Effect of the Dry Ashing Method on Cadmium Isotope Measurements in Soil and Plant Samples

A dry ashing method is commonly used to remove organic material from samples prior to geochemical analysis. In the course of this study, the Cd isotope ratios of a series of soil and plant reference materials and samples were studied to evaluate the effect of the dry ashing method on measurement results of Cd isotope ratios. The samples were pre‐treated using the dry ashing method and high‐pressure bomb for comparison. The results show that the digestion using high‐pressure bombs did not lead to Cd loss, but using the dry ashing method would cause different proportions of Cd loss. The whole range of Cd isotope difference between two methods was from ?0.07‰ to 3.01‰. There was also an obvious difference in measured Cd isotope ratios from the same leaf sample pre‐treated independently by the dry ashing method, indicating that the amount of Cd loss and the effect on Cd isotope measurement during dry ashing is related to the properties of the samples. Therefore, dry ashing may not be appropriate for the removal of organic material in Cd isotope ratio measurement, especially for samples with high organic contents. The δ^114/110Cd values of reference materials NIST SRM 1573a and GSD‐30 are reported for the first time in this study.     

Boron isotopic composition of subduction-zone metamorphic rocks

Many arc lavas contain material derived from subducted oceanic crust and sediments, but it remains unresolved whether this distinctive geochemical signature is transferred from the subducting slab by aqueous fluids, silicate melts, or both. Boron isotopic measurements have the potential to distinguish between slab transfer mechanisms because ¹¹B fractionates preferentially into aqueous fluids whereas little fractionation may occur during partial melting. Previous studies have shown that δ¹¹B values of island arc lavas (−6 to +7) overlap the range of δ¹¹B values for altered oceanic crust (−5 to +25) and pelagic sediments and turbidites (−7 to +11). Secondary ion mass spectrometry (SIMS) analyses of minerals in subduction-zone metamorphic rocks yield δ¹¹B=−11 to −3 suggesting that slab dehydration reactions significantly lower the δ¹¹B values of subducted oceanic crust and sediments. In order to explain the higher δ¹¹B values reported for arc lavas as compared to subduction-zone metamorphic rocks, the B-bearing component derived from the metamorphosed slab must be enriched in ¹¹B relative to the slab, favoring an aqueous fluid as the slab transfer mechanism.     

U–Pb ages of Neoarchean granitoids from the Black Hills, South Dakota, USA: implications for crustal evolution in the Archean Wyoming province

Zircons in basement rocks from the eastern Wyoming province (Black Hills, South Dakota, USA) have been analyzed by ion microprobe (SHRIMP) in order to determine precise ages of Archean tectonomagmatic events. In the northern Black Hills (NBH) near Nemo, Phanerozoic and Proterozoic (meta)sedimentary rocks are nonconformably underlain by Archean biotite–feldspar gneiss (BFG) and Little Elk gneissic granite (LEG), both of which intrude older schists. The Archean granitoid gneisses exhibit a pervasive NW–SE-trending fabric, whereas an earlier NE–SW-trending fabric occurs sporadically only in the BFG, which is intruded by the somewhat younger LEG. Zircon crystals obtained from the LEG and BFG exhibit double terminations, oscillatory zoning, and Th/U ratios of 0.6±0.3—thereby confirming a magmatic origin for both lithologies. In situ analysis of the most U–Pb concordant domains yields equivalent ²⁰⁷Pb/²⁰⁶Pb ages (upper intercept, U–Pb concordia) of 2559±6 and 2563±6 Ma (both ±2σ) for the LEG and BFG, respectively, which constrains a late Neoarchean age for sequential pulses of magmatism in the NBH. Unzoned (in BSE) patches of 2560 Ma zircon commonly truncate coeval zonation in the same crystals with no change in Th/U ratio, suggesting that deuteric, fluid-assisted recrystallization accompanied post-magmatic cooling. A xenocrystic core of magmatic zircon observed in one LEG zircon yields a concordant age of 2894±6 Ma (±2σ). This xenocryst represents the oldest crustal material reported thus far in the Black Hills. Whether this older zircon originated as unmelted residue of 2900 Ma crust that potentially underlies the Black Hills or as detritus derived from 2900 Ma crustal sources in the Wyoming province cannot be discerned. In the southern Black Hills (SBH), the peraluminous granite at Bear Mountain (BMG) of previously unknown age intrudes biotite–plagioclase schist. Zircon crystals from the BMG are highly metamict and altered, but locally preserve small domains suitable for in situ analysis. A U–Pb concordia upper intercept age of 2596±11 Ma (±2σ) obtained for zircon confirms both the late Neoarchean magmatic age of the BMG and a minimum age for the schist it intrudes. Taken together, these data indicate that the Neoarchean basement granitoids were emplaced at 2590–2600 Ma (SBH) and 2560 Ma (NBH), most likely in response to subduction associated with plate convergence (final assembly of supercontinent Kenorland?). In contrast, thin rims present on some LEG–BFG zircons exhibit strong U–Pb discordance, high common Pb, and low Th/U ratios—suggesting growth or modification under hydrothermal conditions, as previously suggested for similar zircons from SE Wyoming. The LEG–BFG zircon rims yield a nominal upper intercept date of 1940–2180 Ma, which may represent a composite of multiple rifting events known to have affected the Nemo area between 2480 and 1960 Ma. Together, these observations confirm the existence of a Paleoproterozoic rift margin along the easternmost Wyoming craton. Moreover, the 2480–1960 Ma time frame inferred for rifting in the Black Hills (Nemo area) corresponds closely to a 2450–2100 Ma time frame previously inferred for the fragmentation of supercontinent Kenorland.     

Assimilation and Fractional Crystallization Controlled by Transport Process of Crustal Melt: Implications from an Alkali Basalt-Dacite Suite from Rishiri Volcano, Japan

Mechanisms of fractional crystallization with simultaneous crustalassimilation (AFC) are examined for the Kutsugata and Tanetomilavas, an alkali basalt–dacite suite erupted sequentiallyfrom Rishiri Volcano, northern Japan. The major element variationswithin the suite can be explained by boundary layer fractionation;that is, mixing of a magma in the main part of the magma bodywith a fractionated interstitial melt transported from the mushyboundary layer at the floor. Systematic variations in SiO₂ correlatewith variations in the Pb, Sr and Nd isotopic compositions ofthe lavas. The geochemical variations of the lavas are explainedby a constant and relatively low ratio of assimilated mass tocrystallized mass (‘r value’). In the magma chamberin which the Kutsugata and Tanetomi magmas evolved, a strongthermal gradient was present and it is suggested that the marginalpart of the reservoir was completely solidified. The assimilantwas transported by crack flow from the partially fused floorcrust to the partially crystallized floor mush zone throughfractures in the solidified margin, formed mainly by thermalstresses resulting from cooling of the solidified margin andheating of the crust. The crustal melt was then mixed with thefractionated interstitial melt in the mushy zone, and the mixedmelt was further transported by compositional convection tothe main magma, causing its geochemical evolution to be characteristicof AFC. The volume flux of the assimilant from the crust tothe magma chamber is suggested to have decreased progressivelywith time (proportional to t^–1/2), and was about 3 x 10^–2m/year at t = 10 years and 1 x 10^–2 m/year at t = 100years. It has been commonly considered that the heat balancebetween magmas and the surrounding crust controls the couplingof assimilation and fractional crystallization processes (i.e.absolute value of r). However, it is inferred from this studythat the ratio of assimilated mass to crystallized mass canbe controlled by the transport process of the assimilant fromthe crust to magma chambers. KEY WORDS: assimilation and fractional crystallization; mass balance model; magma chamber; melt transport; Pb isotope     

Detection of petroleum contamination in river sediments from Quebec City region using GC–IRMS

Isotopic analysis by compound specific gas chromatography–isotope ratio mass spectrometry (GC–IRMS) is used to detect and characterize petroleum pollution in surficial sediments along the St Lawrence River, near Quebec City. Unusually mature n-alkane distributions have been found in some recent intertidal sediments in the region. GC–IRMS results suggest that the n-alkanes are not derived from indigenous organic sources because they carry δ¹³C values between −30.0 and −27.0‰, as well as very small isotopic differences between odd and even numbered n-alkanes, which are both typically associated with petroleum products. Comparison of these sediments with bunker fuel, an oil used in the shipping industry, has shown a close isotopic correlation in some sites, which is further supported by biomarkers. Overall, the contamination has been dispersed along the river but is generally localized around the industrial region where hydrocarbon transfer from shore storage to ships takes place. This study illustrates how GC–IRMS can be used effectively in the detection and characterization of petroleum pollutants in sediments.     

古气候变化与地下热水中氢氧稳定同位的关系

关中盆地地下热水中氢氧稳定同位素的研究表明：研究区地下热水中氢氧稳定同位素组成变化与当地古气候变化具有良好的对应关系,古气候变化直接影响了地下热水接受补给时的氢氧稳定同位素组成。研究区地下热水的补给为更新世前古代大气降水。大约在8.2～10.2 kaB.P.和18.1～19.2 kaB.P.这两个时间段,可能是由于当时温度较低导致关中盆地地下热水补给偏少;关中盆地地下热水的补给过程受古气候的变化影响呈现非等速补给特征,可能存在一定的古地下水形成期。     

Petrogenesis of granitoids from the Lachlan Fold Belt,southeastern Australia: The role of disequilibrium melting

S- and I-type granites from the Lachlan Fold Belt, southeastern Australia, have been investigated to assess the role of disequilibrium melting in their petrogenesis. Differences between the median initial εHf compositions of magmatic zircon populations and the host bulk-rock (ΔεHf^blk-zrc) range from −0.6 to +2.5 ε units, providing evidence for intra-sample (and hence inter-phase) Hf-isotopic heterogeneity. Linear variations on Harker diagrams and O and Hf isotope compositions of magmatic zircon preserved in many I- and S-type granites are inconsistent with assimilation or simple mixing hypotheses. In contrast, isotopic disequilibrium between the melt and a restite assemblage can explain the bulk-rock versus zircon differences observed in these samples.Assuming that magmatic zircon records the melt composition, differences between the bulk-rock εHf and εHf of magmatic zircon (ΔεHf^blk-zrc values) measured for I-type granites (0.4–2.5) can largely be explained by disequilibrium amphibole dehydration melting of meta-igneous protoliths that were either isotopically heterogenous at the time they were formed, or perfectly homogeneous before being aged in the crust for 0.4–1.0 billion years prior to partial melting. The Currowong Suite exhibits petrographic features and preserves geochemical and isotopic compositions that do not lend themselves to simple restite model or magma mixing explanations; however, these observations could be explained by the restite unmixing of magma batches generated from a single source rock if, as modelling has suggested, separate batches contain different melt compositions.By investigating the application of disequilibrium melting to granite genesis, this study demonstrates that isotopic heterogeneity at various sampling scales should actually be expected for the production of granites from a single source, rather than necessitating the involvement of multiple sources and mixing processes. As a result great care should be taken in the interpretation of isotope data from granitic bulk-rocks or their zircons.     

Ubiquitous post-peak zircon in an eclogite from the Kumdy-Kol,Kokchetav UHP-HP Massif (Kazakhstan): Significance of exhumation-related zircon growth and modification in continental-subduction settings

U–Pb geochronological, trace-element and Lu–Hf isotopic studies have been made on zircons from ultrahigh-pressure (UHP) mafic eclogite from the Kumdy-Kol area, one of the diamond-facies domains of the Kokchetav Massif (northern Kazakhstan). The peak eclogitic assemblage equilibrated at > 900 °C, whereas the bulk sample composition displays light rare-earth element (LREE) and Th depletion evident of partial melting. Zircons from the eclogite are represented by exclusively newly formed metamorphic grains and have U–Pb age spread over 533–459 Ma, thus ranging from the time of peak subduction burial to that of the late post-orogenic collapse. The major zircon group with concordant age estimates have a concordia age of 508.1 ±4.4 Ma, which corresponds to exhumation of the eclogite-bearing UHP crustal slice to granulite- or amphibolite-facies depths. This may indicate potentially incoherent exhumation of different crustal blocks within a single Kumdy-Kol UHP domain. Model Hf isotopic characteristics of zircons (ε_Hf(t) +1.5 to +7.8, Neoproterozoic model Hf ages of 1.02–0.79 Ga) closely resemble the whole-rock values of the Kumdy-Kol eclogites and likely reflect in situ derivation of HFSE source for newly formed grains. The ages coupled with geochemical systematics of zircons confirm that predominantly late zircon growth occurred in Th–LREE-depleted eclogitic assemblage, that experienced incipient melting and monazite dissolution in melt at granulite-facies depths, followed by amphibolite-facies rehydration during late-stage exhumation-related retrogression.     

Pathways and distances of fluid flow during low-grade metamorphism: evidence from pyrite deposits of the Cameros Basin, Spain

New geochemical and sulphur isotopic data are presented for a number of pyrite deposits from the Late Jurassic–Early Cretaceous Cameros Basin, Spain. The deposits were formed at, or close to, the peak of metamorphism and are always related to sandstone units in the mainly metapelite sequence. Iron remained immobile and conservative, pyrite iron being derived by sulphidation of chlorite in the host metapelites. Reduced sulphur, however, was supplied from two external sources: thermochemical reduction of sulphate and release of S during metamorphism of sedimentary sulphides. These sources provided isotopically heavy and light S, respectively, with variation in pyrite isotopic composition between different deposits resulting from differences in their relative importance at each site. During metamorphism, the sandstone units acted as aquifers, carrying the sulphidic pore waters to locations where permeability provided by syn-depositional fractures on a scale of 0.5–5  m allowed its interaction with the metapelites. Transport distances for sulphide during metamorphism were of the order of hundreds of metres.     

Constraints of stable isotopes on the origin of alunite from advanced argillic alteration systems in Bulgaria

Voluminous areas of advanced argillic alteration (AAA) constitute major exploration targets for surficial Cu–Au epithermal and potentially underlying porphyry-type deposits. In Bulgaria, more than 30 alunite occurrences are recognised, few of them being associated with a mineralised system. A mineralogical study combined with a stable isotopic (O, H, S) study has been carried out on nine alunite occurrences of advanced argillic zones hosted by volcanic rocks of Late Cretaceous age in the Srednogorie belt and of Oligocene age in the Rhodopes belt. This work was realised in order to constrain the origin of alunite and to define criteria to discriminate alunite from ore deposits and alunite from large barren alteration systems.Mineralogy of the nine occurrences consists of alunite + quartz + minor alumino-phospho-sulphates, associated with more or less kaolinite, dickite, pyrophyllite, diaspore and zunyite, depending on formation temperature. Alunite generally occurs as tabular crystals but is also present as fine-crystalline pseudocubic phases at Boukovo and Sarnitsa, in Eastern Rhodopes. In the advanced argillic alterations associated with economic ore, the presence of zunyite in the deeper parts indicates acid–fluorine–sulphate hydrothermal systems, whereas it is absent in uneconomic and barren advanced argillic alteration. All occurrences are formed at temperatures between 200 and 300 °C.(H, O, S) isotopic signatures of alunite combined with mineralogical features from all the studied occurrences, whatever their type, show characteristics of magmatic-hydrothermal systems. Sulphur data indicate essentially a magmatic origin for sulphur. Oxygen and hydrogen data suggest that hydrothermal fluids result from a mixing between magmatic fluids and an external component, which is identified as seawater-derived fluids or meteoric water in the vicinity of a sea. In most of the alunite occurrences, magmatic fluids are dominant and H₂S/SO₄ ratios are estimated to be higher than 2. Two exceptions exist in the Rhodopes. At Boukovo and Sarnitsa, where the estimated formation temperatures of alunite are the lowest, the external fluids are dominant and H₂S/SO₄ratios are estimated to be lower than or close to 1.At this stage of the work, the mineralogical and isotopic criteria do not enable a clear distinction between economic and uneconomic systems. However, some features are common in the economic ore deposits: the presence of zunyite in the deeper part of the system, the relatively high temperatures suggested by the zunyite + pyrophyllite + alunite + diaspore assemblages, the (O, H, S) signature of alunite, which is characteristic of dominant magmatic–hydrothermal acid–sulphate–fluorine systems.