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Immiscible silicate liquids: K and Fe distribution as a test for chemical equilibrium and insight into the kinetics of magma unmixing
Contributions to Mineralogy and Petrology ( IF 3.5 ) Pub Date : 2021-05-24 , DOI: 10.1007/s00410-021-01798-1
Alexander Borisov , Ilya V. Veksler

Silicate liquid immiscibility leading to formation of mixtures of distinct iron-rich and silica-rich liquids is common in basaltic and andesitic magmas at advanced stages of magma evolution. Experimental modeling of the immiscibility has been hampered by kinetic problems and attainment of chemical equilibrium between immiscible liquids in some experimental studies has been questioned. On the basis of symmetric regular solutions model and regression analysis of experimental data on compositions of immiscible liquid pairs, we show that liquid–liquid distribution of network-modifying elements K and Fe is linked to the distribution of network-forming oxides SiO2, Al2O3 and P2O5 by equation: \(\log K_{{\text{d}}}^{{\text{K/Fe}}} = \, 3.796\Delta X_{{{\text{SiO}}_{2} }}^{{{\text{sf}}}} + \, 4.85\Delta X_{{{\text{Al}}_{2} {\text{O}}_{3} }}^{{{\text{sf}}}} + \, 7.235\Delta X_{{{\text{P}}_{2} {\text{O}}_{5} }}^{{{\text{sf}}}} - \, 0.108,\)where \(K_{{\text{d}}}^{{\text{K/Fe}}}\) is a ratio of K and Fe mole fractions in the silica-rich (s) and Fe-rich (f) immiscible liquids: \(K_{d}^{{\text{K/Fe}}} = \, \left( {X_{{\text{K}}}^{s} /X_{{\text{K}}}^{f} } \right)/ \, \left( {X_{{{\text{Fe}}}}^{s} /X_{{{\text{Fe}}}}^{f} } \right)\) and \(\Delta X_{{\text{i}}}^{sf}\) is a difference in mole fractions of a network-forming oxide i between the liquids (s) and (f): \(\Delta X_{i}^{sf} = X_{i}^{s} - X_{i}^{f}\). We use the equation for testing chemical equilibrium in experiments not included in the regression analysis and compositions of natural immiscible melts found as glasses in volcanic rocks. Departures from equilibrium that the test revealed in crystal-rich multiphase experimental products and in natural volcanic rocks imply kinetic competition between liquid–liquid and crystal–liquid element partitioning. Immiscible liquid droplets in volcanic rocks appear to evolve along a metastable trend due to rapid crystallization. Immiscible liquids may be closer to chemical equilibrium in large intrusions where cooling rates are lower and crystals may be spatially separated from liquids.



中文翻译:

不混溶的硅酸盐液体:钾和铁的分布作为化学平衡测试和对岩浆解体动力学的洞察力

在岩浆演化的晚期,玄武岩和安山岩岩浆普遍存在硅酸盐液体的不混溶性,导致形成独特的富铁和富二氧化硅液体的混合物。动力学问题阻碍了对不溶混性的实验建模,并且在一些实验研究中,对不溶混液体之间的化学平衡的实现提出了质疑。在不对称液体对组成的对称正则解模型和实验数据的回归分析的基础上,我们表明网络修饰元素K和Fe的液-液分布与网络形成氧化物SiO 2,Al的分布有关由等式2 O 3和P 2 O 5\(\ log K _ {{\ text {d}}} ^ {{\ text {K / Fe}}} = \,3.796 \ Delta X _ {{{\ text {SiO}} _ {2}}} ^ { {{\ text {sf}}}} + \,4.85 \ Delta X _ {{{\ text {Al}} _ {2} {\ text {O}} _ {3}}} ^ {{\ text { sf}}}} + \,7.235 \ Delta X _ {{{\ text {P}} _ {2} {\ text {O}} _ {5}}} ^ {{{\ text {sf}}}}} -\,0.108,\)其中\(K _ {{\ text {d}}} ^ {{\ text {K / Fe}}} \\)是富含二氧化硅(s)和富铁(f)不混溶液体:\(K_ {d} ^ {{\ text {K / Fe}}}} = \,\ left({X _ {{\ text {K}}} ^ {s} / X _ {{\ text {K}}} ^ {f}} \ right)/ \,\ left({X _ {{{\ text {Fe}}}}} ^ {s} / X _ {{{\ text { Fe}}}} ^ {f}} \ right)\)\(\ Delta X _ {{\ text {i}}} ^ {sf} \)是形成网络的氧化物i的摩尔分数之间的差异液体(s)和(f):\(\ Delta X_ {i} ^ {sf} = X_ {i} ^ {s}-X_ {i} ^ {f} \)。在回归分析中未包括的实验中,我们使用该方程式测试化学平衡。偏离平衡的是,该测试在富含晶体的多相实验产品和天然火山岩中揭示,这暗示着液-液和晶-液元素分配之间的动力学竞争。由于快速结晶,火山岩中不混溶的液滴似乎沿亚稳态趋势演化。在冷却速度较低且晶体与液体在空间上分离的大型侵入体中,不混溶的液体可能更接近化学平衡。

更新日期:2021-05-24
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