高压下玄武岩浆的不混溶及其对双峰式火山岩的成因意义  

Immiscibility of basalt magma at high pressure and its significance to the bimodal volcanic rock.

作　　者：郑海飞[1] 陈斌[1] 孙樯[1] 谢鸿森[2] 

机构地区：[1]造山带与地壳演化教育部重点实验室,北京大学地质学系北京100871 [2]中国科学院地球化学研究所,贵阳550002

出　　处：《岩石学报》2003年第4期745-751,共7页Acta Petrologica Sinica

基　　金：国家自然科学基金(批准号:10299040;40173019)

摘　　要：我们在温度850～1300℃,压力1.0～3.0GPa和含约2％水的条件下的碱性玄武岩熔融实验产物中发现:在高于2.0GPa压力下玄武岩熔融后的淬火产物中存在着两种不同成分的玻璃,其中一种呈褐色,另一种为浅色或无色。玻璃的电子探针分析结果表明,褐色玻璃具有相对高的MgO、FeO和相对低的SiO_2、K_2O、Na_2O含量及Na_2O>K_2O的特征。熔体的不混溶间隙与温度无明显关系,但与压力关系密切,即在大于2.0GPa的压力条件下两种玻璃的不混溶间隙突然增大。因此,由不混溶作用形成的双峰式火山岩应来自大于2.0GPa的上地幔,且是能够提供酸性端员物质以及快速上升的构造环境。这与双峰火山岩可以产生于大陆裂谷环境或主要产于俯冲带的弧后拉张环境而不是岛弧前缘一致。The experimental result shows that there are two kinds of glasses in,the melting product of alkali basalt at the temperatures of 850similar to1300degreesC and the pressures of 1.0similar to3.0degrees GPa. One of the glasses is brown and the other almost colorless. The analysis of electron microprobe indicates that the brown one has higher content of MgO, FeO and lower SiO2, K2O, and characteristic of Na2O>K2O by comparison with the colorless glasses. The immiscibility gap of the melt is not controlled by temperature, but by pressure; that is, at the pressure greater than 2.0 GPa, the gap becomes larger suddenly. Therefore, the bimodal volcanic rock formed by magma immiscibility may come from the upper mantle depth at the pressure greater than 2.0 GPa. And also the mantle source could provide the silicic end of the substance and should be in the tectonic environment that magma could quickly raise. This is consistent with that bimodal volcano exists in the continental rift valley or in back arc but not in front arc of the subduction zone.

关 键 词：玄武质岩石 中酸性岩 高压熔融实验 高压不混溶 

分 类 号：P589.1[天文地球—岩石学] P588.145[天文地球—地质学]