常压下硫酸盐与基性硅酸盐熔体不混溶的实验研究  

Immiscibility of sulfate and basic silicate melt under atmospheric pressure and distribution behavior of REEs in immiscible phase

作　　者：汪道博 谢玉玲[1] 陈缓 WANG Dao-bo;XIE Yu-ling;CHEN Huan(Civil and Resource Engineering School,University of Science and Technology Beijing,Beijing 100083,China)

机构地区：[1]北京科技大学土木与资源工程学院,北京100083

出　　处：《岩石矿物学杂志》2023年第1期71-82,共12页Acta Petrologica et Mineralogica

基　　金：国家自然科学基金重点项目(41930427)。

摘　　要：自然界中广泛存在硅酸盐与硫酸盐之间的熔体不混溶现象,这种不混溶过程控制了岩浆演化过程的氧逸度变化和岩浆中S的含量,同时也对金属元素的富集具有重要意义。前人对硫酸盐与硅酸盐的熔体不混溶过程及稀土元素在不混溶相分配行为的研究主要集中在碱性硅酸盐与硫酸盐体系,但对基性硅酸盐与硫酸盐的熔体不混溶行为及稀土元素在此过程中的分配规律研究仍相对薄弱。本文选择基性硅酸盐样品与实验用Na_(2)SO_(4)按质量比1∶1制成混合实验样品粉末,并添加少量H_(3)BO_(3)作为助熔剂,通过马弗炉加热至1200℃,使粉末完全熔融,并在1200℃恒温12 h后在马弗炉中快速冷却至常温。对加温-冷却后的样品进行详细的岩矿相、SEM/EDS和不同相态的原位LA-ICP-MS分析。研究结果表明,在1200℃的条件下,硫酸盐与基性硅酸盐熔体可以大比例混溶,且降温会造成两者的不混溶,在不混溶过程中Na、Ca、K、REEs等趋向于进入硫酸盐熔体。不混溶形成的硫酸盐熔体中,稀土元素含量明显高于残余硅酸盐熔体,但轻重稀土元素没有明显的分异。不混溶硅酸盐熔体冷凝形成的玻璃相中除硫酸盐球粒外,还可见镁铁矿、蓝方石、赤铁矿等矿物斑晶,表明富硫酸盐的基性硅酸盐熔体在降温过程中可以造成蓝方石、镁铁矿和赤铁矿的结晶分离,使残余硅酸盐熔体中S、Fe、Mg含量和岩浆氧逸度降低,这可能是地壳浅部出露的幔源基性脉岩中通常S含量不高和相对还原的原因。The immiscibility between silicate melt and sulfate melt widely exists in the nature,which is closely related to the enrichment of metalic elements and mineralization.Currently,the experimental study on the immiscibility process between basic silicate and sulfate melt is still weak,as well as the partition behavior of elements in such process.In this study,we designed a geochemical simulation experiment,which is kept normal pressure and 1200℃for 12 hours,and then cooled to room temperature within 5 minutes.Cooled samples were analyzed in detail by micrography,EPMA and in-situ LA-ICP-MS.The results show that sulfate can be miscible with basic silicate melt in a certain proportion at 1200℃,and cooling will cause immiscibility between them.After cooling,the sample forms two distinct layers.The upper layer is mainly sulfate phase and contains magnesioferrite and forsterite crystals.The lower layer is mainly silicate glass phase,and contains sulfate globes and mineral crystals such as magnesioferrite,hauyne and hematite.The crystallization separation of hauyne caused by cooling and the immiscibility between sulfate melt and silicate melt may be the reason for the low S content and relative reduction of sulfur rich and relatively oxidized mantle derived basic shallow intrusive and volcanic rocks.The analysis results of major and trace elements show that Na,Ca,K and REEs tend to enter the sulfate melt in the immiscibility process.The content of rare earth elements in early immiscible sulfate melt is not high,which has little effect on the content of rare earth elements in silicate melt.However,the content of rare earth elements in the immiscible sulfate melt formed in the late stage and low relative temperature is significantly higher than that in the early exsolved sulfate melt and residual silicate melt.The partition behavior of metalic elements in sulfate melt has a certain correlation with electronegativity,and the partition coefficient increases with the decrease of electronegativity.The partition behavior of r

关 键 词：基性硅酸盐 硫酸盐 熔体不混溶 蓝方石 稀土元素富集 

分 类 号：P589.1[天文地球—岩石学]