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Chemical geology
Elsevier
Chemical geology

Elsevier

0009-2541

Chemical geology/Journal Chemical geologySCIISTPAHCIEI
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    Ba/Ca profiles in shells of Pecten maximus - A proxy for specific primary producers rather than bulk phytoplankton

    Jochum, Klaus PeterChauvaud, LaurentSchoene, Bernd R.Froehlich, Lukas...
    16页
    查看更多>>摘要：Molar barium-to-calcium ratios in bivalve shells (Ba/Ca-shell) have been proposed in a variety of studies to serve as a potential proxy for the reconstruction of phytoplankton dynamics. However, the link between phytoplankton and Ba/Ca-shell profiles remains unclear and needs to be deciphered more accurately. In this study, we analyzed the relationship between Ba/Ca-shell peaks and specific phytoplankton species, and assessed the applicability of Ba/Ca-shell ratios as a species-specific phytoplankton proxy. The timing of peaks in highly resolved Ba/Ca-shell time-series in Pecten maximus shells (Bay of Brest, France) from two years (2011, 2012) were compared to the chlorophyll a concentration and the occurrence of individual diatom and dinoflagellate species. In addition, Monte Carlo simulations were used to approximate a species-specific contribution to the measured Ba/Ca-shell peaks. The results clearly demonstrated that the Ba/Ca-shell profiles cannot be explained either by the chlorophyll a concentration nor by the total diatom or dinoflagellate abundance. Instead, time-series of specific phytoplankton, especially diatoms, revealed a high degree of synchronicity with Ba/Ca-shell peaks when temporally lagged by 8 to 13 days (depending on species). The Monte Carlo simulations suggested that the Ba/Ca-shell peak heights cannot be fully explained by the observed phytoplankton cell concentration, but rather by individually weighted phytoplankton time-series, most likely caused by inter-species differences such as cell size and chemical ability to adsorb Ba. Moreover, the approximated species-specific weighting factors agreed well between the studied years. According to our findings, Ba/Ca-shell peaks are likely associated with blooms of specific phytoplankton taxa, with a time lag of ca. one to two weeks, and the amount of cell-associated Ba varies between phytoplankton species. These conclusions provide further insights into the formation of Ba enrichments in bivalve shells and improves the applicability of Ba/Ca-shell profiles as a species-specific proxy of past phytoplankton dynamics.
      原文链接:
    • NSTL
    • Elsevier

    Mapping intrashell variation in Mg/Ca of brachiopods to external growth lines: Mg enrichment corresponds to seasonal growth slowdown

    Muller, TamasTomasovych, AdamCorrea, Matthias LopezMertz-Kraus, Regina...
    17页
    查看更多>>摘要：Determining secular changes in seawater temperature and Mg/Ca ratio is of high importance in paleooceanographic, paleoenvironmental and paleoecological studies. On the one hand, intra-shell variation in Mg/Ca in brachiopod shells is used to reconstruct historical changes in temperature and in seasonality. On the other hand, intra-shell analyses show a weak relationship between Mg/Ca and delta O-18, but a positive correlation between Mg/Ca and Sr/Ca, and an increase of Mg/Ca at growth lines, indicating that Mg/Ca does not simply track seawater temperature alone. Internal growth lines are rarely visible in brachiopod shells, and the relation between growth lines that mark growth slowdown or cessation and Mg/Ca fluctuation is thus not properly understood. Here, using four species of extant brachiopods from the NE Pacific and the SW Pacific, from environments with low seasonality in seawater temperature (<5 degrees C), and two species of Late Triassic brachiopods (Western Carpathians), we measure intra-shell ontogenetic variation in Mg/Ca, S/Ca and Sr/Ca with laser ablation ICP-MS and electron microprobe. In contrast to previous studies, we map variation in Mg, S, and Sr to external growth lines. We find that, first, zones of Mg/Ca enrichment in the secondary layer form 10-50 mu m-thick discrete bands or clusters of micrometric bands that consistently terminate at major external, predominantly annual growth lines that are associated with distinct notches that correspond to mantle retractions in most species. These bands generate distinct peaks in Mg/Ca that stand above low, more uniformly distributed background Mg/Ca values along laser-ablation and microprobe transects. The magnitude of Mg/Ca in these bands is comparable to Mg/Ca in the primary layer, exceeding background values of Mg/Ca in the secondary layer by 5-20 mmol/mol (by a factor of two to five). Second, Mg enrichment in discrete bands is closely associated with an increase in S concentrations in the secondary layer. Therefore, even when Mg and S uptake is expected to be highest under fast precipitation rate, these empirical observations show that Mg and S enrichment occur in the zones that were precipitated at times of a significant growth reduction (e.g., in cold-temperate brachiopods studied here during winters when primary productivity is low). The mechanisms for this paradox are unclear but can be related to uptake of Mg by amorphous calcium carbonate (ACC) to delay the transformation of ACC to calcite during the formation of growth lines. Therefore, intra-shell variation in Mg/Ca in brachiopods with major growth lines is primarily determined by seasonal decline or cessation in shell growth and not by seasonal changes in seawater temperature.
      原文链接:
    • NSTL
    • Elsevier

    Matrix effects and improved calibration procedures for SIMS titanite U-Pb dating

    Ling, Xiao-XiaoLi, Qiu-LiHuyskens, Magdalena H.Liu, Yu...
    10页
    查看更多>>摘要：Titanite is a U-bearing accessory mineral that can serve as a geochronometer. Compared with zircon, titanite is more likely to contain multi-stage fluid-induced growth. Thus, individual titanite grains may yield complex U-Pb ages that reveal the entire geological history of the rock. Secondary ion mass spectrometry (SIMS), which has high sensitivity and spatial resolution, is a good choice for U-Pb titanite analyses. A titanite U-Pb standard named YQ82 was developed with Pb-206/U-238 age of 1837.6 +/- 1.0 Ma (2 standard error (SE), n = 4) and Pb-207/Pb-206 age of 1845. 0 +/- 1.1 Ma (2SE, n = 4), as determined herein using isotope dilution isotope ratio mass spectrometry (ID-IRMS). However, U-Pb dating of YQ82 via SIMS yielded a bias of up to 14% compared to the ID-IRMS value, indicating a matrix effect. To correct for this matrix effect, six U-Pb titanite dating reference materials (RMs) were analyzed. Based on the correlations between the age biases and the Fe contents of the six RMs, a calibration protocol was established using the index of the Fe content (FeO+/IP, intensity of primary beam) and a power-law relationship between the calibrated Pb+/U+ and UO+/U+. Using this calibration protocol, the U-Pb ages decreased the age bias from approximately +/- 14% to +/- 1.4%. To test the protocol, a titanite sample (BMB108) with a low Fe content (Fe2O3, 0.4 wt%) was calibrated using MKED1 (Fe2O3, 1.5 wt%) as the RM. Without the calibration, the apparent SIMS Pb-206/U-238 age was 10% younger than the thermal ionization mass spectrometry (TIMS) Pb-206/U-238 age (1890 +/- 4 Ma). Using the calibration method, the apparent SIMS U-Pb age was 1898 +/- 14 Ma (2SE, n = 19), which is consistent with the TIMS age. This study shows that a considerable matrix effect related to Fe content is present in the SIMS titanite U-Pb dating results. The calibration protocol developed herein improved the SIMS titanite U-Pb dating accuracy, yielding a more accurate thermal history of the samples.
      原文链接:
    • NSTL
    • Elsevier

    U-Pb isotopic dating of cassiterite: Development of reference materials and in situ applications by LA-SF-ICP-MS

    Yang, MingRomer, Rolf L.Yang, Yue-HengWu, Shi-Tou...
    19页
    查看更多>>摘要：Cassiterite, the economically most important tin mineral, typically has moderate U and variable common Pb contents, making it amenable for U-Pb dating. Cassiterite has extremely low Th/U ratios (Th/U < 0.01) and its Pb-208 is dominantly common Pb. This is particularly helpful as there is significant interference of tungsten oxides on Hg-202 and Pb-204. The feasibility of the Pb-208 correction procedure is discussed in detail. The Pb-208 corrected LA-SF-ICP-MS data are in good agreement with intercept ages in the Tera-Wasserburg diagram and Pb-207 corrected ages. Twelve cassiterite samples were investigated using the ID-TIMS and LA-SF-ICP-MS methods. The ID-TIMS results of Pit-AB, Rond-A, RG-114, BB#7 and 19GX cassiterite are reported for the first time in this study. RG-114, BB#7 and 19GX cassiterite have very low common Pb contents and are recommended for use as primary reference materials for in situ cassiterite. Pit-AB, Rond-A and Yankee cassiterite contain a small amount of common Pb, produce reliable and consistent ages and are suitable as primary reference materials. The remaining five cassiterite samples (Kard, Zinnwald, Els, XBD-W and Y724) were only investigated using the LA-SF-ICP-MS method and produce ages consistent with published age data from the host rocks associated with the tin deposits and with published U-Pb ages of cassiterite from the same deposits. We present an ID-TIMS U-Pb of 154.3 +/- 0.7 Ma for the commonly used cassiterite reference material AY-4. This age differs from previously reported ID-TIMS ages. This age discrepancy is caused by different initial common Pb compositions rather than age heterogeneity.
      原文链接:
    • NSTL
    • Elsevier

    Geochemistry of trace metals and Rare Earth Elements in shallow marine water affected by hydrothermal fluids at Vulcano (Aeolian Islands, Italy)

    Falcone, E. E.Federico, C.Boudoire, G.
    17页
    查看更多>>摘要：Trace metals and Rare Earth Element (REE) are amply discharged by submarine hydrothermal vents, sometimes leading to the formation of ore deposits of economic interest. We report on first data on the geochemical processes involving REE and trace metals, at the solid-liquid interface, in the hydrothermal area of Levante Bay at Vulcano Island (Aeolian Archipelago, Italy). Samples were collected from several submarine springs and seeps, a mud pool and one thermal well, and analyzed for Al, Si, Ti, V, Cr, Mn, Fe, Co, As, Rb, Sr, Cs, Ba, U and REE, besides major ions. Within the bay, hydrothermal fluids contaminate seawater and promote the leaching of metals from sediments through the dissolution of CO2 and H2S, while the particulate matter removes several elements from the water. The leaching of the bottom sediments and the contribution of steam-heated water produce an enrichment of some metals and REE in the Levante Bay with respect to the concentrations expected in the ambient seawater. An enrichment up to one order of magnitude is measured for Fe, Al, Ba, Cs and Rb, and up to two orders of magnitude for Mn in the submarine samples. Other transition metals (Ti, V, Co, Cr), U, As and Sr have concentrations similar or slightly lower than the ambient seawater. REE are in concentrations higher than in ambient seawater up to two orders of magnitude. Despite being significantly higher than uncontaminated seawater, the concentrations of some metals (namely Fe, Al, Ti, Cr, V, Co, U) and REE in most samples are lower than expected by the mixing between seawater and the steam-heated water, discharging from submarine springs. Indeed, equilibrium and reaction path modeling indicate the likely precipitation of Fe-oxyhydroxides, able to remove minor elements, such as Ti, Cr, Co, V and As, and REE. The last ones are significantly removed by newly-forming solid phases, due to the presence of a large amount of Fe released by the acidic fluids through the leaching of sediments. The low pH limits the formation of solution complexes of REE with carbonate ions (the main complexing agent for REE in seawater), whereas the sorption onto particles is still effective, even at close distance from the submarine springs and seeps. This study brings new insights on the geochemical processes occurring in submarine hydrothermal systems, in particular, those in subduction-related context.
      原文链接:
    • NSTL
    • Elsevier

    Application of multivariate data analysis to biogeochemical exploration at the Twin Lakes Deposit, Monument Bay Gold Project, Manitoba, Canada

    Ghorbani, ZohrehGholizadeh, FatemehCasali, JulianaHao, Chunyi...
    12页
    查看更多>>摘要：The Twin Lakes Deposit, located in the northwest of the Superior Province, is a promising Archean Greenstone-hosted orogenic gold deposit. Biogeochemical and statistical techniques were used for the first time in this area to assess the geochemical dispersion patterns of Au, As, Bi, Se, Sb, Tl, Fe, Co, Ni, Cr, Mo, Cd, Pb, Zn, Mn, K, P, B, Mg, Ca, Ba, and Cu in black spruce samples collected perpendicularly to the overall trend of shear-hosted gold mineralization. ICP-MS analysis of ashed black spruce bark, twigs, and needles has outlined a biogeochemical response of black spruce to the concealed mineralization. Most elements are accumulated preferentially in twigs and bark, while B, K, Mg, Mn, and P are highly concentrated in needles. High Au and Tl enrichments were identified in all sample types compared to the background level. The scatter plot matrix demonstrated a positive bivariate relationship between Au and Fe, Mg, Cu, Mo, and Zn; and horizontal to slightly negative correlation with Ca, K, Mn, P, and B. Investigations on the spatial distribution pattern of Au and its pathfinder elements using a robust log-transformed inverse distance weighted (IDW) interpolation method revealed the presence of zones of gold enrichment in the north to the northwest of the survey, where Au is accompanied by TI-As-Cd > Bi, Pb, Se > Sb. According to the Robust RQ-mode principal component analysis (PCA), two main components control the distribution of elements in black spruce, including (1) a geological factor that mostly reflects bedrock composition; and (2) a physiological factor that reflects biological processing of essential elements. Of importance is that Au and its pathfinder elements, including As, Se, Tl, and to a lesser extent, Bi, Cd, and Pb were well-separated as a subgroup of the geological factor with a clear spatial relationship to gold mineralization. This study demonstrates a biogeochemical response in black spruce is associated with gold mineralization at the Monument Bay Gold Project using multivariate statistical methods. The results of this study are promising and can be extrapolated to wider surveys across the Monument Bay Gold Project to vector toward new gold mineralized zones or extensions of the Twin Lakes gold deposit. The results also provide a suite of gold pathfinder elements that support exploration efforts and regional target generation.
      原文链接:
    • NSTL
    • Elsevier

    Optimizing( 40)Ar/Ar-39 analyses using an Isotopx NGX-600 mass spectrometer

    Jicha, Brian R.Tootell, DamianSinger, Brad S.Mixon, Emily E....
    11页
    查看更多>>摘要：Ar-40/Ar-39 geochronology relies on magnetic sector mass spectrometers to determine relative isotopic abundances. Ongoing technological developments within noble gas mass spectrometers over the last decade have led to analysis of increasingly smaller samples and higher precision, but also result in more complex data correction and interpretation. We describe a new multi-collector noble gas spectrometer, the Isotopx NGX-600, that is configured to optimize Ar-40/Ar-39 measurements. The NGX-600 is equipped with 9 Faraday collectors and one ion counting electron multiplier. Each Faraday is equipped with Isotopx ATONA (R) amplifier technology, enabling measurements spanning a dynamic range of amplified beam current from below 10(-16) A to above 10(-9) A. The performance of the NGX-600 is evaluated using both a conventional Nier-type ion source, and a next generation low temperature ion source, which allows for trap current variation from 200 mu A to 1000 mu A. We have performed over 3000 analyses of atmospheric argon to: (1) assess optimal measurement and integration times for blanks, baselines, and air aliquots of various ion intensities, (2) quantify the sensitivity via measurements of first principles Ar-40/Ar-39 standards, (3) compare the sensitivity between the conventional and new low temperature Nier-type ion sources, and (4) evaluate corrections associated with inter-Faraday biases, instrumental mass bias, and Faraday-multiplier gain. In addition to optimization experiments, we report a comparative analysis of both single crystal fusion and incremental heating data from Quaternary volcanic rocks obtained using both the 5-collector Nu Instruments Noblesse and the NGX-600 spectrometers.
      原文链接:
    • NSTL
    • Elsevier

    Chemical and boron isotopic variations of tourmaline deciphering magmatic-hydrothermal evolution at the Gejiu Sn-polymetallic district, South China

    Guo, JiaXiang, LuZhang, RongqingYang, Tao...
    20页
    查看更多>>摘要：The world-class Gejiu Sn-polymetallic district (southwest China), with ca. 3.27 Mt Sn metal, is a magmatic-hydrothermal deposit associated with the Late Cretaceous biotite granites; however, the magmatic and hydrothermal processes involved in its formation remain poorly understood. Tourmaline is a ubiquitous phase in the Gejiu district, and here we employ in-situ major, trace element, and boron isotope compositions of tourmaline to investigate the source and evolution of the mineralizing fluid, and late-magmatic and hydrothermal processes related to ore formation. Based on petrographic observations, four types of tourmaline are identified: i) latemagmatic tourmaline (Tur I) disseminated in the biotite granites, ii) hydrothermal tourmaline (Tur II) from quartz veins with fracture-filling structure in the biotite granites, iii) hydrothermal tourmaline (Tur IIIa-1, IIIa-2 and Tur IIIb-1, IIIb-2) from fluorite-quartz veins with typical replacement texture against the greisenized granites, among which Tur IIIb-2 is directly related to cassiterite formation, and iv) hydrothermal tourmaline (Tur IV) from sulfide-calcite veins hosted by proximal skarn. Tourmaline (Tur Ito Tur III) from the granites belongs mostly to the alkali group and schorl-dravite solid-solution series, with Fe/(Fe + Mg) ranging from 0.83-1.00 for Tur I, 0.68-1.00 for Tur II, and 0.19-1.00 for Tur III, respectively. By contrast, Tur IV belongs mostly to the vacancy group and has foitite-Mg-foitite composition, with Fe/(Fe + Mg) of 0.10-1.00. Large variations of Fe/(Fe + Mg) as well as Na/(Na + Ca) in hydrothermal tourmaline (Tur II to Tur IV), are related to different degrees of interaction between B-rich fluids with host rocks (solidified granite or carbonate). Most trace elements in tourmaline do not correlate with Fe/(Fe + Mg) and Na/(Na + Ca) ratios, implying the trace element compositions are predominantly controlled by melt/fluid compositions and local fluid-rock reactions. Relative to late-magmatic Tur I, hydrothermal tourmaline (Tur II to Tur IV) is enriched in Sr and depleted in Nb, Ta, and (REE + Y), whereas their Li, Be, Sc, V, and Sn concentrations are largely overlapped. The delta B-11 values in different types of tourmaline fall in a narrow range from -17.8 to -13.7 parts per thousand, in favor of B-rich fluids episodically exsolved from the granitic melt. Tin enrichment in both late-magmatic and hydrothermal tourmaline grains, especially in those (Tur IIIb) from Sn-mineralized veins, is related to magmatic differentiation and late-stage fluid exsolution. Among all types of tourmaline, Tur IIIb-2 coexisting with cassiterite has elevated Fe3+/(Fe3+ + Fe2+) ratios (mean 0.23), indicating that cassiterite precipitated under relatively oxidized conditions. The distinct chemical (i.e. high Mg, Ti, V, Sc, Sr, and Sn contents) and B-isotope compositions (slightly lower delta B-11 values of -17.8 to -15.0 parts per thousand) in syn-ore Tur IIIb-2, combined with the co-occurrence of liquid- and vapor-rich inclusions triggered by fluid boiling in Sn-mineralized veins, suggest that fluid boiling and acid-consuming reaction could be major processes triggering cassiterite precipitation in the quartz vein and greisen. Overall, the chemical and B-isotope signatures of tourmaline are ideal tracers to unravel the magmatic-hydrothermal evolution in the Gejiu Sn-polymetallic district, which could be further employed as a potential prospecting guide for Sn-W deposits.
      原文链接:
    • NSTL
    • Elsevier

    Metal recycling tracked by mercury and helium isotopes in platinum-palladium nuggets from Corrego Bom Sucesso, Brazil

    Cabral, Alexandre RaphaelDeng, ChangzhouYin, RunshengYakubovich, Olga, V...
    8页
    查看更多>>摘要：The enigmatic botryoidal nuggets of platinum (Pt) and palladium (Pd) from Corrego Born Sucesso in the southern Serra do Espinhaco, Minas Gerais, Brazil, are considered to have formed during supergene alteration of placer deposits. This requires hitherto unidentified precious-metal enrichment processes and is inconsistent with Pt-Os age of 180 Ma that entails formation at depth, as recently proposed. Here we report the first mercury (Hg) and helium (He) isotopic determinations of Pt-Pd nuggets. Mercury isotopic compositions have a mass-independent fractionation (MIF) signature with an odd-mass deficit (Delta Hg-199 = - 0.22 +/- 0.04; 1SD, n = 15), which requires aqueous photochemical reduction of Hg (II). Extremely low He-3/He-4 (<0.001 R-a) and extremely high concentrations of He (up to 1.9 x 10(17) at/g) are indicative of nugget formation from He-enriched fluids within the quartzite sequence of the Espinhaco basin, not from meteoric surface water. The data are consistent with a nugget-forming setting in the deep biosphere, as a result of groundwater interaction with Pt-Pd-Hg minerals in Pan-African-Brasiliano post-orogenic veins. We propose that the negative Hg-MIF signature was inherited from the vein minerals that originally acquired their Hg from Earth's surface during the intracratonic sedimentation of the Proterozoic Espinhaco basin.
      原文链接:
    • NSTL
    • Elsevier