Bulk Mineral Research Articles

The Onano eruption (Latera volcano, Central Italy): an example of magma mixing/mingling as dominant process in a caldera-forming eruption

The Onano eruption (~ 0.17 Ma) is the second to last caldera-forming eruption of the Latera volcano, in the northernmost sector of the Roman Comagmatic Province (Latium, Italy). The stratigraphic sequence, from base to top, includes ash and pumice-rich flow deposits, spatter-rich flow deposits and lag breccias associated with ash-rich flow deposits. By combining major and trace element compositions of the bulk rocks, matrix glasses and minerals of juvenile components from the different depositional units, we reconstruct the pre-eruptive evolution of the magma chamber and the syn-eruptive magma dynamics. Juvenile clasts with heterogeneous glass composition and/or mineral assemblage are a ubiquitous feature of the Onano eruption. The products cover a large compositional range from phonotephrite to phonolite. They are crystal poor, with felsic paragenesis associated to Mg-rich olivine (Fo82–90) and diopside (Fs4–7), these last not in equilibrium with the erupted melts. The mafic mineral assemblage suggests that the pre-eruption magma reservoir was periodically perturbed by the arrival of a primitive magma carrying on Mg-rich minerals and/or remobilizing a mafic crystal mush at the bottom of the reservoir. According to the results obtained from both rhyolite-MELTS and Rayleigh crystal fractionation modeling, we infer that the system evolved from phonotephrite to phonolite both via crystal fractionation and magma mixing between the two end members. Crystallization mostly proceeded at the wall of the reservoir, while magma mixing in the middle zones of the chamber generated the intermediate tephriphonolitic melts. The pre-eruptive chemical zoning was disrupted during the course of the eruption due to the simultaneous withdrawal of magma from different portions of the reservoir. During the first phase of the eruption, high-silica tephriphonolitic to phonolitic melts, residing in the upper part of the reservoir, were emitted with less involvement of the deeper phonotephrite. Phonotephritic magmas, with only a minor contribution of evolved melts, were later erupted as spatter-rich pyroclastic flows that preceded the main caldera collapse. During the caldera collapse, the whole reservoir was involved, leading to extensive and intimate syn-eruption mingling of the different melts. A comparison between the Onano eruption and other eruptions of the Italian high potassic volcanism provides new insights into the evolution of the Roman Comagmatic Province magmas and their eruptive processes.

Cavitation Erosion Characteristics of High Bulk Modulus Oils: Preliminary Evaluation Experiment for Hydraulic Equipment

Abstract A high bulk modulus oil, which promises to enhance the responsibility and efficiency of hydraulic systems, has been developed because of high pressure operation. Although cavitation erosion in the hydraulic components is a serious concern, the phenomena and behaviors of the oils for the erosion have not been examined. This report investigates the characteristics of cavitation erosion caused by a cavitating jet in terms of high bulk modulus oils produced with different compounding agents and molecular weights and mineral oils prepared as a reference. The experiment was conducted in a test rig, applying the jet-cavitation method under an upstream absolute pressure of 10.1 MPa. The diameter and thickness of the aluminum alloy specimens were 15 and 5 mm, respectively. The mass loss was maximized at a specific standoff distance, and the surface erosion was ring-like, as previously reported for general hydraulic oils and municipal water. The mass losses in the high bulk modulus oils and mineral oils increased with increasing liquid temperature and decreasing cavitation number under the test conditions. Although the erosion is intrinsically enhanced because of the high bulk modulus oils with high density, the erosion can be reduced by adding high molecular-weight polymer.

Heterogeneous nickel isotopic compositions in the terrestrial mantle – Part 1: Ultramafic lithologies

High precision nickel stable isotopic compositions (δ60/58Ni) are reported for 22 peridotite xenoliths from the USA (Kilbourne Hole, New Mexico), Tanzania, and Cameroon. For a subset of these, δ60/58Ni is also reported for their constituent mineral separates (olivine, orthopyroxene, clinopyroxene, and spinel). Bulk peridotites show significant heterogeneity in Ni isotopic composition, ranging from +0.02‰ to +0.26‰. Unmetasomatised fertile peridotites from three localities, define an average δ60/58Ni of +0.19±0.09‰ (n = 18). This value is comparable to previous estimates for the δ60/58Ni of the bulk silicate earth (BSE), but is unlikely to be representative, given observed heterogeneity, presented here and elsewhere. Samples with reaction rims and interstitial glass (interpreted as petrographic indications of minor metasomatism) were excluded from this average; their Ni isotopic compositions extend to lighter values, spanning nearly the entire range observed in peridotite worldwide. Dunites (n = 2) are lighter in δ60/58Ni than lherzolites and harzburgites from the same location, and pyroxenites (n = 5) range from +0.16‰ to as light as −0.38‰.The δ60/58Ni in the Kilbourne Hole xenoliths correlate negatively with bulk-rock Fe concentration and positively with 143Nd/144Nd, providing evidence that light δ60/58Ni is associated with mantle fertility and enrichment. The trend between δ60/58Ni and Fe concentration in bulk rocks appears to be global, replicated across the peridotites in this work from other localities, and in literature data.The inter-mineral fractionations are small; the maximum difference between heaviest and lightest phase is 0.12‰. This provides evidence that bulk rock δ60/58Ni variation does not result from differences in modal mineralogy, fractional crystallization or degrees of partial melting. The δ60/58Ni fractionation appears to be an equilibrium effect and usually is in the decreasing order spinel > olivine = orthopyroxene > clinopyroxene. However, the fractionation between clinopyroxene and orthopyroxene varies in magnitude and sign, and is correlated with pyroxene Si/Fe positively, and Fe/Mg negatively. The magnitude of inter-pyroxene fractionation also correlates with other pyroxene compositional ratios (e.g. La/Smclinopyroxene); as well as bulk rock δ60/58Ni, and [U]. These data provide evidence that Ni isotopes fractionate at the bulk rock and mineral scale in response to mantle enrichment processes, possibly related to recycling of isotopically light subducted components.

Ureilite meteorites provide a new model of early planetesimal formation and destruction

Ureilite meteorites are ultramafic rocks derived from parts of the depleted silicate mantle of their parent planetesimal. We used Monte Carlo modelling to explain the observed array of oxygen isotopes and major element chemistry shown by bulk ureilites, after restoration of their missing core and silicate melt components. Despite using a wide range of primitive nebular material, our modelling shows that only a combination of proxy material resembling Allende-type FeO-rich and MgO-rich chondrules, can account for the ureilite oxygen isotope trend and the reconstructed ureilite major element chemistry. Our model predicts formation of a radial gradient in major elements and oxygen isotopes within the planetesimal, with a more Mg-rich silicate interior and a more Fe-rich silicate exterior. Temperatures recorded by ureilites were not high enough to form a magma ocean but were sufficiently high to form a metallic core and silicate melts. The ureilite parent planetesimal was then disrupted by impact. Re-accretion of the outer layers of more Fe-rich silicate material, at the expense of the more MgO-rich material and the core, explains the observed distribution of bulk rock and mineral compositions.

Grain assemblages and diagenesis in organic-rich mudrocks, Upper Pennsylvanian Cline shale (Wolfcamp D), Midland Basin, Texas

Grain assemblages in the organic-rich Cline shale in the Midland Basin are dominated by components of extrabasinal derivation (11.4 to 98.5 vol. %; average volume: 82.6%). Major extrabasinal components include K-rich clay minerals, detrital quartz, albite, K-feldspar, micas, and lithic fragments. Intrabasinal components include mainly biosiliceous allochems (sponge spicules and radiolarians), agglutinated foraminifera, Ca-phosphate peloids, clay-rich peloids, organomineralic aggregates, intraclasts, and other biocalcareous allochems. Authigenic minerals are most evident as grain replacements, euhedral ankerite, Ca-phosphate cement, and precipitates in large pores. A strongly localized spatial distribution of diagenetic products at micron to centimeter scales is observed in most siliciclastic samples, except in biosiliceous allochem-rich ones in which abundant intergranular pore-filling clay-size microquartz cement is observed. Compaction is evident in the Cline shale because of low porosity and generally low cement volumes. Neither textural variation nor bulk mineral composition alone is sufficient to confidently decipher the rock bulk property (e.g., total organic carbon) and reservoir quality variation (e.g., porosity and permeability). However, a good negative relationship between the ratio of extrabasinal to intrabasinal grains and favorable reservoir properties is observed in the Cline shale. Specifically, higher porosity, permeability, and total organic carbon are observed in samples representing the extreme end members of intrabasinal-derived biosilica-rich layers. Nickel, a recognized proxy for paleoproductivity, exhibits a positive relationship with intrabasinal grain content and reservoir properties. X-ray fluorescence–based analysis of nickel can be a rapid and cost-effective way to delineate favorable unconventional reservoir quality in the Cline shale.

Vestiges of a fore-arc oceanic crust in the Western Mediterranean: Geochemical constraints from North-East Algeria

The present day architecture of the Western Mediterranean mainly results from the interplay of different lithospheric plates and the Cenozoic consumption of various branches of the Alpine Tethys and Neo-Tethys Oceans. Identifying relics of these oceanic domains in the peri-Mediterranean belts enables pinpointing the earliest stages of this evolutionary framework. In NE Algeria, the Kef Lakhal Complex (Edough Massif) is composed of amphibolites and meta-gabbros metamorphosed under amphibolite facies conditions and thrust onto the northern African margin during earliest Miocene. Geochemical analyses reveal that amphibolites and metagabbros have major and trace element signatures characteristic of tholeiitic basalts and gabbros and display N-MORB (La/SmN = 0.7–0.9) to E-MORB (La/SmN = 1.1–1.2) affinities. Enrichments in U and alkalis (Cs, Rb) and high 87Sr/86Sri ratios on bulk rocks and minerals (up to 0.70856) are typical of seawater alteration and indicate that the Kef Lakhal Complex represents a piece of altered oceanic crust. Parental magmas were derived from a Depleted Mantle source (WR εNdi = 8.2–9.9 and εHfi = 10.4–14.4) that preserved some canonical MORB ratios (Ti/Eu = 5700–7600 and Y/Ho = 27.4–29.0), but which was contaminated by a component akin to recycled oceanic crust and associated sediments (Ce/Pb = 10.5–22.2; 206Pb/204Pbi = 18.41–18.74; 207Pb/204Pbi = 15.49–15.59; 208Pb/204Pbi = 37.73–38.37). The sedimentary component is consistent with a volcaniclastic origin and substantiates an intra-oceanic setting. The geochemical characteristics of the Kef Lakhal rocks are comparable to present day (Mariana, W Pacific Ocean) or fossil (Xigaze, SE Tibet) fore-arc oceanic crust. An overview of the Western Mediterranean geodynamic indicates that the most likely period for initiating formation of a fore-arc corresponds either to the Early/Late Cretaceous or to the Middle Eocene. 40Ar/39Ar dating on amphibole further indicate that after thrusting onto the African margin, the Kef Lakhal Complex cooled down to c.550 °C at 18.14 ± 0.27 Ma (2σ).

A New Method for Calculating the Moduli of Mixed Minerals under Abnormal Pressure Applying to the Marine Reservoir

Li, L.; Li, D.; Ma, J.; Wang, D., and Tan, M., 2020. A new method for calculating the moduli of mixed minerals under abnormal pressure applying to the marine reservoir. In: Yang, Y.; Mi, C.; Zhao, L., and Lam, S. (eds.), Global Topics and New Trends in Coastal Research: Port, Coastal and Ocean Engineering. Journal of Coastal Research, Special Issue No. 103, pp. 339–345. Coconut Creek (Florida), ISSN 0749-0208.Abnormal fluid pressure commonly exists in oil and gas-bearing basins. Abnormally high pressure often coexists with undercompaction, whereas abnormally low pressure is often related to rock porosity rebound. The Voigt-Reuss-Hill (V-R-H) equation, which is the arithmetic mean of the Voigt upper bound and the Reuss lower bound, is often used to calculate the bulk and shear moduli of mixed minerals. In this study, it is considered that the moduli of mixed minerals are closer to the Reuss lower bound in abnormal high-pressure reservoirs and the Voigt upper bound in abnormal low-pressure reservoirs. Hence, a new equation is proposed for calculating the moduli of mixed minerals. Then, the Digby model and the Gassmann equation are used to predict shear-wave (S-wave) velocities, which are subsequently compared with the measured results. The results show that the errors in S-wave velocities predicted by the proposed method are lower than 6%.

Effect of crystallographic orientation on atom probe tomography geochemical data?

The recent application of atom probe tomography (APT) to minerals is becoming a powerful tool to unravel geological information from the nanoscale perspective. Yet, there are still unknown fundamental aspects of this microscopy technique for geological applications and the potential crystallographic orientation effect is a significant one. Here, the influence of the crystallographic orientation on the quality of atom probe tomography geochemical data is investigated for two minerals with the same crystal system and different morphology: spinel (isometric, hexoctahedral, octahedron morphology) and galena (isometric, hexoctahedral, cube morphology). Two separate crystals of barite (orthorhombic, dipyramidal, prism morphology) were also analyzed to test the reproducibility of APT data. Despite the general absence of expected stoichiometry, overall bulk and isotopic chemical composition are not affected by crystallographic orientation. 3D data reconstructions of the specimens showed similar spatial distribution of the ion species for each mineral and 2D density maps showed identical (barite, galena) or specular (spinel) patterns for each pair of planes analyzed. Our findings indicate a negligible effect of the crystallographic orientation in APT geochemical data for standard highly symmetric minerals but also suggest the possible influence of the crystallographic structure and composition on the mineral stoichiometry and elements spatial distribution density.

Iterative thermodynamic modelling—Part 1: A theoretical scoring technique and a computer program (Bingo‐Antidote)

AbstractThis paper introduces the software solution Bingo‐Antidote for thermodynamic calculations at equilibrium based on iterative thermodynamic models. It describes a hybrid strategy combining the strength of Gibbs energy minimization (GEM) and inverse thermobarometry models based on the comparison between the modelled and observed mineral assemblage, modes and compositions. The overall technique relies on quantitative compositional maps acquired by electron probe micro‐analyser for obtaining a mutually consistent set of observed data such as bulk rock and mineral compositions. Thus it offers the opportunity to investigate metamorphic rocks on a microscale. The scoring part Bingo integrates three statistical model quality factors for the assemblage, for the mineral modes, for the mineral compositions combined in a global evaluation criterion that quantifies how the model reproduces the observations for the investigated volume. The input parameters of GEM affecting the model quality such as pressure, temperature and eventually some components of the bulk composition (e.g. the molar amount of hydrogen, carbon or oxygen) or activity variables of fluids and gases (e.g. , , f(O2)) can be optimized by inversion in Antidote using several mapping stages followed by a direct search optimization. Examples of iterative models based on compositional maps processed with Bingo‐Antidote demonstrate the utility of the program. In contrast to the qualitative interpretation of phase diagrams, the inversion maximizes the benefits of GEM and permits the derivation of statistically ‘optimal’ pressure–temperature conditions for well‐equilibrated samples. In addition, Bingo‐Antidote opens new avenues for petrological investigations such as the generation of chemical potential landscape maps.

Low-temperature oceanic crust alteration and the isotopic budgets of potassium and magnesium in seawater

Low-temperature (<100°C) alteration of oceanic crust plays an important role in determining the chemical composition of the oceans. Although a major sink of seawater potassium, little is known about the effects of low-temperature basalt alteration on the potassium isotopic composition of seawater (δ41K∼0‰), which is ∼0.50‰ enriched relative to bulk silicate Earth (BSE, δ41K= -0.54‰). Here, we present a suite of isotopic systems (δ41K, δ26Mg, δ7Li, 87Sr/86Sr) and major/minor elements in bulk rock, veins and mineral separates from the upper volcanic section of Cretaceous (Troodos ophiolite) and Jurassic (Ocean Drilling Program Hole 801C) oceanic crust. We use these data to estimate the K isotopic fractionation associated with low-temperature oceanic crust alteration and provide new constraints on the role of this process in the global geochemical cycles of Mg and K in seawater. We find that hydrothermally altered basalts from the Troodos ophiolite and ODP Hole 801C, most of which are enriched in K relative to the unaltered glass compositions, have δ41K values both higher and lower than BSE, ranging from +0.01‰ to -1.07‰ (n=83) and +0.04‰ to -0.88‰ (n=17), respectively. Average δ41K values of bulk-rock samples from Troodos and Hole 801C are indistinguishable from each other at ∼-0.50‰, indicating that low-temperature basalt alteration is a sink of 39K from seawater, and explaining, in part, why seawater has a higher 41K/39K than BSE. In contrast to K, average δ26Mg values for both Troodos (∼0.00‰) and Hole 801C (∼0.20‰) indicate that altered oceanic crust (AOC) is a sink of 26Mg from seawater, likely contributing to the light δ26Mg composition of seawater (∼-0.8‰) relative to BSE (∼-0.2‰). We observe isotopically heavy δ26Mg values in basalt samples characterized by small to no changes in bulk Mg content, consistent with extensive isotopic exchange of Mg between seawater and oceanic crust during low-temperature oceanic crust alteration. Finally, we find that variability in δ7Li and δ41K across three sites in the Troodos ophiolite can be explained by different styles of alteration that appear to be related to the timing of sedimentation and its effects on chemical and isotopic exchange between seawater and oceanic crust.

Micro-analytical characterization of thorium-rich aggregates from Norwegian NORM sites (Fen Complex, Telemark)

In this study we performed microscopic characterization of mineral particles that were collected in the thorium-rich Fen Complex in Norway and identified and isolated based on autoradiography in function of their radioactivity. For this we combined information obtained with X-ray absorption μ-CT, μ-XRF and μ-XRD, both in bi- and in three-dimensional (tomographic) mode. We demonstrate that radionuclides and metals are heterogeneously distributed both within soil samples and within individual Th-enriched aggregates, which are characterised as low-density mineral bulk particles with high density material inclusions, where Th as well as several metals are highly concentrated. For these sites, it is important to take into account how these inhomogeneous distributions could affect the overall environmental behaviour of Th and progeny upon weathering due to human or environmental factors. Moreover, the estimated size of the Th-containing inclusions as determined in this work represents information of importance for the characterization of radionuclides and toxic metals exposure, as well as for assessing the viability of mining for Th and rare-earth metals in the Fen Complex and the associated environmental impact.

Dynamic volatility spillovers among bulk mineral commodities: A network method

The volatility spillover effects among bulk mineral commodities is an important and hot issue in mineral resource policy. This paper applies a network theory approach that incorporates a bivariate spillover model to establish a bulk mineral spillover network that can not only reveal the structure of magnitude and direction of spillovers across nineteen bulk mineral futures prices but can also investigate their dynamic evolutionary process. Our findings indicate that the structure of a network changes with time. In general, an energy bulk mineral commodity such as natural gas acts as the net highest spillover transmitter, while an industries metal commodity such as U.S. Steel acts as the net highest spillover receiver. The overall structure of a network indicates that there are a few spillover flows across bulk mineral markets in which any market tends to have more spillovers among interconnected groups of neighbors. Although the overall structure became complex during the European debt and oil crisis collapses of 2014–2016, it has been simple since the end of 2017 when its range of fluctuation increased. Our research not only provides a process orientation to explore the nonlinear dynamic process of spillovers across markets but also offers important implications for the pricing mechanics of bulk mineral-related products and market management.

Electronic and Vibrational Contributions to the Bulk Stabilities of Trivalent 3d Transition Metal Oxyhydroxides from Electronic Structure Calculations

Any understanding of the thermodynamic driving forces active in the global cycles of earth abundant metals requires attendant knowledge of the bulk mineral free energies. This can be particularly c...

The impact of Mg2+ ions on equilibration of Mg-Ca carbonates in groundwater and brines

At temperatures below 50 °C, the log10 (aMg2+/aCa2+) values in groundwater and brines, irrespective of their origin – either carbonaceous or siliceous rocks/sediments – cover the range between −1.5 and +1.0. Calculations of thermodynamic equilibria between the minerals calcite, aragonite, dolomite and huntite suggest a spread of log10 (aMg2+/aCa2+) between minus infinity and +2.3. Log10(aMg2+/aCa2+) in solution of dissolving ordered dolomite at 25 °C fits the thermodynamical equilibrium between disordered dolomite and calcite and nearly corresponds to that of pure calcite with a dolomitic surface layer due to exchange of Ca2+ against Mg2+ in Mg2+-containing solutions. This observation suggests that the solubility of Mg-Ca carbonates is controlled by the composition of their monomolecular surface layers in equilibrium with the ambient aqueous phase. Incongruently dissolving minerals such as dolomite attain equilibrium between individual surface compositions of different carbonates. The bulk composition of these carbonates hardly if ever equilibrates with the ambient solution due to extremely low ion mobility in the lattice. Because the thermodynamical equilibria are based on the composition of bulk minerals, their estimates of equilibria between carbonates, i.e., log10 (aMg2+/aCa2+) in solution, differ significantly from values established by the chemical composition and structure of the surface layer of carbonates.

A mineralogical and chemical investigation of road dust in Philadelphia, PA, USA.

Road dust was investigated within Philadelphia, a major United States city with a long history of industrial activities, in order to determine pollution levels. Almost all of the investigated minor elements were enriched relative to the continental crust. Furthermore, mean concentrations of Cr, Co, Cu, and Pb were high compared with those reported in cities in other countries. Lead pollution should be investigated further in Philadelphia, where 8 of the 30 sample sites, including those heavily trafficked by civilians, were at or above the EPA's child safety threshold for Pb in bare soil. High Spearman correlations between Zn and Cu, Zn and Cr, Cu and Cr, and Sn and V, as well as factor analysis of minor elements suggests that the primary sources of these elements were anthropogenic. Potential sources included the breakdown of alloys, non-exhaust traffic emissions, paint, smelting, and industry. We found that higher organic content in road dust may be related to higher traffic densities, which could be due to tire-wear particles. Additionally, higher mean concentrations of Fe, Cr, Cu, and Zn were found at sites with elevated traffic densities. Land use impacted some of the elements not influenced by traffic density, including Co, Sn, and Pb. Bulk mineral content was similar across different land uses and traffic densities and, thus, did not appear to be influenced by these factors. Our research emphasized the complexity of road dust and utilized a more comprehensive approach than many previous studies. This study established fundamental groundwork for future risk assessment in Philadelphia, as it identified several key pollutants in the city. Overall, this assessment serves as an informative reference point for other formerly heavily industrialized cities in the USA and abroad.

Anisotropy of Full and Partial Anhysteretic Remanence Across Different Rock Types: 2—Coercivity Dependence of Remanence Anisotropy

AbstractMagnetic fabrics are powerful tools in structural geology and tectonic studies, because they provide a fast and efficient measurement of mineral alignment, which helps interpret a rock's (de)formation history. The magnetic fabric of remanence‐carrying minerals provides useful information when these grains record different deformation stages than the bulk minerals in a rock. When rocks contain several subpopulations of remanence‐carrying minerals, each of these potentially displays a distinct fabric. This can lead to complex remanence anisotropies, being a superposition of all subpopulations' individual anisotropies. Characterization of partial remanence anisotropies has been used to investigate changes in fabric with grain size. However, most studies still report one bulk remanence anisotropy tensor per sample, and it remains to be determined how commonly different populations of remanence‐carrying grains reflect different subfabrics. Based on a large sample collection including 93 specimens from different lithologies, we have investigated the coercivity dependence of anisotropy of (partial) anhysteretic remanent magnetization A(p)ARM. We find that the principal directions, degree, and shape of A(p)ARM are generally dependent on the coercivity window used to impart the anhysteretic remanent magnetizations (ARMs). Depending on the carrier minerals and their fabrics, ARM anisotropy can either increase or decrease when the ARMs are applied over larger coercivity windows. Additionally, the coercivity fraction that dominates the ARM anisotropy is not always the coercivity fraction that acquires the strongest mean ARM. This illustrates the complexity of characterizing remanence anisotropy, and highlights the importance of carefully choosing experimental parameters in A(p)ARM determination for both magnetic fabric and anisotropy correction studies.

Thermodynamics of Metal Carbonates and Bicarbonates and Their Hydrates for Mg, Ca, Fe, and Cd Relevant to Mineral Energetics.

The heats of formation of the carbonate, bicarbonate, and bicarbonate/hydroxide metal complexes, including hydrates of Mg2+, Ca2+, Fe2+, and Cd2+, and the oxides, dichlorides, and dihydroxides are predicted from atomization energies using correlated molecular orbital theory at the CCSD(T) level extrapolated to the complete basis set limit following the Feller-Peterson-Dixon (FPD) approach. Using the calculated gas phase values and the available experimental solid-state values, we predicted the cohesive energies of selective minerals. The gas phase decomposition energies of MO, CO2, and H2O follow the order Mg ≈ Ca > Cd ≈ Fe and correlate with the hardness of the metal +2 ions. Gas phase hydration energies show that the order is Mg > Fe > Ca ≈ Cd. There are a number of bulk hydrated Mg and Ca complexes that occur as minerals but there are few if any for Fe and Cd, suggesting that a number of factors are important in determining the stability of the bulk mineral hydrates. The FPD heats of formation were used to benchmark a range of density functional theory exchange-correlation functionals, including those commonly used in solid-state mineral calculations. None of the functionals provided chemical accuracy agreement (±1 kcal/mol) with the FPD results. The best agreement to the FPD results is predicted for ωB97X and ωB97X-D functionals with an average unsigned error of 10 kcal/mol. The worst functionals are PW91, BP86, and PBE with average unsigned errors of 32-36 kcal/mol.

Geology, mineralogy, S and Sr isotope geochemistry, and fluid inclusion analysis of barite associated with the Lemarchant Zn–Pb–Cu–Ag–Au-rich volcanogenic massive sulphide deposit, Newfoundland, Canada

Barite in the approximately 513 Ma Lemarchant volcanogenic massive sulphide (VMS) deposit, Newfoundland, consists of granular and bladed barite intimately associated with mineralization. Regardless of type, the composition of barite is homogeneous at bulk rock and mineral scale containing predominantly Ba, S, and Sr, with minor Ca and Na. The barite has homogeneous sulphur isotope compositions (δ34Smean = 27‰), similar to Cambrian seawater sulphate (25–35‰) and Sr isotope compositions (87Sr/86Sr = 0.706905 to 0.707485). These results are consistent with barite having formed from fluid–fluid mixing between Cambrian seawater and VMS-related hydrothermal fluids. The 87Sr/86Sr values in the barite are lower than mid-Cambrian seawater, which suggests that some of the Sr was derived from the underlying Neoproterozoic basement. Fluid inclusions in bladed barite are low-salinity, CO2-rich inclusions with homogenization temperatures between 245°–250 °C, and average salinity of 1.2 wt.% NaCl equivalent. Estimated minimum trapping pressures of between 1.7 to 2.0 kbars were calculated from aqueous–carbonic fluid inclusion assemblages. The fluid inclusion results reflect regional metamorphic reequilibration during younger Silurian regional metamorphism, rather than primary fluid signatures, despite the preservation of primary barite and fluid inclusion textures. These results illustrate that barite in VMS deposits records the physicochemical processes associated with VMS formation and the sources of fluids in ancient VMS deposits, as well as seawater sulphate and basement isotopic compositions. The results herein are not only relevant for the Lemarchant deposit but also for other barite-rich VMS deposits globally.

An optimal configuration for spark-induced breakdown spectroscopy of bulk minerals aimed at planetary analysis

Spark-induced breakdown spectroscopy (SIBS) utilizes an electric spark to induce a strong plasma for collecting atomic emissions.

Lawsonite-rich layers as records of fluid and element mobility in subducted crust (Sivrihisar Massif, Turkey)

Lawsonite is an abundant hydrous mineral in subducted oceanic crust and sediments and is an important carrier of water (∼11.5 wt%) and trace elements (REE, U, Th, Sr, Pb) into the mantle. Despite its widespread stability at the high-pressure/low-temperature conditions of subduction, it is rarely preserved in the geologic record, particularly in eclogite. Therefore, scarce fresh lawsonite eclogites in which the composition of lawsonite can be directly determined are important geochemical archives of fluid-mediated processes during subduction. Of particular interest are lawsonite-rich veins and layers that may represent former fluid pathways or metasomatic zones and that therefore may record element mobility related to fluid-rock interaction during subduction and/or incipient exhumation. In the Sivrihisar (Turkey) subduction complex, lawsonite-rich veins and layers comprised of ∼30–50 % lawsonite occur at pod margins where lawsonite eclogite has been partially transformed to lawsonite blueschist. To understand the petrogenesis of these lawsonite-rich assemblages, we conducted a petrographic and geochemical study of a representative lawsonite-rich layer at the margin of a meter-scale lawsonite eclogite pod. Bulk rock and mineral major and trace element analyses were conducted along a transect consisting of the lawsonite-rich layer (Lws + Grt + Ph), its glaucophane-rich margin (Gln + Grt + Lws + Ph + Rt), and the lawsonite eclogite host at varying distances from the layer (∼1–2 cm away and >10 cm away). The bulk-rock composition of the lawsonite eclogite indicates a basaltic protolith that experienced interactions with (meta-)sedimentary rocks before or during the crystallization of high-pressure phases such as garnet and rutile. Integrated major and trace element composition and zoning in high-pressure phases indicate that the lawsonite-rich layer and its associated glaucophane-rich margin likely formed at or near peak eclogite-facies conditions (2.2–2.4 GPa, 520 °C) as a result of fluid-mediated processes that scavenged Al2O3, MnO, Y, Th, HFSE, and REE from the eclogite immediately adjacent to the vein (∼1–2 cm away). Mass balance calculations also suggest the addition of LILE and transition metals (Ni, Cr, Zn) to the lithologic layers at the pod margin; these elements were likely supplied from an external source, such as serpentinites and/or sediments, both of which occur interspersed with mafic rocks in the Sivrihisar complex. Fluid-rock interaction may have also driven changes in fO2, as omphacite, glaucophane, lawsonite, and phengite from the pod margin record core-to-rim increases in Fe3+. These changes in bulk composition preferentially stabilized glaucophane-rich (blueschist) and LREE- and LILE-enriched lawsonite-rich assemblages at the pod margin.