The sorption of alkaline earth cations Ca2+, Sr2+, Ba2+, and Ra2+ was investigated on Na-converted biotite, the main sorbing mineral in granitic bedrock at the nuclear waste disposal site in Olkiluoto, Finland. Batch sorption experiments were conducted under CO2-free conditions by varying the concentration of the alkaline earth cation (Ca2+, Sr2+ and Ba2+ 10−8 M–10−2 M; Ra2+ 10−10 M–10−7 M) in 0.01 M, 0.1 M and 1.0 M NaClO4 solutions buffered to pH 8. Additional experiments were conducted on four Olkiluoto bedrock reference groundwater solutions. The retention of alkaline earth cations on biotite decreased upon their increasing concentration in the liquid phase and on increasing NaClO4 solution concentration. The Kd values in NaClO4 solutions and in glacial melt water (ionic strength 0.12 mmol/L) and fresh groundwater (4.3 mmol/L) systematically decreased in the order Ra2+ ≥ Ba2+ > Sr2+ ≥ Ca2+. No apparent sorption occurred except a slight sorption activity for Ba2+ in brackish (91.4 mmol/L) and saline (515 mmol/L) reference waters. The sorption of Ca2+, Sr2+, Ba2+, and Ra2+ was modelled in 0.01 M, 0.1 M, and 1.0 M NaClO4 solutions and in Olkiluoto reference groundwaters using a mechanistic three-site cation exchange model previously developed for Cs+ sorption on biotite. The model gave a generally good fit with experimental results in NaClO4 solutions and in glacial melt water and fresh groundwater reference solutions. The model, however, gave overestimated values because sorption was not detected in the experimental sorption tests for Ca2+ in 1 M NaClO4 solutions and for the other alkaline earth metals in brackish and saline reference waters.
The origins of groundwater in the aquifers beneath the Osaka Plain, Japan, were investigated based on the spatial distribution and relationship of δ2H, δ18O and Cl− concentration in groundwater and river water. Groundwaters beneath the Osaka Plain have unique geochemical characteristics in each aquifer, which can be categorized into three types: shallow (AI), middle (AII) and deep (AIII). Groundwater in AI aquifers originated from surface water, including local precipitation, seawater and riverwater. AII aquifers are abundantly recharged with freshwater, while seawater also infiltrated through the Uemachi Fault. In the AIII aquifers, two types of saline groundwater in addition to freshwater were present beneath the whole plain: fossil seawater and deep-seated groundwater similar to Arima-type brine. The Arima-type brine is known to exhibit distinct oxygen isotope shifts; i.e., the brine is enriched in 18O and 2H compared with the local meteoric water, shifting the water isotope signature away from the LMWL. The fossil seawater type groundwater was not considerably altered, whereas the deep-seated type groundwater had experienced high-temperature alteration. The both saline groundwaters were diluted by freshwater with lower δ2H and δ18O than present local meteoric waters. Such freshwaters have lower δ18O in AIII aquifers than in AII aquifers, suggesting ultrafiltration, which causes the O-isotopic fractionation when passing through thick sedimentary formations. This study revealed the geochemical characteristics of the groundwaters, which originated from different sources and recharged into different aquifers, in a whole sedimentary basin.
To understand the contribution of carbonate, oxide, organic and siliceous matter to the vertical transportation of elements, sequential chemical leaching was applied to sediment-trap samples collected in and near the Bering Sea. A total of eleven samples representing periods of high to low opal flux were analyzed for elements in carbonate, oxide and silicate. Two samples from high and low opal flux periods were analyzed further for those in organic matter. Siliceous matter may be an important carrier for most elements (Li, Mg, Al, Sc, Ti, V, Cr, Fe, Co, Ge, As, Rb, Zr, Nb, Sn, Sb, Cs, Hf, Ta, W and Th); carbonate may be important for Na, K, Ca, Mn, Ni, Sr, Pd, Ag, Cd, Ba, Pb, Bi and U and organic matter for B, P, Cu and Ga. Carbonate, oxide and siliceous fractions almost equally contribute to transportation of Y and lanthanides (REEs). The concentrations of most elements in the siliceous fraction exhibit hyperbolic relationships against opal flux, where the asymptotes are always non-zero. The non-zero asymptotes lead to two possible interpretations. 1) Diatom frustules are pure opal; when diatom production is extremely high, it is accompanied with supply of terrigenous matter whose amount is proportional to diatom production. 2) Diatom frustules are not pure opal (SiO2), but contain significant amounts of other elements; the observed increase of elemental concentrations at smaller productivity results from selective dissolution of opal or terrigenous matter mixing. We argue that the second interpretation is more plausible, and the composition of diatom frustules is estimated as that of the silicate fraction of the high productive group, with an opal flux greater than 200 mg m−2 day−1. This indicates the potential of diatom frustules as effective vertical carriers of multiple elements.
The petrogenesis and tectonic setting of Middle-Late Jurassic granitoids on Liaodong Peninsula, NE China, are debated. To resolve these uncertainties, this study analyzed geochemistry, zircon U-Pb ages and Hf-O isotopic compositions of the Wulong two-mica monzogranite on the Liaodong peninsula. Zircon U-Pb ages indicate that the Wulong monzogranite was intruded at ca. 168–162 Ma, and included an inherited population of Paleoproterozoic zircon grains (ca. 2.5–2.0 Ga). Wulong monzogranite is characterized by quartz, plagioclase, K-feldspar and minor muscovite and biotite. This S-type granitic intrusion has high SiO2 (>70 wt.%) contents, is high-K calc-alkaline, shows high zircon δ18O values (>7.5‰), and is weakly peraluminous (A/CNK = 1.01–1.08). The relatively low Sm/Nd (0.13–0.21), Rb/Sr (0.17–0.53) and FeOT/MgO (2.54–8.51) ratios suggest that Wulong monzogranite is weakly fractionated. Geochemical characteristics indicate that the monzogranite is probably derived from partial melting of a clay-poor, plagioclase- and biotite-rich psammitic source, such as metasedimentary rocks of the North China Craton. Moreover, the Wulong monzogranite displays high initial 87Sr/86Sr ratios (0.7099–0.7181), δ18O values (7.8–9.4‰), low εHf(t) values (–34.7 to –23.0) with two-stage Hf model ages (TDM2) of 2.67–3.40 Ga and εNd(t) (–24.9 to –21.2) values with two-stage Nd model ages (TDM2) of 2.67–2.96 Ga, suggesting that they may be the product of the melting of the Neoarchen-Paleoproterozoic metasedimentary basement of the North China Craton. Zircon saturation temperature and Ti-in-zircon thermometry indicate that Wulong monzogranite formed at ~700°C, and high Sr/Y ratio and low HREE imply it was formed at depths within the stability field of garnet in a thickened crust. Taking the regional tectonic and magmatic activities into account, it is proposed that the Wulong monzogranite was generated in an active continental margin that involved the subduction of the Izanagi slab during the Middle-Late Jurassic.
We conducted a laboratory hydrothermal experiment that simulated generation of low molecular-weight hydrocarbons during seafloor sediment alteration at 275–361°C and 30 MPa. The abundance and carbon and hydrogen stable isotope composition of low molecular weight thermogenic hydrocarbons in the fluids were determined. In general, the abundance of C1−C4 alkanes increased with time. The abundance of CH4 relative to C2−C4 alkanes as reflected by C1/C2+ ratios showed progressive increases from 1.2 to 4.3 with continued sediment heating. Alkenes were enriched in early phase and decreased with time. Carbon isotope ratios (δ13C) of thermogenic CH4 ranged between –42.0~−24.2‰. Carbon isotope ratios of C2H6 and C3H8 were similar to each other throughout the experiment (δ13C = –28.0~−20.3‰). In general, the carbon isotope ratios of C1−C4 alkanes were more close to those of substrate organic matter in larger carbon numbers and at later periods of the experiment. Hydrogen isotope ratios (δD) of CH4 varied from –325~−262‰, more negative than those expected at the isotope equilibrium between CH4 and H2O. Compared with results from the experiment, natural hydrothermal fluids show higher C1/C2+ ratio, more diverse δ13CCH4 values among the fields, higher δ13CC2 values, and higher δDCH4 values. The differences likely result from lower maturity of the experimental fluid and biogenic methane contribution to the natural fluids.