Groundwater is used extensively in the Central Kenya Rift for domestic and agricultural demands. In these active rift settings groundwater can exhibit high fluoride levels. In order to address water ...security and reduce human exposure to high fluoride in drinking water, knowledge of the source and geochemical processes of enrichment are required. A study was therefore carried out within the Naivasha catchment (Kenya) to understand the genesis, enrichment and seasonal variations of fluoride in the groundwater. Rocks, rain, surface and groundwater sources were sampled for hydrogeochemical and isotopic investigations, the data was statistically and geospatially analyzed. Water sources have variable fluoride concentrations between 0.02–75mg/L. 73% exceed the health limit (1.5mg/L) in both dry and wet seasons. F− concentrations in rivers are lower (0.2–9.2mg/L) than groundwater (0.09 to 43.6mg/L) while saline lake waters have the highest concentrations (0.27–75mg/L). The higher values are confined to elevations below 2000masl. Oxygen (δ18O) and hydrogen (δD) isotopic values range from −6.2 to +5.8‰ and −31.3 to +33.3‰, respectively, they are also highly variable in the rift floor where they attain maximum values. Fluoride base levels in the precursor vitreous volcanic rocks are higher (between 3750–6000ppm) in minerals such as cordierite and muscovite while secondary minerals like illite and kaolinite have lower remnant fluoride (<1000ppm). Thus, geochemical F− enrichment in regional groundwater is mainly due to a) rock alteration, i.e. through long residence times and natural discharge and/or enhanced leakages of deep seated geothermal water reservoirs, b) secondary concentration fortification of natural reservoirs through evaporation, through reduced recharge and/or enhanced abstraction and c) through additional enrichment of fluoride after volcanic emissions. The findings are useful to help improve water management in Naivasha as well as similar active rift setting environments.
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•Rocks and water samples from the Central Kenya Rift, were analyzed for F−.•73% of water sources are above the NEMA health guideline (1.5mg/L) on fluoride.•Fluoride concentration is highly variable in waters in the rift (0.22–74.98mg/L).•Minerals (cordierite, muscovite, villiaumite) have high fluoride 308–6366ppm.•Fluoride enriches in water by rock alteration, evaporation and geothermal mixing.
Fluid-mediated mineral dissolution and reprecipitation processes are the most common mineral reaction mechanism in the solid Earth and are fundamental for the Earth's internal dynamics. Element ...exchange during such mineral reactions is commonly thought to occur via aqueous solutions with the mineral solubility in the coexisting fluid being a rate limiting factor. Here we show in high-pressure/low temperature rocks that element transfer during mineral dissolution and reprecipitation can occur in an alkali-Al-Si-rich amorphous material that forms directly by depolymerization of the crystal lattice and is thermodynamically decoupled from aqueous solutions. Depolymerization starts along grain boundaries and crystal lattice defects that serve as element exchange pathways and are sites of porosity formation. The resulting amorphous material occupies large volumes in an interconnected porosity network. Precipitation of product minerals occurs directly by repolymerization of the amorphous material at the product surface. This mechanism allows for significantly higher element transport and mineral reaction rates than aqueous solutions with major implications for the role of mineral reactions in the dynamic Earth.
Large‐scale impact events are some of the most catastrophic and instantaneous geological processes in nature, and leave in their wake conspicuous geological structures with characteristic magnetic ...anomalies. Despite magnetic anomalies in craters being well‐documented, their relationship with the magnetic mineral composition of the target and impactites is not always straightforward. Furthermore, the influence of impact shock and post‐impact events in the magnetism of natural craters remains elusive. In the Ries crater, Germany, the negative magnetic anomalies are attributed to a reverse polarity remanent magnetization in the impact‐melt bearing lithologies. We report new chemical, rock‐, and mineral‐magnetic data from the shocked basement and impactites, from surface samples, NR73 and SUBO‐18 boreholes, and explore how temperature and hydrothermalism may influence the magnetic mineralogy in the crater. We identified shocked, pure magnetite in the basement, and low‐cation substituted magnetite in the impactites as the main magnetic carriers. The shocked basement is demagnetized but remains largely unaltered by post‐impact hydrothermalism, while the impactites show weak magnetization and are extensively altered by neutral‐to‐reducing post‐impact hydrothermalism. We suggest that the magnetic mineralogy of the demagnetized uplifted basement may contribute significantly to the magnetic anomaly variation, in line with recent findings from the Chicxulub peak‐ring.
Here we report on a set of six apatite reference materials (chlorapatites MGMH#133648, TUBAF#38 and fluorapatites MGMH#128441A, TUBAF#37, 40, 50) which we have characterised for their chlorine ...isotope ratios; these RMs span a range of Cl mass fractions within the apatite Ca10(PO4)6(F,Cl,OH)2 solid solution series. Numerous apatite specimens, obtained from mineralogical collections, were initially screened for 37Cl/35Cl homogeneity using SIMS followed by δ37Cl characterisation by gas source mass spectrometry using both dual‐inlet and continuous‐flow modes. We also report major and key trace element compositions as determined by EPMA. The repeatability of our SIMS results was better than ± 0.10‰ (1s) for the five samples with > 0.5 % m/m Cl and ± 0.19‰ (1s) for the low Cl abundance material (0.27% m/m). We also observed a small, but significant crystal orientation effect of 0.38‰ between the mean 37Cl/35Cl ratios measured on three oriented apatite fragments. Furthermore, the results of GS‐IRMS analyses show small but systematic offset of δ37ClSMOC values between the three laboratories. Nonetheless, all studied samples have comparable chlorine isotope compositions, with mean 103δ37ClSMOC values between +0.09 and +0.42 and in all cases with 1s ≤ ± 0.25.
Key Points
Six apatite reference materials having various Cl mass fractions were characterised for chlorine isotope ratios by SIMS and three GS‐IRMS laboratories.
A small, but significant, crystal orientation effect was recorded by SIMS analyses.
Correlation of instrumental mass fractionation factor with Cl mass fraction is visible along the apatite solid solution series.
► Experiments to examine interaction between minerals, brine and impure/pure scCO2. ► Resulting formation of sulfuric and nitric acids is reflected in low pH. ► A larger amount of cations dissolved ...from the mineral phases when impure scCO2 used. ► Dissolution of carbonates; silicates do not exhibit significant alterations. ► Anhydrite partly transformed to gypsum during experiments with scCO2+NO2.
The aim of this experimental study was to evaluate and compare the geochemical impact of pure and impure CO2 on rock forming minerals of possible CO2 storage reservoirs. This geochemical approach takes into account the incomplete purification of industrial captured CO2 and the related effects during injection, and provides relevant data for long-term storage simulations of this specific greenhouse gas. Batch experiments were conducted to investigate the interactions of supercritical CO2, brine and rock-forming mineral concentrates (albite, microcline, kaolinite, biotite, muscovite, calcite, dolomite and anhydrite) using a newly developed experimental setup. After up to 42day (1000h) experiments using pure and impure supercritical CO2 the dissolution and solution characteristics were examined by XRD, XRF, SEM and EDS for the solid, and ICP–MS and IC for the fluid reactants, respectively. Experiments with mixtures of supercritical CO2 (99.5vol.%) and SO2 or NO2 impurities (0.5vol.%) suggest the formation of H2SO4 and HNO3, reflected in pH values between 1 and 4 for experiments with silicates and anhydrite and between 5 and 6 for experiments with carbonates. These acids should be responsible for the general larger amount of cations dissolved from the mineral phases compared to experiments using pure CO2. For pure CO2 a pH of around 4 was obtained using silicates and anhydrite, and 7–8 for carbonates. Dissolution of carbonates was observed after both pure and impure CO2 experiments. Anhydrite was corroded by approximately 50wt.% and gypsum precipitated during experiments with supercritical CO2+NO2. Silicates do not exhibit visible alterations during all experiments but released an increasing amount of cations in the reaction fluid during experiments with impure CO2. Nonetheless, precipitated secondary carbonates could not be identified.
Carbonate minerals are ubiquitous in most sediment-hosted mineral deposits. These deposits can contain a variety of carbonate types with complex paragenetic relationships. When normalized to ...chondritic values (CN), rare-earth elements and yttrium (REE+Y
CN
) can be used to constrain fluid chemistry and fluid-rock interaction processes in both low- and high-temperature settings. Unlike other phases (e.g., pyrite), the application of in situ laser ablation-inductively coupled plasma-mass spectroscopy (LA-ICP-MS) data to the differentiation of pre-ore and hydrothermal carbonates remains relatively untested. To assess the potential applicability of carbonate in situ REE+Y data, we combined transmitted light and cathodoluminescence (CL) petrography with LA-ICP-MS analysis of carbonate mineral phases from (1) the Proterozoic George Fisher clastic dominated (CD-type) massive sulfide deposit and from (2) correlative, barren host rock lithologies (Urquhart Shale Formation). The REE+Y
CN
composition of pre-ore calcite suggests it formed during diagenesis from diagenetic pore fluids derived from ferruginous, anoxic seawater. Hydrothermal and hydrothermally altered calcite and dolomite from George Fisher is generally more LREE depleted than the pre-ore calcite, whole-rock REE concentrations, and shale reference values. We suggest this is the result of hydrothermal alteration by saline Cl
-
-rich mineralizing fluids. Furthermore, the presence of both positive and negative Eu/Eu
*
values in calcite and dolomite indicates that the mineralizing fluids were relatively hot (>250°C) and cooled below 200–250°C during ore formation. This study confirms the hypothesis that in situ REE+Y data can be used to differentiate between pre-ore and hydrothermal carbonate and provide important constraints on the conditions of ore formation.
Tourmaline is a locally abundant hydrothermal mineral in the wallrocks surrounding the W-Sn-Cu mineralized veins in the Panasqueira deposit (Portugal) and a minor phase within the veins themselves. ...Tourmaline chemical and boron-isotope compositions have been determined from three settings: (1) pervasive fine-grained tourmalinization zones in wallrocks within 10cm of the veins, (2) coarser tourmaline in wallrock-hosted fault zones, and (3) needle-shaped tourmaline from late-stage vugs in the quartz veins. All tourmalines from Panasqueira have ferromagnesian compositions with significant octahedral aluminum contents and variable X-site vacancies. Tourmaline compositions show significant chemical variations at the grain and sample scale but are homogeneous on the deposit scale. Tourmaline is typically optically and chemically zoned, showing significant increases in Fe and F and decrease in Mg, Ca and Al from core to rim. Moreover, as a general trend, Al increases and Mg decreases with proximity to the vein contact. The observed chemical variation seems to have been controlled mainly by the vectors: FeMg−1; X□AlNa−1(R2+)−1; X□Al(OH)Na−1(R2+)−1F−1; R2+(OH,F)Al−1O−1 and X□Al2O Na−1(R2+)−2(OH,F)−1.
The total range in tourmaline δ11B values from the Panasqueira W-Sn-Cu deposit is from −3.7 to −12.7‰, including tourmaline from the three settings. The calculated B-isotope composition of the hydrothermal fluid based on the average tourmaline composition of −9‰ and an estimated alteration temperature in the wallrocks, based on Ti-in-quartz thermometry, of 500°C is −7.1‰. This isotopic composition is permissive of a boron source from local metasediments or an S-type granite, but the high boron concentration in the wallrocks, with >50vol% tourmaline, along with the typical element association of Panasqueira (W, Sn, F, Nb, Ta, Rb) makes a granitic source more likely. Late-stage tourmaline needles in vugs within the mineralized veins show the same patterns and ranges in both chemical and boron isotope composition, suggesting that exsolved magmatic fluids were injected in at least two pulses.
The variations in chemical and boron isotopic compositions of tourmaline combined with estimates from Ti-in-quartz thermometry suggest that hydrothermal ore formation is associated with fluid cooling and geochemical fluid-rock interactions.
•Chemical and boron isotopic compositions of tourmaline and Ti-in-quartz temperatures at Panasqueira W-Sn-Cu deposit.•Compositional variations indicate fluid cooling and fluid-rock interaction as key processes related to ore formation.•Boron isotopic data are compatible with a single fluid source related to the adjacent S-type granite.•Compositional trends suggest multiple pulses of magmatic fluids.
Apatite is a ubiquitous phase in granite plutons and in most adjacent country rocks, thus contamination of a granite magma with wall-rock material results in two genetic types of apatite in the ...magma: cognate and foreign. These two textural and chemical varieties of apatite undergo textural and compositional changes to reach physical and chemical equilibrium (perfect assimilation) in the melt. Our experiments replicate the conditions in such contaminated granites. The starting materials consist of a peraluminous synthetic SiO2-Al2O3-Na2O-K2O (SANK 1.3) granite gel with A/NK of 1.3, synthetic F-apatite, synthetic Cl-apatite, and natural Durango apatite. Initial experiments in cold-seal hydrothermal pressure vessels at magmatically realistic temperatures of 750°C and pressures of 200 MPa produced negligible reactions, even after run times of 2000 h. Instead, we used an argon-pressurized internally heated pressure vessel with a rapid-quench setup at temperatures of 1200°C, pressure of 200 MPa, and run durations of 192 h. An advantage of this high temperature is that it exceeds the liquidus for quartz and feldspar; therefore, apatite is the only solid phase in the run products. The starting composition of each run was 90 wt% SANK 1.3 granite gel and 10 wt% crushed apatite (consisting of one, two, or three varieties), with and without 4 wt% added H2O. Run products were examined by SEM for texture and by EMPA and LA-ICP-MS for composition. The starting synthetic granite composition contains no Ca, F, Cl, or REEs thus, in every run, apatite was initially undersaturated in the melt. In all experiments, most large apatite grains consisted of anhedral shards with rounded corners, most small apatite grains were round, and a small proportion of apatite grains developed one or more crystal faces. In experiments with two or three apatite compositions, the run-product apatite grains had compositions intermediate between those of the starting-material grains, and they were homogeneous with respect to Cl, and probably F, but not with respect to REEs. The processes to reach textural equilibrium consist of dissolution until the melt is saturated in apatite, followed by Ostwald ripening to eliminate small grains and to develop crystal faces on larger ones. The processes to reach chemical equilibrium consist of dissolution of apatite, diffusion of cations (Ca, P, REE) and anions (F, Cl, OH) through the silicate melt, and solid-state diffusion in the undissolved apatite grains. The halogens approached chemical equilibrium in all experiments, but in the experiments containing Durango apatite, the REEs have not. Models involving radial diffusion into spherical apatite grains at the temperatures of the experiments show complete re-equilibration of the halogens, but changes in the REE concentrations affecting only the outer few micrometers. We conclude that the rate of chemical equilibrium for the halogens is greater than the rate of physical equilibrium for texture, which in turn is greater the rate of chemical equilibrium for REEs. We illustrate these processes with a natural example of contaminated granite from the South Mountain Batholith in Nova Scotia. Given that all granites are contaminated rocks, we propose that future petrogenetic studies focus on developing techniques for a minerals-based quantitative estimation of contamination (QEC).
To define the liquidus and solidus of the system CaCO3-MgCO, rotating multi-anvil experiments were performed at 6 GPa in the temperature range 1300 to 1800 °C under anhydrous conditions. ...Additionally, experiments under hydrous conditions were performed in the Mg-rich part of the phase diagram. To determine the melting point of the end-member magnesite at 6 GPa falling sphere/body experiments were performed. The run products were analyzed using electron microprobe, Raman spectroscopy, and X-ray diffraction. Some of the run products were investigated by transmission electron microscopy (TEM). Previous studies report tremendous quenching problems in melting experiments of carbonates, as the primary grown carbonates could not be distinguished from the quenched melt. With the help of rotating multi-anvil experiments the primary grown crystals could be separated from the melt phase and the compositions of both phases could be analyzed by electron microprobe. Compared with the results of static experiments the corresponding phase diagram under anhydrous conditions is significantly different. The anhydrous melting point of MgCO3 at 6 GPa could be located between 1750 and 1800 °C. Under hydrous conditions liquidus and solidus moved to lower temperatures compared to anhydrous conditions and the melting point of hydrous MgCO3 at 6 GPa is located between 1700 and 1750 °C.
The origin of Libyan Desert Glass (LDG) found in the western parts of Egypt close to the Libyan border is debated in planetary science. Two major theories of its formation are currently competing: ...(1) melting by airburst and (2) formation by impact-related melting. While mineralogical and textural evidence for a high-temperature event responsible for the LDG formation is abundant and convincing, minerals and textures indicating high shock pressure have been scarce. This paper provides a nanostructural study of the LDG, showing new evidence of its high-pressure and high-temperature origin. We mainly focused on the investigation of Zr-bearing and phosphate aggregates enclosed within LDG. Micro- and nanostructural evidence obtained with transmission electron microscopy (TEM) are spherical inclusions of cubic, tetragonal, and orthorhombic (
or OII) zirconia after zircon, which indicate high-pressure, high-temperature decomposition of zircon and possibly, melting of ZrO
. Inclusions of amorphous silica and amorphous Al-phosphate with berlinite composition (AlPO
) within mosaic whitlockite and monazite aggregates point at decomposition and melting of phosphates, which formed an emulsion with SiO
melt. The estimated temperature of the LDG melts was above 2750 °C, approaching the point of SiO
boiling. The variety of textures with different degrees of quenching immediately next to each other suggests an extreme thermal gradient that existed in LDG through radiation cooling. Additionally, the presence of quenched orthorhombic OII ZrO
provides direct evidence of high-pressure (>13.5 GPa) conditions, confirming theory 2, the hypervelocity impact origin of the LDG.
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