Rebecca A. Lange

A thermodynamic model for the plagioclase-liquid hygrometer/thermometer

Abstract A new thermodynamic model for the plagioclase-liquid exchange reaction between the albite (NaAlSi3O8) and anorthite (CaAl2Si2O8) components is presented, which can be used as a plagioclaseliquid hygrometer or thermometer. The model incorporates calorimetric and volumetric data for the pure liquid and crystalline components, which permits the effect of temperature and pressure on the exchange reaction to be calculated independently from the effect of composition. This allows a more accurate assessment of the effect of melt composition (including dissolved water concentration) on the exchange reaction from plagioclase-liquid equilibrium experiments. Activity-composition relations for the plagioclase solid solution are taken from Holland and Powell (1992). The new hygrometer is calibrated on 71 plagioclase-liquid experiments, of which 45 are hydrous and 26 are anhydrous. Three filters were applied to the phase-equilibrium data: (1) crystallanities <30%; (2) pure H2O fluidsaturated; and (3) compositional totals (including H2O component) of 97-101% for hydrous quenched glasses. The final data set spans a wide range of liquid compositions (46-74 wt% SiO2), plagioclase compositions (An93-An37), temperatures (825-1230 °C), pressures (0-300 MPa), and dissolved melt water concentrations (0-7 wt% H2O). The standard error of estimate (SEE) for the model is ±0.32 wt% H2O, and all liquid compositions are fitted equally well. When the model is used as a thermometer, all measured temperatures are recovered equally well within ±14 °C on average. The model is only recommended for applications that fall within the compositional bounds of the calibration data set (i.e., metaluminous basalts through rhyolites in equilibrium with An95-An35). It is not yet calibrated for rhyolites crystallizing plagioclase more sodic than An30, owing to an absence of phase-equilibrium experiments on rhyolites that pass the required filters. The new plagioclase-liquid hygrometer/thermometer is available as a Visual Basic program that runs on Excel 2004.

The influence of pressure and composition on the viscosity of andesitic melts

The effect of pressure and composition on the viscosity of both anhydrous and hydrous andesitic melts was studied in the viscosity range of 108 to 1011.5 Pa · s using parallel plate viscometry. The pressure dependence of the viscosity of three synthetic, iron-free liquids (andesite analogs) containing 0.0, 1.06, and 1.96 wt.% H2O, respectively, was measured from 100 to 300 MPa using a high-P-T viscometer. These results, combined with those from Richet et al. (1996), indicate that viscosities of anhydrous andesitic melts are independent of pressure, whereas viscosities of hydrous melts slightly increase with increasing pressure. This trend is consistent with an increased degree of depolymerization in the hydrous melts. Compositional effects on the viscosity were studied by comparing iron-free and iron-bearing compositions with similar degrees of depolymerization. During experiments at atmospheric and at elevated pressures (100 to 300 MPa), the viscosity of iron-bearing anhydrous melts preequilibrated in air continuously increased, and the samples became paramagnetic. Analysis of these samples by transmission electron microscopy showed a homogeneous distribution of crystals (probably magnetite) with sizes in the range of 10 to 50 nm. No significant difference in the volume fractions of crystals was found in samples after annealing for 170 to 830 min at temperatures ranging from 970 to 1122 K. An iron-bearing andesite containing 1.88 wt.% H2O, which was synthesized at intrinsic fO2 conditions in an internally heated pressure vessel, showed a similar viscosity behavior as the anhydrous melts. The continuous increase in viscosity at a constant temperature is attributed to changes of the melt structure due to exsolution of iron-rich phases. By extrapolating the time evolution of viscosity down to the time at which the run temperature was reached, for both the anhydrous (at 1055 K) and the hydrous (at 860 K) iron-bearing andesite, the viscosity is 0.7 log units lower than predicted by the model of Richet et al. (1996). This may be explained by differences in structural properties of Fe2+ and Fe3+ and their substitutes Mg2+, Ca2+, and Al3+, which were used in the analogue composition. The effect of iron redox state on the viscosity of anhydrous, synthetic andesite melts was studied at ambient pressure using a dilatometer. Reduced iron-bearing samples were produced by annealing melts in graphite crucibles in an Ar/CO atmosphere for different run times. In contrast to the oxidized sample, no variation of viscosity with time and no exsolution of iron oxide phases was observed for the most reduced glasses. This indicates that trivalent iron promotes the exsolution of iron oxide in supercooled melts. With decreasing Fe3+/ΣFe ratio from 0.58 to 0.34, the viscosity decreases by ∼1.6 log units in the investigated temperature range between 964 and 1098 K. A more reduced glass with Fe3+/ΣFe = 0.21 showed no additional decrease in viscosity. Our conclusion from these results is that the viscosity of natural melts may be largely overestimated when using data obtained from samples synthesized in air.

Magma eruption rates constrained by 40Ar/39Ar chronology and GIS for the Ceboruco–San Pedro volcanic field, western Mexico

40Ar/39Ar geochronology is coupled with quantitative volume determinations (utilizing field mapping, digital elevation models, orthophotos, and geographic information system [GIS] software) to constrain magma eruption rates at the Ceboruco–San Pedro volcanic field (1600 km2) in the western Trans-Mexican arc. Ages are reported for 40 volcanic units, including Volcan Ceboruco (an active, andesitic stratovolcano), peripheral domes, shields, cinder cones, and fissure-fed flows. After a hiatus of ∼3 m.y., volcanic activity recommenced to produce 80.5 ± 3.5 km3 of magma at a rate of 63 m3/km2 per year over the past 0.8 m.y. However, 75% of this volume erupted in the past 100 k.y., including the 51 ± 2.5 km3 of Volcan Ceboruco, equivalent to an eruption rate of ∼377 m3/km2 per year. There have been at least two stages of cone-building activity at Volcan Ceboruco. The main edifice is composed of ∼38 km3 of precaldera andesites, the youngest dated at 45 ± 8 ka. Their eruption was followed by a hiatus, interrupted by a Plinian eruption at 1 ka. The Plinian eruption and subsequent lava flows are andesite to dacite in composition and constitute ∼13 km3 of the total volume of Volcan Ceboruco. Overall, the relative proportions of lava types erupted in the past 0.8 m.y. are 18%–19% basaltic andesite, 56%–60% andesite, 18%–22% dacite, and 3% rhyolite. The peripheral lavas are each of small volume, geochemically diverse, and show little evidence of prior storage in an upper-crustal chamber. The eruptive sequence, proportions of lava types, phenocryst assemblages, textures, and geochemistry imply that the lavas do not reflect the differentiation of a single parental liquid in a long-lived magma chamber. The distinct geochemical signatures were present prior to magma emplacement in the upper crust, whereupon subsequent degassing and crystallization led to variable phenocryst abundances and assemblages.

An updated calibration of the plagioclase-liquid hygrometer-thermometer applicable to basalts through rhyolites

Abstract An updated and expanded data set that consists of 214 plagioclase-liquid equilibrium pairs from 40 experimental studies in the literature is used to recalibrate the thermodynamic model for the plagioclase- liquid hygrometer of Lange et al. (2009); the updated model is applicable to metaluminous and alkaline magmas. The model is based on the crystal-liquid exchange reaction between the anorthite (CaAl2Si2O8) and albite (NaAlSi3O8) components, and all available volumetric and calorimetric data for the pure end-member components are used in the revised model. The activities of the crystalline plagioclase components are taken from Holland and Powell (1992). Of the 214 experiments, 107 are hydrous and 107 are anhydrous. Four criteria were applied for inclusion of experiments in the final data set: (1) crystallinities <30%; (2) pure-H2O fluid saturated; (3) compositional totals (including H2O component) of 97-101% for hydrous quenched glasses and 98.5-101 for anhydrous quenched glasses; and (4) melt viscosities ≤5.2 log10 Pa·s. The final data set spans a wide range in liquid composition (45-80 wt% SiO2; 1-10 wt% Na2O+K2O), plagioclase composition (An17-95), temperature (750-1244 °C), pressure (0-350 MPa), and H2O content (0-8.3 wt%). The water solubility model of Zhang et al. (2007) was applied to all hydrous experiments. The standard error estimate on the hygrometer model is 0.35 wt% H2O, and all liquid compositions are fitted equally well. Application of the model as a thermometer recovers temperatures to within ±12°, on average. Tests of the hygrometer on anhydrous piston-cylinder experiments in the literature, not included in the regression, show that the model is accurate at all pressures where plagioclase is stable. Applications of the hygrometer are made to natural rhyolites (Bishop Tuff, Katmai, and TobaTuff) with reported H2O analyses in quartz-hosted melt inclusions from the literature; the results show agreement. Applications of the hygrometer/thermometer are additionally made to natural rhyolites from Iceland and Glass Mountain, California. The updated model can be downloaded either as a program in Excel format or as a MatLab script from the Data Repository.

Multi-technique equation of state for Fe2SiO4 melt and the density of Fe-bearing silicate melts from 0 to 161 GPa

We have conducted new equation of state measurements on liquid Fe_(2)SiO_4 in a collaborative, multi-technique study. The liquid density (ρ), the bulk modulus (K), and its pressure derivative (K′) were measured from 1 atm to 161 GPa using 1-atm double-bob Archimedean, multi-anvil sink/float, and shock wave techniques. Shock compression results on initially molten Fe_(2)SiO_4 (1573 K) fitted with previous work and the ultrasonically measured bulk sound speed (C_o) in shock velocity (U_S)-particle velocity (u_p) space yields the Hugoniot: U_S = 1.58(0.03) u_p + 2.438(0.005) km/s. Sink/float results are in agreement with shock wave and ultrasonic data, consistent with an isothermal K_T = 19.4 GPa and K′ = 5.33 at 1500°C. Shock melting of initially solid Fe_(2)SiO_4 (300 K) confirms that the Gruneisen parameter (γ) of this liquid increases upon compression where γ = γ_o(ρ_(o)/ρ)^q yields a q value of –1.45. Constraints on the liquid fayalite EOS permit the calculation of isentropes for silicate liquids of general composition in the multicomponent system CaO-MgO-Al_(2)O_3-SiO_2-FeO at elevated temperatures and pressures. In our model a whole mantle magma ocean would first crystallize in the mid-lower mantle or at the base of the mantle were it composed of either peridotite or simplified “chondrite” liquid, respectively. In regards to the partial melt hypothesis to explain the occurrence and characteristics of ultra-low velocity zones, neither of these candidate liquids would be dense enough to remain at the core mantle boundary on geologic timescales, but our model defines a compositional range of liquids that would be gravitationally stable.

A Pliocene ignimbrite flare-up along the Tepic-Zacoalco rift: Evidence for the initial stages of rifting between the Jalisco block (Mexico) and North America

The Tepic-Zacoalco rift, a NW-trending corridor ~50 × ~250 km, is one arm of a triple-rift system in western Mexico. Together with the Colima rift and the Middle America Trench, it bounds the Jalisco block, a portion of western Mexico that may be moving independently of North America. The predominant basement rock types in the TepicZacoalco rift are rhyolitic ash-fl ow tuffs and lavas, which were previously assumed to be Oligocene-Miocene in age, related to the Sierra Madre Occidental volcanic province, or older. New 40 Ar/ 39 Ar dates on 41 volcanic samples reveal a previously unrecognized, voluminous fl are-up of rhyolitic ignimbrites between 5 and 3 Ma throughout the entire corridor of the Tepic-Zacoalco rift; they are often associated with Pliocene high-Ti basalts. The eruption rate during this Pliocene time period was an order of magnitude higher (hundreds of m/m.y.) than that documented in the Tepic-Zacoalco rift over the last 1 m.y. The Pliocene ash-fl ow tuffs have been faulted along NW-trending lineaments, producing vertical offsets up to at least 500 m. The voluminous ignimbrite fl are-up in the TepicZacoalco rift at 5–3 Ma may refl ect the initial stages of rifting of the Jalisco block away from North America, analogous to what occurred in the proto-gulf region at 12–6 Ma, prior to the transfer of Baja California from North America to the Pacifi c plate. Additionally, new 40 Ar/ 39 Ar dates show that the Sierra Madre Occidental volcanic province extends across the entire width of the Tepic-Zacoalco rift and terminates abruptly at the northern boundary of the Jalisco block near the Rio Ameca. In contrast, PaleoceneEocene basement from the Jalisco block extends northward into the Tepic-Zacoalco rift, where it is locally overlain by Sierra Madre Occidental rhyolites.

Heat capacities of TiO2-bearing silicate liquids: Evidence for anomalous changes in configurational entropy with temperature

Abstract The heat capacities of several TiO 2 -bearing silicate glasses and liquids containing Cs 2 O, Rb 2 O, Na 2 O, K 2 O, CaO, MgO, or BaO have been measured to 1100 K using a differential scanning calorimeter and to 1800 K using a Setaram HT-1500 calorimeter in step-scanning mode. The results for liquids of M 2 O-TiO 2 -2SiO 2 composition ( M — Na, K, Cs) are compared to those for liquids of M 2 O-3SiO 2 composition. The presence of TiO 2 has a profound influence on the heat capacity of simple three-component silicate liquids over the temperature range 900–1300 K. Specifically, replacement of Si 4+ by Ti 4+ leads to doubling of the magnitude of the jump in C p at the glass transion ( T g ); this is followed by a progressive decrease in liquid C p for over 400 K, until C p eventually becomes constant and similar to that in Ti-free systems. The large heat capacity step at T g in the TiO 2 -bearing melts suggests significant configurational rearrangements in the liquid that are not available to TiO 2 -free silicates. In addition, these “extra” configurational changes apparently saturate as temperature increases, implying the completion of whatever process is responsible for them, or the attainment of a random distribution of structural states. Above 1400 K, however, where the heat capacities of TiO 2 -bearing and TiO 2 -free alkali silicate liquids are similar, their configurational entropies differ by ~3.5 J/g.f.w.-K. The larger configurational entropy of the TiO 2 -bearing alkali silicate liquids relative to the TiO 2 -free liquids is energetically equivalent to raising the liquid temperature by more than 300 degrees. This result clearly demonstrates the energetic magnitude of the configurational changes apparent in the supercooled liquid region and their impact on the thermodynamic properties of the stable liquid. Consideration of both density measurements on liquids and spectroscopic data on quenched glasses (from the literature) suggests that the anomalous configurational rearrangements may involve the breakdown of alkali and alkaline earth titanate complexes and changes in Ti 4+ coordination.

The partial molar volume and thermal expansivity of TiO2 in alkali silicate melts: Systematic variation with Ti coordination

Abstract The densities of seven K2O-TiO2-SiO2 (KTS) liquids and seven Na2O-TiO2-SiO2 (NTS) liquids have been measured between 1236 and 1771 K using the double-bob Archimedean method. In addition, the low-temperature density of these liquids at their limiting fictive temperature (T′f; near the glass transition) were also measured. Compositions range from 15 to 39 mol % K2O and Na2O, 10 to 35 mol % TiO2, and 33 to 61 mol % SiO2. Plots of molar volume vs. temperature are linear for all samples in the stable liquid region, but there is an increase in the slope (∂V/∂T) with decreasing temperature in the supercooled liquid region, analogous to that observed for the heat capacity of similar alkali titanosilicate liquids. Derived values of V TiO 2 (at 1373 K) decrease systematically from 32.5 to 26.5 cm3/mol between 39 and 15 mol% K2O and from 29.6 to 25.7 cm3/mol between 39 and 25 mol% Na2O (and are nearly constant at ∼25 ± 0.5 cm3/mol between 25 and 15 mol% Na2O). Values of V TiO 2 do not appear to be a function of the TiO2 concentration, but instead depend strongly on the nature of the alkali present (larger by ∼3 cm3/mol in KTS vs. NTS liquids) and the degree of polymerization (increase with NBO/T). By comparing our values of V TiO 2 at 1373 K to published Ti coordination numbers on samples of similar composition, we calculate that the average Ti coordination number varies from ∼4.0 to 5.1 in the KTS liquids and from ∼4.6 to 5.5 in the NTS liquids. The value of ∂ V TiO 2 ∂T in the stable liquid region correlates strongly with V TiO 2 at 1373 K (and therefore with Ti coordination). It is near zero when the average coordination is close to four, but reaches a maximum of ∼6 to 8 × 10−3 cm3/mol-K when the average Ti coordination number is close to five. In all corresponding glasses, the value of ∂ V TiO 2 ∂T is zero. Together, these features require that ∂ V TiO 2 ∂T in the liquid is associated with the presence of [5]Ti and involves structural changes not available to solids.

The albite fusion curve re-examined: New experiments and the high-pressure density and compressibility of high albite and NaAlSi3O8 liquid

Abstract Experimental brackets on the melting temperature of high albite (NaAlSi3O8) were determined at 2.33 ± 0.03 GPa (1360-1370 °C) and 2.79 ± 0.03 GPa (1370-1389 °C) in a piston-cylinder apparatus. All run products that quenched to a glass were analyzed by Fourier-transform infrared spectroscopy and found to contain ≤500 ppm H2O. In addition, new X-ray diffraction experiments on fully disordered albite are reported to 7.6 GPa; the fitted results lead to a zero-pressure bulk modulus (K0) of 56.4 ± 0.7 and a pressure derivative (K0′) of 3.9 ± 0.3 in a third-order Birch-Murnaghan equation of state. Revised values for the enthalpy and entropy of fusion of high albite at one bar and 1100 °C [ΔHTf = 64.5 ± 2.1 kJ/mol and ΔSTf = 47.0 J/(mol·K)] are recommended on the basis of improved heat capacity equations for NaAlSi3O8 glass and liquid. On the basis of these new results on the fusion curve and thermodynamic data for high albite, the pressure dependence of the NaAlSi3O8 liquid compressibility (K0′) is constrained to be 10.8 ± 1.5 in a third-order Birch-Murnaghan equation of state. The uncertainty in K0 ′ of ±1.5 contributes an error to melt density at 3 GPa (2.543 ± 0.010 g/cm3 at 1500 °C) of ±0.4%.

TEMPERATURE INDEPENDENT THERMAL EXPANSIVITIES OF SODIUM ALUMINOSILICATE MELTS BETWEEN 713 AND 1835 K

Abstract The thermal expansivities of eight sodium aluminosilicate liquids were derived from the slope of new volume data at low temperatures (713−1072 K) combined with the high temperature (1300−1835 K) volume measurements of Stein et al. (1986) on the same liquids. Melt compositions range from 47−71 wt% SiO2, 0−31 wt% A1203, and 17−33 wt% Na2O; the volume of albite supercooled liquid at 1092 K was also determined. The low temperature volumes were derived from measurements of the glass density of each sample at 298 K, followed by measurements of the glass thermal expansion coefficient from 298 K to the respective glass transition interval. This technique takes advantage of the fact that the volume of a glass is equal to the volume of the corresponding liquid at the limiting fictive temperature (T′f), and that T′f can be approximated as the onset of the rapid rise in thermal expansion at the glass transition in a heating curve (Moynihan, 1995). No assumptions were made regarding the equivalence of enthalpy and volume relaxation through the glass transition. The propagated error on the volume of each supercooled liquid at T′f is ∼0.25%. Combination of these low temperature data with the high temperature measurements of Stein et al. (1986) allowed a constant thermal expansivity of each liquid to be derived over a wide temperature interval (763−1001 degrees) with a fitted 1σ error of 0.6–4.6%; in every case, no temperature dependence to dV/dTliq could be resolved. Calibration of a linear model equation leads to fitted values ± 1σ (units of cm3/mole) for V ¯ S i O 2 (26.91 ± .04), V ¯ A l 2 O 3 (37.49 ± .12), V ¯ N a 2 O (26.48 ± .06) at 1373 K, and d V ¯ N a 2 O / d T (7.64 ± .08 × 10-3 cm3/mole-K). The results indicate that neither Si02 nor Al2O3 contribute to the thermal expansivity of the liquids, and that dV/dTliq is independent of temperature between 713–1835 K over a wide range of liquid composition. Calculated volumes based on this model recover both low and high temperature measurements with a standard deviation