Volume 101, Issue 1

Constraining the spin state of Fe in Earth's lower mantle is critical to understanding the chemistry and dynamics of Earth's interior. In the October 2015 issue of American Mineralogist , Dorfman et al. present an experimental study of the effect of iron concentration on the spin transition in bridgmanite. Their experiments involved two different bridgmanite compositions (38 and 74% FeSiO 3 ). Based on the total spin moment determined by synchrotron-based X-ray emission spectroscopy, they show that Fe 2+ in bridgmanite is in the high-spin state in the lower mantle but transition pressure decreases within highly enriched iron concentrations.

Portland cement clinker is primarily used in building and construction, with a global annual production of ∼4 billion tonnes. Part of it is also used for immobilization of hazardous materials (e.g., industrial slurries, harbor bottom sludge, shredded tires), notably those containing toxic metals like e.g., zinc, cadmium, barium (Trezza and Scian 2000). Limited amounts may be added to the raw meal and get incorporated into the clinker minerals upon burning (Taylor 1997), assuming they remain immobilized in the high-pH concrete or mortar environment after hydration and setting of the cement. Extensive laboratory tests seem to confirm effective mobilization, but are generally of limited duration for reason of practicality. Access to sample materials from centennia- or millennia-old structures is limited for reasons of cultural heritage preservation, and none of these structures were originally designed for environmental reasons. The paper by Khoury et al. (2016) analyzes uncommon combustion-metamorphic rocks from Jordan closely resembling cement-immobilized waste, exposed to supergene weathering and alteration over time spans far exceeding experimental practicality.

Spinel is ubiquitous as a rock-forming mineral in terrestrial, lunar, and planetary basalts and closely associated meteoritic equivalents. A major unknown is whether these rocks formed under similar conditions of partial melting of primary or modified mantle, whether redox environments played a role in evolutionary trends, and did mineral crystal chemistry have any influence on elemental partition between solids and liquids? In a novel approach by Papike et al. (2015), spinel is used as an informative, albeit complex indicator of oxygen fugacity, site occupancy of multiple valence elements, and spinel structural types. Planetary basalts may be reduced (IW-3), oxidized (Earth at FMQ), or of intermediate redox state (Mars). Taking an expansive view, the spinel approach holds enormous promise in understanding the magmatic differentiation of asteroids.

Earth's atmosphere contains 27–30% of the planet's nitrogen and recent estimates are that about one-half that amount (11–16%) is located in the continental and oceanic crust combined. The percentage of N in the mantle is more difficult to estimate, but it is thought to be near 60%, at very low concentrations. Knowledge of the behavior of N in various fluid-melt-rock settings is key to understanding pathways for its transfer among the major solid Earth reservoirs. Nitrogen initially bound into various organic materials is transferred into silicate minerals during burial and metamorphism, often as NH4+${\rm{NH}}_4^ + $ substituting for K + in layer silicates (clays and micas) and feldspars. Low-grade metamorphic rocks appear to retain much of this initial organic N signature, in both concentrations and isotopic compositions, thus in some cases providing a relatively un- or little-modified record of ancient biogeochemical cycling. Devolatilization can release significant fractions of the N initially fixed in crustal rocks through organic diagenesis, during progressive metamorphism at temperatures of ∼350–550 °C (depending on pressure). Loss of fractionated N during devolatilization can impart an appreciable isotopic signature on the residual rocks, producing shifts in δ 15 N values mostly in the range of +2 to +5‰. These rocks then retain large fractions of the remaining N largely as NH4+${\rm{NH}}_4^ + $, despite further heating and ultimately partial melting, with little additional change in δ 15 N. This retention leads to the storage of relatively large amounts of N, largely as NH4+${\rm{NH}}_4^ + $, in the continental crust. Nitrogen can serve as a tracer of the mobility of organic-sedimentary components into and within the upper mantle. This contribution focuses on our growing, but still fragmentary, knowledge of the N pathways into shallow to deep continental crustal settings and the upper mantle. We discuss the factors controlling the return of deeply subducted N to shallower reservoirs, including the atmosphere, via metamorphic devolatilization and arc magmatism. We discuss observations from natural rock suites providing tests of calculated mineral-fluid fractionation factors for N. Building on our discussion of N behavior in continental crust, we present new measurements on the N concentrations and isotopic compositions of microporous beryl and cordierite from medium- and high-grade metamorphic rocks and pegmatites, both phases containing molecular N 2 , and NH4+${\rm{NH}}_4^ + $-bearing micas coexisting with them. We suggest some avenues of investigation that could be particularly fruitful toward obtaining a better understanding of the key N reservoirs and the more important pathways for N cycling in the solid Earth.

Metamorphic chronology or petrochronology has steadily evolved over several decades through ever improving analytical techniques and more complete understanding of the geochemical and petrologic evolution of metamorphosing rocks. Here, the principal methods by which we link metamorphic temperatures ( T ) and ages ( t ) are reviewed, focusing primarily on accessory minerals. Methods discussed include textural correlation, inversion of diffusion profiles, chemical correlation, and combined chronologic and thermometric microanalysis. Each method demonstrates remarkable power in elucidating petrologic and tectonic processes, as examples from several orogens illustrate, but limitations must also be acknowledged and help define future research directions. Correlation methods are conceptually simple, but can be relatively non-specific regarding pressure-temperature conditions of formation. A new consideration of errors indicates that modeling of chronologic diffusion gradients provides relatively precise constraints on cooling rates, whereas models of chemical diffusion gradients can lead to large (factor of 2 or more) cooling rate uncertainties. Although arguably the best method currently in use, simultaneous T-t measurements are currently limited to zircon, titanite, and rutile. Directions for future improvement include investigation of diffusion profiles for numerous trace element-mineral systems using now-routine depth profiling. New trace element models will help improve chemical correlation methods. The determination of inclusion entrapment P-T conditions based on Raman spectroscopic measurement of inclusion pressures (“thermoba-Raman-try”) may well revolutionize textural correlation methods.

Pyroclastic material in the form of altered volcanic ash or tephra has been reported and described from one or more stratigraphic units from the Proterozoic to the Tertiary. This altered tephra, variously called bentonite or K-bentonite or tonstein depending on the degree of alteration and chemical composition, is often linked to large explosive volcanic eruptions that have occurred repeatedly in the past. K-bentonite and bentonite layers are the key components of a larger group of altered tephras that are useful for stratigraphic correlation and for interpreting the geodynamic evolution of our planet. Bentonites generally form by diagenetic or hydrothermal alteration under the influence of fluids with high-Mg content and that leach alkali elements. Smectite composition is partly controlled by parent rock chemistry. Studies have shown that K-bentonites often display variations in layer charge and mixed-layer clay ratios and that these correlate with physical properties and diagenetic history. The following is a review of known K-bentonite and related occurrences of altered tephra throughout the timescale from Precambrian to Cenozoic.

Hundred-micrometer-sized calcium-aluminum-silicates (CAS) inclusions occur in moissanite-4 H , moissanite-15 R , and moissanite-6 H from Turkey. These inclusions commonly consist of tabular exsolution lamellae of two different minerals. By combined electron microprobe and Raman spectroscopy analysis, at least eight different, essentially Mg- and Fe-free Ca-Al-silicate or Al-silicate phases have been discerned. The most common phase is dmisteinbergite, a hexagonal modification of CaAl 2 Si 2 O 8 , occurring in association with lamellae of Ca x (Al,Si) 1−x O 3 or Ca 1−x (Al,Si) 2+x O 5 compositions. All three phases contain significant amounts of BaO (up to 2 mol% of celsiane component in dmisteinbergite), SrO, SO 3 , and light rare earth elements (LREE). In particular, Ca 1−x (Al,Si) 2+x O 5 contains up to 2.1 wt% of LREE, 3.9 wt% of F, and significant traces of Cl, while it is also associated to osbornite (TiN). Pure ghelenite, Ca 2 Al 2 SiO 7 , and three additional compositions, namely CaAl 4–x Si x O 7 , Ca 1–x (Al,Si) 3+x O 6 , and Ca 3–x (Al,Si) 6+x O 14 have been found, either occurring as single grains or forming exsolution lamellae. They also contain significant amounts of BaO, SrO, SO 3 , and LREE. One last intriguing phase is composed in average of 65.9 wt% SiO 2 , 17.4% Al 2 O 3 , 3.0% alkalis, 6.0% BaO, 2.0% CaO+MgO, 0.9% ZrO 2 , and up to 0.5% LREE. Dmisteinbergite and ghelenite show Raman peaks in very good agreement with literature data, Ca x (Al,Si) 1–x O 3 shows main Raman modes at 416 and 1009 cm −1 , Ca 1–x (Al,Si) 3+x O 6 at 531 and 1579 cm −1 while Ca 3–x (Al,Si) 6+x O 14 has a strong peak at 553 cm −1 . CaAl 4–x Si x O 7 shows a weak Raman pattern, while Ca 1–x (Al,Si) 2+x O 5 has no detectable Raman modes. Since the association moissanite-CAS is thermodynamically not stable at ambient pressure and moissanite crystals hosting the CAS phases have δ 13 C values typical of deep-mantle origin, we interpret the CAS inclusions as partially retrogressed HP minerals. Striking analogies exist between observed CAS compositions and experimentally obtained HP-HT mineralogy. For instance, Ca x (Al,Si) 1–x O 3 contains up to 25 mol% of Al 2 O 3 , which is considered as the upper limit of alumina solubility in Ca-perovskite. The study confirms that CAS phases are an important mantle depository for large ion lithophile elements (LILE) and LREE.

Interlayered eclogite and symplectitic garnet rock that is interpreted as former garnetite are found in the Gföhl Unit of the Bohemian Massif. They show unusual Fe–Ti-rich compositions, characterized by TiO 2 contents up to 2.34 wt%, and Mg# of 59.8 and 51.6, respectively. Equilibration conditions of 1250 °C and 4.0 GPa are calculated for eclogite. The petrogenesis of this rock association can be best explained as high-temperature and ultrahigh-pressure magmatic cumulates. Highly decoupled Sr-Nd isotopic composition with nearly constant radiogenic 87 Sr/ 86 Sr values and a slightly negative e Nd value suggests interaction of aqueous fluid most likely derived from a subducting slab and/or from parental magmas. The garnetite contains large (up to 0.5 mm) Fe–Ti-rich spheroids of ilmenite–magnetite–spinel, interpreted as frozen droplets of a melt incorporated in the growing garnet. The interstices between these garnet crystals are filled by ilmenite–magnetite–spinel aggregates, with concave outer surfaces with trapped Fe–Ti-rich melt. These ilmenite–magnetite–spinel spheroids represent possibly the first record of such an oxidized assemblage in mantle rocks, and probably the first description of Fe–Ti-rich melt in eclogite-garnetite mantle rocks. A calculation based on mineral proportions in the spheroids and mineral composition indicates that the immiscible Fe–Ti-rich melt consisted of 28.7 TiO 2 , 3.7 Al 2 O 3 , 0.2 Cr 2 O 3 , 27.9 Fe 2 O 3 , 37.0 FeO, 0.8 MnO, and 1.7 MgO wt%. Petrology and geochemistry of the garnetite indicates an unusual composition for an upper mantle melt with a high oxygen fugacity and relatively high Fe content.

Olenitic tourmaline with high amounts of tetrahedral B (up to 2.53 [4] B pfu) has been synthesized in a piston-cylinder press at 4.0 GPa, 700 °C, and a run duration of 9 days. Crystals are large enough (up to 30 × 150 μm) to allow for reliable and spatially resolved quantification of B by electron microprobe analysis (EMPA), single-crystal X-ray diffraction, and polarized single-crystal Raman spectroscopy. Tourmalines with radial acicular habit are zoned in [4] B-concentration [core: 2.53(25) [4] B pfu; rim: 1.43(15) [4] B pfu], whereas columnar crystals are chemically homogeneous [1.18(15) [4] B pfu]. An amount of 1.4(1) [4] B pfu was found in the columnar tourmaline by single-crystal structure refinement (SREF) ( R = 1.94%). The EMPA identify [T] Si −1 [V,W] O −1 [T] B 1 [V,W] (OH) 1 as the main and [X]⃞ -1 [T] Si –1 [X] Na 1 [T] B 1 as minor exchange vectors for [4] B-incorporation, which is supported by the SREF. Due to the restricted and well-defined variations in chemistry, Raman bands in the OH-stretching region (3000–3800 cm −1 ) are unambiguously assigned to a specific cation arrangement. We found the sum of the relative integrated intensity ( I rel ) of two low-frequency bands at 3284–3301 cm −1 (ν1) and 3367–3390 cm −1 (ν2) to positively correlate with the [4] B concentrations: [4] B [pfu] = 0.03(1) × [ I rel (ν1) + I rel (ν2)]. Hence, those bands correspond to configurations with mixed Si/B occupancy at the T site. Our semi-quantitative correlation also holds for well-characterized natural [4] B-bearing tourmaline from the Koralpe, Austria. This work shows the potential for Raman spectroscopy as a non-destructive method for the chemical classification of (precious) natural tourmaline, and as a tool to rapidly characterize chemical zonation of tourmalines in thin section.

The complex, nanometer-scale structural changes resulting from long-term α-particle bombardment of the mineral biotite are revealed for the first time using high-resolution transmission electron microscope (HRTEM) imaging. Radiohaloes are the product of high-energy α-particles emitted from radioactive inclusions penetrating into the surrounding mineral over long (∼1.8 Ga) timescales, resulting in intense discoloration attributed to ionization events and structural damage. HRTEM analysis of these radiohaloes reveals the long-term breakdown of the biotite structure into three distinct domains. Nanometer-scale, neo-phase regions of dilated and contracted mica structure, with periodicities comparable to 1:1 phyllosilicates, are bound by semi-amorphous, high-defect density domains. These are periodically interspersed with areas of near-original biotite structure. Across the halo region, damaged crystallites have become misoriented, revealing changes in the mica layer-to-layer spacing. This nanoscale response of the mica structure has profound implications for understanding the performance of phyllosilicates in barrier systems employed in the safe isolation of nuclear waste, as materials such as these will be relied upon to retard radionuclide migration over the lifetime of a geological disposal facility.

This work presents a spreadsheet that calculates the mole fractions of end-member components for simple Na-Ca-Li-Mg-Fe 2+ -Al tourmalines from electron microprobe data. The input includes the B 2 O 3 concentration obtained either from direct analysis or by estimation on the basis of stoichiometry. The concentration of Li 2 O can either be input from other analysis or estimated by the spreadsheet. The spreadsheet does not address the mole fractions of Cr, V, oxidized or deprotonated tourmaline species, nor account for species involving tetrahedral boron or aluminum. Therefore, the spreadsheet is not a comprehensive tool that includes all IMA approved tourmaline species, and so is not intended for naming tourmalines according to IMA convention. The present method includes a useful subset of end-member species that can be described simply from electron microprobe data and so, akin to a normative mineralogical analysis for rock composition, the calculations are intended to provide a normative result that serves as simple basis for comparing tourmalines that is more direct than names derived from the most abundant species present.

It is generally believed that clay minerals can protect organic matter from degradation in redox active environments, but both biotic and abiotic factors can influence the redox process and thus potentially change the clay-organic association. However, the specific mechanisms involved in this process remain poorly understood. In this study, model organic compound 12-Aminolauric acid (ALA) was selected to intercalate into the structural interlayer of nontronite (an iron-rich smectite, NAu-2) to form an ALA-intercalated NAu-2 composite (ALA-NAu-2). Shawanella putrefaciens CN32 and sodium dithionite were used to reduce structural Fe(III) to Fe(II) in NAu-2 and ALA-NAu-2. The bioreduced ALA-NAu-2 was subsequently re-oxidized by air. The rates and extents of bioreduction and air re-oxidation were determined with wet chemistry methods. ALA release from ALA-NAu-2 via the redox process was monitored. Mineralogical changes after iron redox cycle were investigated with X-ray diffraction, infrared spectroscopy, and scanning and transmission electron microscopy. At the beginning stage of bioreduction, S. putrefaciens CN32 reductively dissolved small and poorly crystalline particles and released intercalated ALA, resulting a positive correlation between ALA release and iron reduction extent (<12%). The subsequent bioreduction (reduction extent from 12∼30%) and complete air re-oxidation showed no effect on ALA release. These results suggest that released ALA was largely from small and poorly crystalline NAu-2 particles. In contrast to bioreduction, chemical reduction did not exhibit any selectivity in reducing ALA-NAu-2 particles, and a considerable amount of reductive dissolution was responsible for a large amount of ALA release (>80%). Because bacteria are the principal agent for mediating redox process in natural environments, our results demonstrated that the structural interlayer of smectite can serve as a potential shelter to protect organic matter from oxidation.

Interactions between organic matter (OM) and clay minerals have received considerable attention in previous studies. The inflence of OM on smectite illitization has been analyzed primarily in simulation experiments rather than in diagenetic studies. The present study explores the influence of OM on smectite illitization during diagenesis. Thirty red and dark mudstone samples from the Dongying Depression were analyzed. X-ray diffraction (XRD) analyses revealed that the illite percentages in mixed-layer illite-smectite (I-S) of both types of samples were dispersive above 3100 m and more convergent below this depth. The stacking mode of I-S in dark mudstones above 3100 m remained primarily at R0–R0.5 ordering with the average number of layers (N ave ) dispersively distributed between 2 and 4.5. In red mudstones, the I-S changed from the R0 to R0.5 mode with the N ave increasing from 2 to 5. Over this range, the smectite illitization in dark mudstones was slower than that in red mudstones. Below 3100 m, the I-S stacking mode of dark mudstones changed from R0.5 to R3 ordering with the N ave increasing sharply from 4 to 8. In red mudstones, the I-S displayed R1.5 and R3 ordering with the N ave varying between 4.5 and 6.5. Over this range, the smectite illitization in dark mudstones accelerated rapidly, whereas the process in red mudstones was retarded. Additionally, the red mudstone samples contained little OM, whereas the dark mudstone samples contained abundant total organic carbon (0.17–4.43%). Thermo-XRD, near-infrared (NIR) as well as mid-infrared (MIR) spectroscopy analyses suggested that the OM in dark mudstones exhibited a significant transition at 3100 m, coincident with the illitization change. Above 3100 m, the smectite illitization in dark mudstones was delayed due to the OM pillar effect in the interlayer spaces of smectite. Below 3100 m, the interlayer OM became varied and desorbed, discharging organic acid. This led to the dissolution of smectite structural layers. Consequently, illitization in the dark mudstone was accelerated. This study revealed that the existence and occurrence of OM could influence the smectite illitization in diagenesis. Further study on the interactions between OM and clay minerals is needed to facilitate our understanding on the mechanism of smectite illitization as well as its geological applications.

Lime-monteponite solid solutions [(Ca,Cd)O ss] with 58.5–73.3 wt% CdO were discovered as an accessory phase in medium-temperature combustion metamorphic (CM) spurrite-fluorellestadite/ fluorapatite marbles from central Jordan. The type locality is situated in the northern part of the Siwaqa complex (Tulul Al Hammam area), the largest area of the “Mottled Zone” Formation in the Dead Sea region. The marbles were derived from bitumen-rich calcareous marine sediments of the Muwaqqar Chalk Marl Formation, which have high Cd, Zn, U, and Ni enrichments and contain Cd-rich wurtzite and sphalerite. Oxidative sintering of these sediments at 800–850 °C gave rise to unusual oxide accessories: lime-monteponite solid solutions, Cd-bearing Ca and Zn aluminate—tululite, zincite, and Zn-, Ni-, and Cu-rich periclase. Cadmium incorporation into different oxides was controlled by steric factors, while Cd [6] → Ca [6] was the principal isomorphic substitution. The intermediate members (Ca 0.645 Cd 0.355 )O–(Ca 0.453 Cd 0.547 )O with a halite-type structure have a cadmium incorporation ratio (K Cd = Cd mineral /Cd rock ) of about 843 and are the main sites for cadmium in CM marbles. In supergene environments, at low water/rock ratios, (Ca 1−x Cd x )(OH) 2 ss ( x ≤ 0.5) constitute the main secondary phase after (Ca,Cd)O ss. At higher water/rock ratios and in the presence of Cl − and F − in the solutions, calcium, and cadmium precipitated as separate phases ﹛fluorite (CaF 2 ) and basic cadmium chloride [Cd(OH) 2–x Cl x )]﹜. A part of cadmium becomes retained in calcium silicate hydrates. The common occurrence of anhydrous (Ca,Cd)O grains in natural rocks, only partly altered to (Ca,Cd)-hydroxide after at least 100 ka exposure to weather and climate, proves that both phases are effective long-term Cd immobilizers.

A detailed ab initio quantum-mechanical characterization is presented of the vibrational properties of pyrope and majorite garnets, using the hybrid B3LYP functional and large all electron Gaussian type basis sets. Discussed quantities include infrared (both TO and LO) and Raman frequencies, normal mode coordinates, spectroscopic intensities, mode Grüneisen parameters, isotopic substitution, and infrared and Raman spectra. Comparison with data available in the literature demonstrates the accuracy of the adopted method. Main spectral features of the two garnets are interpreted in terms of either symmetry analysis or structural contributions to the vibrational modes. Missing peaks in the experiments are discussed in light of the simulated spectra. The present high-quality vibrational data can be used to compute thermal expansivities at high-pressure and high-temperature conditions. Calculated values for majorite at the bottom of the mantle transition zone (α V = 2.2 × 10 −5 K −1 at T = 1500 K and P = 20 GPa) turn out to be sensibly greater (up to three times) than those currently adopted in geophysical thermodynamic databases, thus calling for a careful revision of the numerical models for thermo-chemical convection of the Earth's mantle.

This paper presents the results of Fourier transform infrared (FTIR) spectroscopy of a synthetic beryl, containing D 2 O molecules in its c -axis channels, which we synthesized under hydrothermal conditions at 600 °C and 1.5 kbar. The frequencies of absorbance bands in the range of the stretching vibrations and their overtones and combination modes for D 2 O and HDO molecules have been assigned for the first time. On the basis of our assignments, the absorbance bands observed for the natural beryl in the range of the OD stretching vibrations have been explained.

We present high-pressure experiments up to 8 GPa that constrain the solidus and liquidus of a composition, Fe 0.69 Ni 0.23 Cu 0.01 S 1.00 , typical of upper mantle sulfide. Solidus and liquidus brackets of this monosulfide are parameterized according to a relation similar to the Simon-Glatzel equation, yielding, respectively, T (°C) = 1015.1 [ P (GPa)/1.88 + 1] 0.206 and T (°C) = 1067.3 [ P (GPa)/1.19 + 1] 0.149 (1 ≤ P ≤ 8). The solidus fit is accurate within ±15 °C over the pressure intervals 1–3.5 GPa and within ±30 °C over the pressure intervals 3.5–8.0 GPa. The solidus of the material examined is cooler than the geotherm for convecting mantle, but hotter than typical continental geotherms, suggesting that sulfide is molten or partially molten through much of the convecting upper mantle, but potentially solid in the continental mantle. However, the material examined is one of the more refractory among the spectrum of natural mantle sulfide compositions. This, together with the solidus-lowering effects of O and C not constrained by the present experiments, indicates that the experimentally derived melting curves are upper bounds on sulfide melting in the Earth's upper mantle and that the regions where sulfide is molten are likely extensive in both the convecting upper mantle and, potentially, the deeper parts of the oceanic and continental lithosphere, including common source regions of many diamonds.

Synchrotron X-ray diffraction (XRD) was used to measure the unit-cell parameters of synthetic pyrope (Mg 3 Al 2 Si 3 O 12 ), grossular (Ca 3 Al 2 Si 3 O 12 ), and four intermediate garnet solid solutions. Garnets synthesized dry in multi-anvil (MA) apparatus at 6 GPa show irregular asymmetric positive quench- able excess volumes, with binary Margules parameters W V grossular = 2.04 ± 0.14 cm 3 /mol, and W V pyrope = 4.47 ± 0.15 cm 3 /mol. The two-parameter Margules equation is only an approximate description of the excess volumes in the pyrope-grossular garnets from this study and from the literature. The values for intermediate garnets in this report are roughly a factor of ∼3 larger than previous literature reports of excess volumes of garnets grown at piston-cylinder (PC) pressures (2–4 GPa) with hydrothermal assistance. The discrepancy between our large dry excess volumes and the smaller hydrothermally as- sisted syntheses previously reported in the literature may be due in part to the hydrothermal assistance. When we do damp syntheses, we too get small excess volume. Although dampness produces smaller excess volumes, the mechanism by which this is achieved remains to be discovered. Does dampness relieve microstrains to relax excess volumes? Analysis of synchrotron X-ray diffraction profiles by using Williamson-Hall plots to test for microstrain shows that peak width does change with dampness and with garnet composition. Microstrain in the garnet structure, rather than grain size variation, is the principal reason for the observed XRD peak broadening. Damp garnets at Py 60 Gr 40 have less microstrain and lower excess volume than dry Py 60 Gr 40 , in accord with the working hypothesis that dampness is responsible for diminished excess volume through strain relief. However garnets with compositions Py 80 Gr 20 and Py 20 Gr 80 , close to the negligibly strained end-members pyrope (Py 100 ) and grossular (Gr 100 ), have large microstrains but relatively small excess volume. This is in contrast to Py 40 Gr 60 , which has the largest excess volume but almost no microstrain. (The end-members have no excess volume, by definition, and little microstrain.) This uncorrelated behavior demonstrates that microstrain in general is not directly related to the excess volumes, whatever else dampness may do to a specific composition, for instance introduce a small amount of clinopyroxene into the mode that partitions Ca preferentially away from garnet. Thus the mechanism responsible for the difference between our large excess vol- umes and the smaller ones in the literature may not be as complete or as simple as dampness relieves microstrain and relaxes excess volume. Our dry intermediate garnets have already been relieved of their microstrain by some other mechanism than dampness and still have large excess volume. The state of Ca-Mg ordering may also change, and we show that this may have a small effect on the ex- cess volume through experiments using variable synthesis time and temperature. A potential test for whether our large, complex excess volumes, of whatever origin, are real (or not) is whether the rise of the solvus with pressure is better described by our large excess volumes or those smaller ones in the literature. We observe garnet phase exsolution at 8 GPa and show it to be reversible, at a temperature consistent with theoretical calculation using the large mixing volume presented in the current study.

Fe-S-P compounds have been observed in many meteorites and could be the important components in planetary cores. Here we investigated the phase stability of Fe 3 (S,P) solid solutions and synthesized high-quality Fe 3 (S 1–x P x ) high-pressure phases in the multi-anvil press. The physical properties of Fe 3 (S 0.5 P 0.5 ) were further studied in the diamond-anvil cell by synchrotron X-ray diffraction and emission spectroscopy. The solubility of S in the Fe 3 (S,P) solid solution increases with increasing pressure. The minimum pressure to synthesize the pure Fe 3 S and Fe 3 (S 0.13 P 0.87 ) is about 21 and 8 GPa, respectively. The observed discontinuity in unit-cell parameters at about 18 GPa is caused by the high-spin to low- spin transition of iron, supported by X-ray emission spectroscopy data. The sulfur solubility in Fe 3 (S,P) solid solutions could be an excellent pressure indicator if such solid solutions are found in nature.

Synchrotron X-ray diffraction data were obtained to simultaneously measure unit-cell volumes of W and WO 2 at pressures and temperatures up to 70 GPa and 2300 K. Both W and WO 2 unit-cell volume data were fit to Mie-Grüneisen equations of state; parameters for W are K T = 307 (±0.4) GPa, KT′ = 4.05(±0.04)$K_{\rm{T}}^{_\prime }\, = \,4.05\left( { \pm 0.04} \right)$, γ 0 = 1.61 (±0.03), and q = 1.54 (±0.13). Three phases were observed in WO 2 with structures in the P 2 1 / c , Pnma , and C 2/ c space groups. The transition pressures are 4 and 32 GPa for the P 2 1 / c-Pnma and Pnma-C 2/ c phase changes, respectively. The P 2 1 / c and Pnma phases have previously been described, whereas the C 2/ c phase is newly described here. Equations of state were fitted for these phases over their respective pressure ranges yielding the parameters K T = 238 (±7), 230 (±5), 304 (±3) GPa, KT′ = 4$K_{\rm{T}}^{_\prime }\, = \,4$, 4 (fixed), 4 (fixed) GPa, γ 0 = 1.45 (±0.18), 1.22 (±0.07), 1.21 (±0.12), and q = 1 (fixed), 2.90 (±1.5), 1 (fixed) for the P 2 1 / c , Pnma , and C 2/ c phases, respectively. The W-WO 2 buffer (WWO) was extended to high pressure using these W and WO 2 equations of state. The T - f O 2 slope of the WWO buffer along isobars is positive from 1000 to 2500 K with increasing pressure up to at least 60 GPa. The WWO buffer is at a higher f O 2 than the iron-wüstite (IW) buffer at pressures lower than 40 GPa, and the magnitude of this difference decreases at higher pressures. This implies an increasingly lithophile character for W at higher pressures. The WWO buffer was quantitatively applied to W metal-silicate partitioning by using the WWO-IW buffer difference in combination with literature data on W metal-silicate partitioning to model the exchange coefficient ( K D ) for the Fe-W exchange reaction. This approach captures the non-linear pressure dependence of W metal-silicate partitioning using the WWO-IW buffer difference. Calculation of K D along a peridotite liquidus predicts a decrease in W siderophility at higher pressures that supports the qualitative behavior predicted by the WWO-IW buffer difference, and agrees with findings of others. Comparing the competing effects of temperature and pressure the results here indicate that pressure exerts a greater effect on W metal-silicate partitioning.

The early eruptive phase of the 2010 eruption at the Fimmvörðuháls Pass, east of Eyjafjallajökull volcano, produced poorly evolved basalts with mildly alkaline affinity, and benmoreitic tephra were emitted during the second explosive phase from the summit vent of the volcano. In this study, textural features and chemical zoning preserved in olivine crystals of the early erupted basalts have been used to define the timescales of differentiation processes and magma ascent before the eruption. These lavas contain a mineral assemblage constituted by olivine (Fo 70–88 ) and plagioclase (An 57–83 ) in similar proportions with scarce clinopyroxene and opaque oxides. Olivine occurs as euhedral or embayed crystals characterized by different core compositions and zoning patterns. Three main olivine populations have been found, namely crystals with: (1) wide Fo 88 cores with normal zoning toward narrow rims (P1); (2) ∼Fo 81 cores with either no zoning or slight reverse zoning patterns toward the rims (P2); (3) ∼Fo 77 cores with reverse zoning at the rims (P3). The olivine reverse zoning indicates that these poorly evolved magmas experienced mixing processes in addition to limited fractional crystallization at different levels of the plumbing system. Timescales of transfer dynamics before the eruption have been estimated through Fe-Mg diffusion modeling on these olivine populations. The olivine-melt equilibration through diffusion was triggered by interaction of magmas differing in their evolutionary degree. P1 and P2 crystals recorded a first event of interaction in a ∼22 km deep reservoir that took place about one month before the emission of the analyzed products. Only part of P2 crystals records reverse zoning due to interaction with more basic magma bearing P1 crystals (which consequently develop normal zoning), suggesting fast timescales of magma mixing that prevented the complete homogenization. A second mixing event, which is evident in the P3 olivines, occurred at shallower levels (5–6 km of depth) ∼15 days before the emplacement and can be considered the triggering mechanism leading to the eruption at the Fimmvörðuháls Pass. Integration of our timescales with seismic data relative to the hypocenter migration indicate rates of magma ascent throughout the deep plumbing system of ∼0.01 m/s. This study provides evidence that magmas emitted at Eyjafjallajökull volcano, and more in general at similar other volcanic systems in ocean ridge settings, can undergo complex processes during their storage and transport in the crust, chiefly due to the presence of a multilevel plumbing system.

Seifertite SiO 2 likely exists as a minor phase near the core–mantle boundary. By simulating the pressure and temperature conditions near the core–mantle boundary, seifertite was synthesized as a minor phase in a coarse-grained, polycrystalline sample coexisting with the (Mg,Fe)SiO 3 post-perovskite (pPv) phase at 129 GPa and 2500 K. Here we report the first in situ single-crystal structure determination and refinement of seifertite at high pressure and after a temperature quench from laser heating. We improved the data coverage of a minor phase from a diamond-anvil cell (DAC) by merging single-crystal data of seifertite from six selected grains that had different orientations. Observed systematic absences of reflections from the six individual grains allowed only one space group: Pbcn . The refined results of seifertite are in good agreement with the predictions from previous first-principles calculations at high pressure. This approach provides a method for structure determination of a minor phase in a mineral assemblage synthesized under P - T conditions representative of the deep Earth.