Volume 101, Issue 4

The thermodynamic and transport properties of the aluminosilicate melts at the heart of most magmatic processes vary in complex ways with composition, temperature, and pressure. Insights into these properties can come from information on the structure of the melts themselves, and more commonly from their glassy, quenched equivalents. Although most such connections remain qualitative or semi-quantitative, they are fundamentally important in interpretation of observations on igneous systems in nature and the laboratory, and in the formulation of physically accurate models. This review presents some of the important concepts of aluminosilicate glass and melt structure and dynamics that are most relevant to furthering our understanding of the igneous processes so central to how our planet has formed and evolved. The relationships among glasses, melts, and crystals are introduced. The structural underpinnings of temperature and pressure effects on melt free energies, densities, and viscosities, constraints on the extent of order/disorder among cations and anions in melts, why silica activity varies so strongly with composition, and how liquid-liquid phase separation can be understood, are discussed. Some simple, but useful, general views are presented on melt disorder and the shapes of liquidus surfaces (key to magmatic phase equilibria), as are links between atomic-scale dynamics and viscous flow and diffusion.

Any quantitative interpretation of the formation conditions of igneous rocks requires methods for determining crystallization temperature. Accurate application of such thermobarometers relies on the attainment of equilibrium in the system to be studied. This may be particularly difficult in silicic magmas, where diffusivities are low and crystallization kinetics sluggish. Moreover, progressive degassing of volatile-rich magmas during ascent can result in continuous changes in effective undercooling, causing particular problems in achieving equilibrium between melt and crystals that grow in response to decompression. We consider these problems in the context of plagioclase-melt equilibria for magmas undergoing decompression and degassing-driven crystallization, using two published thermometers. The two thermometers show similar trends with key parameters but absolute temperatures can vary significantly. Analysis of decompression experiments conducted at constant temperature shows systematic variations in calculated temperature and equilibrium constant with varying decompression rate and quench pressure. This indicates that an unrecognized lack of equilibration could result in significant temperature overestimates and potentially spurious results. This highlights the need to assess for equilibrium, and we discuss problems associated with some commonly used indicators of equilibration. Finally, retrospective analysis of published plagioclase-hosted melt inclusion suites from five subduction zone volcanoes shows systematic increases in calculated temperature and decreases in equilibrium constant with decreasing H 2 O concentration. While this could represent the signature of latent heat of crystallization, we suggest that such patterns should be treated with caution unless there is clear evidence of sustained equilibrium between plagioclase and melt during decompression.

Nitrogen is fundamental to the evolution of Earth and the life it supports, but for reasons poorly understood, it is cosmochemically the most depleted of the volatile elements. The largest reservoir in the bulk silicate Earth is the mantle, and knowledge of its nitrogen geochemistry is biased, because ≥90% of the mantle nitrogen database comes from diamonds. However, it is not clear to what extent diamonds record the nitrogen characteristics of the fluid/melts from which they precipitate. There is ongoing debate regarding the fundamental concept of nitrogen compatibility in diamond, and empirical global data sets reveal trends indicative of nitrogen being both compatible (fibrous diamonds) and incompatible (non-fibrous monocrystalline diamonds). A more significant and widely overlooked aspect of this assessment is that nitrogen is initially incorporated into the diamond lattice as single nitrogen atoms. However, this form of nitrogen is highly unstable in the mantle, where nitrogen occurs as molecular forms like N 2 or NH4+${\rm{NH}}_4^ + $, both of which are incompatible in the diamond lattice. A review of the available data shows that in classic terms, nitrogen is the most common substitutional impurity found in natural diamonds because it is of very similar atomic size and charge to carbon. However, the speciation of nitrogen, and how these different species disassociate during diamond formation to create transient monatomic nitrogen, are the factors governing nitrogen abundance in diamonds. This suggests the counter-intuitive notion that a nitrogen-free (Type II) diamond could grow from a N-rich media that is simply not undergoing reactions that liberate monatomic N. In contrast, a nitrogen-bearing (Type I) diamond could grow from a fluid with a lower N abundance, in which reactions are occurring to generate (unstable) N atoms during diamond formation. This implies that diamond’s relevance to nitrogen abundance in the mantle is far more complicated than currently understood. Therefore, further petrological investigations are required to enable accurate interpretations of what nitrogen data from mantle diamonds can tell us about the deep nitrogen budget and cycle.

The Barchi-Kol terrain is a classic locality of ultrahigh-pressure (UHP) metamorphism within the Kokchetav metamorphic belt. We provide a detailed and systematic characterization of four metasedimentary samples using dominant mineral assemblages, mineral inclusions in zircon and monazite, garnet zonation with respect to major and trace elements, and Zr-in-rutile and Ti-in-zircon temperatures. A typical diamond-bearing gneiss records peak conditions of 49 ± 4 kbar and 950–1000 °C. Near isothermal decompression of this rock resulted in the breakdown of phengite associated with a pervasive recrystallization of the rock. The same terrain also contains mica schists that experienced peak conditions close to those of the diamond-bearing rocks, but they were exhumed along a cooler path where phengite remained stable. In these rocks, major and trace element zoning in garnet has been completely equilibrated. A layered gneiss was metamorphosed at UHP conditions in the coesite field, but did not reach diamond-facies conditions (peak conditions: 30 kbar and 800–900 °C). In this sample, garnet records retrograde zonation in major elements and also retains prograde zoning in trace elements. A garnet-kyanite-micaschist that reached significantly lower pressures (24 ± 2 kbar, 710 ± 20 °C) contains garnet with major and trace element zoning. The diverse garnet zoning in samples that experienced different metamorphic conditions allows to establish that diffusional equilibration of rare earth element in garnet likely occurs at ~900–950 °C. Different metamorphic conditions in the four investigated samples are also documented in zircon trace element zonation and mineral inclusions in zircon and monazite. U-Pb geochronology of metamorphic zircon and monazite domains demonstrates that prograde (528–521 Ma), peak (528–522 Ma), and peak to retrograde metamorphism (503–532 Ma) occurred over a relatively short time interval that is indistinguishable from metamorphism of other UHP rocks within the Kokchetav metamorphic belt. Therefore, the assembly of rocks with contrasting P-T trajectories must have occurred in a single subduction-exhumation cycle, providing a snapshot of the thermal structure of a subducted continental margin prior to collision. The rocks were initially buried along a low geothermal gradient. At 20–25 kbar they underwent near isobaric heating of 200 °C, which was followed by continued burial along a low geothermal gradient. Such a step-wise geotherm is in good agreement with predictions from subduction zone thermal models.

Fluids buffered by a plagioclase matrix are experimentally reacted with olivine megacrysts at 800 °C and 800 MPa (piston-cylinder press, CaF 2 assembly) to form secondary veins of orthopyroxene ± clinopyroxene in the olivine. Fluids utilized were varied in both amount (0–2 wt%) and salinity (0–8 M NaCl). Assuming equilibrium with the plagioclase matrix, they are presumed enriched in Si, Al, Ca, Na, and Cl and are thereby similar in composition to slab-derived fluids. The experiments provide controlled, multi-component analogs of Si-metasomatism in the mantle wedge above subduction zones. The veins are dominated by orthopyroxene with minor clinopyroxene and form complex interconnected networks along fractures in the olivine. The reaction is rate limited by interfacial process of dissolution and precipitation. Porosity is developed throughout the veins and along sub-grain boundaries in the olivine megacrysts. These veins strongly resemble the textures observed in secondary metasomatic orthopyroxene veins widely reported in upper mantle xenoliths within arc magmas. A review of literature data on natural samples and experiments suggests that orthopyroxene ± clinopyroxene veins primarily form between 750–950 °C and over a large pressure range from 0.8–3.4 GPa. The abundance and composition of these metasomatic veins may vary as a function of pressure, variances in the fluid-rock partition coefficients, and/or by modification of the metasomatic fluid during the reaction.

Mantle potential temperatures ( T p ) provide insights into mantle circulation and tests of whether Earth is the only planet to exhibit thermally bi-modal volcanism—a distinctive signature of modern plate tectonics. Planets that have a stagnant lid, for example, should exhibit volcanism that is uni-modal with T p , since mantle plumes would have a monopoly on the genesis of volcanism. But new studies of magmatic ferric-ferrous ratios (XFe2O3liq/XFeOliq$X_{{\text{F}}{{\text{e}}_{\text{2}}}{{\text{O}}_{\text{3}}}}^{{\text{liq}}}{\text{/}}X_{{\text{FeO}}}^{{\text{liq}}}$) (Cottrell and Kelley 2011) and the olivine-liquid Fe-Mg exchange coefficient, K D (Fe-Mg) Ol-liq (or K D ) (Matzen et al. 2011) indicate that re-evaluations of T p are needed. New tests and calibrations are thus presented for oxygen fugacity ( f O 2 ), XFe2O3liq/XFeOliq$X_{{\text{F}}{{\text{e}}_{\text{2}}}{{\text{O}}_{\text{3}}}}^{{\text{liq}}}{\text{/}}X_{{\text{FeO}}}^{{\text{liq}}}$, potential temperature ( T p ), melt fraction ( F ), K D , and peridotite enthalpies of fusion (∆ H fus ) and heat capacities ( C P ). The new models for XFe2O3liq/XFeOliq$X_{{\text{F}}{{\text{e}}_{\text{2}}}{{\text{O}}_{\text{3}}}}^{{\text{liq}}}{\text{/}}X_{{\text{FeO}}}^{{\text{liq}}}$ and f O 2 reduce error by 25–30%, and residual error for all models appears random; this last observation supports the common, but mostly untested, assumption that equilibrium is the most probable of states obtained by experiment, and perhaps in nature as well. Aggregate 1s error on T p is as high as ~±77 °C, and estimates of F , and mantle olivine composition, are the greatest sources of error. Pressure and ∆ H fus account for smaller, but systematic uncertainties (a constant ∆ H fus can under-predict T excess = Tpplume−Tpambient$T_{\text{p}}^{{\text{plume}}} - T_{\text{p}}^{{\text{ambient}}}$; assumptions of 1 atm can under-predict T p ). However, assumptions about whether parental magmas are incremental, accumulated, or isobaric batch melts induces no additional systematic error. The new models show that maximum T p estimates on the oldest samples from Earth, Mars, Moon, and Vesta, decrease as planet size decreases. This may be expected since T p should scale with accretion energy and reflect the Clausius-Clapeyron slope for the melting of silicates and Fe-Ni alloys. This outcome, however, occurs only if shergottites (from Mars) are 4.3 Ga (e.g., Bouvier et al. 2009; Werner et al. 2014), and the highest MgO komatiites from Earth’s Archean era (27–30% MgO; Green et al. 1975) are used to estimate T p . With these assumptions, Earth and Mars exhibit monotonic cooling, and support for Stevenson’s (2003) idea that smaller planets cool at similar rates (~90–135 °C/Ga), but at lower absolute temperatures. T p estimates for Mars and Earth are also important in two other ways: Mars exhibits non-linear cooling, with rates as high as 275–550 °C/Ga in its first 0.5 Ga, and Archean volcanism on Earth was thermally bi-modal. Several hundred Archean volcanic compositions are in equilibrium with Fo92–94 olivine, and yield T p modes at 1940 and 1720 °C, possibly representing plume and ambient mantle, respectively. These estimates compare to modern T p values of 1560–1670 °C at Mauna Loa (plume) and 1330–1450 °C at MORB (ambient). We conclude that plate tectonics was active in some manner in the Archean, and that assertions of an Archean “thermal catastrophe” are exaggerated. Our new models also show that the modern Hawaiian source, when compared at the same T , has a lower f O 2 compared to MORB, which would discount a Hawaiian source rich in recycled pyroxenite.

Calcic, igneous amphiboles are of special interest as their compositional diversity and common occurrence provide ample potential to investigate magmatic processes. But not all amphibole-based barometers lead to geologically useful information: recent and new tests reaffirm prior studies (e.g., Erdman et al. 2014), indicating that amphibole barometers are generally unable to distinguish between experiments conducted at 1 atm and at higher pressures, except under highly restrictive conditions. And the fault might not lie with experimental failure. Instead, the problem may relate to an intrinsic sensitivity of amphiboles to temperature ( T ) and liquid composition, rather than pressure. The exceptional conditions are those identified by Anderson and Smith (1995): current amphibole barometers are more likely to be useful when T < 800 °C and Fe# Amp = Fe Amp / (Fe Amp +Mg Amp ) < 0.65. Experimentally grown and natural calcic amphiboles are here used to investigate amphibole solid solution behavior, and to calibrate new thermometers and tentative amphibole barometers that should be applicable to igneous systems generally. Such analysis reveals that amphiboles are vastly less complex than may be inferred from published catalogs of end-member components. Most amphiboles, for example, consist largely of just three components: pargasite [NaCa 2 (Fm 4 Al)Si 6 Al 2 ], kaersutite [NaCa 2 (Fm 4 Ti)Si 6 Al 2 O 22 (OH)], and tremolite + ferro-actinolite [Ca 2 Fm 5 Si 8 O 22 (OH) 2 , where Fm = Fe+Mn+Mg]. And nearly all remaining compositionalvariation can be described with just four others: alumino-tschermakite [Ca 2 (Fm 3 Al 2 )Si 6 Al 2 O 22 (OH) 2 ], a Na-K-gedrite-like component [(Na, K)Fm 6 AlSi 6 Al 2 O 22 (OH) 2 ], a ferri-ferrotschermakite-like component [Ca 2 (Fm 3 Fe 2 )Si 6 Al 2 O 22 (OH) 2 ], and an as yet unrecognized component with 3 to 4 Al atoms per formula unit (apfu), 1 apfu each of Na and Ca, and <6 Si apfu, here termed aluminous kaersutite: NaCaFm 4 Ti(Fe 3+ , Al) Si 5 Al 3 O 23 (OH). None of these components, however, are significantly pressure ( P ) sensitive, leaving the Al-in-amphibole approach, with all its challenges, the best existing hope for an amphibole barometer. A new empirical barometer based on D Al successfully differentiates experimental amphiboles crystallized at 1 to 8 kbar, at least when multiple P estimates, from multiple amphibole compositions, are averaged. Without such averaging however, amphibole barometry is a less certain proposition, providing ±2 kbar precision on individual estimates for calibration data, and ±4 kbar at best for test data; independent checks on P are thus needed. Amphibole compositions, however, provide for very effective thermometers, here based on D Ti , D Na , and amphibole compositions alone, with precisions of ±30 °C. These new models, and tests for equilibrium, are collectively applied to Augustine volcano and the 2010 eruption at Merapi. Both localities reveal a significant cooling and crystallization interval (>190–270 °C) at pressures of 0.75 to 2.2 kbar at Augustine and Merapi, respectively, perhaps the likely depths from which pre-eruption magmas are stored. Such considerable intervals of cooling at shallow depths indicate that mafic magma recharge is not a proximal cause of eruption. Rather, eruption triggering is perhaps best explained by the classic “second boiling” concept, where post-recharge cooling and crystallization drive a magmatic system toward vapor saturation and positive buoyancy.

Melt degassing by bubble nucleation and growth is a driving mechanism of magma ascent. Therefore, decompression experiments with hydrous silicate melts were used to investigate the onset and the dynamics of H 2 O degassing. Nominally H 2 O undersaturated trachytic Campi Flegrei and phonolitic Vesuvius melts representative for the magma compositions of the Campi Flegrei volcanic system were decompressed at a super-liquidus temperature of 1050 °C from 200 MPa to final pressures ( P final ) of 100, 75, and 60 MPa using continuous decompression rates of 0.024 and 0.17 MPa/s. Experiments started from either massive glass cylinders or glass powder to demonstrate the infl of the starting material on melt degassing. Glass powder can be used to shorten the equilibration time ( t eq ) prior to decompression for dissolution of H 2 O in the melt. The decompressed samples were quenched and compared in terms of bubble number density ( N V ), porosity, and residual H 2 O content in the melt. Decompression of all glass cylinder samples led to homogeneous bubble nucleation with high N V of ~10 5 mm –3 . The supersaturation pressures for homogeneous bubble nucleation were estimated to be <76 MPa for the trachytic and <70 MPa for the phonolitic melt. In contrast to glass cylinders, the usage of glass powder equilibrated for 24 h before decompression prevented homogeneous bubble nucleation during decompression. We suggest that trapped air in the powder pore space resulted in the formation of tiny H 2 O-N 2 bubbles throughout the samples prior to decompression. Degassing of these glass powder samples was facilitated by diffusive growth of these pre-existing bubbles and thus did not require signifi H 2 O supersaturation of the melt. This is evidenced by several orders of magnitude lower N V and lower residual H 2 O contents at correspondingly higher porosities compared to the glass cylinder samples. However, a signifi extension of t eq to 96 h in the glass powder experiments led to degassing results comparable to the glass cylinder samples. This effect is probably due to Ostwald ripening, coalescence, and the ascent of the pre-existing bubbles during the extended t eq prior to decompression. The N V of the glass cylinder samples were used to test the applicability of the vesiculation model provided by Toramaru (2006). For the applied decompression rates, the experimental N V are up to 5 orders of magnitude higher than the values predicted by the model. This may be mainly attributed to the usage of the macroscopic surface tension and the total H 2 O diffusivity in the model to describe the molecular process of bubble nucleation. A signifi increase in modeled N V can be achieved by application of a reduced surface tension in combination with the lower diffusivity of network formers as a limiting parameter for the formation of a bubble nucleus. This study demonstrates that the investigation of homogeneous bubble nucleation necessitates an optimized experimental protocol. We strongly recommend performing experiments with massive glass cylinders as starting material. The timescale of decompression is a limiting parameter and must be short enough to minimize the opportunity for a reduction of N V by bubble coalescence. Considering our comparably high N V , the samples of many previous experimental studies that were used to calibrate models for homogeneous bubble nucleation were probably subject to signifi N V reduction. Newly derived data from optimized experiments will require improved models for homogeneous bubble nucleation during magma ascent.

Concentrations of H 2 O, F, Cl, C, P, and S have been measured by secondary ion mass spectrometry (SIMS) in experimentally produced peridotite phases (including clinopyroxene, orthopyroxene, olivine, garnet, amphibole, and mica) and coexisting basanitic glasses. Because only two experiments produced glasses on quenching (with the melt phase in others reverting to felt-like crystallite masses) H 2 O concentrations in melts were also separately determined from mass-balance relationships and by assuming constant H 2 O/La in melts and starting materials. The resulting values were used to calculate mineral/melt partition coefficients ( D values) for H 2 O [where DH2Ocrystal/melt$D_{{{\text{H}}_{\text{2}}}{\text{O}}}^{{\text{crystal/melt}}}$ = (mass fraction of H 2 O in crystal)/(mass fraction of H 2 O in melt)] for conditions of 1025–1190 °C and 1.0–3.5 GPa. These gave 0.0064–0.0164 for clinopyroxene, 0.0046–0.0142 for orthopyroxene, 0.0015–0.0016 for olivine, and 0.0016–0.0022 for garnet. Although less information was obtained for the other volatiles, F was found to be significantly more compatible than H 2 O during peridotite melting, whereas Cl is significantly less compatible. S also has small but appreciable solubilities in amphiboles and micas, but not in pyroxenes or olivine. The solubility of C in silicate minerals appears to be negligible, although C was present in coexisting melts (~0.5 wt% as CO 2 ) and as residual graphite during experiments. The D values for H 2 O in clinopyroxene and orthopyroxene are positively correlated with iv Al but negatively correlated with the H 2 O concentrations of melts (when considered as wt%). These relationships are consistent with the broad trends of previously published partitioning data. Although some of the concentration dependence can be related to cross-correlation between iv Al in pyroxenes and H 2 O concentrations in melts (via the latter’s control of liquidus temperatures) this relationship is too inconsistent to be a complete explanation. A concentration dependence for DH2Omineral/melt$D_{{{\text{H}}_{\text{2}}}{\text{O}}}^{{\text{mineral/melt}}}$ can also be independently predicted from speciation models for H 2 O in silicate melts. Thus it is likely that DH2Opyx/melt$D_{{{\text{H}}_{\text{2}}}{\text{O}}}^{{\text{pyx/melt}}}$ is influenced by both ivAl and the absolute concentration of H 2 O in melts. D H 2 O / D Ce for clinopyroxene is inversely correlated with M2 site radii. Because the latter decrease with increasing pressure and temperature, relatively hot and/or deeply derived melts should be enriched in Ce relative to H 2 O when compared to melts from cooler and shallower mantle sources. Conversely, melts from H 2 O-rich settings (e.g., subduction zones) should have higher H 2 O/Ce than their source rocks. When combined with previously obtained partitioning data for non-volatile elements (from the same experiments), our data are consistent with the enrichment of intraplate basalt sources in both volatile and non-volatile incompatible elements by small-degree melts derived from local mid-ocean ridge basalt sources. In this way, volatiles can be seen to play an active role (via their promotion of partial-melting and metasomatic processes) in the auto-regulation of incompatible element concentrations in the depleted upper mantle.

Studies in mineral ecology exploit mineralogical databases to document diversity-distribution relationships of minerals—relationships that are integral to characterizing “Earth-like” planets. As carbon is the most crucial element to life on Earth, as well as one of the defining constituents of a planet’s near-surface mineralogy, we focus here on the diversity and distribution of carbon-bearing minerals. We applied a Large Number of Rare Events (LNRE) model to the 403 known minerals of carbon, using 82 922 mineral species/locality data tabulated in http://mindat.org (as of 1 January 2015). We find that all carbon-bearing minerals, as well as subsets containing C with O, H, Ca, or Na, conform to LNRE distributions. Our model predicts that at least 548 C minerals exist on Earth today, indicating that at least 145 carbon-bearing mineral species have yet to be discovered. Furthermore, by analyzing subsets of the most common additional elements in carbon-bearing minerals (i.e., 378 C + O species; 282 C + H species; 133 C + Ca species; and 100 C + Na species), we predict that approximately 129 of these missing carbon minerals contain oxygen, 118 contain hydrogen, 52 contain calcium, and more than 60 contain sodium. The majority of these as yet undescribed minerals are predicted to be hydrous carbonates, many of which may have been overlooked because they are colorless, poorly crystalized, and/or water-soluble. We tabulate 432 chemical formulas of plausible as yet undiscovered carbon minerals, some of which will be natural examples of known synthetic compounds, including carbides such as calcium carbide (CaC 2 ), crystalline hydrocarbons such as pyrene (C 16 H 10 ), and numerous oxalates, formates, anhydrous carbonates, and hydrous carbonates. Many other missing carbon minerals will be isomorphs of known carbon minerals, notably of the more than 100 different hydrous carbonate structures. Surveys of mineral localities with the greatest diversity of carbon minerals, coupled with information on varied C mineral occurrences, point to promising locations for the discovery of as yet undescribed minerals.

Pyroxene is arguably the most powerful, single-phase geochemical and petrologic recorder of Solar System processes, from nebular condensation through planetary evolution, over a wide range of temperatures, pressures, and f O 2 . It is an important mineral phase in the crusts and mantles of evolved planets, in undifferentiated and differentiated asteroids, and in refractory inclusions—the earliest Solar System materials. Here, we review the valence state partitioning behavior of Cr (Cr 2+ , Cr 3 +), Ti (Ti 3+ , Ti 4+ ), and V (V 2+ , V 3+ , V 4+ , V 5+ ) among crystallographic sites in pyroxene over a range of f O 2 from ap-proximately fayalite-magnetite-quartz (FMQ) to ~7 log units below iron-wüstite (IW-7), and decipher how pyroxene can be used as a recorder of conditions of planetary and nebular environments and planetary parentage. The most important crystallographic site in pyroxene with respect to its influence on mineral/melt partitioning is M2; its Ca content has a huge effect on partitioning behavior, because the large Ca cation expands the structure. As a result, distribution coefficients ( D s) for Cr and V increase with increasing Ca content from orthopyroxene to pigeonite to augite. In addition, it is noted that V 3+ is favored over V 4+ in olivine and pyroxene. In pyroxene in refractory inclusions, Ti 3+ is favored over Ti 4+ and incorporation of Ti is facilitated by the high availability of Al for coupled substitution. The most important results from analysis of pyroxene in martian meteorites (e.g., QUE 94201) are the oxygen fugacity estimates of IW+0.2 and IW+0.9 derived from partitioning and valence data for Cr and V, respectively, obtained from experiments using appropriate temperatures and melt compositions. Inangrites, changes in V valence state may translate to changes inf O 2 , fromIW-0.7 during early pyroxene crystallization, to IW+0.5 during later episodes of pyroxene crystallization. In addition to f O 2 , the partitioning behavior of Cr, V, and Ti between pyroxene and melt is also dependent upon availability of other cations, especially Al, for charge-balancing coupled substitutions.

We experimentally explored phase relations in the System FeOOH-TiO 2 at pressures of 16 GPa and temperatures of 1000–1600 °C, which corresponds to conditions of the Earth’s mantle transition zone. Analyses of the recovered samples revealed that, in the studied conditions, there are two stable iron-titanium oxyhydroxide phases whose estimated composition is expressed by (FeH) 1-x , Ti x O 2 . One is the Fe-rich solid solution (x < 0.23) with ε-FeOOH type crystal structure (orthorhombic, P 2 1 nm) , and the other is the more Ti-rich solid solution (x > 0.35) with α-PbO 2 type structure (orthorhombic. Pbcn) . The ε-FeOOH phase is stable up to -1100 °C irrespective of chemical composition, whereas the α-PbO 2 type phase is stable up to 1500 °C for a composition of x = 0.5 and at least to 1600 °C for x = 0.75, and thus the α-PbO 2 type phase is stable at average mantle temperature in the Earth’s mantle transition zone. Iron-titanium-rich phases found previously in basalt + H 2 O System are estimated to be the ε-FeOOH and α-PbO 2 type phases where the phase with iron-rich composition found at relatively low temperature (< 1100 °C) is ε-FeOOH phase and the phase with titanium-rich composition is α-PbO 2 type phase. The α-PbO 2 type and ε-FeOOH phases may be stable in the subducted basaltic crust at pressures in the mantle transition zone under water-rich conditions.

A critical radiation damage assessment of the materials that will be present in a Geological Disposal Facility (GDF) for radioactive waste is a priority for building a safety case. Detailed analysis of the effects of high-energy α-particle damage in phyllosilicates such as mica is a necessity, as these are model structures for both the clay-based backfill material and the highly sorbent components of a crystalline host rock. The α-radiation stability of biotite mica [general formula: K(Mg, Fe) 3 (Al, Si 3 O 10 )(F, OH) 2 ] has been investigated using the 5 MV tandem pelletron at the University of Manchester’s Dalton Cumbrian Facility (DCF) and both the microfocus spectroscopy (I18) and core X-ray absorption spectroscopy (B18) beamlines at Diamond Light Source (U.K.). Microfocus X-ray diffraction mapping has demonstrated extensive structural aberrations in the mica resulting from controlled exposure to the focused 4 He 2+ ion (α-particle) beam. Delivered doses were comparable to α-particle fluences expected in the highly active, near-field of a GDF. At doses up to 6.77 displacements per atom (dpa) in the region of highest particle fluence, biotite mica displays a heterogeneous structural response to irradiation on a micrometer scale, with sequential dilation and contraction of regions of the structure perpendicular to the sheets, as well as a general overall contraction of the phyllosilicate layer spacing. At the peak of ion fluence, the structure collapses under a high point defect density and amorphous areas are pervasive among altered domains of the original lattice. Such structural alterations are likely to affect the material’s capacity to sorb and retain escaped radionuclides over long timescales; increased edge site availability may favor increased sorption while interlayer uptake will likely be reduced due to collapse. Radiation-induced reduction of structural iron at the region of highest structural damage across an α-particle’s track has been demonstrated by Fe K -edge X-ray absorption near edge spectroscopy (XANES) and local structural disorder has been confirmed by analysis of both potassium K -edge XANES and Fe K -edge extended X-ray absorption fine structure analysis. An infrared absorption study of deformations in the OH – stretching region, along with electron probe microanalysis complements the synchrotron data presented here.

To develop Raman spectroscopy as a quantitative tool in both geosciences and planetary sciences the effect of iron oxidation state (Fe 3+ /Fe tot ) on the Raman spectra of basaltic and pantelleritic glasses has been investigated. We have used remelted pantellerite from Pantelleria Island and synthetic iron-rich basaltic glasses [from Chevrel et al. (2014)]. The Raman spectra of pantelleritic glasses reveal dramatic changes in the high wavelength region of the spectrum (800–1200 cm –1 ) as iron oxidation state changes. In particular the 970 cm –1 band intensity increases with increasing oxidation state of the glass (Fe 3+ /Fe tot ratio from 0.24 to 0.83). In contrast, Raman spectra of the basaltic glasses do not show the same oxidation state sensitivity (Fe 3+ /Fe tot ratio from 0.15 to 0.79). A shift, however, of the 950 cm –1 band to high wavenumber with decreasing iron oxidation state can be observed. We present here two empirical parameterizations (for silica- and alkali-rich pantelleritic glasses and for iron-rich basaltic glasses) to enable estimation of the iron oxidation state of both anhydrous and hydrous silicate glasses (up to 2.4 wt% H 2 O). The validation of the models derived from these parameterizations have been obtained using the independent characterization of these melt samples plus a series of external samples via wet chemistry. The “pantelleritic” model can be applied within SiO 2 , FeO, and alkali content ranges of ~69–75, ~7–9, and ~8–11 wt%, respectively. The “basaltic” model is valid within SiO 2 , FeO, and alkali content ranges of ~42–54, ~10–22, and ~3–6 wt%, respectively. The results of this study contribute to the expansion of the compositionally dependent database previously presented by Di Genova et al. (2015) for Raman spectra of complex silicate glasses. The applications of these models range from microanalysis of silicate glasses (e.g., melt inclusions) to handheld in situ terrestrial field investigations and studies under extreme conditions such as extraterrestrial (i.e., Mars), volcanic, and submarine environments.

A high-temperature rheometer equipped with a graphite furnace, characterized by an air-bearing-supported synchronous motor, has been enhanced by a custom-made Pt-Au concentric cylinder assembly. With this adaptation, viscosity measurements of highly fluid melts can be achieved at high temperatures, up to 1273 K. Due to the air-bearing-supported motor, this apparatus can perform measurements of extremely low torque ranging between 0.01 μNm and 230 mNm (resolution of 0.1 nNm), extending the typical range of viscosity measurements accessible in the present configuration to 10- 3.5 -10 3.5 Pa.s and shear rates up to 10 2 of s -1 . We calibrated the system with distilled water, silicone oils, and the DGG-1 standard glass. We further present new data for the viscosity of Na 2 CO 3 , K 2 CO 3 , and Li 2 CO 3 liquids. Finally, a comparison between our results and literature data is provided, to illustrate the effect of chemical composition and oxygen fugacity on the viscosity of alkali carbonate melts, which serve as analogs for both carbonatitic melts and molten carbonates of industrial relevance. This study substantially improves the database of alkali carbonate melts and dramatically increases the accuracy with respect to previous measurement attempts. The very low viscosity range data and their temperature dependence also helps to constrain very well the activation energy of these highly fluid systems and confirms the estimate of a universal pre-exponential factor for non-Arrhenian visco sity-temperature relationships.

Experiments in high silica systems at temperatures close to the solidus often produce crystals and melt pools that are too small for in situ analysis. Oscillating the temperature during an experimental run speeds up recrystallization of magma by dissolving small and increasing the size of larger crystals, dramatically changing the crystal size distribution. This principle of periodic heating and cooling, caused for example by repeated injection of hot magma, is also a potential acceleration for the formation of phenocrystic textures in natural rocks. Here we show that temperature cycling has the potential to significantly enlarge melt pools and crystals in a fluid saturated dacitic system. Using a natural dacite dredged from the Pacific-Antarctic Rise as starting material, we performed crystallization experiments applying temperature cycling systematically for two different temperatures and different water activities at 200 MPa. For experiments at 950 °C (with a H2O ~1, ~0.3, and <0.1) an internally heated pressure vessel was used, experiments at 800 °C (with a H2O ~1, ~0.5) were performed in a cold-seal pressure vessel. Comparative experiments at equilibrium conditions with constant temperature were performed for both approaches. For all other experiments temperature was cycled with amplitudes of 20 K for different time intervals but constant total run duration after initial equilibration at constant temperature. Additionally, for one experiment at 800 °C, the temperature was increased several times by 50 K to study the potential of dissolving tiny crystals in the matrix. As a result of the temperature cycling, tiny crystals in the matrix were preferentially dissolved, leading to large melt pools with only rare mineral inclusions enabling microprobe analysis using a defocused beam. With regard to the area of the 10 largest crystals of each cycling experiment, clinopyroxene crystals were up to 19 times larger, and plagioclase crystals even up to 69 times when comparing to experiments performed at constant temperature. Grain sizes of FeTi-oxide phases are less influenced by this technique. Essential requirements for applying temperature cycling routinely are identical phase relations and compositions in runs with constant and cycled temperature. For all studied temperatures and water activities, the phase assemblage was the same and compositions of all phases are identical within the analytical error. Thus, the temperature cycling technique opens interesting perspectives, especially in facilitating in situ analysis in near solidus systems.

A detailed Raman spectroscopic, electron microprobe, and laser ablation-induced coupled plasma-mass spectrometric study of 46 natural tourmalines [XY 3 Z 6 (T 6 O 18 )(BO 3 ) 3 V 3 W] from 10 subgroups was performed to evaluate the potential of the Raman scattering, in particular of the OH bond stretching vibrations, for the identification of tourmaline species and site-occupancy analysis. The widespread chemical variety of the studied samples is reflected in the different spectral shapes. The positions and intensities of the observed vibrational modes can be used for tourmaline species identification. Taking into account the charge of the Y- and Z-site cations as well as the X-site occupancy, the Raman peaks generated by the bond stretching mode of the V OH groups were attributed to different YZZ-YZZ-YZZ cationic configurations, while the peaks originating from W OH stretching is due to chemically different YYY triplets next to an X-site vacancy, X Na, or X Ca. It is shown that the integrated intensities of the V OH-stretching peaks can be used to calculate the contents of the major Y-site elements Mg, (Fe 2+ +Mn 2+ ), Li, and Al. The analysis of the V OH-peak positions yields information on the X-site occupancy. The fitted linear equations can be used to determine the content of X (Na+Ca) and X-site vacancy per formula unit. Guidelines for how to gain crystallochemical information from the Raman spectra of tourmaline are suggested. This study, along with Part 1 dedicated to amphiboles (Leissner et al. 2015), reveals that Raman spectroscopy is well suited as a non-destructive, preparation-free, and easy-to-handle method for species identification and site-occupancy analysis in complex hydrous silicate. Our results demonstrate that the chemistry on the non-tetrahedral positions substantially influences the Raman-active H-O bond stretching phonon modes, which allows for quantitative compositional analysis, including the content of lithium.

A new compound, ideally CuZnCl(OH) 3 , was found on a metallic mining artifact of copper composition at the Rowley mine, Maricopa County, Arizona, U.S.A., and studied with electron microprobe analysis, single-crystal X-ray diffraction, and Raman spectroscopy. It is isostructural with botallackite [Cu 2 Cl(OH) 3 ] with space group P2 1 /m and unit-cell parameters a = 5.6883(5), b = 6.3908(6), c = 5.5248(5) Å, β = 90.832(2)°, V= 200.82(3) Å 3 . The crystal structure of CuZnCl(OH) 3 , refined to R 1 = 0.018, is characterized by brucite-type octahedral sheets made of two distinct and considerably distorted octahedra, Ml and M2, which are coordinated by (5OH + 1C1) and (4OH + 2C1), respectively. The octahedral sheets are parallel to (100) and connected by O-H∙∙∙Cl hydrogen bonding. The major structural difference between CuZnCl(OH) 3 and botallackite is the complete replacement of Cu 2+ in the highly angle-distorted Ml site by non-Jahn-Teller distorting Zn 2+ . The CuZnCl(OH) 3 compound represents the highest Zn content ever documented for the atacamite group of minerals, in conflict with all previous reports that botallackite (like atacamite) is the most resistant, of all copper hydroxylchloride Cu 2 Cl(OH) 3 polymorphs, to the substitution of Zn 2+ for Cu 2+ , even in the presence of large excess of Zn 2+ . Its discovery, along with the recently described new mineral iyoite, CuMnCl(OH) 3 , implies that more botallackite-type compounds or minerals with the chemical formula CuMCl(OH) 3 (M= Ni 2+ , Co 2+ , Fe 2+ , Mn 2+ , Cd 2+ , and Mg 2+ ) may be synthesized or found in nature.

The mantle transition zone, at depths between 410 to 660 km, is characterized by two prominent discontinuities in seismic-wave velocity in addition to a relatively steep velocity gradient. Throughout this region garnet will be an abundant mineral, the composition of which will change depending on both depth and lithology. It is important, therefore, to be able to characterize the effects of these changes on seismic velocities, which means that models must incorporate reliable elasticity data on the dominant mineral end-members that can be accurately employed at mantle conditions. In this study elastic wave velocities of synthetic polycrystalline pyrope garnet (Mg 3 Al 2 Si 3 O 12 ) have been measured using ultrasonic interferometry combined with energy-dispersive synchrotron X-ray diffraction in a 1000-ton multi-anvil press. Measurements were performed at pressures up to 24 GPa, conditions compatible with the base of the transition zone, and at temperatures up to 1300 K. Least-squares refinement of the ambient-temperature data to a third-order finite strain equation yields values for the bulk and shear moduli and their pressure derivatives of K S0 = 172.0 ±1.6 GPa, G 0 = 89.1 ±0.5 GPa, δ K S / δ P = 4.38 ±0.08, and δ G/ δ P = 1.66 ±0.05. The determined temperature derivatives are δ K S /δ T = –17.8 ±2.0 MPa/K and δ G/ δ T = –7.9 ±1.0 MPa/K. High-temperature data were fitted to extract parameters for a thermodynamic model. As several high-pressure and -temperature studies have been performed on pyrope, fitting all of the available data provides a more robust assessment of the accuracy of velocity measurements and allows the uncertainties that are inherent in the various methodologies to be realized. When this model is used to determine pyrope velocities at transition zone conditions the propagated uncertainties are approximately 1.5 and 2.5% for v p and v s , respectively. To reduce these uncertainties it is important not only to measure velocities as close as possible to mantle temperatures but also to understand what causes the difference in velocities between studies. Pyrope v P and v S at mantle transition zone conditions are found to be approximately 2.4 and 3.7%, respectively, larger than recent determinations of majoritic garnet at the same conditions, implying a significant variation with chemistry that is mainly realized at high temperatures.