NaAlSi3O8熔体粒子扩散行为压力效应的分子动力学研究

为了研究压力对钠长石成分熔体粒子微观扩散行为的影响，本文用分子动力学方法，探讨了2001．5K温度下，压力由50MPa上升到19862MPa的过程中，熔体的微观结构、粒子自扩散系数随压力的变化规律。研究表明，在升压过程中，低次配位体Al和Si向5次和6次配位体转变，Al的含量在15GPa达到极大值，而Si含量的极大值在20GPa仍未出现。Na的自扩散系数随压力升高单调下降，Al、Si和O的自扩散系数随压力升高先增后减，在8～10GPa左右达到极大值。网架形成粒子自扩散系数的极大值与Al和Si含量的极大值对应的压力点不一致。所有粒子的自扩散系数与它们与O之间键的寿命呈线性负相关。     

熔融SiO2中原子自扩散性质的分子动力学模拟

研究地幔熔体中元素的扩散性质有着重要的意义,因其影响着元素的交换和分馏过程。SiO2 作为地幔组成的重要组
分之一,其物理化学行为对于地幔动力学过程有着重要的意义。本文研究了SiO2 熔体中元素的扩散机制和自扩散系数与
压力的关系,采用Morse stretch 势场对含有4500 个原子的熔融SiO2 体系进行了分子动力学模拟,计算了硅氧自扩散系数在
3000 K 温度下随压力的变化。模拟结果显示,在0.0001~40 GPa的压力区间,硅氧元素的自扩散系数均先上升后下降,在
17.5 GPa 时达到最大值,O 原子的扩散速率略高于Si 原子。硅氧元素的扩散方式为缺陷控制运移机制,其中硅原子的五配位
结构的形成是关键,为导致扩散系数随压力增大而上升的主要原因,扩散系数的最大值意味着SiO2 熔体中5 配位硅形成机
制的改变。本文也计算了单位［SiO2］的平均体积和压力的关系,结果与实验很吻合。     

高温高压下闪长岩的相变与纵波波速

利用脉冲透射-反射法,在YJ-3000 t高压装置弹性波速测量系统上,测量了0.6、1.0和2.0 GPa,最高1141℃条件下闪长岩的纵波波速(vp)。结果显示,高压下闪长岩的vp随温度升高首先缓慢降低,分别到769℃(0.6 GPa)、810℃(1.0 GPa)和925℃(2.0 GPa)后转而快速下降。实验产物观测显示,0.6 GPa下岩石在758℃时发生脱水熔融并有新生单斜辉石生成,1.0 GPa和2.0 GPa下,闪长岩分别在865℃和921℃的实验产物中出现熔体,新生矿物有单斜辉石和石榴子石。温度升高导致闪长岩中熔体含量增加,斜长石、角闪石和绿泥石等逐渐减少直至消失,单斜辉石和石榴子石呈先增加后减少趋势。探针分析显示,熔体含水量较高,且随温度升高熔体成分向基性方向演化。单斜辉石化学成分变化不明显,2.0 GPa下,随温度从1030℃升高到1138℃,新生石榴子石成分逐渐向钙铝榴石变化。vp变化和熔体含量关系表明,熔体含量增加导致了闪长岩在高温阶段波速的持续快速降低。     

一个标准大气压下含铁和铝的钙硅酸盐体的粘度和结构

CaO-SiO_2-Fe-O体系熔馆体的温度-粘度关系已经确定,其数值可与那些组成中代替Si~(4十)的是Al~(3+)而不是Fe~(3+)的类似熔体对比.温度在液相线之上时,含铁熔体粘度的变化范围在5和15泊之间,作为Ca/Si、铁含量和温度(1400—1600℃)的函数,这些熔体粘滞流的活化能在8—50千卡/摩尔之间.与无铁熔体相比,当加入5%(重量)Fe_2O_3时,含铁熔体的粘度值明显增加,如继续加入三价铁就会引起这一趋势的逆转.恒温时,粘滞流的活化能随Fe~(3+)含量的增加而减小,当铁含量固定时,活化能则随温度的增加而减小.而已发表的CaO-Al_2O_3-SiO_2体系熔体的粘度数据表明,随Al_2O_3含量的增加,其粘度值持续增加.这些     

Fe-Mg-Ca在橄揽石和熔体之间的分配及交换平衡

本文通过实验研究了Fe－Mg－Ca在橄榄石和熔体之间的分配及交换平衡，且从热力学角度对其结果进行了理论解释。实验结果显示，Fe和Mg在橄榄石和熔体之间的分配系数随温度的升高而减小，而Ca在它们之间的分配不受温度的影响；Fe－Mg、Ca－Mg及Ca－Fe在橄榄石－熔体之间的交换系数随温度的升高而增大；压力（＜o．10GPa）对以上分配及交换系数的影响可忽略不计。理论解释表明，各分配及交换系数亦随馆体中SiO2的活度增大而增大；由于温度、压力和熔体成分三者之间相互关联，Fe－Mg－Ca在橄榄石和熔体之间的分配及交换系数与温度、压力和熔体成分的关系其实是三者作用的综合效应结果；在不同的研究中，温度、压力及格体成分的范围不同，即各因素作用的程度不一致，且温度和压力与熔体成分的关系不一样，它们作用的综合效应也就有了差别。因此，在各研究之间，Fe－Mg－Ca在橄榄石－熔体之间分配和交换系数的大小，以及它们与温度、压力，或熔体成分的关系，即有吻合也有相左。     

高温高压下斜长角闪岩的Vp:熔体含量和形态的综合影响

在YJ3000t高压装置上,利用超声波脉冲透射-反射法测量恒压0.6GPa,1.0GPa和2.0GPa,室温至1 195℃条件下斜长角闪岩的纵波波速(Vp),统计了实验中间产物中各组分的体积百分含量和熔体形态(二面角),并根据主要矿物含量和弹性参数,利用VRH平均模型计算了高温高压下斜长角闪岩的Vp。结果显示,不同压力下,样品的Vp随温度升高首先缓慢降低,在温度达约850℃～950℃时转而快速下降。实验产物观测显示,随温度升高熔体含量显著增加,二面角不断减小,熔体由封闭囊状演变为连通薄膜,部分熔融是导致岩石Vp快速降低的主要因素。高温高压下Vp计算结果与测量结果有相同的Vp-T变化趋势,其对比研究表明,岩石初始熔融时,熔体尚未连通,此时熔体含量控制着岩石Vp的降低。部分熔融加剧导致熔体逐渐连通,此时不同压力下熔体导致Vp下降有差异,这可能与熔体连通过程中熔体薄膜的形态因子变化有关。     

950℃、1.0～3.5GPa下斜长角闪岩脱水熔融——压力和时间对结构的影响

利用YJ-3000t和JL-3600t多顶砧压力机,以哀牢山造山带南部红河县大白能—乐育剖面上的天然块状斜长角闪岩为初始样品,在950℃、1.0～3.5GPa、恒温20～300h条件下进行了两个系列的斜长角闪岩块状样品脱水部分熔融实验:(1)保持温度T=950℃,加热时间t=100h不变,改变压力(1.0～3.5GPa)的实验;(2)保持温度T=950℃,压力p=3.0GPa不变,改变加热时间(20～300h)的实验。结果表明,1.0～3.5GPa、950℃、恒温100h的条件下,随压力升高,斜长角闪岩中依次生成了单斜辉石+石榴石+熔体的矿物组合(1.0～1.5GPa)和单斜辉石+石榴石+熔体+硬玉+SiO2矿物+蓝晶石(2.0～3.5GPa)的矿物组合。3.0GPa、950℃条件下,随加热时间增加,实验产物中依次生成了单斜辉石+石榴石+熔体+硬玉+SiO2矿物+蓝晶石的矿物组合(20～100h)和单斜辉石+石榴石+熔体的矿物组合(150～300h)。斜长角闪岩的原岩结构决定了实验产物中新生矿物和熔体的分布。依据实验产物的矿物组合和新生矿物的分布特征,讨论了950℃、1.0～3.5GPa、恒温(20～300h)条件下,斜长角闪岩部分熔融过程的结构变化、变质反应以及石榴石冠状体的成因。     

磷在硅酸盐矿物与熔体之间分配系数的研究进展

硅酸盐矿物与熔体之间的磷分配系数对研究岩浆演化和结晶分异程度具有重要意义,也是了解地幔磷储库和建立地球各圈层间磷运移模式的基础。本文分析和总结了前人采用天然样品和合成实验样品研究不同硅酸盐矿物和熔体间的磷分配系数的成果,分析了不同物理化学参数对分配系数的影响,包括熔体组成(如Mg O、Al2O3含量)、矿物结构(分配系数与[Si O4]4-聚合度呈负相关关系)、温度、压力和氧逸度等,指出当前研究中有关更高压力条件(15 GPa)及有流体存在时分配系数的研究是不足的。     

0.6～2.0GPa压力下部分熔融角闪辉长岩的纵波波速

利用超声波透射-反射法,测量了0.6～2.0 GPa、最高1 085℃条件下角闪辉长岩的纵波波速(vp),详细统计了部分熔融阶段实验产物组分的体积百分含量,利用矿物含量和弹性参数,计算了角闪辉长岩的纵波波速.实验测量和理论计算显示了较一致的vp-t关系,即高压下角闪辉长岩的vp随温度升高先缓慢降低,在温度约800～900℃后转而大幅下降.实验产物显示,样品在温度达812℃(0.6 GPa)、865℃(1.0 GPa)和919℃(2.0 GPa)后发生矿物脱水和部分熔融,熔体含量随温度升高显著增加.熔体是导致高温阶段岩石vp快速降低的主要原因.在初熔阶段vp随熔体增加而降低尤为显著,可能是初熔时矿物脱水生成的自由水及含水量高的熔体,以微细熔体薄膜浸润矿物边界或裂隙所导致.     

中酸性硅酸盐熔体-水体系氢同位素分馏的压力效应

对0．2－2000MPa条件下钠长石熔体，钾长石熔体以及0．2－150MPa条件下流纹岩熔体－－水体系的氢同位素分馏实验数据进行了筹压拟合，发现硅酸盐熔体与水之间的氢同位素分馏存在显著的压力效应，在800，1000和1200度条件下对钠长石熔体，水体系和流夺熔体－－水体系氢同位素分馏压力方程进行的等温拟合表明，只有在特定的压力条件下才可以用钠长石熔体－水体系来近似流纹岩熔体－－水体系的氢同位素分馏行为，当压力超过临界值时，硅酸盐熔体－水体系氢同位素分馏会发生变化，本文拟合的硅酸盐熔体－水体系氢同位素分馏等值线在P－T空间的形态变化特征与矿物－水体系存在较大差异，依据流纹岩熔体与水之间氢同位素分馏的压力效应，成功地模拟了美国西部Glass Creek流纹岩δD值和水含量变化规律与岩浆去气之间的关系。     

Viscosity of silicate melts in CaMgSi2O6–NaAlSi2O6 system at high pressure

In situ X-ray viscometry of the silicate melts was carried out at high pressure and at high temperature. The viscosity of the silicate melts in the diopside(Di)–jadeite(Jd) system was determined in the pressure range from 1.88 GPa to 7.9 GPa and in the temperature range from 2,003 K to 2,173 K. The viscosity of the Di 25%–Jd 75% melt decreases continuously to 5.0 GPa, whereas the viscosity of the Di 50%–Jd 50% melt increases over 3.5 GPa. The viscosity of the Di50%–Jd 50% melt reaches a minimum around 3.5 GPa. Since the amounts of silicon in the two melts are the same, the difference in the pressure dependence of the viscosity may be controlled by another network-forming element, i.e., aluminum. The difference in the pressure dependence of the viscosities in the melts with two intermediate compositions in the Di–Jd system is estimated to be due to the difference in the melt structures at high pressures and high temperatures.     

Viscosity of albite melt at high pressure and high temperature

 The viscosity of albite (NaAlSi₃O₈) melt was measured at high pressure by the in situ falling-sphere method using a high-resolution X-ray CCD camera and a large-volume multianvil apparatus installed at SPring-8. This system enabled us to conduct in situ viscosity measurements more accurately than that using the conventional technique at pressures of up to several gigapascals and viscosity in the order of 10⁰ Pa s. The viscosity of albite melt is 5.8 Pa s at 2.6 GPa and 2.2 Pa s at 5.3 GPa and 1973 K. Experiments at 1873 and 1973 K show that the decrease in viscosity continues to 5.3 GPa. The activation energy for viscosity is estimated to be 316(8) kJ mol⁻¹ at 3.3 GPa. Molecular dynamics simulations suggest that a gradual decrease in viscosity of albite melt at high pressure may be explained by structural changes such as an increase in the coordination number of aluminum in the melt. Received: 6 January 2001 / Accepted: 27 August 2001     

北淮阳地区不同变形-变质煤的元素分布及其影响因素

通过对大别造山带前陆北淮阳地区石炭纪高煤级煤的X射线荧光光谱分析,探讨了不同变形-变质类型煤中常量元素的分布规律及其影响因素。结果表明:构造-热作用具有使Al3+、K+、Si4+、Ti4+在煤体中富集,P5+、Fe2++Fe3+、Mn2+Na+、Ca2+、Mg2+向煤体外迁移的趋势;在弱应力带富集Al3+、K+、Ti4+、Na+、Ca2+;在强应力带富集Si4+、P5+、Fe2++Fe3+、Mg2+。岩浆热变质类型煤和构造-热变质类型煤的w(SiO₂)/w(Al₂O₃)和w(K₂O)/w(Na₂O)值显著增大,与岩浆岩中SiO₂及K₂O质量分数偏高有重要的成因联系。      

Diffusion and viscosity of Mg2SiO4 liquid at high pressure from first-principles simulations

We investigate two key transport properties, self-diffusion and viscosity, of Mg₂SiO₄ liquid as a function of temperature and pressure using density functional theory-based molecular dynamics method. Liquid dynamics in a 224-atom supercell was captured in equilibrium simulations of relatively long durations (50-300 ps) to obtain an acceptable convergence. Our results show that Mg and Si are, respectively, the most and least mobile species at most conditions studied and all diffusivities become similar at high pressure. With increasing temperature from 2200 to 6000 K at ambient pressure, the self-diffusivities increase by factors of 25 (Mg), 80 (Si) and 65 (O), and the viscosity decreases by a factor of 30. The predicted temperature variations of all transport coefficients closely follow the Arrhenian law. However, their pressure variations show a significant non-Arrhenian behavior and also are sensitive to temperature. At 3000 K, the diffusivity (viscosity) decreases (increases) by more than one order of magnitude between 0 and 50 GPa with their activation volumes increasing on compression. Over the entire mantle pressure range, the variations at 4000 K are of two orders of magnitude with nearly constant activation volumes whereas the variations at 6000 K are within one order of magnitude with decreasing activation volumes. The predicted complex dynamical behavior of Mg₂SiO₄ liquid can be associated with the structural changes occurring on compression. We also estimate the diffusivity and viscosity profiles along a magma ocean isentrope, which suggest that the melt transport properties vary modestly over the relevant magma ocean depth ranges.     

Pressure and temperature dependence of the viscosity of a NaAlSi<Subscript>2</Subscript>O<Subscript>6</Subscript> melt

The viscosity of a silicate melt of composition NaAlSi₂O₆ was measured at pressures from 1.6 to 5.5 GPa and at temperatures from 1,350 to 1,880°C. We employed in situ falling sphere viscometry using X-ray radiography. We found that the viscosity of the NaAlSi₂O₆ melt decreased with increasing pressure up to 2 GPa. The pressure dependence of viscosity is diminished above 2 GPa. By using the relationship between the logarithm of viscosity and the reciprocal temperature, the activation energies for viscous flow were calculated to be 3.7 ± 0.4 × 10² and 3.7 ± 0.5 × 10² kJ/mol at 2.2 and 2.9 GPa, respectively.     

Energetics of multicomponent diffusion in molten CaO-Al2O3-SiO2

The energetics of multicomponent diffusion in molten CaO-Al₂O₃-SiO₂ (CAS) were examined experimentally at 1440 to 1650°C and 0.5 to 2 GPa. Two melt compositions were investigated: a haplodacitic melt (25 wt.% CaO, 15% Al₂O₃, and 60% SiO₂) and a haplobasaltic melt (35% CaO, 20% Al₂O₃, and 45% SiO₂). Diffusion matrices were measured in a mass-fixed frame of reference with simple oxides as end-member components and Al₂O₃ as a dependent variable. Chemical diffusion in molten CAS shows clear evidence of diffusive coupling among the components. The diffusive flux of SiO₂ is significantly enhanced whenever there is a large CaO gradient that is oriented in a direction opposite to the SiO₂ gradient. This coupling effect is more pronounced in the haplodacitic melt and is likely to be significant in natural magmas of rhyolitic to andesitic compositions. The relative magnitude of coupled chemical diffusion is not very sensitive to changes in temperature and pressure.To a good approximation, the measured diffusion matrices follow well-defined Arrhenius relationships with pressure and reciprocal temperature. Typically, a change in temperature of 100°C results in a relative change in the elements of diffusion matrix of 50 to 100%, whereas a change in pressure of 1 GPa introduces a relative change in elements of diffusion matrix of 4 to 6% for the haplobasalt, and less than 5% for the haplodacite. At a pressure of 1 GPa, the ratios between the major and minor eigenvalues of the diffusion matrix λ₁/λ₂ are not very sensitive to temperature variations, with an average of 5.5 ± 0.2 for the haplobasalt and 3.7 ± 0.6 for the haplodacite. The activation energies for the major and minor eigenvalues of the diffusion matrix are 215 ± 12 and 240 ± 21 kJ mol⁻¹, respectively, for the haplodacite and 192 ± 8 and 217 ± 14 kJ mol⁻¹ for the haplobasalt. These values are comparable to the activation energies for self-diffusion of calcium and silicon at the same melt compositions and pressure. At a fixed temperature of 1500°C, the ratios λ₁/λ₂ increase with the increase of pressure, with λ₁/λ₂ varying from 2.5 to 4.1 (0.5 to 1.3 GPa) for the haplodacite and 4 to 6.5 (0.5 to 2.0 GPa) for the haplobasalt. The activation volumes for the major and minor eigenvalues of the diffusion matrix are 0.31 ± 0.44 and 2.3 ± 0.8 cm³ mol⁻¹, respectively, for the haplodacite and −1.48 ± 0.18 and −0.42 ± 0.24 cm³ mol⁻¹ for the haplobasalt. These values are quite different from the activation volumes for self-diffusion of calcium and silicon at the same melt compositions and temperature. These differences in activation volumes between the two melts likely result from a difference in the structure and thermodynamic properties of the melt between the two compositions (e.g., partial molar volume).Applications of the measured diffusion matrices to quartz crystal dissolution in molten CAS reveal that the activation energy and activation volume for quartz dissolution are almost identical to the activation energy and activation volume for diffusion of the minor or slower eigencomponent of the diffusion matrix. This suggests that the diffusion rate of slow eigencomponent is the rate-limiting factor in isothermal crystal dissolution, a conclusion that is likely to be valid for crystal growth and dissolution in natural magmas when diffusion in liquid is the rate-limiting factor.     

Pressure dependence of viscosity of rhyolitic melts

Viscosity of silicate melts is a critical property for understanding volcanic and igneous processes in the Earth. We investigate the pressure effect on the viscosity of rhyolitic melts using two methods: indirect viscosity inference from hydrous species reaction in melts using a piston cylinder at pressures up to 2.8 GPa and direct viscosity measurement by parallel-plate creep viscometer in an internally-heated pressure vessel at pressures up to 0.4 GPa. Comparison of viscosities of a rhyolitic melt with 0.8 wt% water at 0.4 GPa shows that both methods give consistent results. In the indirect method, viscosities of hydrous rhyolitic melts were inferred based on the kinetics of hydrous species reaction in the melt upon cooling (i.e., the equivalence of rheologically defined glass transition temperature and chemically defined apparent equilibrium temperature). The cooling experiments were carried out in a piston-cylinder apparatus using hydrous rhyolitic samples with 0.8-4 wt% water. Cooling rates of the kinetic experiments varied from 0.1 K/s to 100 K/s; hence the range of viscosity inferred from this method covers 3 orders of magnitude. The data from this method show that viscosity increases with increasing pressure from 1 GPa to 3 GPa for hydrous rhyolitic melts with water content ?0.8 wt% in the high viscosity range. We also measured viscosity of rhyolitic melt with 0.13 wt% water using the parallel-plate viscometer at pressures 0.2 and 0.4 GPa in an internally-heated pressure vessel. The data show that viscosity of rhyolitic melt with 0.13 wt% water decreases with increasing pressure. Combining our new data with literature data, we develop a viscosity model of rhyolitic melts as a function of temperature, pressure and water content.     

Water diffusion in Mount Changbai peralkaline rhyolitic melt

Diffusion couple experiments with wet half (up to 4.6 wt%) and dry half were carried out at 789–1,516 K and 0.47–1.42 GPa to investigate water diffusion in a peralkaline rhyolitic melt with major oxide concentrations matching Mount Changbai rhyolite. Combining data from this work and a related study, total water diffusivity in peralkaline rhyolitic melt can be expressed as:

$ D_{{{\text{H}}_{ 2} {\text{O}}_{\text{t}} }} = D_{{{\text{H}}_{ 2} {\text{O}}_{\text{m}} }} \left( {1 - \frac{0.5 - X}{{\sqrt {[4\exp (3110/T - 1.876) - 1](X - X^{2} ) + 0.25} }}} \right), $

$ {\text{with}}\;D_{{{\text{H}}_{ 2} {\text{O}}_{\text{m}} }} = \exp \left[ { - 1 2. 7 8 9- \frac{13939}{T} - 1229.6\frac{P}{T} + ( - 27.867 + \frac{60559}{T})X} \right], $

where D is in m² s^?1, T is the temperature in K, P is the pressure in GPa, and X is the mole fraction of water and calculated as X = (C/18.015)/(C/18.015 + (100 ? C)/33.14), where C is water content in wt%. We recommend this equation in modeling bubble growth and volcanic eruption dynamics in peralkaline rhyolitic eruptions, such as the ~1,000-ad eruption of Mount Changbai in North East China. Water diffusivities in peralkaline and metaluminous rhyolitic melts are comparable within a factor of 2, in contrast with the 1.0–2.6 orders of magnitude difference in viscosities. The decoupling of diffusivity of neutral molecular species from melt viscosity, i.e., the deviation from the inversely proportional relationship predicted by the Stokes–Einstein equation, might be attributed to the small size of H₂O molecules. With distinct viscosities but similar diffusivity, bubble growth controlled by diffusion in peralkaline and metaluminous rhyolitic melts follows similar parabolic curves. However, at low confining pressure or low water content, viscosity plays a larger role and bubble growth rate in peralkaline rhyolitic melt is much faster than that in metaluminous rhyolite.

     

Experimental study of the CaMgSi2O6–CO2 system at 3–8 GPa

The melting relationships in the system CaMgSi₂O₆ (Di)–CO₂ have been studied in the 3–8 GPa pressure range to determine if there is an abrupt decrease in the temperature of the solidus accompanying the stabilization of carbonate as a subsolidus phase. Such a decrease has been observed previously in peridotitic and some eclogitic systems. In contrast, the solidus in the Di–CO₂ system was found to decrease in a gradual fashion from 3 to 8 GPa. This decrease accompanies an evolution in the composition of the melt at the solidus from silicate-rich with minor CO₂ at 3 GPa to carbonatitic at 5.5 GPa, where the carbonation reaction Diopside + CO₂ = Dolomite (Dol) + Coesite (Cst) intersects the solidus. The near-solidus melt remains carbonatitic at higher pressure, consistent with carbonate being the dominant contributor to the melt. Based on previous studies in both eclogitic and peridotitic systems, this conclusion can be extended to more complicated systems: once carbonate is a stable subsolidus phase, it plays a major role in controlling both the temperature of melting and the composition of the melt produced.