An experimental study of crystalline basalt dissolution from 2 ? pH ? 11 and temperatures from 5 to 75 °C

Steady-state element release rates from crystalline basalt dissolution at far-from-equilibrium were measured at pH from 2 to 11 and temperatures from 5 to 75 °C in mixed-flow reactors. Steady-state Si and Ca release rates exhibit a U-shaped variation with pH where rates decrease with increasing pH at acid condition but increase with increasing pH at alkaline conditions. Silicon release rates from crystalline basalt are comparable to Si release rates from basaltic glass of the same chemical composition at low pH and temperatures ?25 °C but slower at alkaline pH and temperatures ?50 °C. In contrast, Mg and Fe release rates decrease continuously with increasing pH at all temperatures. This behaviour is interpreted to stem from the contrasting dissolution behaviours of the three major minerals comprising the basalt: plagioclase, pyroxene, and olivine. Calcium is primarily present in plagioclase, which exhibits a U-shaped dissolution rate dependence on pH. In contrast, Mg and Fe are contained in pyroxene and olivine, minerals whose dissolution rates decrease monotonically with pH. As a result, crystalline basalt preferentially releases Mg and Fe relative to Ca at acidic conditions. The injection of acidic CO₂-charged fluids into crystalline basaltic terrain may, therefore, favour the formation of Mg and Fe carbonates rather than calcite. Element release rates estimated from the sum of the volume fraction normalized dissolution rates of plagioclase, pyroxene, and olivine are within one order of magnitude of those measured in this study.     

Response of the coccolithophores Emiliania huxleyi and Coccolithus braarudii to changing seawater Mg and Ca concentrations: Mg/Ca, Sr/Ca ratios and δCa, δMg of coccolith calcite

Calcium and magnesium concentrations in seawater have varied over geological time scales. On short time scales, variations in the major ion composition of seawater influences coccolithophorid physiology and the chemistry of biogenically produced coccoliths. Validation of those changes via controlled laboratory experiments is a crucial step in applying coccolithophorid based paleoproxies for the reconstruction of past environmental conditions. Therefore, we examined the response of two species of coccolithophores, Emiliania huxleyi and Coccolithus braarudii, to changes in the seawater Mg/Ca ratio (≈0.5 to 10 mol/mol) by either manipulating the magnesium or calcium concentration under controlled laboratory conditions. Concurrently, seawater Sr/Ca ratios were also modified (≈2 to 40 mmol/mol), while keeping salinity constant at 35. The physiological response was monitored by measurements of the cell growth rate as well as the production rates of particulate inorganic and organic carbon, and chlorophyll a. Additionally, coccolithophorid calcite was analyzed for its elemental composition (Sr/Ca and Mg/Ca) as well as isotope fractionation of calcium and magnesium (Δ^44/40Ca and Δ^26/24Mg). Our results reveal that physiological rates were substantially influenced by changes in seawater calcium rather than magnesium concentration within the range estimated to have occurred over the past 250 million years when coccolithophores appear in the fossil record. All physiological rates of E. huxleyi decreased at a calcium concentration above 25 mmol L⁻¹, whereas C. braarudii displayed a higher tolerance to increased seawater calcium concentrations. Partition coefficient of Sr was calculated as 0.36 ± 0.04 (±2σ) independent of species. Partition coefficient of Mg²⁺ increased with increasing seawater Ca²⁺ concentrations in both coccolithophore species. Calcium isotope fractionation was constant at 1.1 ± 0.1‰ (±2σ) and not altered by changes in seawater Mg/Ca ratio. There is a well-defined inverse linear relationship between calcium isotope fractionation and partition coefficient of Sr²⁺ in all experiments, suggesting similar controls on both proxies in the investigated species. Magnesium isotope ratios were relatively stable for seawater Mg/Ca ratios ranging from 1 to 5, with a higher degree of fractionation in Emiliania huxleyi (by ≈0.2‰ in Δ^26/24Mg). Although Mg/Ca ratios in the calcite of coccolithophores and foraminifera are similar, the former have considerably higher Δ^26/24Mg (by >+3‰), presumably due to differences in calcification mechanisms between the two taxa. These observations suggest, a physiological control over magnesium elemental and isotopic fractionation during the process of calcification in coccolithophores.     

Are the seismological and geological observations of the Al Hoceima (Morocco, Rif) 2004 earthquake (M = 6.3) contradictory?

Seismic hazard is associated with recent and present fault activity in mountain ranges. In the Betic-Rif alpine mountain chain, tectonic activity started in the Cretaceous, and topographic uplift continues since Tortonian times as a consequence of the NW-SE oblique convergence between Africa and Eurasia. The deformation is active and produces seismicity that sometimes has catastrophic consequences. The Al Hoceima earthquake (February 24, 2004), considered one of the largest earthquakes ever recorded instrumentally in the westernmost Mediterranean (M = 6.3), caused great damage in the region. Seismological studies agree that the main shock was situated on land, at the limit between the External and Internal Zones of the Rif, at a depth of 10-14 km. The focal mechanism points to a strike-slip solution with a NW-SE oriented P axis, quite similar to those of the significant 1994 earthquake swarm located to the north. The epicenter aftershocks distribution would signal the presence of a N-S oriented sinistral fault, activated by the NW-SE regional compression associated to plate boundary convergence. In this setting, the seismogenic fault ruptures related to these seismic events are expected to have reached the Earth's surface. However, detailed field work carried out 1 month after the earthquake does not evidence any N-S strike-slip coseismic fault in the epicentral area. The main observed effects were landslides, damages to constructions, and locally open cracks indicating an unexpected NW-SE extension. Scarce N-S faults are normal, the main ones being located several kilometers away from the epicentral area. To explain this apparent contradiction between geological and seismological observations, we propose a decoupled tectonic model with crustal detachments that separate a deep brittle crust from an upper crust undergoing uplift, and the development of large folds and normal faults. This geological setting, common to internal zones of cordilleras, may need to be taken into account in future paleoseismicity studies and in the assessment of seismic hazard.     

Hydrated goethite (α-FeOOH) (1 0 0) interface structure: Ordered water and surface functional groups

Goethite(α-FeOOH), an abundant and highly reactive iron oxyhydroxide mineral, has been the subject of numerous studies of environmental interface reactivity. However, such studies have been hampered by the lack of experimental constraints on aqueous interface structure, and especially of the surface water molecular arrangements. Structural information of this type is crucial because reactivity is dictated by the nature of the surface functional groups and the structure or distribution of water and electrolyte at the solid-solution interface. In this study we have investigated the goethite (1 0 0) surface using surface diffraction techniques, and have determined the relaxed surface structure, the surface functional groups, and the three dimensional nature of two distinct sorbed water layers. The crystal truncation rod (CTR) results show that the interface structure consists of a double hydroxyl, double water terminated interface with significant atom relaxations. Further, the double hydroxyl terminated surface dominates with an 89% contribution having a chiral subdomain structure on the (1 0 0) cleavage faces. The proposed interface stoichiometry is ((H₂O)(H₂O)OH₂OHFeOOFeR) with two types of terminal hydroxyls; a bidentate (B-type) hydroxo group and a monodentate (A-type) aquo group. Using the bond-valence approach the protonation states of the terminal hydroxyls are predicted to be OH type (bidentate hydroxyl with oxygen coupled to two Fe³⁺ ions) and OH₂ type (monodentate hydroxyl with oxygen tied to only one Fe³⁺). A double layer three dimensional ordered water structure at the interface was determined from refinement of fits to the experimental data. Application of bond-valence constraints to the terminal hydroxyls with appropriate rotation of the water dipole moments allowed a plausible dipole orientation model as predicted. The structural results are discussed in terms of protonation and H-bonding at the interface, and the results provide an ideal basis for testing theoretical predictions of characteristic surface properties such as pK_a , sorption equilibria, and surface water permittivity.     

Devil in the detail; The 1150–1000 Ma magmatic and structural evolution of the Ngaanyatjarra Rift,west Musgrave Province,Central Australia

Mafic and felsic rocks units of the Musgrave Province originally attributed to the c. 1075 Ma Giles Event of the greater Warakurna Large Igneous Province (LIP) are shown to be part of a complex sequence of magmatic and tectonic events punctuated over a span of at least 50 m.y. New geochronology and mapping resolve a sequence of at least 10 magmatic pulses with hiati of up to 10 m.y. consistent with a long-lived intracontinental rift setting. This rift, here named the Ngaanyatjarra Rift, features giant layered mafic-ultramafic Giles intrusions cut by a 10 km wide mafic-felsic magmatic shear zone. The latter is temporally related to the Warakurna LIP, however it is not clear that the Giles intrusions actually form part of the Warakurna LIP. Macroscopic folding and the formation of the large synmagmatic transpressional shear zone attest to synmagmatic basin inversion in the early stages of the rift. The extensive mafic to felsic volcanic rocks of the Tollu Group (traditionally grouped with the Giles Event) were emplaced 25–50 m.y. later than the c. 1075 Ma Warakurna LIP.     

The origin of high δ<Superscript>18</Superscript>O zircons: marbles,megacrysts, and metamorphism

The oxygen isotope ratios (δ¹⁸O) of most igneous zircons range from 5 to 8‰, with 99% of published values from 1345 rocks below 10‰. Metamorphic zircons from quartzite, metapelite, metabasite, and eclogite record δ¹⁸O values from 5 to 17‰, with 99% below 15‰. However, zircons with anomalously high δ¹⁸O, up to 23‰, have been reported in detrital suites; source rocks for these unusual zircons have not been identified. We report data for zircons from Sri Lanka and Myanmar that constrain a metamorphic petrogenesis for anomalously high δ¹⁸O in zircon. A suite of 28 large detrital zircon megacrysts from Mogok (Myanmar) analyzed by laser fluorination yields δ¹⁸O from 9.4 to 25.5‰. The U–Pb standard, CZ3, a large detrital zircon megacryst from Sri Lanka, yields δ¹⁸O = 15.4 ± 0.1‰ (2 SE) by ion microprobe. A euhedral unzoned zircon in a thin section of Sri Lanka granulite facies calcite marble yields δ¹⁸O = 19.4‰ by ion microprobe and confirms a metamorphic petrogenesis of zircon in marble. Small oxygen isotope fractionations between zircon and most minerals require a high δ¹⁸O source for the high δ¹⁸O zircons. Predicted equilibrium values of Δ¹⁸O(calcite-zircon) = 2–3‰ from 800 to 600°C show that metamorphic zircon crystallizing in a high δ¹⁸O marble will have high δ¹⁸O. The high δ¹⁸O zircons (>15‰) from both Sri Lanka and Mogok overlap the values of primary marine carbonates, and marbles are known detrital gemstone sources in both localities. The high δ¹⁸O zircons are thus metamorphic; the 15–25‰ zircon values are consistent with a marble origin in a rock-dominated system (i.e., low fluid_(external)/rock); the lower δ¹⁸O zircon values (9–15‰) are consistent with an origin in an external fluid-dominated system, such as skarn derived from marble, although many non-metasomatized marbles also fall in this range of δ¹⁸O. High δ¹⁸O (>15‰) and the absence of zoning can thus be used as a tracer to identify a marble source for high δ¹⁸O detrital zircons; this recognition can aid provenance studies in complex metamorphic terranes where age determinations alone may not allow discrimination of coeval source rocks. Metamorphic zircon megacrysts have not been reported previously and appear to be associated with high-grade marble. Identification of high δ¹⁸O zircons can also aid geochronology studies that seek to date high-grade metamorphic events due to the ability to distinguish metamorphic from detrital zircons in marble.     

Inner-Helmholtz potential development at the hematite (α-Fe2O3) (0 0 1) surface

Electric potentials of the (0 0 1) surface of hematite were measured as a function of pH and ionic strength in solutions of sodium nitrate and oxalic acid using the single-crystal electrode approach. The surface is predominantly charge-neutral in the pH 4-14 range, and develops a positive surface potential below pH 4 due to protonation of μ-OH⁰ sites (pK_1,1,0,int = −1.32). This site is resilient to deprotonation up to at least pH 14 (−pK_−1,1,0,int ? 19). The associated Stern layer capacitance of 0.31-0.73 F/m² is smaller than typical values of powders, and possibly arises from a lower degree of surface solvation. Acid-promoted dissolution under elevated concentrations of HNO₃ etches the (0 0 1) surface, yielding a convoluted surface populated by sites. The resulting surface potential was therefore larger under these conditions than in the absence of dissolution. Oxalate ions also promoted (0 0 1) dissolution. Associated electric potentials were strongly negative, with values as large as −0.5 V, possibly from metal-bonded interactions with oxalate. The hematite surface can also acquire negative potentials in the pH 7-11 range due to surface complexation and/or precipitation of iron species (0.0038 Fe/nm²) produced from acidic conditions. Oxalate-bearing systems also result in negative potentials in the same pH range, and may include ferric-oxalate surface complexes and/or surface precipitates. All measurements can be modeled by a thermodynamic model that can be used to predict inner-Helmholtz potentials of hematite surfaces.     

U-Pb systematics of zircon and titanite from the Gardnos impact structure, Norway: Evidence for impact at 546 Ma?

Zircon and titanite were investigated in impactites of the Gardnos structure, a crater formed in Sveconorwegian (ca. 1 Ga) crust, which was then overridden in the Devonian by Caledonian nappes. Observed deformation features in zircons are granular texture, planar microstructures, and likely the incorporation of organic carbon during impact causing black staining of the zircon grains. The grains were studied by scanning electron microscopy (SEM) and cathode luminescence (CL) and dated by U-Pb isotope dilution - thermo-ionization mass spectrometry (ID-TIMS). Zircon grains without impact related features have U-Pb data showing moderate discordance (5-13%) and indicating formation ages mostly in the range of 1600-1000 Ma, except detrital zircon ages as old as >2481 Ma, reflecting the diversity of target rocks in the area. Titanite with concordant ages of 995-999 Ma dates metamorphism during final juxtaposition of the Telemarkia on the Idefjorden terrane to the east. Zircon grains with demonstrated or presumed shock features yield highly discordant (14-40%) U-Pb data, with a majority of them plotting along an array with a lower intercept of about 340 Ma reflecting the influence of the Caledonian orogeny and recent Pb-loss. One zircon grain was totally reset at 379 Ma during late Caledonian metamorphism, which also caused local growth of new titanite. A specific group of zircon grains yields data with relatively high discordance for moderate U contents, and five of these analyses, including that of a grain with proven granular or aggregate texture, fit a discordia line with an upper intercept of 546 ± 5 Ma. These features are interpreted as indicating zircon break-down to an amorphous state during impact, with subsequent recrystallization into microcrystalline aggregates causing extensive to complete Pb loss. We further suggest that their crystallinity prevented Pb loss during the Caledonian orogeny, while the small subgrain size and increasing metamictisation allowed more recent disturbances. We thus interpret the 546 Ma age as the approximate time of impact.     

Spatio-temporal variations of δ<Superscript>2</Superscript>H and δ<Superscript>18</Superscript>O in precipitation and shallow groundwater in the Hilly Loess Region of the Loess Plateau,China

Groundwater is of utmost significance to socio-economic development and ecological recovery for the Loess Plateau. However, studies regarding the mechanism governing groundwater recharge over this area appear to be inadequate. This study is to examine the spatio-temporal variations of δ²H and δ¹⁸O in precipitation and shallow groundwater. On the basis of this, the mechanisms governing shallow groundwater recharge were explored. Precipitation and groundwater were sampled monthly from May to October during the period 2004–2006 at 13 sites in the Chabagou Catchment (187 km²). In the Caopingxigou Experimental Watershed (0.1 km²), meteorological variables were observed and rainfall larger than 5 mm was sampled immediately after each rain event. Across the area, 90% of the precipitation occurred from May to September primarily in the form of heavy rains or rainstorms with great spatial variability. There were about 30 localized rains in each year. It was indicated that there existed notable seasonality and pronounced spatial variability in precipitation isotopic compositions. Contributing factors and indications of isotopic compositions, as well as their climatic indications such as monsoon intensities and mixing processes of water vapor, were investigated. The δ²H–δ¹⁸O relation of groundwater was found to be δ²H = 3.22 × δ¹⁸O − 38.1, deviating from the local meteoric water line δ²H = 7.57 × δ¹⁸O + 3.9. The range of δ values in groundwater is shrunken to be 15–21% of that in individual precipitations, and groundwater in the middle reaches shows a wider range of δ values. Isotopic results showed that groundwater originates from precipitation with hydrogen and oxygen isotopic compositions being −69 and −9.7‰, respectively, and most groundwater experiences serious evaporation and adequate mixing with old water during infiltration or percolation in the aerated zone. It was also founded that obvious fluctuations of isotopic compositions in groundwater mainly appear in the middle reaches especially at sites that are close to valleys, suggesting varying sources of groundwater from precipitation, precipitation runoff, isotopically enriched surface water and/or lateral recharge of adjacent groundwater.     

Sr chemostratigrphy, δ<Superscript>13</Superscript>C,and δ<Superscript>18</Superscript>O of rocks in the Crimean carbonate platform (Late Jurassic,northern Peri-Tethys)

The paper presents the results of study of the Sr, C, and O isotope compositions in Upper Jurassic carbonate rocks of the Baidar Valley and Demerdzhi Plateau in the Crimean Mountains represented by different facies of the carbonate platform at the northern active margin of the Tethys. The ⁸⁷Sr/⁸⁶Sr value in them varies from 0.70699 to 0.70728. Based on the Sr chemostratigraphic correlation, the age of massive and layered limestones in the western part of the Ai-Petri and Baidar yailas (pastures) is estimated as late Kimmeridgian–early Tithonian, whereas the age of flyschoids of the Baidar Valley are estimated as late Tithonian–early Berriasian. The nearly synchronous formation of carbonate breccias of the Baidar Valley and Demerdzhi Plateau in late Tithonian–early Berriasian is substantiated. A summary section of Upper Jurassic rocks is compiled based on the Sr chemostratigraphic data. It has been established that δ¹⁸O values in the studied carbonate sediments vary from–2.9 to 1.3‰ (V-PDB). At the same time, shallow-water sediments in the internal part and the edge of the Crimean carbonate platform are depleted in ¹⁸O (–2.9 to +0.1‰) relative to sediments on the slope and foothill (–0.5 to +1.3‰). It is demonstrated that δ¹³C values do not depend on the facies properties and decrease in younger carbonate sediments from 3–3.5‰ to 1–1.5‰ in line with the Late Jurassic general trend. The δ¹³C values obtained for the Crimean carbonate platform turned out to be 0.5–1‰ higher than the values typical of the deep-water marine setting at the western margin of the Tethys. These discrepancies are likely related to peculiarities of water circulation and high bioproductivity in marine waters of the northern Peri-Tethys.     

Reconstruction of palaeohydrological conditions in a lagoon during the 2nd Zechstein cycle through simultaneous use of δD values of individual n-alkanes and δ<Superscript>18</Superscript>O and δ<Superscript>13</Superscript>C values of carbonates

For the first time ¹⁸O and ¹³C values from carbonates and D values of individual n-alkanes were used to reconstruct palaeohydrological conditions in a lagoon at the southern margin of the Central European Zechstein Basin (CEZB). A 12-m core covering the complete Ca2 interval and adjacent anhydrites (A1 and A2) was analyzed for ¹⁸O and ¹³C values of dolomitized carbonates and D values of individual n-alkanes. ¹⁸O_carb values (+2 to +5 vs. VPDB) were strongly influenced by evaporation and temporal freshwater input into the lagoon. The ¹³C_carb values (–1 to +4 vs. VPDB) were controlled mainly by changes in primary production. Both isotopic ratios show an inverse relationship throughout most of the core, contradicting diagenetic alteration, since ¹³C_carb values are not altered significantly during dolomitization. Assuming a temperature range of 35–40 °C in the lagoon, ¹⁸O_carb values of +2.5 to +8 (vs. VSMOW) for the lagoonal water can be reconstructed. The lagoon may have desiccated twice during the Ca2 interval, as indicated by very high ¹⁸O_carb and low ¹³C_carb values, coinciding with increasing amount of anhydrite in the analyzed sample. These events seem to reflect not just local but a regional intra-Ca2 cyclicity. Measured D values of the short-chain n-alkanes, namely n-C₁₆ and n-C₁₈ which are widely used as indicators for photosynthetic bacterial and algal input, reflect the isotopic composition of the lagoonal water. Assuming constant fractionation during incorporation of hydrogen into lipids of –160, an average D value of +70 (vs. VSMOW) can be reconstructed for the lagoonal water, accounting for very arid conditions. The long-chain n-alkanes n-C₂₇, n-C₂₈, n-C₂₉ and n-C₃₀, thought to be derived from the leaf waxes of terrestrial higher plants, were shown to be depleted in D relative to the short-chain alkanes, therefore indicating a different hydrogen source. Terrestrial plants in arid areas mainly use water supplied by precipitation. By using a smaller fractionation of –120 due to evaporation processes in the leaves, reconstructed values vary between –74 and –9 (vs. VSMOW). These values are not indicating extremely arid conditions, implying that the long-chain n-alkanes were transported trough wind and/or rivers into the lagoon at the Zechstein Sea coast. D_water values, reconstructed using the n-C₁₆ alkane and ¹⁸O _water values, independently reconstructed on the same sample using the temperature dependant fractionation for dolomites are good agreement and suggest high amounts of evaporation affecting the coastal lagoon. Altogether, our results indicate that hydrogen isotopic ratios of n-alkanes give information on their origin and are a useful proxy for palaeoclimatic reconstruction.     

A 30,000 yr record of land–ocean interaction in the eastern Gulf of Guinea

A 30,000 yr dinocyst and pollen record from the eastern equatorial Atlantic (off Cameroon) has been investigated in order to identify land–ocean linkages during the last deglacial transition. A strong correlation between the abundance of Brigantedinium spp. and the Ca/Fe ratio during the last glacial period suggests enhanced marine productivity in association with cool seawater temperatures and nutrient input linked to coastal upwelling and/or a proximal river mouth. Dry conditions are recorded on the adjacent continent with a significant representation of open vegetation indicators and the Afromontane taxon Podocarpus. After 17 cal ka BP these indicators register a sharp decline as a result of a climatic transition from the dry/cooler conditions of the last glacial period to the wetter/warmer conditions of the deglaciation. Simultaneously, dinocysts show a significant shift from dominant heterotrophs to an increasing abundance of autotrophs, reflecting warmer conditions. Significant changes are observed during the Younger Dryas, with a return to drier conditions and higher salinities. The start of the Holocene is marked by very low-salinity conditions, reflecting optimal monsoonal conditions over west equatorial Africa. The end of the African Humid Period is observed between 6 and 5 cal ka BP, followed by significant fluctuations in both terrestrial and oceanic proxies.     

Origin and evolution of oilfield brines from Tertiary strata in western Qaidam Basin: Constraints from <Superscript>87</Superscript>Sr/<Superscript>86</Superscript>Sr, δD, δ<Superscript>18</Superscript>O, δ<Superscript>34</Superscript>S and water chemistry

Chemistry of major and minor elements, ⁸⁷Sr/⁸⁶Sr, δD, δ¹⁸O and δ³⁴S of brines were measured from Tertiary strata and Quaternary salt lakes in the western Qaidam Basin. The water chemistry data show that all oilfield brines are CaCl₂ type. They were enriched in Ca²⁺, B³⁺, Li⁺, Sr²⁺, Br⁻, and were depleted in Mg²⁺, SO₄ ²⁻, which indicated that these brines had the characteristics of deeply circulated water. The relationship between δD and δ¹⁸O shows that all data of these brines decline towards the Global Meteoric Water Line (GWL) and Qaidam Meteoric Water Line (QWL), and that the intersection between oilfield brines and Meteoric Water Lines was close to the local spring and fresh water in the piedmont in the western Qaidam Basin. The results suggest that oilfield brines has initially originated from meteoric water, and then might be affected by water-rock metamorphose, because most oilfield brines distribute in the range of metamorphosing water. The ⁸⁷Sr/⁸⁶Sr values of most oilfield brines range from 0.71121 to 0.71194, and was less than that in salt lake water (>0.712), but close to that of halite in the study area. These imply that salt dissolution occurred in the process of migration. In addition, all oilfield brines have obviously much positive δ³⁴S values (ranging from 26.46‰ to 54.57‰) than that of salt lake brines, which was caused by bacterial sulfate reduction resulting in positive shift of δ³⁴S value and depleteed SO₄ ²⁻ in oilfield brines. Combined with water chemical data and δD, δ¹⁸O, ⁸⁷Sr/⁸⁶Sr, δ³⁴S values, we concluded that oilfield brines mainly originate from the deeply circulated meteoric waters, and then are affected by salt dissolution, water-rock metamorphose, sulfate reduction and dolomitization during the process of migration. These processes alter the chemical compositions of oilfield brines and accumulate rich elements (such as B, Li, Sr, Br, K and so on) for sustainable utilization of salt lake resources in the Qaidam Basin.     

Interdiffusion of Na,K,and Ca Between Granitic Melts and NaCl Liquid

This study is aimed at determining the diffusion coefficient of net-work modifiers (mainly Na, K, and Ca) in a two-phase melt-NaCl system, in which the melts are granitic and the system is NaCl-rich in composition. The diffusion coefficients of Na, K, and Ca were measured at the temperatures of 750 – 1400°C, pressures of 0.001 × 10⁸ – 2 × 10⁸ Pa, and initial H₂O contents of 0 wt% –6.9 wt% in the granitic melts. The diffusion coefficients of Fe and Mg were difficult to resolve. In all experiments a NaCl melt was present as well. In the absence of H₂O, the diffusion of net-work modifiers follows an Arrhanious equation at 1 × 10⁵ Pa: lgD_ca=−3. 88−5140/T, lgD_k =−3. 79−4040/T, and lgD_Na, =−4.99−3350/T, where D is in cm² /s andT is in K. The diffusion coefficients of Ca, Na, K, and Fe increase non-linearly with increasing H₂O content in the melt. The presence of about 2 wt% H₂O m the melt will lead to a dramatical increase in diffusivity, but higher H₂O content has only a minor effect. This change is probably the result of a change in the melt structure when H₂O is present. The diffusion coefficients measured in this study are significantly different from those in previous works. This may be understood in terms of the “transient two-liquid equilibrium” theory. Element interdiffusion depends not only on its concentration, but also on its activity co-efficient gradient, which is reflected by the distribution coefficient, of the two contacting melts.     

Dissolution of sandstone powders in deionised water over the range 50–350 °C

Sandstone dissolution is a common water–rock reaction in the Earth’s crust, but a thorough understanding of this phenomenon is constrained by poorly determined kinetic data. To this end, kinetic data were determined for the dissolution of arkosic sandstone powders in deionised water (pH was about 7.0–7.3 and electrical conductivity was between 0.95 and 1.00 μS/cm). Release rates of dissolved elements were determined over the range 50–350 °C at 20, 15, and 10 MPa using a column flow-through pressure vessel reactor. The conductivity of the outlet solution, measured at room temperature, is dependent on the charge of major cations such as Na⁺, K⁺, Ca²⁺ and Mg²⁺ at these conditions. The conductivity of the outlet solution was used to determine the steady state of the dissolution of sandstone powders. The pH values of the outlet solutions at the steady state, measured ex situ at room temperature, were about 7.7, 8.3, 8.4, 8.4 and 7.6 at 75, 100, 150, 200 and 250 °C, respectively, at 10 MPa. Silicon, Na, K, Ca, Al and Mg are the major ions found in the solution at low temperatures, but Si is the only major ion retained at higher temperatures (>150 °C). Compared with static experiments, the flowing dissolution experiments occurred at conditions far from equilibrium. The relationship between temperature and dissolution rates of arkosic sandstone powders was described as log R = 0.005469t − 10.50 where R is the dissolution rates of sandstone powders in kg/(m² s), t is temperature in °C which ranged from 100 to 350 °C at 20 and 15 MPa, and the dissolution rates of sandstone powders were measured only for the major dissolved elements without oxygen in the outlet solutions.     

U–Pb geochronology of the Western Channel Diabase,northwestern Laurentia: Implications for a large 1.59 Ga magmatic province,Laurentia's APWP and paleocontinental reconstructions of Laurentia,Baltica and Gawler craton of southern Australia

U–Pb baddeleyite ages of 1592 ± 3 and 1590 ± 4 Ma are reported for paleomagnetic sites in sheets and dykes of Western Channel Diabase (WCD) that intrude Proterozoic rocks of the flat-lying Hornby Bay Group in the Hornby Bay basin and the deformed volcanic-plutonic Great Bear Magmatic Zone of Wopmay orogen of northwestern Laurentia. A published WCD paleomagnetic pole at 9°N, 115°W (A₉₅ = 6°) has been demonstrated primary. The new ages indicate that the WCD pole falls midway in time between poles for the 1.74 Ga Cleaver dykes and 1.48–1.42 Ga Elsonian-aged plutons, filling an important gap in the Proterozoic apparent polar wander path (APWP) for Laurentia. The WCD pole can be compared with poles reported from similar-aged magmatic units on other cratons in order to test paleocontinental reconstructions. A comparison of the Laurentian WCD pole with primary ca. 1.63 Ga and ca. 1.575 Ga poles for Baltica, along with an earlier comparison of precisely dated 1.27–1.255 Ga poles for Laurentia and Baltica, suggests that the two cratonic blocks drifted as a single entity with Baltica adjacent to eastern Greenland during the ca. 1.59–1.27 Ga interval. On the basis of less well constrained ca. 1.84–1.83 Ga poles from Laurentia and Baltica, it is possible that this reconstruction existed as early as ca. 1.83 Ga. The WCD is the same age as Wernecke breccias of the Wernecke and Ogilvie Mountains of northwestern Laurentia and bimodal Gawler Range Volcanics (GRV) and related Olympic Dam breccias of the Gawler craton. It has been proposed by others that the Gawler craton lay adjacent to northwestern Laurentia at 1.59 Ga, with the Olympic Dam and Wernecke breccias forming a large hydrothermal province. The primary WCD pole provides an opportunity to test Laurentia–Gawler craton reconstructions at 1.59 Ga. A paleopole has been reported for the GRV, although its primary or secondary nature is open to interpretation. If primary, or if acquired as an overprint during the later stages of 1.60–1.58 Ga Hiltaba-GRV magmatism, then a position for the Gawler craton adjacent to northwestern Laurentia is permitted. If the GRV pole is a later secondary overprint then a reliable comparison with Laurentian poles cannot be made.     

The record of temperature, wind velocity and air humidity in the δD and δO of water inclusions in synthetic and Messinian halites

Deuterium and oxygen isotope fractionations between liquid and vapor water were experimentally-determined during evaporation of a NaCl solution (35 g L⁻¹) as a function of water temperature and wind velocity. In the case of a null wind velocity, slopes of δD-δ¹⁸O trajectories of residual waters hyperbolically decrease with increasing water temperatures in the range 23-47 °C. For wind velocities ranging from 0.8 to 2.2 m s⁻¹, slopes of the δD-δ¹⁸O trajectories linearly increase with increasing wind velocity at a given water temperature. These experimental results can be modeled by using Rayleigh distillation equations taking into account wind-related kinetics effects. Deuterium and oxygen isotope compositions of water inclusions trapped by the precipitated halite crystals were determined by micro-equilibration techniques.These isotopic compositions accurately reflect those of the surrounding residual waters during halite growth. Isotopic compositions of water inclusions in twenty natural halites from the Messinian Realmonte mine in Sicily suggest precipitation temperatures of that match the homogenization temperatures obtained by microthermometry (median = 34 ± 5 °C). The similarity between the measured and experimental slopes of the δD-δ¹⁸O evaporation trajectories suggests that the effect of wind was negligible during the genesis of these halite deposits. Hydrogen and oxygen isotope compositions of water inclusions from Realmonte halite also define a linear trend whose extrapolation until intersection with the Mediterranean Meteoric Water Line allows the characterization of the water source with δD and δ¹⁸O values of −70 ± 10‰ and −11.5 ± 1.5‰, respectively. These results reveal that the huge amounts of salts deposited in Sicily result from the evaporation of seawater mixed with a dominant fraction (?50%) of meteoric waters most likely deriving from alpine fluvial discharge.     

Petroleum geochemistry of the Potwar Basin, Pakistan: 1. Oil-oil correlation using biomarkers, δC and δD

Geochemical characterisation of 18 crude oils from the Potwar Basin (Upper Indus), Pakistan is carried out in this study. Their relative thermal maturities, environment of deposition, source of organic matter (OM) and the extent of biodegradation based on the hydrocarbon (HC) distributions are investigated. A detailed oil-oil correlation of the area is established. Gas chromatography-mass spectrometry (GC-MS) analyses and bulk stable carbon and hydrogen isotopic compositions of saturated and aromatic HC fractions reveals three compositional groups of oils. Most of the oils from the basin are typically generated from shallow marine source rocks. However, group A contains terrigenous OM deposited under highly oxic/fluvio-deltaic conditions reflected by high pristane/phytane (Pr/Ph), C₃₀ diahopane/C₂₉Ts, diahopane/hopane and diasterane/sterane ratios and low dibenzothiophene (DBT)/phenanthrene (P) ratios. The abundance of C₁₉-tricyclic and C₂₄-tetracyclic terpanes are consistent with a predominant terrigenous OM source for group A. Saturated HC biomarker parameters from the rest of the oils show a predominant marine origin, however groups B and C are clearly separated by bulk δ¹³C and δD and the distributions of the saturated HC fractions supporting variations in source and environment of deposition of their respective source rocks. Moreover, various saturated HC biomarker ratios such as steranes/hopanes, diasteranes/steranes, C₂₃-tricyclic/C₃₀ hopane, C₂₈-tricyclic/C₃₀ hopane, total tricyclic terpanes/hopanes and C₃₁(R + S)/C₃₀ hopane show that two different groups are present. These biomarker ratios show that group B oils are generated from clastic-rich source rocks deposited under more suboxic depositional environments compared to group C oils. Group C oils show a relatively higher input of algal mixed with terrigenous OM, supported by the abundance of extended tricyclic terpanes (up to C₄₁₊) and steranes.Biomarker thermal maturity parameters mostly reached to their equilibrium values indicating that the source rocks for Potwar Basin oils must have reached the early to peak oil generation window, while aromatic HC parameters suggest up to late oil window thermal maturity. The extent of biodegradation of the Potwar Basin oils is determined using various saturated HC parameters and variations in bulk properties such as API gravity. Groups A and C oils are not biodegraded and show mature HC profiles, while some of the oils from group B show minor levels of biodegradation consistent with high Pr/n-C₁₇, Ph/n-C₁₈ and low API gravities.     

Fluid Composition, δD of Channel H2O,and δ18O of Lattice Oxygen in Beryls: Genetic Implications for Brazilian,Colombian, and Afghanistani Emerald Deposits

The fluid composition, δD of channel H₂O, and δ₁₈O of lattice oxygen have been determined in beryl and emerald from a variety of geological environments and used to constrain the origin of the parental fluids from which beryl has grown. Step-heating analyses performed by quadrupolar mass spectrometry were used to quantify the composition of the fluid phases in beryl from granitic pegmatites and greisens and emerald from Brazil, Colombia, and Afghanistan. An important conclusion is that beryl and emerald have a similar fluid composition, with concentrations of H₂O being greater than 90% of the total water in the mineral irrespective of the age of formation (2.0 Ga to 32 Ma) and tectonic settings. However, the Brazilian Santa Terezinha shear-zone emerald deposit contains abundant CO₂, up to 13 wt% of the total fluid. A second conclusion is that the channel H₂O content for some Brazilian emeralds is higher than the range defined for beryl in the literature, especially for those related to the shear-zone type (2.99 lt; H₂O < 3.16 wt%) and the pegmatite type from the Pombos, Pela Ema, and Pirenopolis deposits (2.78 < H₂O < 3.01 wt%). Colombian emeralds have very low H₂O contents (1.30 < H₂O < 1.96 wt%), among the lowest in the world.

Brazilian, Colombian, and Afghanistani emeralds have contrasting and restricted ranges of δ¹⁸O values. In Brazil, emeralds related to pegmatites have a systematic δ¹⁸O inter-deposit variability (+6.3 < δ¹⁸O < +12.4‰). The calculated δ¹⁸O of the fluid was buffered by the host ultrabasic rocks during fluid-rock interaction. Emerald and cogenetic phlogopite related to shear-zone-type deposits have a quite restricted δ¹⁸O range (+12.0 < δ¹⁸O 7lt; +12.4‰); the calculated is interpreted to represent the original isotopic composition of the hydrothermal fluid. Relative to Brazil, the δ¹⁸O of Colombian and Afghanistani emeralds shows strong enrichment in ¹⁸O (+13.4 < δ¹⁸O < +23.6‰), and the high calculated δ¹⁸O of the fluid suggests extensive reaction with 18O-rich sedimentary or metasedimentary rocks.

In Brazil, the δD composition of channels in emerald and the calculated δ¹⁸O^H₂O for phlogopite are compatible with both magmatic and metamorphic origins. A magmatic origin is supported for emeralds associated with the pegmatitic Socotó and Carnaiba deposits (mean δD = ?37.8 ± 8‰) and a metamorphic origin is suggested for the Santa Terezinha shear-zone type (mean δD = ?32.4 ± 3‰). A metamorphic origin is proposed for Colombian emeralds. Afghanistani emeralds have a δD composition of channels (mean δD = ?46.3 ± 1.3‰) that is compatible with both magmatic and metamorphic origins.     

Controls on δ34S and δ18O of dissolved sulfate in aquifers of the Murray Basin,Australia and their use as indicators of flow processes

The usefulness of stable isotopes of dissolved SO₄ (δ³⁴S and δ¹⁸O) to study recharge processes and to identify areas of significant inter-aquifer mixing was evaluated in a large, semi-arid groundwater basin in south-eastern Australia (the Murray Basin). The distinct isotopic signatures in the oxidizing unconfined Murray Group Aquifer and the deeper reducing Renmark Group confined aquifer may be more sensitive than conventional chemical tracers in establishing aquifer connections. δ³⁴S values in the unconfined Murray Group Aquifer in the south and central part of the study area decrease along the hydraulic gradient from 20.8 to 0.3‰. The concomitant increasing SO₄/Cl ratios, as well as relatively low δ¹⁸O_SO4 values, suggest that vertical input of biogenically derived SO₄ via diffuse recharge is the predominant source of dissolved SO₄ to the aquifer. Further along the hydraulic gradient towards the discharge area near the River Murray, δ³⁴S values in the unconfined Murray Group Aquifer increase, and SO₄/Cl ratios decrease, due to upward leakage of waters from the confined Renmark Group Aquifer which has a distinctly low SO₄/Cl and high δ³⁴S (14.9–56.4‰). Relatively positive δ³⁴S and δ¹⁸O_SO4 values, and low SO₄/Cl in the Renmark Group Aquifer is typical of SO₄ removal by bacterial reduction. The S isotope fractionation between SO₄ and HS⁻ of ∼24‰ estimated for the confined aquifer is similar to the experimentally determined chemical fractionation factor for the reduction process but much lower than the equilibrium fractionation (∼70‰) even though the confined groundwater residence time is >300 Ka years. Mapping the spatial distribution of δ³⁴S and SO₄/Cl of the unconfined Murray Group Aquifer provides an indicative tool for identifying the approximate extent of mixing, however the poorly defined end-member isotopic signatures precludes quantitative estimates of mixing fractions.