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1.
Mg–Fe interdiffusion rates have been measured in wadsleyite aggregates at 16.0–17.0 GPa and 1230–1530 °C by the diffusion couple method. Oxygen fugacity was controlled using the NNO buffer, and water contents of wadsleyite were measured by infrared spectroscopy. Measured asymmetric diffusion profiles, analyzed using the Boltzmann–Matano equation, indicate that the diffusion rate increases with increasing iron concentration and decreasing grain size. In the case of wadsleyite containing 50–90 weight ppm H2O, the Mg–Fe interdiffusion coefficients at compositions of Mg/(Mg + Fe)=0.95 in the coarse-grained region (about 60 m) and 0.90 in the fine-grained region (about 6 m) were determined to be a DXmg = 0.95 (m2 s–1)=1.24 × 10–9 exp[–172 (kJ mol–1)/RT] and DXmg = 0.90 (m2 s–1)=1.77 × 10–9 exp[–143 (kJ mol–1)/RT], respectively. Grain-boundary diffusion rates were estimated to be about 4 orders of magnitude faster than the volume diffusion rate. Grain-boundary diffusion dominates when the grain size is less than a few tens of microns. Results for the nominally dry diffusion couple in the present study are roughly consistent with previous studies, taking into account differences in pressure and grain size, although water contents of samples were not clear in previous studies. We observed that the diffusivity is enhanced by about 1 order of magnitude in wadsleyite containing 300–2100 wt. ppm H2O at 1230 °C, which is almost identical to the enhancement associated with a 300 °C increase in temperature. It is still not conclusive that a jump in diffusivity exists between olivine and wadsleyite because water contents of olivine in previous diffusion studies and effects of water on the olivine diffusivity are uncertain. 相似文献
2.
Lynn A. Silver Phillip D. Ihinger Edward Stolper 《Contributions to Mineralogy and Petrology》1990,104(2):142-162
Infrared spectroscopy was used to determine the concentrations of molecular water and hydroxyl groups in hydrous rhyolitic, orthoclasic, jadeitic, and Ca–Al-silicate glasses synthesized by quenching of melts from elevated presure and temperature. The rhyolitic glasses and some of the Ca–Al-silicate glasses were quenched from water-vapor-saturated melts and used to determine the solubility of water in melts of these compositions. For all compositions studied, hydroxyl groups are the dominant hydrous species at low total water contents, whereas molecular water dominates at elevated water contents. Although the trends in species concentrations in all these compositions are similar, the proportions of the two hydrous species are influenced by silicate chemistry: increasing silica content and K relative to Na both favor molecular water over hydroxyl. Results on rhyolitic glass demonstrate that molecular water is also favored by decreasing temperature at T<850°C. For rhyolitic glasses quenched from vapor-saturated melts, the mole fraction of molecular water is proportional to water fugacity for P(H2O)1500 bars, demonstrating that the behavior of molecular water is approximately Henrian at total water contents up to at least several weight percent. Data on water solubility for albitic, orthoclasic, and Ca–Al-silicate melts to higher pressures can also be fit by assuming Henrian behavior for molecular water and can be used to set constraints on the partial molar volume of water in these melts. The demonstration of Henry's law for molecular water in these liquids provides a link between spectroscopic measurements of microscopic species concentrations and macroscopic thermodynamic properties. 相似文献
3.
The Mogan and Fataga formations on the island of Gran Canaria, Canary Islands, represent a sequence of approximately 30 intercalated pyroclastic and lava flows (total volume about 500 km3 dense-rock equivalent) including subalkaline rhyolitic, peralkaline rhyolitic and trachytic pyroclastic flows, nepheline trachyte lavas and a small volume of alkali basaltic lavas and tephra deposits. The eruption of the intermediate to silicic rocks of the Mogan and Fataga formations follows the roughly 4 Ma duration of basaltic shield volcanism. The most common assemblage in the evolved (Mogan and Fataga) rocks is anorthoclase+ edenitic amphibole+ilmenite+magnetite±augite±hypersthene +apatite+pyrrhotite. A few flows also contain plagioclase, biotite, or sphene. Coexisting Fe-Ti oxides yield equilibrium temperatures between 835 and 930° C and log
between –11.2 and –12.6. The lowermost pyroclastic flow of the Mogan formation is zoned from a rhyolitic base (848° C) to a basaltic top (931° C). Unit P1 has an oxygen isotope feldspar-magnetite temperature (850° C) very close to its Fe-Ti oxide temperature. One of the youngest Mogan flows is zoned from a comendite (836° C) at the base to a comenditic trachyte (899° C) at the top. The Fataga formation pyroclastic flows show only slight compositional zonation, and one flow has the same Fe-Ti oxide compositions at top and base.Calculations using the reaction 1/3 magnetite+SiO2 (melt)=ferrosilite+1/6 O2 indicate total pressures of 1–4 (±3) kb for six of the Mogan flows and one of the Fataga flows. For four of the pyroclastic flows, equilibria involving tremolite-SiO2-diopside-enstatite-H2O and phlogopite-SiO2-sanidine-enstatite-H2O imply water contents of 0.9 to 2.6 (±0.5) wt% and
between 80 and 610 bars, which indicates that magma within the Tejeda reservoir was H2O-undersaturated throughout the entire history of Mogan to Fataga volcanism. The fluorine contents of amphibole, biotite, and apatite, and chlorine contents of apatite reveal thatf
HF/
andf
HCl/
high compared to most igneous rocks and are consistent with the peralkaline nature of most of the volcanics. Thef
HCl estimate for one flow is 10–2 to 10–1 bars andf
HF for six of the flows ranges from about 10–1 to 6 bars. Pyrrhotite compositions yield estimates for log
between –1 and –3, log
between –2 and 1.5, and log
between 0.5 and 3, which fall in the range of most intermediate to silicic systems. The lack of a systematic trend with time for magma composition, Fe-Ti oxide temperatures, water contents, phenocryst abundances, and ferromagnesian phase composition indicate that the Tejeda magmatic system was open and kept at nearly the same conditions by the periodic addition of more primitive melts.The intensive thermodynamic parameters estimated from coexisting phenocryst equilibria are used to constrain the eruption dynamics based on solution of the conservation equations for a vapor plus pyroclast mixture. The estimates of magma reservoir temperature, pressure, and water concentration, when combined with a one-dimensional fluid dynamical model of a pyroclastic eruption, imply that the velocities of the ash flows at the vent exit were on the order of 100 to 200 m s–1, and the mass flow rates were about 107 kg s–1 for an assumed vent radius of 10 m. 相似文献
4.
Divariant oxide plus metal assemblages potentially make useful redox sensors for use in hydrothermal and other high pressure experiments. Here we report the calibration of the (Ni, Mn)O/Ni redox sensor in which the Ni/NiO (NNO) oxygen buffer is displaced to lower oxygen chemical potentials (O2), by the solid solution of MnO in the oxide phase. This assemblage was chosen because: (1) it covers a useful range of O2; (2) the system can be calibrated very accurately. Values of O2 defined by the (Ni, Mn)O/Ni assemblage were determined electrochemically, from 900 to 1300 K, using calcia-stabilized zirconia solid electrolytes. The oxide compositions (8 in total, ranging from 0.1X
NiO0.8) were analysed afterwards by electron microprobe, and were checked for internal consistency by measuring the lattice parameters (a0), using powder XRD. The accuracies of the measurements, both assessed theoretically and established empirically, are (1): ±80J/mol in O2, ±0.0002 Å in a0 and ±0.002 to 0.005 in X
NiO. Activity-composition relations were fitted to the Redlich-Kister formalism. There is a slight asymmetry (corresponding to a subregular model) across the solution with A
0
G
=9577(±45) J/mol, and A
1
G
=–477(±80) J/mol. The experimental data were also used to derive the parameters Vex, Hex and Sex. There is no obvious relationship between excess volumes and enthalpies of mixing, nor between excess volumes and excess entropies. The experimental data from this study have been used to formulate the (Ni, Mn)O/Ni redox sensor expression: O2 = 2(NNO) + 2RTlnX
NiO + 2(1 – X
NiO)2[11483 – 1.697T] – 477(4X
NiO – 1)(900 < T(K) < 1300) where O2(NNO)=–478967+248.514T–9.7961 T In T, from O'Neill and Pownceby(1993a). 相似文献
5.
The Brixen Quartzphyllite, basement of the Southern Alps (Italy), consists of metasediments which had suffered progressive deformation and low grade metamorphism (p
max4 kbar, T
max375±25° C) during the Palaeozoic. It has been excavated by pre-Permian erosion, buried again beneath a pile of Permo-mesozoic to Cainozoic sediments (estimated T
max150° C), and is now exposed anew due to late Alpine uplift and erosion. The behavior of the K-Ar system of white micas is investigated, taking advantage of the narrow constraints on their thermal history imposed by the geological/stratigraphic reference systems.The six structurally and petrographically differing samples come from a single outcrop, whose position is roughly two kilometers beneath the Permian land-surface. White mica concentrates from five grain size fractions (<2 , 2–6 , 6–20 , 20–60 , 60–75 ) of each sample have been analyzed by the conventional K-Ar method, four selected concentrates additionally by the 40Ar/39Ar stepwise heating technique; furthermore, Ar content and isotopic composition of vein quartz were determined.The conventional ages of the natural grain size fractions (20–60 , 60–75) are in the range 316±8 Ma, which corresponds to the 40Ar/39Ar plateau age of 319.0±5.5 Ma within the error limits. The finer grain size fractions yield significantly lower ages, down to 233 Ma for fractions <2 . Likewise low apparent ages (down to 83 Ma) are obtained for the low temperature 40Ar/39Ar degassing steps.There is no correlation between microstructural generation of white mica prevailing in the sample and apparent age. This favours an interpretation of the 316±8 Ma values as cooling age; progressive deformation and metamorphism must be respectively older and their timing cannot be resolved by these methods. The data preclude any significant influence of a detrital mica component as well as of excess argon.The lower ages found for the fine grain-size fractions (respectively the low-T degassing steps) correspond to a near-surface period (p-T-minimum); the values are geologically meaningless. The effect is interpreted to result from partial Ar loss due to reheating during Mesozoic-Cainozoic reburial. A model based on diffusion parameters derived from the outgassing experiments and Dodson's (1979) equation yields a closure temperature of 284±40 °C for a cooling rate of 18° C/Ma. Furthermore, this model suggests that the observed argon loss of up to 5% may in fact have been induced by reheating to 150 °C for 50 Ma. 相似文献
6.
Olivier Paillat Stephen C. Elphick William L. Brown 《Contributions to Mineralogy and Petrology》1992,112(4):490-500
The solubility of water in melts in the NaAlSi3O8–H2O system at high P and T was deduced from the appearance of quenched products and from water concentrations in the quenched glasses measured by ion probe, calibrated by hydrogen manometry. Starting materials were gels with sufficient water added to ensure saturation of the melts under the run conditions. Experiments were carried out for 10–30 h in an internally heated argon pressure vessel (eight at 1400° C and 0.2–0.73 GPa and three at 0.5 GPa and 900–1200° C) and for 1 h in a piston-cylinder apparatus (three at 1200° C, 1–1.3 GPa). No bubbles were observed in the glasses quenched at P<0.5 GPa or from T<1300° C at 0.5 GPa. Bubble concentration in glasses quenched from 1400° C was low at 0.5, moderate at 0.55 GPa and very high at 0.73 GPa and still higher in glasses quenched in the piston cylinder. Water concentration was measured in all glasses, except for the one at 0.55 GPa, for which it was only estimated, and for those at 0.73 GPa because bubble concentration was too high. Inferred water solubilities in the melt increase strongly with increasing P at 1400° C (from 6.0 wt% at 0.2 GPa to 15 at 0.55 GPa) and also with increasing T at 0.5 GPa (from 9.0 wt% at 900° C to 12.9 at 1400° C). The T variation of water solubility is fundamental for understanding the behaviour of melts on quenching. If the solubility decreases with T at constant P (retrograde solubility), bubbles cannot form by exsolution on isobaric quenching, whereas if the solubility is prograde they may do so if the cooling rate is not too fast. It is inferred from observed bubble concentrations and from our and previous solubility data that water solubility is retrograde at low P and prograde at and above 0.45 GPa; it probably changes with T from retrograde below to prograde above 900° C at 0.5 GPa. Moreover, the solubility is very large at higher pressures (possibly>30 wt% at 1.3 GPa and 1200° C) and critical behaviour is approached at 1.3 GPa and 1200° C. The critical curve rises to slightly higher P at lower T and intersects the three-phase or melting curve at a critical end point near 670° C and 1.5 GPa, above which albite coexists only with a supercritical fluid. 相似文献
7.
Glassy orthopyroxene granodiorite-tonalite (named pincinite after type locality) was described from basaltic lapilli tuffs of the Pliocene maar near Pinciná village in the Slovakian part of the Pannonian Basin. Two pincinite types exhibit a qualitatively similar mineral composition (quartz, An20–55 plagioclase, intergranular silicic glass with orthopyroxene and ilmenite, ±K-feldspar), but strongly different redox potential and formation PT conditions. Peraluminous pincinite is reduced (6–7% of total iron as Fe3+ in corundum-normative intergranular dacitic glass) and contains ilmenite with 8–10 mol% Fe2O3 and orthopyroxene dominated by ferrosilite. High-density (up to 0.85 g/cm3) primary CO2 inclusions with minor H2, CH4, H2S, CO and N2 (<2 mol% total) are present in Qtz and Plg. Equilibrium PT conditions inferred from the intergranular Opx–Ilm–Glass assemblage and fluid density correspond to 1,170±50°C, 5.6±0.4 kbar, respectively. Metaluminous pincinite is more oxidised (25–27% of total iron as Fe3+ in diopside-normative intergranular glass of rhyolite–trachyte–dacite composition) and contains Fe2O3-rich ilmenite (17–29 mol%) associated with enstatite. Fluid inclusions are composed of CO2–H2O mixtures with up to 38 mol% H2O. Raman spectroscopy revealed H2S along with dominant CO2 in the carbonic phase. Equilibrium PT parameters for the intergranular Opx–Ilm–Glass assemblage correspond to 740±15°C, 2.8±0.1 kbar, respectively. Reducing gas species (<2 mol% total) in the CO2-inclusions of the peraluminous pincinite resulted from hydrogen diffusion due to fH2 gradient imposed during decrease of redox potential from the log fO2 values near QFM during Qtz + Plg growth, to QFM-2 incidental to the superimposed Opx + Ilm assemblage in the intergranular melt. The decrease in oxygen fugacity was recorded also in the metaluminous pincinite, where log fO2 values changed from ~QFM + 2.6 to QFM + 0.4, but hydrogen diffusion did not occur. Absence of OH-bearing minerals, major and trace element abundances (e.g. REE 300–320, Nb 55–57, Th 4–31, Zr 240–300 ppm, FeOtot/MgO up to 11), and Sr–O isotope ratios in the pincinites are diagnostic of high-temperature anorogenic magmas originated by dehydration melting of biotite in quartz-feldspathoid crust (87Sr/86Sr>0.705–0.706, 18O>9 V-SMOW) around alkali basalt reservoir in depths between 17 and 20 km, and around late stage derivatives of the basalt fractionation, intruding the crust up to depths of 10–11 km. Low water activity in the pincinite parental melt was caused by CO2-flux from the Tertiary basaltic reservoirs and intrusions. The anatexis leads to generation of a melt-depleted granulitic crust beneath the Pannonian Basin, and the pincinites are interpreted as equivalents of igneous charnockites and enderbites quenched at temperatures above solidus and unaffected by sub-solidus re-equilibration and metamorphic overprint. 相似文献
8.
Stephen C. Elphick Colin M. Graham Paul F. Dennis 《Contributions to Mineralogy and Petrology》1988,100(4):490-495
The diffusion rate of 18O tracer atoms in anorthite (An97Ab03) under anhydrous conditions has been measured using SIMS techniques. The tracer source was 18O2 98.4% gas at 1 bar, in the temperature range 1300° C–850° C. The measured diffusion constants are D
0=1
–0.6
+1
×10–9 m2s–1
Q=236±8 kJ mol–1 Comparison of these values with published data for 18O diffusion in anorthite under hydrothermal conditions shows that dry oxygen diffusivities are orders of magnitude lower than equivalent wet values at similar temperatures. The effect of these differences on oxygen isotope equilibration during cooling is discussed. 相似文献
9.
Diffusion of lead in plagioclase and K-feldspar: an investigation using Rutherford Backscattering and Resonant Nuclear Reaction Analysis 总被引:3,自引:0,他引:3
D. J. Cherniak 《Contributions to Mineralogy and Petrology》1995,120(3-4):358-371
Chemical diffusion of Pb has been measured in K-feldspar (Or93) and plagioclase of 4 compositions ranging from An23 to An93 under anhydrous, 0.101 MPa conditions. The source of diffusant for the experiments was a mixture of PbS powder and ground feldspar of the same composition as the sample. Rutherford Backscattering (RBS) was used to measure Pb diffusion profiles. Over the temperature range 700–1050°C, the following Arrhenius relations were obtained (diffusivities in m2s-1):Oligoclase (An23): Diffusion normal to (001): log D = ( – 6.84 ± 0.59) – [(261 ± 13 kJ mol –1)/2.303RT]Diffusion normal to (010): log D = ( – 3.40 ± 0.50) – [(335 ± 11 kJ mol –1)/2.303RT]Andesine (An43): Diffusion normal to (001): log D = ( – 6.73 ± 0.54) – [(266 ± 12 kJ mol –1)/2.303RT] Diffusion normal to (010) appears to be only slightly slower than diffusion normal to (001) in andesine.Labradorite (An67): Diffusion normal to (001): log D = ( – 7.16 ± 0.61) – [(267 ± 13 kJ mol –1)/2.303RT] Diffusion normal to (010) is slower by 0.7 log units on average.Anorthite Diffusion normal to (010): log D = ( – 5.43 ± 0.48) – [(327 ± 11 kJ mol –1)/2.303RT]K-feldspar (Or93): Diffusion normal to (001): log D = ( – 4.74 ± 0.52) – [(309 ± 16 kJ mol –1)/2.303RT] Diffusion normal to (010): log D = ( – 5.99 ± 0.51) – [(302 ± 11 kJ mol –1)/2.303RT]In calcic plagioclase, Pb uptake is correlated with a reduction of Ca, indicating the involvement of PbCa exchange in chemical diffusion. Decreases of Na and K concentrations in sodic plagioclase and K-feldspar, respectively, are correlated with Pb uptake and increase in Al concentration (measured by resonant nuclear reaction analysis), suggesting a coupled process for Pb exchange in these feldspars. These results have important implications for common Pb corrections and Pb isotope systematics. Pb diffusion in apatite is faster than in the investigated feldspar compositions, and Pb diffusion rates in titanite are comparable to both K-feldspar and labradorite. Given these diffusion data and typical effective diffusion radii for feldspars and accessory minerals, we may conclude that feldspars used in common Pb corrections are in general less inclined to experience diffusion-controlled Pb isotope exchange than minerals used in U-Pb dating that require a common Pb correction. 相似文献
10.
In this paper, we present boron isotope analyses of variably degassed rhyolitic glasses from Long Valley, California. The following results indicate that pre-eruptive boron isotopic signatures were preserved in degassed glasses: (1) averaged secondary ionization mass spectrometry (SIMS) measurements of H2O-rich (~3 wt%) melt inclusions from late erupted Bishop Tuff pumice are indistinguishable from positive thermal ionization mass spectrometry (PTIMS) analysis of vesiculated groundmass glass (11B=+5.0±0.9 and +5.4±5, respectively); (2) SIMS spot-analyses on H2O-poor obsidian (~0.15 wt% H2O) from younger Glass Mountain Dome YA (average 11B=+5.2±1.0) overlap with compositionally similar late Bishop Tuff melt inclusions; and (3) four variably degassed obsidian samples from the 0.6 ka Mono Craters (H2O between 0.74 and 0.10 wt%) are homogeneous with regard to boron (average 11B=+3.2±0.8, MSWD=0.4). Insignificant variations in 11B between early and late Bishop Tuff melt inclusion glasses agree with published experimental data that predict minor 11B depletion in hydrous melts undergoing gas-saturated fractional crystallization. Melt inclusions from two crystal-rich post-caldera lavas (Deer Mountain and South Deadman Dome) are comparatively boron-rich (max. 90 ppm B) and have lower 11B values (average 11B=+2.2±0.8 and –0.4±1.0 ) that are in strong contrast to the boron isotopic composition of post-caldera crystal-poor rhyolites (27 ppm B; 11B=+5.7±0.8). These variations in 11B are too large to be caused by pre-eruptive degassing. Instead, we favor assimilation of 11B depleted low-temperature hydrothermally altered intrusive rocks subsequent to fresh rhyolite recharge.Editorial responsibility: J. HoefsAn erratum to this article can be found at 相似文献
11.
The Archean Eye Dashwa Lakes pluton (2672±24 Ma) has domains of mineralogically fresh isotropic granite, domains that have undergone bulk hydrothermal alteration, and at least eleven sets of sequential fracture arrays, each with distinctive mineral assemblages. Fresh granite is characterized by whole rock
18O=8.1 to 8.6 and primary magmatic quartz-feldspar (+1.3), quartz-biotite (5.2 to 5.4) and quartz-magnetite (+9.8) fractionations. Magmatic fluids had a calculated isotopic composition of
18O=7.9±0.5, and D=–80±5. These isotropic volumes of the granite have not experienced significant incursion of external thermal waters. Pegmatites, quartz-molybdenite veins, and phlogopite-muscovite coated fractures are sporadically distributed in the granite, and were precipitated from high-temperature magmatic fluids where
18O=8.0 to 10.3 and D=–80±5.The most abundant variety of fracture filling assemblage is epidote-quartz-chlorite±muscovite: fractures are bounded by domains of mineralogically similar bulk hydrothermal alteration of the granite. These minerals formed at 160 to 280° C, in the presence of NaCl, and NaCl-MgCl2 brines (up to 25 wt% NaCl equivalent) of probable evolved marine water origin (
18O=+0.4 to +3.8, D=–10 to –35) undergoing transient boiling. Upper plateau 40Ar/39Ar ages for the muscovite are 2650±15 Ma. Sequentially in the chronology of fracture-infiltration events, calcite-fluorite veins were deposited from boiling fluids at 340 to 390° C, isotopically characterized by
18O=4.7 and
13C=–5; and rare prehnite-chlorite lined fractures formed at 250 to 290° C. A generation of adularia-bearing veins precipitated at 140 to 230° C, from CaCl2-NaCl brines, where
18O=0 to –6.5 and D=–10 to –30. Incremental 40Ar/39Ar age spectra on the K-feldspar yield an upper plateau of 1100 Ma. Subsequently, hematite developed during reactivation of earlier fractures, at 140 to 210° C in the presence of fluids characterized by
18O=–0.4 to –5.4 and D=–15 to –25. Arrays of open fractures partially occupied by gypsum and goethite reflect a fluid infiltration event at temperatures <50° C. Many of the earlier generations of fracture minerals have transgranular fracture infillings which record the presence of low temperature (88–190° C), hypersaline CaCl2-NaCl brines. Narrow fractures lined with clays±calcite are sites for seepage of modern ground-waters. The isotopic signature of clay (
18O=12 to 20, D=–80±5) plots near the line for modern kaolinites, confirming its formation in the presence of recent surface waters. Calcites coexisting with the clay minerals, and in fractured pegmatite show a common isotopic signature (
18O=23±0.5,
13C=–13.6), indicating precipitation from modern groundwaters, where reactivated fractures have acted as conduits for infiltration of surface waters to depths of 200 m. Intermittent fracture-infiltration has occurred over 2.7 Ga. The early sequences of fracture-related fluid flow are interpreted in terms of devolatilization of the granite, followed by thermal contraction fracturing, incursion of marine water and convective cooling in the Archean. Hematite and adularia fracture fillings correspond to a stage when meteoric water infiltrated the volcanicplutonic terrain during Proterozoic and later times. Episodic fracture-fluid expulsion events may have been driven by seismic pumping, in response to magmatically and tectonically induced stresses within the Shield, with surface waters penetrating to depths of 15 km in the crust. 相似文献
12.
Water diffusion in silicate melts is important for understanding bubble growth in magma, magma degassing and eruption dynamics of volcanos. Previous studies have made significant progress on water diffusion in silicate melts, especially rhyolitic melt. However, the pressure dependence of H2O diffusion is not constrained satisfactorily. We investigated H2O diffusion in rhyolitic melt at 0.95–1.9 GPa and 407–1629 °C, and 0.2–5.2 wt.% total water (H2Ot) content with the diffusion-couple method in a piston-cylinder apparatus. Compared to previous data at 0.1–500 MPa, H2O diffusivity is smaller at higher pressures, indicating a negative pressure effect. This pressure effect is more pronounced at low temperatures. Assuming H2O diffusion in rhyolitic melt is controlled by the mobility of molecular H2O (H2Om), the diffusivity of H2Om (DH2Om) at H2Ot ≤ 7.7 wt.%, 403–1629 °C, and ≤ 1.9 GPa is given by
DH2Om=D0exp(aX),