and exchange of isotopes (Craig, 1966); and lastly, equilibrium evaporation at about 255°C of either sea water or local surface water. The first of these models is considered most unlikely because of the incompatible logarithmic relationship between the chloride content and the oxygen isotope results. The second model has some merit, but the available evidence favours the last of the three models in which the 1965–67 samples derive from surface water or vapour from a boiling chloride water aquifer of sea water origin and the 1968 samples derive from the boiling chloride water, because of disturbance of the system during the tephra eruptions.     

Calibration method affects the measured δ2H and δ18O in soil water by direct H2Oliquid–H2Ovapour equilibration with laser spectroscopy

The direct H₂O_liquid–H₂O_vapour equilibration method utilizing laser spectroscopy (DVE-LS) is a way to measure soil pore water stable isotopes. Various equilibration times and calibration methods have been used in DVE-LS. Yet little is known about their effects on the accuracy of the obtained isotope values. The objective of this study was to evaluate how equilibration time and calibration methods affect the accuracy of DVE-LS. We did both spiking and field soil experiments. For the spiking experiment, we applied DVE-LS to four soils of different textures, each of which was subjected to five water contents and six equilibration times. For the field soil experiment, we applied three calibration methods for DVE-LS to two field soil profiles, and the results were compared with cryogenic vacuum distillation (CVD)-LS. Results showed that DVE-LS demonstrated higher δ²H and δ¹⁸O as equilibration time increased, but 12 to 24 hr could be used as optimal equilibration time. For field soil samples, DVE-LS with liquid waters as standards led to significantly higher δ²H and δ¹⁸O than CVD-LS, with root mean square error (RMSE) of 8.06‰ for δ²H and 0.98‰ for δ¹⁸O. Calibration with soil texture reduced RMSE to 3.53‰ and 0.72‰ for δ²H and δ¹⁸O, respectively. Further, calibration with both soil texture and water content decreased RMSE to 3.10‰ for δ²H and 0.73‰ for δ¹⁸O. Our findings conclude that the calibration method applied may affect the measured soil water isotope values from DVE-LS.     

Orographic distillation and spatio‐temporal variations of stable isotopes in precipitation in the North Atlantic

Little is known about the spatial and temporal variability of the stable isotopic composition of precipitation in the North Atlantic and its relationship to the North Atlantic Oscillation (NAO) and anthropogenic climate change. The islands of the Azores archipelago are uniquely positioned in the middle of the North Atlantic Ocean to address this knowledge gap. A survey of spatial and temporal variability of the stable isotope composition of precipitation in Azores is discussed using newly presented analyses along with Global Network of Isotopes in Precipitation data. The collected precipitation samples yield a new local meteoric water line (δ²H = 7.1 * δ¹⁸O + 8.46) for the Azores region and the North Atlantic Ocean. The annual isotopic mean of precipitation shows a small range for the unweighted and precipitation mass‐weighted δ¹⁸O‐H₂O values. Results show an inverse relation between the monthly δ¹⁸O‐H₂O and the amount of precipitation, which increases in elevation and into the interior of the island. Higher amounts of precipitation (from convective storm systems) do not correspond to the most depleted values of stable isotopes in precipitation. Precipitation shows an orographic effect with depleted δ¹⁸O‐H₂O values related to the Rayleigh effect. Monthly δ¹⁸O‐H₂O values for individual precipitation sampling stations show little relationship to air temperature. Results show a local source of moisture during the summer with the characteristics of the first vapour condensate. The stable isotope composition of precipitation is strongly correlated to the NAO index, and δ¹⁸O‐H₂O values show a statistically significantly trend towards enrichment since 1962 coincident with the increased air temperature and relative humidity due to climate change. Results are in line with observations of increasing sea surface temperature and relative humidity.     

The δ13C–δ18O variations in marble in the Hida Belt,Japan

Marble has a great potential to understand a history of various geological events occurring during tectonic processes. In order to decode metamorphic–metasomatic records on C–O isotope compositions of marble at mid-crustal conditions, we conducted a C–O–Sr isotope study on upper amphibolite-facies marbles and a carbonate–silicate rock from the Hida Belt, which was once a part of the crustal basement of the East Asian continental margin. Carbon and oxygen isotope analyses of calcite from marbles (Kamioka area) and a carbonate–silicate rock (Wadagawa area) show a large variation of δ¹³C [VPDB] and δ¹⁸O [VSMOW] values (from −4.4 to +4.2 ‰ and +1.6 to +20.8 ‰, respectively). The low δ¹³C values of calcites from the carbonate–silicate rock (from −4.4 to −2.9 ‰) can be explained by decarbonation (CO₂ releasing) reactions; carbon–oxygen isotope modeling suggests that a decrease of δ¹³C strongly depends on the amount of silicate reacting with carbonates. The occurrence of metamorphic clinopyroxene in marbles indicates that all samples have been affected by decarbonation reactions. All δ¹⁸O values of calcites are remarkably lower than the marine-carbonate values. The large δ¹⁸O variation can be explained by the isotope exchange via interactions between marble, external fluids, and/or silicates. Remarkably low δ¹⁸O values of marbles that are lower than mantle value (~+5 ‰) suggest the interaction with meteoric water at a later stage. Sr isotope ratios (⁸⁷Sr/⁸⁶Sr = 0.707255–0.708220) might be close to their protolith values. One zircon associated with wollastonite in a marble thin-section yields a U–Pb age of 222 ± 3 Ma, which represents the timing of the recrystallization of marble, triggered by H₂O-rich fluid infiltration at a relatively high-temperature condition. Our isotope study implies that the upper amphibolite-facies condition, like the Hida Belt, might be appropriate to cause decarbonation reactions which can modify original isotope compositions of marble if carbonates react with silicates.     

δO of carbonate, quartz and phosphate from belemnite guards: implications for the isotopic record of old fossils and the isotopic composition of ancient seawater

Belemnite guards of Cretaceous and Jurassic age were found to contain varying amounts of quartz deposited both on the external surface and inside the rostrum. The oxygen isotopic composition of coexisting carbonate, quartz and phosphate from the same rostrum was measured according to well-established techniques. None of these compounds showed isotopic values in equilibrium with one another. Assuming δ¹⁸O values of the diagenetic water within the range of meteoric waters, the δ¹⁸O(SiO₂) yield temperatures in agreement with the apparent secondary origin of this phase. The δ¹⁸O(CO₃²⁻) range, with a certain continuity, between −10.8 and +0.97 PDB-1 with most of the intermediate values being within the range of the carbonate isotopic values of Mesozoic fossils. The most positive isotopic results obtained from phosphate are close to +23/+24‰ (V-SMOW). They can hardly be related to a secondary origin of the phosphate, or to the presence of diagenetic effects, since these results are among the most positive ever measured on phosphate. As far as we know there is no widespread diagenetic process determining an ¹⁸O enrichment of phosphate. The very low concentration of phosphate did not allow the determination of its mineralogical composition. All the available δ¹⁸O(PO₄³⁻) values from belemnite and non-belemnite fossils of marine origin of Tertiary and Mesozoic age are reported along with the newly measured belemnites. The following conclusions may be drawn from the data reported: (1) the pristine oxygen isotope composition of fossil marine organisms (either carbonate or phosphate) may easily undergo fairly large changes because of oxygen isotope exchange processes with diagenetic water; this process is apparent even in the case of geologically recent fossils; (2) the δ¹⁸O(PO₄³⁻) of belemnite rostra seems to be, at least in the case of the most positive results, in isotopic equilibrium with environmental water because of the similarity between the results from Cretaceous belemnites and the results from Cretaceous and Lower Tertiary pelecypods and fish teeth; 3) if so, the only feasible interpretation that can be suggested for the ¹⁸O enriched data is the possibility of a relatively large variation of the oxygen isotopic composition of ocean paleowater from Jurassic to recent time.     

Oxygen isotope geochemistry of cherts from the Onverwacht Group (3.4 billion years), Transvaal,South Africa,with implications for secular variations in the isotopic composition of cherts

δ¹⁸O values for 87 chert samples from the 3.4-b.y.-old Onverwacht Group, South Africa, range from +9.4 to +22.1‰. δ-values for cherts representing early silicified carbonates and evaporites, and possible primary precipitates range from +16 to +22‰ and are distinctly richer in¹⁸O than silicified volcaniclastic debris and cherts of problematical origin. The lower δ-values for the latter two chert types are caused by isotopic impurities such as sericite and feldspar, and/or late silicification at elevated temperature during burial. Cherts with δ-values below +16‰ are thus not likely to yield geochemical data relevant to earth surface conditions.Fine-grained chert is less than 0.7‰ depleted in¹⁸O relative to coexisting coarse drusy quartz. Because coarse quartz preserves its isotopic composition with time, the maximum amount of post-depositional lowering of the δ-values of cherts by long-term isotopic exchange with meteoric groundwaters does not exceed 0.7‰ in 3.4 b.y. In response to metamorphism the δ-values of Onverwacht cherts appear to remain unchanged or to have increased by as much as 4‰. Neither metamorphism nor long-term isotopic exchange with groundwaters can explain why Onverwacht cherts are depleted in¹⁸O relative to their Phanerozoic counterparts.Meteoric waters with a δ¹⁸O range of at least 3‰ appear to have been involved in Onverwacht chert diagenesis. δ-values for possible primary cherts or cherts representing silicified carbonates and evaporites are compatible with the depositional and diagenetic environments deduced from field and petrographic evidence. Onverwacht cherts appear to have formed with δ-values at least 8‰ lower than Phanerozoic cherts.The new Onverwacht data combined with all published oxygen isotope data for cherts suggest a secular trend similar to that initially suggested by Perry (1967) in which younger cherts are progressively enriched in¹⁸O. However, Precambrian cherts appear to be richer in¹⁸O than Perry's original samples and can be reasonably interpreted in terms of declining climatic temperatures from ～70°C at 3.4 b.y. to present-day values, as initially suggested by Knauth and Epstein (1976). This surface temperature history is compatible with existing geological, geochemical, and paleontological evidence.     

Helium,oxygen, strontium and neodymium isotopic relationships in Icelandic volcanics

³He/⁴He ratios have been obtained for basaltic, intermediate and acid volcanic glasses from Iceland. Basaltic glasses exhibit a wide range of ³He/⁴He ratios (4 < R/R_a < 24), which is consistent with the previously recorded range for Icelandic geothermal systems. In contrast the glasses with intermediate and acid compositions have ³He/⁴He values close to the atmospheric value (R_a) with the exception of a 13-Ma sample which has R/R_a= 0.07. ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd ratios and δ¹⁸O values are reported for the same samples.³He/⁴He does not correlate with either ⁸⁷Sr/⁸⁶Sr or ¹⁴³Nd/¹⁴⁴Nd ratio and radiogenic components of He, Sr and Nd have apparently been decoupled. Interaction of Icelandic magmas with hydrothermally altered and older Icelandic crust is the preferred explanation for variable and often low δ¹⁸O values. It is suggested that primary ³He/⁴He ratios may have been modified by incorporation of radiogenic helium developed within the Icelandic crust to impose a larger range of ³He/⁴He ratios on the erupted products than was actually inherited from the mantle beneath Iceland. Intermediate and acid samples have all been severely contaminated by atmospheric helium, most probably at very shallow levels within the crust.