沉积硼酸盐热作用过程中的硼同位素变化  

Changes in boron isotopes during the thermal process of sedimentary borate

作　　者：彭章旷 马云麒[2,3] 彭齐鸣 PENG Zhangkuang;MA Yunqi;PENG Qiming(School of Earth Sciences and Resources,China University of Geosciences,Beijing 100083,China;Key Laboratory of Comprehensive and Highly Efficient Utilization of Salt Lake Resources,Qinghai Institute of Salt Lakes,Chinese Academy of Sciences,Xining,Qinghai 810008,China;Salt Lake Chemical Analyzing and Testing Center,Qinghai Institute of Salt Lakes,Chinese Academy of Sciences,Xining,Qinghai 810008,China)

机构地区：[1]中国地质大学地球科学与资源学院,北京100083 [2]中国科学院青海盐湖研究所,中国科学院盐湖资源综合高效利用重点实验室,青海西宁810008 [3]中国科学院青海盐湖研究所盐湖化学分析测试中心,青海西宁810008

出　　处：《地质学报》2021年第7期2160-2168,共9页Acta Geologica Sinica

基　　金：国家自然科学基金项目(编号41473117);中国科学院功能开发技术创新项目(编号2020g102);第二次青藏高原综合科学考察研究(编号2019QZKK0805)联合资助的成果。

摘　　要：建立温度与硼同位素变化的关系是研究沉积硼酸盐成岩-变质作用中硼同位素地球化学行为的关键,这有助于准确认识硼酸盐δ^(11)B值所代表的地质意义。本文以自然产出的三方硼砂和钠硼解石为材料,结合热分解特征研究了不同热作用过程中硼同位素变化。200℃下硼酸盐经历结晶水的脱水反应,此过程中三方硼砂和钠硼解石δ^(11)B值分别由6.48±0.14‰降低到5.41±0.27‰和由-13.27±0.11‰降低到-15.93±0.23‰。1‰~3‰的硼同位素变化与自然界观察到的由脱水作用形成的次生硼酸盐比原生硼酸盐δ^(11)B值低2‰~4‰的现象一致。200~300℃间经历羟基的脱水过程,伴随着1‰~2‰的硼同位素降低,此过程中三方硼砂和钠硼解石δ^(11)B值分别由5.41±0.27‰降低到3.73±0.26‰和由-15.93±0.23‰降低到-17.11±0.11‰。300~670℃间硼酸盐经历物相转化过程,三方硼砂和钠硼解石δ^(11)B值分别由3.73±0.26‰升高到4.59±0.08‰和-17.11±0.11‰升高到-16.08±0.20‰。此过程1‰的硼同位素升高是由硼-氧骨架结构改变造成的。本实验结果可以指示沉积硼酸盐的形成过程,以及埋藏过程中的硼同位素变化规律。硼酸盐脱水过程中会形成具有更高δ^(11)B值的流体,其可能显著影响内生硼矿的硼同位素组成,因此讨论硼同位素地质意义不能与海相-非海相环境中的硼同位素组成进行简单对比,需要考虑流体演化过程中的硼同位素变化。Establishing the relationship between temperature andδ^(11)B value change is the key to understanding the boron isotope geochemical behavior of sedimentary borate during diagenesis-metamorphism.Natural tincalconite and ulexite used to study this relationship help to correctly understand the geological significance ofδ^(11)B value.When dehydration reaction of crystallization water occurs under 200℃,theδ^(11)B values of the tincalconite and ulexite decrease from 6.48±0.14‰to 5.41±0.27‰and from-13.27±0.11‰to-15.93±0.23‰,respectively.The 1‰~3‰δ^(11)B value change is consistent with the phenomenon observed in nature that theδ^(11)B value of secondary borate formed by dehydration is 2‰~4‰lower than that of primary borate.When the dehydration of hydroxyl group occurs between 200~300℃,it causes 1‰~2‰δ^(11)B value reduction,and theδ^(11)B values of tincalconite and ulexite decrease from 5.41±0.27‰to 3.73±0.26‰and from-15.93±0.23‰to-17.11±0.11‰,respectively.At 300~670℃,the borate undergoes amorphization and recrystallization,theδ^(11)B values of the tincalconite and ulexite increase from 3.73±0.26‰to 4.59±0.08‰and from-17.11±0.11‰to-16.08±0.20‰,respectively.The 1‰δ^(11)B value increase is caused by the change of boron-oxygen framework.The results can indicate the formation process of the sedimentary borate and the variation trend ofδ^(11)B value during the burial process.The fluid with higherδ^(11)B values is formed during the dehydration of borate,and may significantly affect the boron isotope composition of endogenous boron ore.Therefore,it is necessary to consider the boron isotope changes during fluid evolution when discussing the geological significance of boron isotope.

关 键 词：沉积硼酸盐 脱水作用 热分解 硼同位素变化 

分 类 号：P597[天文地球—地球化学]