87Sr86Sr Ratios Research Articles

5125 Verifying CTV-PTV margins for isocentric breast cancer radiotherapy, using an off-line correction protocol and fixed couch height

Continental flood volcanism of the Paraná basin (Lower Cretaceous) is represented by two-pyroxene tholeiitic basalts (90 vol. %). The Northern Paraná Province (NPP) is dominated by basalts high in TiO2 and incompatible elements (HTiB), while the Southern Paraná Province (SPP) is dominated by basalts low in TiO2 and incompatible elements (LTiB).NPP basalts show relatively small variations of initial (120 Ma) 87Sr86Sr (R0) and 143Nd144Nd (Nd∗) ratios (R0=0.7051−0.7062 and Nd∗=0.5124−0.5125, respectively) relative to those occurring in SPP (R0=0.7046−0.7120 and Nd∗=0.5122−0.5128, respectively). The latter basalts show significant positive correlations between R0 vs. SiO2, K2O, Rb and Ba, and negative correlations between R0 vs. (Cr + Ni) and mg-value, believed to be due to crustal “granitic” contamination. In general, the effects of contamination are pronounced in SPP and tend to vanish towards NPP. δ18O-values range from +6.5 to +10.0‰ in the basalts from NPP and SPP and essentially reflect water-magma interactions.“Uncontaminated” LTiB and HTiB basalts from NPP are isotopically distinct from the “uncontaminated” analogues from SPP (LTiB) (R0=0.7055 vs. 0.7046; Nd∗=0.5124 vs. 0.5128, respectively). All chemical data consistently indicate distinct sources and a large-scale mantle heterogeneity. The NPP mantle source is expected to have relatively high content of “enriched” components, possibly related to small-volume melts and metasomatic fluids. Tentatively, the last stabilization age of the Paraná mantle heterogeneity is 0.5–1.0 Ga old.The existence of high- and low-TiO2 basalt suites in both Paraná and Karoo provinces in Brazil and southern Africa, respectively, indicates a large-scale heterogeneity in the subcontinental mantle, and suggests that basalt generation occurred in the lithospheric mantle.

A multi-proxy study of Pliocene sediments from Île de France, North-East Greenland

A multi-technique approach has been used to study a Pliocene shallow water marine deposit, designated the Île de France Formation, in North-East Greenland. The sequence is correlated on the basis of 87Sr–86Sr ratios in shells and palaeomagnetic studies with the Gauss normal polarity chron, which is dated to between 2.60 and 3.58 Ma years BP. This dating is in accordance with amino acid epimerisation and evidence from dinoflagellates, foraminifers and molluscs. Sediments, marine molluscs and foraminifers show that the sequence was deposited on the inner shelf, below storm wave base. Seawater temperatures were much higher than today, as demonstrated by the occurrence of a number of southern extra-limital species. The same applies to air temperature, and the few remains of land plants may indicate a forested upland with Picea and Thuja. A number of extinct taxa are present, including Nucula jensenii that is erected as a new species.

Geochemical and isotopic (Sr, Nd and Pb) characteristics of volcanic rocks from southwestern Ethiopia

Major, trace and isotope data are presented for volcanic rocks from southwestern Ethiopia. The studied samples consist of mafic and felsic rocks, with the mafic lavas having generally transitional chemical characteristics between alkaline and tholeiitic, a feature that is typical of the Ethiopian province. The mafic samples have low Mg numbers (Mg#<55, excluding sample E145) and low compatible element contents, such as Ni and Cr suggesting that these lavas have undergone fractionation en route to the surface. All the samples are strongly enriched in LREE and in other incompatible elements as well as in HFSE, such as in Nb and Ta. 87Sr 86Sr ratios show a relatively small range (0.7031–0.7039). 143Nd 144Nd ratios show limited variation (0.5127–0.5129). All analysed lavas are notable for their radiogenic and wide variation in Pb isotope ratios; 206Pb 204Pb from 18.44 to 19.94, 207Pb 204Pb from 15.57 to 15.72 and 208Pb 204Pb from 38.37 to 39.71. The slightly radiogenic 207Pb 204Pb and 207Pb 204Pb ratios can not be attributed to crustal contamination, because 87Sr 86Sr and 143Nd 144Nd ratios do not indicate evidence of contamination by crustal materials. Moreover, Sr and Nd isotopic ratios show essentially no correlation with major and trace element compositions. The slightly elevated Pb isotopic compositions in the southwestern Ethiopian mafic lavas are consistent with their derivation from a time-integrated U and Th enriched mantle source, which may be attributed to a hot mantle plume. Nonetheless, a subcontinental lithospheric mantle source is also involved for those mafic lavas with low 143Nd 144Nd and 206Pb 204Pb values. The felsic samples show substantial similarity in isotopic ratios with the mafic rocks, implying that they are petrogenetically related to the associated mafic lavas.

Crustal recycling beneath continental arcs: silica-rich glass inclusions in ultramafic xenoliths from the Sierra Nevada, California

We describe silica-rich (up to ∼69.5% SiO 2) glass inclusions trapped as grain boundary films and within-grain pockets in ultramafic xenoliths hosted by Pliocene basalts from the Sierra Nevada, California. The ultramafic xenoliths are lherzolites which equilibrated in the Sierra Nevada upper mantle at 1150–1180°C and ∼1.4-1.8 GPa. The glass inclusions have trachytic compositions, similar to previously described silicic melts from mantle xenoliths [1–9]. We have determined the Sr and Nd isotope compositions of the grain boundary films using a leaching technique, and calculated the glass isotopic compositions. The glass 87Sr 86Sr (0.7077-0.7085) and 143Nd 144Nd (≈0.51244) ratios are higher than in the ultramafic xenoliths and distinct from the host basalt ratios. Glasses are characterized by Nb depletions ( N/b nNb n *∼0.15 ), enrichment of light rare earth elements ( L/a nYb n≈ 50 ), and the presence of negative Eu anomalies ( E/u nEu n *≈ 0.7-0.86 ), indicating a crustal origin for the melt source. The Nd isotope ratios ( ɛ Nd≈ −4 ) are inconsistent with an oceanic crust as the source for these former melts. The source rocks must have been continental materials recycled in the mantle, either foundered lower crust or subducted sediment. Low Rb/Sr (0.036-0.077) and high Sr/Nd (>35) ratios observed in the glasse are suggesting a lower crustal source. The Sierra Nevada lowermost crust (amphibole-bearing garnet pyroxenites and other dense Mesozoic cumulate mafic-ultramafic rocks), as defined by studies of older, Miocene xenolith-bearing volcanic rocks from the same area [M.N. Ducea, J. Saleeby, J. Geophys. Res. 101 (1996) 8229–8244], has isotopic compositions similar to the glass inclusions. Geologic [M.N. Ducea, J. Saleeby, J. Geophys. Res. 101 (1996) 8229–8244] and geophysical [G. Zandt, S. Ruppert, EOS Trans. AGU 77 (1996) 831] evidence indicate that the Sierra Nevada has lost its eclogitic arc root, probably by foundering in the mantle. We propose here that the silica-rich glasses were formed by low percent partial melting of the dense, cold Sierran batholithic lowermost crust during root delamination. Further tests need to be aimed at addressing the viability of the main alternative to our interpretation, i.e. derivation of glasses from melting subducted sediments.

Tracing the source of gypsum in New Caledonian soils by REE contents and SSr isotopic compositions

Gypsum minerals forming in pedogenic environments in New Caledonia were examined for their S and Sr isotope compositions and REE distributions. Three possible sources were identified: volcanic, soil-derived and marine. However, differentiation based only on the REE contents and normalized patterns was not possible, but, the best preserved volcanic sample yields a negative Eu Eu ∗ anomaly and the lowest Gd N Yb N ratio due to the high content of organic matter. Mixing calculations of the δ 34S values and 87Sr 86Sr ratios confirmed that the marine gypsum has a δ 34S value close to + 21‰ and an 87Sr 86Sr ratio close to 0.70912, and that the volcanic gypsum has a δ 34S value of 0‰ and an 87Sr 86Sr ratio of about 0.70331. The soil-derived gypsum samples yield intermediate values, as a result of a mixing between the marine and volcanic end members. Negative δ 34S values for the volcanic gypsum are explained by low f O 2 of the source rocks, while high 87Sr 86Sr ratios for the same gypsum are explained by probable atmospheric input. The data confirm also that biological fractionation of δ 34S, if any, is limited.

Sediment dewatering and pore fluid migration along thrust faults in a foreland basin inferred from isotopic and elemental geochemical analyses (Eocene southern Pyrenees, Spain)

The lower Eocene Ainsa basin was formed during the first stages of the south-Pyrenean foreland basin evolution due to southwestward migration of imbricated thrust-folds. Isotopic and elemental geochemistry of syn-kinematic veins (calcite and celestite) and their marly host-rock, sampled in three thrust-fault zones and one footwall syncline, allows us to characterize the origin of pore fluids and the early stages of their evolution and circulation during the early deformation of the basin-fill. The isotopic composition of sulfur and the 87Sr 86Sr ratios of calcites and celestites from the veins in the footwall syncline show that the original fluid had the isotopic composition of Eocene seawater. The different 87Sr 86Sr ratio in veins from the thrust-fault zones compared with the same ratio in the marly host-rock of the footwall syncline indicates that the thrust-fault zones acted as conduits for advective fluids. The relatively high 87Sr 86Sr ratio in the veins related to the thrust-fault zones indicates that the fluid originated from the interaction of seawater with an external fluid coming from deeper sources or from the meteoric weathering of the emerged part of the belt. δ 18O and δ 13C values of calcites show that the isotopic composition of the calcite-cements in veins was controlled by the isotopic composition of the marly host sediment. Depletion of both δ 18O and δ 13C with respect to Eocene seawater composition, together with elemental geochemistry of calcite cements in the veins, points to burial transformations of a seawater-derived fluid to a formation water composition. The distribution of δ 18O and δ 13C values of the marly host-rock and calcite cements in veins of the four outcrops probably resulted from differences in the meteoric water influences. The hydrogeological regime at the toe of the submarine thrust system was dominated by tectonically-induced dewatering of the foreland basin sediments. The thrust-fault zones were the channelizing paths for migration of fluids expelled from the surrounding sediments, as well as fluids derived from more internal parts of the belt.

Occurrence of mineral water springs in the stream channel of the Allier River (Massif Central, France): chemical and Sr isotope constraints

The French Massif Central has long been recognized as an area with numerous mineral water springs. It is often very difficult to measure the flow rate of mineral springs since they are spread out over extentive areas. Water from the Allier River has been sampled monthly over a 13-month period and a spatial cross-section survey carried out at base flow conditions in order to estimate the discharge of mineral water in the Allier River streambed. For the monthly survey, the 87Sr 86Sr ratios vary between 0.709511 and 0.712656. A good correlation exists between 87Sr 86Sr and 1 Sr ratios and defines binary mixing between mineral waters and the Allier River sensu-stricto. The mixing processes have been studied using the Sr isotopic systematic and a conservative species (chloride). Except for one sample, the mineral water input never exceeds 2%. However, the mineral water discharge into the Allier streambed could not be precisely estimated because effluent seepage of mineral water may occur in a part of the channel representing only a small volume of the cross-sections. Variations in the solute concentrations across the river channel were measured at 14 sites chosen as being representative of areas where: (a) mineral water emerges, (b) no mineral water emerges, and (c) mixing is complete. A model developed for the monthly study using Cl as a reference was used in the spatial survey. The input of mineral water in the Allier streambed ranges from 0.1–0.15% in most of the samples and is greater than 2% at two points. The general mass balance of the proportions of mineral water and Allier River water ( sensu-stricto) using the sum of cations and anions was calculated and the input of mineral water into the Allier streambed was determined at the cross-section nearest the Allier gauging station (0.134%). The discharge of the Allier River at the gauging station was about 17.7 m 3/s during the sampling period, which gives a discharge of mineral water of about 0.02 m 3/s into the Allier channel.

SmNd, PbPb and RbSr geochronology and petrogenesis of the mafic dyke swarm of Mahbubnagar, South India: implications for Paleoproterozoic crustal evolution of the Eastern Dharwar Craton

The SmNd, RbSr and PbPb isotopic systematics of whole rock and separated mineral fractions from four tholeiitic mafic dykes of the Mahbubnagar swarm have been investigated. This dyke swarm intrudes the granitic basement of the Cuddapah basin in the Eastern Dharwar Craton (EDC). The SmNd data on whole rock samples of four gabbros and one dolerite and 15 mineral fractions yielded a linear array with an isochron age of 2173 ± 64 Ma and an initial ϵ Nd value of 0.4 ± 0.3. The SmNd data of a metapyroxenite and its three mineral fractions plot parallel to this array yielding an isochron age of 2189 ± 123 Ma with a negative initial ϵ Nd value of − 1.8 ± 1.4. RbSr and PbPb internal isochron ages of three gabbros and a PbPb internal isochron age of a metapyroxenite dyke range between 2031 and 2284 Ma and, within the limits of experimental error, broadly conform to the SmNd isochron age. By contrast, RbSr internal isochron ages of three other samples range between 1474 and 1952 Ma, and one sample yields a PbPb internal isochron age of 3446 Ma, showing the effects of partial isotopic resetting, possibly due to hydrothermal alteration. These data suggest that mafic dykes of the Mahbubnagar swarm were emplaced at c. 2170 Ma, marking an episode of the crustal extension and fracturing which ultimately resulted in the formation of the Cuddapah basin. Different petrological and geochemical variants of mafic dykes seen in the area most probably represent generation and/or crystallization of the progressively evolved batches of magma during a relatively short time interval. The initial ϵ Nd values calculated at 2170 Ma range from 0.6 to − 2.0 and can be explained as reflecting contamination of depleted mantle derived magma by crustal rocks of EDC. The initial 87Sr 86Sr ratios (0.7027–0.7064) and model μ 1 values (8.3–8.5) of apparently undisturbed samples are also consistent with this suggestion.

RbSr isotope systematics of a granitic soil chronosequence: The importance of biotite weathering

The RbSr isotope systematics of bedrock, soil digests, and the cation exchange fraction of soils from a granitic glacial soil chronosequence in the Wind River Mountains, Wyoming, USA, were investigated. Six soil profiles ranging in age from 0.4 to ∼300 kyr were studied and revealed that the 87Sr 86Sr ratio of exchangeable strontium in the B-horizons decreased from 0.7947 to 0.7114 with increasing soil age. Soil digests of the same samples showed much smaller variation in 87 Sr 86 Sr from 0.7272 to 0.7103 and also generally decreased with increasing soil age. Elevation of the 87 Sr 86 Sr ratios of Sr released by weathering over the soil digest and bedrock values results from the rapid weathering of biotite to form hydrobiotite and vermiculite in the younger soils. Biotite is estimated to weather at aaproximately eight times the rate of plagioclase (per gram of mineral) in the youngest soil profile and decreases to a rate of only ∼20% of that of plagioclase in the oldest soil. 87 Rb 86 Sr ratios of the soil cation exchange fraction are estimated to be depleted by factors of up to 11 over the 87 Rb 86 Sr ratios released by weathering, due to ion exchange partitioning. This study demonstrates that the 87 Sr 86 Sr ratio released by weathering of crystalline rocks can deviate significantly from bedrock values, and that in soils less than ∼20 kyr in age which contain biotite in the soil parent material, weathering-derived 87 Sr 86 Sr values can be elevated so dramatically that this factor must be considered in estimations of weathering rates based on strontium isotopes.

Geochemical constraints on the formation of Late Cenozoic ferromanganese micronodules from the central Arctic Ocean

In order to determine geochemical compositions of Late Cenozoic Arctic seawater, oxide fractions were chemically separated from 15 samples of hand-picked ferromanganese micronodules (50–300 μm). The success of the chemical separation is indicated by the fact that > 97% of the Sr in the oxide fraction is seawater-derived. Rare-earth element (REE) abundances of the Arctic micronodule oxide fractions are much lower than those of bulk Fe-Mn nodules from other ocean basins of the world (e.g., 33 vs. 145 ppm Nd), but the Arctic oxides are enriched in Ce relative to Nd (CeNNdN = 2.2 ± 0.5) and have convex-upward, shale-normalized REE patterns (NdNGdN = 0.61 ± 0.06, GdNYbN = 1.5 ± 0.2, NdNYbN = 0.9 ± 0.2), typical of other hydrogenous and diagenetic marine Fe-Mn-oxides. Bulk sediment samples from the central Arctic Ocean have REE abundances and patterns that are characteristic of those of post-Archean shale. Non-detrital fractions (calcite + oxide coatings) of Recent Arctic foraminifera have REE abundances and patterns similar to those of Recent foraminifera from the Atlantic Ocean.Electron microprobe analyses (n = 178) of transition elements in 29 Arctic Fe-Mn micronodules from five different stratigraphic intervals of Late Cenozoic sediment indicate that oxide accretion occurred as a result of hydrogenetic and diagenetic processes close to the sediment-seawater interface. Transition element ratios suggest that no oxide accretion occurred during transitions from oxic to suboxic diagenetic conditions. Only K is correlated with Si and Al, and ratios of these elements suggest that they are associated with illite or phillipsite. Ca and Mg are correlated with Mn, which indicates variable substitution of these elements from seawater into the manganate phase. The geochemical characteristics of Arctic Fe-Mn micronodules indicate that the REEs of the oxide fractions were ultimately derived from seawater. However, because of minute contributions of Sr from siliciclastic detritus during diagenesis or during the chemical leaching procedure, Sr isotope compositions of the oxide fractions cannot be used to trace temporal changes in the 87Sr86Sr ratio of Arctic seawater or to improve the chronostratigraphy.

Massive barite deposits in the convergent margin off Peru: Implications for fluid circulation within subduction zones

The convergent margin of Peru, characterized by an extensional tectonic regime and the lack of a well-developed accretionary prism, has been investigated by a deep-sea submersible during the Nautiperc cruise (March–April, 1991). This allowed the collection of fluid samples, soft sediments, and barite concretions in the vicinity of biological communities associated with fluid steps. Major and trace element contents as well as strontium, oxygen, hydrogen, and sulfur isotopic compositions have been measured on fluid and/or solid samples to constrain the nature and origin of fluid circulating in this extensional tectonic context. Chemical variations with respect to bottom seawater composition have been recorded in the fluid samples and suggest the presence of a nonlocal component in the fluid expelled at the seafloor. The major variations correspond to elevations of the Cl, Na, and the Ba contents as well as the 87Sr 86Sr ratios. This is interpreted as the expulsion of a radiogenic, continent-related (basinal brine and/or meteoric water) fluid. Massive barite concretions have been collected at the seafloor in two areas of major fluid venting. The radiogenic signature (strontium isotopic composition) of the barite concretions implies that they are related to the nonlocal deep fluid component identified in the fluid samples. Furthermore, it is shown that these barite deposits testify to a hot, short, and intensive fluid circulation event. Compared to subduction zones that exhibit venting fluid with a strong oceanic water signature, the nature and origin of venting fluid along the subduction zone of Peru are different. The extensional tectonic regime of the Peru continental margin, locally associated with a dense E-W trending fault network, is an agent which may help to drain continent-related fluid as deep as the subduction scarp at the trench-slope boundary.

High-Mg subduction-related Tertiary basalts in Sardinia, Italy

The Oligo-Miocene volcanics (32-15 Ma), which occur in the Oligo-Miocene Sardinian Rift, were interpreted in the literature as an intracontinental volcanic arc built upon continental crust about 30 km thick. They are characterized by a close field association of dominantly andesites and acid ignimbrites, with subordinate basalts. In this paper we deal with the origin and evolution of recently discovered high-magnesia basalts aged ca. 18 Ma occurring in the Montresta area, northern Sardinia, relevant to the petrogenesis of the Cenozoic volcanics of Sardinia. The igneous rocks of the Montresta area form a tholeiitic, subduction-related suite. Major-element variation from the high-magnesia basalts (HMB) to high-alumina basalts (HAB) are consistent with crystal/liquid fractionation dominated by olivine and clinopyroxene. Proportions of plagioclase and titanomagnetite increase from HAB to andesites. Initial 87Sr 86Sr ratios increase with differentiation from 0.70398 for the HMB to 0.70592 for the andesites. This suggests concomitant crustal contamination. The geochemical characteristics of the high-magnesia basalts are typical of subduction-related magmas, with negative Nb, Zr and Ti spikes in mantle-normalized diagrams. It is proposed that these high-magnesia basalts were produced by partial melting of a mantle source characterized by large-ion lithophile elements (LILE) enrichment related principally to dehydration of subducted oceanic crust. Chondrite-normalized rare earth elements (REE) patterns indicate that the lavas are somewhat enriched in light rare earth elements (LREE), with flat heavy rare earth elements (HREE) patterns. This evidence is consistent with a spinel-bearing mantle source. The sub-parallel chondrite-normalized patterns show enrichment with differentiation, with a greater increase of LREE than HREE. The occurrence of high-magnesia basalts at 18 Ma in Sardinia appears to be correlated with and favoured by pronounced extensional tectonics at that time.

Evolution of the Mount Etna magma: Constraints on the present feeding system and eruptive mechanism

Volcanism in the Mount Etna area began some 500,000 years ago with sparse effusions of subaphyric olivine tholeiites showing primary characteristics (0.3–0.4% K 2O, 12–10% MgO, 500 to 400 ppm Cr and 200–350 ppm Ni). At 300 ky BP, pigeonite tholeiites were emitted, soon followed by increasingly porphyric transitional tholeiites (0.4–0.7% K 2O), slightly evolved porphyritic alkali basalts (0.6–1.2% K 2O), and trachybasalts (1.3–2.2% K 2O) close to hawaiites, though rich in calcic plagioclase phenocrysts. All these ancient lavas, either tholeiitic or alkaline, cover the same range of 87Sr 86Sr ratios (0.7030–0.7032). Since 200 ky BP, porphyritic trachybasalts have composed most of the various parts of Mt. Etna proper. They were accompanied from time to time by more differentiated products (porphyritic or aphanitic trachyandesites and trachytes) whose eruptions eventually culminated in caldera collapse. For the last 14 ky, Etna has continued to erupt porphyritic trachybasalts and rarely aphyric basalts, some of which are strongly enriched in K, Rb, Cs, and have higher 87Sr 86Sr (0.7033 to 0.7037). The gradual shift in chemical and mineralogical composition from tholeiites to alkaline types is consistent either with a change in the melting degree of an initially homogeneous mantle source, or more likely with melting of upper mantle levels metasomatized by previous infiltrations of K-rich, small-degree melts from the same source. The primary magma eventually evolved to alkali olivine basalt from which the porphyritic alkali basalts and trachybasalts are shown to be derived by high-pressure (8–10 kbar) fractional crystallization, involving clinopyroxene and olivine as dominant liquidus phases. The younger trachyandesites and trachytes are products of low-pressure fractionation of minerals, mainly plagioclase, present as phenocrysts in porphyric types. Sudden increases in K, Rb, Cs, and 87Sr 86Sr ratios, like those in the post-1971 period, may be explained by selective assimilation, through a fluid phase, of particular crustal levels beneath the volcanic pile. It is suggested that upwelling of the asthenosphere first caused extensive melting of a mantle diapir, allowing tholeiitic magma to accumulate near the mantle-crust interface. Then, increasingly alkaline basalt was generated and fed the entire volcanism of Mt. Etna by undergoing continuous but limited differentiation (trachybasalts) in a subcrustal reservoir, possibly at the top of the mantle diapir. Superimposed on this basic mechanism, more pronounced differentiation (trachyandesites and trachytes) occurred in temporary, superficial crustal chambers, for which there is geophysical and morphological evidence (calderas). At present, the 20–30 km deep subcrustal reservoir appears of critical importance in controlling volcanic activity: Variations of magmatic pressure within it (input/output of magma) should trigger opening of fractures in the crust, exchange with phreatic fluids and selective assimilation, and finally fissure eruptions. A ‘volcano-tectonic’ model is presented that accounts for the various eruptive styles.

Calcite in fractures in a volcanic environment (Vulcano Island, Italy): contribution of geochronological and isotopic studies to volcanotectonics

The southern and southeastern rim of the present caldera (La Fossa caldera) at Vulcano Island (Aeolian Islands, Italy) is crosscut by a network of fractures filled with calcite and minor chalcedony. Fluid inclusion studies indicate that both mineral phases were deposited from hydrothermal solutions. Several calcite samples were analyzed and their U, Sr, Nd, O and C isotopic composition was determined. U Th geochronological data suggest two discrete depositional episodes: one at 50 ka (southern rim) and the other at 25 ka (southeastern rim). Oxygen isotopic compositions, 87Sr 86Sr and 234U 238U ratios exclude that seawater was involved significantly in the hydrothermal system, in agreement with fluid inclusion studies which did not document the presence of NaCl. The Nd and Sr isotopic composition of calcite reflects the composition of the host rocks. The oxygen isotopic composition excludes a post-depositional interaction with meteoric water and suggests a depositional temperature consistent with that obtained from the fluid inclusion study (160–170 °C), assuming equilibrium conditions and a fluid isotopic composition close to present-day steam emissions. The positive correlation between U content and C isotopic composition indicates a rapid change in CO 2 pressure during calcite deposition. This observation is consistent with a rapid fluid ascent, likely connected to two phases of collapse of the La Fossa caldera. The ages of the calcites which fill the fractures at the La Fossa caldera indicate the activation time of the fracture systems which, in turn, are connected to the above mentioned phases of the caldera collapse.

Late Cenozoic Sr isotope evolution of the Arctic Ocean: constraints on water mass exchange with the lower latitude oceans

Direct measurements of the Sr isotope composition of the different water masses that comprise the chemically and thermally stratified modern Arctic Ocean yield 87Sr86Sr ratios that are homogeneous and identical (i.e. within an error of < 1 × 10−5) to the 87Sr86Sr ratio of the lower latitude oceans. Variations in the 87Sr86Sr ratios of planktonic foraminifera from the Arctic Ocean, which define the Sr isotope composition of shallow Arctic seawater, follow the Sr isotope variations of the lower latitude oceans over the past 0.9 Ma. These new Sr isotope data support the chronostratigraphy previously established for the Pleistocene Arctic Ocean. Moreover, the results indicate that although the Arctic Ocean is nearly an enclosed basin and river-runoff constitutes a significant portion of the modern surface waters, the Arctic Ocean has been in Sr isotope equilibrium with the world ocean throughout most of the Quaternary. This conclusion is clearly supported by data for samples younger than 0.9 Ma, and is probably valid for the Sr isotope compositions of samples that are as old as 1.6 Ma. Because the Sr isotope data suggest water mass exchange between the Arctic and the world ocean during the Pleistocene, it follows that the Arctic Ocean has probably been an important factor in global climate over this time interval.

Post-Brasiliano (Pan-African) high-K granitic magmatism in Central Brazil: the role of late Precambrian-early Palaeozoic extension

In western Goiás, Brazil, the emplacement of large, high-K postorogenic granites and associated small gabbro-dioritic intrusions, followed immediately after the last deformational events of the Brasiliano-Pan-African orogeny at ∼600 Ma. Well-fitted whole-rock RbSr isochrons indicate ages which suggest two discrete intrusive events: the older between ∼588 and 560 Ma and the younger between ∼508 and 485 Ma. The older granites display general petrographic and geochemical characteristics of highly differentiated calc-alkaline I-type granitoids, whereas the younger intrusions are more alkaline, similar to A-type granites. Initial 87Sr 86Sr ratios vary from ∼0.703 to 0.710 and initial Nd isotope ratios yield ϵ Nd(T) values in the range between −4.0 and +3.0. There are no major differences in initial isotopic compositions between the two granite groups, suggesting that the parental magmas for both groups of rocks mostly originated by refusion of crustal sources isotopically similar to the ∼940-640 Ma basement arc-type metatonalites-metagranodiorites and associated arc metavolcanics. The major and trace element compositional differences between the two granite groups is explained in terms of modifications in the melting conditions within the crust, with younger melts being produced by the refusion of anhydrous, depleted crustal sources left after the extraction of previous batches of more hydrated calc-alkaline magmas. The heat input required to promote extensive remelting of the continental crust was, most likely, provided by mantle-derived mafic magmas that invaded and probably underplated the crust, during uplift and extension. The two intrusive events are bimodal in nature and are interpreted as shallow-level extension-related events associated with regional uplift and denudation occurring just after two orogenic pulses at the end of the Proterozoic and early Palaeozoic, the older at ∼600 Ma and the younger between ∼550 and 510 Ma. The onset of the granite magmatism at ∼590 Ma, shortly post-dating the Brasiliano orogeny (∼600 Ma), is broadly coeval with the initial stages of sedimentation of the terrigenous and carbonatic rocks of the ensialic Paraguay Belt in Brazil and its correlative in Bolivia, the Tucavaca Belt, which probably correspond to rift deposits related to the break up of Laurentia from Gondwana at the end of the Proterozoic and beginning of the Palaeozoic. Granites of the younger group cut the deformational structures of the Paraguay Belt metasediments and pre-date, by between ∼40 and 20 Ma, the initial stages of subsidence and sedimentation of the Paraná Basin.

Isotopic and elemental systematics of Sr and Nd in 454 Ma biogenic apatites: implications for paleoseawater studies

Pristine conodonts (CAI ≤ 1.5), inarticulate brachiopods, and conulariids, all from a single hand sample of Ordovician limestone, define a co-varying trend of 87Sr 86Sr and Sr and Sr concentration. Most of the apatitic fossils have 87Sr 86Sr ratios that are more radiogenic than the enclosing whole-rock limestone, indicating a general susceptibility of biogenic apatites to post-depositional Sr exchange. The largest isotopic shifts were measured in inarticulate brachiopods and conulariids, and deduced for conodont basal body material. Conodont crown material exhibits the smallest effects. The Sr exchange effects are strongly dependent on differences in apatite composition, as revealed by contrasting Ca P ratios. Although conodont crown material (with low Ca P ratios) is less prone to isotopic disturbance relative to other types of coexisting apatite fossils, high resolution X-ray mapping reveals that even conodont crowns exchange Sr, as is shown by a gradient of decreasing Sr concentration from crown rim to core. In contrast to Sr, all coexisting fossil apatites have identical initial 143Nd 144Nd ratios over a wide range of Nd concentration. No relationship between 87 Sr 86 Sr and 143Nd 144Nd was observed despite a pronounced antithetic pattern of Sr and Nd distribution both between the fossil types, and within individual conodonts containing preserved basal body material. In agreement with earlier studies, it is concluded that the bulk of the Nd in fossil apatites is from seawater that originally overlay the depositional site.

Unusually large Nb [sbnd]Ta depletions in North Chile ridge basalts at 36°50′ to 38°56′S: major element, trace element, and isotopic data

We present new major and trace element data for 28 basalts and 10 basaltic glasses recovered from 16 locations from the North Chile Ridge (NCR) at 36°50′ to 38°56′S. This part of the Chile Ridge consists of three short ridge segments, which are characterized by deep (3200–4100 m) axial valleys. Chemical compositions of the basaltic glasses vary from primitive to moderately fractionated basalts (MgO = 9.54−7.28 wt%). All rocks are incompatible element depleted, mid-ocean ridge basalts (MORB) with average chondrite-normalized (La/Sm) N ratios of 0.46 ± 0.08. The radiogenic isotopic ratios of 10 representative samples display a narrow range in 87Sr 86Sr ratios from 0.70241 to 0.70249. 143Nd 144Nd ratios also vary within a small range from 0.51312 to 0.51318, and the 206Pb 204Pb ratios range from 18.2 to 18.6, with one exception which has a 206Pb 204Pb of 19.1. Overall, isotopic compositions are similar to average depleted MORB from the EPR (East Pacific Rise) and MAR (Mid-Atlantic Ridge), but 207Pb 204Pb ratios are significantly higher. Pb isotopic systematics of East Pacific MORB reflect large-scale heterogeneities, which are probably the result of long-lived differences in Th/U ratios in the mantle. Significant differences exist in the inferred primary melt compositions between the NCR basalts and depleted MORB from the South EPR and Galapagos Spreading Centre (GSC). For a given MgO content, basalt glasses from the NCR have systematically higher Na and Ti and lower Ca concentrations than those from the South EPR and the GSC. This has been interpreted as indicating relatively low average degrees of melting, which is possibly the result of cooling the shallow asthenosphere near transform offsets. NCR basalts are, on average, more primitive than basalts from the EPR and GSC, implying the lack of a robust magmatic system in this part of the Chile Ridge. This, together with the characteristic ridge topography and short average segment length, suggests that the magmatic system is short-lived, similar to slow-spreading ridges. This can be attributed to lower upwelling rates and, consequently, low magma supply rates near the transform offsets. The majority of the basalts from the NCR are unusually depleted in Nb and Ta relative to average depleted MORB. Trace element modelling shows that the distinct trace element characteristics of the NCR lavas could be the result of melting a mantle which has experienced a previous melting episode. This episode was possibly related to upwelling and melting of the mantle beneath the Pacific-Farallon ridge more than a million years ago.

Neogene Himalayan weathering history and river 87Sr 86Sr : impact on the marine Sr record

Clastic sediments in the Bengal Fan contain a Neogene history of erosion and weathering of the Himalaya. We present data on clay mineralogy, major element, stable and radiogenic isotope abundances from Lower Miocene-Pleistocene sediments from ODP Leg 116. Nd and Sr isotope data show that the Himalayan provenance for the eroded material has varied little since > 17 Ma. However, from 7 to 1 Ma smectite replaces illite as the dominant clay, while sediment accumulation decreased, implying an interval of high chemical weathering intensity but lower physical erosion rates in the Ganges-Brahmaputra (GB) basin. O and H isotopes in clays are correlated with mineralogy and chemistry, and indicate that weathering took place in the paleo-Gangetic flood plain. The 87Sr 86Sr ratios of pedogenic clays (vermiculite, smectite) record the isotopic composition of Sr in the weathering environment, and can be used as a proxy for 87Sr 86Sr in the paleo-GB basin. The Sr data from pedogenic clays shows that river 87Sr 86Sr values were near 0.72 prior to 7 Ma, rose rapidly to ≥ 0.74 in the Pliocene, and returned to ≤ 0.72 in the middle Pleistocene. These are the first direct constraints available on the temporal variability of 87Sr 86Sr in a major river system. The high 87Sr 86Sr values resulted from intensified chemical weathering of radiogenic silicates and a shift in the carbonate-silicate weathering ratio. Modeling of the seawater Sr isotopic budget shows that the high river 87Sr 86Sr values require a ca. 50% decrease in the Sr flux from the GB system in the Pliocene. The relationship between weathering intensity, 87Sr 86Sr and Sr flux is similar to that observed in modern rivers, and implies that fluxes of other elements such as Ca, Na and Si were also reduced. Increased weathering intensity but reduced Sr flux appears to require a late Miocene-Pliocene decrease in Himalayan erosion rates, followed by a return to physically dominated and rapid erosion in the Pleistocene. In contrast to the view that increasing seawater 87Sr 86Sr results from increased erosion, Mio-Pliocene to mid-Pleistocene changes in the seawater Sr budget were the result of reduced erosion rates and Sr fluxes from the Himalaya.

Mantle heterogeneities beneath the South Atlantic: a Nd [sbnd]Sr [sbnd]Pb isotope study along the Mid-Atlantic Ridge (3°S–46°S)

We report on 55 Nd and Sr isotope analyses of Mid-Atlantic Ridge (MAR) basalt glasses from 3°S to 46°S, using the same samples on which Pb and He isotope ratios were reported earlier (Hanan et al. [1] and Graham et al. [2]). Eighteen new Pb, Sr, and Nd isotope analyses are also reported on basalt glasses from 17 stations from the same region. 87Sr 86Sr ratios range from 0.70212 to 0.70410 and 143Nd 144Nd from 0.51285 to 0.51331. The along-ridge long wavelength 87Sr 86Sr variation delineated by light-REE depleted basalts increases progressively southward toward Tristan da Cunha. Short wavelength, spike-like, positive 87Sr 86Sr anomalies composed of light-REE enriched basalts are found opposite the Tristan, St. Helena, and Circe (Ascension) hotspots (as for the Pb isotopes). The short and long wavelength 143Nd 144Nd variations anti-correlate with those of 87Sr 86Sr . The 17 new Pb isotope analyses confirm both the short and long wavelength trends previously reported by Hanan et al. [1]. These spatial variations, as well as the variations in Nd Sr Pb isotopic space fully confirm the mantle plume-ridge interaction model and upper mantle mixing conditions beneath the South Atlantic inferred previously on the basis of Pb isotopic data only (Hanan et al. [1]). However, in Nd Sr isotopic subspace the Circe and the St. Helena mixing vector are not distinguishable. The short wavelength Nd Sr Pb He anomalies suggest recent lateral sublithospheric channeled flows from these off-ridge plumes to the migrating MAR axis. The long wavelength variations reflect a broad pollution of the asthenosphere by Pb and Sr radiogenic, isotope-rich, mantle material, which has been partly depleted of incompatible elements relatively recently. This broad pollution may be related to the partial melting and dispersion of the Tristan and St. Helena plume heads into the subcontinental asthenosphere, prior to the opening of the South Atlantic. In this two-stage model, the mixing relations in Nd Sr Pb space further require that the incompatible element depletion and dispersion of the St. Helena plume into the asthenosphere occurred before that of the Tristan plume head. We emphasize that this two-stage model is based purely on isotope systematics. The implied thermal and dynamic aspects of this model remain to be evaluated and tested.