Table1_Insight into differentiation in alkalic systems: Nephelinite-carbonate-water experiments aimed at Ol Doinyo Lengai carbonatite genesis.XLSX

Ol Doinyo Lengai (ODL, Tanzania, East African Rift) is the only known volcano currently erupting carbonatite on Earth with 30 yr. cycles alternating between quiescent carbonatite effusion and explosive, compositionally-zoned silicate eruptions. We performed isothermal crystallization and thermal gradient experiments involving ODL nephelinite, Na2CO3 and H2O to understand magmatic differentiation in this system using SEM-EDS x-ray analysis, x-ray tomography, SIMS and LA-ICPMS to characterize samples. Isothermal crystallization experiments document that hydrous liquids coexist with nepheline+feldspar; as peralkalinity increases, temperatures decrease. Presence of Na2CO3 increases the solubility of water in the liquid. Experiments placing nephelinite with H2O+ Na2CO3 in a 1,000–350°C thermal gradient show that rapid reaction occurs, resulting in virtually melt-free mineral aggregates having mineral layering reflecting systematic differentiation throughout the capsule. Both types of experiments argue that a continuous interconnected melt exists over a large temperature range in alkalic magmatic systems allowing for differentiation in a reactive mush zone process. Liquid compositions change from carbonate-water bearing nephelinites at high temperature down to hydrous carbonate silicate liquids at <400°C. We propose a model for ODL eruption behavior: 1) nephelinite magmas pond and build a sill complex downward with time; 2) hydrous carbonate melts form in the mush and buoyantly rise, ultimately erupting as natrocarbonatites observed; 3) H2O contents build up in melt at the bottom of the sill complex, eventually leading to water vapor saturation and explosive silicate eruptions. The model accounts for eruption cycling and the unusual compositional zoning of ODL silicate tephras.

奥杜因约伦盖火山（Ol Doinyo Lengai，ODL，坦桑尼亚东非裂谷）是目前地球上已知唯一正在喷发碳酸盐岩的火山，其活动以30年为周期，在沉寂的碳酸盐岩溢流与爆发式、成分分带的硅酸盐喷发之间交替进行。我们针对该火山的霞石岩（nephelinite）、碳酸钠（Na₂CO₃）与水（H₂O）开展了等温结晶与热梯度实验，以探究该体系中的岩浆分异作用；实验过程中采用扫描电子显微镜-能谱仪（SEM-EDS）X射线分析、X射线断层扫描（X-ray tomography）、二次离子质谱（SIMS）与激光剥蚀电感耦合等离子体质谱（LA-ICPMS）对样品进行表征。等温结晶实验结果表明，含水熔体与霞石+长石共存；随着过碱度升高，体系温度降低。碳酸钠的存在可提升水在熔体中的溶解度。将添加H₂O+Na₂CO₃的霞石岩样品置于1000~350℃的热梯度环境中开展的实验显示，体系发生快速反应，生成几乎不含熔体的矿物集合体，其矿物层理特征反映了整个实验容器内的系统性岩浆分异作用。两类实验均证实，在碱性岩浆体系中，连续连通的熔体在较大温度范围内均稳定存在，使得反应熔泥带过程中的岩浆分异作用得以发生。熔体成分从高温下的含水碳酸盐霞石岩，逐渐转变为低于400℃时的含水碳酸盐硅酸盐熔体。我们提出了奥杜因约伦盖火山的喷发行为模型：1）霞石岩岩浆逐渐聚集并随时间向下形成岩床复合体；2）含水碳酸盐熔体在熔泥带中形成并以浮力方式上升，最终以被观测到的钠碳酸盐岩（natrocarbonatite）形式喷发；3）水含量在岩床复合体底部的熔体中不断累积，最终达到水汽饱和并引发爆发式硅酸盐喷发。该模型解释了奥杜因约伦盖火山的喷发周期以及其硅酸盐火山碎屑（tephras）独特的成分分带特征。