Correlation of magnetitic susceptibility with18O/16O data in late orogenic granites of the southern Appalachian Piedmont

Initial magnetic susceptibility (generally indicative of magnetite content) has been determined for 445 samples from 17 granites located in the southern Appalachian Piedmont of Georgia and South Carolina. These values have been correlated with whole rock δ¹⁸O data from the same plutons, yielding a pronounced inverse relationship. It has previously been shown for the southern Piedmont that low oxygen isotopic (¹⁸O-enriched) values usually occur in S-type granites (Wenner [1], this issue). It follows, then, that I-type granites are characterized by high susceptibilities (χ > 1 × 10^?4 G/Oe), and S-type granites by low susceptibilities (χ < 1 × 10^?4 G/Oe). An interesting result of this work has been the observation that some S-type granites exhibit good within-site clusters of remanent magnetic directions while I-type granites generally do not.     

Thermomagnetic characteristics in late orogenic granites and gneisses of the southern Appalachian Piedmont

Thermomagnetic curves have been obtained for samples from 25 granites and gneisses in Georgia, South Carolina, and North Carolina. This data set can be divided into two distinct curve types: Type I includes rocks which exhibit no distinct Curie points and a linear decrease in magnetization with increasing temperature; Type II curves exhibit a distinct Curie point over a range of temperatures expected for magnetite (500–580°C). Synthetic samples constructed from magnetite and hematite powder exhibit Type I behavior when the hematite to magnetite ratio is high (e.g. 15 : 1). Examination of polished sections shows relatively coarse-grained magnetite only in Type II rocks. We interpret our data to indicate that Type I thermomagnetic curves are dominated by relatively large hematite/magnetite ratios while Type II granites are characterized by relatively coarse-grained magnetite. Type I granite samples have low magnetic susceptibility values (less than 4 × 10^?4 cgs) while most Type II granite samples have higher values. The Type I granites are invariably¹⁸O-enriched whereas Type II granites typically exhibit low¹⁸O/¹⁶O ratios. These relationships are consistent with previously reported correlations of susceptibility (generally indicative of magnetite content) and oxygen isotopic trends in the southern Appalachian Piedmont.     

Contrasting18O enrichment and origins of High Himalayan and Transhimalayan intrusives

Twelve analysed leucogranites of the High Himalaya in Bhutan (Chung La, Mönlakarchung) and Garhwal (Badrinath) are among the most¹⁸O-enriched granites known (11.5–12.4‰ δ¹⁸O_SMOW with two exceptions) and separate minerals show good isotopic concordance. The data strongly support an origin of the granites by anatexis of continental basement such as the Indian crystalline basement sheet or slab, undercut by the Main Central Thrust, of which five samples were analysed. In contrast, the pre-collision Transhimalayan (Gangdese) batholiths to the north of the Indus-Tsangpo suture, as exemplified by the Ladakh intrusives, show an initially oceanic trend of δ¹⁸O vs. SiO₂ that becomes gradually somewhat enriched with respect to Hachijo-Jima. While not completely outside the range of enrichment that seems possible by fractional crystallisation, this could tie in with the⁸⁷Sr inhomogeneities reported by Honegger et al. [9], which may be due to assimilation of variably radiogenic Eurasian continental basement. For both the leucogranites and Ladakh intrusives the¹⁸O levels and the concordance between minerals rule out significant cumulative water/rock ratios in syn- or post-magmatic interaction with subsurface waters.     

Sr and O isotopic characteristics of porphyries in the Qinling molybdenum deposit belt and their implication to genetic mechanism and type

A great deal of Mesozoic hypobatholithic granites and hypabyssal porphyries develop in the Qinling Mountains. The former has long been regarded as transformation type (or S-type), and the latter associated with Mo-mineralization regarded as syntexis type (or I-type) granitoids. Statistics show that Sr₁ and σ¹⁸O of hypabyssal porphyries respectively range from 0.705 to 0.714, and from 7.2‰ to 12.1‰, agreeing with those of hypobatholithes (Sr₁=0.705–0.710, σ¹⁸O=6.1‰–10.4‰), which indicates that they share similar material sources and petrogenic mechanism. Based on analysis of lithological, mineralogical and geochemical characteristics of these granitoids and on study of their petrogenic tectonic background and regional geophysical data, we argue that both the shallow-seated porphyries and deep-seated batholithes were the products of Mesozoic collision between South China and North China paleocontinents. Subsequently, all these grantitoids should be attributed to collision type.     

Isotopic geochemistry (O,Sr, Pb) of the Golda Zuelva and Mboutou anorogenic complexes,North Cameroun: mantle origin with evidence for crustal contamination

The annular (6–8 km diameter) Golda Zuelva and Mboutou anorogenic complexes of North Cameroun are composed of a suite of alkaline plutonic rocks ranging from olivine gabbro to amphibole and biotite granite. For the Mboutou complex there are two overlapping centres. In the Golda Zuelva complex the plutonic rocks are associated with a later hawaiite to rhyolite volcanic suite. A Rb/Sr whole rock isochron gives an age of 66±3 Ma for the Golda Zuelva granites, with initial⁸⁷Sr/⁸⁶Sr ratio of 0.7020, and demonstrates that plutonism and volcanism were essentially contemporaneous and probably cogenetic. For Golda Zuelva and the north Mboutou centre¹⁸O/¹⁶O (5.6–6.2),⁸⁷Sr/⁸⁶Sr (0.7030–0.7045) and Pb isotopic ratios (²⁰⁷Pb/²⁰⁴Pb: 15.60–15.64) support a mantle origin for the initial magmas. Unlike Sr isotopes, the O isotopic ratios of the granitic end members at Golda Zuelva (～7.5) indicate crustal contamination. Post-magmatic alteration was not significant.For the younger south Mboutou centre the O-, Sr- and Pb-isotopic data indicate more extensive magma-crust interaction and in a different (higher level?) crustal environment with δ¹⁸O granite=3.3‰,⁸⁷Sr/⁸⁶Sr ratios up to 0.706 and Pb isotopic ratios more markedly displaced from the oceanic volcanic field. The low-¹⁸O granites probably record, at least in part, a magmatic process with subsequent minor post-magmatic alteration effects. The major and trace element systematics between the north and south Mboutou centres are directly comparable. The evolution of the magmas were dominated by fractional crystallisation and progressive crustal contamination processes.     

Role of mantle-derived magma in genesis of early Yanshanian granites in the Nanling Range,South China: <Emphasis Type="Italic">in situ</Emphasis> zircon Hf-O isotopic constraints

Although a number of petrographic observations and isotopic data suggest that magma mixing is common in genesis of many granite plutons, it is still controversial whether the mantle-derived magmas were involved in granites. We carried out in this study a systematic analysis of in situ zircon Hf-O isotopes for three early Yanshanian intrusions dated at ca. 160 Ma from the Nanling Range of Southeast China. The Qinghu monzonite has very homogeneous zircon Hf-O isotopic compositions, εHf(t) =11.6±0.3 and δ18O=5...     

Oxygen and carbon isotope evidence for seawater-hydrothermal alteration of the Macquarie Island ophiolite

Rocks of the Miocene Macquarie Island ophiolite, south of New Zealand, have oxygen and carbon isotopic compositions comparable to those of seafloor rocks. Basalt glass and weathered basalts have δ¹⁸O values at 5.8–6.0‰ and 7.9–9.5‰, respectively, similar to drilled seafloor rocks including samples from the Leg 29 DSDP holes near Macquarie Island. Compared to the basalt glass, the greenschist to amphibolite facies metaintrusives are depleted in¹⁸O (δ¹⁸O=3.2–5.9‰) similar to dredged seafloor samples, whereas the metabasalts are enriched (δ¹⁸O=7.1–9.7‰). Although the gabbros are only slightly altered in thin-section they have exchanged oxygen with a hydrothermal fluid to a depth of at least 4.5 km. There is an approximate balance between¹⁸O depletion and enrichment in the exposed ophiolite section. The carbon isotopic composition of calcite in the weathered basalts (δ¹³C=1.0–2.0‰) is similar to those of drilled basalts, but the metamorphosed rocks have low δ¹³C values (?14.6 to 0.9‰).These data are compatible with two seawater circulation regimes. In the upper regime, basalts were weathered by cold seawater in a circulation system with high water/rock ratios (?1.0). Based on calcite compositions weathering temperatures were less than 20°C and the carbon was derived from a predominantly inorganic marine source. As previously suggested for the Samail ophiolite, it is postulated that the lower hydrothermal regime consisted of two coupled parts. At the deeper levels, seawater circulating at low water/rock ratios (0.2–0.3) and high temperatures (300–600°C) gave rise to¹⁸O-depleted gabbro and sheeted dikes via open system exchange reactions. During reaction the seawater underwent a shift in oxygen isotopic composition (δ¹⁸O=1.0–5.0‰) and subsequently caused¹⁸O enrichment of the overlying metabasalts. In the shallower part of the hydrothermal regime the metabasalts were altered at relatively high water/rock ratios (1.0–10.0) and temperatures in the range 200–300°C. The relatively low water/rock ratios in the hydrothermal regime are supported by the low δ¹³C values of calcite, interpreted as evidence of juvenile carbon in contrast to the inorganic marine carbon found in the weathered basalts.     

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 isotope fractionation in decarbonation metamorphism: the Mottled Zone event

Calculated univariant equilibria and oxygen isotope compositions of silicates and carbonates support the proposal that the “Mottled Zone Event” is a low-pressure (1–25 atm), high-temperature (200° < T < 1300°C) metamorphism of calcareous siliceous sediments in which the thermal energy is provided by combustion of organic matter. δ¹⁸O of silicates decreases systematically with increasing metamorphic grade from averages of 18.1‰ in protolith shales, to 16.6‰ in grossular-diopside-zeolite rocks, 15.6‰ in wollastonite and anorthite-diopside-gehlenite-grossular fels, 14.1‰ in spurrite-brownmillerite marbles and 11.7‰ in the highest-grade larnite-gehlenite-brownmillerite assemblages. Decarbonation is the principal mechanism influencing the oxygen isotope compositions. The progressive decrease of δ¹⁸O in silicates can be modelled as a Rayleigh distillation of CO₂ approximately 16‰ enriched in ¹⁸O relative to whole rock assemblages i.e., of initial isotopic composition 8.5‰ heavier than the parent carbonates. The mineral assemblage of one sample with an unusual granoblastic texture is in apparent isotopic equilibrium at a temperature of 540°C.     

Differences in stable isotope compositions of freshwater snails from surface sediments of two Polish shallow lakes

The study reports and discusses the differences in δ¹³C and δ¹⁸O values of shells between several species of freshwater snails. Shells were derived from sediment samples collected from depths of 0.5, 1, 2 and 3 m along transects in two shallow eutrophic lakes located in mid-western Poland. Mean δ¹³C values of the shells ranged between −7.5 and −3.8‰ in Lake Jarosławieckie and between −8.1 and −5.2‰ in Lake Rosnowskie Duże, whereas mean δ¹⁸O values ranged between −2.2 and −0.2‰ and between −2.2 and 0.4‰ respectively in the studied lakes. A similar order of species in terms of shell isotope values, from least to most ¹³C and ¹⁸O-depleted was observed in both lakes and seems to indicate constancy of the factors controlling the stable isotope compositions of snail shells. We postulate that the nearly 4‰ difference in the mean carbon stable isotope values between the species was primarily controlled by the amount of metabolic carbon incorporated into the shells and the δ¹³C values of the snail food. Different growth cessation temperatures and microhabitats of the species studied result in temporally and spatially varied DIC δ¹³C values, water δ¹⁸O values and water temperature of shell precipitation, and may thus differentiate the δ¹³C and δ¹⁸O values of shells. The range of δ¹³C and δ¹⁸O values of individual shells from a sediment sample (mean 2.35 and 2.15‰, respectively) is interpreted as reflecting an intraspecific variability of isotope compositions in shells from a population and changes of the ambient conditions during the accumulation of the sediment layer. The species-specificity and intraspecific variability in C and O isotopic compositions of shells allow concluding that in palaeolimnological studies, stable isotope analyses should be performed on a set of mono-specific shells representing mean isotope compositions of the species for the interval studied rather than single shells or multispecific bulk shell material.     

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.     

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”.     

Factors controlling diurnal variation in the isotopic composition of atmospheric water vapour observed in the taiga,eastern Siberia

Deciduous forest covers vast areas of permafrost under severe dry climate in eastern Siberia. Understanding the water cycle in this forest ecosystem is quite important for climate projection. In this study, diurnal variations in isotopic compositions of atmospheric water vapour were observed in eastern Siberia with isotope analyses of precipitation, sap water of larch trees, soil water, and water in surface organic layer during the late summer periods of 2006, 2007, and 2008. In these years, the soil moisture content was considerably high due to unusually large amounts of summer rainfall and winter snowfall. The observed sap water δ¹⁸O ranged from ?17.9‰ to ?13.3‰, which was close to that of summer precipitation and soil water in the shallow layer, and represents that of transpired water vapour. On sunny days, as the air temperature and mixing ratio rose from predawn to morning, the atmospheric water vapour δ¹⁸O increased by 1‰ to 5‰ and then decreased by about 2‰ from morning to afternoon with the mixing ratio. On cloudy days, by contrast, the afternoon decrease in δ¹⁸O and the mixing ratio was not observed. These results show that water vapour that transpired from plants, with higher δ¹⁸O than the atmospheric water vapour, contributes to the increase in δ¹⁸O in the morning, whereas water vapour in the free atmosphere, with lower δ¹⁸O, contributes to the decrease in the afternoon on sunny days. The observed results reveal the significance of transpired water vapour, with relatively high δ¹⁸O, in the water cycle on a short diurnal time scale and confirm the importance of the recycling of precipitation through transpiration in continental forest environments such as the eastern Siberian taiga. Copyright © 2012 John Wiley & Sons, Ltd.     

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).     

Early Cretaceous I‐type granites in the southwest Fujian Province: new constraints on the late Mesozoic tectonic evolution of southeast China

Zircons from two samples of the Sukeng pluton in the southwest Fujian Province, China, were analyzed by LA–ICP–MS with the aim of determining the timing of formation. The zircons from the two samples yield similar U–Pb ages of 100.47 ± 0.42 and 102.46 ± 0.69 Ma, indicating that the Sufeng pluton is contemporaneous with the Sifang and Luoboling plutons, all of which are also related to Cu–Au–Pb–Zn–Mo mineralization within the study area. All three plutons have geochemical features of I‐type granites, are high‐ to mid‐K calc‐alkaline metaluminous rocks, and have average molar Al₂O₃/ (CaO+Na₂O+K₂O) values of 0.95, initial ⁸⁷Sr/⁸⁶Sr ratios of 0.70465–0.70841, εNd(t) values at 101 Ma from –1.72 to –7.26, and two‐stage Nd model ages (T_2DM) from 1.16 to 1.60 Ga. Zircons within these plutons have εHf(t) values at 101 Ma from –3.5 to 6.25 and T_2DM ages from 0.74 to 1.46 Ga, suggesting these I‐type granites formed from magmas generated by partial melting of Mesoproterozoic to Neoproterozoic continental crust that mixed with mantle‐derived magmas. The magmatism was associated with thickening of the lower crust caused by collisions between microcontinents in the Cathaysian Block, which were driven by Early Cretaceous subduction of the Pacific Plate.     

Oxygen isotope constraints on the origin and differentiation of the Moon

We report new high-precision laser fluorination three-isotope oxygen data for lunar materials. Terrestrial silicates with a range of δ¹⁸O values (− 0.5 to 22.9‰) were analyzed to independently determine the slope of the terrestrial fractionation line (TFL; λ = 0.5259 ± 0.0008; 95% confidence level). This new TFL determination allows direct comparison of lunar oxygen isotope systematics with those of Earth. Values of Δ¹⁷O for Apollo 12, 15, and 17 basalts and Luna 24 soil samples average 0.01‰ and are indistinguishable from the TFL. The δ¹⁸O values of high- and low-Ti lunar basalts are distinct. Average whole-rock δ¹⁸O values for low-Ti lunar basalts from the Apollo 12 (5.72 ± 0.06‰) and Apollo 15 landing sites (5.65 ± 0.12‰) are identical within error and are markedly higher than Apollo 17 high-Ti basalts (5.46 ± 0.11‰). Evolved low-Ti LaPaz mare-basalt meteorite δ¹⁸O values (5.67 ± 0.05‰) are in close agreement with more primitive low-Ti Apollo 12 and 15 mare basalts. Modeling of lunar mare-basalt source composition indicates that the high- and low-Ti mare-basalt mantle reservoirs were in oxygen isotope equilibrium and that variations in δ¹⁸O do not result from fractional crystallization. Instead, these differences are consistent with mineralogically heterogeneous mantle sources for mare basalts, and with lunar magma ocean differentiation models that result in a thick feldspathic crust, an olivine–pyroxene-rich mantle, and late-stage ilmenite-rich zones that were convectively mixed into deeper portions of the lunar mantle. Higher average δ¹⁸O (WR) values of low-Ti basalts compared to terrestrial mid ocean ridge basalts (Δ=0.18‰) suggest a possible oxygen isotopic difference between the terrestrial and lunar mantles. However, calculations of the δ¹⁸O of lunar mantle olivine in this study are only 0.05‰ higher than terrestrial mantle olivine. These observations may have important implications for understanding the formation of the Earth–Moon system.     

Granitic plutonism as an indicator of microplates in the Palaeozoic of central and eastern Maine

Oxygen and strontium isotope ratios have been used to characterize source regions for granitic magmas for a transect across the northern Appalachian orogen in central and eastern Maine. The northwestern plutons (Katahdin and Seboeis) have δ¹⁸O values of 10.3–13.3 and initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7083 and 0.7066, respectively. The central plutons (Bottle Lake and Center Pond) have lower δ¹⁸O values (8.2–9.9) and initial ⁸⁷Sr/⁸⁶Sr ratios (0.7043–0.7055). The southeastern plutons (Lucerne and Deblois) have δ¹⁸O values (9.0–11.0) but initial ⁸⁷Sr/⁸⁶Sr ratios (0.7077 and 0.7041, respectively) which are intermediate between the northwestern and central plutons.Source models derived from these results and other petrological and geochemical data reflect the juxtaposition of discrete source regions by transcurrent faulting, which may be related to oblique plate motions. This model illustrates the importance of microplate accretion in the Palaeozoic history of the northern Appalachian orogen.     

Use of stable isotopes to understand run‐off generation processes in the Red River Delta

This paper presents the use of stable isotopes of water for hydrological characterization and flow component partitioning in the Red River Delta (RRD), the downstream section of the Red River. Water samples were collected monthly during 2015 from the mainstream section of the river and its right bank tributaries flowing through the RRD. In general, δ¹⁸O and δ²H river signatures were depleted in summer–autumn (May–October) and elevated in winter–spring (November–April), displaying seasonal variation in response to regional monsoon air mass contest. The Pacific equatorial–maritime air mass dominates in summer and the northern Asia continental air mass controls in winter. Results show that water of the RRD tributaries stems solely from local sources and is completely separated from water arriving from upstream subbasins. This separation is due to the extensive management of the RRD (e.g., dykes and dams) for the purposes of irrigation and inundation prevention. Mainstream river section δ¹⁸O and δ²H compositions range from ?10.58 and ?73.74‰ to ?6.80 and ?43.40‰, respectively, and the corresponding ranges inside the RRD were from ?9.35 and ?64.27‰ to ?2.09 and ?15.80‰. A combination of data analysis and hydrological simulation confirms the role of upstream hydropower reservoirs in retaining and mixing upstream water. River water inside the RRD experienced strong evaporation characterized by depleted d‐excess values, becoming negative in summer. On the other hand, the main stream of the Red River has d‐excess values around 10‰, indicating moderate evaporation. Hydrograph separation shows that in upstream subbasins, the groundwater fraction dominates the river flow composition, especially during low flow regimes. Inside the RRD, the river receives groundwater during the dry season, whereas groundwater replenishment occurs in the rainy season. Annual evaporation obtained from this hydrograph separation computation was about 6.3% of catchment discharge, the same order as deduced from the difference between subbasin precipitation and discharge values. This study shows the necessity to re‐evaluate empirical approaches in large river hydrology assessment schemes, especially in the context of climate change.     

Groundwater evolution and mean water age inferred from hydrochemical and isotopic tracers in a tropical confined aquifer

The coastal confined aquifer in the Gulf of Urabá (Colombia) is an important water source for the banana agro‐industry as well as for urban and rural communities. However, the main processes controlling recharge and mixing in the aquifer are still poorly understood. Hydrochemical analyses and stable isotope monitoring were conducted to (a) determine groundwater recharge origin, mean groundwater age, and the main processes governing groundwater chemistry and the potential mixing of marine water and the influence of diffusive processes from the two surrounding aquitard layers. Hydrochemical data indicate that the main processes affecting the dissolved chemical composition include cation exchange, dissolution of carbonated and CO₂, and silicate weathering. δ¹⁸O and δ²H compositions combined with ¹⁴C data highlight the differences in climatic conditions between the recharge zone and the confined section of the aquifer, which is close to the Atlantic Ocean. Groundwater samples with ¹⁴C ages from recent to 28,300 years BP show a depleted isotopic trend ranging from ?6.43‰ to ?9.14‰ in δ¹⁸O and from ?43.2‰ to ?65.7‰ in δ²H. The most depleted δ¹⁸O and δ²H compositions suggest a cooler recharge climate than the current conditions (corresponding to the last glacial period of the late Pleistocene). Depleted δ¹³C values in the total dissolved inorganic carbon indicate the existence of organic material oxidation processes within the geologic formation. These results can be used or transferred to enhance groundwater modelling efforts in other confined coastal aquifers of South America where scarcity of long‐term monitoring data limits water resources planification under a changing climate.     

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.