Volume 104, Issue 4

What are the key reactions driving the global carbon cycle in Earth, the only known habitable planet in the Solar System? And how do chemical reactions govern the transformation and movement of carbon? The special collection “Earth in Five Reactions: A Deep Carbon Perspective” features review articles synthesizing knowledge and findings on the role of carbon-related reactions in Earth’s dynamics and evolution. These integrative studies identify gaps in our current understanding and establish new frontiers to motivate and guide future research in deep carbon science. The collection also includes original experimental and theoretical investigations of carbon-bearing phases and the impact of chemical and polymorphic reactions on Earth’s deep carbon cycle.

The Earth in Five Reactions Workshop posed two significant challenges: (1) the formulation of a conceptual definition of “reaction” and (2) the identification and ranking of the “most important reactions” in the context of planetary evolution. Attempted answers to those challenges, collated in this collection of articles, reflect both the opportunities and hurdles when scientists deal with questions of meaning and value.

Iron-oxide apatite (IOA) deposits are mined for iron (Fe) and can also contain economically exploitable amounts of Cu, P, U, Ag, Co, and rare earth elements (REE). Recently, it has been proposed based on trace element zonation in magnetite grains from the Los Colorados Kiruna-type IOA deposit, Chile, that ore formation is directly linked to a magmatic source. The model begins with the crystallization of magnetite microlites within an oxidized volatile-rich (H 2 O+Cl) andesitic magma reservoir, followed by decompression, nucleation of fluid bubbles on magnetite microlite surfaces, segregation of a Fe-Cl-rich fluid-magnetite suspension within the magma reservoir, and subsequent ascent of the suspension from the magma chamber via pre-existing structurally enhanced dilatant zones that act as conduits. Emplacement and precipitation of the suspension results in the formation of magnetite grains with core-to-rim features that record a transition from purely igneous to magmatic-hydrothermal conditions within IOA deposits. Here we test this model by using in situ femtosecond laser ablation MC-ICP-MS measurements of Fe isotopes to determine grain-to-grain and intra-grain Fe isotope variations in magnetite grains from the Los Colorados IOA deposit. All in situ δ 56 Fe values ( 56 Fe/ 54 Fe relative to IRMM-14) plot within the magmatic range (0.06 to 0.50‰), in agreement with previously published bulk Fe isotope analyses in magnetite from the Los Colorados IOA deposit. Different trace element signatures of these magnetite grains indicate an igneous or magmatic-hydrothermal origin, respectively. Although data partly overlap, the assigned igneous magnetites yield on average higher δ 56 Fe values (0.24 ± 0.07‰; n = 33), when compared to magmatic-hydrothermal magnetites (0.15 ± 0.05‰; n = 26). Some magnetite grains exhibit a distinct core-to-rim trend from higher toward lower δ 56 Fe signatures. Furthermore, the δ 56 Fe of the igneous magnetites correlate negatively with trace elements contents typical for igneous formation (Ti, Al, Ga, V, Mn, Zn); igneous magnetites become isotopically heavier with decreasing concentrations of these elements, indicating a trend toward higher δ 56 Fe in the magnetite with magma evolution. Model calculations of the δ 56 Fe evolution in melt, magnetite, and fluid further constrain the magmatic-hydrothermal origin of Kiruna-type IOA deposits.

Geophysical investigations and laboratory experiments provide strong evidence for subduction of ancient oceanic crust, and geological and mineralogical observations suggest that subducted oceanic crust is recycled into the upper mantle. This model is supported by some direct petrologic and mineralogical evidence, principally the recovery of super-deep diamonds from kimberlites and the presence of crustal materials in ophiolitic chromitites and peridotites, but many details are still unclear. Here we report the discovery of ophiolite-hosted diamonds in the podiform chromitites of the Skenderbeu massif of the Mirdita ophiolite in the western part of Neo-Tethys. The diamonds are characterized by exceedingly light C isotopes (δ 13 C PDB ~ −25‰), which we interpret as evidence for subduction of organic carbon from Earth’s surface. They are also characterized by an exceptionally large range in δ 15 N air (−12.9‰ to +25.5‰), accompanied by a low N aggregation state. Materials sparsely included in diamonds include amorphous material, Ni-Mn-Co alloy, nanocrystals (20 × 2 nm) of calcium silicate with an orthorhombic perovskite structure (Ca-Pv), and fluids. The fluids coexisting with the alloy and Ca-Pv provide clear evidence that the diamonds are natural rather than synthetic. We suggest that the Skenderbeu diamonds nucleated and grew from a C-saturated, NiMnCo-rich melt derived from a subducted slab of ocean crust and lithosphere in the deep mantle, at least in the diamond stability field, perhaps near the top of the mantle transition zone. The subsequent rapid upward transport in channeled networks related to slab rollback during subduction initiation may explain the formation and preservation of Skenderbeu diamonds. The discovery of diamonds from the Mirdita ophiolite not only provides new evidence of diamonds in these settings but also provides a valuable opportunity to understand deep cycling of subducted oceanic crust and mantle composition.

This study investigates Ti mobility in the presence of halogens, as shown by the hydrothermal alteration of magmatic rutile in syenite. The syenite pegmatite studied intrudes gabbro, is preserved as a tectonic block in a major strike-slip fault zone, and formed in a back-arc environment in which there was widespread A-type granite plutonism. Rutile was studied by SEM and Raman spectroscopy, trace elements were analyzed by LA-ICP-MS, and age was determined by in situ U-Pb analysis. Magmatic rutile in the syenite forms millimetric-scale crystals rimmed by magmatic titanite and magnetite and also occurs as smaller interstitial crystals. Hydrothermal alteration occurred preferentially along crystal margins and fractures by a layer-by-layer dissolution-reprecipitation process resulting in high Zr contents (~5000 ppm) in the rutile, together with enrichment in U and depletion in high field strength elements. The magmatic emplacement age of the syenite was ~360 Ma (dated rutile G) and no younger than 353.9 ± 5.7 Ma (mean Concordia age of interstitial rutile). The syenite was synchronous with the later phases of regional A-type granite plutonism. Most magmatic rutile has REE patterns either (1) with 1–50 times chondrite enrichment, LREE > HREE and a Eu anomaly, resulting from felsic melt inclusions, or (2) flat patterns with 0.1–10 times chondrite enrichment, present in ilmenite exsolution lamellae or inclusions. Later hydrothermal halogen-rich fluids, derived from dissolution of halite, produced widespread metasomatic scapolite in the syenite. These fluids also leached Ti and other HFSE, together with REE, from large fractured rutile crystals. Such fluids resulted in local dissolution-reprecipitation of Ti and Zr and resetting of the U-Pb system in the altered rutile, at 337.4 ± 3.5 Ma. Normalized REE abundances in the hydrothermal rutile show a U-shaped pattern, with the greatest depletion in the MREE. Variations in dissolution and transport of Zr and Ti by halogen-rich fluids affect the Zr in rutile geothermometer, which yields unrealistic temperatures when applied in this study. More generally, the complexities of rutile chemistry in this hydrothermal setting could be reproduced in deeper subduction settings as a result of variations in halogen content of fluids released by prograde metamorphism.

The exchange of halogens between fluids and solid silicates holds considerable potential to shed light on fluid-rock interactions associated with various geological processes, including seawater-ocean-crust interaction, crustal and mantle metasomatism, and economic deposit formation. This study reports on how variations in formation conditions (temperature, pressure, hydrogen fugacity), bulk composition (Na and K ratio), and choice of starting material salts affect the Cl contents of calcium amphiboles synthesized specifically from ferro-pargasite and hastingsite bulk compositions. Syntheses were attempted over the range of 600–950 °C and 0.1–0.45 GPa at log f H 2 of 1.4 to 2.4 (equivalent to –0.9 to –2.1 log f O 2 below the fayalite-magnetite-β-quartz oxygen buffer, or ΔFMβQ) for durations of 111–672 h. Amphiboles were characterized by powder X-ray diffraction and electron microprobe, with cation proportions calculated on the basis of an assumed 18% ferric iron content. Amphiboles formed from the ferro-chloro-pargasite bulk composition [NaCa 2 (Fe 4.0 Al)(Al 2 Si 6 )O 22 Cl 2 ] had Cl contents of only about 0.5 atoms per formula unit (apfu), compared to the intended 2.0, and whose stabilities were about 70 °C lower at 0.1–0.2 GPa than reported in a previous study of Cl-free (OH-bearing) ferro-pargasite. Syntheses on the ferro-pargasite bulk composition in the presence of a brine with a nominal mole fraction of Cl ( X Cl ) of 0.3 over the range of 700–950 °C at 0.2 GPa showed that temperature had less effect on the Cl content of the amphibole than small variations in the brine concentration assessed after treatment. For the chloro-hastingsite bulk composition [NaCa 2 (Fe 4.0 Fe 3+ )(Al 2 Si 6 )O 22 Cl 2 ], the Cl content of the product amphibole was unaffected by the specific choice of chloride salt or salt combinations (NaCl, CaCl 2 , FeCl 2 ), but showed a direct correlation with the substitution of K for Na. Experiments done over the range of 0.10–0.45 GPa at 700 °C and at –1.3 log f O 2 ΔFMβQ showed an increased rate of nucleation of amphibole with increasing pressure for a hastingsite bulk composition with 40% substitution of K for Na, but no variation in the Cl content of the amphibole. Classification of the amphiboles formed in this study showed that several were well outside their intended field, with some of those formed from the ferro-pargasite bulk composition straddling the boundary between hastingsite and ferro-pargasite, while several formed from the hastingsite bulk composition were actually ferro-ferri-hornblendes. These results confirm that K more so than Na is important for the incorporation of Cl into calcium amphiboles, and that Cl-bearing ferro-ferri-hornblende, with low A-site Na + K, can form even from mixtures with abundant Na + K. Combining these observations with the strong correlation between Cl content and Fe# [= Fe 2+ /(Fe 2+ +Mg)] noted in previous studies, a general correlation was found to exist between the Cl content and the FeAlK index, defined as Fe#( T Al + K), which combines the effects of Fe#, tetrahedrally coordinated Al ( T Al), and K content. A linear trend is observed once a minimum value of about 0.34 in the FeAlK index is reached. The implication is that the crystal-chemical controls for Cl incorporation in calcium amphiboles are dominated by substitution of Fe 2+ for Mg, T Al for Si, and K for Na into the crystallographic A site with a linear dependence at the rate of 0.45 Cl per FeAlK index above a minimum value of about 0.34.

We carried out three series of amphibole crystallization experiments from hydrous basaltic melt to calibrate the dependence of crystal growth rate on temperature and pressure in amphibole-bearing igneous rocks. One series of 100 h duration multi-anvil experiments were carried out at a constant pressure of 0.6 GPa and variable temperatures from 860 to 970 °C. The second series was conducted at a constant temperature of 970 °C and variable pressures from 0.6 to 2.6 GPa. The third series examined the time dependence at 970 °C and 0.6 GPa, with durations from 1 to 100 h. A verification experiment showing both reproducibility and the ability of these three series to predict behavior at novel conditions was performed in a piston cylinder at 1.0 GPa and 900 °C for 63 h. All experiments yielded mostly amphibole in a quenched glass of granitic to granodioritic composition. We used the two-dimensional thin section method to measure the crystal size distribution (CSD) of amphibole in the experimental products. Concave-down CSD curves at small sizes indicate a textural coarsening process during the crystallization. The CSD data were inverted using canonical CSD theory for CSD growth rate; maximum and average growth rates of amphibole were also inferred directly from the maximum and average grain size and crystallization time. The maximum growth rate is, of course, always larger than the average growth rate, which is in turn slightly larger than the CSD growth rate, suggesting that CSD growth rate is an adequate measure of the average growth rate of a mineral in magmatic rocks. The CSD growth rate increases with increasing temperature in the isobaric series and with increasing pressure at constant temperature. However, the growth rate is negatively correlated with crystallization time at constant temperature and pressure. Based on the experimental results, a functional form for evaluating growth rate at known pressure and temperature from an observed amphibole CSD was developed and applied to a diorite collected from the eastern Tianshan Mountains, Xinjiang Uygur autonomous region, NW China. The estimated growth rate of amphibole is between 1.6 × 10 –9 mm/s and 5.6 × 10 –7 mm/s, and combined with petrological constraints on pressure and temperature, the corresponding crystallization time was between 0.1 and 4.3 yr in the natural diorite.

This paper describes the plagioclase alteration process with a focus on the role of micropores, mass transfer, and reaction rate in the Toki granitic pluton in central Japan. The plagioclase alteration process involves albitization, K-feldspathization, and the formation of illite, calcite, fluorite, and epidote, which is classified into three categories based on their distribution: overall alteration throughout the plagioclase grain (Type A), alteration at the cores of the grain (Type B), and the partial alteration at the rims of the grain (Type C). Micropores form during the incipient stage of plagioclase alteration by dissolution of the anorthite component, and then contribute to the infiltration of hydrothermal fluid into the plagioclase, resulting in further progress of the alteration. The distribution of the micropores within the plagioclase is a key factor of the alteration, resulting in the Type A–C distribution patterns. Another factor in plagioclase alteration is the mass transfer of the components released from biotite by chloritization. The overall reactions lead to the quantitative assessment of mass transfer between the reactant and product minerals during the alteration: the alteration involves the inflow of H 4 SiO 4 , Al 3+ , Fe 2+ , Mn 2+ , Mg 2+ , K + , CO 2 , and F – , and the outflow of H 2 O, H + , and Ca 2+ . The age of the plagioclase alteration was determined to be 59.2 ± 1.4 Ma by illite K-Ar geochronology. The combination of this age and the time-temperature ( t - T ) path determined by thermochronometry yields a temperature range from 290 to 305 °C. Chronological data suggest that the serial alteration processes from biotite chloritization to plagioclase alteration occurred during 68–51 Ma and at a temperature of 180–350 °C within the rock body. The biotite chloritization and plagioclase alteration led to sequential variations in the fluid chemistry: the concentrations of aluminum, iron, manganese, and magnesium ions in the hydrothermal fluid decrease gradually, and the concentrations of calcium and fluorine ions in the fluid increase gradually as the biotite chloritization and plagioclase alteration proceed. Both processes also cause variations in the pH of the hydrothermal fluid that affect the dissolution of plagioclase. The infiltration rate of the hydrothermal fluid (HFI rate) and the potassium transfer rate ( K T rate) through the micropores into the plagioclase represent the mass transfer rate of the alteration: the maximum HFI rate ranges from 38.0 × 10 –6 to 68.1 × 10 –6 μm/year and the maximum K T rate ranges from 36.3 × 10 –6 to 64.1 × 10 –6 μm/year. These rates can be used to determine the rate of the plagioclase alteration reaction, which ranges from 9.25 × 10 –13 to 3.45 × 10 –12 gram atom oxygen cm –2 s –1 . The mass transfer rate and reaction rate are possible indicators that relate the extent of hydrothermal alteration in the plagioclase to the timescale.

We present experimental data on the diffusivity of Pb in CaTiO 3 perovskite, which is commonly used for dating kimberlites and carbonatites. Experiments were performed on oriented synthetic and natural CaTiO 3 single crystals. The Pb-source was either a laser deposited (Ca 0.83 Pb 0.07 )Ti 1.05 O 3 thin film or a (Ca 0.9 Pb 0.1 )TiO 3 powder reservoir. The crystals were annealed in a high-temperature furnace between 736 and 1135 °C and for durations from 2 to 283 h. The diffusion profiles were measured with Rutherford backscattering and time-of-flight secondary ion mass spectrometry in the depth-profiling mode. The concentration profiles measured on the same samples with the two analytical methods are in agreement. The measured concentration profiles show two regions—a steep gradient at the diffusion interface that transitions sharply (at ~50 to 150 nm from the surface) to a low concentration tail that penetrates deeper into the crystal. This diffusion behavior could be modeled best using diffusion coefficients that are a function of the Pb concentration, with a different set of diffusion coefficient for the high and the low concentration region of the profile, respectively. The diffusion coefficients extracted from the thin film and powder source experiments are similar within uncertainties. Pb diffuses slower at concentrations between 8.5 and 0.6 wt% and 1.6 to 2.6 log units faster below ~0.5 wt% Pb. Temperature dependency for each region is discussed in the text, and the Arrhenius relation for the fast diffusion regime that is most relevant for natural samples is DPbfast=2.5× 10− 13× exp⁡ (− 158(24)kJ/mol/RT)m2/s.$$\begin{array}{} D_{\text{Pb}}^{\text{fast}}=2.5 \times 10^{-13}\times \exp \Big(-158\big(24\big)\text{kJ/mol}/RT\Big)m^2/s. \end{array} $$ We found a distinct change in the structure of CaTiO 3 in the surface region of the single crystal that is coincidental with the change in diffusivity. This initial region is dominated by planar defects. We propose that Pb is trapped in planar defects that have formed due to the high strain introduced into the perovskite structure caused by the mismatch in the ionic radius between Ca 2+ and Pb 2+ . The activation energies obtained here yield closure temperature for Pb in CaTiO 3 between 300 and 400 °C for a range of different cooling scenarios if diffusive resetting of Pb in CaTiO 3 occurs at all. At typical cooling rates of hours to days for ascending kimberlite, the age of crystal growth is preserved, with closure temperatures similar to the magma temperature.

We discuss differences between the bulk etch rate ( ν B ) and an alternative radial etch rate ( ν R ) model for fission-track etching in apatite. A skeletal ν R -model, based on the inferred orientations of the ν R minima and maxima, accounts for the main geometrical features of etched fission tracks, including the track-surface intersections, track channels and their terminations, and the outlines of confined tracks. It unifies the diverse appearances of etched tracks as variations of a basic plan, governed by the orientation of the etched surface and that of the track. The ν R -model also embeds fission-track etching in the mainstream theories of crystal growth and dissolution. However, in contrast to the ν B -model, the ν R -model does not provide bottom-up criteria for discriminating between tracks that are counted by an observer or a computer program and those that are not. Moreover, abandoning the ν B -model implies that basic assumptions of fission-track dating must be reconsidered, in particular that track counting efficiencies depend only on a critical dip angle, and are thus independent of the track registration geometry and the length distribution.

Nickel is an abundant element in the bulk earth, and nickel-dominant olivine, liebenbergite, is the only igneous nickel-rich silicate found in nature. In this study, we used high-pressure single-crystal diffraction to explore the compressional behavior of a synthetic liebenbergite sample up to 42.6 GPa at ambient temperature. Over the studied pressure range, the liebenbergite sample retains the orthorhombic Pbnm structure, and no phase transition is observed. A third-order Birch-Murnaghan equation of state was used to fit the pressure behavior of the unit-cell volume, lattice parameters, the polyhedral volume, and the average bond length within each polyhedron. The best-fit bulk modulus K T0 = 163(3) GPa and its pressure derivative KT0′$\begin{array}{} K_{{\rm T}0}' \end{array} $ = 4.5(3). We find that liebenbergite is the most incompressible olivine-group silicate reported thus far, and Ni 2+ tends to increase the isothermal bulk modulus of both olivine- and spinel-structured silicates. Consequently, Ni-rich olivine has a higher density compared to Ni-poor olivine at the upper mantle P - T conditions; however enrichment of Ni in mantle olivine is generally too low to make this density difference relevant for fractionation or buoyancy.

The Fe-bearing wadsleyite-ringwoodite phase transition loop under dry conditions in a temperature range of 1473 and 1873 K was determined by in situ X-ray diffraction experiments at the synchrotron facility SPring-8. Pressure at high temperature was precisely determined within a 0.23 GPa error using in situ X-ray diffraction of MgO as a pressure standard. Under dry conditions, assuming an equilibrium chemical composition of wadsleyite and ringwoodite coexisting with garnet in a pyrolite model and an adiabatic temperature gradient with a potential temperature of 1550–1650 K, the phase transition depth and effective width of the seismic discontinuity were found to be 500–514 and 20–22 km, respectively. This effective width, which is three times greater than that of the olivine-wadsleyite phase boundary, can reflect a seismic wave of approximately 0.25 Hz. The wider transition loop between wadsleyite and ringwoodite could create a broad seismic discontinuity. Considering wet and oxidized conditions, the depth of the wadsleyite-ringwoodite phase boundary could be greater than 520 km assuming the small temperature dependency on water and oxygen fugacity effects. Variation in the depth of seismic anomaly may be attributed to water content or oxygen fugacity of the transition zone.

The crystal structures of natural Nb-rich chevkinite-(Ce) from the Biraya rare-metal deposit, Russia, crystallizing in space groups C 2/ m and P 2 1 / a , were solved and refined to R 1 = 0.03 and R 1 = 0.07, respectively, from data collected with a single-crystal diffractometer. X-ray photoelectron spectroscopy was used to determine the oxidation states of the following ions: Ce 3+ , Fe 2+ , Fe 3+ , Ti 4+ , and Ti 3+ . Typically, Nb-rich chevkinite-(Ce) has space group C 2/ m , but specimens with space group P 2 1 / a were also identified. While XPS shows that both C 2/ m and P 2 1 / a crystal structures contain Ti 4+ and Ti 3+ , it also indicates that Ti 2+ may occur in the P 2 1 / a phases. The general formula for the Nb-rich chevkinite-(Ce) having space group C 2/ m is (Ce 3+ ,La 3+ ,Nd 3+ ,Pr 3+ ,Ca 2+ ,Na + ) ∑4 Fe 2+ (Fe 3+ ,Nb 5+ ,Al 3+ ) ∑2 (Ti 4+ ,Ti 3+ ,Nb 5+ ,Mg 2+ ) ∑2 (Si 2 O 7 ) 2 O 8 and with P 2 1 / a is (Ce 3+ ,La 3+ ,Nd 3+ ,Pr 3+ ,Ca 2+ ,Na + ) ∑4 (Fe 2+ ,Mg 2+ ,Ti 2+ ) ∑1 (Fe 3+ ,Nb 5+ ,Al 3+ ) ∑2 (Ti 4+ ,Ti 3+ ,Nb 5+ ) ∑2 (Si 2 O 7 ) 2 O 8 . It is proposed that, in addition to the substitution C Fe 3+ + D Ti 4+ ↔ C Fe 2+ + D Nb 5+ , niobium can also be incorporated into chevkinite-(Ce) by the substitution 2 D Ti 4+ ↔ D Nb 5+ + D Ti 3+ , leading to substantial Nb-enrichment. The study has revealed that the distribution of cations between the various structural sites is considerably more complex than that envisaged in standard models of the chevkinite group, and that cations such as Ti can have more than one valency in the structure. Lighter elements with larger ionic radii (Mg 2+ ) tend to occupy the D site in the C 2/ m crystal structure, whereas in the P 2 1 / a structure these elements (Mg 2+ , Ti 2+ ) tend to enter the B site. Niobium is distributed between the C and D sites. The techniques employed provide a comprehensive representation of the distribution of cations and can be used to examine similar complexity in other mineral groups. The present study has shown that Nb can also be fractionated from residual melts.

Meyrowitzite, Ca(UO 2 )(CO 3 ) 2 ⋅5H 2 O, is a new mineral species from the Markey mine, Red Canyon, San Juan County, Utah, U.S.A. It is a secondary phase found on calcite-veined asphaltum in association with gypsum, markeyite, and rozenite. Meyrowitzite occurs as blades up to about 0.2 mm in length, elongate on [010], flattened on {100}, and exhibiting the forms {100}, {001}, {101}, {110}, and {011}. The mineral is yellow and transparent with vitreous luster and very pale yellow streak. Fluorescence under a 405 nm laser is from weak greenish yellow to moderate greenish blue. The Mohs hardness is ca. 2, tenacity is brittle, fracture is irregular, and there is one perfect cleavage, { 1 01}. The measured density is 2.70(2) g/cm 3 . The mineral is optically biaxial (+) with α = 1.520(2), β = 1.528(2), and γ = 1.561(2) (white light). The 2 V (meas) = 53.0(6)°; weak dispersion, r > ν ; optical orientation: Z = b , Y ^ a = 19° in obtuse β; pleochroism pale yellow, X ≈ Y < Z . Electron microprobe analyses provided the empirical formula Ca 0.94 (U 1.00 O 2 )(CO 3 ) 2 ⋅5(H 2.02 O) on the basis of U = 1 and O = 13 apfu, as indicated by the crystal structure determination. Meyrowitzite is monoclinic, P 2 1 / n , a = 12.376(3), b = 16.0867(14), c = 20.1340(17) Å, β = 107.679(13)°, V = 3819.3(12) Å 3 , and Z = 12. The structure ( R 1 = 0.055 for 3559 I o > 2σ I ) contains both UO 7 pentagonal bipyramids and UO 8 hexagonal bipyramids, the latter participating in uranyl tricarbonate clusters (UTC). The two kinds of bipyramids and the carbonate groups link to form a novel corrugated heteropolyhedral sheet. This is the first structural characterization of a uranyl-carbonate mineral with a U:C ratio of 1:2. Meyrowitzite is apparently dimorphous with zellerite.

Although hydrogen is the most abundant element in the solar system, the mechanisms of exchange of this element between the deep interior and surface of Earth are still uncertain. Hydrogen has profound effects on properties and processes on microscopic-to-global scales. Here we report the discovery of the first hydride (VH 2 ) ever reported in nature. This phase has been found in the ejecta of Cretaceous pyroclastic volcanoes on Mt Carmel, N. Israel, which include abundant xenoliths containing highly reduced mineral assemblages. These xenoliths were sampled by their host magmas at different stages of their evolution but are not genetically related to them. The xenoliths are interpreted as the products of extended interaction between originally mafic magmas and CH 4 +H 2 fluids, derived from a deeper, metal-saturated mantle. The last stages of melt evolution are recorded by coarse-grained aggregates of hibonite (CaAl 12 O 19 ) + grossite (CaAl 4 O 7 ) + V-rich spinels ± spheroidal to dendritic inclusions of metallic vanadium (V 0 ), apparently trapped as immiscible metallic melts. The presence of V 0 implies low oxygen fugacities and suggests crystallization of the aggregates in a hydrogen-rich atmosphere. The presence of such reducing conditions in the upper mantle has major implications for the transport of carbon, hydrogen and other volatile species from the deep mantle to the surface.

Pigeonite ( P 2 1 / c clinopyroxene) crystallizes in various terrestrial and extraterrestrial rocks. However, because it breaks down (“inverts”) in slowly cooled rocks, bulk natural samples of pigeonite from coarse-grained rocks are not available. We have synthesized eight samples of pigeonite with compositions of Wo 8 and Wo 10 [where Wo (mol%) = 100Ca/(Ca+Mg+Fe 2+ )] and X ranging from 20 to 60 [where X = 100Fe 2+ /(Mg+Fe 2+ )]. These samples are suitable for spectroscopic and other studies that require bulk samples. Because of relatively fine grain size (mainly 5–50 μm) and slight grain-to-grain variation in composition, they are generally not suitable for studies requiring individual crystals. We will make samples available for appropriate investigations, especially if the techniques used are non-destructive and the samples can be returned after use.

In this issue This New Mineral Names has entries for 8 new minerals, including fengchengite, ferriperbøeite-(Ce), genplesite, heyerdahlite, millsite, saranchinaite, siudaite, vymazalováite and new data on lavinskyite-1 M