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Barium (Ba) isotopes have been used to trace water mass mixing and export productivity in the oceans. However, the sources, isotopic signature and seasonal variation of dissolved Ba in large rivers remain poorly constrained. In order to improve our understanding of sources and fractionation of riverine Ba isotopes at the continental scale, weekly sampling of river water was carried out in the middle Yellow River over the full hydrological year of 2013. Dissolved Ba was mainly sourced from silicate dissolution, whose flux was correlated with physical erosion rate in this arid to semi-arid basin, largely covered by loess. More than half of the annual dissolved Ba flux was transported during the monsoon season (June to mid-September), in particular during a storm event period (accounting for 4% of the annual dissolved Ba flux in 4 days). The dissolved Ba isotopic composition (δ138Barw) ranged from +0.17‰ to +0.46‰, all higher than δ138Ba of loess (0.00 ± 0.04‰). The seasonal variation in δ138Barw is best explained via an adsorption model of light Ba isotopes onto solids in the river, which is sensitive to erosion of loess, in particular during the storm event and the spring-time ice melting intervals, with a potential role of barite precipitation in groundwater during the dry seasons. With one to three orders of magnitude higher particulate matter fluxes during the monsoon seasons in rivers such as the Yellow River, desorption of Ba could result an unaccounted strong variation of Ba concentration and isotopic composition in coastal environments, which further study requires.

International audience; The water contents have been measured by Fourier transform infrared spectrometry (FTIR) on the minerals of peridotite xenoliths hosted by late Mesozoic basalts in three localities, Daxizhuang (74 Ma), Junan (67 Ma) and Qingdao (82 Ma), of the eastern North China Craton (NCC). The water content (H2O by weight) of the Daxizhuang samples is from 103 to 311 ppm for clinopyroxene (cpx), from 30 to 141 ppm for orthopyroxene (opx) and from 21 to 66 ppm for whole rocks. In contrast, the samples from Junan and Qingdao have high water content, from 466 to 746 ppm for cpx, 187 to 346 ppm for opx and 99 to 232 ppm for whole rocks. These results, combined with previously reported data of lithospheric mantle samples of the eastern NCC at ~ 120 Ma and < 40 Ma, suggest a temporal variation of water content in the lithospheric mantle since the peak time (~ 120 Ma) of the destruction of the NCC. At that time, the lithospheric mantle was hydrous (> 1000 ppm); after its destruction was complete at the end of the Mesozoic, the water content of the lithospheric mantle was extremely low (< 50 ppm); during the period of destruction, the lithospheric mantle had intermediate water content. The temporal variation is thought to be linked with the process of the craton destruction: (1) at the peak time of destruction, the hydrated lithospheric mantle creates a low-viscosity prerequisite for its own destruction; (2) during the destruction, the upwelling asthenosphere erodes the lowermost lithospheric mantle and heats the overlying lithospheric mantle continuously, resulting in lithospheric thinning and dewatering of the relict lithospheric mantle till to the cease of the destruction which is at least partly due to its low water content. The upwelling asthenosphere, however, cools at weak zones (e.g., deep faults) and transforms into newly accreted lithospheric mantle with higher water content than the dewatered relict mantle.

International audience; The subduction of oceanic slabs is widely accepted to be a main reason for chemical heterogeneities in the mantle. However, determining the contributions of slabs in areas that have experienced multiple subduction events is often difficult due to possible overlapping imprints. Understanding the temporal and spatial variations of source components for widespread intraplate small volume basalts in eastern China may be a basis for investigating the influence of the subducted Pacific slab, which has long been postulated but never confirmed. For this purpose, we investigated the Chaihe-aershan volcanic field (including more than 35 small-volume Quaternary basaltic volcanoes) in NE China and measured the oxygen isotopes and water content of clinopyroxene (cpx) phenocrysts using secondary ion mass spectrometry (SIMS) and Fourier transform infrared spectroscopy (FTIR), respectively. The water content of magma was then estimated based on the partition coefficient of H2O between cpx and the basaltic melt. The δ18O of cpx phenocrysts (4.28‰ to 8.57‰) and H2O content of magmas (0.19 wt.%-2.70 wt.%) show large variations, reflecting the compositional heterogeneity of the mantle source. The δ18O values and H2O content within individual samples also display considerable variation, suggesting the mixing of magmas and that the magma mixing occurred shortly before the eruption. The relation between the δ18O values of cpx phenocrysts and the H2O/Ce ratio, Ba/Th ratio and Eu anomaly of whole rocks demonstrates the contributions of three components to the mantle source (hydrothermally altered upper oceanic crust and marine sediments, altered lower gabbroic oceanic crust, and ambient mantle). The proportions of these three components have varied widely over time (∼1.37 Ma to ∼0.25 Ma). The Pacific slab is constantly subducted under eastern Asia and continuously transports recycled materials to the deep mantle. The temporal heterogeneity of the source components may be caused by ongoing Pacific slab subduction. Combined with other basalt localities in eastern China (Shuangliao basalts, Taihang basalts and Shangdong basalts), the contributions of recycled oceanic components in their mantle source are heterogeneous. This spatial heterogeneity of mantle sources may be induced by variable alterations and dehydration during the recycling process of the Pacific slab. Our results show that the source components of Cenozoic intraplate small-volume basalts in eastern China are temporally and spatially heterogeneous, which is likely induced by the ongoing subduction of the Pacific slab. This demonstrates that integrating the temporal variations in geochemical characteristics and tectonic history of a study region can identify the subducted oceanic plate that induced enriched components in the mantle source of intraplate basalts.

© 2015 Elsevier B.V. The Wonoka-Shuram Anomaly represents the largest negative carbon isotope excursion recognized in the geologic record and is associated with the emergence and diversification of metazoan life ca. 580 million years ago (Ma). The origin of the anomaly is highly debated, with interpretations ranging from primary to diagenetic, each having unique and potentially transformative implications for early life. Here, we apply carbonate clumped isotope thermometry to three sections expressing the anomaly in order to constrain mineral formation temperatures and thus directly calculate water oxygen isotope compositions (δ18Ow) with which carbonate minerals equilibrated. With δ18Ow known, it is possible to address previous hypotheses for the origin of the anomaly. In each section, precipitation temperatures correlate positively with reconstructed δ18Ow. Previous hypotheses, based on the covariance of δ18Ocarb vs. δ13Ccarb (uncorrected for temperature effects), suggested a meteoric diagenetic origin for the anomaly. However, reconstructed δ18Ow values do not covary with carbon isotope compositions (δ13Ccarb) within anomaly facies. Rather, the oxygen isotope and temperature data are consistent with carbonate recrystallization and equilibration under increasingly rock-buffered conditions. Based on simple modeling and comparison to modern formation fluids, recrystallization may have occurred in an environment far removed from the initial depositional or early diagenetic regime. In addition, although clumped isotope temperatures vary significantly and reach elevated values consistent with burial diagenesis, it is unclear to what degree, if at all, carbon isotope values were reset during recrystallization. Ultimately, these new data indicate that Wonoka-Shuram-aged carbonates experienced equilibration with fluids under increasingly closed-system conditions. The clumped isotope data do not provide a means to distinguish previous hypotheses outright, but provide additional context for the evaluation of geochemical signatures within these ancient carbonate rocks.

The genesis of Cenozoic continental basalts in Eastern China is highly debated. Subducted oceanic crust (most probably the Pacific oceanic slab) has been increasingly suggested to be involved in their mantle source. The Taihang Mountains are located at a surface position corresponding to the western edge of a stagnant subducted Pacific slab revealed by geophysical investigations, and thus provide an opportunity to study the spatial extent of the effect of Pacific subduction on the mantle characteristics of Eastern China. In previous studies, alkali basalts from the Taihang Mountains were considered to be the products of interaction between asthenosphere-derived melts and the old, enriched lithospheric mantle; their magnesium isotope signature suggested the contribution of oceanic carbonate. To explore the possible contribution from recycled oceanic crust in the genesis of the Taihang basalts we estimated the water contents of the magmas by a phenocryst-based approach ( ${\rm C}_{water}^{melt}={\rm C}_{water}^{mineral}/{\rm C}_{water}^{mineral/melt}$ ) and measured the oxygen isotope compositions of clinopyroxene phenocrysts by secondary ion mass spectrometry for the same sample suite as used in previous studies. The calculated water contents (H 2 O by weight) of the parental magmas range from 0·20 to 1·07 wt %, and the corresponding H 2 O/Ce ratios correlate well with (Nb/La) n (where n represents primitive mantle normalization), (Ba/Th) n and eNd. The oxygen isotope ratios (δ 18 O SMOW ) range from 5·8 to 7·4‰, and the average value for each sample ranges from 6·6 to 7·0‰. These new data indicate a large contribution from extremely dehydrated recycled oceanic crust together with entrained sediments. The minor element compositions (Ca, Mn and Ni contents) of olivine phenocrysts in the Taihang basalts suggest a pyroxenite source that was probably formed by the interaction between oceanic crust-derived melt and ambient mantle peridotite.

Os isotopic compositions and Re and Os concentrations were determined for Cenozoic basalt-borne ultramafic xenoliths from the Subei Basin of east central China. Re–Os analyses were coupled with whole rock major and trace element and S abundance determinations, and with characterization of rock textures, modal phase proportions and sulfide petrography. The two main sampling areas, Lianshan and Panshishan, separated by only 6 km, have similar textures and major and moderately incompatible lithophile trace element compositions. The Os isotopic ratios of these two areas plot on the same trends relating 187Os/188Os to indices of melt extraction such as whole rock Yb content. These Os isotopic systematics suggest that both areas were affected by an early Proterozoic (∼1.8 Ga) melt extraction event. Thus the two areas apparently shared the same long term lithospheric history. Nevertheless, the sulfide abundances and whole rock S, Os and Re concentrations are strikingly lower in Lianshan than in Panshishan, and the two localities have different incompatible lithophile trace element signatures. These differences resulted from contrasting melt percolation styles between the two areas. Lianshan was affected by extensive percolation of sulfur undersaturated melts that removed Re, Os and S, while Panshishan experienced interaction with low degree or highly evolved melts that added Re, Cu and S, but had no affect on Os abundances. The lack of correlation between 187Os/188Os and 187Re/188Os, compared with the good correlation between 187Os/188Os and Yb, indicates that the perturbation of the Re and Os concentrations was fairly recent, and perhaps related to Mesozoic or Cenozoic lithospheric thinning in eastern China. The Lianshan Os concentrations are typical of those of off-cratonic mantle xenoliths, while the Panshishan Os concentrations are closer to those of orogenic peridotites. This suggests that the low Os concentrations, and by extension, the low concentrations of all of the highly siderophile elements (HSE) typically observed in ultramafic mantle xenoliths, may result from recent melt percolation processes, probably directly or indirectly related to the magmatism that brought the xenoliths to the surface. Thus ultramafic xenoliths may not provide reliable estimates of the HSE contents of the upper mantle, and variations in HSE abundances between xenolith localities should not be used to define global scale processes. Os and other HSE abundances may prove to be sensitive indicators of melt percolation, and may provide information about the degree of sulfur saturation of the melts. Despite the loss of Os during recent percolation, the Os isotopic ratios of most samples are nearly unmodified by this process, confirming the utility of this system for dating ancient melt extraction events.

International audience; Selenium (Se) isotopes can yield substantial isotopic fractionation (up to 20‰) confirmed by experiments and field investigations, depending on various biotic or abiotic redox transformations. Therefore, it is expected that redox changes in the ancient oceans would induce significant isotopic fractionation, and the Se isotopic signatures recorded in old sedimentary rocks might provide new insight into how the redox state of the ancient ocean has evolved. However, previous studies have shown that Se is slightly enriched in the lighter isotope relative to the bulk earth values in most deposited conditions (oxic, anoxic, and even sulfidic). Here, our results reveal that ferruginous conditions can result in excessive accumulation of Se in sediments with an elevated Se/S ratio and significant isotope fractionation (about 6‰), which leads us to propose that Se isotopes are an appropriate geochemical proxy to trace unique oceanic conditions over times. Accordingly, Se isotopic variations measured in three Early Cambrian formations in southern China suggest that anoxic waters with ferruginous conditions must have been present in early Cambrian ocean along the eastern margin of the Yangtze platform, and oceanic circulation was stepwise reorganized. This may have triggered biological diversification from the Ediacaran to the Early Cambrian.

International audience; To understand the solidification processes of natural magma and the texture evolution of igneous rocks, we have carried out in situ observation of the crystallization of a high-K basaltic melt cooling from ~1,240 °C in a moissanite cell. In a series of experiments with different thermal history, olivine or clinopyroxene (cpx) appeared as the liquidus phase before the formation of plagioclase. During cooling at 100 °C/h, the morphology of olivine and cpx transited from tabular to hopper habit. To first order approximation, crystal grow rate (2 × 10−9 to 7 × 10−9 m/s for olivine and 6 × 10−9 to 17 × 10−9 m/s for cpx), probably limited by chemical diffusion, is proportional to crystal size. In one experiment dominated by olivine crystallization, the good image quality allows the analysis of texture evolution over an extended period. Nucleation of olivine occurred only in a narrow temperature and time interval below the liquidus. Two-dimensional length- and area-based crystal size distributions (CSDs) show counterclockwise rotation around axes of 8 μm and 100 μm2, which is consistent with the proportionate crystal growth. Both CSDs and direct observation show the dissolution of small crystals and Ostwald ripening. These data suggest that conventional analyses of crystal size distributions of igneous rocks may be in error—the slope of the CSD cannot be interpreted in terms of a uniform growth rate, and the intercept with the vertical axis does not correspond to a nucleation density.

The Raobazhai ultramafic massif of the ultrahigh pressure Sulu–Dabie orogenic belt, central China, is thought to be a segment of subcontinental lithospheric mantle that was subducted and exhumed during the Triassic collision of the North China and Yangtze cratons. We performed a Re–Os isotopic study of peridotites from the massif, associated with major and trace element analysis and textural examination. Os (1.02 to 6.28 ppb) and Re (0.004 to 0.376 ppb) concentrations are typical of orogenic lherzolite values, and 187Os/188Os ratios (0.1157 to 0.1283) are all similar to or lower than the proposed primitive upper mantle value. 187Os/188Os is roughly correlated with 187Re/188Os, and strongly correlated with Al2O3. These correlations can be explained by radiogenic ingrowth of 187Os since an ancient partial melting event. TMA model ages (1.7 to 2.0 Ga) of refractory peridotites from the lower massif are consistent with the model age (1.8 Ga) obtained from the 187Os/188Os vs. Al2O3 correlation at ~1% Al2O3. This age cannot distinguish the cratonic provenance of the Raobazhai massif, since similar Re–Os model ages have been obtained from both the North China and the Yangtze cratons. The poor quality of the 187Os/188Os vs. 187Re/188Os correlation indicates that the Re/Os ratios were disturbed, perhaps during Triassic subduction. The mainly lherzolitic samples of the upper massif, which were most strongly affected by this process, have porphyroclastic textures with fine-grained olivine, pyroxene and amphibole neoblasts, suggesting Re mobility during recrystallization in the presence of fluids. Previous studies of ultramafic xenoliths from arc volcanics demonstrate that slab-derived melts or fluids can both scavenge mantle Os and add substantial amounts of radiogenic Os to the suprasubduction mantle. In Raobazhai, both trace element patterns and the abundance of hydrous phases provide evidence for extensive interaction with fluids during subduction and/or exhumation. Nevertheless, the strong correlation between 187Os/188Os and Al2O3, and the high Os concentrations of these rocks indicate that Os isotopic ratios, and probably even Os concentrations, were essentially unaffected by this process. Assuming that the arguments favoring a suprasubduction setting for the Raobazhai massif are valid, these data provide evidence that Os systematics are sometimes surprisingly robust, even above subduction zones.

In situ analysis of major elements, platinum-group elements and Os isotope composition data for 170 grains of Ru–Os–Ir alloys from podiform chromitites in the Luobusa and Dongqiao ophiolites, Tibet, provide new constraints on the sources of the alloy grains, the Os-isotope composition of the Neo-Tethyan oceanic upper mantle, and the timing of melt–rock reactions in the ultramafic peridotites of the host ophiolites. Most alloy grains are osmiridium and iridosmine. Based on elemental and isotopic compositions, two populations can be identified. Group I, found in both ophiolite bodies, has fractionated PGE patterns typical of alloys and homogeneous 187 Os/ 188 Os 0.12645±4 (2s; n=145). Group II alloys from the Dongqiao ophiolites show smooth PGE patterns and 187 Os/ 188 Os ranges from 0.12003 to 0.12194 (yielding Re-depletion ages ≥1.1 Ga). Both the PGE patterns and the Os isotopic compositions of Group II resemble those of sulfides residual after melting. Group I alloys represent an early cumulate phase during the crystallization of magmas and are interpreted as derived from an asthenospheric melt. In contrast, Group II alloys are inferred to derive from the breakdown of residual sulfides from the mantle section of the ophiolite, which may represent relics of a sub-continental lithospheric mantle. The coexistence of the two groups of alloys in Dongqiao samples reflects interaction between asthenosphere-derived melts and old depleted lithospheric mantle and suggests that the chromitites originated through melt–rock reactions. The data on the Group I alloys support the hypothesis that the Re–Os evolution of the convective oceanic mantle resembles that of enstatite chondrites. © 2007 Elsevier B.V. All rights reserved.