Sr Isotope Values Research Articles

Diagenetic evolution and associated mineralization in Middle Devonian carbonates, southwestern Ontario, Canada

Abstract The Middle Devonian Lucas Formation of southwestern Ontario, Canada, forms the main hydrocarbon reservoir rocks in the region. Integrated field, petrographic, fluid inclusion and isotope geochemistry reveal a variety of diagenetic processes that include the formation of dolomite, calcite, celestine and fluorite under diverse geochemical conditions. The host carbonate strata display two types of replacive dolomite fabrics: 1) fine-crystalline matrix dolomite (5–20 μm); and 2) medium-crystalline dolomite (50–150 μm) with an average size of 100 μm. Stable oxygen and carbon isotope results for both dolomite fabrics show a wide range of values (δ18O = −9.4‰ to −3.9‰ VPDB, δ13C = 0.4‰ to 3.4‰ VPBD). For late fracture-filling calcite, δ 18O and δ 13C values range from −9.4 to −5.9‰ VPDB and −7.7‰ to −0.04‰ VPDB, respectively. The 87Sr/86Sr ratios of selected dolomite and calcite samples range from 0.70798–0.70838, preserving Middle Devonian seawater values while some samples have slightly more radiogenic ratios than contemporaneous seawater. Field and petrographic evidence and Sr isotope ratios of dolomite suggest early dolomitization from evaporated seawater in a sabkha setting (shallow seepage reflux or evaporitive pumping). However, the depletion in δ18O (varying from −3.9 to −9.4‰ VPDB), and radiogenic Sr isotope values of the Lucas Formation dolomite are considered to be the result of recrystallization, which is interpreted to occur at elevated temperatures in a burial environment. The 18O-depleted values and slightly radiogenic 87Sr/86Sr ratios of calcite cements suggest precipitation from hot, saline basinal/hydrothermal brines. In addition, the 13C-depleted calcites indicate that carbon was possibly formed by oxidation of organic matter. Fluid inclusions in some of the late calcite cements fluoresce under UV light, indicating the presence of hydrocarbons. Homogenization temperatures of these inclusions vary from 87.1°C to 171.9°C with an average value of 126°C. Average salinity of fluid inclusions is 19.7 wt. % NaCl eq. Celestine samples have lower salinities relative to calcite, averaging 14.5 wt. % NaCl eq. with an average homogenization temperature of 219.5°C. Based on the geochemical and fluid inclusion results, hot, saline basinal/hydrocarbon bearing brines originated from central part of the basin and migrated to Devonian carbonates via fractures and faults. Celestine is a product of thermochemical sulfate reduction and formed due to mixing of saline, Sr and sulfate bearing brines and meteoric waters.

Diverse Sr isotope signatures preserved in mid-oceanic-ridge basalt plagioclase

At mid-ocean ridges, a variety of crustal processes overprint mantle-derived melts and can obscure original mantle compositions. To address the nature of this crustal filter, we report 87 Sr/ 86 Sr ratios from plagioclase phenocrysts and host glasses in mid-oceanic-ridge basalts from the Juan de Fuca Ridge, Blanco Transform Zone, and the Southwest Indian Ridge. Microdrilled isotopic analyses reveal significant inter- and intracrystalline disequilibria within individual samples. These disequilibria suggest that a range of isotopically distinct melt components contribute to individual plagioclase crystals and to the magmas that transport them to the surface. Low Cl/K values both in the host glass and in plagioclase-hosted melt inclusions largely rule out incorporation of seawater-derived material as an explanation for differences in 87 Sr/ 86 Sr. Instead, the observed heterogeneity implies derivation of magmas from isotopically diverse mantle sources. Importantly, the range of Sr isotope values preserved in a single sample is similar to the range of compositions seen at the ridge segment scale. Unlike analyses of host glass compositions, which are the result of extensive crustal processing, isotopic analyses of phenocryst phases record fine-scale aggregation of these distinct mantle-derived melts and are thus an important and underutilized tool in interpreting the nature of the mid-oceanic-ridge basalts.

A basement-interacted fluid in the N81 deposit, Pine Point Pb-Zn District, Canada: Sr isotopic analyses of single dolomite crystals

The Mississippi Valley-type Pb-Zn deposits of the Pine Point district (Northwest Territories, Canada) are located close to the eastern edge of the present day Western Canadian Sedimentary Basin. The deposits are thought to have formed as the result of basin-wide fluid flow in the Presqu’ile Barrier, the host to the ore deposits. A laser ablation multi-collector inductively coupled plasma mass spectrometric study of 87Sr/86Sr ratios of ore-related dolomites from the N81 deposit at Pine Point indicates that at least two sources of Sr were present in the mineralizing system. One fluid has a range in Sr isotopic values from 0.07073 to 0.71200 and is interpreted to be derived from Middle Devonian seawater that interacted with clastic units in the basin. The second fluid has higher Sr isotopic values (up to 0.71520), similar to those found in some Canadian Shield brines, and is interpreted to represent an evaporated seawater-derived brine which has interacted with crystalline basement rocks. Reactivation of old structures in the basement may have provided a pathway for cross-formation fluid flow to the site of mineralization. The data suggest that the stratigraphic location of the Pine Point District, near the interface between the Western Canadian Sedimentary Basin and its basement, may have exerted a fundamental control on the formation of these deposits.

An emplacement mechanism for the mega‐block zone within the Chicxulub crater, (Yucatán, Mexico) based on chemostratigraphy

Abstract– To better constrain the emplacement mechanism of the so‐called “mega‐block zone,” a structurally complex unit of target rocks within the Chicxulub impact structure, the stratigraphic coherence of this zone is tested using its strontium isotopic composition. Forty‐eight samples across the 616 m sequence of deformed Cretaceous rocks in the lower part of the Yaxcopoil‐1 core, drilled by ICDP in 2002, were analyzed for their 87Sr/86Sr isotope ratio. The oceanic anoxic event 2 (OAE2 event), located near the base of the core forms the only stratigraphic anchor point. From this point upward to approximately 1050 m depth, the 87Sr/86Sr trend shows small oscillations, between approximately 0.7074 and 0.7073, characteristic of Cenomanian to Santonian values. This is followed by an increase to approximately 0.7075, similar to the one reported in the seawater strontium curve during the Campanian. Scattered Sr isotope ratios are attributed to local diagenetic effects, such as those expected from the possible presence of hot, impact‐induced dikes and hydrothermal fluid flow, originating from the thick central melt sheet. The absence of Upper Maastrichtian Sr isotope values may result from the removal of upper target lithologies during the impact cratering process. Based on these results, the displaced Cretaceous sequence in Yax‐1 appears to have preserved its stratigraphic coherence. During the modification stage, it probably moved as a whole into the annular basin during collapse of the crater wall, thereby breaking up into discrete units along previously weakened detachment zones. This model is consistent with the emplacement mechanism postulated by Kenkmann et al. (2004).

Northwestern Junggar Basin, Xiemisitai Mountains, China: A geochemical and geochronological approach

We report SIMS U–Pb zircon, elemental and Nd–Sr–Pb isotopic data for the volcanic rocks and their subvolcanic units from the Xiemisitai Mountains in the West Junggar Region (Xinjiang, China), aiming to determine their ages, source regions and tectonic setting. Zircon U–Pb dating results of the volcanic rocks (422.5±1.9Ma–411.2±2.9Ma) indicate that the volcanic strata occurred in the Xiemisitai Mountains at Late Silurian–Early Devonian time rather than Middle Devonian as suggested previously. They are probably the eastern extension of the early Paleozoic (Cambrian–Silurian) Boshchekul–Chingiz volcanic arc of East Kazakhstan in China.The Xiemisitai volcanic rocks include andesite, rhyolite, and their pyroclastic equivalents. The subvolcanic units are felsite and granite porphyry. Andesite is moderately LREE-enriched, with a marked negative Nb anomaly and Th/Yb-enrichment. Rhyolite, felsite and granite porphyry are enriched in LREE and Th and depleted Nb. The initial Sr isotope values of the Xiemisitai volcanic rocks and their subvolcanic units range from 0.702294 to 0.704456, εNd(420Ma) from +0.19 to +1.88 with Nd model ages ranging from 600 to 803Ma. Initial Pb isotope ratios (206Pb/204Pb) are 17.77–17.96, (207Pb/204Pb) are 15.49–15.54 and (208Pb/204Pb) are 37.56–37.80. These features suggest that the Xiemisitai volcanic rocks and their subvolcanic units derived from a mantle wedge significant mixed by subducted material (EMI) in a subduction zone. They also indicate significant involvement of Neoproterozoic primitive crust in the formation of the Xiemisitai magmatic arc. These rocks are associated with a Late Silurian–Early Devonian southward subduction of the oceanic lithosphere in the northern West Junggar Region. This gives rise to an EW-trending Boshchekul–Chingiz volcanic arc and associated metallogenic belt in the northern West Junggar Region and adjacent Kazakhstan.

Mapping 87Sr/86Sr variations in bedrock and water for large scale provenance studies

Although variation in 87Sr/86Sr has been widely pursued as a tracer of provenance in environmental studies, forensics, archeology and food traceability, accurate methods for mapping variations in environmental 87Sr/86Sr at regional scale are not available. In this paper, we build upon earlier efforts to model 87Sr/86Sr in bedrock by developing GIS-based models for Sr isotopes in rock and water that include the combined effects of lithology and time. Using published data, we fit lithology-specific model parameters for generalized equations describing the concentration of radiogenic Sr in silicate and carbonate rocks. The new model explained more than 50% of the observed variance in measured Sr isotope values from independent global databases of igneous, metaigneous, and carbonate rocks, but performed more poorly (explaining 33% of the variance) for sedimentary and metasedimentary rocks. In comparison, a previously applied model formulation that did not include lithology-specific parameters explained only 20% and 8% of the observed variance for igneous and sedimentary rocks, respectively, and exhibited an inverse relationship with measured carbonate rock values. Building upon the bedrock model, we also developed and applied equations to predict the contribution of different rock types to 87Sr/86Sr variations in water as a function of their weathering rates and strontium content. The resulting water model was compared to data from 68 catchments and shown to give more accurate predictions of stream water 87Sr/86Sr (R2=0.70) than models that did not include lithological weathering parameters. We applied these models to produce maps (“isoscapes”) predicting 87Sr/86Sr in bedrock and water across the contiguous USA, and compared the mapped Sr isotope distributions to data on Sr isotope ratios of US marijuana crops. Although the maps produced here are demonstrably imperfect and leave significant scope for further refinement, they provide an enhanced framework for lithology-based Sr isotope modeling and offer a baseline for provenance studies by constraining the 87Sr/86Sr in strontium sources at regional scales.

Petrography and geochemistry of the magnesites and dolostones of the Ediacaran Ibor Group (635 to 542 Ma), Western Spain: Evidences of their hydrothermal origin

The Ediacaran deposits (between 635 and 542 Ma) of the Central Zone of the Iberian Massif consist of alternating siliciclastic and carbonate beds. These carbonates are dolostones and magnesites which are interpreted to have been formed by the replacement of primary peritidal limestones. Through petrographic and geochemical analyses, we recognize different types of dolomites (D1 to D4) and magnesites (M1 and M2). Despite distinct petrographic features of the four types of dolomite, their oxygen and carbon isotopes overlap with δ 18O values ranging from + 15.45 to + 17.51‰ (SMOW) and δ 13C from − 0.13 to 3.21‰ PDB. Sr isotope values for D1 and D2 range from 0.7028 to 0.7091. Magnesites (M1 and M2) show oxygen values higher than + 17.87.0‰, and δ 13C values show the same variability as for the dolomites. D3 and D4 oxygen isotope values are between + 18.91 and 19.61, and the carbon isotope values range are similar to the other diagenetic phases. Sr isotope values for the magnesites and late dolomites (D3 and D4) are 0.7095 to 0.7104, being higher than those of the D1 and D2 dolomites. D1 is a relatively early dolomite phase formed by the replacement of fine grained peritidal limestones. The coarser crystal size of D2, which shows similar geochemical features as D1, suggests formation by dolomitization of coarser grained limestones. The replacement of D1 and D2 by M1 and M2 advanced along stylolites, fractures and bedding planes. This replacement is interpreted to have occurred by hydrothermal fluids, which is suggested by the presence of talc and forsterite. D3, a coarse dolomite, completely destroyed any previous texture and D4 (dolomite cement) post-dates magnesite formation. Interactions of hydrothermal fluids with the prior carbonates reset the oxygen isotopes of the earlier dolomite. The study of these magnesites and related dolostones may offer new insights into the model of formation of sparry magnesites hosted by mixed siliciclastic–carbonate platform deposits. The establishment of the factors and mechanism that control the diagenetic evolution of these carbonates has a great importance in order to understand and predicts porosity and permeability variations of rocks formed under similar geological conditions.

Mesoproterozoic carbonatitic magmatism in the Bayan Obo deposit, Inner Mongolia, North China: Constraints for the mechanism of super accumulation of rare earth elements

The Bayan Obo mine in North China contains the largest rare-earth element (REE) resources in the world. The mechanism of giant REE enrichment in such a restricted region has been the focus of several studies. Carbonatites are known to contain high concentrations of REE. Three types of carbonatite dykes occur around the Bayan Obo deposit, including dolomite, calcite and calcite-dolomite carbonatite varieties. The contact relations show that the intrusion of calcite carbonatite dykes post-date the dolomite dykes during the process of evolution of the carbonatite magma. The geochemical data show that the content of Sr and light (L) REE in these dykes gradually increases from dolomite [(La/Yb) N values range from 1.6 to 3.8], through calcite-dolomite [(La/Yb) N ranging from 51 to 57], to calcite type [(La/Yb) N ranging from 85 to 4617]. This evolutionary trend suggests the crystal fractionation of the carbonatite magma, which might have played a critical role for the giant REE accumulation in the Bayan Obo region. The ε Nd(t) values of the carbonatitic dykes show a tight cluster between − 0.47 and 0.65, whereas the initial Sr isotope values show a broad range from 0.703167 to 0.708871. The massive ore-hosting dolomite marbles show comparable element content and Nd isotope composition, and a Sm–Nd isochron age similar to that of the carbonatite dykes, implying a close relationship during their magmatic origin. This interpretation is also supported by the intrusive contact between the ore-hosting dolomite marble and Mesoproterozoic Bayan Obo group, as well as the presence of wall-rock xenoliths in the dolomite marble.

Strontium-, carbon- and oxygen-isotope compositions of marbles from the Cycladic blueschist belt, Greece

AbstractThe Cycladic blueschist belt, Greece, is mostly submerged below sea level and regional correlations are difficult to establish. Marbles are widespread within the belt and locally used as marker horizons to subdivide monotonous schist sequences. However, owing to the lack of distinctive petrographic characteristics, the marbles have not been used for island-to-island correlations. This study aims to investigate the potential of Sr-, C- and O-isotope compositions of marbles as a tool for unravelling the litho- and/or tectonostratigraphic relationships across the Cycladic islands, and as a proxy for the time of sediment formation. For this purpose, we have studied metamorphic carbonate rocks from the islands of Tinos, Andros, Syros, Sifnos and Naxos. Identical87Sr/86Sr values for certain marble horizons occurring on Tinos, Andros and Sifnos are interpreted to document coeval regional carbonate precipitation. The87Sr/86Sr values of the apparently least altered samples intersect the seawater curve multiple times within the most likely time interval of original carbonate precipitation (< 240 Ma; as indicated by previously published ion probe U–Pb zircon data) and thus an unequivocal age assignment is not possible. Very broad temporal correlations are possible, but more subtle distinctions are not feasible. On Andros, the overlapping Sr-isotope values of marbles representing the lowest and highest parts of the metamorphic succession are in accordance with a model suggesting isoclinal folding or thrusting of a single horizon, or very fast sedimentation. In contrast, distinct87Sr/86Sr values for samples from Tinos, representing different levels of the metamorphic succession, suggest that these rocks represent a temporal succession and not the tectonic repetition of a single horizon. Based on Sr-, O- and C-isotope characteristics alone the time equivalence of marbles occurring on different islands could not be documented unambiguously. However, by using various combinations of these parameters, some occurrences can be discriminated from the overall sample population. The new data further accentuate the general potential of coupled Sr-, C- and O-isotope characteristics for identification of archaeological provenance and complement existing datasets for Aegean marbles.

Strontium and neodymium isotopic signatures indicate the provenance and depositional process of loams intercalated in coastal dune sand, western Japan

Isotopic analyses of strontium and neodymium suggest that layers of loam intercalated in dune sand on the Japan Sea coast at Tottori, western Japan, consist of a mixture of Asian dust from China and locally derived sediment. An outcrop exposure shows a succession of late Pleistocene dune sand, a lower loam layer, Daisen–Kurayoshi Pumice (DKP; 50–55 ka or older), an upper loam, Aira–Tn tuff (ca. 30 ka), and Holocene dune sand, in ascending order. Bulk samples of the loam layers show an upward increase in 87Sr/ 86Sr, suggesting that the contribution of Asian dust increases upward. The Sr isotopic values also suggest a greater contribution of Asian dust in the silt fraction than in the bulk sample. Asian dust transported by westerly jet from the Taklamakan or Gobi desert is the main constituent of the upper part of the lower loam, of which isotopic values of silicate portion is isotopically identical to those of desert sand in China ( 87Sr/ 86Sr, 0.717–0.719; ε Nd, −9.5 to −9.4). In contrast, the Sr and Nd isotopic values of DKP ( 87Sr/ 86Sr, 0.705; ε Nd, −2.6 to 0.6) are close to those of the volcanic rocks of Mt. Daisen, which is regarded as the source of the tephra. The isotopic signature suggests that Asian dust also have contributed to the upper part of the upper loam layer. The upward increase of 87Sr/ 86Sr within each of the loam layers suggests that the contribution of Asian dust increased as the proportion of reworked deposits from the underlying layer (dune sand or DKP) decreased by burial. In contrast, 87Sr/ 86Sr in DKP shows little vertical change, suggesting very rapid deposition without entrainment of the underlying lower loam layer.

Chapter 62 The Port Askaig Formation, Dalradian Supergroup, Scotland

Abstract The Port Askaig Formation (Fm.) is a thick glaciogenic succession within the Dalradian Supergroup that consists of over 700 m of variably dolomitic diamictite, conglomerate, sandstone mudstone and minor dolomite, and is bounded by mixed siliciclastic–carbonate successions of the Islay (Lossit) and Bonahaven formations. These strata are exposed in the metamorphic Caledonides of Scotland, although excellent preservation of sedimentary structures can be found at several sites. An extensional setting for this succession has been proposed based on stratigraphic and structural arguments. The available chemostratigraphic data include the Chemical Index of Alteration, δ 13 C and Sr-isotope values. Palaeomagnetic analyses have been shown to be subject to post-depositional Caledonian overprinting. There is also continued debate over the regional palaeogeographical reconstructions of the Scottish promontory for this time period. The succession is chronologically poorly constrained with U–Pb analyses of stratigraphically much higher or lower deposits. The thick succession is thought to record glacially influenced marine sedimentation and reworking of unstable sediments in a tectonically active setting with evidence of ice-margin fluctuations. Alternative palaeoenvironmental interpretations that focus on glacial terrestrial processes and emphasize climatic influence instead of tectonic activity have also been proposed. The overlying carbonate is a lithologically diverse coastal complex and so does not fit the Neoproterozoic norm. Research has to date focused on the stratigraphic and sedimentological aspects of this succession, as well as some of the broader palaeogeographical and structural features of the Dalradian basin. Future efforts should focus on the chronological, structural and palaeogeographical constraints of this succession.

Multiple crust–mantle interactions for the destruction of the North China Craton: Geochemical and Sr–Nd–Pb–Hf isotopic evidence from the Longbaoshan alkaline complex

In situ zircon U–Pb ages and Hf isotopic data, major and trace elements, and Sr–Nd–Pb isotopic compositions are reported for the Longbaoshan alkaline intrusive complex in the western Shandong Province (Luxi Block), southeastern North China Craton. The Longbaoshan complex, which consists of quartz syenite, aegirine–augite syenite, hornblende syenite, monzonite, and syenodiorite, was emplaced at 129.4–131.7Ma. The complex is characterized by high concentrations of SiO2, K2O+Na2O, Al2O3, LILEs (e.g., Sr and Ba), and LREEs, and low concentrations of CaO, Fe2O3, MgO, and HFSEs (e.g., Nb, Ta, P and Ti). The Sr, Nd, Pb, and Hf isotopic values of this complex are similar to the nearby mafic rocks, which were derived from EM2-type lithospheric mantle, indicating that the Longbaoshan complex was mainly derived from partial melting of the EM2 source. Combined with geochemical and isotopic features, the high Nb/Ta ratios (average 19.2) of the Longbaoshan complex suggest that the EM2 source was induced by modification of crust-derived melts, which were in equilibrium with rutile-bearing eclogite. This was attributed to the subduction of the Yangtze continental crust. The presence of inherited zircons (2.51–2.64Ga) with positive εHf(t) values (0.2–6.2) suggest that the ancient crust of the North China Craton was also involved in the formation of the Longbaoshan complex. Magma mixing modelling shows that those 10–35% crustal materials were assimilated in the complex. This complex may have experienced hornblende, apatite/monazite, and titanite crystal fractionation before emplacement in the shallow crust level. Two stages of crust–mantle interaction can be identified through the formation of the Longbaoshan complex, including (1) the lithospheric mantle was metasomatized by the melts/fluids derived from the subducted crust and then transformed into an EM2 counterpart, and (2) magma derived from the EM2 source underplated the ancient crust and assimilated the crust-derived materials. We thus propose that multiple crust–mantle interactions are the essential mechanism for the destruction of the NCC. The first intensive crust–mantle interaction was aroused by crustal subduction and was responsible for the lithospheric mantle destruction, and the second major crust–mantle interaction was induced by mantle-derived magma underplating and was responsible for the crustal activation.

Hydrothermal diagenesis of Paleozoic seamount carbonate components

We present geochemical results of Paleozoic carbonate components (brachiopods—biogenic low-Mg calcite: bLMC, crinoids—biogenic high-Mg calcite: bHMC and whole rock—admixture of aragonite and calcite: wrAC) from open ocean Permo-Carboniferous seamounts of Japan situated in the tropical mid-Panthalassic Ocean. The diagenetic history of these and other carbonate allochems was determined by visual/optical inspection of their external morphology and luminescence, by evaluation of their microstructures, and by assessment of their trace element and stable isotope distributions. The degree and trends of the diagenetic alteration tests were then reconciled with their concomitant Sr isotope values and compared to those of the established seawater–Sr isotope curve. The altered carbonate material from three of the four time slices support the novel concept of diagenesis in the presence of marine/hydrothermal fluids leading to less radiogenic Sr isotope values. In addition, Sr isotope values of unaltered bLMC from three of the four time slices do not concur with the values for the concurrent time of the established seawater 87Sr/ 86Sr curve. An adjustment of their stratigraphic position is untenable and unsatisfactory in light of their biostratigraphic assignment, which suggests that Sr isotope diagenesis is much more complex and complicated than is espoused in the literature. Consequently, the concept of less radiogenic Sr isotope values in diagenetic marine carbonates has profound implications for the veracity of the established seawater 87Sr/ 86Sr profile and curve, and it needs to be re-evaluated in light of the findings.

Springwater geochemistry at Honey Creek State Natural Area, central Texas: Implications for surface water and groundwater interaction in a karst aquifer

A two and a half year study of two adjacent watersheds at the Honey Creek State Natural Area (HCSNA) in central Texas was undertaken to evaluate spatial and temporal variations in springwater geochemistry, geochemical evolution processes, and potential effects of brush control on karst watershed hydrology. The watersheds are geologically and geomorphologically similar, and each has springs discharging into Honey Creek, a tributary to the Guadalupe River. Springwater geochemistry is considered in a regional context of aquifer components including soil water, cave dripwater, springwater, and phreatic groundwater. Isotopic and trace element variability allows us to identify both vadose and phreatic groundwater contributions to surface water in Honey Creek. Spatial and temporal geochemical data for six springs reveal systematic differences between the two watersheds. Springwater Sr isotope values lie between values for the limestone bedrock and soils at HCSNA, reflecting a balance between these two primary sources of Sr. Sr isotope values for springs within each watershed are consistent with differences between soil compositions. At some of the springs, consistent temporal variability in springwater geochemistry (Sr isotopes, Mg/Ca, and Sr/Ca values) appears to reflect changes in climatic and hydrologic parameters (rainfall/recharge) that affect watershed processes. Springwater geochemistry was unaffected by brush removal at the scale of the HCSNA study. Results of this study build on previous regional studies to provide insight into watershed hydrology and regional hydrologic processes, including connections between surface water, vadose groundwater, and phreatic groundwater.

Chronology and Evolution of Caldera-forming and Post-caldera Magma Systems at Okataina Volcano, New Zealand from Zircon U–Th Model-age Spectra

U–Th disequilibrium model-age data are presented for zircons from four young eruptive units from Okataina volcano, New Zealand. These data highlight contrasts in the time-scales over which eruptible silicic magma bodies are generated and accumulated below a highly active rhyolite volcano prior to and following the ~61 ka caldera-forming Rotoiti eruption (80 – 120 km 3 magma). The Rotoiti event was followed by ≥12 explosive eruptions of the Mangaone Subgroup between ~45 and 30 ka. A change in eruptive styles between ~30 and 25 ka brackets the major Oruanui eruption of Taupo volcano ~80 km away; subsequently eight rhyolitic, lava-dominated and two basaltic explosive eruptions occurred between 25 ka and 1886 CE. We present (238U–230Th) zircon model-age data determined by secondary-ion mass spectrometry (SIMS) and thermal ionization mass spectrometry (TIMS) from the Rotoiti, Ngamotu (Unit B) (~45 ka), Mangaone (Unit I) (33 ka) and Rotorua (15•4 ka) eruptive units. We couple these data with published and new compositional information to trace magma crystallization and storage patterns. Population density curves of SIMS model ages from zircons in the two pumice types (biotite-free and biotite-bearing) from the Rotoiti eruption differ. Zircons from the former yield a model-age peak coincident with eruption age, whereas zircons from the latter show a peak at 70–90 ka and little variation in TIMS model-age values from different crystal size fractions. Concentration weighted means of model ages from the two pumices, however, are the same within 1 SD error, and their Sr isotopic values overlap at 2 SD precision, suggesting that they are genetically linked. Model-age spectra for the Ngamotu, Mangaone and Rotorua pumices are complex, indicating recycling of crystals from multiple older populations that largely pre-date the 61 ka caldera-forming eruption. Superimposed on the older age spectra are variably developed younger pre-eruptive suites of ages reflecting varying amounts of crystallization prior to each eruption. A lack of commonality in these younger peaks in the post-caldera eruptive rocks, along with compositional and isotopic differences between (and sometimes within) the eruptive units, collectively precludes their origin from a single melt-bearing mush system. Okataina contrasts with Taupo, where comparable-age eruption deposits have simpler age spectra, consistent with larger-scale crystallization cycles and thermal events in the magma chambers there. When compared with other caldera-related silicic systems for which suitable data are available, Okataina differs in lacking a simple pre-eruptive history prior to its caldera-forming event and having a complex, non-coherent magmatic history of post-caldera eruptions.

Tectonic setting of the Jurassic bimodal magmatism in the Sakarya Zone (Central and Western Pontides), Northern Turkey: A geochemical and isotopic approach

The Lower to Middle Jurassic Mudurnu formation of the Sakarya Zone (Northern Turkey) was deposited in an extensional basin. This unit crops out along the southern Pontide range and consists of marine sedimentary rocks including debris flows, lignite-bearing clastic rocks and Ammonitico Rosso horizons alternating with mafic and felsic volcanic and volcaniclastic rocks. Magmatic rocks of the Mudurnu formation comprise two compositionally different groups; 1) a mafic group including diabase–microgabbro–basaltic lavas and their pyroclastic equivalents, and 2) a felsic group including granite porphyries and felsic pyroclastic rocks. All the magmatic members of the Mudurnu formation are subalkaline and display a calc-alkaline affinity. They are bimodal, with a significant silica gap between the mafic and felsic members with the exception of a few samples. These magmatic rocks display enrichment in LILE and depletion in Nb, Ta, P and Ti, implying a subduction-related magmatic signature. Melting modelling for the mafic rocks indicates that they originated possibly from subcontinental lithospheric mantle (SCLM) composed of spinel lherzolite. εNd(i) values (+ 1.5 to + 4.3) imply that the mafic volcanic and hypabyssal rocks were possibly derived from a time-integrated LREE-depleted mantle source. The initial Sr and Nd isotope values, and εNd(i) of the felsic hypabyssal rocks are comparable to the mafic ones. The isotope data point to a genetic relationship between the felsic and mafic members. Results obtained from the geochemical modelling of incompatible versus compatible trace elements show that the felsic rocks were derived from the mafic melts by fractional crystallization (FC) process. In the light of their regional geological setting and these geochemical characteristics, we propose that the magmatic rocks of the Mudurnu formation formed in an extensional basin situated on an active and/or just ended subduction zone during the Jurassic period. The Mudurnu formation is a good example of extensional magmatism derived from a depleted mantle source that displayed subduction signatures and that was modified by fluids from a long-lived subduction event.

Application of multi-isotope ratios to study the source and quality of urban groundwater in Metro Manila, Philippines

Abstract To characterize water quality in terms of dissolved elements and to investigate both the origin of the water and the source and behavior of groundwater contaminants in Metro Manila, 33 water (groundwater and surface water) samples were analyzed for ion and element concentrations, H and O isotope ratios (δD-H2O and δ18O-H2O), SO 4 2 - isotope ratios (δ34S- SO 4 2 - and δ18O- SO 4 2 - ), and Sr isotope ratio (87Sr/86Sr). The chemical measurements showed that the primary important environmental concerns within Metro Manila are groundwater salinization for both shallow and deep aquifers, and As concentrations (up to 22.5 μg/L) in shallow groundwaters. Comparison of SO 4 2 - and Sr isotope values with possible source materials revealed that contamination by man-made materials such as fertilizers and detergents are present in some shallow groundwaters. Shallow groundwater having higher δD-H2O and δ18O-H2O values (av. −44‰ ± 5.6‰ and −6.8‰ ± 0.6‰, respectively, n = 15) than deep groundwater (av. −48‰ ± 4.4‰ and −7.4‰ ± 0.7‰, respectively, n = 7) suggests that the origins of H2O in both groundwaters are different from each other. Since the mixing of shallow and deep groundwater does not commonly occur under Metro Manila, the contaminants in the shallow aquifers are unlikely to be transported into the deep aquifers. Sulfate reduction by bacterial activity was observed for some groundwaters, resulting in a maximum elevation in δ34S- SO 4 2 - values of around 10‰. By using SO 4 2 - isotope ratios as an indicator of changes in redox conditions and Sr isotope ratio as a source indicator, it was shown that As was dissolved from unconsolidated sedimentary deposits of volcanic origin which enclose shallow unconfined aquifers, but was not the result of changes in redox conditions. The study demonstrated that multi-isotope ratios are useful for evaluating water quality problems in urban groundwaters.

Bioapatite 87Sr/86Sr of the last woolly mammoths—Implications for the isolation of Wrangel Island

The movements of terminal Pleistocene and Holocene mammoths on Wrangel Island, off northeastern Siberia, were investigated using the isotopic composition of Sr in skeletal remains. A significant shift toward more radiogenic 87 Sr/ 86 Sr ratios, from 0.71218 ± 0.00103 to 0.71491 ± 0.00138 in bioapatite, marks the beginning of the Holocene. The higher Sr isotope values in mid-Holocene mammoth remains are interpreted as an influence of radiogenic Sr derived from the Neoproterozoic rocks in the central part of Wrangel Island. The shift implies a change in the ranging and feeding areas of the animals, ultimately reflecting the inundation of the mainland connection and isolation of the population. The shift indicates also that the late Pleistocene animals were not permanent occupants in the territory that makes up present-day Wrangel Island.

African kimberlites revisited: In situ Sr-isotope analysis of groundmass perovskite

New in situ Sr-isotope data are presented for perovskite grains from 54 samples, representing 42 separate localities of Group I, Group II and transitional kimberlites from southern Africa. Although broadly consistent with published whole-rock data from the same intrusions, a clear offset towards less radiogenic compositions is evident in the perovskite results. This suggests that even bulk-rock data acquired after the most careful selection and treatment of chips can be compromised by alteration and contamination processes. Consistent with the bulk-rock data, on- and off-craton samples from the Group I kimberlites of our study cannot be distinguished on the basis of initial 87Sr/ 86Sr ratios. Our dataset also contains 4 samples of transitional kimberlite from 3 off-craton intrusions. The perovskite results from Springbank lie within the range for Group I samples, whereas Droogfontein and Silvery Home are more typical of the Group II kimberlites. An apparent gap in 87Sr/ 86Sr between ~ 0.705 and ~ 0.706 suggests that at least in terms of Sr-isotope values, so-called transitional kimberlites need not have intermediate compositions between Group I and Group II magmas.

CRETACEOUS RIDGE SUBDUCTION ALONG THE LOWER YANGTZE RIVER BELT, EASTERN CHINA

The Lower Yangtze river belt is one of the most important metallogenic belts in China. The mechanisms responsible for ore genesis and the formation of related Cretaceous igneous rocks, such as adakite, A-type granitoid, and Nb-enriched basalt, remain controversial. Mesozoic granitoids in the Lower Yangtze river belt were mostly formed in the Early Cretaceous (140–125 Ma), and three granitoid belts—the inner, the south, and the north—have been defined according to petrological and geochemical characteristics. Previously, based mainly on negative eNd and high initial Sr isotope values, the adakitic rocks were generally attributed to partial melting of thickened or delaminated lower crust, both of which require crustal thickening. Mesozoic crustal thickening, however, is not supported by the development of extensional basins in the region. From the Late Jurassic to Cretaceous, eastern China was closely associated with subduction of the Pacific plate in the south and the Izanagi plate in the north. The midocean ridge (MOR) between these two plates was drifting toward and likely subducting under the Lower Yangtze river belt. A ridge subduction model can therefore explain the distribution of different magmatic rocks and ore deposits in the belt. Partial melting of subducting young, hot oceanic slabs close to the ridge formed adakitic rocks. The negative eNd values of adakitic rocks can be plausibly interpreted by mixing between adakitic magmas and enriched components in the lithospheric mantle, and/or crustal materials through AFC process. A slab window opened during ridge subduction as indicated by A-type granitoids in the center of the inner belt. Nb-enriched basalt found in the belt was likely formed by partial melting of a mantle wedge metasomatized by fluids released from the subducting slab at shallow depths.