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A telepresence-enabled cruise using remotely operated vehicle (ROV) exploration discovered an unusual tubular deposit of Fe-rich hydrothermal nontronite on a young seamount, Mashi, of the Wolf-Darwin lineament in the Galapagos Islands. X-ray diffraction, ICP-MS, ICP-AES, and SEM-EDS analyses show that this deposit is chemically and mineralogically similar to other deep-sea hydrothermal nontronites, indicating a likely formation temperature of about 30° to 50 °C by diffuse hydrothermal activity. These deposits contain mixtures of Fe-rich, Al-poor nontronite and poorly crystalline Fe-Si-oxyhydroxides with bulk compositions of 38–51 wt% SiO 2 and 40–50 wt% Fe 2 O 3 *. The presence of filamentous and spherical structures in the samples suggests that mineral deposition was in part facilitated by chemosynthetic microbes. Although hydrothermal nontronite has been sampled at a number of seafloor sites by coring and dredging, this is the first in situ documentation of its unusual sinuous, tubular structure, on the seafloor. Quantitative image-analysis of ROV imagery indicates that hydrothermal fluid pathways, developed through an underlying pillow lava sequence, likely control the distinctive sinuous morphology.

We provide measurements of thermal diffusivity (D), heat capacity (CP), and viscosity (η) for 12 remelted natural lavas and 4 synthetic glasses and melts, ranging in composition from leucogranite to low-silica basalt, and calculate their thermal conductivity. Both viscosity and the glass transition temperature decrease with decreasing melt polymerization. For basaltic glasses, D is low, ~ 0.5 mm2 s− 1 at room temperature, decreases slightly with increasing temperature, and then drops upon melting to ~ 0.25 to 0.35 mm2 s− 1. Other samples behave similarly. Despite scatter, clear correlations exist between D of glass or melt with Si content, density, NBO/T, and, most strongly, with fragility (m). Glass thermal diffusivity is represented by D = FT− G + HT, where F, G and H are fitting parameters. For melts, ∂ D/∂ T was resolved only for dacite-andesite and MORB: a positive slope is consistent with other iron-bearing samples. Glass and liquid CP depend on density and other physical properties, but not exactly in the same manner as D. We calculate thermal conductivity (k) from these data and demonstrate that k for glasses is described by a Maier-Kelly formula. Large scatter exists for k at 298 K, but silicic to intermediate melts have k between 1.8 and 1.3 Wm− 1 K− 1, whereas basaltic melts are constrained to ~ 1.4 ± 0.1 Wm− 1 k− 1. Low values for thermal diffusivity and viscosity for basaltic melts suggests that basalts transfer heat much more efficiently by advection than by conduction alone, and that partially molten zones in the mantle quickly become more thermally insulating than non-molten zones, potentially contributing to melt localization during decompression melting.

We determined the viscosity and heat capacity of a series of two basaltic liquids containing H 2 O, F, H 2 O + CO 2 , H 2 O + F, and H 2 O + CO 2 + F. One was a natural calc-alkaline basalt from Fuego volcano, Guatemala, and the other was an Fe-free synthetic analog. The viscosity measurements were performed in the low-temperature, high-viscosity range (~ 10 9 –10 12 Pa s) just above the glass transition, where the kinetics of volatile exsolution are slow. Differential scanning calorimetry measurements were performed at atmospheric pressure from room temperature up to ~ 100 K above the glass transition. The water contents ranged from nominally anhydrous to 3 wt.% H 2 O, with F contents up to 2 wt.%, and CO 2 contents up to 0.2 wt.%. Volatiles do not noticeably affect the heat capacity of glasses. The glass transition temperatures obtained from calorimetry and viscometry are in good agreement. Water has a strong viscosity-reducing effect on basaltic melts. F has a measurable viscosity-reducing effect in basaltic melts, but it is significantly smaller than that of water. The combined effects of H 2 O and F on viscosity appear to be additive on a wt.% basis. Both the effects of H 2 O and F on basaltic melts are smaller than those for more polymerized melts. Small quantities of CO 2 do not measurably affect basaltic melt viscosity, at least in the presence of > 1 wt.% water. Future viscosity models incorporating fluorine need to account for the compositional dependence of its effects on dry and hydrous melts.

We determined the heat capacities of four series of glasses and liquids of basaltic and basaltic andesite compositions from remelted volcanic rock samples and Fe-free synthetic analogues. The samples are low-alkali, Ca- and Mg-rich aluminosilicates with non-bridging oxygen to tetrahedrally-coordinated cation ratios (NBO/T) ranging between 0.33 and 0.67. Differential scanning calorimetry measurements were performed at atmospheric pressure between room temperature and ~ 100 K above the glass transition for hydrous samples and up to ~ 1800 K for dry samples. The water contents investigated range up to 5.34 wt.% (16.4 mol%). Water does not measurably affect the heat capacity of glasses. We derived a new value of the partial molar heat capacity of water in silicate glasses of C ¯ P , H 2 O glass = 82.804 + 10 − 3 T − 48.274 × 10 − 5 T − 2 (J/mol K) using our new data in combination with literature data on more and less polymerized compositions. The increase in heat capacity at the glass transition is of the order of ~ 30–40% and generally increases with increasing water content. The onset of the glass transition in hydrous samples occurs below the Dulong–Petit limit of 3R/g atom. The configurational heat capacity, i.e., the magnitude of the change in heat capacity observed at the glass transition, generally increases as polymerization decreases and as water content increases. We obtained a partial molar heat capacity of water in silicate liquids of basaltic composition of ~ 86 J/mol K. This value is comparable to the partial molar values for the major oxides which range from ~ 79 to 230 J/mol K. The partial molar heat capacity of water in silicate liquids appears to be compositionally-dependent, increasing as melt polymerization decreases. Such a dependence is certainly linked to the speciation and structural roles of water in complex silicate melts, however, a single value of ~ 93 J/mol K could reproduce the heat capacity of hydrous liquids of a wide range of NBO/T (0–1.51) at temperatures up to ~ 100 K above the glass transition and water contents of 0–3.76 wt.% with a root-mean square deviation of only 3.23 J/mol K.

The viscosity of a series of 6 Fe-free, synthetic basaltic andesite liquids, containing up to 3.76 wt.% dissolved water, was measured in the range of the glass transition (10 8 –10 13 Pa s) by parallel-plate viscometry. Concentric-cylinder and falling-sphere viscometry provided high-temperature measurements (10–10 3 Pa s) on basaltic andesite liquids containing up to 2 wt.% dissolved water. The viscosity ( η in Pa s) of Fe-free basaltic andesite can be described as a function of temperature ( T in Kelvin) and water content ( w in wt.%) by the expression log( η ) = − 4.81 + 6940.7/( T − {491.9 − 272.5 log[ w + 0.49]}).This parameterization reproduces 55 viscosity data with a root-mean-square-deviation (RMSD) of 0.24 log units in viscosity. The results of this viscometry study suggest that basaltic andesite liquids should remain very fluid, even while undergoing equilibrium degassing, to pressures as low as 50 MPa (i.e., less than 2 km depth). Only a modest increase in viscosity of at most a factor of 100 would occur in the last 2 km of ascent. Furthermore, our results show that water affects the viscosity of a wide range of depolymerized melts to a similar degree. For example, the addition of 2 wt.% dissolved H 2 O reduces the viscosity of andesite, basaltic andesite, basalt and their alkalic counterpart liquids by a factor of ~ 15–50.

We present a suite of 21 high-temperature, uniaxial deformation experiments performed on 25 by 50 mm unjacketed cores of porous (Φ ∼ 0.8) sintered rhyolitic ash. The experiments were performed at, both, atmospheric (dry) and elevated water pressure conditions (wet). Experiments used a constant displacement rate of 2.5 × 10− 6 m s− 1 corresponding to a strain rate (ɛo) of ∼ 5 × 10− 5 s− 1; a single experiment was run at 2.5 × 10− 5 m s− 1 (ɛo ∼ 5 × 10− 4 s− 1). Dry experiments were conducted mainly at 900 °C, but also included a suite of lower temperature experiments at 850, 800 and 750 °C. Wet experiments were performed at ∼ 650 °C under water pressures of 1, 2.5, and 5 MPa, and at a fixed PH2O of ∼ 2.5 MPa for temperatures of ∼ 385, 450, and 550 °C. During deformation, strain is manifest by shortening of the cores, reduction of porosity, flattening of ash particles, and radial increase of the cores. The continuous reduction of porosity leads to a dynamic transient strain-dependent rheology and requires strain to be partitioned between a volume (porosity loss) and a shear (radial increase) component. These data demonstrate the effect of porosity on the rheology of dry and hydrous melts. The effect of increasing porosity is to expand the window for ductile deformation for dry melts by delaying the onset of brittle deformation by ∼ 50 °C (875 °C to 825 °C). The effect is more pronounced in hydrous melts (∼ 0.67–0.78 wt.% H2O) where the ductile to brittle transition is depressed by ∼ 140 to 150 °C. Increasing water pressure also delays the onset of strain hardening due to compaction-driven porosity reduction. These rheological data are pertinent to any volcanic processes involving high-temperature porous magmas (e.g., magma flow in conduits, welding of pyroclastic materials).

We describe an experimental apparatus used to perform deformation experiments relevant to the volcanological sciences. The apparatus supports low-load, high-temperature deformation experiments under dry and wet conditions on natural and synthetic samples. The experiments recover the transient rheology of complex (melt ± porosity ± solids) volcanic materials during uniaxial deformation. The key component to this apparatus is a steel cell designed for high-temperature deformation experiments under controlled water pressure. Experiments are run under constant displacement rates or constant loads; the range of accessible experimental conditions include: 25–1100 °C, load stresses 0 to 150 MPa, strain rates 10 −6 to 10 −2 1/s, and fluid pressures 0–150 MPa. The apparatus is calibrated against standard values of viscosity using constant-load experiments on cores of NIST (NBS) 717a borosilicate glass. We also report results of constant-displacement rate (~10 −6 m/s) experiments on porous (~70%) sintered cores of ash from the Rattlesnake Tuff. The cores of volcanic ash were deformed in experiments under ambient (“dry”) and elevated water pressures (“wet”). Dry experiments at ~870 °C show an increase in effective viscosity (10 9.5 to 10 10.4 Pa·s) with increasing strain (~30%) due to porosity reduction during compaction. Experiments under ~1–3 MPa P H 2 O recover lower values of apparent viscosity (10 9.2 to 10 9.4 Pa·s) despite being run at lower temperatures (640–665 °C). The wet experiments also do not show a rise in viscosity with increased strain (decreasing porosity) as observed in dry experiments. Rather, the presence of an H 2 O fluid phase expands the window of viscous deformation and delays the onset of strain hardening that normally accompanies porosity reduction.