共查询到20条相似文献，搜索用时 281 毫秒
1.
通过野外观察,结合薄片、铸体薄片等手段研究,发现海拉尔盆地呼伦湖凹陷西界上库力组火山通道相次火山亚相的柱状节理流纹岩和喷溢相上部亚相的球粒流纹岩中发育两种不同结构的球粒,两种球粒均形成于高的过冷却动力学效应下。柱状节理流纹岩中的球粒肉眼不可见,球粒直径小于0. 2 mm,球粒中心发育结晶核,组成球粒的纤维体多为隐晶质。条带状球粒流纹岩中的球粒在野外露头上醒目,圆形、椭圆形特征明显,球粒直径在0. 5～2 mm之间,均匀分布,结晶核不明显,组成球粒的纤维体为霏细质。本文认为导致这两种球粒结构差异性的主要原因是受熔浆过冷度大小的影响,柱状节理流纹岩过冷度较条带状球粒流纹岩大,因此成核速率高,成核密度大,球粒直径小,排列紧密,条带状球粒流纹岩较之相反。同时,流纹质熔浆的均质性程度影响了球粒间孔的充填特征。此外,研究发现,球粒间孔的发育程度和球粒形态直接相关。柱状节理流纹岩基质中的球粒结构紧密排列,球粒受相互生长制约呈不规则状,由于柱状节理本身由十分致密的球粒构成,储集空间不发育,因此不含油。由于条带状球粒流纹岩的球粒排列相对疏松,孔隙大量发育,可能含有丰富的油气,储集空间主要类型为球粒间孔和球粒溶蚀孔。 相似文献
2.
P. L. Clay B. O’Driscoll R. Gertisser H. Busemann S. C. Sherlock S. P. Kelley 《Contributions to Mineralogy and Petrology》2013,165(2):373-395
Spherulitic textures in the Rocche Rosse obsidian flow (Lipari, Aeolian Islands, Italy) have been characterized through petrographic, crystal size distribution (CSD) and in situ major and volatile elemental analyses to assess the mode, temperature and timescales of spherulite formation. Bulk glass chemistry and spherulite chemistry analyzed along transects across the spherulite growth front/glass boundary reveal major-oxide and volatile (H2O, CO2, F, Cl and S) chemical variations and heterogeneities at a ≤5 μm scale. Numerous bulk volatile data in non-vesicular glass (spatially removed from spherulitic textures) reveal homogenous distributions of volatile concentrations: H2O (0.089 ± 0.012 wt%), F (950 ± 40 ppm) and Cl (4,100 ± 330 ppm), with CO2 and S consistently below detection limits suggesting either complete degassing of these volatiles or an originally volatile-poor melt. Volatile concentrations across the spherulite boundary and within the spherulitic textures are highly variable. These observations are consistent with diffusive expulsion of volatiles into melt, leaving a volatile-poor rim advancing ahead of anhydrous crystallite growth, which is envisaged to have had a pronounced effect on spherulite crystallization dynamics. Argon concentrations dissolved in the glass and spherulites differ by a factor of ~20, with Ar sequestered preferentially in the glass phase. Petrographic observation, CSD analysis, volatile and Ar data as well as diffusion modeling support continuous spherulite nucleation and growth starting at magmatic (emplacement) temperatures of ~790–825 °C and progressing through the glass transition temperature range (T g ~ 750–620 °C), being further modified in the solid state. We propose that nucleation and growth rate are isothermally constant, but vary between differing stages of spherulite growth with continued cooling from magmatic temperatures, such that there is an evolution from a high to a low rate of crystallization and low to high crystal nucleation. Based on the diffusion of H2O across these temperature ranges (~800–300 °C), timescales of spherulite crystallization occur on a timescale of ~4 days with further modification up to ~400 years (growth is prohibitively slow <400 °C and would become diffusion reliant). Selective deformation of spherulites supports a down-temperature continuum of spherulite formation in the Rocche Rosse obsidian; indeed, petrographic evidence suggests that high-strain zones may have catalyzed progressive nucleation and growth of further generations of spherulites during syn- and post-emplacement cooling. 相似文献
3.
变质岩中的球状结构及其对变质作用的指示 总被引：1，自引：1，他引：0
球状结构是指由纤维状晶体组成的放射状集合体,其为一种高度不平衡的结构。它们常见于火山岩、假玄武玻璃、沉积岩以及热液沉积物中,在不同变质级别的变质岩中也有广泛报道。变质岩中的球状结构记录了丰富的变质作用信息,然而其在变质岩中的成因机制和对变质作用的指示意义尚缺少详细的研究。球状结构的生长需要温度、压力或成分的改变使得结晶体系远离平衡,它的结晶动力学过程受物质的扩散控制,要求晶体生长速率远大于物质扩散速率。详细解析球状结构和分析导致不平衡的因素有助于限定变质作用的条件和过程。本文拟从变质岩中报道的代表性球状结构出发,基于球状结构结晶所需的热力学和动力学条件,总结变质岩中球状结构的四种可能成因机制:流体结晶、变质熔体结晶、冲击变质作用和高度不平衡的变质反应。组成球状结构的纤维状晶体具有大的表面能,在持续保持温压和流体条件不变的情况下会快速重结晶使得球状结构消失,球状结构得以保留意味着其所处的环境要迅速转变为它不再结晶的条件。因此,变质岩中的球状结构指示寄主岩石经历了持续时间很短的非平衡变质作用过程。借鉴其它学科研究球状结构的定量方法和在变质作用条件下开展球状结构的实验研究是变质岩中球状结构研究的潜在方向。 相似文献
4.
Mizuhiko Akizuki 《Lithos》1983,16(4):249-254
Spherulites consisting of fibrous alkali feldspar and silica minerals are produced by devitrification of rhyolite glass under hydrothermal conditions. The alkali feldspars (Ab72.5Or23.0An4.5, Ab81.7Or14.0An4.3) in spherulites from two localities in Japan consist of triclinic anorthoclase showing fine cross-hatched twinning and monoclinic sanidine showing fine cross-hatching not attributable to twinning. The cross-hatching, which corresponds to albite and pericline twinning, is produced in the process of transition from a monoclinic to a triclinic phase. The spherulite may develop at a temperature lower than about 200°C because the co-existing silica mineral is not quartz, but metastable tridymite. According to the phase diagram of the alkali feldspars by MacKenzie (1952), the alkali feldspars should have been triclinic during growth. However, the textures show that the alkali feldspar grew as a disordered monoclinic phase. Because of the high growth rate, the Al/Si disordered structure was produced during growth and afterwards transformed into a triclinic structure with cross-hatched twinning. 相似文献
5.
6.
Relaxation geospeedometry using differential scanning calorimetry (DSC) has been applied to quantify the cooling history across the glass transition of flow ramps at the front of the calc-alkaline rhyolite Rocche Rosse flow of Lipari, Aeolian Islands, Italy. Modelled cooling rates for the obsidian retrieved from two profiles range between 0.2 and 0.03 K min–1 . Cooling at the flow front appears to be dominated by conductive heat loss of individual flow ramps forming individual cooling units. Cooling rates of tens of Kelvins per day appear to have controlled the last stage of viscous deformation before the entire flow came to rest. It is inferred that cooling rates slower than those modelled are required to sustain flow in highly viscous rhyolitic lavas. 相似文献
7.
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:
where is the water diffusivity at 1 wt% H2O
t
in m2/s, T is the temperature in K and P is the pressure in MPa. The above equation reproduces the experimental data (14 runs in total) with a standard fit error
of 0.15 log units. It can be employed to model degassing of peralkaline melts at water contents up to 2 wt%. 相似文献
8.
We have analysed the kinetics of Argon and CO2 diffusion in simplified iron free rhyolitic to hawaiitic melts using the diffusion couple technique. The concentration distance profiles of Ar and CO2 were measured with electron microprobe analysis and Fourier Transform Infrared Spectroscopy, respectively. Error functions were fitted to the symmetrical concentration distance profiles to extract the diffusion coefficients.In the temperature range 1373 to 1773 K the activation energies for Ar diffusion range from 169 ± 20 to 257 ± 62 kJ mol−1. Ar diffusivity increases exponentially with the degree of depolymerisation. In contrast, the mobility of total CO2, that is identical to Ar mobility in rhyolitic melt, keeps constant with changing bulk composition from rhyolite to hawaiite. CO2 speciation at 1623 K and 500 MPa was modeled for the range of compositions studied using the diffusion data of Ar and total CO2 in combination with network former diffusion calculated from viscosity data. Within error this model is in excellent agreement with CO2 speciation data extrapolated from temperatures near the glass transition temperature for dacitic melt composition. This model shows that even in highly depolymerised hawaiitic and tholeiitic melts molecular CO2 is a stable species and contributes 70 to 80% to the total CO2 diffusion, respectively. 相似文献
9.
The average local structure of a rhyolitic composition glass has been determined at 25°C using X-ray radial distribution analysis (RDA) and quasi-crystalline modelling and is best described as similar to that in a stuffed framework composed principally of six-membered rings of Si and Al tetrahedra (basically a stuffed tridymite-like model). Using this model it is possible to calculate a density (2.41 g/cm3) which compares well with the measured density (2.40 g/cm3); a structural model based on four-membered rings (an albite-like model) results in a substantially higher calculated density (2.60 g/cm3). We suggest that the rhyolite glass structural model is appropriate for rhyolitic melts, based on evidence from the recent literature. New viscosity data for an anhydrous rhyolite composition measured between 1200°C and 1500°C are presented and interpreted in terms of our proposed structural model and previous melt structure models for the major normative components of rhyolite. A mechanism for diffusion and viscous flow in framework silicate melts (including rhyolite composition) is proposed on the basis of recent molecular orbital calculations and molecular dynamics simulations of silicate and fluoride melts. 相似文献
10.
Daniel R. Neuville Philippe Courtial Donald B. Dingwell Pascal Richet 《Contributions to Mineralogy and Petrology》1993,113(4):572-581
The heat capacities of a rhyolite and an andesite glass and liquid have been investigated from relative-enthalpy measurements made between 400 and 1800 K. For the glass phases, the experimental data agree with empirical models of calculation of the heat capacity. For the liquid phases, the agreement is less good owing to strong interactions between alkali metals and aluminum, which are not currently accounted for by empirical heat capacity models. The viscosity of both liquids has been measured from the glass transition to 1800 K. The temperature dependence of the viscosity is quantitatively related to the configurational heat capacity (determined calorimetrically) through the configurational entropy theory of relaxation processes. For both rhyolite and andesite melts, the heat capacity and viscosity do not differ markedly from those obtained by additive modeling from components with mineral compositions. 相似文献
11.
Results are reported from an experimental study in which the partitioning of U and Mg between aragonite and an aqueous solution were determined as a function of crystal growth rate. Crystals, identified as aragonite by X-ray diffractometry and micro-Raman spectroscopy, were grown by diffusion of CO2 from an ammonium carbonate source into a calcium-bearing solution at temperatures of 22 and 53 °C. Hemispherical bundles (spherulites) of aragonite crystals were produced, the growth rates of which decreased monotonically from the spherulite interiors to the edges and thus provide the opportunity to examine the influence of growth rate on crystal composition. Element concentration ratios were measured using electron microprobe (EMP) and fluid composition was determined by inductively coupled plasma-mass spectrometry (ICP-MS) and atomic absorption (AA). Growth rates were determined directly by addition of a Dy spike to the fluid during the experiment that was subsequently located in an experimentally precipitated spherulite using secondary ion mass spectrometry (SIMS). At 22 °C both U/Ca and Mg/Ca partition coefficients exhibited a strong growth rate dependence when crystal growth rates were low, and became independent of growth rate when crystal growth rates were high. The U/Ca ratios in aragonite increase between 22 and 53 °C; in contrast Mg/Ca ratios show inverse dependence on temperature. 相似文献
12.
新西兰北岛Waiotapu地热区位于Taupo火山带中部,以发育众多的第四纪酸性火山岩而闻名。通过 X衍射、电子探针、扫描电镜、激光拉曼等方法对区内一处于活动间歇期的小型火山口堆积物岩矿特征及特殊结构进行研究。结果表明,火山口堆积物可分为3类:1)球粒流纹岩,矿物组成为:α方石英-PO鳞石英—正长石、奥长石、紫苏辉石—钛铁矿(钛磁铁矿);2)流纹质晶屑熔结凝灰岩,矿物组成为:奥长石晶屑、塑性玻屑;3)硫磺土,矿物组成为:单质硫、长英质细粒岩屑。结合前人实验矿物学结论后认为,球粒流纹岩中的球粒体为富火山玻璃的流纹岩喷出堆积后在热水作用下脱玻化形成的,是酸性火山岩早成岩期表生蚀变阶段的标志;而流纹质熔结凝灰岩则为岩浆喷溢出地表后,在塑性流动中冷凝结晶形成的,是酸性火山岩早成岩期冷凝固结阶段的标志。在此基础上,提出了Waiotapu地热区酸性火山岩的表生蚀变模式,为酸性火山岩冷凝固结—表生蚀变阶段的岩矿特征及蚀变作用研究提供现代对比实例。 相似文献
13.
Huaiwei Ni Hans Keppler M. A. G. M. Manthilake Tomoo Katsura 《Contributions to Mineralogy and Petrology》2011,162(3):501-513
We report the first study of electrical conductivities of silicate melts at very high pressures (up to 10 GPa) and temperatures
(up to 2,173 K). Impedance spectroscopy was applied to dry and hydrous albite (NaAlSi3O8) glasses and liquids (with 0.02–5.7 wt% H2O) at 473–1,773 K and 0.9–1.8 GPa in a piston-cylinder apparatus, using a coaxial cylindrical setup. Measurements were also
taken at 473–2,173 K and 6–10 GPa in two multianvil presses, using simple plate geometry. The electrical conductivity of albite
melts is found to increase with temperature and water content but to decrease with pressure. However, at 6 GPa, conductivity
increases rapidly with temperature above 1,773 K, so that at temperatures beyond 2,200 K, conductivity may actually increase
with pressure. Moreover, the effect of water in enhancing conductivity appears to be more pronounced at 6 GPa than at 1.8 GPa.
These observations suggest that smaller fractions of partial melt than previously assumed may be sufficient to explain anomalously
high conductivities, such as in the asthenosphere. For dry melt at 1.8 GPa, the activation energy at T > 1,073 K is higher than that at T < 1,073 K, and the inflection point coincides with the rheological glass transition. Upon heating at 6–10 GPa, dry albite
glass often shows a conductivity depression starting from ~1,173 K (due to crystallization), followed by rapid conductivity
enhancement when temperature approaches the albite liquidus. For hydrous melts at 0.9–1.8 GPa, the activation energies for
conductivity at ≥1,373 K are lower than those at <973 K, with a complex transition pattern in between. Electrical conductivity
and previously reported Na diffusivity in albite melt are consistent with the Nernst–Einstein relation, suggesting the dominance
of Na transport for electrical conduction in albite melts. 相似文献
14.
E. S. Persikov P. G. Bukhtiyarov A. N. Nekrasov G. V. Bondarenko 《Geochemistry International》2014,52(5):365-371
The diffusion of water in natural obsidian and model dacitic melts (Ab90Di8Wo2, mol %) has been studied at water vapor pressure up to 170 MPa, temperatures of 1200°C, H2O contents in melts up to ～6 wt % using a high gas pressure apparatus equipped with a unique internal device. The experiments were carried out by a new low-gradient technique with application of diffusion hydration of a melt from fluid phase. The water solubility in the melts and its concentration along $C_{H_2 O} $ diffusion profiles were determined using IR microspectrometry by application of the modified Bouguer-Beer-Lambert equation. A structural-chemical model was proposed to calculate and predict the concentration dependence of molar absorption coefficients of the hydroxyl groups (OH?) and water molecules (H2O) in acid polymerized glasses (quenched melts) in the obsidian-dacite series. The water diffusion coefficients $D_{H_2 O} $ were obtained by the mathematical analysis of concentration profiles and the analytical solution of the second Fick diffusion law using the Boltzman-Matano method. It was shown experimentally that $D_{H_2 O} $ exponentially increases with a growth of water concentration in the obsidian and dacitic melts within the entire range of diffusion profiles. Based on the established quantitative correlation between $D_{H_2 O} $ and viscosity of such melts, a new method was developed to predict and calculate the concentration, temperature, and pressure dependences of $D_{H_2 O} $ in acid magmatic melts (obsidian, rhyolite, albite, granite, dacite) at crustal T, P parameters and water concentrations up to 6 wt %. 相似文献
15.
黄羊山矿床是最近在新疆发现的一个超大型晶质石墨矿床,预测晶质石墨矿物量至少为72.64 Mt。该矿床赋存于花岗岩内,90%的石墨呈球粒状构造,球粒直径最高达20 cm,世界罕见。通过钻孔岩芯编录、探槽编录、镜下观察和锆石U-Pb定年,研究了该矿床矿化情况、矿物组合和成岩年代,探讨了矿床成因。研究表明,黄羊山石墨矿床成岩于(306±4)Ma,属晚石炭世。石墨球粒和基质的岩性相同,皆为碱长花岗岩,只是石墨球粒内较为富集黑云母、角闪石和单斜辉石。与石墨伴生的金属矿物主要为磁黄铁矿、黄铜矿、钛铁矿和赤铁矿。由于石墨的强还原性,这些金属矿物多分布于石墨球粒内,形成典型的环带结构。石墨矿化可分为岩浆热液期和热液叠加期2期,前者是主成矿期,形成球粒状和浸染状构造石墨,后者形成脉状构造石墨。石墨晶体呈片状和胶状结构,片状石墨横截面呈针状,定向性明显。石墨矿石的全岩碳同位素呈负低值,表明构成石墨的碳来自地层有机物。岩浆在上侵过程中同化混染了地层有机物,在岩浆演化晚期熔体相与流体相分离时,碳质溶入流体相中,当温度和压力降低时石墨从岩浆热液中沉淀成矿。中粒钠铁闪石花岗岩、细粒黑云母花岗岩和中粒黑云母花岗岩中皆含石墨球粒,黄羊山岩体仍具有巨大找矿潜力。 相似文献
16.
Deformation mechanism maps for feldspar rocks 总被引：6，自引：0，他引：6
Deformation mechanism maps for feldspar rocks were constructed based on recently published constitutive laws for dislocation and grain boundary diffusion creep of wet and dry plagioclase aggregates. The maps display constant temperature contours in stress-grain size space for strain rates ranging from 10−16 to 10−12 s−1.Two fields of dominance of grain boundary diffusion-controlled creep and dislocation creep are separated by a strongly grain size-sensitive transition zone. For wet rocks, diffusion-controlled creep dominates below a grain size of about 0.1–1 mm, depending on temperature, stress, strain rate and feldspar composition. Plagioclase aggregates containing up to 0.3 wt.% water as often found in natural feldspars are more than 2 orders of magnitude weaker than dry rocks. The strength of water-bearing feldspar rocks is moderately dependent on composition and water fugacity.For a grain size range of about 10–50 μm commonly observed in natural ultramylonites, the deformation maps predict that diffusion-controlled creep is dominant at greenschist to granulite facies conditions. Low viscosity estimates of 1018–1019 Pa·s from modeling postseismic stress relaxation and channel flow of the continental lower crust can only be reconciled with laboratory experiments assuming dislocation creep at high temperatures >900 °C or, at lower temperatures, diffusion creep of fine-grained rocks possibly localized in abundant high strain shear zones. For similar thermodynamic conditions and grain size, lower crustal rocks are predicted to be less than order of magnitude weaker than upper mantle rocks. 相似文献
17.
Rate laws in metamorphism 总被引：1，自引：0，他引：1
George W. Fisher 《Geochimica et cosmochimica acta》1978,42(7):1035-1050
Once a new mineral has nucleated in a metamorphic rock, further growth generally involves three mechanisms, operating simultaneously: (1) diffusion of components; (2) supply of heat needed for chemical reactions; and (3) precipitation on the nucleus. The resulting structure can grow no faster than is permitted by the slowest mechanism, the rate-determining step. During growth, the relative rates of these mechanisms change systematically, so that metamorphic structures pass through a definite sequence of stages, each characterized by a different rate-determining step. Spherical structures tend to pass through three successive stages: (1) an initial reaction-controlled stage, in which growth is linearly proportional to time; (2) an intermediate diffusion-controlled stage, in which growth is proportional to the square root of time; and (3) a final heat-flow-controlled stage, with growth proportional to the cube root of time. Planar reaction zones follow a two-stage sequence beginning with either a reaction-controlled or a heat-flow-controlled stage, both characterized by linear growth laws; and culminating with a diffusion-controlled stage, in which growth is proportional to the square root of time.Estimated values of the kinetic coefficients governing each of these steps suggest that in most metamorphic processes the reaction-controlled stage will end long before the structure is large enough to detect, and that most growth will occur by either a heat-flow-controlled or diffusion-controlled mechanism. Increasing distance between the initial nuclei, increasing grain size, decreasing diffusion coefficients, and increasing heat flow rates all tend to favor diffusion control over heat-flow control. The rate-determining step can be expected to vary both with local and regional changes in these variables; and attempts to delineate domains in which different mechanisms constitute the ratedetermining step may provide valuable insight into the scale on which these parameters vary in metamorphism. 相似文献
18.
Yu. L. Gulbin 《Geology of Ore Deposits》2013,55(8):625-636
Results from the modeling of compositional zoning patterns in garnet porphyroblasts from the medium-grade metapelitic schist of northern Ladoga area are considered. The P-T pseudosections in the model KMnFMASH system were calculated for this purpose using THERMOCALC software (Powell et al., 1998). Particular emphasis is placed upon the effect of garnet growth kinetics on the model zoning profiles for Mn (Gulbin, 2013). They fit the observed profiles if intergranular diffusion-controlled growth is assumed for porphyroblasts. Additionally, a model of metamorphic fractional crystallization is used to characterize the oscillations in both the garnet core and rim. Starting from the assumption that a reservoir, where garnet grows, consists of chlorite, and that this mineral is intensely replaced with biotite and staurolite at the onset of crystallization, a partial release of Mn from the chlorite structure and the concentration of this component in intergranular space is inferred. In terms of the model under consideration, the coefficient of the Mn partition between garnet and reservoir temporarily increases at the early stage of garnet growth, giving rise to the enrichment of the intermediate zone of porphyroblasts in Mn. In addition to the modeling of garnet growth zoning, its subsequent diffusion modification is estimated on the basis of intracrystalline diffusion profile simulation. The reverse zoned, Mn-rich and Mg-poor garnet rims are related to retrograde growth of garnet at the late stage of porphyroblast formation. The data obtained are used to constrain metamorphic evolution and the P-T-t path of staurolite-bearing rocks in the northern domain of the studied area. 相似文献
19.
James M. Watkins Donald J. DePaolo Christian Huber 《Geochimica et cosmochimica acta》2009,73(24):7341-3118
Liquid phase diffusion experiments were carried out to determine whether diffusive isotopic fractionation of a major chemical element (Ca) varies with chemical composition in high-temperature molten silicates. The objective was to determine how differences in silicate liquid structure, such as the ratio of bridging to non-bridging oxygen atoms, as well as bulk transport properties such as viscosity, relate to isotope discrimination during diffusion. This information, in turn, may relate to the lifetimes and sizes of multi-atom structures in the liquid. Diffusion couples consisting of juxtaposed natural mafic and felsic liquids were held at T = 1450 °C and P = 1.0 GPa for durations of 12-24 h in a standard piston-cylinder assembly. Experiments were done using different mafic endmember compositions (two tholeiitic basalts and a ugandite) and a single rhyolite composition. Major-element diffusion profiles and Ca isotope profiles were measured on the recovered quenched glasses. The starting materials were isotopically indistinguishable, but 44Ca/40Ca variations of ca. 5‰ arose due to a mass dependence of the Ca diffusion coefficients. Results indicate that the mass dependence of Ca diffusion coefficients varies with the magnitude and direction of aluminum gradients and the viscosity of the liquid. Some Ca fractionations result mainly from Al gradients.A simplified multicomponent diffusion model was used to model the experimental results. The model allows for diffusion of Ca in response to gradients in the concentrations of both CaO as well as Al2O3, and the model results are consistent with the inferred existence of at least two distinct species of Ca. The magnitude of isotopic discrimination during diffusion also appears to be stronger on the rhyolite versus the basalt/ugandite side of diffusion couples. The results can largely be accounted for by an adaptation of the model of Dingwell (1990), whereby in high silica liquids, Ca diffuses largely by site hopping through a quasi-stationary aluminosilicate matrix, producing strong isotopic effects because the Ca diffusion is not strongly correlated with the movement of the framework atoms. In low-silica liquids, Ca diffusion is correlated with the movement of the other components and there is less mass discrimination. Combining our Ca results with Ca, Mg, and Li data from previous studies, we show that this model can explain most of the cation- and composition-dependence of diffusive isotopic fractionations observed thus far. A key parameter controlling isotopic discrimination is the ratio of the elemental (Ca, Mg, Li) diffusivity to the Eyring (or Si) diffusivity. However, all experiments done so far also exhibit isotopic features that are not yet fully explained; some of these may relate to small temperature gradients in the capsules, or to more complex coupling effects that are not captured in simplified diffusion models. 相似文献
20.
Diffusion rates for sulfur in rhyolite melt have been measured at temperatures of 800–1100° C, water contents of 0–7.3 wt%,
and oxygen fugacities from the quartz-fayalite-magnetite buffer to air. Experiments involved dissolution of anhydrite or pyrrhotite
into rhyolite melt over time scales of hours to days. Electron microprobe analysis was used to measure sulfur concentration
profiles in the quenched glasses. Regression of the diffusion data in dry rhyolite melt gives Dsulfur=0.05·exp{−221±80RT}, which is one to two orders of magnitude slower than diffusion of other common magmatic volatiles such as H2O, CO2 and Cl-. Diffusion of sulfur in melt with 7 wt% dissolved water is 1.5 to 2 orders of magnitude faster than diffusion in the anhydrous
melt, depending on temperature. Sulfur is known to dissolve in silicate melts as at least two different species, S2− and S6+, the proportions of which vary with oxygen fugacity; despite this, oxygen fugacity does not appear to affect sulfur diffusivity
except under extremely oxidizing conditions. This result suggests that diffusion of sulfur is controlled by one species over
a large range in oxygen fugacity. The most likely candidate for the diffusing species is the sulfide ion, S2−. Re-equilibration between S2− and S6+ in oxidized melts must generally be slow compared to S2− diffusion in order to explain the observed results. In a silicic melt undergoing degassing, sulfur will tend to be fractionated
from other volatile species which diffuse more rapidly. This is consistent with analyses of tephra from the 1991 eruption
of Mount Pinatubo, Philippines, and from other high-silica volcanic eruptions.
Received: 26 April 1995 / Accepted: 1 November 1995 相似文献