A Nd,Sr and O isotopic investigation into the causes of chemical and isotopic zonation in the Bishop Tuff,California

The Bishop Tuff represents a single eruption of chemically zoned rhyolitic magma. Six whole rock samples spanning the compositional and temperature range yield initial⁸⁷Sr/⁸⁶Sr of 0.7060–0.7092 andδ¹⁸O of 5.9–10.3‰. Six constituent sanidines yield smaller ranges of initial⁸⁷Sr/⁸⁶Sr of 0.7061–0.7069 andδ¹⁸O of 6.7–7.9. In contrast¹⁴³Nd/¹⁴⁴Nd ratios for the six whole rocks and two constituent magnetites exhibit negligible variation with a mean of0.51258 ± 1. These data are used to show that the phenocrysts were precipitated from an already chemically zoned liquid, that the zoning process involved negligible assimilation of, or exchange with, country rocks and that the extreme Sr and O isotopic disequilibria are probably the result of post-eruptive interaction with meteoric water. The parent magma had?Nd = ?0.9, ?Sr = +23 andδ¹⁸O = 7‰ and was formed from mantle-derived magmas and/or melts of lower crustal rocks isotopically similar to parts of the Sierra Nevada Batholith.     

The isotopic cycle of oceanic phosphate,I

Steady-state conditions seem to exist in the Atlantic and Pacific Oceans in the case of the¹⁸O content of dissolved phosphate. The measured δ¹⁸O(PO₄^3?) values are fairly constant at all latitudes and depths. A constant difference of about 1‰ exists between average Atlantic and Pacific results. Oxygen isotope measurements carried out on phosphate in the organic matter of fish, showed an average difference of about 3‰ when compared with dissolved oceanic phosphate. Such a difference can be attributed to biological fractionation effects in the metabolism of phosphate. However, the equilibrium fractionation factors in the phosphate-water system are poorly known and consequently we do not know whether this difference represents an approach to, or a deviation from, isotopic equilibrium conditions.     

Strontium,lead and oxygen isotopic investigation of the Skaergard intrusion,East Greenland

New Pb, Sr and O isotopic analyses of rocks from the Skaergard intrusion indicate the following: (1) initial⁸⁷Sr/⁸⁶Sr of the gabbroic magma was less than or equal to 0.7041; (2) limited contamination of magma with crustal Sr and Pb may have occurred in a deep reservoir below the presently exposed parts of the intrusion; (3) marked crustal contamination occurred at high level in marginal border group rocks, but these rocks effectively shielded the main magma body from further interaction with country rock gneisses; (4) subsolidus interaction between Skaergard gabbros and hydrothermal fluids modified δ¹⁸O values but had little effect on Sr and perhaps Pb isotopic ratios; (5) late-stage melanogranophyres may be comagmatic with the Skaergard magma, but silicic granophyres are not; (6) silicic granophyres contain large and varied proportions of crustal Sr and Pb; some may be largely anatectic melts derived from the deep crust whereas others may represent mixing of such anatectic melts with late-stage differentiated liquids of the Skaergard intrusion (e.g. Sydtoppen sill).     

Sm-Nd isotopic evolution of chondrites and achondrites,II

The¹⁴⁷Sm¹⁴³Nd and¹⁴⁶Sm¹⁴²Nd isotope systematics have been investigated in five chondrites and the achondrites Moama and Angra dos Reis (ADOR). The new chondrite data and those we have reported before are all consistent with our previously reported reference values for CHUR (“chondritic uniform reservoir”) of (¹⁴³Nd/¹⁴⁴Nd)_CHUR⁰ = 0.511847 and (¹⁴⁷Sm/¹⁴⁴Nd)_CHUR⁰ = 0.1967. Most of the bulk chondrites analyzed have ¹⁴³Nd/¹⁴⁴Nd and ¹⁴⁷Sm/¹⁴⁴Nd within 0.5 ε-units and 0.15% of the CHUR values, respectively. This strongly suggests that the CHUR evolution is now known to within these error limits throughout the history of the solar system. The St. Severin chondrite yields an SmNd internal isochron age of T = 4.55 ± 0.33AE and an initial ε_Nd = 0.11 ? 0.26. Much larger variations in Sm/Nd ratios were measured in mineral separates of the Moama and ADOR achondrites. Thus, very precise ages of 4.46 ± 0.03AE and4.564 ± 0.037AE were obtained for these meteorites, respectively. The initial ε_Nd values obtained for Moama and ADOR are 0.03 ? 0.25and0.14 ? 0.20, respectively. The values obtained on these meteorites are fully consistent with the CHUR evolution curve. Initial ε_Nd data on terrestrial igneous and meta-igneous rocks demonstrates that positive initial ε_Nd values occur throughout the past 4 AE. This confirms our earlier report that a light rare earth element-depleted layer has existed throughout most of the Earth history and is the source of present-day mid-ocean ridge basalts. The inferred shape of the ε_Nd vs. age curve for the depleted mantle suggests profound changes in tectonic regimes with time; in particular, it suggests a much higher rate of recycling of continental materials into the mantle during the Archean as compared to later time periods.¹⁴⁶Sm¹⁴²Nd systematics of ADOR and Moama are supportive of the hypothesis that¹⁴⁶Sm was present in the early solar system and suggests a ¹⁴⁶Sm/¹⁴⁴Sm ratio of about 0.01 for the solar system ～ 4.56 AE ago. This inferred high¹⁴⁶Sm abundance cannot be explained as a late injection from a supernova and must be due to galactic nucleo-synthesis.     

Strontium isotopic geochemistry of intrusive rocks,Puerto Rico,Greater Antilles

The strontium isotopic geochemistry is given for three Puerto Rican intrusive rocks: the granodioritic Morovis and San Lorenzo plutons and the Rio Blanco stock of quartz dioritic composition. The average calculated initial⁸⁷Sr/⁸⁶Sr ratios are 0.70370, 0.70355 and 0.70408, respectively. In addition, the San Lorenzo data establish a whole-rock isochron of71 ± 2m.y., which agrees with the previously reported K-Ar age of 73 m.y. Similarity of most of the intrusive rocks in the Greater Antilles with respect to their strontium isotopic geochemistry regardless of their major element composition indicates that intrusive magmas with a wide range of composition can be derived from a single source material. The most likely source material, in view of the available isotopic data, is the mantle wedge overlying the subduction zone.     

Carbon isotopic variations of Proterozoic carbonates in Jixian,Tianjin, China

Along the Proterozoic stratigraphic sections of Jixian, Tianjin, the δ13C values of carbonates are mostly negative in the Changchengian (Changcheng System), varying approximately within a range of (0±1 )‰in the Jixianian (Jixian System), and have commonly positive values of (2±2)‰ in the Qingbaikouan (Qingbaikou System). From Chuanlinggou Formation to Dahongyu Formation, i.e. from ca. 1700 Ma to 1600 Ma, the δ13C values of carbonates increase from about -3‰ to 0‰, but oscillate with large excursion in Dahongyu Formation. Two large excursions occur in the Jixianian, one in Yangzhuang Formation and the other in the third sub-member of Wumishan Formation overlaying. These integrated and continuous records of carbon isotope reflect secular variations of δ13C value in ancient seawater from ca. 1700 Ma to ca. 800 Ma. The increase and subsequent oscillation of seawater δ13C value occurring from the ages of ca. 1700 to ca. 1600 Ma and <1300 Ma may signify two correlative global tectonic events.     

Stable isotopic studies of dinosaur eggshells from the Nanxiong Basin, South China

Stable carbon and oxygen isotopes have been observed in dinosaur eggshell samples,identified as Macroolithus yaotunensis,collected in two sections of the Nanxiong Basin,Guangdong Province,South China.Multiple positiveδ18O perturbations that occurred during the K/T transition of about 150 ka suggest that there may have been at least three periods of the extreme dry climate with a mean annual air temperature over 27℃.δ13C ranges from -8.37‰ to -11.60‰ of the dinosaur eggshells,added to the metabolic enrichment of 16‰,may indicate that the dinosaurs represented by this type of eggshells probably consumed the plant species of C3 type with the δ13C values of -24.3‰- -27.6‰.Moreover,the trend toward more negative δ13C values of the eggshells during the K/T transition could be an indirect consequence of a lasting increase in atmospheric CO2 concentration.     

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.     

Two stages of isotopic exchanges experienced by the Ertaibei granite pluton, northern Xinjiang, China

¹⁸O/¹⁶O and D/H of coexisting feldspar, quartz, and biotite separates of twenty samples collected from the Ertaibei granite pluton, northern Xinjiang, China are determined. It is shown that the Ertaibei pluton experienced two stages of isotopic exchanges. The second stage of¹⁸O/¹⁶O and D/H exchanges with meteoric water brought about a marked decrease in the δ¹⁸O values of feldspar and biotite from the second group of samples. The D/H of biotite exhibits a higher sensitivity to the meteoric water alteration than its¹⁸O/¹⁶O. However, the first stage of¹⁸O/¹⁶O exchange with the¹⁸O-rich aqueous fluid derived from the dehydration within the deep crust caused the Δ¹⁸O_{Quartz-Feldspar} reversal. It is inferred that the dehydration-melting may have been an important mechanism for anatexis. It is shown that the deep fluid encircled the Ertaibei pluton like an envelope which serves as an effective screen to the surface waters.     

The isotopic signature of monsoon conditions,cloud modes,and rainfall type

This work provides a comprehensive physically based framework for the interpretation of the north Australian rainfall stable isotope record (δ¹⁸O and δ²H). Until now, interpretations mainly relied on statistical relationships between rainfall amount and isotopic values on monthly timescales. Here, we use multiseason daily rainfall stable isotope and high resolution (10 min) ground‐based C‐band polarimetric radar data and show that the five weather types (monsoon regimes) that constitute the Australian wet season each have a characteristic isotope ratio. The data suggest that this is not only due to changes in regional rainfall amount during these regimes but, more importantly, is due to different rain and cloud types that are associated with the large scale circulation regimes. Negative (positive) isotope anomalies occurred when stratiform rainfall fractions were large (small) and the horizontal extent of raining areas were largest (smallest). Intense, yet isolated, convective conditions were associated with enriched isotope values whereas more depleted isotope values were observed when convection was widespread but less intense. This means that isotopic proxy records may record the frequency of which these typical wet season regimes occur. Positive anomalies in paleoclimatic records are most likely associated with periods where continental convection dominates and convection is sea‐breeze forced. Negative anomalies may be interpreted as periods when the monsoon trough is active, convection is of the oceanic type, less electric, and stratiform areas are wide spread. This connection between variability of rainfall isotope anomalies and the intrinsic properties of convection and its large‐scale environment has important implications for all fields of research that use rainfall stable isotopes.     

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.     

Implication of the temporal sulphur isotopic variation during the 2000 eruption of Miyakejima Volcano, Japan

Abstract   The variation of sulphur isotopic composition during the 2000 eruption of the Miyakejima Volcano was examined in order to monitor the temporal change of the volcanic activity. The δ³⁴S values of water-soluble sulphate leached from volcanic ash effused during intermittent eruptions from July to September 2005 range from +5 to +11‰ with a fluctuation of ca 3‰ within a single eruption. The δ³⁴S value of sulphuric acid mist collected with 'Cu-metal trap' placed on the flank of the volcano from December 2000 to January 2001 is +6.2‰. These sulphur isotopic compositions of sulphate, which were isotopically equilibrated in the subvolcanic hydrothermal system, indicate that the temperature of the hydrothermal system beneath the caldera increased after the period of intermittent phreatic and phreatomagmatic eruptions. Then, the δ³⁴S value of sulphuric acid trapped from January to March 2001 was +9.0‰ and the δ³⁴S value of water-soluble sulphate on volcanic ash emitted with minor eruption in May 2001 was +11.0‰, suggesting a decrease in temperature of the subvolcanic hydrothermal system.     

Some isotopic and hydrological changes associated with the 1999 Chi-Chi earthquake, Taiwan

Abstract   The changes in the isotopic composition of, and the groundwater level in, the Choshui River alluvial fan near the ruptured Chelungpu Fault during and following the 1999 ( M _w = 7.5) Chi-Chi earthquake in Taiwan are reported. Three aspects of the hydrological changes are noticed. First, following the Chi-Chi earthquake, the lower aquifers beneath the Choshui River fan showed a significant shift in isotopic composition towards that of the surface water in the Choshui River, suggesting enhanced exchanges of water between the river and the groundwater. Second, in some wells, water levels and isotopic compositions in different aquifers converged to the same respective values during the Chi-Chi earthquake, suggesting coseismic exchanges of water between the different aquifers, which implies enhanced permeability due perhaps to the fracturing and breaching of aquitards between the aquifers. Third, the pattern of the coseismic water-level response is distinctly different from that of the shift in the isotopic composition, suggesting that they were produced by different mechanisms.     

Geochemical and isotopic evidence for seawater contamination of the hydrothermal system of Taal Volcano, Luzon, the Philippines

 The hydrologic structure of Taal Volcano has favored development of an extensive hydrothermal system whose prominent feature is the acidic Main Crater Lake (pH<3) lying in the center of an active vent complex, which is surrounded by a slightly alkaline caldera lake (Lake Taal). This peculiar situation makes Taal prone to frequent, and sometimes catastrophic, hydrovolcanic eruptions. Fumaroles, hot springs, and lake waters were sampled in 1991, 1992, and 1995 in order to develop a geochemical model for the hydrothermal system. The low-temperature fumarole compositions indicate strong interaction of magmatic vapors with the hydrothermal system under relatively oxidizing conditions. The thermal waters consist of highly, moderately, and weakly mineralized solutions, but none of them corresponds to either water–rock equilibrium or rock dissolution. The concentrated discharges have high Na contents (>3500 mg/kg) and low SO₄/Cl ratios (<0.3). The Br/Cl ratio of most samples suggests incorporation of seawater into the hydrothermal system. Water and dissolved sulfate isotopic compositions reveal that the Main Crater Lake and spring discharges are derived from a deep parent fluid (T≈300  °C), which is a mixture of seawater, volcanic water, and Lake Taal water. The volcanic end member is probably produced in the magmatic-hydrothermal environment during absorption of high-temperature gases into groundwater. Boiling and mixing of the parent water give rise to the range of chemical and isotopic characteristics observed in the thermal discharges. Incursion of seawater from the coastal region to the central part of the volcano is supported by the low water levels of the lakes and by the fact that Lake Taal was directly connected to the China sea until the sixteenth century. The depth to the seawater-meteoric water interface is calculated to be 80 and 160 m for the Main Crater Lake and Lake Taal, respectively. Additional data are required to infer the hydrologic structure of Taal. Geochemical surveillance of the Main Crater Lake using the SO₄/Cl, Na/K, or Mg/Cl ratio cannot be applied straightforwardly due to the presence of seawater in the hydrothermal system. Received: 12 February 1997 / Accepted: 26 January 1998     

Additional note on the isotopic composition of strontium in McCartys flow,Valencia County,New Mexico

Strontium isotopic analyses of additional samples of the McCartys basalt flow (Holocene) exhibit no evident correlation with distance from the source crater or with strontium content. This confirms our previous comments urging caution in sampling of one basalt flow in order to determine its “source” initial⁸⁷Sr/⁸⁶Sr.     

Hydrochemical and isotopic characterization of groundwater in the southeastern part of the Plateaux Region,Togo

Hydrochemical, inverse geochemical modelling and isotopic approaches are used to assess the hydrogeochemical evolution of groundwater from the basement aquifers in the southeastern part of the Plateaux Region, Togo. Groundwater originates from present-day rainwater infiltration and is mostly fresh and slightly acidic to neutral. Hydrochemical facies are predominantly mixed cations-HCO₃ associated with Ca/Mg-Cl, Na-HCO₃ and Na-Cl water types in equilibrium with kaolinite and Ca/Mg-smectites. They are related to silicates hydrolysis, anthropogenic contamination, nitrification/denitrification, mixing along flowpaths and dissolution/precipitation of secondary minerals. The pattern of flow paths is in accordance with an increasing trend in total dissolved solids (TDS) toward the potentiometric depression located in the central and southern parts of the aquifer system. Inverse geochemical modelling using the NETPATH-WIN model showed the relative importance of biotite, plagioclase and amphibole weathering and dissolution of secondary carbonate minerals along the flowpath, suggesting that an abundance of minerals is not necessarily the main factor controlling the groundwater chemistry evolution.     

The strontium isotopic composition of seawater,and seawater-oceanic crust interaction

The⁸⁷Sr/⁸⁶Sr ratio of seawater strontium (0.7091) is less than the⁸⁷Sr/⁸⁶Sr ratio of dissolved strontium delivered to the oceans by continental run-off (～0.716). Isotope exchange with strontium isotopically lighter oceanic crust during hydrothermal convection within spreading oceanic ridges can explain this observation. In quantitative terms, the current⁸⁷Sr/⁸⁶Sr ratio of seawater (0.7091) may be maintained by balancing the continental run-off flux of strontium (0.59 × 10¹² g/yr) against a hydrothermal recirculation flux of 3.6 × 10¹² g/yr, during which the⁸⁷Sr/⁸⁶Sr ratio of seawater drops by 0.0011. A concomitant mean increase in the⁸⁷Sr/⁸⁶Sr ratio of the upper 4.5 km of oceanic crust of 0.0010 (0.7029–0.7039) should be produced. This required⁸⁷Sr enrichment has been observed in hydrothermally metamorphosed ophiolitic rocks from the Troodos Massif, Cyprus.The post-Upper Cretaceous increase in the strontium isotopic composition of seawater(～0.7075–0.7091) covaries smoothly with inferred increase in land area. This suggests that during this period the main factor which has caused variability in the⁸⁷Sr/⁸⁶Sr ratio of seawater strontium could have been variation in the magnitude of the continental run-off flux caused by variation in land area. Variations in land area may themselves have been partly a consequence of variations in global mean sea-floor spreading rate.     

Origins of groundwater inferred from isotopic patterns of the Badain Jaran Desert, northwestern China

There are many viewpoints about the sources of groundwater in the Badain Jaran Desert (BJD), such as precipitation and snowmelt from the Qilian Mountains (the upper reaches [UR] of the Heihe River Basin [HRB]) and precipitation from the BJD and the Yabulai Mountains. To understand the source of the groundwater of the BJD and their possible associations with nearby bodies of water, we analyzed variations of stable isotope ratios (δD and δ(18) O) and the deuterium excess (d-excess) of groundwater and precipitation in the BJD, of groundwater, precipitation, river and spring water in the UR, and of groundwater and river water in the middle and lower reaches (MR and LR) of the HRB. In addition, the climatic condition under which the groundwater was formed in the BJD was also discussed. We found obvious differences in δD, δ(18) O, and d-excess among groundwater in the BJD, nearby water bodies and the HRB. The groundwater δD-δ(18) O equation for the BJD was δD = 4.509δ(18) O-30.620, with a slope and intercept similar to that of nearby areas (4.856 and -29.574), indicating a strong evaporation effect in the BJD and its surrounding areas. The equation's slope of the BJD was significantly lower than those of HRB groundwater (6.634), HRB river water (6.202), precipitation in the BJD and Youqi (7.841), and the UR of the HRB (7.839). The d-excess (-17.5‰) of the BJD was significantly lower than those of nearby groundwater (-7.4‰), HRB groundwater (12.1‰), precipitation in the BJD (5.7‰) and in the UR of the HRB (15.2‰), and HRB river water (14.4‰). The spatial patterns of δ(18) O and d-excess values in the BJD suggest mixing and exchange of groundwater between the BJD and neighboring regions, but no hydraulic relationship between the BJD groundwater and water from more distant regions except Outer Mongolia, which is north of the BJD. Moreover, we conclude that there is little precipitation recharge to groundwater because of the obvious d-excess difference between groundwater and local precipitation, low precipitation, and high evaporation rates. The abnormally negative d-excess values in groundwater of the BJD indicate that this water was formed in the past under higher relative humidity and lower temperatures than modern values.     

Large scale isotopic Sr, Nd and O isotopic anatomy of altered oceanic crust: DSDP/ODP sites417/418

Large-scale compositional domains at DSDP/ODP drill sites 417A, 417D and 418A were analyzed for O, Sr and Nd isotope ratios, and REE, U, K, Rb and Sr abundances, to constrain the bulk chemical composition of the oceanic crust that is recycled at subduction zones. The combination of the three sites gives the composition of the upper oceanic crust in this region over a distance of about 8 km. The δ¹⁸O_SMOW and⁸⁷Sr/⁸⁶Sr_meas of compositional domains 10–100 m in size correlate well, with a range of 7.7–19.2 and 0.70364–0.70744, and mean of 9.96 and 0.70475, respectively. The Rb inventory of the upper crust increases by about an order of magnitude, while Sr contents remain constant. U abundances increase moderately under oxidizing alteration conditions and nearly triple in the commonly reducing alteration environments of the upper oceanic crust. REEs are influenced by alteration only to a small extent, and recycled oceanic crust is similar to MORB with respect to¹⁴³Nd/¹⁴⁴Nd. Even though the average composition of the upper oceanic crust is well defined, the large scale composition varies widely. Highly altered compositional domains may not have a large impact on the average composition of the oceanic crust, but they may preferentially contribute to fluids or partial melts derived from the crust by prograde metamorphic reactions.