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Chemical geology
Elsevier
Chemical geology

Elsevier

0009-2541

Chemical geology/Journal Chemical geologySCIISTPAHCIEI
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    Characterizing groundwater recharge sources using water stable isotopes in the North Basin of Lake Kivu, East Africa

    Balagizi, Charles M.Kasereka, Marcellin M.Kyambikwa, Albert M.Cuoco, Emilio...
    13页
    查看更多>>摘要：The delta O-18 and delta H-2 of rivers and springs were investigated in order to characterize the groundwater recharge sources around Nyiragongo and Nyamulagira volcanoes, in the Democratic Republic of the Congo and Rwanda. Water samples were collected monthly between November 2013 and October 2014 from 5 major rivers, 3 major cold springs, 3 tepid springs and 1 hot spring. The temperatures of each spring were nearly constant over the sampling period attesting for their groundwater character, while the temperatures of the rivers were much more variable. The rivers monthly delta H-2 and delta O-18 range from -6.8 parts per thousand to 1.9 parts per thousand and -3.1 parts per thousand to 1.6 parts per thousand, respectively, while springs showed depleted values that span from -10.2 to -1.1 parts per thousand for delta H-2 and - 3.6 to -1.9 parts per thousand for delta O-18. Catchment morphology (formed of depression, upper footslope and medium to high gradient-mountains) and the local tectonic discontinuity (fissures and faults) regulate the surface runoff and subsurface flow, control the precipitation infiltration zones and hence the aquifers recharge areas. Chemical and isotopic (delta O-18 and delta H-2) compositions of springs and rivers reveal the presence of shallow and deep aquifers, with some waters having intermediate isotope composition. Three different recharge zones characterized by different altitudes were identified: the first is found at low altitude ranging from similar to 1800 m to similar to 2150 m, the second and intermediate recharge zone in the altitude range from similar to 2180 m to similar to 2500 m at the upper footslope area, while the third and highest recharge area is located in the altitudes range from similar to 2620 to similar to 3220 m. The two upper recharge areas are the most fractured and fissured zones allowing rapid infiltration of depleted precipitations which recharge deep aquifers found in the tepid and hot springs. Based on their chemical and isotopic composition, waters from the shallow and deep aquifers have been considered representative of mixing end members. During their ascent to the surface, water from the deep aquifer mixes with that of shallow aquifer yielding the tepid springs of intermediate chemical and isotopic composition, while the other keep their original fingerprint corresponding to the isotopically depleted hot spring.
      原文链接:
    • NSTL
    • Elsevier

    Geochemical interactions among water, minerals, microbes, and organic matter in formation of speleothems in volcanic (lava tube) caves

    Kulkarni, Harshad VijayFord, JoshuaBlank, Jennifer G.Park, Minkyu...
    15页
    查看更多>>摘要：Volcanic (lava tube) caves at Lava Beds National Monument (N. CA, USA) provide a valuable terrestrial analog for volcanic caves on Mars and the Moon. Terrestrial volcanic caves host a diverse microbial life, liquid water, and a variety of secondary mineral deposits (speleothems) with diverse morphologies and chemical compositions. Speleothems may preserve records of past and present microbial life and signatures of paleoenvironmental changes in terrestrial volcanic caves. Distinguishing between speleothems via chemical processes and microbially-mediated processes in terrestrial volcanic caves will provide valuable insights for future exploration of martian volcanic caves. To elucidate the formation of speleothems, we studied the chemical makeup (inorganic and organic) of cave waters in seven volcanic caves of variable ages, temperature, moisture content, light intensity, and frequency of human visitation. Cave water was characterized by stable isotopic composition (delta O-18 and delta H-2), concentrations of major and trace elements, cations, anions, and characteristics of dissolved organic matter (DOM). A forward reaction model (PHREEQC) was used to test possible pathways for secondary mineral precipitation that formed these speleothems. The source of cave water was primarily regional meteoric precipitation that entered the caves through cave openings or through the cave overburden and fractured basalt walls as indicated by cave floor puddle water line delta H-2 = 8.32*delta O-18 + 9.55 parallel to the global meteoric water line (GMWL, delta H-2 = 8.3*delta O-18 + 10). A line formed by cave ceiling drip water delta H-2 = 3.39*delta O-18 44.77 intersecting the GMWL indicated that the water may be undergoing evaporation within the caves. Silicate weathering was found to be a primary process resulting in cave water enriched in Si (22 +/- 7 mg/L), and contained trace levels of Al, Fe, Zn, Li, Sr, Cu, B, V, Ba, Cr and Mn. Geochemical calculations indicated that cave waters were undersaturated with respect to both amorphous silica (SiO2am) and calcite (CaCO3) which were the major components of speleothems observed within the caves. Results of a forward reaction model showed that evaporation of cave waters could lower the solubility of SiO2am and CaCO3 by increasing their saturation and ultimately precipitate these two secondary minerals forming the speleothems. The cave water DOM was characterized by high concentrations of dissolved organic carbon (DOC, 12 +/- 8 mg/L) with a molar C/N ratio ranging from 2 to 22. The DOM was found to be aromatic (SUVA254, 1.2-2.9 L/mg.m), terrestrially derived and humic-like (humification index, 7-26) and contained molecules of 100 Da and 5000 Da approximate molecular weight (AMU). Our results indicated that the terrestrially derived carbonaceous organic matter transported into the caves was not utilized for heterotrophic microbial metabolisms as DOC was accumulated over dissolved inorganic carbon (DIC). Both findings suggest that with minimal heterotrophy, chemo-litho-autotrophy may be important pathways that cycle the elements within these volcanic caves with low light conditions. Together, this study proposes a potential pathway of speleothem precipitation through the interaction of water, dissolved mineral constituents, and microbial life where dissolved ions are concentrated in cave drip water through cyclic condensation-vaporization processes.
      原文链接:
    • NSTL
    • Elsevier

    The influence of black shale weathering on riverine barium isotopes

    Charbonnier, QuentinBouchez, JulienGaillardet, JeromeCalmels, Damien...
    11页
    查看更多>>摘要：Barium and its isotopes are receiving growing interest in their capacity to record the variation of the oceanic biological pump through geological timescales. Nonetheless, only little is known about the continental sources and processes that can drive the Ba flux and isotope composition to the ocean. Whereas the role of processes such as secondary phase formation, sorption and biological uptake on Ba isotopes has been recently investigated, the potential of various rock sources in producing a range of Ba isotope compositions remains poorly known. In particular, black shales often exhibit enrichment in Ba that might potentially be associated with various Ba isotope signatures. In this study, we report the isotope composition of Ba in rivers with different relative contribution of black shale rocks in their catchments, located in the Mackenzie Basin (Northwest Canada). Both the dissolved and solid loads of the Mackenzie tributaries are enriched in heavy Ba isotopes with respect to the continental crust, which is consistent with an additional source to the weathering of igneous and siliciclastic rocks. Although the dissolved Ba abundance is partially driven by weathering processes, the river dissolved Ba isotope composition rather reflects a binary mixing between 1) a "classical" siliciclastic source (with a Ba isotope composition close to that of the upper continental crust (delta Ba-138 similar to 0.15 +/- 0.05 parts per thousand) and 2) an isotopically heavier source especially present in the Mackenzie Mountains (with a (delta Ba-138 similar to 0.40 parts per thousand). The positive relationships between dissolved (delta Ba-138 and radiogenic osmium isotope ratios, as well as with sulphuric acid production, indicate that the heavy Ba source is tightly linked to the weathering of black shale. Although the exact reason for such heavy Ba isotope signatures remains elusive, this study emphasises the potential of black shale weathering in shifting the Ba isotope composition of the dissolved flux to the ocean towards heavier values.
      原文链接:
    • NSTL
    • Elsevier

    Impact of hydro-chemical conditions on structural and hydro-mechanical properties of chalk samples during dissolution experiments

    Leger, M.Roubinet, D.Jamet, M.Luquot, L....
    14页
    查看更多>>摘要：The importance of karst reservoirs for water resources, and their complexity in terms of structural properties and hydraulic responses, require a better understanding of the formation and location of conduits in these systems, in particular for chalk reservoirs. For this purpose, we conduct laboratory experiments of acid solutions injection in chalk core samples using a homemade experimental device. Three different flow rates and two acid concentrations are applied on six samples at atmospheric conditions with Peclet numbers ranging from 0.1 to 1.7 and Damkohler numbers from 10(-6) to 10(-3). Hydraulic and chemical measurements are carried out during the experiments, while petrophysical characterization is conducted on the samples before and after the reactive percolation experiments. The analysis of these data shows an increase in permeability of the rock samples due to calcite dissolution, which is associated with the creation of preferential conduits. We also observe that the initial structural properties control the dissolution regime with the following main conclusions. For homogeneous systems, the overall dissolution rate and percolation time are directly linked to the considered flow rate and injected acid concentration. For heterogeneous systems, (i) pre-existing large-scale structural heterogeneities control the location of newly created paths while reducing the value of the dissolution rate, and (ii) micro-scale heterogeneities result in highly heterogeneous general structures, which are responsible for channels formation while applying low Damkohler numbers.
      原文链接:
    • NSTL
    • Elsevier

    Progressive formation of authigenic carbonate with depth in siliciclastic marine sediments including substantial formation in sediments experiencing methanogenesis

    Loyd, Sean J.Smirnoff, Marissa N.
    15页
    查看更多>>摘要：Authigenic carbonate (AC) forms in siliciclastic marine sediments in part as a result of the degradation of organic matter and methane. Degradation pathways vary considerably and each impact the chemical evolution of sediments and porewater differently. The relative importance of these reactions in contributing to carbonate authigenesis are poorly understood, especially from a global perspective. Modern porewater geochemical data and authigenic carbonate carbon isotope compositions (delta C-13(ac)) of globally distributed marine sediment sites allow direct assessment of the relative importance of diagenetic pathways. Common correlations between bulk delta(13)C(ac )and depth reveal that AC tends to form progressively by multiple reaction pathways, rather than within a particular sediment horizon. A general lack of shallow AC within sediments 1) containing porewater sulfate and 2) exhibiting decreasing dissolved inorganic carbon (DIC) delta C-13 with depth suggest that organotrophic sulfate reduction does not promote significant authigenesis. Instead, the anaerobic oxidation of methane (AOM) may (in part) account for most AC expressing C-13-depleted isotope compositions, including those with delta C-13 values between -25 (the approximate marine organic matter value) and 0 parts per thousand VPDB. Widespread increases in delta Ca-13, with depth in conjunction with values that exceed seawater DIC compositions indicate precipitation in sediments exhibiting methanogenesis, likely aided by contemporaneous marine silicate weathering. Deeper AC formation may occur in sediments experiencing thermal decarboxylation. When data from all sites are considered collectively, sediments experiencing AOM and methanogenesis emerge as the most significant in yielding AC. The relative importance of authigenesis pathways controls potential impacts of AC deposition on marine carbon budgets and provides insight into the geochemical signatures exhibited by ancient carbonate concretions.
      原文链接:
    • NSTL
    • Elsevier

    Copper and gallium isotopic behavior in highly weathered soils

    Liu, Jin-HuaCheng, JieZhou, LianFeng, Lan-Ping...
    20页
    查看更多>>摘要：The trace elements copper (Cu) and gallium (Ga) are of potential utility in geological studies related to weathering and pedogenic processes, as well as to paleoclimate changes. However, the behavior of Cu and Ga and their isotopic fractionation mechanisms during chemical weathering are not well understood at present. To address this issue, coupled Cu and Ga isotopes were measured in 17 samples from a sedimentologically and geochemically well-characterized weathering profile developed on Cenozoic basalts in Hainan Island, South China. Each sample was subjected to a sequential extraction procedure yielding four fractions of Cu and Ga (i.e., exchangeable, adsorbed, organic-bound, and silicate-hosted), for which isotopic compositions were separately determined in addition to analyses of the bulk sample. Relative to bedrock (delta Cu-65/63 = +0.07 +/- 0.04 parts per thousand relative to NIST 976), soil samples are both enriched and depleted in the heavy isotopes of Cu (delta Cu-65/63(bulk) = -0.29 to +0.24 parts per thousand). A positive correlation (r = +0.74) of delta Cu-65/63(bulk) with tau(Cu,Th) [= [(C-Cu/C-Th)sample / (C-Cu/C-Th)(parent) - 1] x 100%] reflects preferential leaching of heavier Cu isotopes out of the weathering profile. Lighter Cu isotopes were retained in the silicate-hosted fraction (delta Cu-65/63(sil) = -0.84 to -0.11 parts per thousand), and heavy Cu isotopes are enriched in the adsorbed fraction (delta Cu-65/63(ads) = +0.27 to +1.59 parts per thousand) relative to the exchangeable fraction (delta Cu-65/63(exch) = +0.06 to +0.44 parts per thousand), which consist mainly of highly crystalline and poorly crystalline Fe-hydroxides, respectively. Relative to bedrock (delta Ga-71/69 = 0.00 +/- 0.05 parts per thousand relative to the Ga-IPGP standard), soil samples are slightly enriched in the heavy isotopes of Ga (delta Ga-71/69(bulk) = +0.01 to +0.20 parts per thousand). In contrast to Cu, weathering preferentially releases the light isotopes of Ga, yielding higher delta Ga-71/69 values for the exchangeable (+0.54 to +1.18 parts per thousand) and adsorbed fractions (-0.01 to +0.55 parts per thousand). Reactions with Fe-hydroxides are the dominant control on Cu and Ga isotopic variation, with preferential uptake of heavier isotopes during adsorption and release of lighter isotopes during desorption. Ga substitution for Fe in tetrahedral and octahedral sites led to the preferential release of isotopically lighter Ga from primary minerals. These results advance our understanding of the isotopic fractionation mechanisms of Cu and Ga during chemical weathering and the potential utility of these isotopic systems as paleo-weathering proxies.
      原文链接:
    • NSTL
    • Elsevier

    Petrogenesis of the late Paleoproterozoic Gleibat Lafhouda dolomite carbonatite (West African Craton Margin, Moroccan Sahara) and its relevance to the onset of fragmentation of the Columbia supercontinent

    Bouabdellah, MohammedChakhmouradian, Anton R.Mouttaqi, AbdellahReguir, Ekaterina P....
    20页
    查看更多>>摘要：The Gleibat Lafhouda dolomite carbonatites of the Moroccan Sahara occur as three separate cone-shaped plugs intruding an autochthonous succession of Archean supracrustal basement rocks. Geochemically, the Gleibat Lafhouda dolomite carbonatites are characterized by a compositional range of 11.3-27.1 wt% MgO, 3.1-29.7 wt % CaO, 3.5-38.0 wt% FeOtot and < 0.1-7.5 wt% SiO2, and enrichment in large-ion lithophile elements (LILE), particularly Sr (2173-11,310 ppm), Ba (174-4537 ppm), U (0.1-296 ppm) and light REEs (LREEs) (131-1295 ppm), but not in the heavy REE (HREEs) and high-field strength elements (HFSE) such as Ti, Zr, and Hf. Nb and Ta show, however, much higher concentrations ranging from 0.5 ppm to 1.0 wt%, and < 0.0 to 199 ppm, respectively, which set them apart from naturally occurring carbonatites and the experimentally derived carbonated melts. The combined stable (delta C-13(V-PDB )= -2.5 to -6.6 parts per thousand, delta O-18(V)-SMOW = 6.0 to 20.7 parts per thousand) and radiogenic( 87)Sr/Sr-86(in), (0.7032-0.7046), Nd-143/Nd-144(in), (0.5105-0.5106) or epsilon Nd(t) (+ 3 to +6), and (206)pb/(204)pb (19.06-49.05), Pb-207/Pb-204 (15.90-18.87), and Pb-208/Pb-204 (37.87-38.50) isotope compositions are consistent with low degree partial melting, at convecting upper mantle conditions, of a predominantly depleted mantle source in a rift-related environment. Based on these geochemical features, it is suggested that the Gleibat Lafhouda dolomite carbonatites represent the earliest manifestation of rifting processes related to the fragmentation of the Columbia supercontinent at 1.85 Ga. Accordingly, we propose that these carbonatitic rocks represent the initial Mg-rich melt in the mantle plume head that derived from decompressional adiabatic melting of a depleted mantle source.
      原文链接:
    • NSTL
    • Elsevier

    Stability and molecular fractionation of ferrihydrite-bound organic carbon during iron reduction by dissolved sulfide

    Ma, Wei-WeiZhu, Mao-XuYang, Gui-PengLi, Tie...
    13页
    查看更多>>摘要：The interactions of organic carbon (OC) with ferric iron (Fe) oxides play an important role in OC stabilization, but reductive dissolution of Fe(III) oxides under anoxic conditions potentially compromises the stabilization of Fe-bound OC (Fe-OC). Up to date, there are very limited studies on the fate of Fe-OC in sulfidic conditions commonly encountered in OC-rich marine sediments. Herein, multi-pronged studies were implemented to quantitatively characterize the capacity of metastable iron sulfide (FeS) to sequester OC extracted from marine sediments and to unravel the fate of ferrihydrite-bound OC complexes (Fh-OCs) when exposed to dissolved sulfide. Results show that, for a given mechanism of OC association (adsorption or coprecipitation), FeS has DOC retention capacity generally comparable to Fh, implying that the metastable phase could serve as an important OC sink in anoxic marine sediments. During reduction of Fh-OCs by dissolved sulfide, Fh reduction was progressively retarded with increasing OC pre-adsorption, while Fh reduction was promoted after DOC pre-coprecipitation. Despite high extents of Fe reduction by dissolved sulfide, OC release from Fh-OC coprecipitates was low due to efficient re-sequestration of released DOC by coprecipitation with FeS, and an increase in OC pre-loading could facilitate Fe2+ sulfidization and thus re-sequestration of released DOC by FeS. When Fh-OC adsorption complexes were exposed to dissolved sulfide, substantial OC release was restricted to the complexes with lower OC pre-adsorption (C/Fe <= 1.5). Overall, reactive Fe reduction by dissolved sulfide may not lead to substantial release of associated OC as long as reduced Fe could be sulfidized to efficiently re-sequester concomitantly released DOC. Results of carbon-1 s near-edge X-ray absorption fine structure spectroscopy indicate that, upon reduction of Fh-OCs by dissolved sulfide and subsequent FeS formation, carboxyl-C was selectively released into the solution but aromatic-C selectively remained on the solid matrixes after Fe reduction, suggesting that OC sink switching from Fe oxides to Fe sulfides could result in molecular fractionations of porewater DOC to some extent, with potential implications for OC biogeochemical cycling.
      原文链接:
    • NSTL
    • Elsevier

    Comparative analysis of exploration potential within the Urumieh Dokhtar Magmatic Arc, Iran, with a detailed example from mineral chemistry of the Marshenan intrusion

    Sarjoughian, FatemehLentz, David R.Kazemi, KazemYousefi, Fazilat...
    20页
    查看更多>>摘要：The Early Miocene Marshenan intrusion, in the central part of the Urumieh-Dokhtar magmatic arc (UDMA), central Iran, includes granodiorite-granite and diorite; it is a classic example of a Cu-Au barren intrusive system. Zircon U-Pb dating indicates that the diorite display an age of 20.32 +/- 0.36 Ma, coeval with granodiorite rocks (20.5 +/- 0.8 Ma). These rocks are composed of feldspar, quartz, amphibole, biotite, titanite, and magnetite. In the granodiorites, the plagioclase composition ranges from oligoclase to bytownite, the amphiboles are magnesiohornblende and biotites is Mg rich, related to calc-alkaline orogenic suites in the region. Plagioclase phenocrysts exhibits oscillatory zoning and marked changes in the abundance of elements, such as Ba, Sr, and Fe, suggesting magma mixing/mingling may have had a role in generating these parental magmas. The average calculated P-T conditions of the granodiorite and diorite rocks are about 730 degrees C and 2.1 kbar and 733 degrees C and 1.7 kbar, respectively, corresponding to near solidus conditions equal to emplacement depths of similar to 7 to 8 km. Magmatic H2O contents and fO(2) calculated from crystallized amphiboles indicate that the Marshenan granodiorite had initial magmatic H2O contents similar to 5 wt% and relatively high fO(2) (Delta NNO; ave. 1.3) and the diorite had initial magmatic H2O contents similar to 4.7 wt% and relatively high fO(2) (Delta NNO; ave. 1.5), both consistent with the presence of phases, such as hornblende, biotite, magnetite, and titanite. In comparison with other intrusions in the UDMA, the Marshenan intrusion formed via the same physico-chemical mechanisms like other barren intrusions, whereas the fertile intrusions exhibit slightly higher temperatures, pressures, fO(2), and H2O values than the barren intrusions. These compiled data suggest that, in spite of the high magmatic H2O and fO(2) contents, the Marshenan and other barren intrusions in the UDMA will not produce porphyry Cu mineralization, unlike the giant Kerman deposit, probably due to magma source, magmatic evolution processes, timing of volatile exsolution, pre-existing crustal-scale fractures, coeval volcanism, and extended duration of volatile saturated crystallization to subsolidus conditions.
      原文链接:
    • NSTL
    • Elsevier

    Crystal-chemical variations of spinel, clinopyroxene, and plagioclase in MORB basaltic melt induced by continuous cooling

    Giuliani, LetiziaVetere, FrancescoIezzi, GianlucaNazzari, Manuela...
    21页
    查看更多>>摘要：In this study we present the compositional changes of clinopyroxene (cpx), plagioclase (plg), spinel (sp), and glass experimentally solidified from an Icelandic MORB melt. The starting material was cooled at P-atm and fO(2) of air, in the thermal range of cooling (Delta Tc) between 1300 degrees C (superliquidus) to 800 degrees C (solidus) with rates (Delta T/Delta t) of 1, 7, 60, 180, 1800, and 9000 degrees C/h. The run products obtained at 1, 7 and 60 degrees C/h are holocrystalline, whilst between 60 and 180 degrees C/h plg disappears, and texture of cpx + sp. shifts from faceted to dendritic. As cooling rate increases, we observe that Fe2O3 decreases and Al2O3 increases in sp. and Al2O3 + Fe2O3 increase and CaO + MgO decrease in cpx. These measured variations mirror changes induced by cooling rate in cation (atoms per formula unit, a.p.f.u.) and molecular abundances of these two crystalline phases. Plg composition shows clear linear trends versus cooling rate. The chemistry of sp., cpx and, to a minor extent, plg solidified from this basaltic liquid is thus strictly related to the cooling rate condition and is similar to those observed in previous investigations on alkaline and evolved basaltic systems. In particular, cpx is the only mineral phase profusely present at all the cooling rates, showing the greatest chemical variations in terms of oxides, cations, and components. The intra-crystalline glass (<= 50 mu m from crystal rims) obtained at 180-1800 degrees C/h shows compositional variations related to the surrounding crystal growth, evidencing strong supersaturation phenomena (such as dendritic texture) due to the establishment of a diffusion-controlled growth regime. Chemical attributes of mineral phases are also quantitatively related with the maximum (G(max)) and average (G(CSD)) growth rates of sp., cpx, and plg. When compared with the starting melt composition, the chemistry of cpx suggests the attainment of near-equilibrium crystallization conditions at cooling rate <= 60 degrees C/h, whereas disequilibrium effects are found at cooling rate > 60 degrees C/h. In contrast, plg is in disequilibrium with the initial melt chemistry in all experiments. By using thermometric models, the calculated crystallization of plg takes place at temperatures much lower than those of cpx, when the crystal content is high and the diffusion of cations in the melt is slow due to the higher (residual) melt viscosity. Under such conditions and due to the effect of cooling, the system cannot return to homogeneous concentrations and, consequently, plg more effectively records the disequilibrium partitioning of cations between the growing crystal surface. The data-set reported here captures the entire (superliquidus to solidus) and intrinsic (heterogeneous site-free silicate liquid) solidification behavior from an actual MORB melt from very rapid to extremely sluggish cooling rate. Finally, all analytical relationships found in this work enable careful reconstruction of the solidification conditions of MORB melts, providing novel geo-speedometers for them at high fO(2).
      原文链接:
    • NSTL
    • Elsevier