Sr Isotope Systematics Research Articles

Neoarchean crustal growth by accretionary processes: Evidence from combined zircon–titanite U–Pb isotope studies on granitoid rocks around the Hutti greenstone belt, eastern Dharwar Craton, India

The Neoarchean Hutti greenstone belt hosts mesothermal gold deposits and is surrounded by granitoid rocks on all sides. Combined U–Pb dating of zircon and titanite from the granitoid rocks constrains their emplacement history and subsequent geologic evolution. The Golapalli and Yelagatti granodiorites occurring to the north of the Hutti greenstone belt were emplaced at 2569±17Ma. The Yelagatti granodiorite yielded a younger titanite age of 2530±6Ma which indicates that it was affected by a post-crystallization thermal event that exceeded the titanite closure temperature. The western granodiorites from Kardikal have identical titanite and zircon ages of 2557±6Ma and 2559±19Ma, respectively. The eastern Kavital granodiorites yielded titanite ages of 2547±6Ma and 2544±24Ma which are identical to the published U–Pb zircon SHRIMP ages. These ages imply that the granitoid rocks surrounding the Hutti greenstone belt were formed as discrete batholiths within a short span of ca. 40Ma between 2570Ma and 2530Ma ago. They were juxtaposed by horizontal tectonic forces against the supracrustal rocks that had formed in oceanic settings at the end of the Archean. The first phase of gold mineralization coincided with the last phase of granodiorite intrusion in the Hutti area. A metamorphic overprint occurred at ca. 2300Ma ago that reset the Rb–Sr isotope system in biotites and possibly caused hydrothermal activity and enrichment of Au in the ore lodes. The eastern Dharwar Craton consists of quartz monzodiorite–granodiorite–granite suites of rocks that are younger than the greenstone belts that are older than ∼2650Ma reported from earlier studies. The granitoid magmatism took place between 2650 and 2510Ma ago indicating accretionary growth of the eastern Dharwar Craton.

Evolution of the Tyrone ophiolite, Northern Ireland, during the Grampian–Taconic orogeny: a correlative of the Annieopsquotch Ophiolite Belt of central Newfoundland?

The Tyrone Plutonic Group of Northern Ireland represents the upper portions of a tectonically dissected suprasubduction-zone ophiolite accreted to the composite Laurentian margin during the Middle Ordovician. Understanding its development and relationship to the Tyrone Central Inlier, an outboard fragment of relatively high-grade, peri-Laurentian continental crust, is essential for reconstructing the closure of the Iapetus Ocean. The Tyrone Plutonic Group is composed of tectonized layered, isotropic and pegmatitic gabbros, sheeted dolerite dykes and rare pillow lavas. New U–Pb zircon thermal ionization mass spectrometry geochronology has yielded an age of 483.68 ± 0.81 Ma from pegmatitic gabbro. Geochemical characteristics, Nd and Sr isotope systematics, and zircon inheritance indicate that the Tyrone Plutonic Group formed above a north-dipping subduction zone, by the propagation of a spreading centre into a microcontinental block. Synkinematic, calc-alkaline tonalitic to granitic material preserved in the contact zone between the Tyrone Plutonic Group and the Tyrone Central Inlier has produced pressure estimates of 2.3–4.0 ± 0.6 kbar and temperatures of 525–610 °C. Coeval arc–ophiolite accretion at c . 470 Ma may explain how sillimanite-grade metamorphic conditions were reached locally in the underlying Tyrone Central Inlier. Strong temporal, geochemical and lithological similarities exist to the Annieopsquotch Ophiolite Belt of Newfoundland. Supplementary materials: Petrographic photographs, whole-rock, isotopic and mineral geochemical data, and U–Th–Pb isotopic data are available at www.geolsoc.org.uk/SUP18646 .

The status of the Makrotantalon Unit (Andros, Greece) within the structural framework of the Attic-Cycladic Crystalline Belt

AbstractThis study focuses on the status of the Makrotantalon Unit (Andros, Greece) within the framework of the Cycladic nappe stack. We document unambiguous evidence that this unit has experienced blueschist-facies metamorphism and identify previously unknown lawsonite ± pumpellyite assemblages in glaucophane-free metasediments. The position of the presumed tectonic contact at the base of this unit is vague, but roughly outlined by serpentinites. Only a single outcrop displays a weak angular unconformity with cohesive cataclasites in the footwall. Rb–Sr geochronology was carried out on 11 samples representing various rock types collected within or close to inferred or visible fault zones. Owing to a lack of initial isotopic equilibration and/or subsequent disturbance of the Rb–Sr isotope systematics, isochron relationships are poorly developed or non-existing. In NW Andros, direct dating of distinct displacement events has not been possible, but a lower age limit of ~ 40 Ma for final thrusting is constrained by the new data. Sporadically preserved Cretaceous ages either indicate regional differences in theP–T–dhistory or a different duration of metamorphic overprinting, which failed to completely eliminate inherited ages. The detachment on the NE coast records a later stage of the structural evolution and accommodates extension-related deformation. Apparent ages of ~ 29–25 Ma for samples from this location are interpreted to constrain the time of a significant deformation increment. On a regional scale, the Makrotantalon Unit can be correlated with the South Evia Blueschist Belt, but assignment to a specific subunit is as yet unconfirmed.

Miocene post-collisional shoshonites and their crustal xenoliths, Yarlung Zangbo Suture Zone southern Tibet: Geodynamic implications

The convergence between the Indian plate and the southern margin of the Eurasian continent created an active continental margin from Late Jurassic until about 40Ma ago, which then evolved to form the Himalaya and the Tibetan Plateau during the continental collision stage. Post-collisional magmatism in southern Tibet, north of the Yarlung Zangbo Suture Zone (YZSZ) has been active since 45Ma and is related to normal faulting and extensional tectonism. To date no such magmatism was reported within the YZSZ itself. This paper reports on the discovery of Miocene shoshonites within the YZSZ. They are significant because the magma traveled, at least in part, through oceanic crust, thus limiting interaction with the continental crust to the mid-crustal level and which affected the post-collisional magmatic rocks occurring in the northern part of the subduction system. In addition, xenoliths and xenocrysts of crustal origin in these rocks constrain the nature of metamorphic rocks underlying the YZSZ at mid-crustal level. The geochemical signatures of the shoshonitic rocks, including Nd and Sr isotope systematics, indicate derivation from a garnet-bearing middle continental crustal source. Crustal imprint complicates modeling of the petrogenetic processes which occurred prior to mid-crustal ponding of the magma which took place between 11 and 17Ma at depths of 40 to 50km. The significant role of crustal contamination raises serious concerns about models proposed for similar magmatic activity elsewhere in the Himalaya and the Tibetan Plateau.

Petrogenetic association of the oldest lunar basalts: Combined Rb–Sr isotopic and trace element constraints

We report a new radiometric Rb–Sr age determination of 4.03±0.04Ga for high-Al basaltic clast 14321,1353, which is a Group B Apollo 14 high-Al basalt based on its whole-rock incompatible-trace-element chemistry. In addition, available literature Rb–Sr isotope data for Apollo 14 high-Al basalts were reprocessed using the same decay constant and data reduction protocol as used for sample 14321,1353. The Rb–Sr isotopic ages of high-Al basalts range from 4.31±0.17 to 3.93±0.06Ga. The trace element and Rb–Sr isotopic compositions of high-Al basalts are distinct from those of Apollo 14 aluminous impact melts that crystallized at 3.86±0.01Ga. The Rb–Sr isotope systematics, coupled with incompatible-trace-element data are consistent with the parental melts of Apollo 14 high-Al basalts being derived from sources consisting of primitive crystallization products of the lunar magma ocean that were variably metasomatized by late-stage KREEP-rich fluids. Our new Rb–Sr isotopic age for 14321,1353 suggests that the KREEP metasomatism must occur in the lunar mantle no later than 4.03±0.04Ga. Collectively, the Apollo 14 high-Al basalts represent the earliest known volcanic products from the Moon and place a time constraint on the formation of the high-Al basalt source through the crystallization of the lunar magma ocean.

Sr-Nd isotopic disequilibrium of clinopyroxenes from the ultrapotassic effusive rocks of the East African rift system: Mixing of melts and source heterogeneity

505 Ultrapotassic magmatism is the deepest type of magmatism showing a number of indicators of mantle source enrichment in incompatible trace elements (1). The western branch of the East African rift system is a classic area of the occurrence of ultrapotassic rocks (2, 3). The presence of rocks with different modal and chemical compositions within a relatively small area reflects kilometerscale heterogeneity in the upper mantle. The most popular model for the source of potassic magmas is a lherzolitic (harzburgitic?) mantle with numerous pyroxenite veins and layers (4). Addi� tional evidence for the heterogeneity of the source of potassic magmas in the western branch of the rift was obtained during the investigation of Sr and Nd isotopic systematics in phenocrysts from the volcanic rocks. Despite extensive geological work in this region, only a few Sr-Nd and Pb isotope analyses were reported for the kamafugite group of rocks (5, 6), and we are not aware of any published isotopic data for the major minerals of these rocks. Some studies were car� ried out for clinopyroxene from East African nephelin� ite lavas from Napak Volcano, eastern Uganda (7), and Mount Elgon Volcano, eastern Uganda-western Kenya (8). This paper reports the results of the investi� gation of Sr and Nd isotopic compositions of the major minerals (clinopyroxene and mica) of kamafugites and an attempt to estimate the composition of their mantle source. Our study focused on ultrapotassic volcanic rocks from the ToroAnkole and Virunga provinces in the northern part of the western branch of the East African rift system. The ToroAnkole volcanics are the youngest rocks of the East African rift system, and their K-Ar and 40 Ar/ 39 Ar age is no higher than 50 000 yr (9). Most samples for this study were collected within the Bun� yaruguru volcanic field. Samples of kamafugite (ugan� dite) and its probable derivatives, melaleucitite and leu� citite, from Visoke Volcano (Virunga Province) were studied for the sake of comparison. 1 The magmatism of the Virunga Province is of Miocene age (10). Among the characteristic features of the kamafugite rocks of the ToroAnkole Province are high Mg # values (up to 0.79), high contents of Ni (up to 780 ppm) and Cr (up to 1170 ppm), and extreme enrichment in incompatible trace elements: Ce/Y is up to 26.38; Zr/Y, up to 26.22; (La/Yb)n , up to 142,69; and (La/Sm) n , up to 7.27 (11).

Sr–Nd–Pb isotopic constraints on the nature of the mantle sources involved in the genesis of the high-Ti tholeiites from northern Paraná Continental Flood Basalts (Brazil)

There has been little research on geochemistry and isotopic compositions in tholeiites of the Northern region from the Paraná Continental Flood Basalts (PCFB), one of the largest continental provinces of the world. In order to examine the mantle sources involved in the high-Ti (Pitanga and Paranapanema) basalt genesis, we studied Sr, Nd, and Pb isotopic systematics, and major, minor and incompatible trace element abundances. The REE patterns of the investigated samples (Pitanga and Paranapanema magma type) are similar (parallel to) to those of Island Arc Basalts' REE patterns. The high-Ti basalts investigated in this study have initial (133 Ma) 87Sr/86Sr ratios of 0.70538–0.70642, 143Nd/144Nd of 0.51233–0.51218, 206Pb/204Pb of 17.74–18.25, 207Pb/204Pb of 15.51–15.57, and 208Pb/204Pb of 38.18–38.45. These isotopic compositions do not display any correlation with Nb/Th, Nb/La or P2O5/K2O ratios, which also reflect that these rocks were not significantly affected by low-pressure crustal contamination. The incompatible trace element ratios and Sr–Nd–Pb isotopic compositions of the PCFB tholeiites are different to those found in Tristan da Cunha ocean island rocks, showing that this plume did not play a substantial role in the PCFB genesis. This interpretation is corroborated by previously published osmium isotopic data (initial γOs values range from +1.0 to +2.0 for high-Ti basalts), which also preclude basalt generation by melting of ancient subcontinental lithospheric mantle. The geochemical composition of the northern PCFB may be explained through the involvement of fluids and/or small volume melts related to metasomatic processes. In this context, we propose that the source of these magmas is a mixture of sublithospheric peridotite veined and/or interlayered with mafic components (e.g., pyroxenites or eclogites). The sublithospheric mantle (dominating the osmium isotopic compositions) was very probably enriched by fluids and/or magmas related to the Neoproterozoic subduction processes. This sublithospheric mantle region may have been frozen and coupled to the base of the Parana basin lithospheric plate above which the Paleozoic subsidence and subsequent Early Cretaceous magmatism occurred.

40Ar–39Ar dating, whole-rock and Sr–Nd–Pb isotope geochemistry of post-collisional Eocene volcanic rocks in the southern part of the Eastern Pontides (NE Turkey): implications for magma evolution in extension-induced origin

The Eocene volcano-sedimentary units in the southern part of the Eastern Pontides (NE Turkey) are confined within a narrow zone of east–west trending, semi-isolated basins in Bayburt, Gumushane, Şiran and Alucra areas. The volcanic rocks in these areas are mainly basalt and andesite through dacite, with a dominant calc-alkaline to rare tholeiitic tendency. 40Ar–39Ar dating of these volcanic rocks places them between 37.7 ± 0.2 and 44.5 ± 0.2 Ma (Middle Eocene). Differences in the major and trace element variations can be explained by the fractionation of clinopyroxene ± magnetite in basaltic rocks and that of hornblende + plagioclase ± magnetite ± apatite in andesitic rocks. Primitive mantle-normalized multi-element variations exhibit enrichment of large-ion lithophile elements and to a lesser extent, of light rare earth elements, as well as depletion of high field strength elements, thus revealing that volcanic rocks evolved from a parental magma derived from an enriched mantle source. Chondrite-normalized rare earth element patterns of the aforementioned volcanic rocks resemble each other and are spoon-shaped with low-to-medium enrichment (LaN/LuN = 2–14), indicating similar spinel lherzolitic mantle source(s). Sr, Nd and Pb isotopic systematics imply that the volcanic rocks are derived from a subduction-modified subcontinental lithospheric mantle. Furthermore, post-collisional thickened continental crust, lithospheric delamination and a subduction-imposed thermal structure are very important in generating Tertiary magma(s). The predominantly calc-alkaline nature of Eocene volcanic rocks is associated with increasing geodynamic regime-extension, whereas tholeiitic volcanism results from local variations in the stress regime of the ongoing extension and the thermal structure, as well as the thickness of the crust and the mantle-crust source regions. Based on volcanic variety and distribution, as well as on petrological data, Tertiary magmatic activity in Eastern Pontides is closely related to post-collisional thinning of the young lithosphere, which, in turn, is caused by extension and lithospheric delamination after collisional events between the Tauride–Anatolide Platform and the Eurasian Plate.

Magmatic history and evolution of continental lithosphere of the Sør Rondane Mountains, eastern Dronning Maud Land, East Antarctica

The Sør Rondane Mountains, eastern Dronning Maud Land, East Antarctica, are situated within the Pan-African suture zone, between West and East Gondwana, and the timing of collision event is regarded as the late Neoproterozoic to early Cambrian. In order to understand the tectonothermal history and evolution of the continental lithosphere, geochemical studies were conducted and zircon U–Pb SHRIMP dating was performed on intrusive rocks with basaltic compositions and associated metamorphic rocks. The metamorphosed tonalite complex exposed in the southwestern part of the mountains comprises tonalite associated with microgabbros, occurring as magmatic enclaves and later dikes that have intruded both the host and magmatic enclaves. Geochemically, the microgabbros are classified as low-Ti and high-Ti types, corresponding to the magmatic enclaves and dikes, respectively. Moreover, the geochemical features of the low-Ti microgabbro resemble those of an oceanic-arc tholeiite, whereas the high-Ti microgabbro has features of a back-arc basalt. The apparent zircon U–Pb ages show c. 990Ma for the low-Ti microgabbro and c. 950Ma for the high-Ti microgabbro. The high-grade metamorphic rocks situated in the northern part of tonalite complex were metamorphosed between 640 and 620Ma, as constrained by zircon overgrowth rims of a migmatite leucosome (620±2Ma) and the previously reported age data. The timing of peak metamorphism corresponds to the early stage of the Pan-African suture event. Postdating the suturing event, unmetamorphosed minette dikes, dated 564±2 Ma, intrude the tonalite complex and high-grade gneisses. On the basis of geochemical investigation, including Sr and Nd isotopic systematics, the microgabbros are considered to have originated from a depleted mantle source, whereas the minette magma is derived from an enriched mantle source. Consequently, the source mantles of the mafic magmas in the Sør Rondane Mountains have fundamentally changed from a depleted source in the early Neoproterozoic to a more enriched source in the late Neoproterozoic. Geochemical and isotopic evidence suggest that this compositional change of source mantle would reflect the interaction between the depleted mantle and the enriched crustal materials, such as subducted crustal rocks, caused by the Pan-African suture event.

Geochemical significance of the Rb–Sr, La–Ce and Sm–Nd isotope systems in A-type rocks with REE tetrad patterns and negative Eu and Ce anomalies: The Cretaceous Muamsa and Weolaksan granites, South Korea

The geochemical implications of Sr, Ce and Nd isotope systematics for the highly fractionated A-type Muamsa and Weolaksan granites are discussed. These samples show REE tetrad patterns, and strongly negative Eu and negative Ce anomalies. The negative Eu anomalies increase with SiO2 contents and are inversely correlated with CaO and TiO2. Rb–Sr and Sm–Nd isotope systematics of the two granites indicate that they remained as a closed system since granite emplacement at 88.1±0.8Ma. The present and initial (T=88.1Ma) ɛCe and ɛNd values of the two granites are similar suggesting that these isotope systems remain undisturbed since the REE tetrads were formed. The Sr–Ce–Nd isotope systematics of the Muamsa and Weolaksan granites indicate that REE tetrad patterns in these granites were formed by magmatic fractionation rather than by hydrothermal alteration. The negative Eu anomalies and the REE tetrad patterns in both granites were thus produced by fractionation between the residual melt and a coexisting aqueous high-temperature fluid during magmatic differentiation from the same source, whereas Ce anomalies were formed after granite emplacement.

Petrogenesis of the Neoproterozoic West Highland Granitic Gneiss, Scottish Caledonides: Cryptic mantle input to S-type granites?

The Neoproterozoic (c. 870Ma) West Highland Granitic Gneiss, exposed in the Northern Highlands Terrane of Scotland, has elemental characteristics that are strikingly similar to those of the host Moine metasediments, which are thus consistent with an origin involving major Moine melting. Most of the constituent bodies have compositions significantly removed from minimum melts of pelites, and trace element constraints suggest variable but significant restite entrainment leading to less silicic bulk compositions with enhanced REE, Zr and Y. However, initial Nd and Hf isotope ratios are not coincident with contemporary Moine and imply a significant juvenile contribution. Close association with a regional suite of metabasites prompts consideration of mafic magma input, for which binary mixing models offer qualitative support. Quantitative difficulties with typical Moine metasediments are eased with radiogenic pelites or by partial melting of the mafic component. A possible alternative is currently unexposed Grenvillian felsic crust. Subsequent interaction of the granitic gneisses with meteoric water has significantly perturbed the oxygen and Sr isotope systems, the timing of which is equivocal but probably occurred during Caledonian events.The elemental characteristics of the West Highland Granitic Gneiss show many similarities with Scandinavian (rift-related?) granites of the same age, but since their geochemistry is largely inherited from the protolith it would be unwise to pursue palaeotectonic attribution on this basis. However, the probable incorporation of significant mantle-derived mafic magma of MORB-like affinity is consistent with an extensional setting.

Seasonal changes in the radiogenic and stable strontium isotopic composition of Xijiang River water: Implications for chemical weathering

Seasonal changes in strontium (Sr) isotopic systematics in large rivers can effectively track temporal variations in regional chemical weathering, and help to better constrain the Sr isotopic budget of global oceans. However, such records are scarce. Here we present a year-long time-series of both radiogenic and stable Sr isotope measurements (87Sr/86Sr and δ88Sr) from river water at Guiping, in the middle reaches of the Xijiang River, South China. Temporal changes in chemical weathering in the drainage basin were investigated by recording seasonal changes in the composition of both Sr isotopes and major ions in the river water. River water 87Sr/86Sr values range from 0.708487 to 0.710336, with most values being <0.7092, indicating a dominant contribution from the weathering of carbonate rocks. High 87Sr/86Sr values (>0.7092) are generally accompanied by high Rb/Sr ratios, and low pH and δ13C of dissolved inorganic carbo (DIC) and occur during periods of large river flux. This indicates that the contribution to the chemistry of river water from the weathering of silicate rocks is enhanced during rainy seasons. Seasonal variation in river water δ88Sr is large, from 0.147‰ to 0.661‰, with higher δ88Sr values generally corresponding to higher 87Sr/86Sr values. Again, low δ88Sr values (<0.3‰) suggest a dominant contribution from the weathering of carbonate rocks, whereas the high δ88Sr values may represent a contribution from intensive weathering of silicate rocks. Using the time-series data, we estimated the flux-weighted averages of Sr concentration, 87Sr/86Sr, and δ88Sr for the Xijiang River to be 1.00μmol/L, 0.70960, and 0.38‰, respectively. The Sr concentration and 87Sr/86Sr results are close to previous estimates, while the δ88Sr value is amongst the highest recorded for the large rivers of the world, and is close to that of seawater. Our results indicate that large rivers draining through tropical/sub-tropical regions could play an important role in regulating the stable Sr isotopic budget of global oceans.

Geochronology of igneous rocks at and near to the Nezhdaninka gold deposit, Yakutia, Russia: U-Pb, Rb-Sr, and Sm-Nd isotopic data

The intrusive rocks associated with the large Nezhdaninka gold deposit (Au > 470 t) hosted in the Permian carbonaceous terrigenous sequence have been dated on zircon and rock-forming minerals with precision U-Pb (ID-TIMS) and Rb-Sr methods. The lamprophyre of the dike complex that occurs in the ore field and spatially is related to gold mineralization has concordant U-Pb zircon age (121 ± 1 Ma) and the same isochron Rb-Sr age (121.0 ± 2.8 Ma). The concordant U-Pb zircon age of granodiorite that dominates in the Kurum pluton is 94 ± 1 Ma, whereas the Rb-Sr isochron age of various intrusive rocks from this pluton is 1–4 Ma younger. This difference is caused by long-term cooling of the Kurum pluton and later closure of Rb-Sr isotopic system of biotite (300–350°C) and other rock-forming minerals as compared with U-Pb isotopic system of zircon (∼ 900°C). The Rb-Sr age of quartz diorite from the Gel’dy group of stocks (92.6 ± 0.8 Ma) coincides within uncertainty limits with the age of the Kurum pluton. Thus, the rocks pertaining to two epochs of magmatic activity, which developed in the South Verkhoyansk Foldbelt and divided by a time span of 25–28 Ma, are documented in the Nezhdaninka ore field. Taking into account that the age of gold mineralization is no less than 120 Ma, the data obtained allow us to specify the previously proposed formation model of the Nezhdaninka deposit. These data give grounds to rule out the Late Cretaceous Kurum pluton and the Gel’dy group of stocks from constituents of the ore-magmatic system, and to suggest that an Early Cretaceous deep-seated magma source existed beneath the deposit. Along with host terrigenous rocks, this magma source participated in the supply of matter to the hydrothermal system. The Nd, Sr, and Pb isotopic systematics of igneous rocks and ore mineralization in the Nezhdaninka ore field show that the Early and Late Cretaceous magma sources were formed in the Precambrian crust dated at ∼1.8 Ga.

Boron isotope evidence for shallow fluid transfer across subduction zones by serpentinized mantle

Serpentinites formed by alteration of oceanic and forearc mantle are major volatile and fluid-mobile element reservoirs for arc magmatism, though direct proof of their dominance in the subduction-zone volatile cycles has been elusive. Boron isotopes are established markers of fluid-mediated mass transfer during subduction. Altered oceanic crust and sediments have been shown to release in the subarc mantle 11 B-depleted fluids, which cannot explain 11 B enrichment of many arcs. In contrast to these crustal reservoirs, we document high δ 11 B values retained in subduction-zone Alpine serpentinites. No 11 B fractionation occurs in these rocks with progressive burial: the released 11 B-rich fluids uniquely explain the elevated δ 11 B of arc magmas. B, O-H, and Sr isotope systems indicate that serpentinization was driven by slab fluids that infiltrated the slab-mantle interface early in the subduction history.

Coal fly ash interaction with environmental fluids: Geochemical and strontium isotope results from combined column and batch leaching experiments

The major element and Sr isotope systematics and geochemistry of coal fly ash and its interactions with environmental waters were investigated using laboratory flow-through column leaching experiments (sodium carbonate, acetic acid, nitric acid) and sequential batch leaching experiments (water, acetic acid, hydrochloric acid). Column leaching of Class F fly ash samples shows rapid release of most major elements early in the leaching procedure, suggesting an association of these elements with soluble and surface bound phases. Delayed release of certain elements (e.g., Al, Fe, Si) signals gradual dissolution of more resistant silicate or glass phases as leaching continues. Strontium isotope results from both column and batch leaching experiments show a marked increase in 87Sr/86Sr ratio with continued leaching, yielding a total range of values from 0.7107 to 0.7138. For comparison, the isotopic composition of fluid output from a fly ash impoundment in West Virginia falls in a narrow range around 0.7124. The experimental data suggest the presence of a more resistant, highly radiogenic silicate phase that survives the combustion process and is leached after the more soluble minerals are removed. Strontium isotopic homogenization of minerals in coal does not always occur during the combustion process, despite the high temperatures encountered in the boiler. Early-released Sr tends to be isotopically uniform; thus the Sr isotopic composition of fly ash could be distinguishable from other sources and is a useful tool for quantifying the possible contribution of fly ash leaching to the total dissolved load in natural surface and ground waters.

Re–Os isotope and highly siderophile element systematics of the Paraná continental flood basalts (Brazil)

Basalts of the Paraná continental flood basalt (PCFB) province erupted through dominantly Proterozoic continental crust during the Cretaceous. In order to examine the mantle source(s) of this major flood basalt province, we studied Os, Sr, Nd, and Pb isotope systematics, and highly siderophile element (HSE) abundances in tholeiitic basalts that were carefully chosen to show the minimal effects of crustal contamination. These basalts define a precise Re–Os isochron with an age of 131.6±2.3Ma and an initial 187Os/188Os of 0.1295±0.0018 (γ187Os=+2.7±1.4). This initial Os isotopic composition is considerably more radiogenic than estimates of the contemporary Depleted Mantle (DM). The fact that the Re–Os data define a well constrained isochron with an age similar to 40Ar/39Ar age determinations, despite generally low Os concentrations, is consistent with closed-system behavior for the HSE. Neodymium, Sr, and Pb isotopic data suggest that the mantle source of the basalts had been variably hybridized by melts derived from enriched mantle components. To account for the combined Os, Nd, Sr, and Pb isotopic characteristics of these rocks, we propose that the primary melts formed from metasomatized asthenospheric mantle (represented by arc-mantle peridotite) that underwent mixing with two enriched components, EM-I and EM-II. The different enriched components are reflected in minor isotopic differences between basalts from southern and northern portions of the province.The Tristan da Cunha hotspot has been previously suggested to be the cause of the Parana continental flood basalt magmatism. However, present-day Tristan da Cunha lavas have much higher 187Os/188Os isotopic compositions than the source of the PCFB. These data, together with other isotopic and elemental data, preclude making a definitive linkage between the Tristan plume and the PCFB.

Volcanic arcs fed by rapid pulsed fluid flow through subducting slabs

During subduction, the seawater-altered lithosphere becomes dehydrated and expels fluids. Isotopic analysis of an exhumed oceanic slab in the Tianshan Mountain Range shows that although subduction can continue for many millions of years, fluids are expelled in short-lived channels over periods of just a few hundred years. At subduction zones, oceanic lithosphere that has interacted with sea water is returned to the mantle, heats up during descent and releases fluids by devolatilization of hydrous minerals. Models for the formation of magmas feeding volcanoes above subduction zones require largescale transport of these fluids into overlying mantle wedges1,2,3. Fluid flow also seems to be linked to seismicity in subducting slabs. However, the spatial and temporal scales of this fluid flow remain largely unknown, with suggested timescales ranging from tens to tens of thousands of years3,4,5. Here we use the Li–Ca–Sr isotope systems to consider fluid sources and quantitatively constrain the duration of subduction-zone fluid release at ∼ 70 km depth within subducting oceanic lithosphere, now exhumed in the Chinese Tianshan Mountains. Using lithium-diffusion modelling, we find that the wall-rock porosity adjacent to the flowpath of the fluids increased ten times above the background level. We show that fluids released by devolatilization travelled through the slab along major conduits in pulses with durations of about ∼ 200 years. Thus, although the overall slab dehydration process is continuous over millions of years and over a wide range of pressures and temperatures, we conclude that the fluids produced by dehydration in subducting slabs are mobilized in short-lived, channelized fluid-flow events.

The suitability of annual tree growth rings as environmental archives: Evidence from Sr, Nd, Pb and Ca isotopes in spruce growth rings from the Strengbach watershed

The combination of the Sr, Nd and Pb isotope systems, recognized as tracers of sources, with the Ca isotope system, known to reveal biology-related fractionations, allowed us to test the reliability of spruce (Picea abies) growth rings as environmental archives through time (from 1916 to 1983) in a forest ecosystem affected by acid atmospheric deposition. Sr and Pb isotopes have already been applied in former tree-ring studies, whereas the suitability of Nd and Ca isotope systems is checked in the present article. Our Sr and Nd isotope data indicate an evolution in the cation origin with a geogenic origin for the oldest rings and an atmospheric origin for the youngest rings. Ca isotopes show, for their part, an isotopic homogeneity which could be linked to the very low weathering flux of Ca. Since this flux is weak the spruces’ root systems have pumped the Ca mainly from the organic matter-rich top-soil over the past century. In contrast, the annual growth rings studied are not reliable and suitable archives of past Pb pollution.

Provenance of late Oligocene to quaternary sediments of the Ecuadorian Amazonian foreland basin as inferred from major and trace element geochemistry and Nd–Sr isotopic composition

Abstract Oligocene to Quaternary deposits from the Oriente Amazonian foreland basin (Ecuador and Peru) were analyzed for major and trace element geochemistry (46 and 32 samples respectively) and Nd–Sr isotopic systematics (n = 10). Chemical Index of Alteration values lower than those of other Amazonian foreland basin sediments and scattering along the AK join in the A–CN–K diagram indicate that the Oriente foreland basin has been continuously fed by poorly to moderately weathered sediments having an overall Andesitic composition since the Oligocene. Chemical ratios such as Cr/Th and Th/Sc as well as Eu anomaly and Nd–Sr isotopic compositions indicate that most of the analyzed sediments contained a greater proportion of volcanic arc rock material than the other Amazonian foreland basin sediments. When compared with the older sediments The Quaternary sediments are characterized by a greater contribution of the volcanic arc source. The composition of the sediments deposited in the Ecuadorian Amazonian foreland basin is mainly controlled by geodynamic processes. We suspect the Late Pliocene-Pleistocene subduction of the Carnegie ridge to be responsible for the back arc volcanism feeding the Amazonian foreland with more basic materials. Input of young Ecuadorian volcanic rocks may explain the difference in Sr and Nd isotopic ratios of suspended sediments between the Solimoes and the Madeira rivers.

Strontium isotopic composition as tracer of weathering processes, a review with respect to James Ross Island, Antarctica

Strontium isotopes (87Sr/86Sr) are routinely used to determine sources and mixing relationships in geochemical studies. They have proven to be useful in determining weathering processes and quantifying end-member mixing processes. A number of studies highlight that Sr isotopes represent a powerful tool helping to constrain weathering reactions, weathering rates, flow pathways and mixing scenarios, even when inherent differences in weathering rates of different minerals, and mineral heterogeneity in natural environments may cause difficulties in defining the weathering component of different geochemical systems. Nevertheless, Sr isotopes are useful when combined with other chemical data, to constrain models of water–rock interaction and mixing as well as geochemical processes such as weathering. This paper presents basic information about Sr isotopic system, new analytical developments, summary of recent published studies in constraining the weathering processes, and indicates studies similar to weathering in polar regions. The aim of this paper is to present rationale of using Sr isotopes as tracer of weathering processes on James Ross Island, Antarctica.