沉积硼酸盐矿物硼同位素组成特征及控制因素研究

Characteristics and Control Factors of Boron Isotope Composition of Sedimentary Borate Minerals

中文关键词:  硼酸盐矿物  硼同位素  硼源  pH值  矿物结构  蚀变反应

英文关键词:Borates  δ¹¹B value  Boron source  pH value  Borate structure  Alteration reaction

中文摘要:

      硼资源主要来自沉积硼酸盐矿床,其硼同位素组成携带着丰富的成矿及蚀变信息。通过室内模拟实验及统计全球沉积硼酸盐矿物δ¹¹B值,分析发现硼酸盐矿物δ¹¹B值主要受硼源、硼酸盐结构、pH值、后期蚀变及瑞利过程影响。不同试剂合成及不同盐湖产出的硼酸盐矿物δ¹¹B值存在显著差异,说明硼源是决定δ¹¹B值的第一要素。同一矿床及全球沉积硼酸盐矿物间δ¹¹B值均表现出钠硼酸盐(硼砂、三方硼砂、贫水硼砂)＞钠—钙硼酸盐(钠硼解石、硼钠钙石)＞镁硼酸盐(多水硼镁石、柱硼镁石、库水硼镁石)＞钙硼酸盐(硬硼钙石、板硼石、白硼钙石、三斜硼钙石)＞钙—镁硼酸盐(砷硼镁钙石、水方硼石),这与硼酸盐矿物中BO₃/BO₄比值及卤水pH值有关。原始沉积硼酸盐后期埋藏过程经历的蚀变反应会改变δ¹¹B值:(1)次生硼酸盐矿物δ¹¹B值比原生硼酸盐矿物低2‰～4‰,这是由于脱水反应中¹¹B倾向进入流体相；(2)次生硼酸盐矿物δ¹¹B值比原生硼酸盐矿物高0‰～1‰,这是因为瑞利过程中¹⁰B优先进入固相造成后期沉积硼酸盐矿物δ¹¹B值升高0.3‰～2.5‰；(3)后期流体作用会显著改变δ¹¹B值,主要表现在硅—锶硼酸盐矿物δ¹¹B值与原生硼酸盐矿物具有显著差异。

英文摘要:

      Sedimentary borate deposits provide the major boron resource,and their δ¹¹B values carry rich mineralization and alteration information.The δ¹¹B values of 234 major sedimentary borate minerals are collected,and some borates are synthesized by chemical reagent and collected from Qinghai-Tibet Plateau.Meanwhile,the change of δ¹¹B values during the thermal decomposition and the precipitation of borates are studied.The δ¹¹B values of borates synthesized by different reagents and produced in different salt lakes are significantly different,which shows that the boron source is the most important factor to effect the δ¹¹B value of borates.The δ¹¹B values of the borates in the single deposit and the global sedimentary deposits all show Na-borates(Borax,Kernite,Tincal)>Na-Ca borate(Ulexite,Probertite)>Mg-borate(Inderite,Pinnoite,Kurnakovite)>Ca-borate(Colemanite,Pandermite,Inyoite,Meyerhofferite)>Ca-Mg borate(Terrugite,Hydroboracite)>other borates(howlite,Veatchite-A),which is related to the BO₃/BO₄ ratio of borates and the pH values of brines.The primary sedimentary borates experience the alteration reaction during the burial process,and it will change the δ¹¹B value:(1) The δ¹¹B value of secondary borates is 2‰～4‰ lower than that of primary borates as δ¹¹B tends to enter the fluid phase during the dehydration reaction；(2) The δ¹¹B value of secondary borates is 0‰～1‰ higher than that of primary borates,which is caused by the preferential entry of ¹⁰B into the solid phase during the Rayleigh process；(3) The reaction between primary borates and later fluid will significantly change the δ¹¹B value,and it is mainly manifested by the significant difference of δ¹¹B value between silicon-strontium borates and the primary borates.The δ¹¹B value of sedimentary borates are mainly affected by boron source,borate structure,pH value,post-alteration and Rayleigh process.