Igneous Lithologies Research Articles

Compositional balance analysis for Cu–Zn(±Co) geochemical anomaly mapping in Trøndelag county, Norway

Trøndelag county has been recently the focus of a new mineral exploration wave given its well-known base metal mineralization potential associated with the occurrence of volcanogenic Cu–Zn (±Co ± Ag ± Au) massive sulfide deposits (VMS). This study evaluates a regional low-density soil sampling survey by implementing principal component analysis (PCA), and compositional balance analysis (CoBA) to target prospective areas for base metal mineralization. Principal component analysis indicates a dominant base metal mineralization signature characterized by a correlation between As and Cu–Cr–Co–Ni, which is consistent with the occurrences of mafic metavolcanic and associated low K–Ca metasedimentary rocks. However, several other Cu–Zn occurrences hosted in K- and/or Ca-rich (meta-)sedimentary (e.g., Røros districts) and felsic (meta-)igneous lithologies have been overlooked by this approach. Data- and knowledge-driven balances (i.e., isometric log-ratios) were then constructed to enhance these “weaker” Cu–Zn (±Co) mineralization fingerprints. Most of the data-driven balances (dcobals) are shown to be very noisy, and less useful for base metal geochemical anomaly mapping compared to their more coherent knowledge-driven counterparts (kcobals). However, the kcobals (e.g., for Cu, Zn and Co) highlight relatively large anomalous areas, which makes them less practical if used as individual exploration vectors. To further define zones of economic interest, common anomalous areas (defined as kcobal >75th percentile) have been filtered out from selected kcobal pairs. This has resulted in the identification of several prospective areas that correspond with 15%–20% of the Trøndelag county.This study not only explores the combined use of CoBA and PCA for detecting base metal anomalies, but also elaborates on some factors that can affect the interpretation and performance of multivariate approaches (e.g., mineralization/alteration extension versus survey resolution, geological framework, and deposit type). The visualization and implementation of CoBA presented in this paper aim to improve geoscientists’ understanding of the reach and limitations of using isometric log-ratios for mineral exploration studies.

Discrimination of tectonic provinces using zircon U-Pb ages from bedrock and detrital samples in the northern Andes

Abstract The northern Andes of southern Colombia contain a rich geologic history recorded by Proterozoic to Cenozoic metamorphic, igneous, and sedimentary rocks. The region plays a pivotal role in understanding the evolution of topography in northwestern South America and the development of large river systems, such as the Amazon, Orinoco, and Magdalena rivers. However, understanding of the basement framework has been hindered by challenging access, security concerns, tropical climate, and outcrop scarcity. Further, an insufficient geochronologic characterization of Andean basement complicates provenance interpretations of adjacent basins and restricts understanding of the paleogeographic evolution of southern Colombia. To address these issues, this paper presents a zircon U-Pb geochronological dataset derived for 24 bedrock samples and 19 modern river samples. The zircon U-Pb results reveal that the Eastern Cordillera of southern Colombia is underlain by basement rocks that originated in various tectonic events since ca. 1.5 Ga, including the accretion of discrete terranes. The oldest rocks, found in the Garzon Massif, are high-grade metamorphic rocks with contrasting Proterozoic protolith crystallization ages. Whereas the SW part of the massif formed during the Putumayo Orogeny (ca. 1.2–0.9 Ga), we report orthogneisses for the NE segment with protoliths formed at ca. 1.5 Ga, representing the NW continuation of the Rio Negro Jurena province of the Amazonian Craton. In contrast, crystalline rocks of the Central Cordillera primarily consist of Permian–Triassic (ca. 270–250 Ma) and Jurassic–Cretaceous (ca. 180–130 Ma) igneous rocks formed in a magmatic arc. In southernmost Colombia, the Putumayo Mountains mainly consist of Jurassic–Cretaceous (180–130 Ma) plutonic and volcanic rocks. Furthermore, we analyzed the heavy mineral abundances in modern river sands in southern Colombia (spanning 1°N–5°N) and found that key minerals such as garnet and epidote can be utilized to trace high-grade metamorphic and igneous lithologies, respectively, in the river catchments. The differentiation of basement ages for separate tectonic provinces, combined with heavy mineral abundances in modern sands, can serve as unique fingerprints in provenance analyses to trace the topographic and exhumational evolution of different Andean regions through time.

A new continental hydrogen play in Damara Belt (Namibia).

Serpentinization is commonly presented as the main source of natural hydrogen (H2) in the continental domains. However, recent works in Australia and Brazil showed that Archean-Paleoproterozoic banded iron formations could be another natural source of H2 gas. Although the reaction that produces hydrogen is similar (Fe2+ oxidation-H2O reduction process), the iron content may be higher in banded iron formations than in mafic igneous lithologies, potentially generating H2 more efficiently. Here, we present structural evidence that reported H2 emissions from Waterberg Basin, Namibia are associated with underlying Neoproterozoic banded iron formations-the Chuos Formation. Magnetite, a known H2-generating mineral, is ubiquitous and accompanied by other suspected H2-generating minerals (biotite and siderite) in Chuos Formation. Magnetite occurs either as pervasive cm to dm continuous metamorphic laminations in foliation and fractures planes and/or diffusely disseminated in metachert and metacarbonate levels. From this, we infer that metamorphism does not negatively affect the Fe2+ content that is required to generate hydrogen. H2 seepages in Waterberg Basin suggest that an active H2-generating system may exist at depth and that the presence of potential traps and reservoirs is likely based on field observations.

Prediction of igneous lithology and lithofacies based on ensemble learning with data optimization

Igneous rocks are widely developed in various Mesozoic and Cenozoic continental and marine basins. Igneous reservoirs are the key reservoirs for current oil and gas development. Accurate prediction of lithology and lithofacies is a prerequisite for the effective exploration of igneous reservoirs. Igneous lithology and lithofacies are complex and correlated. The existing single-label igneous rock identification methods only consider the prediction of individual properties, and less consideration is given to the correlation of reservoir properties. Therefore, lithology and lithofacies prediction based on conventional logging data is regarded as a typical class-imbalanced multilabel classification problem when considering both attribute correlation in algorithms and evaluation metrics. To solve this problem, an ensemble method of data optimization combined with multigrained cascade forest (CF) is used in this study to develop a new multilabel lithology and lithofacies prediction model based on data from nine conventional logs in the eastern depressional reservoirs of the Liaohe Basin, and satisfactory results are obtained. The imbalance problem of the conventional logging data sets is first solved by using K-means and synthetic minority oversampling technique methods; then, the model is trained by scenario transformation and stripping with multigrained CF; and next, a multilabel classification evaluation index with multiple perspectives is introduced. The differences between the model and typical intelligent algorithms such as CF, adaptive boosting, random forest, and support vector machine are compared in simulated wells, and the new model is found to have obvious advantages. The model is finally applied to an actual well, and the accurate prediction results illustrate that the new model designed and trained for the class imbalance multilabel classification problem in this paper has application value in the lithology and lithofacies multilabel prediction of igneous rocks and also provides a theoretical basis for more complex multilabel reservoir evaluation using machine learning in the future.

Vanadian–Chromian Spinels (Sludyanka Metamorphic Complex, South Baikal Area). Magnesian Spinel

Abstract —The paper presents data on two groups of magnesian Cr–V spinel occurring as rock-forming minerals in various types of Cr–V bearing rocks of the Sludyanka metamorphic complex (South Baikal area, Siberia, Russia): (i) low-Al magnesiochromite–magnesiocoulsonite spinels and (ii) Al spinels varying from V-bearing aluminous magnesiochromite and Al–Cr coulsonite to spinel proper with minor contents of Cr and V, and with complex zoning patterns. Spinels in the Sludyanka rocks are also present as iron-magnesian accessories, among which high-Cr varieties are compositionally similar to mantle-derived spinel from igneous rocks and to inclusions in diamonds. Comparison of formation conditions for mantle spinels in igneous lithologies and spinel phases in metamorphic rocks shows that their mineral chemistry is mainly controlled by the bulk composition of the igneous and sedimentary protoliths, respectively. Detrital spinel may be a false indicator poorly applicable to paleogeodynamic reconstructions and diamond potential evaluation.

A c. 1900 Ma Tethyan-type ophiolite in the Penokean Orogen, Pembine, Wisconsin (USA): Insights from the volcanic stratigraphy

Orogenic ophiolites are a hallmark of Phanerozoic plate tectonics, containing igneous lithologies that provide constraints on fundamental tectono-magmatic processes. The c. 1900Ma Pembine Ophiolite (Wisconsin, USA) is associated with the Penokean Orogen and represents a rare example of a proposed Paleoproterozoic ophiolite. The Penokean Orogen shares broad characteristics with Phanerozoic (<541 Ma) orogens, but the origin of the Pembine Ophiolite remains unclear, with the mafic volcanic rocks interpreted as representing either an intra-oceanic arc or continental back arc setting. To test these hypotheses, we present the results of petrography, bulk-rock geochemistry and mineral chemistry for a suite of 34 Pembine rocks, as well as U-Pb zircon geochronology for two samples. Based on trace elements established as immobile in the studied rocks, we demonstrate that mafic volcanism progressed (up-stratigraphic-section) from mid-ocean ridge-like to boninitic. The chemical evolution is identical to that observed in < 250 Ma ophiolites in the Himalayan–Alpine Orogen, which record forearc spreading during the nascent stages of subduction in the Tethys Ocean. We interpret the Pembine Ophiolite as forearc lithosphere formed during subduction initiation and obducted to the margin of the Superior Craton during the Penokean Orogeny. The processes responsible for forming (and preserving) this example of a Paleoproterozoic ophiolite may not have been dissimilar to those operating on the Phanerozoic Earth.

In Situ Geologic Context Mapping Transect on the Floor of Jezero Crater From Mars 2020 Perseverance Rover Observations

AbstractIn situ geologic context mapping based on rover and helicopter observations provides documentation of a nearly continuous record of geology and exposed surface structure over a 120 m‐wide corridor along the traverse of the Mars 2020/Perseverance rover. The results record the geologic context of Mars 2020 campaign sites and sample sites, including the local extent of bedrock outcrops, stratigraphy, attitude, and structure from imaging and rover‐based remote sensing, and outcrop lithology based on in situ proximity science. Mapping identifies a sequence of igneous lithologies including (a) early mafic, possibly intrusive, rocks; (b) pervasively fractured and deeply altered massive bedrock of undetermined protolith; (c) buried and exhumed lava flows with pahoehoe and aa textures; (d) several varieties of regolith; and (e) small impact craters.

Apatite laser ablation Lu[sbnd]Hf geochronology: A new tool to date mafic rocks

Mafic rocks are the most common type of igneous rocks on Earth, however, constraining the crystallization age of mafic rocks can be challenging. Apatite is a common accessory phase in mafic rocks and is amenable to dating using the U–Pb system. However, the U–Pb system in apatite has a relatively low closure temperature (∼350°-550 °C) and is therefore prone to resetting by later thermal and metasomatic events. Here, a recently developed LuHf dating method using laser ablation reaction-cell mass spectrometry is applied to apatite from mafic rocks. The LuHf system in apatite has a higher closure temperature (∼650°-750 °C) compared to U–Pb, increasing the chances of obtaining primary crystallization ages. Furthermore, the laser-ablation method allows rapid data collection compared to traditional solution-based LuHf dating techniques. Four study areas were selected to compare the LuHf vs U–Pb systematics of apatite in mafic igneous rocks: the Paleoproterozoic Sudbury Igneous Complex (Canada), the Neoproterozoic Borborema Province (NE Brazil), the Paleoproterozoic Fennoscandian Shield (Finland), the Archean Yilgarn Craton and adjacent Mesoproterozoic Albany Fraser Orogen (Western Australia). For all analyzed samples that have apatite trace element compositions typical of an undisturbed primary mafic igneous lithology, the LuHf system retains primary igneous apatite crystallization ages, whereas the U–Pb system in the same grains often records isotopic disturbance or a cooling age. In few cases, the LuHf system has also been disturbed in response to recrystallization, however, such disturbance is readily detected with trace element data. Hence, this study demonstrates the potential of laser ablation apatite LuHf dating to obtain primary crystallization ages for otherwise difficult to date mafic rocks.

Crustal remelting origin of highly silicic magmatism on the Moon

Orbital observations of non-mare, highly silicic volcanic constructs on the Moon challenge the conventional view of the formation of silicic crusts in the presence of water and plate tectonics—the former the Moon has in very short supply and the latter it likely never ever had. Revealing the silica-rich magma origin may reshape our understanding of early planetary crust formation. However, the cause of lunar silicic magmatism remains enigmatic. Here we conduct phase equilibrium simulations for potential lunar igneous lithologies that demonstrate that the compositions of silicic volcanic constructs can be produced by partial melting of KREEP (potassium, rare earth elements, and phosphorus) basalts. Either internal radiogenic heating or external impact bombardment can provide the heat needed for KREEP basalt melting. Combined with silica-rich materials identified on other rocky planets, we suggest that highly evolved crusts might be more prevalent in the early solar system than anticipated as they can originate from crustal remelting in the absence of water and plate tectonics.

Samples Collected From the Floor of Jezero Crater With the Mars 2020 Perseverance Rover

AbstractThe first samples collected by the Mars 2020 mission represent units exposed on the Jezero Crater floor, from the potentially oldest Séítah formation outcrops to the potentially youngest rocks of the heavily cratered Máaz formation. Surface investigations reveal landscape‐to‐microscopic textural, mineralogical, and geochemical evidence for igneous lithologies, some possibly emplaced as lava flows. The samples contain major rock‐forming minerals such as pyroxene, olivine, and feldspar, accessory minerals including oxides and phosphates, and evidence for various degrees of aqueous activity in the form of water‐soluble salt, carbonate, sulfate, iron oxide, and iron silicate minerals. Following sample return, the compositions and ages of these variably altered igneous rocks are expected to reveal the geophysical and geochemical nature of the planet's interior at the time of emplacement, characterize martian magmatism, and place timing constraints on geologic processes, both in Jezero Crater and more widely on Mars. Petrographic observations and geochemical analyses, coupled with geochronology of secondary minerals, can also reveal the timing of aqueous activity as well as constrain the chemical and physical conditions of the environments in which these minerals precipitated, and the nature and composition of organic compounds preserved in association with these phases. Returned samples from these units will help constrain the crater chronology of Mars and the global evolution of the planet's interior, for understanding the processes that formed Jezero Crater floor units, and for constraining the style and duration of aqueous activity in Jezero Crater, past habitability, and cycling of organic elements in Jezero Crater.

Geological History of the Great Altai: Implications for Mineral Exploration

The Great Altai region, located at the boundary of Russia, Mongolia, China, and Kazakhstan, belongs to the system of the Central Asian Orogenic Belt. It has undergone a long complex geological and metallogenic history. Extremely rich resources of base, precious, and rare metals (Fe, Cu, Pb, Zn, Ag, Au, Li, Cs, Ta, Nb, REE, etc.) maintain developed mining and metallurgical industry, especially in East Kazakhstan, which is the key metallogenic province. The East Kazakhstan province comprises the Rudny Altai, Kalba-Narym, West-Kalba, and Zharma-Saur metallogenic belts, each having its typical mineralization profiles and deposits. The reconstructed geodynamic and metallogenic history of the Great Altai province, along with the revealed relationships between tectonic settings and mineralization patterns, allowed us to formulate a number of geodynamic, structural, lithostratigraphic, magmatic, mineralogical, and geochemical criteria for exploration and appraisal of mineral potential in Eastern Kazakhstan. Geodynamic criteria are based on the origin of different mineralization types in certain geodynamic settings during the Late Paleozoic–Early Mesozoic orogenic cycle. Structural criteria mean that the location of base-metal deposits in Rudny Altai, gold deposits in the West Kalba belt, rare and base metals in the Kalba-Narym and Zharma-Saur zones is controlled by faults of different sizes. Lithostratigraphic criteria consist of the relation of orebodies with certain types of sedimentary or volcanic-sedimentary rocks. Magmatic criteria are due to the relation between mineralization types and igneous lithologies. Mineralogical and geochemical criteria include typical minerals and elements that can serve as tracers of mineralization. The joint use of all these criteria will open new avenues in prospecting and exploration at a more advanced level.

Magma mixing and exsolution phenomena in peralkaline rhyolites: insights from the Gold Flat Tuff, Nevada

The distribution and compositions of chevkinite-group minerals (CGMs) in the pantelleritic Gold Flat Tuff, Nevada, USA, are used to examine three aspects of the evolution of the tuff, which we feel are of general significance in peralkaline magmatism. First, both chevkinite-(Ce) and perrierite-(Ce) occur in certain facies, although normally these phases almost invariably occur in different igneous lithologies. Their co-occurrence in the tuff is due to the mixing of pantelleritic and intermediate magmas. Second, the tuff is the first recorded occurrence of a CGM in a pantellerite eruptive, with possible implications for the crystallization conditions. In particular, low values of aSiO 2 may have stabilized ilmenite + chevkinite rather than aenigmatite, although the unusually high LREE contents (∑La–Sm ≤ 1517 ppm) in the pantellerite may have played a role. Third, an unusual lamellar texture in the CGM is revealed by Atomic Force Microscopy to be formed by a rutile-like phase. The lamellae may have formed by exsolution from a rutile-like layer in the crystal structure. An electron back-scattered diffraction study of a single crystal showed a structural dislocation not apparent optically or by electron back-scattered imaging. This may have wider implications in mineralogical studies.

Testing ASTER and Sentinel-2 MSI Images to Discriminate Igneous and Metamorphic Rock Units in the Chadormalu Paleocrater, Central Iran

In the last fifty years, satellite images have been used to map the Earth’s surface at a variety of scales. Two satellite multispectral sensors (Sentinel-2 MSI and ASTER) have great utility for lithological discrimination in areas of good rock exposures. This study was conducted in order to test the ability of these sensors to discriminate igneous and metamorphic lithologies in the Chadormalu paleocrater and evaluate the image types and processing methodologies. The MNF (Minimum Noise Fraction) transform, Mathematical Evaluation Method (MEM), Spectral Angle Mapper (SAM), Mixture Tuned Matched Filter (MTMF), and band ratios were performed on near and short wave infrared ASTER and Sentinel-2 bands. Comparison of the results from several methods demonstrates that the MEM method can detect lithological units with very low false detection and better matching with ground truth data. Moreover, this study indicates that the results produced by the MEM algorithm on Sentinel-2 MSI data are more accurate than the results produced with ASTER data in the same area. Therefore, the MEM algorithm seems to be well suited for image classification involving multispectral databases such as ASTER and Sentinel-2 images.

On the paragenetic modes of minerals: A mineral evolution perspective

AbstractA systematic survey of 57 different paragenetic modes distributed among 5659 mineral species reveals patterns in the diversity and distribution of minerals related to their evolving formational environments. The earliest minerals in stellar, nebular, asteroid, and primitive Earth contexts were dominated by relatively abundant chemical elements, notably H, C, O, Mg, Al, Si, S, Ca, Ti, Cr, and Fe. Significant mineral diversification subsequently occurred via two main processes, first through gradual selection and concentration of rarer elements by fluid-rock interactions (for example, in hydro-thermal metal deposits, complex granite pegmatites, and agpaitic rocks), and then through near-surface biologically mediated oxidation and weathering.We find that 3349 mineral species (59.2%) are known from only one paragenetic context, whereas another 1372 species (24.2%) are associated with two paragenetic modes. Among the most genetically varied minerals are pyrite, albite, hornblende, corundum, magnetite, calcite, hematite, rutile, and baryte, each with 15 or more known modes of formation.Among the most common paragenetic modes of minerals are near-surface weathering/oxidation (1998 species), subsurface hydrothermal deposition (859 species), and condensation at volcanic fumaroles (459 species). In addition, many species are associated with compositionally extreme environments of highly differentiated igneous lithologies, including agpaitic rocks (726 species), complex granite pegmatites (564 species), and carbonatites and related carbonate-bearing magmas (291 species). Biological processes lead to at least 2707 mineral species, primarily as a consequence of oxidative weathering but also through coal-related and other taphonomic minerals (597 species), as well as anthropogenic minerals, for example as byproducts of mining (603 minerals). However, contrary to previous estimates, we find that only ~34% of mineral species form exclusively as a consequence of biological processes. By far the most significant factor in enhancing Earth’s mineral diversity has been its dynamic hydrological cycle. At least 4583 minerals—81% of all species—arise through water-rock interactions.A timeline for mineral-forming events suggests that much of Earth’s mineral diversity was established within the first 250 million years. If life is rare in the universe, then this view of a mineralogically diverse early Earth provides many more plausible reactive pathways over a longer timespan than previous models. If, however, life is a cosmic imperative that emerges on any mineral- and water-rich world, then these findings support the hypothesis that life on Earth developed rapidly in the early stages of planetary evolution.

Study of seismic attributes recognition method of high quality igneous rock reservoir

Aimed target area is deeply buried, complex lithology, dual media, reservoir development degree is controlled by a variety of factors, meanwhile, lateral thickness and lithofacies change rapidly, and strata formation is poor. Therefore, igneous rock reservoir has difficulty in predicting, since seismic is complicated to track trace, reservoir attribute analysis is hard to determine the time window, and inversion modeling requires sophisticated. By analyzing, the basalt in the target research area accounts for the principal component of the igneous rock, however, the igneous rocks with relatively developed reservoirs are mostly distributed in the trachyte breccia which has good productivity. The results of petrophysical study indicate that frequency-dependent AVO inversion method is an important means to identify fluid and reservoir prediction, notwithstanding it is difficult to distinguish high-quality reservoirs barely by P-wave impedance. Consequently, AVOF inversion method is appropriately proposed to identify igneous rock reservoir. Foremost, eliminating the effects of algorithm,frequency, spectrum balancing and other factors, then put the improved three-term Aki&amp;Richards frequency-dependent AVO inversion method applying to distinguish igneous reservoir fluid and lithology, for the purpose of carrying out the identification of high-quality reservoirs.

Study on lithology identification method of carboniferous in the west wing chepaizi salient in western margin of junggar basin

Reservoir characteristics are important indexes for oil and gas exploration and development. Therefore, the identification of igneous lithology of target formation is carried out by using intersection diagram and mechanical parameter method according to the Carboniferous logging data of the west wing chepaizi salient in western margin of junggar basin. The results show that the study area is dominated by volcanic clastic rocks and sedimentary rocks. The cross-section and mechanical parameters method can be used to identify the lithology and obtain the corresponding physical parameters. Finally, this work has guiding significance for the exploration and development of Carboniferous reservoirs in the west wing chepaizi salient in western margin of junggar basin.

Differentiation and magmatic history of Vesta: Constraints from HED meteorites and Dawn spacecraft data

Quantifying the amounts of various igneous lithologies in Vesta’s crust allows the estimation of petrologic ratios that describe the asteroid’s global differentiation and subsequent magmatic history. The eucrite:diogenite (Euc:Diog) ratio measures the relative proportions of mafic and ultramafic components. The intrusive:extrusive (I:E) ratio assesses the effectiveness of magma ascent and eruption. We estimate these ratios by counting numbers and masses of eucrites, cumulate eucrites, and diogenites in the world’s meteorite collections, and by calculating their proportions as components of crustal polymict breccias (howardites) using chemical mixing diagrams and petrologic mapping of multiple thin sections. The latter two methods yield a Euc:Diog ratio of ∼2:1, although meteorite numbers and masses give slightly higher ratios. Surface lithologic maps compiled from spectra of Dawn spacecraft instruments (VIR and GRaND) yield Euc:Diog ratios that bracket estimates of Euc:Diog from the meteorites. The I:E ratios from HEDs lie between 0.5–2.1:1, due to uncertainties in identifying cumulate eucrite. Gravity mapping of Vesta by the Dawn spacecraft supports the existence of diogenite plutons in the crust. Quantifying the proportion of high-density diogenitic crust in the gravity map yields I:E ratios of 0.8-1:2:1, values which are bracketed by calculations based on HEDs. The I:E ratio for Vesta is lower than for Earth and Mars, consistent with physical modeling of asteroid-size bodies. Nevertheless, it indicates a significant role for pluton emplacement during the formation of Vesta’s crust. These results are inconsistent with simple differentiation models that produce the crust by crystallization of a global magma ocean, unless residual melts are extracted into crustal magma chambers.

Dating igneous rocks using the Potassium-Argon Laser Experiment (KArLE) instrument: A case study for ~380Ma basaltic rocks.

We report new K-Ar isochron data for two ~380Ma basaltic rocks, using an updated version of the Potassium-Argon Laser Experiment (KArLE), which is being developed for future in situ dating of planetary materials. These basalts have K contents comparable with those of lunar KREEP basalts or igneous lithologies found by Mars rovers, whereas previous proof-of-concept studies focused primarily on more K-rich rocks. We aim to measure these analogous samples to show the advancing capability of in situ K-Ar geochronology. Combining laser-induced breakdown spectroscopy (LIBS), mass spectrometry (MS), and microscopic analyses, we measured the abundance of K and 40 Ar from 23 spots on the basalt samples. We then constructed K-Ar isochron plots from these rocks. The breadboard instrument consists of flight-equivalent devices including a 30-mJ Nd:YAG laser and a quadrupole mass spectrometer. Despite much lower K abundances than in previous studies, the isochron slopes yielded 380±44Ma and 398±50Ma for 380.7-Ma and 373.5-Ma rocks, respectively, indicating that accuracy better than 25Ma (<7%) is achievable with our instrument. The isochron intercepts both yielded trapped 40 Ar approximately 1×10-6 cm3 STP/g. Our experimental results demonstrate that accurate and precise measurements are possible using the KArLE approach on basaltic rocks, which are ubiquitous on planetary surfaces, and are useful in addressing a wide range of questions in planetary science.

Multi-isotope survey on the metallogenesis of the hydrothermal Co-Ni deposits in the Alpine Central Pyrenees of Spain

Thrust systems, related to thin-skinned tectonics are the main structural control for the development of Co-Ni deposits (Co-Ni arsenides, diarsenides and sulpharsenides) in the Alpine Central Pyrenees. Co-Ni ores occur in the hanging wall of these thrusts, hosted either by carbonate (San Juan de Plan and Crescencia mines) or siliciclastic rocks (Solita Mine), whereas the footwall is formed by black shales. Previous studies have applied graphite geothermometry and crystal-chemical criteria (the presence of alloclasite and the extent of the cobaltite–gersdorffite solid-solution) to determine that the complex assemblages of the studied deposits were formed at temperatures between 300 °C and 600 °C. The possible sources of the mineralizing fluids and metals have been investigated with a multi-isotope approach. Stable isotope data (C, O) support a primary magmatic source for the ore-forming fluids although interaction with both the Variscan black shales and carbonate host-rocks is also evidenced. Nd and Sr isotope compositions suggest that these fluids reacted with basement siliciclastic and igneous lithologies (and to a lesser extent with carbonate rocks). This reaction scavenged sulfur from the Variscan basement as a whole (positive δ34S values) and specifically from the underlying local black shales (negative δ34S values) and local magmatic rocks (δ34S values close to 0‰). Metals were extracted also from the Variscan black shales underneath each deposit, as suggested by the close correspondence between the metal ratios in the Co-Ni deposits and in the underlying black shales.The ore-forming activity responsible for the studied Co-Ni deposits spans an age range (Upper Cretaceous, 86 ± 27 Ma) similar to that of other high temperature events, developed in a context of thinned crust in the Pyrenean realm (i.e. High T/low P metamorphism, mantle exhumation and alkaline magmatism). These processes promoted high temperature hydrothermalism and formation of ore deposits, prior to the tectonic inversion and thrusting during the compressive stage.

Molybdenum, vanadium, and uranium weathering in small mountainous rivers and rivers draining high-standing islands

Rivers draining high standing islands (HSIs) and small mountainous rivers (SMRs) are known to have extremely high sediment fluxes, and can also have high chemical weathering yields, which makes them potentially important contributors to the global riverine elemental flux to the ocean. This work reports on the riverine concentrations, ocean flux, and weathering yields of Molybdenum (Mo), Vanadium (V), and Uranium (U) in a large number of small but geochemically important rivers using 338 river samples from ten lithologically-diverse regions. These redox-sensitive elements are used extensively to infer paleo-redox conditions in the ocean, and Mo and V are also important rock-derived micronutrients used by microorganisms in nitrogen fixation. Unlike in large river systems, in which dissolved Mo has been attributed predominately to pyrite dissolution, Mo concentrations in these rivers did not correlate with sulfate concentrations. V was found to correlate strongly with Si in terrains dominated by silicate rocks, but this trend was not observed in primarily sedimentary regions. Many rivers exhibited much higher V/Si ratios than larger rivers, and rivers draining young Quaternary volcanic rocks in Nicaragua had much higher dissolved V concentrations (mean=1306nM) than previously-studied rivers. U concentrations were generally well below the global average with the exception of rivers draining primarily sedimentary lithologies containing carbonates and shales. Fluxes of U and Mo from igneous terrains of intermediate composition are lower than the global average, while fluxes of V from these regions are higher, and up to two orders of magnitude higher in the Nicaragua rivers. Weathering yields of Mo and V in most regions are above the global mean, despite lower than average concentrations measured in some of those systems, indicating that the chemical weathering of these elements are higher in these SMR watersheds than larger drainages. In regions of active boundaries with andesite/dacite lithologies, rivers draining young Pleistocene rocks had higher concentrations than did older Miocene-Pliocene rocks of a similar composition. This work shows that weathering yields of Mo, V, and U from SMRs are slightly higher than from large rivers, and the age of igneous lithologies in these regions exhibits a measurable control on riverine concentrations of these elements.