K-rich Minerals Research Articles

Calibration methods for laser ablation Rb–Sr geochronology: comparisons and recommendation based on NIST glass and natural reference materials

Abstract. In situ rubidium–strontium (Rb–Sr) geochronology, using laser ablation–inductively coupled plasma–tandem mass spectrometry (LA-ICP-MS/MS) technology, allows rapid dating of K-rich minerals such as micas (e.g. biotite, muscovite, and phlogopite) and K-feldspar (potassium-containing feldspar). While many studies have demonstrated the ability of the method, analytical protocols vary significantly, and to date, no studies have provided an in-depth comparison and synthesis in terms of precision and accuracy. Here we compare four calibration protocols based on commonly used reference materials (RMs) for Rb–Sr dating. We demonstrate that downhole fractionation trends (DHFs) for natural biotite, K-feldspar, and phlogopite contrast with that for the commonly used Mica-Mg nano powder reference material. Consequently, Rb–Sr dates calibrated to Mica-Mg can be up to 5 % inaccurate, and the degree of inaccuracy appears to be unsystematic between analytical sessions. Calibrating to Mica-Mg also introduces excess uncertainty that can be avoided with a more consistent primary calibration material. We propose a calibration approach involving (1) NIST-610 glass as the primary reference material (PRM) for normalisation and drift correction and (2) a natural mineral with similar DHF characteristics to the analysed samples as matrix correction RM (MCRM) to correct the Rb/Sr ratio for matrix-induced offsets. In this work, MDC phlogopite (the source mineral for Mica-Mg nano powder) was used as the MCRM, consistently producing accurate Rb–Sr dates for a series of natural biotites and K-feldspars with well-characterised expected ages. Biotite from the Banalasta Adamellite, Taratap Granodiorite, and Entire Creek pegmatite are also suitable RMs for Rb/Sr ratio calibration purposes, with consistently <1.5 % fully propagated uncertainties in our methodological approach. Until calibration using isochronous natural standards as the primary RM becomes possible in data reduction software, the two-step calibration approach described here is recommended.

Fungal organic acid uptake of mineral-derived K is dependent on distance from carbon hotspot.

Fungal species are foundational members of soil ecosystems with vital contributions that support interspecies resource translocation. The minute details of these biogeochemical processes are poorly investigated. Here, we addressed this knowledge gap by probing fungal growth in a novel mineral-doped soil micromodel platform using spatially-resolved imaging methodologies. We found that fungi uptake K from K-rich minerals using organic acids exuded in a distance-dependent manner from a carbon-rich hotspot. While identification of specific mechanisms within soil remains challenging, our findings demonstrate the significance of reduced complexity platforms such as the mineral-doped micromodel in probing biogeochemical processes. These findings provide visualization into hyphal uptake and transport of mineral-derived nutrients in a resource-limited environment.

A Mineral-Doped Micromodel Platform Demonstrates Fungal Bridging of Carbon Hot Spots and Hyphal Transport of Mineral-Derived Nutrients.

Soil fungi facilitate the translocation of inorganic nutrients from soil minerals to other microorganisms and plants. This ability is particularly advantageous in impoverished soils because fungal mycelial networks can bridge otherwise spatially disconnected and inaccessible nutrient hot spots. However, the molecular mechanisms underlying fungal mineral weathering and transport through soil remains poorly understood primarily due to the lack of a platform for spatially resolved analysis of biotic-driven mineral weathering. Here, we addressed this knowledge gap by demonstrating a mineral-doped soil micromodel platform where mineral weathering mechanisms can be studied. We directly visualize acquisition and transport of inorganic nutrients from minerals through fungal hyphae in the micromodel using a multimodal imaging approach. We found that Fusarium sp. strain DS 682, a representative of common saprotrophic soil fungus, exhibited a mechanosensory response (thigmotropism) around obstacles and through pore spaces (~12 μm) in the presence of minerals. The fungus incorporated and translocated potassium (K) from K-rich mineral interfaces, as evidenced by visualization of mineral-derived nutrient transport and unique K chemical moieties following fungus-induced mineral weathering. Specific membrane transport proteins were expressed in the fungus in the presence of minerals, including those involved in oxidative phosphorylation pathways and the transmembrane transport of small-molecular-weight organic acids. This study establishes the significance of a spatial visualization platform for investigating microbial induced mineral weathering at microbially relevant scales. Moreover, we demonstrate the importance of fungal biology and nutrient translocation in maintaining fungal growth under water and carbon limitations in a reduced-complexity soil-like microenvironment. IMPORTANCE Fungal species are foundational members of soil microbiomes, where their contributions in accessing and transporting vital nutrients is key for community resilience. To date, the molecular mechanisms underlying fungal mineral weathering and nutrient translocation in low-nutrient environments remain poorly resolved due to the lack of a platform for spatial analysis of biotic weathering processes. Here, we addressed this knowledge gap by developing a mineral-doped soil micromodel platform. We demonstrate the function of this platform by directly probing fungal growth using spatially resolved optical and chemical imaging methodologies. We found the presence of minerals was required for fungal thigmotropism around obstacles and through soil-like pore spaces, and this was related to fungal transport of potassium (K) and corresponding K speciation from K-rich minerals. These findings provide new evidence and visualization into hyphal transport of mineral-derived nutrients under nutrient and water stresses.

Analysis of the Radiological, Mineralogical and Long-Term Sustainability of Several Commercial Aswan Granites Used as Building Materials

The widespread usage of granite in the building sector motivated us to conduct this research and examine the material’s sustainability in terms of the investigated characteristics. The purpose of this paper is to discuss the statistical analysis results for the mineralogical impact on radiological hazards indices, such as the equivalent of radium, absorbed gamma dose rate, annual effective dose, internal and external hazard indices, as well as the gamma-ray index, that were calculated to estimate the environmental risks associated with these granites used as building materials, to protect the public from excessive radioactivity exposure. We focused primarily on statistical significance at a 95% confidence level. We employed a non-parametric test (Kruskal–Wallis Test) rather than a one-way ANOVA, to determine the statistical significance of the samples due to the lack of homogeneity or normality among them. To assess the difference between the samples, we used the Mann–Whitney Test on each pair of samples. Additionally, Pearson correlation coefficients for all the mineralogical results are computed. The presence of K-rich minerals (Kefeldspars, biotite) and accessories such as uranophane, uranothorite, allanite, xenotime, fergusonite, aeschynite, zircon, cassiterite, apatite, and sphene, which are mostly found in granitic rocks, determines the level of natural radioactivity of the investigated granites. Most of the rock samples analyzed have indicators of radioactive dangers that are within the acceptable level range, indicating that they are suitable for use as building materials. On the other hand, some samples have environmental criteria that are higher than international standards, indicating that they are unsuitable for use as construction materials.

Constraints from in-situ Rb-Sr dating on the timing of tectono-thermal events in the Umm Farwah shear zone and associated Cu-Au mineralisation in the Southern Arabian Shield, Saudi Arabia

• In-situ Rb-Sr dating constrains tectonic and igneous events in mineralised systems. • Umm Farwah Major Shear Zone was developed between 671 Ma and 631 Ma. • Ablah pegmatite was emplaced following the shear zone between 626 and 611 Ma. • Greisen and Cu-Au deposits in the Mount Ablah were formed due to two separate hydrothermal events. Constraining the tectonic history and timing of major shear zone structures in the Arabian Shield is critical for better understanding of the origin, tectonic history and distribution of ore deposits within this Precambrian crystalline crustal block. Here, we used a novel in-situ Rb-Sr dating of micas and K-feldspar to constrain the age of the Umm Farwah Shear Zone, which represents a regional tectonic feature that extends about 200 km across the southern part of the Arabian Shield. We also constrain the timing of Mount Ablah mineralisations which are exposed within the Umm Farwah Shear Zone and contains two styles of mineralisations including a greisen and Cu-Au mineralisation. These ore deposits are hosted in a pegmatite body that formed during the development of the Umm Farwah Shear Zone. Two main groups of muscovite (i.e., Group I and Group II) and K-feldspar mineral phases were identified based on ages and elemental variations in the mineralised zone and the host rocks. Results of in-situ Rb-Sr dating in the above K-rich minerals show that the Umm Farwah Shear Zone was initiated at ca. 651 Ma, and the emplacement of the Mount Ablah pegmatite occurred between 626 and 611 Ma, followed by the formation of greisen at 601 ± 12 Ma. In-situ Rb–Sr data also highlight a younger ‘alteration’ event occurred at ca. 556 ± 23 Ma, which partially reset the Rb–Sr system in the pegmatite and may thus reflect hydrothermal circulation event or overprint due to formation of Cu-Au mineralisation.

Revealing mineralization and subsequent hydrothermal events: Insights from 40Ar/39Ar isochron and novel gas mixing lines of hydrothermal quartzs by progressive crushing

Abstract Increasingly more data have indicated that the 40Ar/39Ar crushing technique provides a very useful approach to determine the mineralization ages of hydrothermal ore deposits. In this study novel gas mixing lines are proposed based on the data of two quartz samples by 40Ar/39Ar crushing. Electron microprobe analyses indicate that the quartz grains contain K-rich silicate microlites. Both quartzs by crushing yield two mixing lines with ages corresponding respectively to those of K-feldspar or sericite (post-mineralization fluids) and muscovite samples (ore-forming fluids). This is the first report that both ages of secondary and primary fluid inclusions (SFIs and PFIs) are confirmed correspondingly by those of the K-rich minerals. This study strongly indicates that the 40Ar/39Ar crushing technique is very useful to determine the ages of geofluid activities. The newly proposed gas mixing patterns are shown on the inverse and normal isochron diagrams, indicating: (1) the mixtures are in different proportions of the PFIs and SFIs in different crushing steps; (2) the gas mixing lines yield PFI ages with low initial 40Ar/36Ar ratios; (3) an intermediate age between PFI and SFI ages cannot be obtained because the progressive crushing technique never release gas mixtures with a constant PFI/SFI proportion for each step. The gas mixing lines are thus interpreted to yield significant geological ages. The mixing patterns will help us better understand the 40Ar/39Ar dating results and be used for references to other isotopic mixing isochron systems.

Geology, Geochronology, and Geochemistry of the Dahongshan Fe-Cu-(Au-Ag) Deposit, Southwest China: Implications for the Formation of Iron Oxide Copper-Gold Deposits in Intracratonic Rift Settings

The large Dahongshan Fe-Cu-(Au-Ag) deposit in the Kangdian iron oxide copper-gold (IOCG) metallogenic province, southwest China, contains approximately 458.3 Mt of ore at 41.0% Fe, 1.35 Mt Cu (metal) at 0.78% Cu, and significant amounts of Au (16 t), Ag (141 t), Co (18,156 t), and Pd + Pt (2.1 t). The deposit consists mainly of two types of ores: (1) lenses of massive or banded magnetite-(hematite) hosted in extensively Na metasomatized metavolcanic rocks, metaarenite, and brecciated rocks, and (2) strata-bound disseminated, stockwork, and banded magnetite-chalcopyrite-(bornite) in mica schist and marble. Both types of orebodies and country rocks underwent extensive hydrothermal alteration, resulting in a similar paragenesis. Pervasive stage I sodic alteration formed widespread albite and local scapolite. It was subsequently replaced by Ca- or K-rich minerals represented by actinolite, K-feldspar, biotite, sericite, and chlorite of stages II and III. Magnetite is slightly younger than and partly overlaps the sodic alteration assemblages. Hematite is texturally later than magnetite, is locally abundant within the massive Fe oxide orebody, and is closely associated with sericite. Copper sulfides are coeval with quartz, biotite, sericite, and chlorite in stage III assemblages. Widespread siderite and ankerite predominate in stages II and III, respectively. Quartz-calcite veins mark the result of waning stage IV hydrothermal alteration. In addition to widespread alteration during the major ore-forming event, the deposit has also undergone extensive overprinting and remobilization during post-ore magmatic and metamorphic events. The Dahongshan orebodies are intimately associated with abundant doleritic dikes and sills that have hydrothermal mineral assemblages similar to those in the ore-hosting rocks. One dolerite sill that cuts a massive Fe orebody has a laser ablation-inductively coupled plasma-mass spectrometry zircon U-Pb age of 1661 ± 7 Ma, which is, within uncertainty, consistent with the age of 1653 ± 18 Ma determined for hydrothermal zircons from stockwork chalcopyrite-magnetite ore. The zircon U-Pb ages are thus considered to mark the timing of major mineralization that formed the Dahongshan deposit. Post-ore modification is recorded by an Re-Os isochron age of 1026 ± 22 Ma for pyrite in discordant quartz-carbonate-sulfide veins, and by younger Neoproterozoic mineralization dated at ca. 830 Ma using Re-Os isotopes on molybdenite. The former age is contemporaneous with late Mesoproterozoic magmatism in the region, whereas the latter is coeval with regional Neoproterozoic metamorphic events in southwest China. Carbon and oxygen isotope values of albitized marble are between those of mantle-derived magmatic carbon and dolostone end members. The ore-forming fluids that equilibrated with stage II magnetite have δ 18 O values of 9.1 to 9.5‰, whereas fluids linked to the deposition of quartz and ankerite during stages III and IV have lower δ 18 O values of 2.9 to 7.3‰. The oxygen isotope data indicate that the ore-forming fluids related to stage II are chiefly magmatically derived and mixed with abundant basinal brine during stages III and IV; this interpretation is consistent with sulfur isotope values of sulfides in the deposits. Pyrite and chalcopyrite from the Dahongshan deposit have a large range of δ 34 S values from −3.4 to +12.4‰, implying mixing of magmatic and external sulfur (likely from basinal brines) in sedimentary rocks. The Dahongshan deposit formed in an intracratonic rift setting due to underplating by mafic magmas that induced large-scale fluid circulation and pervasive sodic-calcic metasomatism in country rocks. Ore metals were derived mainly from a deep-seated magma chamber and partly from country rocks. Hydrothermal brecciation of the country rocks formed at the top of the dolerite intrusions and along zones of weakness within the country rocks owing to overpressure imposed by the ore fluids. Magnetite and hematite precipitated early near the dolerite intrusions, whereas Cu sulfides formed later in country rocks where sulfide saturation was favored. We propose that this genetic model may be widely applicable to Precambrian IOCG deposits elsewhere that formed in intracratonic rift settings.

Metasomatic column with the apobasite listvenites end-member from the Slovinky-Gelnica ore field (Gemeric Superunit, Western Carpathians) and comparison to their type localities from Ural

A metasomatic column (or metasomatic zone pattern) is the complete sequence of metasomatic zones characterising an individual metas omatic facies (Zharikov, 2007). The genesis of this column is the result of a series of processes, in the case of basic rock protolith leading to the formation of apobasite listvenites (in the sense of Sazonov, 1975, 1978). It includes three partial processes: 1) carbonatization, 2) silicification and 3) formation of mica (e.g. Plissart et al., 2009) involving the infiltration of a CO 2 -rich fluids and shows similarity to processes of listvenitisation (e.g. Clayton, 1993). Field research in Slovinky-Gelnica ore field has revealed the presence of such a metasomatic column (cf. Ivan, 1987) . It comprises metabasites as the protolith, carbonatised metabasites, listvenite-type metasomatites and listvenites s.s. The metabasites have features of subvolcanic rock types ( Ivan, 2009) , what makes them comparable with similar rocks from Ural. The petrographic study of selected samples has showed that amphibole, chlorite and albite became progressively unstable in this sequence of alterations, replaced by carbonate, silica and lastly by the muscovite minerals, sericite or fuchsite (Ivan, 1987). Carbonates change their quantity as well as mineralogical composition with the development of the column in two trends of changes: towards Mg-Fe carbonates or Mg-Ca carbonates. Regarding the samples from Ural, Fe-dolomites and ankerites predominate, the metasomatic columns derived from gabbros and dolerites are essentially the same. The geochemical study of rock samples from the study area (22 whole rock analyses) as well as the comparison with similarly metasomatised rock series from Ural (56 analyses) allowed as to give more detailed characterization of the metasomatic column. The most obvious change is a clear increase of K with decreasing Na as a sign of the albite break-up and K-rich minerals (such as muscovite) formation. The increase of LOI and CO 2 indicate the process of carbonatisation. Listvenitisation is often associated with the hydrothermal ore veins formation and the rare metal mineralization with concentration of elements such as Au, Co, Sb, Cu, Ni (e.g. Dinel, 2008; Buckman & Ashley 2010 ). This could be significant as regards the genesis of stratiform ore deposits within the Slovinky-Gelnica ore field.

Snow shielding factors for cosmogenic nuclide dating inferred from long-term neutron detector monitoring

The depth-dependent attenuation of the secondary cosmic-ray particle flux due to snow cover and its effects on production rates of cosmogenic nuclides constitutes a potential source of uncertainty for studies conducted in regions characterized by frequent seasonal snow burial. Recent experimental and numerical modelling studies have yielded new constraints on the effect of hydrogen-rich media on the production rates of cosmogenic nuclides by low- and high-energy neutrons (<10−3 MeV and >102 MeV, respectively). Here we present long-term neutron-detector monitoring data from a natural setting that we use to quantify the effect of snow cover on the attenuation of fast neutrons (0.1–10 MeV), which are responsible for the production of 21Ne from Mg and 36Cl from K. We use data measured between July 2001 and May 2008 at seven stations located throughout the Ecrins-Pelvoux massif (French Western Alps) and its surroundings, at elevations ranging from 200 to 2500 m a.s.l. From the cosmic-ray fluxes recorded during summer, when snow is absent, we infer an apparent attenuation length of 148 g cm−2 in the atmosphere at a latitude of ∼45°N and for altitudes ranging from ∼200 to 2500 m a.s.l. Using snow water-equivalent (SWE) values obtained through snow-coring campaigns that overlap in time the neutron monitoring for five stations, we show that fast neutrons are much more strongly attenuated in snow than predicted by a conventional mass-shielding formulation and the attenuation length estimated in the atmosphere. We suggest that such strong attenuation results from boundary effects at the atmosphere/snow interface induced by the high efficiency of water as a neutron moderator. Finally, we propose an empirical model that allows calculating snow-shielding correction factors as a function of SWE for studies using 21Ne and 36Cl analyses in Mg- and K-rich minerals, respectively. This empirical model is of interest for studies with a focus on cosmic-ray exposure dating, particularly if the target rocks are made up of mafic to ultramafic units where seasonal snow-cover is a common phenomenon.

Groundwater recharge and hydrogeochemical evolution in the Ejina Basin, northwest China

Groundwater plays a dominant role in the eco-environmental protection of arid/semi-arid areas. Understanding sources and mechanisms of groundwater recharge in the Ejina Basin, an arid inland river basin in northwest China, is important for water resource planning in this ecologically sensitive area. In this study, 90 water samples were collected from rainfall, rivers and lakes, and springs and pumping wells in 2009. Analysis of the aquifer system and hydrological conditions, together with hydrogeochemical and isotope techniques were used to investigate groundwater sources and their associated recharge processes. Our results show that shallow phreatic and deep confined groundwater differ greatly in their compositions, with a distinct spatial heterogeneity of phreatic groundwater TDS (from 365 mg/L to 5833 mg/L), which increase along the shallow groundwater flow paths. Groundwater chemical evolution is mainly controlled by rock dominance and the evaporation-crystallization process, and the dominant anion species change systematically from HCO3 to SO4 to Cl, and the dissolved ions within the groundwater system from Na- and K-rich minerals and sulfate phases also contribute significantly to the groundwater composition. The stable isotope levels (delta O-18 and delta H-2) of the surface water and the shallow phreatic groundwater confirm that the Heihe River and Badain Jaran Desert groundwater are the main sources recharging the phreatic aquifer in the Ejina Basin. Thus, river infiltration and desert front recharge should be considered as the two main recharge mechanisms of the Ejina aquifer. However, recharge from the Badain Jaran Desert aquifer to the Ejina Basin has occurred at a lower rate due to acidification since the middle Holocene. For this reason, the sustainable improvement of the ecological environment should be based on the shallow groundwater recharge of the phreatic aquifer in the Ejina Delta, which mainly takes place via seepage through the riverbed and direct infiltration during periods of environmental flow control. (C) 2012 Elsevier B.V. All rights reserved.

The implication of sodium-rich plagioclase minerals contaminating K-rich feldspars aliquots in luminescence dating

Luminescence dating was performed on three different sediments, collected in Italy. Measurements were accomplished on K-rich minerals extract, where we dispensed a small amount of grains on each aliquots. Anomalous fading was measured for each aliquot. We observed a negative correlation between the measured age and associated rate of fading, for each sample. In two samples, a few outliers were identified, where they displayed a lower measured age and fading rate. We positively identified the presence of Na-rich plagioclase minerals dominating the luminescence output of these outliers.

Origin of Cenozoic alkaline potassic volcanic rocks at KonglongXiang, Lhasa terrane, Tibetan Plateau: Products of partial melting of a mafic lower-crustal source?

Abstract It is generally believed that Cenozoic potassic and ultrapotassic volcanic rocks of the Tibetan Plateau were generated by partial melting of enriched mantle. This paper reports new data regarding the elemental and isotopic compositions of the KonglongXiang volcanic rocks (KVR; K2O/Na2O > 2 and low MgO contents) in the middle–western Lhasa terrane, Tibetan Plateau. Ar–Ar isotope dating of a sample from the KVR yields an age of 21.3 Ma, indicating that the KVR lavas erupted during the Early Miocene. The KVR consists mainly of trachyte and rhyolite. A comparison of the KVR with Cenozoic potassic volcanic rocks of the Tibetan Plateau and the results of melting experiments involving K-rich volcanic rocks, as well as other K-rich volcanic rocks from throughout the world, reveals that the KVR trachytes originated from a mafic lower crust containing K-rich minerals rather than from a mantle source. The KVR rhyolites were probably derived from the middle–upper crust. Based on age data, compositional data, and the regional tectonic setting, we propose that the formation of the KVR and associated N–S grabens was triggered by Early Miocene E–W regional extension in the area of the southern Tibetan Plateau.

Precise dating of biotite in distal volcanic ash: Isolating subtle alteration using 40Ar/39Ar laser incremental heating and electron microprobe techniques

Precise dating of K-rich minerals in volcanic rocks via the 40 Ar/ 39 Ar geochronometer has become crucial to resolving many geological problems. In some cases, ash beds containing biotite phenocrysts but lacking sanidine are the only datable horizons at key stratigraphic intervals, necessitating comparison of sanidine and biotite ages within the same chronostratigraphic framework. To assess the integrity of biotite and evaluate the accuracy of ages obtained from this often problematic mineral, incremental heating 40 Ar/ 39 Ar experiments were performed on large, millimeter-sized, euhedral biotite crystals from four key Eocene tuffs in the Green River and Wind River Basins and are compared with sanidine 40 Ar/ 39 Ar ages. Unaltered biotite crystals with homogenous K-compositions from two tuff beds yielded concordant plateau ages that are indistinguishable from cogenetic sanidine ages. In contrast, biotite crystals from two other tuffs yielded discordant spectra, with relatively young initial steps followed by older, down-stepping apparent ages. Plateau ages from the discordant experiments are older than the sanidine ages by 1 to 14%. Integrated (total fusion) ages from these experiments exhibit scatter toward both younger and older ages that correlate with the degree of spectral discordance. Electron microprobe transects reveal that biotite crystals yielding discordant age spectra contain 1-10 μm thick K-depleted (&lt;8% K 2 O) alteration zones along internal cleavage planes that are absent in biotite crystals yielding concordant age spectra. We propose that these altered zones promote open-system behavior. Phenomena such as loss of K or 40 Ar*, 39 Ar K recoil into internal K-depleted phases, and 39 Ar K recoil entirely out of internally corroded biotite crystals are all potential mechanisms that can be related to this alteration. Due to the presence of these multiple potential pathways that promote the gain or loss of isotopes, such altered biotite crystals are unsuitable for high-resolution 40 Ar/ 39 Ar dating, comparisons with sanidine ages, or calibration of the geomagnetic polarity timescale.

Conjunctive K–Ca and Rb–Sr dating of glauconies

The glaucony series (of which glauconitic mica is the K-rich end member) is one of the few widely occurring minerals in the sedimentary record. So their isotopic ages determined mainly by the K–Ar method represent a major database for the geologic time scale. Many K–Ar ages were, however, found to be too young and often also discordant with available corresponding Rb–Sr ages. This discrepancy may reflect the disparate responses of the chemically contrasting daughter elements, Ar and Sr to postdepositional processes, and renders either age equivocal. A potentially promising approach to evaluate the intrinsic potential of glauconies and other K-rich minerals for direct and reliable dating of sediments is the conjunctive use of the chemically very similar K–Ca and Rb–Sr systems. The relationship between K–Ca and Rb–Sr ages may show a coherence or regularity indicative of a geologically meaningful age like in the case of the well known chemically identical dual U–Pb decay systems. As a crucial first step, this report presents the first successful precise K–Ca dating in conjunction with Rb–Sr dating of an Ordovician glauconite using the same sample aliquot, chemical procedure and mass spectrometer.

Controls on 87Sr/ 86Sr ratios of groundwater in silicate-dominated aquifers: SE Murray Basin, Australia

87Sr/ 86Sr ratios of groundwater in the southeast Riverine Province of the Murray Basin, Australia are between 0.7107 and 0.7191. The 87Sr/ 86Sr ratios vary between different subcatchments and generally decline with distance northwards from the basin margins. There are few carbonates in this region, and Sr is primarily derived from silicate minerals. The spatial variation in 87Sr/ 86Sr ratios reflects the distribution of K-rich minerals, such as biotite and K-feldspar in the aquifers. However, major ion chemistry implies that silicate weathering is only a minor process. Sr isotope ratios are most probably controlled by exchange on clays derived from weathering of the silicate minerals. Ion exchange is promoted by the low groundwater flow rates in the Riverine Province and the clay-rich nature of many of the aquifers.

Ion Microprobe Analysis of Rb and Sr Isotopes in K-rich Minerals: An Attempt of Radiometric Dating

We report here in situ measurements of Rb and Sr isotopes in K-rich minerals (muscovite, phlogopite, and K-feldspar) using Sensitive High-Resolution Ion MicroProbe (SHRIMP). Several grains of the K-rich minerals were extracted from eight granitic rocks with various formation ages and mounted on epoxy-resin disks. Chemical compositions were measured by an electron probe microanalyzer (EPMA). Then they were set in the source chamber of SHRIMP. A 4-5 nA mass filtered O2- primary beam was used to sputter a 30-μm-diameter flat-bottomed crater and secondary positive ions were extracted by a 10 kV accelerating voltage for mass analysis. A mass resolution of 8900-9300 at 1% peak height was attained to reduce isobaric hydride interferences such as 87RbH+ on 88Sr+. The magnet was cyclically peak-stepped from mass 84 (84Sr and 28Si56Fe) to mass 88 (88Sr and 30Si58Fe), including the background, mass 85 for 85Rb and 29Si56Fe, mass 86 for 86Sr and 30Si56Fe, and mass 87 for 87Rb, 87Sr, and 29Si58Fe, respectively. No SiFe compounds of any mass number contributed significantly for the samples analyzed. Observed 87A/85A ratios (where A is an arbitrary element) were plotted against 88A/85A ratios for each sample. A least-square fitting was made in the diagram. The gradient and Y-intercept correspond to initial 87Sr/88Sr and initial 87Rb/85Rb ratios or present (87Rb/85Rb)+(87Sr*/85Rb) ratios, respectively, where “ * ” denotes radiogenic. The initial 87Sr/88Sr ratios are mostly consistent with the upper mantle 87Sr/88Sr ratios of various ages within experimental error. There is a positive correlation between the initial 87Rb/85Rb ratios and formation ages of the granitic rock samples, as it is expected from 87Rb decay. Further evaluation of the instrumental isotopic mass fractionation and relative sensitivities for Rb and Sr are required before the in situ 87Rb-87Sr measurement is routinely applied as a dating method.

The Elatsite porphyry copper deposit in the Panagyurishte ore district, Srednogorie zone, Bulgaria: U-Pb zircon geochronology and isotope-geochemical investigations of magmatism and ore genesis

Abstract Single zircons from several porphyry dykes bracketing the time of formation of the Elatsite porphyry Cu-Au deposit (Bulgaria) were dated by high-precision U-Pb isotope analysis, using thermal ionization mass spectrometry (TIMS). On the basis of cross-cutting relationships, and the mineralogy and geochemistry of igneous and altered rocks, five dyke units are distinguished. The earliest porphyry dyke is associated with, and overprinted by, the main stage of ore-related veining and potassic alteration. U-Pb analyses of zircons yield a mean 206 Pb/ 238 U age of 92.1 ± 0.3 Ma, interpreted to reflect the time of intrusion. Zircons of the latest ore forming dyke, crosscutting the main stage veins but still associated with minor potassic alteration and veining, give an intrusion age of 91.84 ± 0.3 Ma. Thus, ore mineralization is confined by individually dated igneous events, indicating that the entire time span for the ore-forming magmatism and high temperature hydrothermal activity extended over a maximum duration of 1.1 Ma, but probably much less. Zircon analyses of a late ore dyke cutting all ore veins and hosting pyrite as the only sulphide mineral give a concordant 206 Pb/ 238 U age of 91.42 ± 0.15 Ma. Based on a spatial relationships of the magnetite-bornite-chalcopyrite assemblage with coarse-grained hydrothermal biotite and K-feldspar, a Rb-Sr age of 90.55 ± 0.8 Ma is calculated using the two K-rich minerals. This age is interpreted as a closing date for the Rb-Sr system at T ≈ 300 °C consistent with published K-Ar data. Therefore the entire lifespan of the magmatic-hydrothermal system is estimated to have lasted about 1.2 Ma. Soon after, the Cretaceous complex was exposed by erosion, as shown by palaeontologically dated (Turonian; 91–88.5 Ma) sandstones containing fragments of porphyry dykes. Geochemical discrimination ratios suggest a mixed mantle and crustal source of the Cretaceous magma. Isotope analyses of Sr, Nd and Hf confirm the conclusion that all porphyry rocks within and around the Elatsite deposit originate from an enriched mantle source at Cretaceous times, with crustal contamination indicated by moderately radiogenic Pb.

Heat Transfer Mechanisms in Magmas: Constraints from Ar Loss in Thermal Aureoles

Thermal exchange between magmas and their country rocks governs important processes such as fractional crystallisation, crustal melting and assimilation. Theoretical models predict extreme differences in wall rock heating between dykes or sills through which magmas flow in either turbulent or laminar mode. Turbulent flow theoretically causes much higher temperatures in wall rocks, and thus assimilation during ascent of magmas. However, such behaviour has never been quantitatively measured in natural samples. Radiogenic argon (4~ loss from K-rich minerals has long been recognised adjacent to igneous intrusions though its ability to measure heat flow quantitatively depends upon the measurement

40Ar/39Ar Dating of Langbeinite [K2Mg2(SO4)3] in Late Permian Evaporites of the Salado Formation, Southeastern New Mexico, USA

Late Permian stratigraphy of the southwestem United States reveals an overall shoaling trend between the Guadelupian Stage and the end of the Permian. The shoaling sequence features an upsection progression between marine carbonates, evaporates, and terrestrial red-beds. The virtual absence of fauna in the evaporates and red-beds, which define the Ochoan Series, precludes biostratigraphic age assignment. Thus the duration of the shoaling sequence, and correlation with the global Late Permian record of events is problematic. 4~ (sanidine) dates for tufts in red-beds of the Quartermaster Fm., at the top of the Ochoan Series in west Texas, bracket the 250.0 Ma 4~ age determined for the Permian/Triassic boundary at the Meishan section in south China (Renne et al., 1995). In an effort to extend the geochronological framework downsection, we have undertaken 4~ analysis of K-rich evaporate minerals from the Salado Formation. The Salado Formation comprises c. 600-700 m of hypersaline evaporites whose top lies c. 100 m below the Quartermaster Fm. through much of west Texas and southeastern New Mexico (e.g. Lowenstein, 1988). Nine horizons of K-rich bittern salts occurring in the McNutt Potash Zone of the Salado Fm. are mined for potash, and commonly contain the mineral langbeinite whose composition is K2M92(S04)3. Langbeinite in other occurrences has been dated by K-Ar (e.g. Pilot and Blank, 1967) and 4~ methods (Lippolt and Oesterle, 1977; Lippolt et al., 1993), and in some cases yielded stratigraphically plausible ages. A sample from the IMC Kalium mine (500 foot level, near the top of the Salado Formation) near Carlsbad N. M. was analysed by incremental heating in a resistance furnace in two experiments. Both samples were about 3 mg single crystals of transparent, highly lustrous langbeinite wrapped in Cu foil. Temperature steps ranged from 450 to 1300~ and were of 600 s and 1800 s duration in the first (IMC-I.1) and second (IMC-1.2) experiments, respectively. Gas release was relatively sluggish up to near the melting temperature, at c. 880~ Based on linear Arrhenius relations (assuming spherical diffusion geometry for this isometric mineral), 39ARK release over the temperature range 600 to 850~ in vacuo appears to occur by volume diffusion, with activation energies of 42.7 _+ 5.7 and 42.6 -t1.9 kcal/mol. These results are indistinguishable from the activation energy of 44 kcal/mol determined by Lippolt and Oesterle (1977) for 4~ diffusion in two different langbeinite samples, and confirm their conclusion that langbeinite is highly argonretentive.

Cosmogenic chlorine-36 production in K-feldspar

The high production rate of in-situ cosmogenic 36Cl from potassium allows K-rich minerals to be dated with high sensitivity and precision. K-rich minerals, such as K-feldspar and biotite are commonly associated with quartz in which 26Al and 10Be are produced, allowing three cosmogenic nuclides, with half-lives ranging from 0.3 Ma to 1.5 Ma, to be measured in a single sample. To calibrate the production rate of 36Cl from potassium we have measured K-feldspar samples from ice-scoured bedrock from three separate sites, covering a range of altitudes, latitudes and exposure ages. The results from each site are internally consistent, with eight glacial pavements from the Sierra Nevada giving a production rate of 1290 ± 220 atom/(gK)/yr at 3000 m and 38°N, three Scottish glacial pavement samples giving 301 ± 27 atom/(gK)/yr at 520 m and 58°N, and two Antarctic bedrock samples giving 1310 ± 50 atom/(gK)/yr at 2000 m and 70°S. Normalising these production rates to sea level and high latitude, results from the Sierra Nevada and Scotland are found to be in good agreement with each other, but the production rate derived from the Antarctic samples is found to be approximately 35% higher. Repeat measurements and stepwise dissolution indicate that the high 36Cl concentration of the Antarctic samples is real and cannot be attributed to meteoric 36Cl. We must therefore conclude that either the 36Cl production rate differ on the 104 and 106 yr time scales, or that the current altitude scaling factor applied to the Antarctic results underestimates the true scaling factor for the Antarctic atmosphere. The production rates derived above are based principally on K-feldspars with low chloride contents. Samples with high chloride contents required corrections for 36Cl produced by neutron capture on 35Cl. The 36ClCl ratio of this component was measured directly in four samples from the Sierra Nevada after crushing the K-feldspar to release Cl-rich fluid from inclusions. The 36ClCl ratios measured appear to be in reasonable agreement with values predicted by recent calibrations of the production by neutron capture on 35Cl, but the final comparison awaits measurements of uranium, thorium and neutron absorber (B, Gd, Sm) concentrations in our samples.