Silicate liquid immiscibility in magmas and in the system K2O-FeO-AI2O3-SiO2: an example of serendipity
Silicate liquid immiscibility in magmas and in the system K2O-FeO-AI2O3-SiO2: an example of serendipity
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岩浆和 K2O-FeO-Al2O3-SiO2 系统中的硅酸盐液体不混溶性：偶然性的一个例子
DOI：
10.1016/0016-7037(78)90250-8
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发表时间：
1978
影响因子：
5
通讯作者：
E. Roedder
中科院分区：
文献类型：
--
作者：
E. Roedder
硅酸盐液体不可渗透性的概念在岩石学历史的早期就被用来解释某些成分不同的岩石对，但是。根据Greig（Am。J. Sci.13，1-44，133-154）和鲍恩（火成岩的演变），它陷入了多年的失宠。在K_2 O-FeO-Al_2O_3-SiO_2系统的实验工作中，发现了在地质上合理的温度范围和成分内的不可熔性，以及在各种月球和地球岩石中发现了不可熔性的证据，恢复了这一过程。代表K_2 O-FeO-Al_2O_3-SiO_2系统的四面体的高硅角的相平衡，以通过四面体的恒定FeO截面的形式呈现，10%的增量这些部分显示了初生相体积的初步关系，它们是基于对519种成分进行的5631次淬火试验，这些成分是在纯氮中的金属铁容器中制成的。涉及十三种结晶化合物，其中至少六种显示出两种或多种晶体修饰。两个分离的相体积，在每个相体积中，两种不混溶的液体，一种富铁，另一种贫铁，存在于液相线处。其中一个体积是完全在四元系统，横跨1：1 K2 O：Al 2 O3平面。没有发现这样的四元化合物，但有证据表明至少有部分四元固溶体，液相线温度迅速降低，从K2 O·Al 2 O3·2SiO 2（钾霞石，六方钾霞石）。从K_2O·Al_2O_3·4SiO_2（白榴石）到K_2O·FeO·5SiO_2，这两个系列显然都涉及四面体配位的取代。一个亚铁离子和一个硅离子交换两个铝离子。在对天然相的分析中发现的一些“杂质”可能反映了这些置换。由于不可混溶体积的几何形状完全位于四元体系内，其附近的组成显示出许多相变和大量的结晶，温度变化很小，通常在1100-1150 ℃范围内。在用Rb或Cs取代K的等价体系中，在一些探索性试验中发现了类似的低温高碱不溶性。本文综述了涉及不相容性（包括稳定和亚稳定）的体系的组成和一般行为，以及天然不相容性的证据。这表明它可能是一个比通常认为的更常见的特征。几个自然的不可渗透性的例子是详细的;大多数产生长英质。碱铝硅酸盐熔体和镁铁质熔体。来自各种各样的玄武质母岩浆，既有过饱和的，也有欠饱和的。不幸的是，陆地岩石中不可熔性的最佳证据，即两种成分不同的玻璃之间的清晰定义的弯月面，就其本质而言是自毁的，因为它被结晶或重力分离和聚结成单独的岩浆有效地消除了。大规模验证外溶或“分裂”过程的操作可能需要在实验室和现场仔细研究同位素和微量元素的分配。
The concept of silicate liquid immiscibility was invoked early in the history of petrology to explain certain pairs of compositionally divergent rocks, but. as a result of papers by Greig (Am. J. Sci.13, 1–44, 133–154) and Bowen (The Evolution of the Igneous Rocks), it fell into disfavor for many years. The discovery of immiscibility in geologically reasonable temperature ranges and compositions in experimental work on the system K2O-FeO-Al2O3-SiO2, and of evidence for immiscibility in a variety of lunar and terrestrial rocks, has reinstated the process.Phase equilibria in the high-silica corner of the tetrahedron representing the system K2O- FeO-Al2O3-SiO2are presented, in the form of constant FeO sections through the tetrahedron, at 10% increments. Those sections, showing the tentative relationships of the primary phase volumes, are based on 5631 quenching runs on 519 compositions, made in metallic iron containers in pure nitrogen. Thirteen crystalline compounds are involved, of which at least six show two or more crystal modifica-tions. Two separate phase volumes, in each of which two immiscible liquids, one iron-rich and the other iron-poor, are present at the liquidus. One of these volumes is entirely within the quaternary system, astride the 1:1 K2O:Al2O3plane. No quaternary compounds as such have been found, but evidence does point toward at least partial quaternary solid solution, with rapidly lowering liquidus temperatures, from K2O·Al2O3· 2SiO2(‘potash nepheline’, kalsilite. kaliophilite) to the isostructural compound K2O·FeO·3SiO2, and from K2O·Al2O3·4SiO2(leucite) to the isostructural compound K2O·FeO·5SiO2, Both of these series apparently involve substitution, in tetrahedral coordination. of a ferrous iron and a silicon ion for two aluminum ions. Some of the ‘impurities’ found in analyses of the natural phases may reflect these substitutions.As a result of the geometry of the immiscibility volume located entirely within the quaternary system, compositions near it show a number of phase changes and large amounts of crystallization with small temperature changes, generally in the range 1100–1150 C. Similar low-temperature, high-alkali immiscibility was discovered in a few exploratory runs in the equivalent systems with Rb or Cs substituting for K. But not in those with Li or Na.A review of the compositions and general behavior of systems involving immiscibility, both stable and metastable, and of the evidence for natural immiscibility. indicates that it may be a much more common feature than generally thought. Several examples of natural immiscibility are detailed; most yield a felsic. alkali-aluminosilicate melt and a mafic melt. from a wide variety of generally basaltic parental magmas, both under- and over saturated. Unfortunately, the best line of evidence for immiscibility in terrestrial rocks, a sharply defined meniscus between two compositionally disparate glasses, is by its very nature self-destructing, since it is effectively eliminated by either crystallization or gravitative separation and coalescence into separate magmas. Verification of operation of the exosolutionor ‘splitting’ process on a large scale will probably require careful study of isotopic and trace element partitioning in both laboratory and field.