Dissolution of Quartz, Albite, and Orthoclase in H2O-Saturated Haplogranitic Melt at 800 C and 200 MPa: Diffusive Transport Properties of Granitic Melts at Crustal Anatectic Conditions
Dissolution of Quartz, Albite, and Orthoclase in H2O-Saturated Haplogranitic Melt at 800 C and 200 MPa: Diffusive Transport Properties of Granitic Melts at Crustal Anatectic Conditions
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800 C 和 200 MPa 下石英、钠长石和正长石在 H2O 饱和单长花岗岩熔体中的溶解：地壳深熔条件下花岗岩熔体的扩散传输特性
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发表时间：
2006
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通讯作者：
T. Dewers
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作者：
Antonio Acosta;D. London;G. Morgan;T. Dewers
我们在800�C和200 Mpa条件下进行了石英、钠长石、斜长石和刚玉在H2O饱和的单长花岗岩熔体中的溶解实验，持续时间为120-1488h，目的是确定地壳重熔温度下花岗岩熔体的扩散输运性质。无水起始玻璃和单一矿物相(石英或长石)的圆柱体沿着平整和抛光的表面并列在金或铂胶囊内，其中添加了�10wt%的H2O。垂直于矿物-玻璃界面的玻璃(淬火熔体)中的浓度分布和与相关相图的比较表明，384h后，界面上的熔体在溶解相中达到饱和，随着时间的延长，浓度分布仅受熔体中组分的扩散控制。浓度分布随时间的演变表明，熔体中的非耦合扩散在氧化物组成空间中沿以下四个线性独立的方向发生：SiO_2、Na2O和K2O轴(分别为Si-、Na-和K-特征向量)，以及Al_2O_3、Na_2O和K_2O轴之间的方向(Al-特征向量)，使得Al/Na摩尔比等于大块熔体的摩尔比，Al/(Na-K)摩尔比等于平衡的ASI(？)熔体中的Al_2O_3/[Na2O_2O_2])。玻璃圆柱体被夹在两个矿物圆柱体--石英和钠长石、石英和钾长石或钠长石和刚玉-之间的实验中，检验了推断的非耦合扩散方向的有效性，并通过化学势梯度探索了熔体中的远程化学通讯。对实验石英和长石溶解数据的扩散方程的有效解的应用分别提供了沿�(2�0-2�8)·10�15m2/S和�(0�6-2�4)·10�14m2/S的硅特征向量和铝特征向量方向的扩散系数。碱的最小扩散系数[�(3-9)·10�11m2/S]比熔体的四面体成分大几个数量级。这里提供的信息决定了当提供足够的热量，扩散是熔体中唯一的质量传输(混合)过程时，地壳深熔发生的速度。计算的扩散系数表明，初始粒度为2-3 mm的石英长石质源岩在800�C(无限供热)下进行流体静力饱和熔融可产生20-30体积。在不到1-10年的时间内，均质熔体的百分比。在H2O不饱和熔体中较慢的扩散将增加这一时间框架。
We have conducted experiments on dissolution of quartz, albite, orthoclase, and corundum into H2O-saturated haplogranite melt at 800 � C and 200MPa over a duration of 120–1488h with the aim of ascertaining the diffusive transport properties of granitic melts at crustal anatectic temperatures. Cylinders of anhydrous starting glass and a single mineral phase (quartz or feldspar) were juxtaposed along flat and polished surfaces inside gold or platinum capsules with � 10 wt % added H2O. Concentration profiles in glass (quenched melt) perpendicular to the mineral–glass interfaces and comparison with relevant phase diagrams suggest that melts at the interface are saturated in the dissolving phases after 384h, and with longer durations the concentration profiles are controlled only by diffusion of components in the melt. The evolution of the concentration profiles with time indicates that uncoupled diffusion in the melt takes place along the following four linearly independent directions in oxide composition space: SiO2 ,N a2O, and K2O axes (Si-, Na-, and K-eigenvectors, respectively), and a direction between the Al2O3, Na2O, and K2O axes (Al-eigenvector), such that the Al/Na molar ratio is equal to that of the bulk melt and the Al/(Na þ K) molar ratio is equal to the equilibrium ASI (¼ mol. Al2O3/[Na2O þ K2O]) of the melt. Experiments in which a glass cylinder was sandwiched between two mineral cylinders—quartz and albite, quartz and K-feldspar, or albite and corundum—tested the validity of the inferred directions of uncoupled diffusion and explored longrange chemical communication in the melt via chemical potential gradients. The application of available solutions to the diffusion equations for the experimental quartz and feldspar dissolution data provides diffusivities along the directions of the Si-eigenvector and Al-eigenvector of � (2� 0–2� 8) · 10 � 15 m 2 /s and � (0� 6–2� 4) · 10 � 14 m 2 /s, respectively. Minimum diffusivities of alkalis [� (3–9) · 10 � 11 m 2 /s] are orders of magnitude greater than the tetrahedral components of the melt. The information provided here determines the rate at which crustal anatexis can occur when sufficient heat is supplied and diffusion is the only mass transport (mixing) process in the melt. The calculated diffusivities imply that a quartzo-feldspathic source rock with initial grain size of 2–3mm undergoing hydrostatic, H2O-saturated melting at 800 � C (infinite heat supply) could produce 20–30 vol. % of homogeneous melt in less than 1–10 years. Slower diffusion in H2O-undersaturated melts will increase this time frame.