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1.
The ion microprobe at Johnson Space Center has been calibrated for in situ water determinations on a 10-μm scale over the range 0.2 wt.% H2O to 1.8, 6.8, and 3.7 wt.%, for basaltic, albitic, and rhyolitic glasses, respectively. The basalt glass calibration curve differs substantially from those of albite and rhyolite glasses, indicating a need to carefully match composition and/or melt structure between H2O standards and unknowns.A value for the diffusivity of water as a function of concentration and time has been calculated from water diffusion profiles measured in rhyolite glasses prepared at 850°C and 700 barsPt(H2O) [1]. Transient diffusion into a semi-infinite medium is described by the equation:?(φ/2)?¸/?φ=?(Dw?¸/?φ)/?φ #x003B8;=1, φ=0, θ→ 0, θ→∞, wherex =distance from the cylinder edge,t =time,C0 =initial concentration,Cs =concentration at the edge,C =concentration at x,θ = C ? C0/Cs ? C0,φ = x/t1/2, andDw =diffusivity of water. An iterative technique has been used to calculate solutions to the diffusion equation as a function ofDw [2]. Comparison of these solutions with the ion probe data indicate that, for0.2wt.% ≤ C ≤ 3.7wt.%H2O,Dw can be described by an exponential function of θ, of the formDw = D0exp(bθ), withD0 (i.e.,Dw at 0.2%) = (0.8?2.2) × 10?8 cm2/s and2 ≤ b ≤ 4. 相似文献
2.
Sixteen sets of apatite/liquid partition coefficients (Dap/liq) for the rare earth elements (REE; La, Sm, Dy, Lu) and six values for Sr were experimentally determined in natural systems ranging from basanite to granite. The apatite + melt (glass) assemblages were obtained from starting glasses artificially enriched in REE, Sr and fluorapatite components; these were run under dry and hydrous conditions of 7.5–20 kbar and 950–1120°C in a solid-media, piston-cylinder apparatus. An SEM-equipped electron microprobe was used for subsequent measurement of REE and Sr concentrations in coexisting apatites and quenched glasses. The resulting partition coefficient patterns resemble previously determined apatite phenocryst/groundmass concentration ratios in the following respects: (1) the rare earth patterns are uniformly concave downward (i.e., the middle REE are more compatible in apatite than the light and heavy REE); (2) DREEap/liq is much higher for silicic melts than for basic ones; and (3) strontium (and therefore Eu2+) is less concentrated by apatite than are the trivalent REE. The effects of both temperature and melt composition on DREEap/liq are systematic and pronounced. At 950°C, for example, a change in melt SiO2 content from 50 to 68 wt.% causes the average REE partition coefficient to increase from ～7 to ～30. A 130°C increase in temperature, on the other hand, results in a two-fold decrease in DREEap/liq. Partitioning of Sr is insenstitive to changes in melt composition and temperature, and neither the Sr nor the REE partition coefficients appear to be affected by variations in pressure or H2O content of the melt.The experimentally determined partition coefficients can be used not only in trace element modelling, but also to distinguish apatite phenocrysts from xenocrysts in rocks. Reported apatite megacryst/host basalt REE concentration ratios [12], for example, are considerably higher than the equilibrium partition coefficients, which suggest that in this particular case the apatite is actually xenocrystic.A reversal experiment incorporated in our study yielded diffusion profiles of REE in apatite, from which we extracted a REEαCa interdiffusion coefficient of 2–4×10?14 cm2/s at 1120°C. Extrapolated downward to crustal temperatures, this low value suggests that complete REE equilibrium between felsic partial melts and residual apatite is rarely established. 相似文献
3.
Carl B. Agee 《Earth and Planetary Science Letters》2008,265(3-4):641-654
High pressure experiments using the sink/float method have bracketed the density of hydrous iron-rich ultrabasic silicate melt from 1.35 to 10.0 GPa at temperatures from 1400 to 1860 °C. The silicate melt composition was a 50–50 mixture of natural komatiite and synthetic fayalite. Water was added in the form of brucite Mg(OH)2 and was present in the experimental run products at 2 wt.% and 5 wt.% levels as confirmed by microprobe analyses of total oxygen. Buoyancy marker spheres were olivines and garnets of known composition and density. The density of the silicate melt with 5 wt.% water at 2 GPa and 1500 °C is 0.192 g cm? 3 less than the anhydrous form of this melt at the same P and T. This density difference gives a partial molar volume of water in silicate melt of ～ 7 cm3 mol? 1, which is similar to previous studies at high pressure. The komatiite–fayalite liquids with 0 and 2 wt.% H2O, have extrapolated density crossovers with equilibrium liquidus olivine at 8 and 9 GPa respectively, but there is no crossover for the liquid with 5 wt.% H2O. These results are consistent with the hypothesis that dense hydrous melts could be gravitationally stable atop the 410 km discontinuity in the Earth. The results also support the notion that equilibrium liquidus olivine could float in an FeO-rich hydrous martian magma ocean. Extrapolation of the data suggests that FeO-rich hydrous melt could be negatively buoyant in the Earth's D″-region or atop the core–mantle-boundary (CMB), although experiments at higher pressure are needed to confirm this prediction. 相似文献
4.
Gases trapped in lavas of three main flows of the Ardoukôba eruption (8 to 15 November, 1978) have been analysed by mass spectrometry. These analyses concern both plagioclase phenocrysts and microcrystalline mesostasis. Fluids are released between 500°C and 1200°C, and consist of H2O, CO2, CO, N2, SO2, HCl, H2, CH4 with traces of hydrocarbons and H2S. The total content is less than 0.3–0.4 wt. % of samples with about 0.1–0.15 wt % of H2O. No significant variation among the three flows is observed. Plagioclase phenocrysts are less abundant in fluids than the mesostasis (～2/3). The gases trapped in these phenocrysts are richer in CO and organic compounds, whereas mesostasis contain more H2O, CO2 and SO2. CO is likely produced by reduction of CO2 and H2O with carbon during either analyses or eruption itself, or is of primary origin. In the latter case, gas composition suggests an entrapment temperature of about 1200°C ± 75°C. Kinetic study of the water and carbon dioxide release allows to calculate the diffusion characteristics of these fluids. Water and carbon dioxide behave rather similarly. Plagioclase gives a single activation energy value (8 Kcal/mole), while mesostasis gives two values (8 Kcal/mole, 15 Kcal/mole). Diffusion coefficients at 20°C are estimated to fall in the range 10?13 · 10?12 cm2 · sec?1. 相似文献
5.
Single-crystal spectra of pyropes, synthesized on the 3:1:3 composition of the system MgO-Al2O3-SiO2 atpH2O = ptotal = 25kbar and 1000°C, show a broad band centered at around 3400 cm?1 due toνH2O of (H2O)n aggregates in fluid inclusions and a sharp intense band near 3600 cm?1 due toνOH of (HO)44? clusters introduced by the hydrogarnet substitutionSi4+ = 4H+. The water contents in the synthetic pyrope, due to (HO)44? clusters are estimated near 0.05 wt.% H2O from spectroscopic data. Si deficits in microprobe analyses of the pyropes studied support the hydrogarnet substitution. These results show that the hydrogarnet substitution in pyrope may contribute to water contents in the mantle. 相似文献
6.
Radiogenic Ar diffusion from phlogopite mica has been measured at 900° and 1080°C at 15 kbars pressure, using a Griggs-type hot creep tester operated at hydrostatic pressure, and followed by standard mass spectrometric analysis. The diffusion coefficients fall within the uncertainty of the diffusion results reported by Giletti (1974) at 1 kbar at 900° and on extrapolation of that Arrhenius plot to 1080°C. That linear plot is given by D0 = 0.75 and Q = 57.9kcal/g-atom Ar. In the observed temperature range, the effect of pressure up to 15 kbars is not significant. A second Ar diffusion, which employed Ca(OH)2-CaO as a water buffer, was run at 1 atm H2O pressure and 550°C for 781 days. The resulting diffusion coefficient agrees, within the uncertainty, with the extrapolation of the earlier curve. This result, and both earlier ones at 600°, are similar in that they fall below the curve. Consequently, despite the agreement within the uncertainty, it is possible that diffusion of Ar from phlogopite below 650°C is somewhat slower than given by the above equation parameters. 相似文献
7.
The vesiculation of a peralkaline rhyolite melt (initially containing ∼0.14 wt.% H2O) has been investigated at temperatures above the rheological glass transition (T
g≈530 °C) by (a) in situ optical observation of individual bubble growth or dissolution and (b) dilatometric measurements
of the volume expansion due to vesiculation. The activation energy of the timescale for bubble growth equals the activation
energy of viscous flow at relatively low temperatures (650–790 °C), but decreases and tends towards the value for water diffusion
at high temperatures (790–925 °C). The time dependence of volume expansion follows the Avrami equation
ΔV (t)∼{1–exp [–(t/τav)
n
]}
with the exponent n=2–2.5. The induction time of nucleation and the characteristic timescale (τav) in the Avrami equation have the same activation energy, again equal to the activation energy of viscous flow, which means
that in viscous melts (Peclet number <1) the vesiculation (volume expansion), the bubble growth process, and, possibly, the nucleation of vesicles, are
controlled by the relaxation of viscous stresses. One of the potential volcanological consequences of such behavior is the
existence of a significant time lag between the attainment of a super-saturated state in volatile-bearing rhyolitic magmas
and the onset of their expansion.
Received: March 20, 1995 / Accepted: October 24, 1995 相似文献
8.
The textures of chondrules have been reproduced by crystallizing melts of three different compositions at 1 atm with cooling rates ranging from 400 to 20°C/min under 10?9 to 10?12 atmPO2. A porphyritic olivine texture has been formed from a melt of olivine-rich composition (SiO2 = 45 wt.%), a barred-olivine texture from melt of intermediate composition (SiO2 = 47 wt.%), and radial-olivine texture from melt of pyroxene-rich composition (SiO2 = 57 wt.%). The cooling rate for producing barred olivine is most restricted; the rate ranges from 120 to 50°C/min. Other textures can be formed with wider ranges of cooling rate. The results of the experiments indicate that some of the major types of textures of chondrules can be formed with cooling rate of about 100°C/min. With this cooling rate, the texture varies depending on the composition of melt. 相似文献
9.
《Journal of Volcanology and Geothermal Research》2005,139(3-4):295-313
Examination of glass and crystal chemistry in the Rotoiti Pyroclastics (>100 km3 of magma) demonstrates that compositional diversity was produced by mingling of the main rhyolite magma body with small volumes of other magmas that had been crystallizing in separate stagnant magma chambers. Most (>90%) of the Rotoiti deposits were derived from a low-K2O, cummingtonite-bearing, rhyolitic magma (T1) discharged throughout the eruption sequence. T1 magma is homogeneous in composition (melt SiO2=77.80±0.28 wt.%), temperature (766±13 °C) and oxygen fugacity (NNO+0.92±0.09). Most T1 phenocrysts formed in a shallow (∼200 MPa), near water-saturated (awater=0.8) storage chamber shortly before eruption. Basaltic scoria erupted immediately before the rhyolites, and glass-bearing microdiorite inclusions within the rhyolite deposits, suggest that basalt emplaced on the floor of the chamber drove vigorous convection to produce the well-mixed T1 magma. Lithic lag breccias contain melt-bearing biotite granitoid inclusions that are compositionally distinct from T1 magma. The breccias which overlie the voluminous T1 pyroclastic flow deposits resulted from collapse of the syn-Rotoiti caldera. Post-collapse Rotoiti pumices contain T1 magma mingled with another magma (T2) that is characterized by high-K glass and biotite, and was cooler and less oxidised (712±16 °C; NNO−0.16±0.16). The mingled clasts contain bimodal disequilibrium populations of all crystal phases. The granitoid inclusions and the T2 magma are interpreted as derived from high-K magma bodies of varying ages and states of crystallization, which were adjacent to but not part of the large T1 magma body. We demonstrate that these high-K magmas contaminated the erupting T1 magma on a single pumice clast scale. This contamination could explain the reported wide range of zircon U–Th ages in Rotoiti pumices, rather than slow crystallization of a single large magma body. 相似文献
10.
Zircon saturation revisited: temperature and composition effects in a variety of crustal magma types 总被引：38，自引：0，他引：38
Hydrothermal experiments in the temperature range 750–1020°C have defined the saturation behavior of zircon in crustal anatectic melts as a function of both temperature and composition. The results provide a model of zircon solubility given by: In DZrzircon/melt= ?3.80?[0.85(M?1)]+12900/T where DZrzircon/melt is the concentration ratio of Zr in the stoichiometric zircon to that in the melt, T is the absolute temperature, and M is the cation ratio (Na + K + 2Ca)/(Al · Si). This solubility model is based principally upon experiments at 860°, 930°, and 1020°C, but has also been confirmed at temperatures up to 1500°C for M = 1.3. The lowest temperature experiments (750° and 800°C) yielded relatively imprecise, low solubilities, but the measured values (with assigned errors) are nevertheless in agreement with the predictions of the model.For M = 1.3 (a normal peraluminous granite), these results predict zircon solubilities ranging from ～ 100 ppm dissolved Zr at 750°C to 1330 ppm at 1020°C. Thus, in view of the substantial range of bulk Zr concentrations observed in crustal granitoids (～ 50–350 ppm), it is clear that anatectic magmas can show contrasting behavior toward zircon in the source rock. Those melts containing insufficient Zr for saturation in zircon during melting can have achieved that condition only by consuming all zircon in the source. On the other hand, melts with higher Zr contents (appropriate to saturation in zircon) must be regarded as incapable of dissolving additional zircon, whether it be located in the residual rocks or as crystals entrained in the departing melt fraction. This latter possibility is particularly interesting, inasmuch as the inability of a melt to consume zircon means that critical geochemical “indicators” contained in the undissolved zircon (e.g. heavy rare earths, Hf, U, Th, and radiogenic Pb) can equilibrate with the contacting melt only by solid-state diffusion, which may be slow relative to the time scale of the melting event. 相似文献
11.
Fugacity-concentration relationship of dilute hydrogen in water at elevated temperature and pressure
The fugacity vs. concentration ratio(Y) of dilute hydrogen dissolved in water was determined at temperatures below 500°C and pressures below 1000 bar by measuring H2 concentrations in magnetite-hematite-water and palladium hydride-water systems. Combining these results with reported solubility data of H2 in water and with the Shaw's expression for the activity-composition relationship of H2H2O mixture at higher temperatures, theP-T-Y diagram was constructed over theP-T range below 900°C and 1000 bar. 相似文献
12.
William L. Romine Alan G. Whittington Peter I. Nabelek Anne M. Hofmeister 《Bulletin of Volcanology》2012,74(10):2273-2287
Thermal diffusivity (D) was measured using laser-flash analysis on pristine and remelted obsidian samples from Mono Craters, California. These high-silica rhyolites contain between 0.013 and 1.10?wt% H2O and 0 to 2?vol% crystallites. At room temperature, D glass varies from 0.63 to 0.68?mm2?s?1, with more crystalline samples having higher D. As T increases, D glass decreases, approaching a constant value of ??0.55?mm2?s?1 near 700?K. The glass data are fit with a simple model as an exponential function of temperature and a linear function of crystallinity. Dissolved water contents up to 1.1?wt% have no statistically significant effect on the thermal diffusivity of the glass. Upon crossing the glass transition, D decreases rapidly near ??1,000?K for the hydrous melts and ??1,200?K for anhydrous melts. Rhyolitic melts have a D melt of ??0.51?mm2?s?1. Thermal conductivity (k?=?D·??·C P) of rhyolitic glass and melt increases slightly with T because heat capacity (C P) increases with T more strongly than density (??) and D decrease. The thermal conductivity of rhyolitic melts is ??1.5?W?m?1?K?1, and should vary little over the likely range of magmatic temperatures and water contents. These values of D and k are similar to those of major crustal rock types and granitic protoliths at magmatic temperatures, suggesting that changes in thermal properties accompanying partial melting of the crust should be relatively minor. Numerical models of shallow rhyolite intrusions indicate that the key difference in thermal history between bodies that quench to obsidian, and those that crystallize, results from the release of latent heat of crystallization. Latent heat release enables bodies that crystallize to remain at high temperatures for much longer times and cool more slowly than glassy bodies. The time to solidification is similar in both cases, however, because solidification requires cooling through the glass transition in the first case, and cooling only to the solidus in the second. 相似文献
13.
R.K. Lowry S.J.B. Reed J. Nolan P. Henderson J.V.P. Long 《Earth and Planetary Science Letters》1981,53(1):36-40
An ion-microprobe-based technique has been used to measure lithium tracer-diffusion coefficients (DLi) in an alkali-basaltic melt at 1300, 1350 and 1400°C. The results can be expressed in the form:DLi=7.5 ×10?2exp(?27,600/RT)cm2S?1The results show significantly faster diffusion rates than those previously recorded for other monovalent, divalent and trivalent cations in a tholeiitic melt. Consequently, diffusive transport of ions acting over a given time in a basaltic melt can produce a wider range of transport distance values than hitherto supposed. Hence, it is concluded that great care should be exercised when applying diffusion data to petrological problems. 相似文献
14.
The electrical conductivity of a single crystal of San Carlos olivine (Fo92, 0.16 wt.% Fe2O3) has been measured as a function of temperature and oxygen fugacity (fO2). After heating to 1338°C at fO2 = 10?12 atm., the conductivity at 950°C was 10?5 (ohm-m)?1, almost 3 orders of magnitude less than that measured in air. This decrease is due to the reduction of Fe3+ to Fe2+. Further heating to 1500°C at fO2 = 10?14 atm., decreased the electrical conductivity at 950°C to 10?6 (ohm-m)?1. When recovered at room temperature, the speciment had Fo96 composition and contained small, opaque blebs distributed throughout the crystal. Derivations of temperature profiles for the earth's mantle from conductivity-depth models must take account of the important role played by iron oxidation state in the electrical conductivity of olivine. 相似文献
15.
The effects of temperature, fO2 and composition on the electrical conductivity of silicate liquids have been experimentally determined from 1200 to 1550°C under a range of fO2 conditions sufficient to change the oxidation state of Fe from predominantly Fe2+ to Fe3+. Oxidation of ferrous to ferric iron in the melt has no measurable effect on the conductivity of melts with relatively low ratios of divalent to univalent cations. Under strongly oxidizing conditions a minor decrease of conductivity is detected inth highΣM2/ΣM+ ratios. It is concluded that for purposes of estimating the conductivity of magmatic liquids, fO2 may be ignored to a first approximation. Both univalent and divalent cation transport is involved in electrical conduction. Melts relying heavily on divalent cations for conduction, i.e. melts with relatively large ΣM2+/ΣM+ ratios, show strong departures from Arrheenius temperature dependence with the apparent activation energies decreasing steadily as the temperature increases. Conductivities dominated by the univalent cations, in melts with relatively small ΣM2+/ΣM+ ratios, show classical Arrhenius temperature dependence. These observations are discussed in terms of the general characteristics of the melt structure.Compositional variations within the magmatic range account for much less than an order of magnitude variation in electrical conductivity at a fixed temperature. This observation, combined with previous measurements of the conductivity of olivine (A. Duba, H.C. Heard and R. Schock, 1974) make it possible to state with reasonable confidence that melts occurring within the mantle will be more conductive by 3–4 orders of magnitude than their refractory residues. Potential applications to geothermometry are discussed. 相似文献
16.
The viscosity of basalts (quartz and olivine tholeiite) was studied under pressure in dry conditions and in the presence of water. In dry conditions at 1400°C when pressure increases to 20 kbars the viscosity reduces by a factor of 2. In conditions of water saturation of basalt melts at 5 kbar the viscosity is smaller by a factor of ～ 50 than that in dry conditions. In the water undersaturated conditions when water content is fixed (3.3% H2O) in melt the viscosity considerably decreases with pressure and takes intermediate value between those in dry and water saturated conditions. Experimental data recently obtained permit us to consider the peculiarities of physical properties of magma in the presence of water on a new base. Ascending magma can reach critical velocities of transition to the turbulent regime under negligible pressure drop, as a result of low viscosity. It is known at present that water influences on the viscosity of acidic melt under pressure of 1–8 kbars and at temperatures between 800–1200°C. Various authors gave physico-chemical evaluation of the dynamics of granite melts on the basis of these data. The viscosity of basalt melts and their dynamics under normal pressure is also well-known. The known new experimental data of basaltic melt viscosity under pressure in dry conditions (Kushiro et al., 1976;Khitarov et al., 1978) and in the presence of water (Khitarov et al., 1976) embrace broader intervals of physico-chemical conditions as on the pressure (up to 20–30 kbar) as well on the content of water (from 3% up to 12 %). These data permitted to evaluate on a new base the dynamics of magmatic melts under pressure. 相似文献
17.
To investigate the influence of microlites on lava flow rheology, the viscosity of natural microlite-bearing rhyolitic obsidians
of calc-alkaline and peralkaline compositions containing 0.1–0.4 wt.% water was measured at volcanologically relevant temperatures
(650–950 °C), stresses (103–105 Pa) and strain rates (10–5 to 10–7 s–1). The glass transition temperatures (T
g
) were determined from scanning calorimetric measurements on the melts for a range of cooling/heating rates. Based on the
equivalence of enthalpic (calorimetric) and shear (viscosity) relaxation, we calculated the viscosity of the melt in crystal-bearing
samples from the T
g
data. The difference between the calculated viscosity of the melt phase and the measured viscosity for the crystal-bearing
samples is interpreted to be the physical effect of microlites on the measured viscosity. The effect of <5 vol.% rod-like
microlites on the melt rheology is negligible. Microlite-rich and microlite-poor samples from the same lava flow and with
identical bulk chemistry show a difference of 0.6 log10 units viscosity (Pa s), interpreted to be due to differences in melt chemistry caused by the presence of microlites. The
only major differences between measured and calculated viscosities were for two samples: a calc-alkaline rhyolite with 1 vol.%
branching crystals, and a peralkaline rhyolite containing crystal-rich bands with >45 vol.% crystals. For both of these samples
a connectivity factor is apparent, with, for the latter, a close packing framework of crystals which is interpreted to influence
the apparent viscosity.
Received: 14 March 1996 / Accepted: 30 May 1996 相似文献
18.
Apatite and phosphorus in mantle source regions: An experimental study of apatite/melt equilibria at pressures to 25 kbar 总被引：1，自引：0，他引：1
E. Bruce Watson 《Earth and Planetary Science Letters》1980,51(2):322-335
The solubility of fluorapatite in a wide variety of basic magmatic liquids was experimentally determined over a range of upper mantle P-T conditions (8–25 kbar, 1275–1350°C). Fluorapatite is stable over the entire range of conditions investigated, but its solubility in melts is variable, depending negatively on SiO2 content of the melt and positively upon temperature, with relatively little sensitivity to pressure above 8 kbar. At upper mantle pressures and a temperature of 1250°C, molten basalt (50% SiO2) will dissolve 3–4 wt.% P2O5 before saturation in apatite is reached. For a magma 100°C cooler or containing 10 wt.% more SiO2, apatite saturation occurs at less than 2 wt.% dissolved P2O5. The observed high solubility of apatite in basic magmas at their normal near-liquidus temperatures virtually precludes the occurrence of residual apatite in mantle source regions. If relatively low-temperature melting conditions prevail (e.g., 1100°C), as might be possible in H2O-bearing regions of the upper mantle, apatite could remain in the residue, but only in amounts too small to have significant effects on the rare earth patterns of the liquids.Because of the high solubility of apatite in basic magmas, phosphorus can be confidently treated as an incompatible element in peridotite melting models. Such models, in combination with observed characteristics of basic lavas, indicate that the upper mantle contains ～200 ppm of phosphorus, much less than the chondritic abundance of ～900 ppm. 相似文献
19.
Brian W. Robinson 《Earth and Planetary Science Letters》1973,18(3):443-450
Studies of the sulphur hydrolysis reaction, 4S + 4H2O /ag 3H2S + HSO4? + H+, were conducted between 200 and 320°C in sealed silica glass tubes. The isotope exchange reaction: H218O + HS16O4? /ag H216O + HS18O4? is so rapid at the low pH (1.5–3) as to be unquenchable. However, the sulphur isotope exchange reaction: H234S+ H32SO4? /ag H232S + H34SO4? gave t12 values of 0.1, 0.3 and 1.7 days at 320, 260 and 200°C respectively and equilibrium H2S - HSO4? sulphur isotope fractionation values of 20.9, 22.4, 24.8, 26.7 and 29.3‰ at 320, 290, 260, 230 and 200°C respectively. This latter data is represented by: 1000lnα(HSO4??H2S) = 5.07 (106T?2) + 6.33, and has valuable applications in geothermal and ore deposit studies. 相似文献
20.
James E. Gardner Kenneth S. Befus James Watkins Marc Hesse Nathan Miller 《Bulletin of Volcanology》2012,74(8):1865-1879
Spherical masses of crystal fibers (spherulites) crystalize from rhyolitic melt/glass mainly in response to significant undercooling while lava cools. Spherulite growth should induce compositional gradients in the surrounding glass from expulsion of incompatible constituents and diffusion of those constituents away from the spherulite. Finite-difference numerical modeling of one-dimensional diffusion, in which diffusivities are allowed to vary with temperature, is used to investigate how compositional gradients reflect spherulite growth and lava cooling. Overall, three forms of gradients are identified. Elements that diffuse quickly are expelled from the spherulite but then migrate away too quickly to become enriched at the boundary of the spherulite. Elements that diffuse slowly are trapped within the growing spherulite. Between those endmembers are elements that are not trapped, yet diffuse slow enough that they become enriched at the contact. Their slow diffusion away then elevates their concentrations in the surrounding glass. How enriched those elements are at the spherulite-matrix interface and how far their enrichments extend outwards into the glass reflect how spherulites grow and thermal conditions during growth. Concentrations of H2O, Rb, F, Li, Cl, Na, K, Sr, Cs, Ba, and Be were measured in and around spherulites in obsidian from a 4.7?±?1?km3 rhyolite lava dome erupted from Tequila volcano, Mexico. Measurable concentration gradients are found for H2O, Rb, and F. Attributes of those gradients and the behaviors of the other elements are in accord with their experimentally constrained diffusivities. Spherulites appear to have grown following radial, rather than volumetric, growth. The observed gradients (and lack of others) are more consistent with growth mainly below the glass transition, which would necessitate the dome cooling at ca. 10?5 to 10?7?°C?s?1. Such slow cooling is consistent with the relatively large volume of the dome. 相似文献