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1.
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。  相似文献   

2.
用分子动力学方法,研究了1999 K下,压力由23 MPa上升到15183 MPa的过程中,CaAl_2Si_2O_8成分熔体的微观结构、剪切粘滞度和粒子自扩散系数的压力效应。在此基础上,探讨压力对剪切粘滞度与粒子自扩散系数之间关系的影响,并将它同微观结构的变化联系起来。结果表明,粒子自扩散系数的压力效应与熔体结构有很强的相关性;压力的挤压效应阻碍了粒子的扩散,而Si-O和Al-O 5次配位体的形成又加速了扩散过程,两种相反的作用相互抵消,造成的结果是在0~5 GPa范围内,Si~(4 ),O~(2-)和Al~(3 )等网架形成粒子的自扩散系数随压力变化不明显;当压力继续增大时,挤压效应占了主导,导致自扩散系数值快速减小。Ca~(2 )作为网架修饰粒子,自扩散系数随压力升高单调下降。压力小于5 GPa时,粒子自扩散系数的大小关系是:D_(Ca)>D_(Al)>D_O>D_(Si)。系统粘滞度随压力的变化与熔体中BO的含量密切相关:BO含量小于域值时,一定范围内BO含量的变化不会对粘滞度产生很大的影响,超过域值,BO含量的微小增加会导致粘滞度值迅速增大。有效应用Eyring方程的关键是方程中粒子跳跃距离的确定,本研究发现,Si~(4 )和O~(2-)的跳跃距离可以通过系统中非桥氧的百分含量来获得。这一发现使得我们能够利用系统中NBO的含量,结合Eyring方程有效进行不同压  相似文献   

3.
异剥橄榄岩化是地幔橄榄岩与硅不饱和熔体发生强烈相互作用的地幔过程。这一过程通常伴随地幔物理-化学性质的显著改变,与克拉通岩石圈减薄和破坏密切相关。本文梳理了前人对异剥橄榄岩化的驱动机理、结构和成分特征及其对大陆稳定性影响等方面的定性认识,并在此基础上通过热力学模拟,刻画了不同属性地幔与熔体在不同温度压力条件下的相互作用过程,目标是为定量限定异剥橄榄岩化的发生条件、物理-化学效应及地球动力学意义。研究显示,异剥橄榄岩化通常会出现磷灰石、角闪石、金云母和碳酸盐等标志性交代矿物,并以斜方辉石被单斜辉石替换为典型特征,还可发育海绵边、熔体囊(已冷却结晶为单斜辉石、橄榄石等细粒矿物)和矿物环带等非平衡结构。热力学模拟表明,熔岩反应过程受原岩、熔体性质以及熔/岩比例的共同控制,不同属性地幔(难熔方辉橄榄岩、饱满二辉橄榄岩)与贫硅(如霞石岩)熔体反应都能生成富单斜辉石橄榄岩,且在较高压力(如2.5GPa和4.0GPa)条件下更容易形成异剥橄榄岩;相比之下,不同性质地幔与富硅(如MORB)熔体反应在任何压力条件下均不能产生异剥橄榄岩。此外,两类熔体参与的熔岩反应均能引起围岩密度逐渐增加,并在高压条件下...  相似文献   

4.
樊祺诚  隋建立  孙谦  李霓  赵勇伟  杜星星 《岩石学报》2008,24(11):2495-2500
根据北部湾涠洲岛晚更新世火山岩中尖晶石二辉橄榄岩的矿物化学和形成条件推测,它们是来自地下约40km-50km的大陆岩石圈地幔样品。涠洲岛地幔橄榄岩中同时出现高硅(64%~68%)和低硅(49%~57%)两种熔体成分,它们普遍富碱、Al2O3和H2O、CO2等挥发组分(2%-5%)。研究认为,两种熔体存在不同的成因机制。高硅熔体可能与软流圈上升的玄武质岩浆和斜方辉石反应或下地壳物质循环进入地幔后的部分熔融有关。低硅熔体直接提供了富硅熔体与橄榄石发生交代作用的证据。这可以用富SiO2熔体+Ol→贫SiO2熔体+Opx反应解释,其结果是消耗橄榄石生成斜方辉石。涠洲岛橄榄岩中两种熔体的发现,进一步提供了发生在大陆岩石圈地幔流(熔)体与橄榄岩交代作用的普遍性、复杂性,以及地幔熔体多样性的新证据。  相似文献   

5.
埃达克岩的Na亏损及其对地幔Na交代的指示意义   总被引:1,自引:3,他引:1  
埃达克岩是玄武质洋壳部分熔融的产物。然而,与实验室玄武岩部分熔融产生的埃达克质熔体相比,天然埃达克岩明显地高Mg、Cr和Ni,这表明埃达克岩浆在上升过程中有地幔成分的加入。本文的观察结果表明,全球新生代埃达克岩的Na2O含量低于5.8%,大约95%的新生代埃达克岩样品Na2O含量小于5.0%。然而,在埃达克岩产生的压力范围(1.5~3.0GPa),实验的玄武岩部分熔体大多数Na2O含量超过5.0%,最高达到9.0%,显示埃达克岩具有明显的Na亏损现象。我们认为这是埃达克熔体在热的地幔楔中与地幔橄榄岩反应的结果。在俯冲带,大洋板片熔融产生的熔体(埃达克熔体)上升并与地幔橄榄岩发生反应,原始的埃达克熔体获得MgO、Cr及Ni等地幔组分,但其Na2O和SiO2等通过反应进入地幔,导致地幔交代作用。根据长英质熔体与橄榄岩反应体系的相关系,我们认为,地幔单斜辉石、橄榄石、尖晶石的混染作用以及钠质角闪石和斜方辉石的分离结晶作用,是改变埃达克熔体组成并导致其Na亏损的一个重要的过程。埃达克岩的Na亏损为地幔Na交代作用和一些富Na的弧岩浆成因提供了重要证据。  相似文献   

6.
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。  相似文献   

7.
王春光  许文良 《地球科学》2019,44(12):4112-4118
使用活塞-圆筒式高温高压装置进行一系列榴辉岩部分熔融熔体与橄榄岩反应实验,可以为深入了解俯冲带壳-幔相互作用的影响因素及地幔不均一性的成因提供重要信息.实验使用反应偶的方法,并在0.8~3.0 GPa和1 200~1 425℃条件下进行.实验结果表明,榴辉岩部分熔融熔体-橄榄岩反应的动力学和结果受控于熔体主量元素成分、熔体中的H2O、温度、压力和橄榄岩的物理状态等因素.大陆俯冲带地幔岩石中斜方辉石的富集是再循环陆壳熔体与上覆地幔反应的结果,地幔岩石中斜方辉石岩脉的形成与含水熔体交代有关,地幔岩石中的石榴辉石岩和石榴石岩可能形成于高压、低温条件下的熔体-橄榄岩反应.   相似文献   

8.
李艳翔  徐峥 《地球化学》2023,(2):238-249
地幔部分熔融过程中亲S元素Ag的分配行为能够限定地幔中Ag含量,同时可以追踪地幔中S含量和去向。在地幔部分熔融过程中, Ag的分配行为与其在地幔硅酸盐矿物–玄武质熔体间的分配系数密切相关。因此准确地测定Ag在除硫化物之外地幔硅酸盐矿物和共存熔体之间的分配系数是预测Ag分配行为的必要条件之一。目前缺乏Ag在地幔矿物(橄榄石、辉石等)和玄武质熔体之间分配系数的报道,阻碍了对Ag在地幔部分熔融过程中分配行为的研究。本研究通过高温高压实验获得了在洋中脊玄武岩(MORB)产生的温度和压力条件下,Ag在橄榄石(ol)、斜方辉石(opx)、单斜辉石(cpx)与玄武质熔体(melt)之间的分配系数,分别为DAgol/melt=0.0005±0.0002、DAgopx/melt=0.007±0.003和DAgcpx/melt=0.046±0.009。根据实验获得的分配系数进行模拟的计算结果表明,太古代科马提岩和洋中脊苦橄岩源区的地幔Ag含量为(8±2)×10-9。如果这个值...  相似文献   

9.
老牛山复式花岗岩基位于华北克拉通南缘小秦岭地区,锆石LA-ICP-MS U-Pb 定年结果表明,该岩基早期花岗岩--
康坪岩体形成于晚三叠世[(207.9±0.72)]Ma,是早中生代岩浆作用的产物。该岩体具有高硅、富碱、准铝的地球化学特征,
铁镁比值较高,富集LILE(尤其是Sr 和Ba)和LREE,Eu 无明显异常,相对亏损HREE 和Y,可归属为高Ba-Sr 花岗岩。但
与典型埃达克岩相比,康坪岩体Cr,Ni 等相容元素含量较低,而K2O 含量明显偏高,属高钾钙碱性系列,重稀土元素分馏
不显著。在成岩过程中,康坪岩体的地球化学行为主要受角闪石(单斜辉石)、斜长石(钾长石)以及极少量石榴子石的分
离结晶控制。元素和同位素地球化学研究表明,老牛山复式花岗岩基康坪岩体成岩物质主要来自含远洋沉积物俯冲板片析
出流体/ 熔体交代的富集地幔以及古老太华群基底物质所组成的混合源区,是大陆碰撞造山晚期后碰撞“松弛”阶段,俯冲
板片断离后下地壳底部物质发生部分熔融形成的岩浆产物。  相似文献   

10.
地幔富硅交代与大陆岩石圈的演化   总被引:4,自引:0,他引:4  
陈立辉  周新华 《地学前缘》2001,8(3):141-146
富硅交代是弧下地幔中熔体岩石相互作用的主要表现形式 ,是造成古老克拉通陆下岩石圈地幔富硅的主要机制。在弧下地幔捕掳体中 ,橄榄岩被来自俯冲洋壳物质部分熔融生成的含水富硅熔体交代后 ,斜方辉石含量的增加使全岩富集SiO2 ,斜方辉石显示异常低的Al2 O3和Cr2 O3,微量元素上表现为强烈富集LILE ,强烈亏损Nb ,Ta和Ti。在古老克拉通地幔岩样品中 ,方辉橄榄岩具过剩的斜方辉石 ,橄榄石的Ni含量与斜方辉石的组成含量成正比 ,而和橄榄石的x(Mg) /x(Mg +Fe2 +)值没有正比关系 ,被解释为亏损的地幔橄榄岩和来自俯冲板片的富硅熔体相互作用的结果。熔体岩石相互作用最终导致了陆下岩石圈地幔富集SiO2 ,这种被含水富硅熔体改造后的地幔岩石的部分熔融可能是造成陆壳富硅富镁的主要原因。含水富硅熔体对岩石圈地幔的影响程度也可能是大陆岩石圈增生或裂解、增厚或减薄的关键因素之一。  相似文献   

11.
Self-diffusion coefficients for Si and O in Di58An42 liquid were measured from 1 to 4 GPa and temperatures from 1510 to 1764°C. Glass starting powders enriched in 18O and 28Si were mated to isotopically normal glass powders to form simple diffusion couples, and self-diffusion experiments were conducted in the piston cylinder device (1 and 2 GPa) and in the multianvil apparatus (3.5 and 4 GPa). Profiles of 18O/16O and 29,30Si/28Si were measured using secondary ion mass spectrometry. Self-diffusion coefficients for O (D(O)) are slightly greater than self-diffusion coefficients for Si (D(Si)) and are often the same within error. For example, D(O) = 4.20 ± 0.42 × 10−11 m2/s and D(Si) = 3.65 ± 0.37 × 10−11 m2/s at 1 GPa and 1662°C. Activation energies for self-diffusion are 215 ± 13 kJ/mol for O and 227 ± 13 kJ/mol for Si. Activation volumes for self-diffusion are −2.1 ± 0.4 cm3/mol and −2.3 ± 0.4 cm3/mol for O and Si, respectively. The similar self-diffusion coefficients for Si and O, similar activation energies, and small, negative activation volumes are consistent with Si and O transport by a cooperative diffusion mechanism, most likely involving the formation and disassociation of a high-coordinated intermediate species. The small absolute magnitudes of the activation volumes imply that Di58An42 liquid is close to a transition from negative to positive activation volume, and Adam-Gibbs theory suggests that this transition is linked to the existence of a critical fraction (∼0.6) of bridging oxygen.  相似文献   

12.
Colin H. Donaldson 《Lithos》1975,8(2):163-174
Concentration gradients in glass adjacent to skeletal olivines in a DSDP basalt have been examined by electron probe. The glass is depleted in Mg, Fe, and Cr and enriched in Si, Al, Na, and Ca relative to that far from olivine. Ionic diffusion coefficients for the glass compositions are calculated from temperature, ionic radius and melt viscosity, using the Stokes-Einstein relation. At 1170°C, the diffusion coefficient of Mg2+ ions in the basalt is 4·5.10?9 cm2/s. Comparison with measured diffusion coefficients in a mugearite suggests this value may be 16 times too small. The concentration gradient data and the diffusion coefficients are used to calculate instantaneous olivine growth rates of 2–6.10?7 cm/s. This is too slow for olivine to have grown in situ during quenching. Growth necessarily preceded emplacement such that the composition of the crystals plus the enclosing glass need not be that of a melt. The computed olivine growth rates are compatible with the rate of crystallization deduced for the Skaegaard intrusion.  相似文献   

13.
We have explored first-principles molecular dynamics simulation data for hydrous MgSiO3 liquid (with 10 wt% water) to gain insight into its structural and dynamical behavior as a function of pressure (0–150 GPa) and temperature (2,000–6,000 K). By visualizing/analyzing a number of parameters associated with short- and mid-range orders, we have shown that the melt structure changes substantially on compression. The speciation of the water component at low pressures is dominated by the isolated structures (with over 90% hydrogen participated) consisting of hydroxyls, water molecules, O–H–O bridging and four-atom (O–H–O–H and H–O–H–O) groups, where every oxygen atom may be a part of polyhedron or free (i.e., bound to only magnesium atom). Hydroxyls favor polyhedral sites over magnesium sites whereas molecular water is almost entirely bound to magnesium sites, and also interpolyhedral bridging (Si–O–H–O–Si) dominates other types of bridging. Water content is shown to enhance and suppress, respectively, the proportions of hydroxyls and molecular water. As compression increases, these isolated structures increasingly combine with each other to form extended structures involving a total of five or more O and H atoms and also containing threefold coordination species, which together consume over 80% hydrogen at the highest compression studied. Our results show that water lowers the mean coordination numbers of different types including all cation–anion environments. The hydrous melt tends to be more tetrahedrally coordinated but with the Si–Si network being more disrupted compared to the anhydrous melt. Protons increase the content of non-bridging oxygen and decrease the contents of bridging oxygen as well as oxygen triclusters (present at pressures above 10 GPa). The calculated self-diffusion coefficients of all atomic species are enhanced in the presence of water compared to those of the anhydrous melt. This is consistent with the prediction that water depolymerizes the melt structure at all pressures. Our analysis also suggests that proton diffusion involves two processes—the transfer of H atoms (requiring the rupture and formation of O–H bonds) and the motion of hydroxyls as hydrogen carriers (requiring the rupture and formation of Si–O and/or Mg–O bonds). Both the processes are operative at low compression whereas only the first process is operative at high compression.  相似文献   

14.
Diffusion of Al in synthetic forsterite was studied at atmospheric pressure from 1100 to 1500 °C in air along [100] with activities of SiO2, MgO and Al2O3 (aSiO2, aMgO and aAl2O3) buffered. At low aSiO2, the buffer was forsterite + spinel + periclase (fo + sp + per) at all temperatures, while at high aSiO2 and subsolidus conditions a variety of three-phase assemblages containing forsterite and two other phases from spinel, cordierite, protoenstatite or sapphirine were used at 1100–1350 °C. Experiments at high aSiO2 and 1400 °C used forsterite + protoenstatite + melt (fo + en + melt), and at 1500 °C, fo + melt. The resulting diffusion profiles were analysed by LA–ICP–MS in scanning mode. Diffusion profiles in the high aSiO2 experiments were generally several hundred microns in length, but diffusion at low aSiO2 was three orders of magnitude slower than in high aSiO2 experiments carried out at the same temperature, producing short profiles only a few microns in length and close to the spatial resolution of the analytical method. Interface concentrations of Al in the forsterite, obtained by extrapolating the diffusion profiles to the crystal/buffer interface, were only a fraction of those expected at equilibrium, and varied among the differing buffer assemblages according to (aAl2O3)1/2 and (aSiO2)3/4, pointing to the substitution of Al in forsterite by an octahedral-site, vacancy-coupled (OSVC) component with the stoichiometry Al 4/3 3+ vac2/3SiO4, whereas the main substitution expected from previous equilibrium studies would be the coupled substitution of 2 Al for Mg + Si, giving the stoichiometry MgAl2O4. It is proposed that this latter substitution is not seen on the length scales of the present experiments because it requires replacement of Si by Al on tetrahedral sites, and is accordingly rate-limited by the slow diffusivity of Si. Instead, diffusion of Al by the OSVC mechanism is relatively fast, and at high aSiO2, even faster than Fe–Mg interdiffusion.  相似文献   

15.
Partitioning of manganese between forsterite and silicate liquid   总被引:1,自引:0,他引:1  
Partition coefficients for Mn between forsterite and liquid in the system MgO-CaO-Na2O-Al2O3-SiO2 (+ about 0.2% Mn) were measured by electron microprobe for a variety of melt compositions over the temperature range 1250–1450°C at one atm pressure. The forsterite-liquid partition coefficient of Mn (mole ratio, MnO in Fo/MnO in liquid, designated Dmnfo?Liq) depends on liquid composition as well as temperature: at 1350°C, DMnFo?Liqranges from 0.60 (basic melt, SiO2 = 47wt%) to 1.24 (acidic melt, SiO2 = 65wt%). At lower temperatures, the partition coefficient is more strongly dependent on melt composition.The effects of melt composition and temperature on DMnfo?Liq can be separately evaluated by use of the Si:O atomic ratio of the melts. A plot of DmnFo?Liq measured at various temperatures vs melt Si:O for numerous liquid compositions reveals discrete, constant-temperature curves that are not well defined by plotting DMnFo?Liq against other melt composition parameters such as melt basicity or MgO content. For constant Si:O in the melt, In DMnFo?Liq vs reciprocal absolute temperature is linear; however, the slope of the plot becomes more positive for higher values of Si:O, indicating a higher energy state for Mn2+ ions in acidic melts than in basic melts.Comparison of Mn partitioning data for the iron-free system used in this study with data of other workers on iron-bearing compositions suggests that the effect of iron on Mn partitioning between olivine and melt is small over the range of basalt liquidus temperatures.  相似文献   

16.
Molecular dynamics simulations based on ab initio interatomic potentials have been performed to study the kinetics and mechanisms of interdiffusion in MgSiO3 and Mg2SiO4 melts. Diffusion coefficients in the presence of a concentration gradient are lower than self-diffusion coefficients due to the requirements of local charge-balance within the system. Extrapolations of Mg2+ diffusion coefficients obtained at high temperatures (5000 to 3000 K) into geologically relevant temperatures (1500 to 2500 K) are reasonably accurate compared to experimental Mg2+ diffusion coefficients in a compositionally similar system. A significant system-size effect is also observed on the rates of diffusion obtained from the molecular dynamics simulations. Diffusion mechanisms involve movement of individual Mg2+ and O2- ions and anionic [SiOn](4–2n) complexes for both self-diffusion and interdiffusion.  相似文献   

17.
       根据大陆下地壳的成分、含水基性岩体系部分熔融的基本原理和实验岩石学资料,本文对大陆下地壳的熔融机制展 开了讨论,并在此基础上对比实验熔体与大别山C 型埃达克岩的成分,进而探讨约束源岩成分、熔融的温压条件和部分熔 融程度。研究结果表明,大陆下地壳总体上是中- 基性(SiO2 50%~60% )和含少量水的,在缺乏流体相条件下伴随含水 矿物脱水的部分熔融是下地壳产生含水长英质熔体和无水残留体的主要机制。角闪岩在中等压力下(1.0~1.2 GPa,相当于 35~40 km)理论上能够产生石榴石含量超过~20% 的熔融残余,从而使得与之平衡的长英质熔体具有低Y,高Sr/Y 和La/Yb 比值等埃达克岩特征。基于水活度模型和变质基性岩p -t 相图的估算显示,含有40%~60% 角闪石的源岩(含水0.8%~1.2%) 在~950 ℃能够得到最大为15%~20% 的熔体,该熔体分数满足熔体分离的要求。大别山C型埃达克岩主要为高钾钙碱性系 列(K2O 3.5%~5%),与实验熔体成分的对比可知,其无法由低钾源岩在合理的部分熔融程度形成。根据钾在角闪岩部分熔 融过程过表现为强不相容元素的原理,利用合理假设的残余体组合得到的分配系数,估算K2O 含量为~1% 的源岩在熔融程 度为15%~20% 的情况下能够得到类似大别山C 型埃达克岩成分的熔体。  相似文献   

18.
With improvements on high-pressure experimental techniques in multi-anvil apparatus and the development of new analytical tools, major progress has been made on diffusion in garnets in the past several decades. The data obtained in the experimental determination of diffusion coefficients in garnets are of fundamental importance for diffusion modeling and timescales of geological and planetary processes. In this review, we have compiled experimental data on self-diffusion (Si, O, cations), trace element diffusion (Li, Y, Ga, Cr, Sr, REEs), and interdiffusion (Ca–Fe/Mg, Si–Al) in garnet in the light of new advances and recent applications. In addition, some empirical relationships among diffusion parameters (pre-exponential factor D 0, activation energy E, ionic radius) are also discussed. We hope that this review can provide a useful data digest and guide to future study of diffusion in garnet.  相似文献   

19.
一种计算NaAlSi3O8熔体粘度的理论方法   总被引:2,自引:0,他引:2  
朱永峰  赵永超 《岩石学报》1997,13(2):173-179
硅酸岩熔体的结构特征是制约熔体粘度的主要因素,熔体结构的变异是其中粘流作用发生的原因,化学成分对熔体粘度的控制是通过改变熔体结构而实现的。SiO2熔体中仅存在Si-O键,而NaAl-Si3O8熔体中存在Si-O键,Al-O键和Na-O键,Na-O键在熔体结构中通过O与Si-O键相联结,并且使与之相联的Si-O键的键强变弱。因此,熔体结构单元中与Na-O键相联结的Si-O键最易断开,并因此导致流变作用的发生。文中计算了不同温度条件下NaAlSi3O8熔体中Si-O键的键强,以SiO2熔体中的Si-O键的键强和SiO2熔体的粘度为标准,建立了计算了NaAlSi3O8熔体粘度的理论模型,依此理论模型求得的熔体粘度在合理的地质温度范围内与实验结果完全吻合  相似文献   

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