Band 102, Heft 12

Efforts to map the vertical distribution of mafic and ultramafic igneous rocks in the Earth’s crust and uppermost mantle have long been hampered by the lack of precise geobarometers for the appropriate mineral assemblages. The average P (av P ) method (Powell and Holland 1994) is a multiple-reaction approach that uses a least-squares minimization to average the pressures derived from individual mineral equilibria, taking into account both their uncertainties and correlations. We applied average P to a carefully selected database of published phase-equilibrium experiments in dry to H 2 O-saturated, andesitic to basaltic and peridotitic systems at P = 0.6–9.3 kbar, T = 940–1240 °C, with log f O 2 from NNO-2.6 to NNO+3.6 log units (where NNO is nickel-nickel oxide buffer). We made minor modifications to the thermodynamic models of clinopyroxene, spinel, and olivine to improve the accuracy and precision of the results given by the av P method. Tests on the experimental database, using the modified thermodynamic models and spinel + clinopyroxene + olivine + plagioclase equilibria, showed that average P can reproduce the experimental P , within the calculated 1σ uncertainties (0.9–2.6 kbar; 1.6 kbar on average), for 67% of the database. No systematic deviations of the calculated pressure ( P ) with temperature ( T ) or mineral compositions are observed. Given the large compositional range of the experimental database, these results suggest that the method can be applied to any gabbroic, pyroxenitic, or peridotitic rocks that contain the appropriate phase assemblage clinopyroxene + olivine + plagioclase ± spinel. For assemblages equilibrated at P < 5 kbar, the calculated P shows a slight dependence on T , which therefore needs to be well constrained to keep the overall P uncertainties as low as possible. T can be estimated using either available independently calibrated geothermometers or a simple calculation routine suggested in this work. Application of average P to gabbroic xenoliths from Dominica, Lesser Antilles, and to gabbroic and peridotitic xenoliths from Wikieup, Arizona, demonstrates the ability of the method to produce precise P estimates for natural assemblages equilibrated at both mid- and lower crustal conditions, respectively. Depending on the errors on mineral composition, appropriateness of the T estimate, and attainment of equilibrium of the assemblage, P uncertainty for natural rocks is ≤1.0 kbar. Such a level of precision can help to discriminate between rival petrogenetic processes in subduction zone, intra-plate, and mid-oceanic ridge settings.

Crystallization of groundmass minerals may record the physicochemical conditions of magmatic processes upon eruption and is thus a topic of interdisciplinary research in the disciplines of mineralogy, petrology, and volcanology. Recent studies have reported that the groundmass crystals of some volcanic rocks exhibit a break in their crystal size distribution (CSD) slopes that range from a few micrometers to hundreds of nanometers. The crystals consisting of the finer parts of the break were defined as nanolites. In this study, we report the presence of nanometer-scale crystals down to 1 nm in the pyroclasts of the 2011 eruption of Shinmoedake, the Kirishima volcano group, based on field emission-scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). We discovered a gap (hiatus) from ~100 to ~30 nm in the size distribution of pyroxene in a dense juvenile fragment of a vulcanian explosion. The pyroxene crystals ~20–30 nm on a diameter were ferroaugite ( C 2/ c ), while those a few hundred nanometers in width had a composite structure consisting of the domains of orthopyroxene ( Pbca ), augite ( C 2/ c ), and sub-calcic augite ( C 2/ c ). In high-angle annular dark-field scanning TEM images of the same sample, bright spots ~1–2 nm in diameter were recognized with a gap in size from ~10–20 nm titanomagnetite ( Fd 3 m ). They are presumed to have Fe-rich compositions, although their phases were too small to be determined. In addition, we found that crystals smaller than a few tens of nanometers for pyroxene and 100 nm for plagioclase did not exist or their number densities were too low for accurate determination. This indicates that there are practical minimum sizes of the crystals. These observations show that nucleation of the nanoscale crystals almost paused (froze) in the late stage of groundmass crystallization, possibly due to a decrease in undercooling, increase in interfacial free energy, and decrease in diffusivity in a dehydrated melt, whereas crystal growth was mostly continuous. In this paper, we introduce the novel term “ultrananolite,” to refer to crystals smaller than 30 nm in diameter, and redefine “nanolite” simply as those 30 nm to 1 µm in width, complementing the size interval of crystals in volcanic groundmass smaller than microlites (1–30 μm). In the transient nucleation process, the presence of subcritical size clusters is required. The observed ultrananolite-sized particles might partly include subcritical clusters. The difference in the slope of CSDs, presence of gaps in size distribution, and minimum crystal size among the eruption styles of the 2011 Shinmoedake eruption may be interpreted by considering the difference in magma residence time and fragmentation pressure in the shallow conduit, and possibly the rewelding process in the crater.

Basaluminite is a poorly crystalline aluminum hydroxysulfate that precipitates in waters affected by acid mine drainage (AMD) and in acid sulfate soils (ASS). Its ability to sequester potentially toxic elements, such as Cu and As, makes it an important component of these systems, with strong environmental implications. Although it was initially described as a mineral, basaluminite is now considered a nanoscale variety of felsöbányaite, a rare mineral. In the present study, chemical analyses of natural and synthetic basaluminites are combined with data from advanced nanoscale characterization techniques such as high-energy X-ray diffraction (HEXD) and their corresponding pair distribution function (PDF) analyses, extended X-ray absorption fine structure (EXAFS), and solid-state nuclear magnetic resonance (ssNMR) spectroscopy. X-ray scattering data are analyzed with reverse Monte Carlo (RMC) modeling to obtain an atomistic representation of the disorder presents in this nanomineral. Sulfur K -edge EXAFS results show that sulfate is coordinated to the aluminum-octahedral framework of basaluminite mainly through outer-sphere ligands, though the existence of inner-sphere ligands seems to be significant in synthetic samples. PDF analyses show that both synthetic and natural basaluminites have identical short-range order, with ~1.2 nm coherent domain size, and share structural characteristics with felsöbányaite. Interestingly, 27 Al ssNMR reveals the presence of, respectively, ~1 and 5% of tetrahedral and pentahedral coordinations. RMC models of basaluminite highlight the presence of structural point defects. The understanding of this nanocrystalline character has important implications in terms of the reactivity of this nanomineral in AMD and ASS. The lack of correlation between the spatial and temporal occurrence of basaluminite and felsöbányaite suggests that the similarities between both mineral structures could be fortuitous, and highlights the need for a re-evaluation of the status of basaluminite as a nanomineral.

Recent studies of arc volcanic systems have shown that major and trace element zoning in calcic amphibole yields information about magmatic processes such as fractional crystallization and mixing. Similar studies of plutonic amphibole are scant, yet hold the potential to yield comparable information. To that end, calcic amphibole from late-stage rocks of the English Peak plutonic complex (EPC; Klamath Mountains, northern California) was analyzed in situ, in textural context. The pluton’s late stage consists of three nested intrusive units inwardly zoned from tonalite to granite. Bulk-rock compositions and U-Pb (zircon) ages are consistent either with internal fractional crystallization of a single magma batch or with episodic emplacement of successively evolved magmas, ± magma mixing. Major and trace element abundances and zoning patterns in hornblende (s.l.) are used to test these two interpretations, identify specific magmatic units, determine the temperature range of hornblende stability, and model magma crystallization. In each mapped unit, euhedral to subhedral hornblende displays prominent olive-brown core zones that crystallized at 880–775 °C. Cores are embayed and rimmed by green hornblende crystallized from 775–690 °C. These distinctions are preserved even in samples with moderate deuteric alteration. Some trace elements (Zr, Hf, Sr, Ti, V) decrease monotonically from core to rim, suggesting co-precipitation of hornblende with plagioclase, ilmenite, and zircon. Others (Ba, Rb) are approximately constant in highest- T core zones, then decrease, consistent with onset of biotite crystallization. In contrast, initial rim-ward decreases in Sc, Y, and REE change to near-constant values within olive-brown cores, a change modeled by a decrease in bulk partition coefficients ( D ) due to onset of biotite crystallization. These elements then increase in abundance in green rims, with as much as a fourfold enrichment. Such enrichments can result from resorption/ re-precipitation attending changing P and T during final emplacement, whereby trace elements in core zones were redistributed to the rims. Although hornblende compositions from the three zones are similar, outer-zone hornblende has higher Ti, Ba, Sc, and REE, whereas interior-zone hornblende has higher Mn. These differences are consistent with episodic ascent of compositionally similar but not identical magmas from a mid-crustal reservoir. Evidence for in situ magma mixing is lacking in hornblende. Core-to-rim decrease in Zr indicates hornblende and zircon crystallized together, at T as high as 880 °C. Because zircon saturation thermometry yields T estimates <720 °C for all EPC samples, many of the analyzed rocks are inferred to be cumulates. This study illustrates the utility of detailed major and trace element analysis of hornblende as a means to identify magmatic units and model petrogenetic processes in calc-alkaline granitic rocks.

29 Si NMR has only rarely been applied to silicate minerals in which the predominant cations have unpaired electron spins (e.g., most transition metals and REE), because of the potential for serious line broadening and signal loss. However, as shown here, spectra for a series of natural and synthetic copper(II) silicate minerals can be readily obtained, have paramagnetic shifts far outside known chemical shift ranges, and potentially are very sensitive to structural details involving interactions of paramagnetic cations and Si sites. Signals from different silicon sites in the structures can be distinguished and quantified. Peak broadening due to magnetic couplings and to disorder can be large, but not to the point of “non-observability.” NMR signal loss can be related to specific, and in some cases improvable, technical issues such as excitation bandwidth, sample spinning speed, and rapid nuclear spin relaxation. Two samples of the “mineraloid” chrysocolla from different copper ore deposits have very similar spectra with significant paramagnetic shifts, suggesting strong Si-Cu interactions and a common stoichiometry and short-range structure.

The CM and CI carbonaceous chondrites are typically dominated by phyllosilicates with variable proportions of tochilinite, anhydrous silicates, carbonates, sulfides, sulfates, oxides, and organic compounds. During thermal metamorphism the phyllosilicates dehydrate and decompose yielding water and olivine/enstatite. The thermal transformation of carbonate is less well understood, especially in the presence of volatile decomposition products, such as CO, CO 2 , SO 2 , H 2 S, and H 2 O. Here is described the mineralogical transformation of calcite (CaCO 3 ) to oldhamite (CaS) and portlandite [Ca(OH) 2 ] during extraterrestrial thermal metamorphism on the Sutter’s Mill parent body. Sutter’s Mill is a regolith breccia consisting of at least two lithologic components: phyllosilicate-calcite-bearing and anhydrous olivine-rich. Evidence suggests that the anhydrous stones were derived from extraterrestrial heating of the phyllosilicate-calcite-bearing material. One of only three Sutter’s Mill stones (SM3) collected prior to heavy rainfall over the recovery site is the focus of this study. Its powder X-ray diffraction patterns are dominated by olivine, with lesser enstatite, Fe-sulfides, magnetite, and oldhamite. Oldhamite is absent in the rained-on stones reflecting its water sensitivity and the pristine nature of SM3. Optical micrographs show whitish to bluish grains of oldhamite and portlandite embedded in dark, fine-grained matrix. The presence of abundant olivine and absence of phyllosilicates, tochilinite, and carbonate indicates that SM3 underwent heating to ~750 °C. At this temperature, calcite would have decomposed to lime (CaO). Volatilization experiments show that CO, CO 2 , SO 2 , and H 2 S evolve from CM and CI chondrites heated above 600 °C. Lime that formed through calcite decomposition would have reacted with these gases forming oldhamite under reducing conditions. Residual lime not converted to oldhamite, would have readily hydrated to portlandite, possibly through retrograde reactions during cooling on the parent body. These reactions have parallels to those in coal-fired electricity generating plants and provide an analogous system to draw comparison. Furthermore, the identification of these minerals, which are sensitive to terrestrial alteration, and determination of their formation is enabled only by the rapid collection of samples from an observed fall and their subsequent curation.

Observations of discordant ages, meaning that an age given by one mineral geochronometer is different from the age given by another geochronometer from the same rock, began in the early days of geochronology. In the late 1950s and 1960s, discordant U-Pb zircon ages were unquestioningly attributed to Pb diffusion at high temperature. Later, the mineralogical properties and the petrogenesis of the zircon crystals being dated was recognized as a key factor in obtaining concordant U-Pb ages. Advances in analytical methods allowed the analysis of smaller and smaller zircon multigrain fractions, then the analysis of individual grains, and even pieces of grains, with higher degrees of concordancy. Further advances allowed a higher analytical precision, a clearer perception of accuracy, and a better statistical resolution of age discordance. As for understanding the cause(s) of discordance, belief revision followed the coupling of imaging, cathodoluminescence (CL), and backscattered electrons (BSE), to in situ dating by secondary ion mass spectrometry (SIMS) or by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Discordant zircon and other accessory minerals (e.g., monazite, apatite, etc.) often consist of young rims accreted onto/into older cores. Age gradients are sharp, and no Pb diffusion gradients are observed. As U-Pb discordance in crystalline, non-radiation damaged grains is caused by diachronous, heterochemical mineral generations, interpretations of mineral ages, based on the exclusive role of diffusion, are superseded, and closure temperatures of zircon and monazite are irrelevant in geological reality. Other isotopic systems (Rb-Sr, K-Ar) were believed, since the 1960s, to be similarly controlled by the diffusivity of radiogenic daughters. When zircon and monazite discordance were recognized as zone accretion/reaction with sharp boundaries that showed little or no high-temperature diffusive re-equilibration, the other chronometric systems were left behind, and interpretations of mineral ages based on the exclusive role of diffusion survived. The evidence from textural-petrologic imaging (CL, BSE) and element mapping by electron probe microanalyzer (EPMA) or high spatial resolution SIMS or LA-ICP-MS provides the decisive constraints. All microcline and mica geochronometers that have been characterized in detail document patchy textures and evidence for mineral replacement reactions. It is important not to confuse causes and effects; heterochemical microstructures are not the cause of Ar and Sr loss; rather, they follow it. Ar and Sr loss by dissolution of the older mineral generation occurs first, heterochemical textures form later, when the replacive assemblage recrystallizes. Heterochemical mineral generations are identified and dated by their Ca/Cl/K systematics in 39 Ar- 40 Ar. Replacive reactions adding or removing Cl, such as, e.g., sericite overgrowths on K-feldspar, retrograde muscovite intergrowths with phengite, etc. are detected by Cl/K vs. Ar/K isotope correlation diagrams. Ca-poor reaction products, such as, e.g., young biotite intergrown with older amphibole, adularia replacing microcline, etc., can be easily identified by Ca/K vs. Ar/K diagrams supported by EPMA analyses. Mixed mineral generations are observed to be the cause of discordant, staircase-shaped age spectra, while step-heating of crystals with age gradients produces concordant plateaus. Age gradients are therefore unrelated to staircase age spectra. There is a profound analogy between the U-Pb, Rb-Sr, and K-Ar systems. Pb and Ar diffusion rates are both much slower than mineral replacement rates for all T < 750 °C. Patchy retrogression textures are always associated with heterochemical signatures (U/Th ratios, REE patterns, Ca/Cl/K ratios). As a rule, single-generation minerals with low amounts of radiation damage give concordant ages, whereas discordance is caused by mixtures of heterochemical, resolvably diachronous, mineral generations in petrologic disequilibrium. This can also include (sub-)grains that have accumulated significant amounts of radiation damage. For accurate geochronology the petrologic characterization with the appropriate technique(s) of the minerals to be dated, and the petrologic context at large, are as essential as the mass spectrometric analyses.

To define the liquidus and solidus of the system CaCO 3 -MgCO, rotating multi-anvil experiments were performed at 6 GPa in the temperature range 1300 to 1800 °C under anhydrous conditions. Additionally, experiments under hydrous conditions were performed in the Mg-rich part of the phase diagram. To determine the melting point of the end-member magnesite at 6 GPa falling sphere/body experiments were performed. The run products were analyzed using electron microprobe, Raman spectroscopy, and X-ray diffraction. Some of the run products were investigated by transmission electron microscopy (TEM). Previous studies report tremendous quenching problems in melting experiments of carbonates, as the primary grown carbonates could not be distinguished from the quenched melt. With the help of rotating multi-anvil experiments the primary grown crystals could be separated from the melt phase and the compositions of both phases could be analyzed by electron microprobe. Compared with the results of static experiments the corresponding phase diagram under anhydrous conditions is significantly different. The anhydrous melting point of MgCO 3 at 6 GPa could be located between 1750 and 1800 °C. Under hydrous conditions liquidus and solidus moved to lower temperatures compared to anhydrous conditions and the melting point of hydrous MgCO 3 at 6 GPa is located between 1700 and 1750 °C.

The electrical conductivity of mudstone before and after dehydration was measured using complex impedance spectroscopy in the frequency range of 10 −1 to 10 6 Hz, and the experiments were carried out at 0.5–2.5 GPa and 623–973 K. Before and after dehydration, the electrical conductivity of mudstone and temperature followed an Arrhenius relation. The influence of pressure on electrical conductivity was weaker than that of temperature. The conductivity slightly increased with increasing pressure. Dehydration at 760–800 K dramatically enhanced the electrical conductivity of mudstone; the dehydration temperature decreased slightly with increasing pressure. Hydrogen-related lattice defects (e.g., HM′$\begin{array}{} \text{H}_{\text M}' \end{array} $ or H • ) are proposed to be the main charge carriers in the mudstone sample before dehydration, whereas H + and OH − are suggested to be the main charge carriers in the dehydration product of mudstone. Finally, the electrical conductivity of the dehydration product of mudstone can be used to interpret high-conductivity layers (HCLs) associated with the Hope and Porters Pass fault zones in Marlborough, New Zealand.

Monazite (CePO 4 ) and xenotime (YPO 4 ) are important hosts for REE and thus can be used to monitor REE mass transfer in various settings. In this investigation, the solubilities of synthetic monazite and xenotime were measured in KCl-H 2 O fluids at 800 °C and 1.0 GPa, using the piston-cylinder apparatus. The experimental results indicate an increase in monazite and xenotime solubility in aqueous fluids with moderate KCl mole fractions ( X KCl ) in agreement with previous investigations of the solubility of these phases in NaCl-H 2 O. Under all conditions, monazite and xenotime dissolve congruently. The solubility of synthetic monazite increases from 8 ppm in pure H 2 O to 335 ppm at X KCl = 0.506. The solubility of synthetic xenotime rises from 46 ppm in pure H 2 O to 126 ppm at X KCl = 0.348, above which it is constant or declines slightly. Monazite and xenotime solubilities are considerably lower in KCl-H 2 O than in NaCl-H 2 O at the same salt concentration. Best-fit equations for the solubilities of the two phases are: cmz=− 464XKCl2+891XKCl+8$$\begin{array}{} \displaystyle c_{\rm mz}=-464\,X^2_{\rm KCl}+891\,X_{\rm KCl}+8 \end{array} $$ and cxt=− 563XKCl2+432XKCl+46$$\begin{array}{} \displaystyle c_{\rm xt}=-563\,X^2_{\rm KCl}+432\,X_{\rm KCl}+46 \end{array} $$ where mz and xt stand for monazite and xenotime, and X KCl = n KCl /( n KCl + n H 2 O ) where n is moles. The change in solubilities with KCl implies that Ce dissolves as an anhydrous chloride complex (CeCl 3 ), whereas Y forms a mixed Cl-OH solute [YCl(OH) 2 ]. The data also imply that H 2 O-NaCl fluids and H 2 O-KCl fluids close to neutral pH can transport substantial amounts of REE and Y, thus obviating the need to invoke low-pH solutions in high-grade environments where they are highly unlikely to occur.

In this study, we quantitatively investigate crystal-melt segregation processes in two upper-crustal, intermediate-to-silicic plutons from the Tertiary Adamello Batholith, Italian Alps, by combining (1) an estimation of the amount of crystallized interstitial liquid using cathodoluminescence images, phase maps, and mass-balance calculations with (2) quantification of crystal preferred orientation using electron backscatter diffraction. Cathodoluminescence images, phase maps, and plagioclase profiles are used together to distinguish early grown primocrysts from overgrowths formed after the rheological “lock-up” of the magma bodies. Mass-balance calculations, taking into account mineral compositions and bulk-rock chemistry, are used as an additional means to quantify the amount of trapped melt. The following features are indicative of crystal accumulation (or melt loss) in some parts of the batholith: (1) The amount of crystallized interstitial liquid can be low and negatively correlated with crystal (and shape) preferred orientations. Locally, up to ca. 27% melt may have been lost. (2) Significant intracrystalline deformation in plagioclase (up to ca. 13° of lattice distortion) is present in strongly foliated samples, resulting from compaction in a highly crystalline mush. These mineralogical and textural features indicative of variability in the degree of crystal accumulation in some areas of the Adamello batholith may explain the highly scattered bulk-rock geochemical patterns (particularly in trace elements). However, the precise quantification of the amount of melt loss remains challenging in felsic plutons, because of the compositional deviation from liquid lines of descent due to multi-scale variations in the degree of crystal-melt segregation and the fact that magmatic textures indicative of crystal accumulation can be subtle.

Scapolites are widespread rock-forming aluminosilicates, appearing in metasomatic and igneous environments, and metamorphic terrains. Marialite (Na 4 Al 3 Si 9 O 24 Cl) is the Cl-rich end-member of the group. Even though Cl-rich scapolite is presumably stable over a wide range of pressure and temperature, little is known about its stability field. Understanding Cl-rich scapolite paragenesis is important since it can help identify subsurface fluid flow, metamorphic, and isotopic equilibration. Due to its metasomatic nature Cl-rich scapolite is commonly reported in economic ore deposits, hence it is of critical interest to the mineral resource industries who seek to better understand processes contributing to mineralization. In this experimental study two reactions were investigated. (1) The anhydrous reaction of albite + halite to form marialite [3NaAlSi 3 O 8 + NaCl = Na4Al 3 Si 9 O 24 Cl]. (2) The hydrothermal equivalent described by H 2 O + Na 4 Al 3 Si 9 O 24 Cl = 3NaAlSi 3 O 8 + liquid, where the liquid is assumed to be a saline-rich hydrous-silicate melt. Experiments were performed using a piston-cylinder press and internally heated gas vessels. The temperature and pressure conditions range from 700–1050 °C and 0.5–2.0 GPa, respectively. The starting materials were synthetic phases including end-member marialite, high-albite, and halite. For reaction 1, marialite was found to be stable above 920 to 990 °C over a pressure range of 0.65 to 2.0 GPa, but unstable between 800 and 950 °C at pressures of 0.5 GPa and lower. For reaction 2, marialite was found to be very intolerant of water, requiring a minimum bulk brine salinity of approximately 0.8 mole fraction of NaCl at 1050 and 1000 °C at pressures of 2.0 and 1.5 GPa, respectively. From the location of reaction 1 in pressure-temperature space, thermochemical data for marialite were extracted. Values for the enthalpy of formation (Δ Hf∘ )$(\!\Delta H{_{\rm f}^{\circ}}\!) $ and third-law entropy ( S °) for marialite at 298 K and 1 atm have been calculated as −12 167.5 ± 1.5 kJ/mol and 0.7579 ± 0.002 kJ/K·mol, respectively, based on existing thermochemical data for high-albite and halite. The main implication of this study is that end-member marialite is only stable at temperatures greater than 920 °C and pressures equivalent to a minimum depth of 18 km under extremely dry conditions. These conditions are not generally realized in typical scapolite-bearing rocks, which occur at shallower-levels and in hydrothermal settings, which may be why pure marialite is rarely observed. This study is the first experimentally determined stability for end-member marialite and provides an important reference for quantifying the stability of Cl-rich scapolites found in nature.

A quantitative knowledge of the equation of state of wadsleyite solid solutions is needed to refine thermodynamic and thermoelastic models for the transition zone in Earth’s upper mantle. Here we present the results of high-pressure single-crystal X-ray diffraction experiments on two crystals of slightly hydrous iron-bearing wadsleyite with Fe/(Mg+Fe) = 0.112(2), Fe 3+ /ΣFe = 0.15(3), and 0.24(2) wt% H 2 O up to 20 GPa. By compressing two wadsleyite crystal sections inside the same diamond-anvil cell, we find a negligible influence of crystal orientation on the derived equation of state parameters. Volume and linear compression curves were analyzed with finite strain theory to demonstrate their mutual consistency for the Reuss bound indicating quasi-hydrostatic stress conditions. The results on the here-studied wadsleyite crystals are incorporated into a multi-end-member model to describe the equation of state for wadsleyite solid solutions in the system Mg 2 SiO 4 -Fe 2 SiO 4 -MgH 2 SiO 4 -Fe 3 O 4 . For the hypothetical ferrous wadsleyite end-member, Fe 2 SiO 4 , we find a substantially larger bulk modulus than expected by extrapolating currently accepted trends. The multi-end-member equation of state model may serve as a basis for the calculation of phase equilibria and the interpretation of seismic observations regarding the transition zone.

This study investigates the effect of tetrahedral B ( [4] B) in synthetic tourmaline on the B-isotope fractionation between tourmaline and fluid. This is important for the correct interpretation of B-isotope variations in natural tourmalines containing “excess” B (greater than three atoms per formula unit), which substitutes for Si at tetrahedral sites. Such tourmalines commonly occur in Li, Al-rich pegmatites and have been reported from glaucophane schists that formed at high pressures during subduction. Tourmaline synthesis experiments were performed in a piston-cylinder apparatus in the system SiO 2 -Al 2 O 3 -B 2 O 3 -NaCl-H 2 O at 4 GPa and 700 °C using different run durations, starting from quartz-γ-Al 2 O 3 -H 3 BO 3 solid mixtures and NaCl-solutions. We were able to produce “olenitic” tourmaline with excess B between 1.2 and 2.5 [4] B per formula unit. The B-isotope compositions of the olenitic tourmaline and coexisting fluids were determined by secondary ion mass spectrometry and multi-collector plasma source mass spectrometry to derive isotope fractionation coefficients. The results indicate that for every 10 mol% of total B in tourmaline in tetrahedral coordination, the value of Δ 11 B tur-fluid is shifted to more negative values by about 1‰ at 700 °C. This is in good agreement with published ab initio calculations and corresponds to an intracrystalline fractionation of B-isotopes between the trigonal B and tetrahedral T sites of tourmaline on the order of 8 ± 5‰, whereby 10 B partitions to the T site.

Arsenic (As), barium (Ba), and sulfate (SO42− $\begin{array}{} \displaystyle \rm SO_4^{2-} \end{array} $), coexisting in natural and mining impacted environments, possibly lead to As-barite coprecipitation. This work investigated the coprecipitation of Ba 2+ , SO42− $\begin{array}{} \displaystyle \rm SO_4^{2-} \end{array} $, and AsO43− $\begin{array}{} \displaystyle \rm AsO_4^{3-} \end{array} $ [As(V)] and the incorporation of As(V) into the barite structure. The As(V) content in the coprecipitates increased with pH and the initial aqueous As(V) concentration. At pH ≤ 5, As(V) was dominantly fixed through isomorphic substitution for SO42− $\begin{array}{} \displaystyle \rm SO_4^{2-} \end{array} $ in the barite structure (<0.32 wt%). At pH > 5, barium (hydrogen) arsenate constituted an appreciable fraction of As(V)-bearing species in addition to the incorporated As(V). FTIR spectroscopy indicated that As(V) in the coprecipitate occurred as mixed phases and the As(V) species incorporated into the barite structure was dominated by HAsO42− $\begin{array}{} \displaystyle \rm HAsO_4^{2-} \end{array} $ species. EXAFS analysis gave As-O and As-OH bond lengths of 1.67 and 1.75 Å for HAsO42− $\begin{array}{} \displaystyle \rm HAsO_4^{2-} \end{array} $ in barite structure, respectively. The FPMS structural refinement reproduced well the As K -edge XANES spectrum and gave bond lengths of As-O at 1.63, 1.64, 1.68, and 1.75 Å with an average bond length of 1.68 ± 0.05 Å in HAsO42− $\begin{array}{} \displaystyle \rm HAsO_4^{2-} \end{array} $ doped barite structure. The findings are of significance for understanding the geochemical cycle of As in As(V), Ba 2+ , and SO42− $\begin{array}{} \displaystyle \rm SO_4^{2-} \end{array} $ coexisting systems.