Heat capacity of hydrous trachybasalt from Mt Etna: comparison with CaAl<Subscript>2</Subscript>Si<Subscript>2</Subscript>O<Subscript>8</Subscript> (An)–CaMgSi<Subscript>2</Subscript>O<Subscript>6</Subscript> (Di) as basaltic proxy compositions

The specific heat capacity (C _p) of six variably hydrated (~3.5 wt% H₂O) iron-bearing Etna trachybasaltic glasses and liquids has been measured using differential scanning calorimetry from room temperature across the glass transition region. These data are compared to heat capacity measurements on thirteen melt compositions in the iron-free anorthite (An)–diopside (Di) system over a similar range of H₂O contents. These data extend considerably the published C _p measurements for hydrous melts and glasses. The results for the Etna trachybasalts show nonlinear variations in, both, the heat capacity of the glass at the onset of the glass transition (i.e., C _p ^g ) and the fully relaxed liquid (i.e., C _p ^l ) with increasing H₂O content. Similarly, the “configurational heat capacity” (i.e., C _p ^c  = C _p ^l  ? C _p ^g ) varies nonlinearly with H₂O content. The An–Di hydrous compositions investigated show similar trends, with C _p values varying as a function of melt composition and H₂O content. The results show that values in hydrous C _p ^g , C _p ^l and C _p ^c in the depolymerized glasses and liquids are substantially different from those observed for more polymerized hydrous albitic, leucogranitic, trachytic and phonolitic multicomponent compositions previously investigated. Polymerized melts have lower C _p ^l and C _p ^c and higher C _p ^g with respect to more depolymerized compositions. The covariation between C _p values and the degree of polymerization in glasses and melts is well described in terms of SM_hydrous and NBO/T _hydrous. Values of C _p ^c increase sharply with increasing depolymerization up to SM_hydrous ~ 30–35 mol% (NBO/T _hydrous ~ 0.5) and then stabilize to an almost constant value. The partial molar heat capacity of H₂O for both glasses (\( C_{{{\text{p}}\;{\text{H}}_{2} {\text{O}}}}^{\text{g}} \)) and liquids (\( C_{{{\text{p}}\;{\text{H}}_{2} {\text{O}}}}^{\text{l}} \)) appears to be independent of composition and, assuming ideal mixing, we obtain a value for \( C_{{{\text{p}}\;{\text{H}}_{2} {\text{O}}}}^{\text{l}} \) of 79 J mol^?1 K^?1. However, we note that a range of values for \( C_{{{\text{p}}\;{\text{H}}_{2} {\text{O}}}}^{\text{l}} \) (i.e., ~78–87 J mol^?1 K^?1) proposed by previous workers will reproduce the extended data to within experimental uncertainty. Our analysis suggests that more data are required in order to ascribe a compositional dependence (i.e., nonideal mixing) to \( C_{{{\text{p}}\;{\text{H}}_{2} {\text{O}}}}^{\text{l}} \).     

The effect of alkalis and polymerization on the solubility of H<Subscript>2</Subscript>O and CO<Subscript>2</Subscript> in alkali-rich silicate melts

The effect of alkalis on the solubility of H₂O and CO₂ in alkali-rich silicate melts was investigated at 500 MPa and 1,250 °C in the systems with H₂O/(H₂O + CO₂) ratio varying from 0 to 1. Using a synthetic analog of phonotephritic magma from Alban Hills (AH1) as a base composition, the Na/(Na + K) ratio was varied from 0.28 (AH1) to 0.60 (AH2) and 0.85 (AH3) at roughly constant total alkali content. The obtained results were compared with the data for shoshonitic and latitic melts having similar total alkali content but different structural characteristics, e.g., NBO/T parameter (the ratio of non-bridging oxygens over tetrahedrally coordinated cations), as those of the AH compositions. Little variation was observed in H₂O solubility (melt equilibrated with pure H₂O fluid) for the whole compositional range in this study with values ranging between 9.7 and 10.2 wt. As previously shown, the maximum CO₂ content in melts equilibrated with CO₂-rich fluids increases strongly with the NBO/T from 0.29 wt % for latite (NBO/T = 0.17) to 0.45 wt % for shoshonite (NBO/T = 0.38) to 0.90 wt % for AH2 (NBO/T = 0.55). The highest CO₂ contents determined for AH3 and AH1 are 1.18 ± 0.05 wt % and 0.86 ± 0.12 wt %, respectively, indicating that Na is promoting carbonate incorporation stronger than potassium. At near constant NBO/T, CO₂ solubility increases from 0.86 ± 0.12 wt % in AH1 [Na/(Na + K)] = 0.28, to 1.18 ± 0.05 wt % in AH3 [Na/(Na + K)] = 0.85, suggesting that Na favors CO₂ solubility on an equimolar basis. An empirical equation is proposed to predict the maximum CO₂ solubility at 500 MPa and 1,100–1,300 °C in various silicate melts as a function of the NBO/T, (Na + K)/∑cations and Na/(Na + K) parameters: \({\text{wt}}\% \;{\text{CO}}_{2} = - 0.246 + 0.014\exp \left( {6.995 \cdot \frac{\text{NBO}}{T}} \right) + 3.150 \cdot \frac{{{\text{Na}} + {\text{K}}}}{{\varSigma {\text{cations}}}} + 0.222 \cdot \frac{\text{Na}}{{{\text{Na}} + {\text{K}}}}.\) This model is valid for melt compositions with NBO/T between 0.0 and 0.6, (Na + K)/∑cation between 0.08 and 0.36 and Na/(Na + K) ratio from 0.25 to 0.95 at oxygen fugacities around the quartz–fayalite–magnetite buffer and above.     

Water diffusion in Mount Changbai peralkaline rhyolitic melt

Diffusion couple experiments with wet half (up to 4.6 wt%) and dry half were carried out at 789–1,516 K and 0.47–1.42 GPa to investigate water diffusion in a peralkaline rhyolitic melt with major oxide concentrations matching Mount Changbai rhyolite. Combining data from this work and a related study, total water diffusivity in peralkaline rhyolitic melt can be expressed as:

$ D_{{{\text{H}}_{ 2} {\text{O}}_{\text{t}} }} = D_{{{\text{H}}_{ 2} {\text{O}}_{\text{m}} }} \left( {1 - \frac{0.5 - X}{{\sqrt {[4\exp (3110/T - 1.876) - 1](X - X^{2} ) + 0.25} }}} \right), $

$ {\text{with}}\;D_{{{\text{H}}_{ 2} {\text{O}}_{\text{m}} }} = \exp \left[ { - 1 2. 7 8 9- \frac{13939}{T} - 1229.6\frac{P}{T} + ( - 27.867 + \frac{60559}{T})X} \right], $

where D is in m² s^?1, T is the temperature in K, P is the pressure in GPa, and X is the mole fraction of water and calculated as X = (C/18.015)/(C/18.015 + (100 ? C)/33.14), where C is water content in wt%. We recommend this equation in modeling bubble growth and volcanic eruption dynamics in peralkaline rhyolitic eruptions, such as the ~1,000-ad eruption of Mount Changbai in North East China. Water diffusivities in peralkaline and metaluminous rhyolitic melts are comparable within a factor of 2, in contrast with the 1.0–2.6 orders of magnitude difference in viscosities. The decoupling of diffusivity of neutral molecular species from melt viscosity, i.e., the deviation from the inversely proportional relationship predicted by the Stokes–Einstein equation, might be attributed to the small size of H₂O molecules. With distinct viscosities but similar diffusivity, bubble growth controlled by diffusion in peralkaline and metaluminous rhyolitic melts follows similar parabolic curves. However, at low confining pressure or low water content, viscosity plays a larger role and bubble growth rate in peralkaline rhyolitic melt is much faster than that in metaluminous rhyolite.

     

Petrological cannibalism: the chemical and textural consequences of incremental magma body growth

The textures of minerals in volcanic and plutonic rocks testify to a complexity of processes in their formation that is at odds with simple geochemical models of igneous differentiation. Zoning in plagioclase feldspar is a case in point. Very slow diffusion of the major components in plagioclase means that textural evidence for complex magmatic evolution is preserved, almost without modification. Consequently, plagioclase affords considerable insight into the processes by which magmas accumulate in the crust prior to their eventual eruption or solidification. Here, we use the example of the 1980–1986 eruptions of Mount St. Helens to explore the causes of textural complexity in plagioclase and associated trapped melt inclusions. Textures of individual crystals are consistent with multiple heating and cooling events; changes in total pressure (P) or volatile pressure ( $P_{{{\text{H}}_{ 2} {\text{O}}}}$ P H 2 O ) are less easy to assess from textures alone. We show that by allying textural and chemical analyses of plagioclase and melt inclusions, including volatiles (H₂O, CO₂) and slow-diffusing trace elements (Sr, Ba), to published experimental studies of Mount St. Helens magmas, it is possible to disambiguate the roles of pressure and temperature to reconstruct magmatic evolutionary pathways through temperature–pressure–melt fraction (T– $P_{{{\text{H}}_{ 2} {\text{O}}}}$ P H 2 O –F) space. Our modeled crystals indicate that (1) crystallization starts at $P_{{{\text{H}}_{ 2} {\text{O}}}}$ P H 2 O  > 300 MPa, consistent with prior estimates from melt inclusion volatile contents, (2) crystal cores grow at $P_{{{\text{H}}_{ 2} {\text{O}}}}$ P H 2 O  = 200–280 MPa at F = 0.65–0.7, (3) crystals are transferred to $P_{{{\text{H}}_{ 2} {\text{O}}}}$ P H 2 O  = 100–130 MPa (often accompanied by 10–20 °C of heating), where they grow albitic rims of varying thicknesses, and (4) the last stage of crystallization occurs after minor heating at $P_{{{\text{H}}_{ 2} {\text{O}}}}$ P H 2 O  ~ 100 MPa to produce characteristic rim compositions of An₅₀. We hypothesize that modeled $P_{{{\text{H}}_{ 2} {\text{O}}}}$ P H 2 O decreases in excess of ~50 MPa most likely represent upward transport through the magmatic system. Small variations in modeled $P_{{{\text{H}}_{ 2} {\text{O}}}}$ P H 2 O , in contrast, can be effected by fluxing the reservoir with CO₂-rich vapors that are either released from deeper in the system or transported with the recharge magma. Temperature fluctuations of 20–40 °C, on the other hand, are an inevitable consequence of incremental, or pulsed, assembly of crustal magma bodies wherein each pulse interacts with ancestral, stored magmas. We venture that this “petrological cannibalism” accounts for much of the plagioclase zoning and textural complexity seen not only at Mount St. Helens but also at arc magmas generally. More broadly we suggest that the magma reservoir below Mount St. Helens is dominated by crystal mush and fed by frequent inputs of hotter, but compositionally similar, magma, coupled with episodes of magma ascent from one storage region to another. This view both accords with other independent constraints on the subvolcanic system at Mount St. Helens and supports an emerging view of many active magmatic systems as dominantly super-solidus, rather than subliquidus, bodies.     

H<Subscript>2</Subscript>O diffusion in peralkaline to peraluminous rhyolitic melts

The diffusion of water in a peralkaline and a peraluminous rhyolitic melt was investigated at temperatures of 714–1,493 K and pressures of 100 and 500 MPa. At temperatures below 923 K dehydration experiments were performed on glasses containing about 2 wt% H₂O _t in cold seal pressure vessels. At high temperatures diffusion couples of water-poor (<0.5 wt% H₂O _t ) and water-rich (~2 wt% H₂O _t ) melts were run in an internally heated gas pressure vessel. Argon was the pressure medium in both cases. Concentration profiles of hydrous species (OH groups and H₂O molecules) were measured along the diffusion direction using near-infrared (NIR) microspectroscopy. The bulk water diffusivity () was derived from profiles of total water () using a modified Boltzmann-Matano method as well as using fittings assuming a functional relationship between and Both methods consistently indicate that is proportional to in this range of water contents for both bulk compositions, in agreement with previous work on metaluminous rhyolite. The water diffusivity in the peraluminous melts agrees very well with data for metaluminous rhyolites implying that an excess of Al₂O₃ with respect to alkalis does not affect water diffusion. On the other hand, water diffusion is faster by roughly a factor of two in the peralkaline melt compared to the metaluminous melt. The following expression for the water diffusivity in the peralkaline rhyolite as a function of temperature and pressure was obtained by least-squares fitting:

Theoretical calculation of equilibrium Mg isotope fractionation between silicate melt and its vapor

Isotope fractionation during the evaporation of silicate melt and condensation of vapor has been widely used to explain various isotope signals observed in lunar soils, cosmic spherules, calcium–aluminum-rich inclusions, and bulk compositions of planetary materials. During evaporation and condensation, the equilibrium isotope fractionation factor (α) between high-temperature silicate melt and vapor is a fundamental parameter that can constrain the melt’s isotopic compositions. However, equilibrium α is difficult to calibrate experimentally. Here we used Mg as an example and calculated equilibrium Mg isotope fractionation in MgSiO₃ and Mg₂SiO₄ melt–vapor systems based on first-principles molecular dynamics and the high-temperature approximation of the Bigeleisen–Mayer equation. We found that, at 2500 K, δ²⁵Mg values in the MgSiO₃ and Mg₂SiO₄ melts were 0.141?±?0.004 and 0.143?±?0.003‰ more positive than in their respective vapors. The corresponding δ²⁶Mg values were 0.270?±?0.008 and 0.274?±?0.006‰ more positive than in vapors, respectively. The general \(\alpha - T\) equations describing the equilibrium Mg α in MgSiO₃ and Mg₂SiO₄ melt–vapor systems were: \(\alpha_{{{\text{Mg}}\left( {\text{l}} \right) - {\text{Mg}}\left( {\text{g}} \right)}} = 1 + \frac{{5.264 \times 10^{5} }}{{T^{2} }}\left( {\frac{1}{m} - \frac{1}{{m^{\prime}}}} \right)\) and \(\alpha_{{{\text{Mg}}\left( {\text{l}} \right) - {\text{Mg}}\left( {\text{g}} \right)}} = 1 + \frac{{5.340 \times 10^{5} }}{{T^{2} }}\left( {\frac{1}{m} - \frac{1}{{m^{\prime}}}} \right)\), respectively, where m is the mass of light isotope ²⁴Mg and m′ is the mass of the heavier isotope, ²⁵Mg or ²⁶Mg. These results offer a necessary parameter for mechanistic understanding of Mg isotope fractionation during evaporation and condensation that commonly occurs during the early stages of planetary formation and evolution.     

H-bonding scheme and cation partitioning in axinite: a single-crystal neutron diffraction and Mössbauer spectroscopic study

The crystal chemistry of a ferroaxinite from Colebrook Hill, Rosebery district, Tasmania, Australia, was investigated by electron microprobe analysis in wavelength-dispersive mode, inductively coupled plasma–atomic emission spectroscopy (ICP–AES), ⁵⁷Fe Mössbauer spectroscopy and single-crystal neutron diffraction at 293 K. The chemical formula obtained on the basis of the ICP–AES data is the following: \( ^{X1,X2} {\text{Ca}}_{4.03} \,^{Y} \left( {{\text{Mn}}_{0.42} {\text{Mg}}_{0.23} {\text{Fe}}^{2 + }_{1.39} } \right)_{\varSigma 2.04} \,^{Z1,Z2} \left( {{\text{Fe}}^{3 + }_{0.15} {\text{Al}}_{3.55} {\text{Ti}}_{0.12} } \right)_{\varSigma 3.82} \,^{T1,T2,T3,T4} \left( {{\text{Ti}}_{0.03} {\text{Si}}_{7.97} } \right)_{\varSigma 8} \,^{T5} {\text{B}}_{1.96} {\text{O}}_{30} \left( {\text{OH}} \right)_{2.18} \). The ⁵⁷Fe Mössbauer spectrum shows unambiguously the occurrence of Fe²⁺ and Fe³⁺ in octahedral coordination only, with Fe²⁺/Fe³⁺ = 9:1. The neutron structure refinement provides a structure model in general agreement with the previous experimental findings: the tetrahedral T1, T2, T3 and T4 sites are fully occupied by Si, whereas the T5 site is fully occupied by B, with no evidence of Si at the T5, or Al or Fe³⁺ at the T1–T5 sites. The structural and chemical data of this study suggest that the amount of B in ferroaxinite is that expected from the ideal stoichiometry: 2 a.p.f.u. (for 32 O). The atomic distribution among the X1, X2, Y, Z1 and Z2 sites obtained by neutron structure refinement is in good agreement with that based on the ICP–AES data. For the first time, an unambiguous localization of the H site is obtained, which forms a hydroxyl group with the oxygen atom at the O16 site as donor. The H-bonding scheme in axinite structure is now fully described: the O16–H distance (corrected for riding motion effect) is 0.991(1) Å and an asymmetric bifurcated bonding configuration occurs, with O5 and O13 as acceptors [i.e. with O16···O5 = 3.096(1) Å, H···O5 = 2.450(1) Å and O16–H···O5 = 123.9(1)°; O16···O13 = 2.777(1) Å, H···O13 = 1.914(1) Å and O16–H···O13 = 146.9(1)°].     

An experimental study of Fe–Ni exchange between sulfide melt and olivine at upper mantle conditions: implications for mantle sulfide compositions and phase equilibria

The behavior of nickel in the Earth’s mantle is controlled by sulfide melt–olivine reaction. Prior to this study, experiments were carried out at low pressures with narrow range of Ni/Fe in sulfide melt. As the mantle becomes more reduced with depth, experiments at comparable conditions provide an assessment of the effect of pressure at low-oxygen fugacity conditions. In this study, we constrain the Fe–Ni composition of molten sulfide in the Earth’s upper mantle via sulfide melt–olivine reaction experiments at 2 GPa, 1200 and 1400 °C, with sulfide melt \(X_{{{\text{Ni}}}}^{{{\text{Sulfide}}}}=\frac{{{\text{Ni}}}}{{{\text{Ni}}+{\text{Fe}}}}\) (atomic ratio) ranging from 0 to 0.94. To verify the approach to equilibrium and to explore the effect of \({f_{{{\text{O}}_{\text{2}}}}}\) on Fe–Ni exchange between phases, four different suites of experiments were conducted, varying in their experimental geometry and initial composition. Effects of Ni secondary fluorescence on olivine analyses were corrected using the PENELOPE algorithm (Baró et al., Nucl Instrum Methods Phys Res B 100:31–46, 1995), “zero time” experiments, and measurements before and after dissolution of surrounding sulfides. Oxygen fugacities in the experiments, estimated from the measured O contents of sulfide melts and from the compositions of coexisting olivines, were 3.0?±?1.0 log units more reduced than the fayalite–magnetite-quartz (FMQ) buffer (suite 1, 2 and 3), and FMQ ??1 or more oxidized (suite 4). For the reduced (suites 1–3) experiments, Fe–Ni distribution coefficients \(K_{{\text{D}}}^{{}}=\frac{{(X_{{{\text{Ni}}}}^{{{\text{sulfide}}}}/X_{{{\text{Fe}}}}^{{{\text{sulfide}}}})}}{{(X_{{{\text{Ni}}}}^{{{\text{olivine}}}}/X_{{{\text{Fe}}}}^{{{\text{olivine}}}})}}\) are small, averaging 10.0?±?5.7, with little variation as a function of total Ni content. More oxidized experiments (suite 4) give larger values of K_D (21.1–25.2). Compared to previous determinations at 100 kPa, values of K_D from this study are chiefly lower, in large part owing to the more reduced conditions of the experiments. The observed difference does not seem attributable to differences in temperature and pressure between experimental studies. It may be related in part to the effects of metal/sulfur ratio in sulfide melt. Application of these results to the composition of molten sulfide in peridotite indicates that compositions are intermediate in composition (\(X_{{{\text{Ni}}}}^{{{\text{sulfide}}}}\)?~?0.4–0.6) in the shallow mantle at 50 km, becomes more Ni rich with depth as the O content of the melt diminishes, reaching a maximum (0.6–0.7) at depths near 80–120 km, and then becomes more Fe rich in the deeper mantle where conditions are more reduced, approaching (\(X_{{{\text{Ni}}}}^{{{\text{sulfide}}}}\)?~?0.28)?>?140 km depth. Because Ni-rich sulfide in the shallow upper mantle melts at lower temperature than more Fe-rich compositions, mantle sulfide is likely molten in much of the deep continental lithosphere, including regions of diamond formation.     

Amphibole stability in primitive arc magmas: effects of temperature,H<Subscript>2</Subscript>O content,and oxygen fugacity

The water-saturated phase relations have been determined for a primitive magnesian andesite (57 wt% SiO₂, 9 wt% MgO) from the Mt. Shasta, CA region over the pressure range 200–800 MPa, temperature range of 915–1,070 °C, and oxygen fugacities varying from the nickel–nickel oxide (NNO) buffer to three log units above NNO (NNO+3). The phase diagram of a primitive basaltic andesite (52 wt% SiO₂, 10.5 wt% MgO) also from the Mt. Shasta region (Grove et al. in Contrib Miner Petrol 145:515–533; 2003) has been supplemented with additional experimental data at 500 MPa. Hydrous phase relations for these compositions allow a comparison of the dramatic effects of dissolved H₂O on the crystallization sequence. Liquidus mineral phase stability and appearance temperatures vary sensitively in response to variation in pressure and H₂O content, and this information is used to calibrate magmatic barometers-hygrometers for primitive arc magmas. H₂O-saturated experiments on both compositions reveal the strong dependence of amphibole stability on the partial pressure of H₂O. A narrow stability field is identified where olivine and amphibole are coexisting phases in the primitive andesite composition above 500 MPa and at least until 800 MPa, between 975–1,025 °C. With increasing H₂O pressure (\({P}_{\text {H}_2{\rm O}}\)), the temperature difference between the liquidus and amphibole appearance decreases, causing a change in chemical composition of the first amphibole to crystallize. An empirical calibration is proposed for an amphibole first appearance barometer-hygrometer that uses Mg# of the amphibole and \(f_{\text {O}_2}\):

$$ P_{\text{H}_{2}{\rm O}}({\rm MPa})=\left[{\frac{{\rm Mg\#}}{52.7}}-0.014 * \Updelta {\rm NNO}\right]^{15.12} $$

This barometer gives a minimum \({P}_{\text{H}_{2}{\rm O}}\) recorded by the first appearance of amphibole in primitive arc basaltic andesite and andesite. We apply this barometer to amphibole antecrysts erupted in mixed andesite and dacite lavas from the Mt. Shasta, CA stratocone. Both high H₂O pressures (500–900 MPa) and high pre-eruptive magmatic H₂O contents (10–14 wt% H₂O) are indicated for the primitive end members of magma mixing that are preserved in the Shasta lavas. We also use these new experimental data to explore and evaluate the empirical hornblende barometer of Larocque and Canil (2010).

     

Equation of state of synthetic qandilite Mg<Subscript>2</Subscript>TiO<Subscript>4</Subscript> at ambient temperature

Using a diamond-anvil cell and synchrotron X-ray diffraction, the compressional behavior of a synthetic qandilite Mg_2.00(1)Ti_1.00(1)O₄ has been investigated up to about 14.9 GPa at 300 K. The pressure–volume data fitted to the third-order Birch–Murnaghan equation of state yield an isothermal bulk modulus (K _T0) of 175(5) GPa, with its first derivative \(K_{T0}^{{\prime }}\) attaining 3.5(7). If \(K_{T0}^{{\prime }}\) is fixed as 4, the K _T0 value is 172(1) GPa. This value is substantially larger than the value of the adiabatic bulk modulus (K _S0) previously determined by an ultrasonic pulse echo method (152(7) GPa; Liebermann et al. in Geophys J Int 50:553–586, 1977), but in general agreement with the K _T0 empirically estimated on the basis of crystal chemical systematics (169 GPa; Hazen and Yang in Am Miner 84:1956–1960, 1999). Compared to the K _T0 values of the ulvöspinel (Fe₂TiO₄; ~148(4) GPa with \(K_{T0}^{{\prime }} = 4\)) and the ringwoodite solid solutions along the Mg₂SiO₄–Fe₂SiO₄ join, our finding suggests that the substitution of Mg²⁺ for Fe²⁺ on the T sites of the 4–2 spinels can have more significant effect on the K _T0 than that on the M sites.     

An experimental study of Ti and Zr partitioning among zircon, rutile, and granitic melt

In order to evaluate the effect of trace and minor elements (e.g., P, Y, and the REEs) on the high-temperature solubility of Ti in zircon (zrc), we conducted 31 experiments on a series of synthetic and natural granitic compositions [enriched in TiO₂ and ZrO₂; Al/(Na + K) molar ~1.2] at a pressure of 10 kbar and temperatures of ~1,400 to 1,200 °C. Thirty of the experiments produced zircon-saturated glasses, of which 22 are also saturated in rutile (rt). In seven experiments, quenched glasses coexist with quartz (qtz). SiO₂ contents of the quenched liquids range from 68.5 to 82.3 wt% (volatile free), and water concentrations are 0.4–7.0 wt%. TiO₂ contents of the rutile-saturated quenched melts are positively correlated with run temperature. Glass ZrO₂ concentrations (0.2–1.2 wt%; volatile free) also show a broad positive correlation with run temperature and, at a given T, are strongly correlated with the parameter (Na + K + 2Ca)/(Si·Al) (all in cation fractions). Mole fraction of ZrO₂ in rutile $ \left( {\mathop X\nolimits_{{{\text{ZrO}}_{ 2} }}^{\text{rt}} } \right) $ in the quartz-saturated runs coupled with other 10-kbar qtz-saturated experimental data from the literature (total temperature range of ~1,400 to 675 °C) yields the following temperature-dependent expression: $ {\text{ln}}\left( {\mathop X\nolimits_{{{\text{ZrO}}_{ 2} }}^{\text{rt}} } \right) + {\text{ln}}\left( {a_{{{\text{SiO}}_{2} }} } \right) = 2.638(149) - 9969(190)/T({\text{K}}) $ , where silica activity $ a_{{{\text{SiO}}_{2} }} $ in either the coexisting silica polymorph or a silica-undersaturated melt is referenced to α-quartz at the P and T of each experiment and the best-fit coefficients and their uncertainties (values in parentheses) reflect uncertainties in T and $ \mathop X\nolimits_{{{\text{ZrO}}_{2} }}^{\text{rt}} $ . NanoSIMS measurements of Ti in zircon overgrowths in the experiments yield values of ~100 to 800 ppm; Ti concentrations in zircon are positively correlated with temperature. Coupled with values for $ a_{{{\text{SiO}}_{2} }} $ and $ a_{{{\text{TiO}}_{2} }} $ for each experiment, zircon Ti concentrations (ppm) can be related to temperature over the range of ~1,400 to 1,200 °C by the expression: $ \ln \left( {\text{Ti ppm}} \right)^{\text{zrc}} + \ln \left( {a_{{{\text{SiO}}_{2} }} } \right) - \ln \left( {a_{{{\text{TiO}}_{2} }} } \right) = 13.84\left( {71} \right) - 12590\left( {1124} \right)/T\left( {\text{K}} \right) $ . After accounting for differences in $ a_{{{\text{SiO}}_{2} }} $ and $ a_{{{\text{TiO}}_{2} }} $ , Ti contents of zircon from experiments run with bulk compositions based on the natural granite overlap with the concentrations measured on zircon from experiments using the synthetic bulk compositions. Coupled with data from the literature, this suggests that at T ≥ 1,100 °C, natural levels of minor and trace elements in “granitic” melts do not appear to influence the solubility of Ti in zircon. Whether this is true at magmatic temperatures of crustal hydrous silica-rich liquids (e.g., 800–700 °C) remains to be demonstrated. Finally, measured $ D_{\text{Ti}}^{{{\text{zrc}}/{\text{melt}}}} $ values (calculated on a weight basis) from the experiments presented here are 0.007–0.01, relatively independent of temperature, and broadly consistent with values determined from natural zircon and silica-rich glass pairs.     

Si and O self-diffusion in hydrous forsterite and iron-bearing olivine from the perspective of defect chemistry

We discuss the experimental results of silicon and oxygen self-diffusion coefficients in forsterite and iron-bearing olivine from the perspective of defect chemistry. Silicon diffusion is dominated by V_O ^··-associated V_Si″″, whereas oxygen diffusion is dominated by hopping of V_O ^·· under anhydrous conditions, and by (OH)_O ^· under hydrous conditions. By considering the charge neutrality condition of [(OH)_O ^·] = 2[V_Me″] in hydrous forsterite and iron-bearing olivine, we get D _Si ∝ (\(C_{{{\text{H}}_{2} {\text{O}}}}\))^1/3 and D _O ∝ (\(C_{{{\text{H}}_{2} {\text{O}}}}\))⁰, which explains the experimental results of water effects on oxygen and silicon self-diffusion rates (Fei et al. in Nature 498:213–215, 2013; J Geophys Res 119:7598–7606, 2014). The \(C_{{{\text{H}}_{2} {\text{O}}}}\) dependence of creep rate in the Earth’s mantle should be close to that given by Si and O self-diffusion coefficients obtained under water unsaturated conditions.     

Redox-controlled iron isotope fractionation during magmatic differentiation: an example from the Red Hill intrusion, S. Tasmania

This study presents accurate and precise iron isotopic data for 16 co-magmatic rocks and 6 pyroxene–magnetite pairs from the classic, tholeiitic Red Hill sill in southern Tasmania. The intrusion exhibits a vertical continuum of compositions created by in situ fractional crystallisation of a single injection of magma in a closed igneous system and, as such, constitutes a natural laboratory amenable to determining the causes of Fe isotope fractionation in magmatic rocks. Early fractionation of pyroxenes and plagioclase, under conditions closed to oxygen exchange, gives rise to an iron enrichment trend and an increase in $ f_{{{\text{O}}_{2} }} $ of the melt relative to the Fayalite–Magnetite–Quartz (FMQ) buffer. Enrichment in Fe³⁺/ΣFe_melt is mirrored by δ⁵⁷Fe, where ^VIFe²⁺-bearing pyroxenes partition ⁵⁷Fe-depleted iron, defining an equilibrium pyroxene-melt fractionation factor of $ \Updelta^{57} {\text{Fe}}_{{{\text{px}} - {\text{melt}}}} \le - 0.25\,\permille \times 10^{6} /T^{2} $ . Upon magnetite saturation, the $ f_{{{\text{O}}_{2} }} $ and δ⁵⁷Fe of the melt fall, commensurate with the sequestration of the oxidised, ⁵⁷Fe-enriched iron into magnetite, quantified as $ \Updelta^{57} {\text{Fe}}_{{{\text{mtn}} - {\text{melt}}}} = + 0.20\,\permille \times 10^{6} /T^{2} $ . Pyroxene–magnetite pairs reveal an equilibrium fractionation factor of $ \Updelta^{57} {\text{Fe}}_{{{\text{mtn}} - {\text{px}}}} \approx + 0.30\,\permille $ at 900–1,000?°C. Iron isotopes in differentiated magmas suggest that they may act as an indicator of their oxidation state and tectonic setting.     

Experimental calibration of Forsterite–Anorthite–Ca-Tschermak–Enstatite (FACE) geobarometer for mantle peridotites

The crystallization of plagioclase-bearing assemblages in mantle rocks is witness of mantle exhumation at shallow depth. Previous experimental works on peridotites have found systematic compositional variations in coexisting minerals at decreasing pressure within the plagioclase stability field. In this experimental study we present new constraints on the stability of plagioclase as a function of different Na₂O/CaO bulk ratios, and we present a new geobarometer for mantle rocks. Experiments have been performed in a single-stage piston cylinder at 5–10 kbar, 1050–1150?°C at nominally anhydrous conditions using seeded gels of peridotite compositions (Na₂O/CaO?=?0.08–0.13; X _Cr = Cr/(Cr?+?Al)?=?0.07–0.10) as starting materials. As expected, the increase of the bulk Na₂O/CaO ratio extends the plagioclase stability to higher pressure; in the studied high-Na fertile lherzolite (HNa-FLZ), the plagioclase-spinel transition occurs at 1100?°C between 9 and 10 kbar; in a fertile lherzolite (FLZ) with Na₂O/CaO?=?0.08, it occurs between 8 and 9 kbar at 1100?°C. This study provides, together with previous experimental results, a consistent database, covering a wide range of P–T conditions (3–9 kbar, 1000–1150?°C) and variable bulk compositions to be used to define and calibrate a geobarometer for plagioclase-bearing mantle rocks. The pressure sensitive equilibrium:

$$\mathop {{\text{M}}{{\text{g}}_{\text{2}}}{\text{Si}}{{\text{O}}_{\text{4}}}^{{\text{Ol}}}}\limits_{{\text{Forsterite}}} +\mathop {{\text{CaA}}{{\text{l}}_{\text{2}}}{\text{S}}{{\text{i}}_{\text{2}}}{{\text{O}}_{\text{8}}}^{{\text{Pl}}}}\limits_{{\text{Anorthite}}~} =\mathop {{\text{CaA}}{{\text{l}}_{\text{2}}}{\text{Si}}{{\text{O}}_{\text{6}}}^{{\text{Cpx}}}}\limits_{{\text{Ca-Tschermak}}} +{\text{ }}\mathop {{\text{M}}{{\text{g}}_{\text{2}}}{\text{S}}{{\text{i}}_{\text{2}}}{{\text{O}}_{\text{6}}}^{{\text{Opx}}}}\limits_{{\text{Enstatite}}} ,$$

has been empirically calibrated by least squares regression analysis of experimental data combined with Monte Carlo simulation. The result of the fit gives the following equation:

$$P=7.2( \pm 2.9)+0.0078( \pm 0.0021)T{\text{ }}+0.0022( \pm 0.0001)T{\text{ }}\ln K,$$

$${R^2}=0.93,$$

where P is expressed in kbar and T in kelvin. K is the equilibrium constant K?=?a _CaTs × a _en/a _an × a _fo, where a _CaTs, a _en, a _an and a _fo are the activities of Ca-Tschermak in clinopyroxene, enstatite in orthopyroxene, anorthite in plagioclase and forsterite in olivine. The proposed geobarometer for plagioclase peridotites, coupled to detailed microstructural and mineral chemistry investigations, represents a valuable tool to track the exhumation of the lithospheric mantle at extensional environments.

     

H2O–CO2 solubility in mafic alkaline magma: applications to volatile sources and degassing behavior at Erebus volcano, Antarctica

We present new equilibrium mixed-volatile (H₂O–CO₂) solubility data for a phonotephrite from Erebus volcano, Antarctica. H₂O–CO₂-saturated experiments were conducted at 400–700 MPa, 1,190 °C, and ~NNO + 1 in non-end-loaded piston cylinders. Equilibrium H₂O–CO₂ fluid compositions were determined using low-temperature vacuum manometry, and the volatile and major element compositions of the glassy run products were determined by Fourier transform infrared spectroscopy and electron microprobe. Results show that the phonotephrite used in this study will dissolve ~0.8 wt% CO₂ at 700 MPa and a fluid composition of $ X_{{{\text{H}}_{ 2} {\text{O}}}} $ ~0.4, in agreement with previous experimental studies on mafic alkaline rocks. Furthermore, the dissolution of CO₂ at moderate to high $ X_{{{\text{H}}_{ 2} {\text{O}}}}^{\text{fluid}} $ in our experiments exceeds that predicted using lower-pressure experiments on similar melts from the literature, suggesting a departure from Henrian behavior of volatiles in the melt at pressures above 400 MPa. With these data, we place new constraints on the modeling of Erebus melt inclusion and gas emission data and thus the interpretation of its magma plumbing system and the contributions of primitive magmas to passive and explosive degassing from the Erebus phonolite lava lake.     

Thermal equation of state of CaIrO<Subscript>3</Subscript> post-perovskite

The pressure–volume–temperature (P–V–T) relation of CaIrO₃ post-perovskite (ppv) was measured at pressures and temperatures up to 8.6 GPa and 1,273 K, respectively, with energy-dispersive synchrotron X-ray diffraction using a DIA-type, cubic-anvil apparatus (SAM85). Unit-cell dimensions were derived from the Le Bail full profile refinement technique, and the results were fitted using the third-order Birth-Murnaghan equation of state. The derived bulk modulus \( K_{T0} \) at ambient pressure and temperature is 168.3 ± 7.1 GPa with a pressure derivative \( K_{T0}^{\prime } \) = 5.4 ± 0.7. All of the high temperature data, combined with previous experimental data, are fitted using the high-temperature Birch-Murnaghan equation of state, the thermal pressure approach, and the Mie-Grüneisen-Debye formalism. The refined thermoelastic parameters for CaIrO₃ ppv are: temperature derivative of bulk modulus \( (\partial K_{T} /\partial T)_{P} \) = ?0.038 ± 0.011 GPa K^?1, \( \alpha K_{T} \) = 0.0039 ± 0.0001 GPa K^?1, \( \left( {\partial K_{T} /\partial T} \right)_{V} \) = ?0.012 ± 0.002 GPa K^?1, and \( \left( {\partial^{2} P/\partial T^{2} } \right)_{V} \) = 1.9 ± 0.3 × 10^?6 GPa² K^?2. Using the Mie-Grüneisen-Debye formalism, we obtain Grüneisen parameter \( \gamma_{0} \) = 0.92 ± 0.01 and its volume dependence q = 3.4 ± 0.6. The systematic variation of bulk moduli for several oxide post-perovskites can be described approximately by the relationship K _T0  = 5406.0/V(molar) + 5.9 GPa.     

Cu diffusivity in granitic melts with application to the formation of porphyry Cu deposits

We report new experimental data of Cu diffusivity in granite porphyry melts with 0.01 and 3.9 wt% H₂O at 0.15–1.0 GPa and 973–1523 K. A diffusion couple method was used for the nominally anhydrous granitic melt, whereas a Cu diffusion-in method using Pt₉₅Cu₅ as the source of Cu was applied to the hydrous granitic melt. The diffusion couple experiments also generate Cu diffusion-out profiles due to Cu loss to Pt capsule walls. Cu diffusivities were extracted from error function fits of the Cu concentration profiles measured by LA-ICP-MS. At 1 GPa, we obtain \({D_{{\text{Cu, dry, 1 GPa}}}}=\exp \left[ {( - {\text{13.89}} \pm {\text{0.42}}) - \frac{{{\text{12878}} \pm {\text{540}}}}{T}} \right],\) and \({D_{{\text{Cu, 3}}{\text{.9 wt\% }}{{\text{H}}_{\text{2}}}{\text{O}},{\text{ 1 GPa}}}}=\exp \left[ {( - 16.31 \pm 1.30) - \frac{{{\text{8148}} \pm {\text{1670}}}}{T}} \right],\) where D is Cu diffusivity in m²/s and T is temperature in K. The above expressions are in good agreement with a recent study on Cu diffusion in rhyolitic melt using the approach of Cu₂S dissolution. The observed pressure effect over 0.15–1.0 GPa can be described by an activation volume of 5.9 cm³/mol for Cu diffusion. Comparison of Cu diffusivity to alkali diffusivity and its variation with melt composition implies fourfold-coordinated Cu⁺ in silicate melts. Our experimental results indicate that in the formation of porphyry Cu deposits, the diffusive transport of magmatic Cu to sulfide liquids or fluid bubbles is highly efficient. The obtained Cu diffusivity data can also be used to assess whether equilibrium Cu partitioning can be reached within certain experimental durations.     

Partial melting of secondary pyroxenite at 1 and 1.5 GPa,and its role in upwelling heterogeneous mantle

We performed partial melting experiments at 1 and 1.5 GPa, and 1180–1400 °C, to investigate the melting under mantle conditions of an olivine-websterite (GV10), which represents a natural proxy of secondary (or stage 2) pyroxenite. Its subsolidus mineralogy consists of clinopyroxene, orthopyroxene, olivine and spinel (+garnet at 1.5 GPa). Solidus temperature is located between 1180 and 1200 °C at 1 GPa, and between 1230 and 1250 °C at 1.5 GPa. Orthopyroxene (±garnet), spinel and clinopyroxene are progressively consumed by melting reactions to produce olivine and melt. High coefficient of orthopyroxene in the melting reaction results in relatively high SiO₂ content of low melt fractions. After orthopyroxene exhaustion, melt composition is controlled by the composition of coexisting clinopyroxene. At increasing melt fraction, CaO content of melt increases, whereas Na₂O, Al₂O₃ and TiO₂ behave as incompatible elements. Low Na₂O contents reflect high partition coefficient of Na between clinopyroxene and melt (\(D_{{{\text{Na}}_{ 2} {\text{O}}}}^{{{\text{cpx}}/{\text{liquid}}}}\)). Melting of GV10 produces Quartz- to Hyperstene-normative basaltic melts that differ from peridotitic melts only in terms of lower Na₂O and higher CaO contents. We model the partial melting of mantle sources made of different mixing of secondary pyroxenite and fertile lherzolite in the context of adiabatic oceanic mantle upwelling. At low potential temperatures (T _P < 1310 °C), low-degree melt fractions from secondary pyroxenite react with surrounding peridotite producing orthopyroxene-rich reaction zones (or refertilized peridotite) and refractory clinopyroxene-rich residues. At higher T _P (1310–1430 °C), simultaneous melting of pyroxenite and peridotite produces mixed melts with major element compositions matching those of primitive MORBs. This reinforces the notion that secondary pyroxenite may be potential hidden components in MORB mantle source.     

New insights on pressure,temperature, and chemical stability of CsAlSi<Subscript>5</Subscript>O<Subscript>12</Subscript>, a potential host for nuclear waste

A Cs-bearing polyphase aggregate with composition (in wt%): 76(1)_CsAlSi5O12 + 7(1)_CsAlSi2O6 + 17(1)_amorphous, was obtained from a clinoptilolite-rich epiclastic rock after a beneficiation process of the starting material (aimed to increase the fraction of zeolite to 90 wt%), cation exchange and then thermal treatment. CsAlSi₅O₁₂ is an open-framework compound with CAS topology; CsAlSi₂O₆ is a pollucite-like material with ANA topology. The thermal stability of this polyphase material was investigated by in situ high-T X-ray powder diffraction, the combined P–T effects by a series of runs with a single-stage piston cylinder apparatus, and its chemical stability following the “availability test” (“AVA test”) protocol. A series of additional investigations were performed by WDS–electron microprobe analysis in order to describe the P–T-induced modification of the material texture, and to chemically characterize the starting material and the run products. The “AVA tests” of the polyphase aggregate show an extremely modest release of Cs⁺: 0.05 mg/g. In response to applied temperature and at room P, CsAlSi₅O₁₂ experiences an unquenchable and displacive Ama2-to-Amam phase transition at about 770 K, and the Amam polymorph is stable in its crystalline form up to 1600 K; a crystalline-to-amorphous phase transition occurs between 1600 and 1650 K. In response to the applied P = 0.5 GPa, the crystalline-to-amorphous transition of CsAlSi₅O₁₂ occurs between 1670 and 1770 K. This leads to a positive Clapeyron slope (i.e., dP/dT > 0) of the crystalline-to-amorphous transition. When the polyphase aggregate is subjected at P = 0.5 GPa and T > 1770 K, CsAlSi₅O₁₂ melts and only CsAlSi₂O₆ (pollucite-like; dominant) and Cs-rich glass (subordinate) are observed in the quenched sample. Based on its thermo-elastic behavior, P–T phase stability fields, and Cs⁺ retention capacity, CsAlSi₅O₁₂ is a possible candidate for use in the immobilization of radioactive isotopes of Cs, or as potential solid hosts for ¹³⁷Cs γ-radiation source in sterilization applications. More in general, even the CsAlSi₅O₁₂-rich aggregate obtained by a clinoptilolite-rich epiclastic rock appears to be suitable for this type of utilizations.     

A multi-isotope approach for understanding sources of water,carbon and sulfur in natural springs of the Central Appalachian region

Natural springs have been reliable sources of domestic water and have allowed for the development of recreational facilities and resorts in the Central Appalachians. The structural history of this area is complex and it is unknown whether these natural springs receive significant recharge from modern precipitation or whether they discharge old water recharged over geological times scales. The main objective of this study was to use stable isotopes of water ( $\delta^{18} {\text{O}}_{{{\text{H}}_{2} {\text{O}}}}$ and $\delta^{2} {\text{H}}_{{{\text{H}}_{2} {\text{O}}}}$ ), dissolved inorganic carbon ( $\delta^{13} {\text{C}}_{\text{DIC}}$ ) and dissolved sulfate ( $\delta^{34} {\text{S}}_{{{\text{SO}}_{4} }}$ and $\delta^{18} {\text{O}}_{{{\text{SO}}_{4} }}$ ) to delineate sources of water, carbon and sulfur in several natural springs of the region. Our preliminary isotope data indicate that all springs are being recharged by modern precipitation. The oxygen isotope composition indicates that waters in thermal springs did not encounter the high temperatures required for O isotope exchange between the water and silicate/carbonate minerals, and/or the residence time of water in the aquifers was short due to high flow rates. The carbon isotopic composition of dissolved inorganic carbon and sulfur/oxygen isotopic composition of dissolved sulfate provide evidence of low-temperature water–rock interactions and various biogeochemical transformations these waters have undergone along their flow path.