Fe(III) mobilisation by carbonate in low temperature environments: Study of the solubility of ferrihydrite in carbonate media and the formation of Fe(III) carbonate complexes

The linkage between the iron and the carbon cycles is of paramount importance to understand and quantify the effect of increased CO₂ concentrations in natural waters on the mobility of iron and associated trace elements. In this context, we have quantified the thermodynamic stability of mixed Fe(III) hydroxo-carbonate complexes and their effect on the solubility of Fe(III) oxihydroxides. We present the results of carefully performed solubility measurements of 2-line ferrihydrite in the slightly acidic to neutral–alkaline pH ranges (3.8–8.7) under constant pCO₂ varying between (0.982–98.154 kPa) at 25 °C.The outcome of the work indicates the predominance of two Fe(III) hydroxo carbonate complexes FeOHCO₃ and Fe(CO₃)₃³⁻, with formation constants log^*β°_1,1,1 = 10.76 ± 0.38 and log β°_1,0,3 = 24.24 ± 0.42, respectively.The solubility constant for the ferrihydrite used in this study was determined in acid conditions (pH: 1.8–3.2) in the absence of CO₂ and at T = (25 ± 1) °C, as log^*K_s,0 = 1.19 ± 0.41.The relative stability of the Fe(III)-carbonate complexes in alkaline pH conditions has implications for the solubility of Fe(III) in CO₂-rich environments and the subsequent mobilisation of associated trace metals that will be explored in subsequent papers.     

Solubility of Fe2(OH)3Cl (pure-iron end-member of hibbingite) in NaCl and Na2SO4 brines

Pure-iron end-member hibbingite, Fe₂(OH)₃Cl(s), may be important to geological repositories in salt formations, as it may be a dominant corrosion product of steel waste canisters in an anoxic environment in Na–Cl- and Na–Mg–Cl-dominated brines. In this study, the solubility of Fe₂(OH)₃Cl(s), the pure-iron end-member of hibbingite (Fe^II, Mg)₂(OH)₃Cl(s), and Fe(OH)₂(s) in 0.04 m to 6 m NaCl brines has been determined. For the reactionFe₂(OH)₃Cl(s) + 3H⁺ ? 3 H₂O + 2 Fe²⁺ + Cl^?,the solubility constant of Fe₂(OH)₃Cl(s) at infinite dilution and 25 °C has been found to be log₁₀ K = 17.12 ± 0.15 (95% confidence interval using F statistics for 36 data points and 3 parameters). For the reactionFe(OH)₂(s) + 2H⁺ ? 2 H₂O + Fe²⁺,the solubility constant of Fe(OH)₂ at infinite dilution and 25 °C has been found to be log₁₀ K = 12.95 ± 0.13 (95 % confidence interval using F statistics for 36 data points and 3 parameters). For the combined set of solubility data for Fe₂(OH)₃Cl(s) and Fe(OH)₂(s), the Na⁺–Fe²⁺ pair Pitzer interaction parameter θ_{Na⁺/Fe²⁺} has been found to be 0.08 ± 0.03 (95% confidence interval using F statistics for 36 data points and 3 parameters). In nearly saturated NaCl brine we observed evidence for the conversion of Fe(OH)₂(s) to Fe₂(OH)₃Cl(s). Additionally, when Fe₂(OH)₃Cl(s) was added to sodium sulfate brines, the formation of green rust(II) sulfate was observed, along with the generation of hydrogen gas. The results presented here provide insight into understanding and modeling the geochemistry and performance assessment of nuclear waste repositories in salt formations.     

Hydrosulfide/sulfide complexes of zinc to 250 °C and the thermodynamic properties of sphalerite

The solubility of synthetic ZnS_(cr) was measured at 25–250 °C and P = 150 bars as a function of pH in aqueous sulfide solutions (~ 0.015–0.15 m of total reduced sulfur). The solubility determinations were performed using a Ti flow-through hydrothermal reactor. The solubility of ZnS_(cr) was found to increase slowly with temperature over the whole pH range from 2 to ~ 10. The values of the Zn–S–HS complex stability constant, β, were determined for Zn(HS)₂⁰_(aq), Zn(HS)₃^?, Zn(HS)₄^2?, and ZnS(HS)^?. Based on the experimental values the Ryzhenko–Bryzgalin electrostatic model parameters for these stability constants were calculated, and the ZnS_(cr) solubility and the speciation of Zn in sulfide-containing hydrothermal solutions were evaluated. The most pronounced solubility increase, about 3 log units at m(S_total) = 0.1 for the temperatures from 25 to 250 °C, was found in acidic solutions (pH ~ 3 to 4) in the Zn(HS)₂⁰_(aq) predominance field. In weakly alkaline solutions, where Zn(HS)₃^? and Zn(HS)₄^2? are the dominant Zn–S–HS complexes, the ZnS_(cr) solubility increases by 1 log unit at the same conditions. It was found that ZnS(HS)^? and especially Zn(HS)₄^2? become less important in high temperature solutions. At 25 °C and m(S_total) = 0.1, these species dominate Zn speciation at pH > 7. At 100 °C and m(S_total) = 0.1, the maximum fraction of Zn(HS)₄^2? is only 20% of the total Zn concentration (i.e. at pH_t ~ 7.5), whereas at 350 °C and 3 <pH_t <10, the fraction of Zn(HS)₄^2? and ZnS(HS)^? is less than 0.05% and 2.5% respectively, of the total Zn concentration and Zn(HS)₂⁰ and Zn(HS)₃^? predominate. The measured equilibrium formation constants were combined with the literature data on the stability of Zn–Cl complexes in order to evaluate the concentration and speciation of Zn in chloride solutions. It was found that at acidic pH, and in more saline fluids having total chloride > 0.05 m, Zn–Cl complexes are responsible for hydrothermal Zn transport with no significant contribution of Zn–S–HS complexes. The hydrosulfide/sulfide complexes will play a more important role in lower salinity (< 0.05 m chloride) hydrothermal solutions which are characteristic of many epithermal ore depositing environments. The value of Δ_fG° (β-ZnS_(cr)) = ? 198.6 ± 0.2 kJ/mol at 25 °C was determined via solubility measurements of natural low-iron Santander (Spain) sphalerite.     

Origin of a barite-sulfide ore deposit in the Mykonos intrusion,cyclades: Trace element,isotopic, fluid inclusion and raman spectroscopy evidence

The polymetallic Mykonos vein system in the Cyclades, Greece, consists of 15 tension-gashes filled with barite, quartz, pyrite, sphalerite, chalcopyrite and galena in ca. 13.5 Ma, I-type, Mykonos monzogranite. Zones of silica and chlorite–muscovite alteration are associated with the veins and overprint pervasive silicification, phyllic and argillic alteration that affected large parts of the monzogranite. The mineralization cements breccias and consists of an early barite–silica–pyrite–sphalerite–chalcopyrite assemblage followed by later argentiferous galena. A combination of fluid inclusion and stable isotope data suggests that the barite and associated mineralization were deposited from fluids containing 2 to 17 wt.% NaCl equivalent, at temperatures of ~ 225° to 370 °C, under a hydrostatic pressure of ≤ 100 bars. The mineralizing fluids boiled and were saturated in H₂S and SO₂.Calculated δ¹⁸O_H2O and δD_H2O, initial ⁸⁷Sr/⁸⁶Sr isotope compositions and the trace and REEs elements contents are consistent with a model in which the mineralizing fluids were derived during alteration of the Mykonos intrusion and subsequently mixed with Miocene seawater. Heterogeneities in the calculated δ³⁴S_SO4^− 2 and δ³⁴S_H2S compositions of the ore fluids indicate two distinct sources for sulfur, namely of magmatic and seawater origin, and precipitation due to reduction of the SO₄^− 2 during fluid mixing. The physicochemical conditions of the fluids were pH = 5.0 to 6.2, logf_S2 = − 13.8 to − 12.5, logf_O2 = − 31.9 to − 30.9, logf_H2S(g) = − 1.9 to − 1.7, logf_Te2 = − 7.9 and logα(SO₄^− 2_(aq)/H₂S_(aq)) = + 2.6 to + 5.5. We propose that retrograde mesothermal hydrothermal alteration of the Mykonos monzogranite released barium and silica from the alkali feldspars. Barite was precipitated due to mixing of SO₄^− 2-rich Miocene seawater with the ascending Ba-rich magmatic fluid venting upwards in the pluton.     

Mafic xenoliths in Proterozoic kimberlites from Eastern Dharwar Craton,India: Mineralogy and P–T regime

Mafic xenoliths of garnet pyroxenite and eclogite from the Wajrakarur, Narayanpet and Raichur kimberlite fields in the Archaean Eastern Dharwar Craton (EDC) of southern India have been studied. The composition of clinopyroxene shows transition from omphacite (3–6 wt% Na₂O) in eclogites to Ca pyroxene (<3 wt% Na₂O) in garnet pyroxenites. Some of the xenoliths have additional phases such as kyanite, enstatite, chromian spinel or rutile as discrete grains. Clinopyroxene in a rutile eclogite has an X_Mg value of 0.70, which is unusually low compared to the X_Mg range of 0.91–0.97 for all other samples. Garnet in the rutile eclogite is also highly iron-rich with an end member composition of Prp_26.5Alm_52.5Grs_14.7Adr_5.1TiAdr_0.3Sps_1.0Uv_0.1. Garnets in several xenoliths are Cr-rich with up to 8 mol% knorringite component. Geothermobarometric calculations in Cr-rich xenoliths yield different P–T ranges for eclogites and garnet pyroxenites with average P–T conditions of 36 kbar and 1080 °C, and 27 kbar and 830 °C, respectively. The calculated P–T ranges approximate to a 45 mW m^?2 model geotherm, which is on the higher side of the typical range of xenolith/xenocryst geotherms (35–45 mW m^?2) for several Archaean cratons in the world. This indicates that the EDC was hotter than many other shield regions of the world in the mid-Proterozoic period when kimberlites intruded the craton. Textural and mineral chemical characteristics of the mafic xenoliths favour a magmatic cumulate process for their origin as opposed to subducted and metamorphosed oceanic crust.     

Zircon U-Pb geochronology,Lu-Hf isotope systematics,and geochemistry of bimodal volcanic rocks and associated granitoids from Kotri Belt,Central India: Implications for Neoarchean–Paleoproterozoic crustal growth

The Bastar Craton of Central India has a thick sequence of volcano–sedimentary rocks preserved in Kotri–Dongargarh belt that developed on a tonalite-trondhjemite-granodiorite (TTG) basement followed upwards by the Amgaon, Bengpal, Bailadila, and Nandgaon Groups of rocks. Here, we report the U-Pb geochronology and Lu-Hf isotope systematics and whole rock geochemistry of volcanic rocks and associated granitoids belonging to the Pitepani basalts, Bijli rhyolites, and Dongargarh granite in the Nandgaon Group of the Kotri belt. The volcanic rocks of the Nandgaon Group are bimodal in nature in which the basalts exhibit intergranular, porphyritic to spherulitic texture composed of pyroxenes, plagioclase, tremolite, actinolite, and chlorite ± Fe oxides. The rhyolites display porphyritic texture consisting of K-feldspar, quartz, and plagioclase as phenocrysts. The associated porphyritic granitoids have K-feldspar, microcline, plagioclase, and biotite phenocrysts within a groundmass of similar composition. The bimodal suite displays LILE, LREE enrichment, and HFSE depletion with significant negative Nb-Ta anomalies combined with slightly fractionated REE patterns in the basalts and highly fractionated patterns and prominent negative Eu anomalies in the rhyolites endorsing their generation in an island-arc/back-arc tectonic setting. The geochemical features of the associated granitoids indicate that these are potassic and classify as within-plate A-type granites. Zircons from the basalts show clear oscillatory zoning in their CL images. They cluster as a coherent group with ²⁰⁷Pb/²⁰⁶Pb spot ages ranging from 2446 to 2522 Ma and weighted mean age of 2471 ± 7 Ma. Zircons from the rhyolite samples are subhedral to euhedral and show simple oscillatory zoning with some heterogeneous fractured domains. The data from two samples define upper intercept ages of 2479 ± 13 Ma and 2463 ± 14 Ma. Zircon grains in the granite show clear oscillatory zoning and their U-Pb data define an upper intercept age of 2506 ± 50 Ma. The Lu-Hf isotopic data on the zircons from the basalts show initial ¹⁷⁶Hf/¹⁷⁷Hf ratios from 0.280925 to 0.281018. Their ε_Hf(t) values are in the range of − 10.0 to − 6.7. The Hf-depleted model ages (T_DM) are between 3038 Ma and 3171 Ma, and Hf crustal model ages (T_DM^C) vary from 3387–3589 Ma. The zircons from the rhyolites show initial ¹⁷⁶Hf/¹⁷⁷Hf ratios from 0.280919 to 0.281020 and from 0.281000 to 0.281103, respectively, with ε_Hf(t) values varying from − 10 to − 6.4 and from − 7.5 to − 3.9. Among these, one sample shows T_DM between 3038 Ma and 3182 Ma, and T_DM^C varies from 3377 to 3596 Ma, whereas the other sample shows ages of 2925 Ma and 3072 Ma with T_DM^C varying from 3208 to 3432 Ma. The initial ¹⁷⁶Hf/¹⁷⁷Hf ratios of the granites range from 0.280937 to 0.281062 with ε_Hf(t) values of − 8.8 to − 4.3. The T_DM shows a range of 2979 Ma and 3170 Ma, and T_DM^C varies from 3269 to 3541 Ma. The predominant negative ε_Hf(t) values of zircons from these rocks suggest that the source material was evolved from the Paleoarchean crust. The geological, geochemical, and geochronological evidence suggests coeval tectonic and magmatic episodes of volcanic and plutonic activity in an island-arc setting where the arc migrated toward the continental margin and played a significant role in the Neoarchean–Paleoproterozoic crustal growth of the Kotri belt of Central India.     

Dielectric loss in ore-forming chromium spinel at temperatures from 20 to 800 °C

Physical, physicochemical, and mineralogical-petrographic methods have been applied to samples of ophiolite-hosted chromite ore from different deposits and occurrences in the Urals. Temperature dependences of dielectric loss obtained for nine chromite ore samples consisting of 95–98% Cr spinel show prominent peaks indicating a relaxation origin of the loss. The analyzed samples have the loss peaks at different temperatures depending mainly on H = (FeO/Fe₂O₃)^? : (FeO/Fe₂O₃)^??, where (FeO/Fe₂O₃)^? and (FeO/Fe₂O₃)^?? are, respectively, the ferrous/ferric oxide ratios in the samples before and after heating to 800 °C, and H is thus the heating-induced relative change in the FeO/Fe₂O₃ ratio. These peak temperatures vary from 550 °C (sample 1, high-Cr chromium spinel with more than 52% Cr₂O₃) to 750 °C (sample 2, aluminous and magnesian spinel with less than 30% Cr₂O₃), and H ranges correspondingly from 1.61 to 5.49. The temperature of the loss peaks is related with H as H = 34.30 ? 11.52N + 1.20N², with an error of σ = 0.19 (N = T · 10^?2, T is temperature in °C).     

Interaction of U(VI) with Äspö diorite: A batch and in situ ATR FT-IR sorption study

Pristine diorite drill cores, obtained from the Äspö Hard Rock Laboratory (HRL, Sweden), were used to study the retention properties of fresh, anoxic crystalline rock material towards the redox-sensitive uranium. Batch sorption experiments and spectroscopic methods were applied for this study. The impact of various parameters, such as solid-to-liquid ratio (2–200 g/L), grain size (0.063–0.2 mm, 0.5–1 mm, 1–2 mm), temperature (room temperature and 10 °C), contact time (5–108 days), initial U(VI) concentration (3 × 10⁻⁹ to 6 × 10⁻⁵ M), and background electrolyte (synthetic Äspö groundwater and 0.1 M NaClO₄) on the U(VI) sorption onto anoxic diorite was studied under anoxic conditions (N₂). Comparatively, U(VI) sorption onto oxidized diorite material was studied under ambient atmosphere (pCO₂ = 10^−3.5 atm). Conventional distribution coefficients, K_d, and surface area normalized distribution coefficients, K_a, were determined. The K_d value for the U(VI) sorption onto anoxic diorite in synthetic Äspö groundwater under anoxic conditions by investigating the sorption isotherm amounts to 3.8 ± 0.6 L/kg which corresponds to K_a = 0.0030 ± 0.0005 cm (grain size 1–2 mm). This indicates a weak U sorption onto diorite which can be attributed to the occurrence of the neutral complex Ca₂UO₂(CO₃)₃(aq) in solution. This complex was verified as predominating U species in synthetic Äspö groundwater by time-resolved laser-induced fluorescence spectroscopy (TRLFS). Compared to U sorption at room temperature under anoxic conditions, U sorption is further reduced at decreased temperature (10 °C) and under ambient atmosphere. The U species in aqueous solution as well as sorbed on diorite were studied by in situ time-resolved attenuated total reflection Fourier-transform infrared (ATR FT-IR) spectroscopy. A predominant sorbing species containing a UO₂(CO₃)₃⁴⁻ moiety was identified. The extent of U sorption onto diorite was found to depend more on the low sorption affinity of the Ca₂UO₂(CO₃)₃(aq) complex than on reduction processes of uranium.     

Hydro-geochemical and isotopic fluid evolution of the Los Azufres geothermal field,Central Mexico

Hydrothermal alteration at Los Azufres geothermal field is mostly propylitic with a progressive dehydration with depth and temperature increase. Argillic and advanced argillic zones overlie the propylitic zone owing to the activity of gases in the system. The deepest fluid inclusions (proto-fluid) are liquid-rich with low salinity, with NaCl dominant fluid type and ice melting temperatures (T_mi) near zero (0 °C), and salinities of 0.8 wt% NaCl equivalent. The homogenization temperature (T_h)  = 325 ± 5 °C. The boiling zone shows T_h = ±300 °C and apparent salinities between 1 and 4.9 wt% NaCl equivalent, implying a vaporization process and a very important participation of non-condensable gases (NCGs), mostly CO₂. Positive clathrate melting temperatures (fusion) with T_h = 150 °C are observed in the upper part of the geothermal reservoir (from 0 to 700 m depth). These could well be the evidence of a high gas concentration. The current water produced at the geothermal wells is NaCl rich (geothermal brine) and is fully equilibrated with the host rock at temperatures between T = 300 and 340 °C. The hot spring waters are acid-sulfate, indicating that they are derived from meteoric water heated by geothermal steam. The NCGs related to the steam dominant zone are composed mostly of CO₂ (80–98% of all the gases). The gases represent between 2 and 9 wt% of the total mass of the fluid of the reservoir.The authors interpret the evolution of this system as deep liquid water boiling when ascending through fractures connected to the surface. Boiling is caused by a drop of pressure, which favors an increase in the steam phase within the brine ascending towards the surface. During this ascent, the fluid becomes steam-dominant in the shallowest zone, and mixes with meteoric water in perched aquifers. Stable isotope compositions (δ¹⁸O–δD) of the geothermal brine indicate mixing between meteoric water and a minor magmatic component. The enrichment in δ¹⁸O is due to the rock–water interaction at relatively high temperatures. δ¹³C stable isotope data show a magmatic source with a minor meteoric contribution for CO₂. The initial isotopic value δ³⁴S_RES = −2.3‰, which implies a magmatic source. More negative values are observed for shallow pyrite and range from δ³⁴S (FeS₂) = −4‰ to −4.9‰, indicating boiling. The same fractionation tendencies are observed for fluids in the reservoir from results for δ¹⁸O.     

Tectonics and thermal structure of western Satpura,India

Increased seismicity and occurrences of hot springs having surface temperature of 36–58 °C are observed in the central part of India (74–81° E, 20–25° N), where the NE trending Middle Proterozoic Aravalli Mobile Belt meets the ENE trending Satpura Mobile Belt. Earlier Deep Seismic Sounding (DSS) studies along Thuadara-Sendhwa-Sindad profile in the area has showed Mesozoic Sediments up to around 4 km depth covered by Deccan Trap and the Moho depth with a boundary velocity (P_n) of 8.2 km/s. In the present study, surface heat flow of 48 ± 4 mW m^?2 has been estimated based on P_n velocity, which agrees with the value of heat flow of 52 ± 4 mW m^?2 based on Curie point isotherms estimates. The calculated temperature-depth profile shows temperature of 80–120 °C at the basement, which is equivalent to oil window temperature in Mesozoic sediments and around 570–635 °C at Moho depth of 38–43 km and the thermal lithosphere is about 110 km thick, which is comparatively higher than those of adjoining regions. The present study reveals the brittle–ductile transition zone at 14–41 km depth (temperature around 250–600 °C) where earthquake nucleation takes place.     

Biomarkers record environmental changes along an altitudinal transect in the wettest place on Earth

The combined application of plant wax δD (δD_wax) and branched glycerol dialkyl glycerol tetraethers (brGDGTs) has been suggested as proxy for paleo-elevation. In some of the altitudinal transects studied so far, nonlinear precipitation gradients, large variations in seasonality, soil and vegetation types introduced substantial amounts of scatter in the relationship between these proxies and altitude. To further evaluate the principal functioning of the proxies, δD_wax and brGDGTs were analysed in surface soils along an altitude gradient (from 28 m up to 1865 m a.s.l.) in Meghalaya, India. The transect experiences limited seasonal temperature variation and receives very high monsoonal precipitation along the whole elevation gradient. The δD_wax show a significant relation with altitude (r² = 0.72). The additional fits with stream water δD (r² = 0.72) and modelled precipitation δD (r² = 0.72) indicate that δD_wax tracks the altitude effect on precipitation. Also the brGDGT distribution shows a correlation with altitude, reflecting the decrease in temperature with higher elevation (r² = 0.65, or r² = 0.66 using the original and recalibrated methylation of branched tetraethers-cyclisation of branched tetraethers (MBT–CBT) proxy). Application of the original MBT–CBT calibration generates calculated air temperatures that overestimate measured air temperature by ∼6 °C, whereas temperatures similar to measured are obtained with the revised calibration. These results indicate that δD_wax and brGDGTs may faithfully and accurately track environmental changes with altitude for transects where seasonal and diurnal temperature variability is relatively limited. Furthermore, proxy application to locations that experience high rainfall increases the suitability as climatic indicators, as it excludes soil moisture availability as a limiting factor.     

Quantifying pyrogenic carbon from thermosequences of wood and grass using hydrogen pyrolysis

Previously studied thermosequences of wood (chestnut) and grass (rice straw) biochar were subjected to hydrogen pyrolysis (hypy) to evaluate the efficacy of the technique for determining pyrogenic carbon (C_P) abundance. As expected, biochar from both wood and grass produced at higher temperature had higher C_P amount. However, the trend was not linear, but more sigmoidal. C_P/C_T ratio values (C_T = total organic carbon) for the wood thermosequence were ⩽0.03 at biochar production temperature (T_CHAR) ⩽ 300 °C. They increased dramatically until 600 °C and remained relatively constant and near unity at higher biochar production temperature. Grass biochar was similar in profile, but C_P/C_T values rose dramatically after 400 °C. The findings are consistent with the hypothesis that hypy residues contain polycyclic aromatic hydrocarbons (PAHs) with a degree of condensation above at least 7–14 fused rings, with labile organic matter and pyrogenic PAHs below this degree of condensation removed by hypy.Both wood and grass thermosequences displayed δ¹³C_P values that decreased with increased T_CHAR, indicating that recalcitrant carbon compounds (pyrogenic aromatic PAHs with a relatively high degree of condensation) were first formed from structural components with relatively high δ¹³C values (e.g. cellulose). Relatively constant δ¹³C values at T_CHAR ⩾ 500 °C suggested the dominant pyrolysis reaction was condensation of PAHs with no additional fractionation. Comparison of hypy with benzene polycarboxylic acid (BPCA), ‘ring current’ NMR and pyrolysis gas chromatography–mass spectrometry (GC–MS) results from the same suite of samples indicated a consistent overview of the structure of C_P, but provided unique and complimentary information.     

Measuring the effective diffusion coefficient of dissolved hydrogen in saturated Boom Clay

Boom Clay is studied as a potential host formation for the disposal of high-and intermediate level long-lived radioactive waste in Belgium. In such a geological repository, generation of gases (mainly H₂ from anaerobic corrosion) will be unavoidable. In order to make a good evaluation of the balance between gas generation vs. gas dissipation for a particular waste form and/or disposal concept, good estimates for gas diffusion coefficients of dissolved gases are essential. In order to obtain an accurate diffusion coefficient for dissolved hydrogen in saturated Boom Clay, diffusion experiments were performed with a recently developed through-diffusion set-up for dissolved gases. Due to microbial activity in the test set-up, conversion of hydrogen into methane was observed within several experiments. A complex sterilisation procedure was therefore developed in order to eliminate microbiological disturbances. Only by a combination of heat sterilisation, gamma irradiation and the use of a microbial inhibitor, reliable, reproducible and accurate H₂(g) diffusion coefficients (measured at 21 °C) for samples oriented parallel (D_eff = 7.25 × 10⁻¹⁰ m²/s and D_eff = 5.51 × 10⁻¹⁰ m²/s) and perpendicular (D_eff = 2.64 × 10⁻¹⁰ m²/s) to the bedding plane were obtained.     

Short-lived radioactive nuclides in meteorites and early solar system processes

Now extinct, short-lived radioactive nuclides, such as ⁷Be (T_1/2 = 53 days), ¹⁰Be (T_1/2 = 1.5 Ma), ²⁶Al (T_1/2 = 0.74 Ma), ³⁶Cl (T_1/2 = 0.3 Ma), ⁴¹Ca (T_1/2 = 0.1 Ma), ⁵³Mn (T_1/2 = 3.7 Ma) and ⁶⁰Fe (T_1/2 = 1.5 Ma), were present in the protosolar nebula when the various components of meteorites formed. The presence of these radioactive isotopes requires a ‘last-minute’ origin, either nucleosynthesis in a massive star dying close in space and time to the nascent solar system or production by local irradiation of part of the protosolar disk by high-energy solar cosmic rays. In this review, we list: (i) the different observations indicating the existence of multiple origins for short-lived radioactive nuclides, namely ⁷Be, ¹⁰Be and ³⁶Cl for irradiation scenario and ⁶⁰Fe for injection scenario; (ii) the constraints that exist on their distribution (homogeneous or heterogeneous) in the accretion disk; (iii) the constraints they brought on the timescales of nebular processes (from Ca–Al-rich inclusions to chondrules) and of the accretion and differentiation of planetesimals.     

Petrogenesis of Carboniferous adakites and Nb-enriched arc basalts in the Alataw area,northern Tianshan Range (western China): Implications for Phanerozoic crustal growth in the Central Asia orogenic belt

Carboniferous volcanic rocks in the Alataw area, Northern Tianshan Range (Xinjiang), consist of early Carboniferous (ca. 320 Ma) adakites and Nb-enriched arc basalts and basaltic andesites (NEBs), and late Carboniferous (ca. 306–310 Ma) mainly high-K calc-alkaline andesites, dacites and rhyolites. The adakites are calc-alkaline, and characterized by high Na₂O/K₂O (1.52–3.32) ratios, negligible to positive Eu anomalies, strong depletion of heavy rare earth elements (e.g., Yb = 0.74–1.47 ppm) and Y (6.7–14.9 ppm), positive Sr and Ba but negative Nb and Ti anomalies, and relatively constant ε_Nd(T) values (+ 3.4–+ 6.6) and (⁸⁷Sr/⁸⁶Sr)_i ratios (0.7035–0.7042). Some andesitic and dacitic adakite samples exhibit high MgO contents similar to magnesian andesites. The NEBs are sodium-rich (Na₂O/K₂O = 2.03–8.06), and differ from the vast majority of arc basalts in their higher Nb, Zr, TiO₂ and P₂O₅ contents and Nb/Th, Nb/La and Nb/U ratios, and minor negative to positive anomalies in Ba, Nb, Sr, Zr and Ti. They have the highest ε_Nd(T) values (+ 6.4–+ 11.6) but varying (⁸⁷Sr/⁸⁶Sr)_i ratios (0.7007–0.7063). The high-K calc-alkaline suite is similar to typical ‘normal’ arc volcanic rocks in terms of moderately fractionated rare earth abundance and distinctly negative Eu, Nb, Sr and Ti anomalies. They have ε_Nd(T) values (+ 1.2–+ 6.4) and (⁸⁷Sr/⁸⁶Sr)_i ratios (0.7018–0.7059). Geochemically, they are similar to coeval I-type granitoids in the Alataw area. Given the presence of early Carboniferous ophiolites in the Northern Tianshan Range, and the isotopically inappropriate compositions of Proterozoic metamorphic basement in the Alataw area, we argue that the Alataw adakites were most probably related to the melting of young subducted crust of the Northern Tianshan Ocean. The NEBs likely originated from mantle wedge peridotites metasomatized by adakites and minor slab-derived fluids. The later high-K calc alkaline suite was generated by AFC processes that acted on melts derived from a mantle wedge metasomatized by hydrous fluids. The larger range of isotopic compositions exhibited by both the NEB and high-K suite, relative to the adakites, suggests that the mantle wedge was heterogeneous prior to slab- or fluid-mediated metasomatism.Continental crustal growth of the Central Asian orogenic belt was dominated by contributions of the juvenile materials from the depleted mantle prior to 270 Ma and possibly afterwards. The results of this study suggest that other Carboniferous Nb-enriched basalts in the Tianshan Range were generated by subduction processes rather than by intraplate tectonics as previously proposed.     

Synthesis and characterization of polycrystalline KAlSi3O8 hollandite [liebermannite]: Sound velocities vs. pressure to 13 GPa at room temperature

A polycrystalline specimen of liebermannite [KAlSi₃O₈ hollandite] was synthesized at 14.5 GPa and 1473 K using glass starting material in a uniaxial split-sphere apparatus. The recovered specimen is pure tetragonal hollandite [SG: I4/m] with bulk density of within 98% of the measured X-ray value. The specimen was also characterized by Raman spectroscopy and nuclear magnetic resonance spectroscopy. Sound velocities in this specimen were measured by ultrasonic interferometry to 13 GPa at room T in a uniaxial split-cylinder apparatus using Al₂O₃ as a pressure marker. Finite strain analysis of the ultrasonic data yielded K_S0 = 145(1) GPa, K₀′ = 4.9(2), G₀ = 92.3(3) GPa, G₀′ = 1.6(1) for the bulk and shear moduli and their pressure derivatives, corresponding to V_P0 = 8.4(1) km/s, V_S0 = 4.9(1) km/s for the sound wave velocities at room temperature. These elasticity data are compared to literature values obtained from static compression experiments and theoretical density functional calculations.     

Fluorite solubility in hydrous haplogranitic melts at 100 MPa

Fluorite is the most common fluoride mineral in magmatic silicic systems and its crystallization can moderate or buffer fluorine concentrations in these settings. We have experimentally determined fluorite solubility and speciation mechanisms in haplogranitic melts at 800–950 °C, 100 MPa and aqueous-fluid saturation. The starting haplogranite compositions: peraluminous (alumina saturation index, ASI = 1.2), subaluminous (ASI = 1.0) and peralkaline (ASI = 0.8) were variably doped with CaO or F₂O₋₁ in the form of stoichiometric mineral or glass mixtures. The solubility of fluorite along the fluorite–hydrous haplogranite binaries is low: 1.054 ± 0.085 wt.% CaF₂ (peralkaline), 0.822 ± 0.076 wt.% (subaluminous) and 1.92 ± 0.15 wt.% (peraluminous) at 800 °C, 100 MPa and 10 wt.% H₂O, and exhibits a minimum at ASI ∼ 1. Fluorite saturation isotherms are strongly hyperbolic in the CaO–F₂O₋₁ space, suggesting that fluorite saturation is controlled by the activity product of CaO and F₂O₋₁, i.e., these components are partially decoupled in the melt structure. The form of fluorite liquidus isotherms implies distinct roles of fluorite crystallization: in Ca-dominant systems, fluorite crystallization is controlled by the fluorine concentration in the melt only and remains nearly independent of calcium contents; in F-rich systems, the crystallization of fluorite is determined by CaO contents and it does not buffer fluorine concentration in the melt. The apparent equilibrium constant, K, for the equilibrium CaO + cF₂O₋₁ = CaF₂ (+ associates) is log K= − (2.449 ± 0.085)·Al₂O₃^exc + (4.902 ± 0.066); the reaction-stoichiometry parameter varies as follows: c= − (0.92 ± 0.11)·Al₂O₃^exc + (1.042 ± 0.084) at 800 °C, 100 MPa and fluid saturation where Al₂O₃^exc are molar percent alumina in excess over alkali oxides. The reaction stoichiometry, c, changes at subaluminous composition: in peralkaline melts, competition of other network modifiers for excess fluorine anions leads to the preferential alkali–F short-range order, whereas in peraluminous compositions, excess alumina associates with calcium cations to form calcioaluminate tetrahedra. The temperature dependence of fluorite solubility is described by the binary symmetric Margules parameter, W = 36.0 ± 1.4 kJ (peralkaline), 39.7 ± 0.5 kJ (subaluminous) and 32.8 ± 0.7 kJ (peraluminous). The strong positive deviations from ideal mixing imply the occurrence of CaF₂–granite liquid–liquid immiscibility at temperatures above 1258 °C, which is consistent with previous experimental data. These experimental results suggest very low solubilities of fluorite in Ca-rich melts, consistent with the lack of fluorine enrichment in peralkaline rhyolites and calc-alkaline batholiths. On the other hand, high CaO concentrations necessary to crystallize fluorite in F-rich peraluminous melts are not observed in nature and thus magmatic crystallization of fluorite in topaz-bearing silicic suites is suppressed. A procedure for calculating fluorite solubility and the liquidus isotherms for a whole-rock composition and temperature of interest is provided.     

Geochronological and geochemical constraints on the petrogenesis of alkaline ultramafic dykes from southwest Guizhou Province,SW China

Geochronological, geochemical and whole-rock Sr–Nd isotopic analyses have been completed on a suite of alkaline ultramafic dykes from southwest (SW) Guizhou Province, China with the aim of characterising their petrogenesis. The Baiceng ultramafic dykes have a LA-ICP-MS zircon ²⁰⁶Pb/²³⁸U age of 88.1 ± 1.1 Ma (n = 8), whereas two phlogopites studied by ⁴⁰Ar/³⁹Ar dating methods give emplacement ages of 85.25 ± 0.57 Ma and 87.51 ± 0.45 Ma for ultramafic dykes from Yinhe and Lurong, respectively. In terms of composition, these Late Mesozoic ultramafic dykes belong to the alkaline magma series due to their high K₂O (3.31–5.04 wt.%) contents. The dykes are characterised by enrichment of light rare earth element (LREE) and large-ion lithosphile elements (LILEs) (Rb and Ba), negative anomalies in high field strength elements (HFSEs), such as, Nb, Ta and Ti relative to primitive mantle, low initial ⁸⁷Sr/⁸⁶Sr ratios (0.7060–0.7063) and positive ε_Nd(t) values (0.3–0.4). Such features suggest derivation from low degree (< 1%) partial melting of depleted asthenospheric mantle (garnet-lherzolite), and contamination to various degrees (～ 10%) by interaction with upper crustal materials.     

Intermediate sulfidation epithermal gold-base metal deposits in Tertiary subaerial volcanic rocks,Sahinli/Tespih Dere (Lapseki/Western Turkey)

Gold in the Sahinli and Tespih Dere intermediate sulfidation gold-base metal deposits in Western Turkey occurs in relatively deep epithermal quartz veins along with base metal minerals which have epithermal textures, including plumose quartz, vug infills, comb and cockade textures and matrix-supported milled breccias. The total sulfide content of the veins in the area is variable ranging from < 1% to 60% and is dominated by pyrite, galena, sphalerite and chalcopyrite. Sphalerite is Fe-poor (0.6 to 1.4 mol% FeS). Minor amounts of Ag-rich tetrahedrite are present. Primary hydrothermal alteration minerals include illite/muscovite, mixed-layer illite/smectite (11.6 Å) and clinochlore towards the east and, alunite, dickite/nacrite and pyrophyllite towards the west at Sahinli; major illite/muscovite and dickite occur at Tespih Dere and Sarioluk, respectively.Fluid inclusions in main-stage quartz at Sahinli are only liquid-rich, with homogenization temperatures ranging from 220 to 322 °C and the majority of T_h values between 250 and 300 °C. Salinity ranges from 4.3 to 6.9 wt.% NaCl equiv. First ice-melting temperatures (T_mf) between ?24.5 and ?19.0 °C indicate that the fluids were dominated by NaCl – H₂O during mineralization. The relatively higher average T_h at the Tespih Dere deposit (295 °C) is attributed to a relatively deeper level of exposure.Calculated δ¹⁸O values indicate that ore-forming hydrothermal fluids in the study area had δ¹⁸O_H2O ranging from + 1.1 to + 9.7‰ (average = 3.8‰), strongly ¹⁸O-enriched compared with present-day hydrothermal meteoric water in the area (δ¹⁸O = ?8.5‰). δD values of fluid inclusions in quartz range from ?58 to ?93‰ and δD values of clay minerals and alunite from ?40 to ?119‰. δD values from intermediate argillic alteration (average = ?68‰) in the study area are very similar to δD values of the present-day local geothermal system (average δD = ?54‰) whereas δD values from advanced-argillic alteration (average δD = ?33‰) are very different from the present-day local geothermal system.The δ³⁴S values in samples from the Sahinli and Tespih Dere deposits average ?2.9‰ for pyrite; ?3.3‰ for chalcopyrite; ?5.4‰ for sphalerite and ?7.6‰ for galena. These data are consistent with derivation of the sulfur from either igneous rocks or possibly from local wallrock.