Progress in Study of Boron Geochemistry in High Temperature Geothermal Fluids
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摘要: 硼是高温地热流体中典型的特征元素之一,探讨其物质来源和富集规律,对认识地热系统的形成与演化以及地热资源的合理开发具有重要的指导意义.同时,硼也是一种典型的有害元素,伴随地热流体排放到地表后,会对地热区及周边环境造成严重的负面效应.近年来,在高温地热资源正在被大规模开发利用的背景下,高温地热流中硼的地球化学起源及其环境效应研究已引起国内外相关学者的广泛关注.本文综述了高温地热流体中硼的地球化学特征、物质来源以及环境地质效应,在此基础上总结了后期需要进一步加强的方向,以期为地热资源的合理开采、地热田周边地区的环境保护提供借鉴思路和指导作用.Abstract: Boron is one of the typical characteristic elements in high temperature geothermal fluids. Its origin and enrichment in geothermal fluids are of great significance to understanding the formation and evolution of geothermal system and the rational development of geothermal resources. At the same time, boron is one of the harmful elements, and will cause serious negative effects on the geothermal area and its surrounding environment as geothermal fluids discharged to the surface. In recent years, under the background of large-scale exploitation and utilization of high temperature geothermal resources, the origin of boron in high temperature geothermal fluids and its environmental effects have attracted extensive attention. In this paper, the geochemistry characteristics, sources and environmental geological effects of boron in geothermal fluids are reviewed, aiming to provide insights for the rational exploitation of geothermal resources and the environmental protection near the geothermal areas.
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Key words:
- geochemistry /
- boron /
- environmental effect /
- geothermal system /
- high temperature geothermal fluid
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在国际地热界,高温地热系统是指热储温度高于150 ℃的地热系统,主要分布于板缘地热带(如环太平洋地热带、地中海‒喜马拉雅地热带、大西洋中脊地热带和红海‒亚丁湾‒东非裂谷地热带)和板内热点地区(如美国黄石国家公园等).在我国,高温地热系统主要分布于滇藏地热带(地中海‒喜马拉雅地热带的一部分)的藏南、川西、滇西以及环太平洋地热带的台湾地区.这类地热系统因其异常高的热储温度,具有巨大的开发利用价值(郭清海,2020;汪新伟等,2022),在地热发电、住房供暖、水产养殖、医疗保健、温泉洗浴等方面得到了广泛应用.与一般中低温地热水相比,高温地热流体往往富集多种特征元素(如硼、氯、砷、锂、钨、锑等),这些特征元素往往蕴含着地热系统形成及演化过程中详尽的地球化学信息(Kaasalainen and Stefánsson,2012;吕苑苑等,2014;Kaasalainen et al., 2015;Barnes et al., 2019;Guo et al., 2019b;Zhao et al., 2019;Cullen et al., 2021;郭清海和杨晨,2021).对特征元素的物质来源和富集规律进行研究,可为深刻认识地热系统的成因机制及演化规律奠定基础,为地热资源的合理开发提供科学依据(刘明亮等,2020;朱喜等,2021).另一方面,某些特征元素(如硼、氟、砷、钨、锑等)伴随地热流体排放到地表后,也可能会对地热区周边环境造成污染(Guo et al., 2008, 2017, 2019a, 2019b, 2020, 2021;张庆等, 2014, 2015),严重威胁当地居民的身体健康以及动植物的生长.综上,高温地热流体中的特征元素具有重要的科学研究意义,近年来已引起国内外相关学者的广泛关注,并已成为地球科学与环境科学领域研究的热点问题(Song et al., 2021).
硼,作为一种易溶不相容元素,是高温地热流体中最典型的特征元素之一,常被用来分析地热流体的起源以及相关的水文地球化学过程(Ellis,1970;Arnórsson,1985;Arnórsson and Andrésdóttir,1995).同时,硼在环境介质中也具有很大的危害性,过量摄入会造成人体和动物慢性中毒,使肝、肾脏受到损害,脑和肺出现水肿,并抑制植物生长(Çöl and Çöl,2003).根据世界卫生组织标准,人体饮用水中硼的含量限值为0.5 mg/L,植被灌溉水的硼含量限值为1 mg/L(World Health Organization,2008).高温地热流体常常具有硼含量很高的特点,从泉口或地热井排出以后将会引发负面的环境效应,尤其当地热能大规模开发利用以后,排放到环境中的硼总量将会更大,造成地热区周边环境介质的硼污染问题也将更为严峻.近年来国内外已开展部分高温地热流体硼地球化学方面的研究工作(吕苑苑等,2014;Yuan et al., 2014;Yamaoka et al., 2015;Zhang et al., 2015;Purnomo et al., 2016;Wu et al., 2016;Liu et al., 2019;Zhao et al., 2019),总的来说这些工作都取得了丰硕的成果,但整体而言还比较零散且缺乏系统性,对高温地热流体中硼的物质来源认识仍比较笼统,对富硼地热水排放引起的环境效应研究也略显匮乏.为此,本文对国内外高温地热流体中硼的地球化学研究进行了概述和评论,在此基础上总结了后期需要进一步加强的工作,以期为地热成因硼污染的治理、地热田周边地区的环境保护、地热资源的合理开采提供借鉴思路和指导作用.
1. 高温地热流体硼的地球化学特征
1.1 地热流体中硼的含量及硼同位素特征
在高温地热流体中,硼是一种易溶元素,其含量变化范围可达几个数量级,从低于1 mg/L至高于1 000 mg/L(表 1).整体而言,高温地热流体常具有硼含量较高的特点,如美国黄石国家公园热泉水硼含量达28.5 mg/L(Palmer and Sturchio, 1990),日本Kagoshima地热区热水硼含量达24.8 mg/L(Oi et al., 1996),土耳其Menderes Massif地热田热水硼含量达54.2 mg/L(Vengosh et al., 2002),新西兰Taupo火山地热区热水硼含量可达82.1 mg/L(Millot et al., 2012),印度尼西亚Java地热田热水硼含量达93.2 mg/L(Purnomo et al., 2016),希腊Milos Island地热田热水硼含量达99.0 mg/L(Wu et al., 2016),墨西哥Los Humeros地热田热水硼含量达725.0 mg/L(Bernard et al., 2011),新西兰Negwha地热田热水硼含量高达1 101.6 mg/L(Aggarwal et al., 2003).而在我国,调查表明西藏自治区广泛分布大量富硼地热系统,拉萨地区羊八井地热井水硼含量达165.4 mg/L(Yuan et al., 2014),山南地区古堆热泉水中硼含量达95.8 mg/L(Liu et al., 2020),日喀则地区搭格架和色米热泉水中硼含量分别高达106.9 mg/L和525.7 mg/L(Liu et al., 2020).不同地热系统中硼含量较大的差异主要与其补给来源密切相关.
表 1 世界范围内典型高温地热系统地热水中B含量和B同位素特征Table Supplementary Table Boron concentrations and δ11B values of geothermal waters in typical high temperature geothermal systems worldwide典型地热区 采样温度(℃) B(mg/L) δ11B(‰) 数据来源 新西兰Negwha地热田 38~180 259.2~1 101.6 ‒3.8~‒3.2 Aggarwal et al., 2003 新西兰Taupo火山地热区 205~320 17.5~82.1 ‒6.7~‒1.9 Bégué et al., 2017 法国Limagne盆地 12~73 0.6~6.9 ‒6.3~12.6 Millot et al., 2007 日本Kagoshimadi地热区 29~102 0.6~24.8 2.1~39.4 Oi et al., 1996 意大利Vulcano地热田 21~99 3.0~9.1 ‒7~1 Leeman et al., 2005 意大利Cimino‒Vico火山区 25~62 0.1~1.6 ‒8.4~‒4.1 Battistel et al., 2016 墨西哥Los Humeros地热田 ‒ 214.0~725.0 ‒1.7~0.3 Bernard et al., 2011 以色列Dead Sea地热区 15~39 20.5~31.3 51.7~54.9 Vengosh et al., 1991 土耳其Menderes Massif地热区 35~224 1.1~54.2 ‒2.3~18.7 Vengosh et al., 2002 希腊Milos Island地热区 63~116 1.7~99.0 2.1~40.5 Wu et al., 2016 美国黄石国家公园 32~140 0.4~28.5 ‒9.3~4.4 Palmer et al., 1990 印度尼西亚Java地热区 33.2~102.0 2.7~93.2 ‒2.4~28.7 Purnomo et al., 2016 中国西藏羊八井地热田 86~87 10.2~165.4 ‒13.8~‒8.4 Yuan et al., 2014;Zhang et al., 2015 中国西藏羊易地热田 79~89 38.5~45.7 ‒9.7~‒5.0 Yuan et al., 2014 中国西藏搭格架地热田 37~86 1.1~106.9 ‒16.3~‒11.7 吕苑苑等,2014;Liu et al., 2019 中国西藏曲卓木地热田 57~77 21.9~44.6 ‒11.3~‒7.1 Liu et al., 2019 中国云南热海地热田 56~96 4.6~10.5 ‒6.3~‒4.5 吕苑苑等,2014 中国吉林长白山火山地热区 21~82 0.2~4.4 ‒13.3~35.9 Zhao et al., 2019 从20世纪80年代起,随着硼同位素分析手段的成熟以及测试精度的提高(Spivack and Edmond, 1986;Xiao et al., 1988;Al-Ammar et al., 2000;吕苑苑等,2008),硼同位素被广泛应用于古气候和古环境反演(Hemming and Hanson, 1992a;Hönisch and Hemming, 2005;Pearson et al., 2009;Trotter et al., 2011;Foster et al., 2012)、矿床成因(Jiang et al., 1999;Hu et al., 2015;Su et al., 2016)、盐湖演化(Liu et al., 2000;Wei et al., 2014;Fan et al., 2015)、地下水污染源示踪(Guinoiseau et al., 2018;Nigro et al., 2018)等领域.近年来在高温地热研究领域,硼同位素的研究和应用也引起相关学者的广泛关注,主要用于追溯地热流体的来源(Purnomo et al., 2016;Wu et al., 2016)、识别地热系统中的相关地球化学过程(Aggarwal et al., 2000;Leeman et al., 2005)、示踪地热水排泄引发的环境效应等(Pennisi et al., 2006;Yuan et al., 2014).不同地热系统往往也呈现出不同的硼同位素特征(表 1),如新西兰Negwha地热田和Taupo火山地热区热水中δ11B范围分别为-3.8‰~-3.2‰(Aggarwal et al., 2003)和-6.7‰~-1.9‰(Millot et al., 2012),意大利Vulcano地热区和Cimino-Vico火山区热水中δ11B范围分别为-7.0‰~1.0‰(Leeman et al., 2005)和-8.4‰~-4.1‰(Battistel et al., 2016),美国黄石国家公园地热水中δ11B范围为-9.3‰~4.4‰(Palmer and Sturchio, 1990),法国Limagne盆地热水中δ11B范围为-6.3‰~12.6‰(Millot et al., 2007),印度尼西亚Java地热田热水中δ11B范围为-2.4‰~28.7‰(Purnomo et al., 2016),日本Kagoshima地热区热水中δ11B范围为2.1‰~39.4‰(Oi et al., 1996),希腊Milos Island地热田热水中δ11B范围为2.1‰~40.5‰(Wu et al., 2016),以色列Dead Sea地热区热水中δ11B范围为51.7‰~54.9‰(Vengosh et al., 1991).近年来国内也已开展部分高温地热流体硼同位素地球化学方面的研究工作,但研究区大多集中在滇藏地热带,如西藏羊八井地热井水中δ11B范围为-13.8‰~-8.4‰(Yuan et al., 2014;Zhang et al., 2015),西藏羊易地热井水中δ11B范围为-9.7‰~-5.0‰(Yuan et al., 2014),西藏搭格架热泉水中δ11B范围为-16.3‰~-11.7‰(Liu et al., 2019;吕苑苑等,2014),西藏曲卓木热泉水中δ11B范围为-11.3‰~-7.1‰(Liu et al., 2019),西藏朗久热泉水δ11B范围为-10.9‰~0.1‰(吕苑苑等,2014),云南腾冲热海热泉水δ11B范围为-6.3‰~-4.5‰(吕苑苑等,2014).不同地热系统硼同位素值的差异同样是由其不同来源引起的,虽然气‒液相分离和粘土矿物或热液蚀变矿物的吸附与共沉淀等诸多过程都会造成一定程度的硼同位素分馏(Palmer et al., 1987),但是这些过程通常不足以影响地热流体硼同位素对物源信息的记录,因此硼同位素常被用于示踪地热流体的物质来源.
1.2 地热流体中硼同位素分馏机理
自然界硼有两种稳定同位素:10B(19.82%)和11B(80.18%),由于它们之间相对较大的质量差,使得δ11B值的变化范围较大(-70‰~+75‰)(Vengosh et al., 1991;Barth,1993;Hogan and Blum, 2003).随着硼同位素分析技术的发展,对硼同位素的地球化学行为及其分馏机理的认识也越来越深入.硼没有价态变化,且不参与氧化还原反应,在一般水环境中,硼主要以H3BO3或B(OH)4-的形态存在(在某些特殊的水环境中可能存在其他硼的形态,下文详述).热力学平衡交换是导致硼同位素分馏的主要原因,反应式为:10B(OH)3 + 11B(OH)4- → 11B(OH)3 + 10B(OH)4-,重同位素11B在H3BO3中富集,而轻硼同位素10B在B(OH)4-中富集.硼同位素的分馏取决于体系中H3BO3和B(OH)4-的相对含量,而两者的相对含量受控于溶液的pH值和硼浓度,其平衡反应表达式为:B(OH)3 + H2O → B(OH)4- + H+.当溶液pH小于7时,硼主要以H3BO3形式存在;当pH大于10时,则以B(OH)4-为主,pH介于7至10时,则两者共存于溶液中.在地热流体中,气‒液相分离、蒸发岩矿物的析出、粘土矿物的吸附、硼与碳酸盐类矿物共沉淀等过程都可能会因硼同位素在不同体系中富集程度的不同而产生分馏.
高温地热流体从深部热储向上运移至排泄出地表的过程中,由于温度、压力的变化会发生沸腾和气‒液相分离,此过程是高温地热系统中造成硼同位素分馏的主要过程,并受到了广泛关注.气‒液相分离过程中,富集11B的H3BO3易于挥发进入地热蒸汽中,而富集10B的B(OH)4-倾向于残留在地热流体中,因此气‒液相分离的结果是地热蒸汽中δ11B值增大,而地热流体中δ11B值减小.实验结果表明在温度140~300 ℃范围内,气‒液相分离过程造成硼同位素分馏值(Δliq-vap)约为-3‰~-1‰,并且随着温度升高分馏效应减小,在温度达到400 ℃以上,分馏效应可忽略不计(Spivack et al., 1990;Liebscher et al., 2005).Purnomo et al.(2016)研究了印度尼西亚Java地热系统硼同位素的变化,结果发现地热蒸汽的δ11B值要比地热水高3.8‰,主要原因即是气‒液相分离过程造成的.Yuan et al.(2014)研究了气‒液分离过程对羊八井地热流体中硼同位素的影响,结果显示随着地热蒸汽比例的增加,地热水硼同位素值降低,同样也说明气‒液分离过程造成了硼同位素的分馏,但分馏值小于0.9‰.上述研究都表明,在高温地热系统中气‒液相分离过程造成的硼同位素分馏效应整体而言不明显,但笔者认为在一些以蒸汽为主的地热系统(如美国Geysers地热田,Moore et al., 2001;印度尼西亚Kamojang地热田,Sofyan et al., 2015),由于强烈的水‒汽分离现象,硼同位素的分馏是不容忽视的,非常有必要补充和完善硼同位素在这类地热系统中分馏理论的研究.
蒸发岩矿物的析出(Swihart et al., 1986;Oi et al., 1989;Liu et al., 2000;肖军等,2012)、粘土矿物的吸附(Palmer et al., 1987;Spivack and Edmond, 1987;Spivack et al., 1987)、硼与碳酸盐类矿物共沉淀(Hemming and Hanson, 1992b;肖应凯等, 2006, 2008)等地球化学过程都会伴随硼同位素的分馏,主要表现为富集10B的B(OH)4-优先进入固相,流体相δ11B值增大,但也存在部分“反分馏”(即液相δ11B值减小)的情况(卿德林等,2012).目前,关于蒸发岩矿物析出过程中硼同位素分馏效应的研究主要集中于盐湖体系,在地热系统中,Purnomo et al.(2016)在研究印度尼西亚Java地热系统硼同位素特征时发现,地热水中硼同位素值高达34.86‰,认为其主要是由于蒸发岩矿物的析出作用导致的.粘土矿物(如高岭石、伊利石、蒙脱石等)的吸附过程主要是水溶液中四面体结构的B(OH)4-优先被吸附在粘土矿物表面,后续被进一步结合到由Si-O四面体和Al-O八面体组合而成的晶格层,最终置换层间Si和Al,进而造成硼同位素的分馏.Palmer et al.(1990)研究了美国黄石公园地热水的硼同位素特征,结果发现Beak Creek和Laduke热泉分别具有最低的硼浓度和最高的硼同位素值,并将其归因于次生矿物的吸附.Aggarwal et al.(2000)认为在地热系统次生矿物的吸附(或共沉淀)过程中硼同位素的分馏与矿物的种类无关,主要取决于体系的温度和pH值,温度升高会降低硼同位素的分馏系数,而pH值则会影响溶液中B(OH)4-和H3BO3的相对含量,最终控制硼同位素的分馏程度.当前,关于硼与碳酸盐类矿物共沉淀过程中硼同位素分馏效应的研究,主要集中于海相碳酸盐沉淀过程,用于重建古海洋和古气候.早期研究表明,硼只以较轻的B(OH)4-的形式掺入海洋碳酸盐,因此可视海水B(OH)4-的B同位素组成等于海洋碳酸盐的B同位素值(Hemming and Hanson, 1992b;Hemming et al., 1995;Sanyal et al., 2000).而近些年越来越多的研究都表明,碳酸盐类矿物(如方解石、文石等)共沉淀过程中存在有较高比例H3BO3的掺入(Pagani et al., 2005;肖应凯等,2008;Mavromatis et al., 2015;Farmer et al., 2019),显然碳酸盐沉淀过程中硼同位素分馏过程还有待进一步研究.在诸多以碳酸盐岩为热储的地热系统中(如西藏曲卓木、卡乌等地热区),泉口广泛分布大面积钙华,地热水中的硼主要以与碳酸盐矿物共沉淀的方式向固相迁移并蓄积.与海相沉积环境相比,高温地热流体温度和硼含量更高,碳酸盐沉淀过程也更快,其同位素分馏效应极大可能有别于海相碳酸盐沉积过程,后续可进一步对此过程开展研究,以丰富地热系统中硼同位素分馏理论.
2. 高温地热流体中硼的来源
2.1 世界范围内典型高温地热系统硼的来源
世界范围内,典型高温水热系统地热水的B含量与δ11B值关系图显示(图 1),地热流体中硼的来源主要包括:岩石的淋滤、岩浆流体的补给以及海水或浅层冷水的混合.整体而言,来源于海水或浅层冷水混合的地热水呈现出较低的B含量和较高的δ11B值,来源于岩浆流体补给的地热水表现出较高的B含量和较低的δ11B值,来源于岩石淋滤的地热水显示出中等的B含量和δ11B值(图 1).首先,不同地热系统地热水的物质来源通常都与岩石的淋滤密切相关(Arnórsson and Andrésdóttir,1995;Yamaoka et al., 2015;Wu et al., 2016),而B作为一种易溶元素,很容易从围岩矿物中淋滤出来,因此岩石的淋滤是地热水中B重要的来源,如新西兰Taupo火山区(Millot et al., 2012)、法国Limagne盆地(Millot et al., 2007)、意大利Cimino-Vico火山地热区(Battistel et al., 2016)、冰岛Nesjavellir地热区(Aggarwal et al., 2000)、美国黄石公园水热系统(Palmer and Sturchio, 1990).对于这种硼来源,围岩的类型及围岩中B的含量是影响地热水中B浓度的重要因素.例如,以玄武岩为热储的地热系统(如冰岛Southern Lowlands)地热水中硼的含量范围为1~10 mg/kg(Arnórsson and Andrésdóttir,1995),以沉积岩和变质岩(如意大利Lardereloo、新西兰Ngahwa)和以英安岩‒流纹岩等火山岩(如墨西哥Los Azufres)为热储的地热系统地热水中硼的含量一般高于100 mg/kg(Bernard et al., 2011).其次,对于诸多岩浆热源型地热系统,岩石的淋滤往往并不能完全解释其异常高的硼浓度,岩浆流体的补给被认为是岩浆热源型地热系统地热水中B的另一重要来源.Leeman et al.(2005)报道了意大利Vulcano地热区热水的B含量达240 mg/L,Bernard et al.(2011)报道了墨西哥Humeros地热田地热流体中B含量范围为100~150 mg/L,Wu et al.(2016)报道了希腊爱琴海Milos岛地热水中B含量达99.4 mg/L,这些异常高的硼浓度都被归因于岩浆流体的补给.然而,到目前为止,由于技术条件的限制,岩浆流体的地球化学组成尚无公认的测试或计算数据,因此岩浆流体的补给对岩浆热源型地热系统中B的贡献仍存在较大的学术争议,后续还需进一步开展岩浆热源型地热系统硼地球化学来源方面的研究.再次,分布于滨海地区的地热系统,如以色列Dead Sead地热区(Vengosh et al., 1991)、印度尼西亚Java地热系统(Purnomo et al., 2016)、日本Kagoshima地热区(Oi et al., 1996),海水或浅层冷水的混合往往也是地热流体中硼的重要来源.
图 1 世界范围内典型高温地热系统地热水中B含量与δ11B值关系数据来源于Oi et al.(1996)、Aggarwal et al.(2000)、Vengosh et al.(2002, 1991)、Leeman et al.(2005)、Millot et al.(2007, 2012)、Bernard et al.(2011)、吕苑苑等(2014)、Yuan et al.(2014)、Zhang et al.(2015)、Battistel et al.(2016)、Wu et al.(2016)Fig. 1. Plot of B concentration vs. δ11B value of geothermal waters in typical high temperature geothermal systems worldwide2.2 中国西藏地热水中硼的富集机制
调查表明,在我国西藏自治区广泛分布大面积高温水热系统(廖志杰和赵平,1999),其中地热水具有硼含量异常高的特点,含量超过10 mg/L的地热区占70%左右(张知非等,1982).对于西藏地热水中硼的来源,目前为止还存在较大争议(张知非等,1982;吕苑苑等,2014;Zhang et al., 2015).张知非等(1982)认为,印度板块与欧亚板块的碰撞造成西藏硅铝质地壳的重熔,重熔岩浆(中酸性岩浆)的硼含量较基性玄武质岩浆高得多,而硼易于伴随岩浆挥发组分进入到深部地热流体中,因此,以重熔岩浆为热源的西藏地热系统具有硼含量很高的特点.吕苑苑等(2014)通过对比分析西藏地热区与世界范围内其他地热田,认为岩浆脱气过程并不能为地热水提供丰富的硼,西藏地热水中的硼主要来源于富硼岩石的淋滤.Zhang et al.(2015)分析了西藏典型地热系统硼的地球化学特征,认为高温水热系统中硼的来源主要为岩浆流体的补给,而低温水热系统主要源自围岩矿物的溶解.笔者近年来对西藏地热水中硼的地球化学特征进行了研究(刘明亮,2018;Liu et al., 2019, 2020),认为西藏地热水中异常高的硼浓度是由多方面因素共同决定的.
首先,位于雅鲁藏布江缝合带北缘和班公错‒怒江缝合带南缘(对应于印度板块的俯冲区域)的大部分西藏地热系统,极大可能是岩浆热源加热而形成(Guo et al., 2019b),例如西藏羊八井和羊易地热系统深部的岩浆热源已被大量地球物理资料所证实(Brown et al., 1996;Chen et al., 1996;Kind et al., 1996).虽然到目前为止,人们在诸多其他西藏高温地热区(如搭格架、色米、古堆等)还未开展地球物理方面的工作,但是大量区域地质、地球化学以及地表显示等证据都指示,这些地热系统深部也极大可能存在岩浆热源,因此岩浆流体的补给是地热水中B的重要来源.然而值得注意的是,同属于滇藏地热带并且已被证实存在岩浆热源的云南热海地热田(白登海等,1994;上官志冠等,2000),中性/弱碱性热泉水中硼的浓度范围为2.0~17.0 mg/L(Guo et al., 2014),远低于岩浆热源型的西藏地热水(如羊八井、羊易、搭格架、色米、古堆等).因此可推测,岩浆房的化学组成可能控制着释出的岩浆流体中的B含量,换言之,地热系统深部不同的岩浆类型(如玄武质、安山质或流纹质岩浆)可能影响着地热流体的B浓度.事实上,由于印度板块与欧亚板块的碰撞,藏南地区形成了巨厚的地壳层(70~80 km;Yuan et al., 1997;Zhao et al., 2001;Zhang et al., 2011),地幔楔向上涌动过程中不可避免地会混染大量壳源物质或者诱发地壳岩石(如酸性岩浆岩和海相沉积岩)的局部重熔,硼作为一种不相容元素倾向于向流体相迁移,从而实现了岩浆流体中硼的富集.与之相对应的是,云南腾冲在大地构造位置上处于俯冲形成的弧后伸展背景(朱炳泉和毛存孝,1983;穆治国等,1987;Lei et al., 2009),地壳厚度仅约30 km左右(Yang et al., 2013),幔源岩浆上升过程中可能只混入了较小比例的壳源物质,腾冲热海地热气体较高的3He/ 4He比(1.01~4.37 Ra)(上官志冠等,2000),而西藏地热气体较低的3He/4He比(0.02~0.98 Ra)(Yokoyama et al., 1999;Hoke et al., 2000;侯增谦和李振清,2004)即是重要佐证.因此,由于壳源岩浆释放的岩浆流体的补给,西藏地热水中硼的含量远高于腾冲热海.另外,青藏高原不同时代不同类型的岩石都具有B含量很高的特点,如前寒武系的变质岩,白垩纪末火成岩以及新生代流纹岩、花岗岩、多种沉积岩中B含量都显著高于全球相应的岩石,尤其是分布于冈底斯的白垩纪末‒古近纪火山岩和高喜马拉雅期含电气石淡色花岗岩(郑绵平等,1989).更为重要的是,西藏地热水中的B含量与热储岩性呈现出良好的对应关系,例如,以海相碳酸岩盐为热储层的拉孜县锡钦曲灿和亚东县康布曲灿热泉B含量分别为0.36 mg/L和0.67 mg/L,远低于其他以酸性岩浆岩为热储的地热系统(吕苑苑等,2014);与之相对应的是,青藏高原碳酸盐岩的B含量(平均值为51×10-6)远低于酸性岩浆岩(平均值为378×10-6)(郑绵平等,1989).由此可见,富硼岩石的淋滤必然也是西藏地热水中B含量异常高的重要原因.
综上,笔者提出了西藏地热水中硼的富集机制(图 2).对于具有岩浆热源的西藏地热系统(如羊八井、羊易、搭格架、色米、古堆等),大量壳源物质的混染使得岩浆流体成分较其他岩浆热源型地热系统(如我国云南热海、美国黄石国家公园等)更为富集B,岩浆流体的补给完成了地热水中B的初步富集;随后,在岩浆热源型地热系统更高的热储温度条件下,地热水与富硼围岩发生更为强烈的水‒岩相互作用,淋滤围岩中大量的B,完成了地热水中B的再次富集.对于非岩浆热源型地热系统(如西藏曲卓木等),地热水中的B主要源于围岩的溶滤,而青藏高原大部分岩石具有硼含量很高的特点,因此地热水中硼含量也相对较高(如曲卓木热泉水中B浓度范围为21.9~44.6 mg/L;Liu et al., 2019),但整体而言低于岩浆热源型地热系统.
图 2 西藏地热水中硼富集机制(改自Liu et al., 2019)Fig. 2. Boron enrichment mechanisms of geothermal waters in Tibet (modified from Liu et al., 2019)3. 高温地热流体中硼的赋存形态及其环境效应
3.1 高温地热流体中硼的赋存形态
近年来,关于水资源中典型有害元素的生态毒理效应研究表明,有害组分的毒性不仅与其总浓度有关,而更取决于它的赋存形态(任福弘等,1996;Planer-Friedrich et al., 2008;王敏黛等,2016).在一般水环境中,硼主要以H3BO3(中酸性条件下)或B(OH)4-(碱性条件下)的形态存在(Tabelin et al., 2014),当总硼含量较高时(pH值介于6~11),则可能会形成多聚硼氧配阴离子(如B3O3(OH)4-、B4O5(OH)42-、B5O6(OH)4-等)(Schmidt et al., 2005),而当水溶液中同时富硼和富氟时,还可能会形成氟硼络合物(如BF(OH)3-、BF2(OH)2-、BF3(OH)-、BF4-等)(Katagiri et al., 2006).随着振动光谱的广泛应用,已有学者对硼酸盐饱和水溶液中多聚硼氧配离子的拉曼光谱进行了研究,并讨论了总硼浓度、pH、温度等因素对硼存在形态以及相互作用的影响(张爱芸和姚燕,2007;张林进和叶旭初,2008;宋月月等,2014;周永全,2014).然而,迄今为止鲜有研究涉及硼在天然水体(尤其是地热流体)中的赋存形态及转化机理.笔者团队近年来通过收集不同形态硼的热力学数据,更新PHREEQC软件的数据库,模拟计算了西藏地热水中不同形态硼的含量(Liu et al., 2020).结果表明,地热水中硼的赋存形态主要包括H3BO3、B(OH)4-、多聚硼氧配阴离子(B3O3(OH)4-、B4O5(OH)42-和B5O6(OH)4-)和氟硼络合物(BF(OH)3-、BF2(OH)2-、BF3(OH)-和BF4-),各种形态硼的相对含量受地热水温度、pH和氟硼浓度控制;H3BO3是所有地热水中硼最主要的存在形态,在碱性条件下,随着温度和硼浓度升高,B(OH)4-和多聚硼氧配阴离子含量增加,而氟硼络合物在所有地热水样品中含量几乎可忽略不计,只在酸性条件下,当氟硼浓度比达到很高时才可能形成氟硼络合物(Liu et al., 2020).需要说明的是,上述对地热流体中硼赋存形态的分析是基于水化学模拟软件的理论计算,进一步的研究工作还需通过振动光谱(如红外光谱、拉曼光谱)等实验方法的定性识别和定量测试.
3.2 富硼地热水排放的环境效应
硼在环境介质中具有很大的危害性,富硼地热水的排放无疑会对地热区周边环境介质(包括水环境和土壤环境)产生严重的负面效应(郭清海,2022).例如,在我国西藏羊八井和羊易地热田,由于地热废水的排放造成羊八井藏布曲(河)下游河水的硼含量达3.8 mg/L(Guo et al., 2008),羊易罗朗河下游河水硼浓度达0.7 mg/L(Guo et al., 2009),均高于世界卫生组织推荐的人体饮用水中硼浓度的上限值(0.5 mg/L),并且已对当地居民和动植物造成了严重的危害(佟伟等,2000).除了水环境以外,地热水的排泄对地热区地表沉积物和土壤环境也会造成一定程度的污染.具体而言,当地热流体从泉口排放到地表以后,流体中的硼能够与沉积物中碳酸盐类矿物、铁锰氧化物、粘土矿物、有机质等表面活跃的羟基官能团进行配位体交换,从而被吸附到固相沉积物中(Chen et al., 2009;Ruiz-Agudo et al., 2012).经过长时间的累积,地热区地表沉积物和土壤往往含有远高于地壳平均丰度的硼含量,如在西藏羊八井、搭格架、色米和古堆地热区采集的泉口沉积物和土壤中硼含量范围为22.5×10-6~1 653.9×10-6(Liu et al., 2020),显著高于全球范围内土壤中硼含量(2×10-6~100×10-6;Türker et al., 2014).值得关注的是,当环境条件发生变化时,沉积物中的硼可能会从固相中释放,重新进入水环境引起水体污染.例如,周期性的降水事件即可淋滤出沉积物中可交换态的硼,而长期的风化过程可能会导致碳酸盐结合态、铁锰氧化物结合态、有机物/硫化物结合态甚至残余态硼从固相中释放出来.到目前为止,关于地热来源硼环境效应方面的研究仍非常有限,对于硼在地表环境中的迁移蓄积过程还有待进一步探讨,尤其在当前高温地热流体正在被大力开发利用的背景下,排放到环境中的硼总量巨大,探讨富硼地热水排泄后的环境地质效应,对于地热田周边地区的环境保护具有重要意义.
4. 高温地热流体硼地球化学研究展望
高温地热流中硼的地球化学研究不仅对深刻认识地热流体的物质来源与形成演化具有重要的理论意义,对地热资源的合理开发、地热田周边地区的环境保护也具有重要的实际意义.为此,本文综述了国内外在高温地热流体硼地球化学研究方面的主要认识和最新进展,以期进一步促进硼地球化学在地热研究中的应用.基于上述的研究进展,笔者初步认为,未来可进一步加强以下几方面的工作:
(1)加强地热流体来源硼的定量贡献研究.目前,国内外学者对地热流体中硼的地球化学起源进行了详细研究,但基本都围绕于定性推断硼的物质来源,而针对上述各种物源端元(如岩浆流体的补给、围岩矿物的溶滤、海水或浅层冷水的混合等)对地热水中硼的定量贡献研究仍非常有限,尤其是对于岩浆热源型的富硼地热系统而言,硼的来源具有争议并且极有可能是多种端元共同作用的结果,定量评估各种物源端元对硼来源的影响程度对于揭示高温地热流体中硼的富集机制具有重要意义.例如,可选取代表性岩浆热源型地热系统(如西藏羊八井、羊易、搭格架等),通过室内实验还原深部流体‒岩石相互作用过程,并将实验结果与基于硼同位素质量平衡的模拟结果(Yamaoka et al., 2015)进行对比分析,探讨围岩矿物的淋滤对地热流体中硼的定量贡献;结合上述流体‒岩石相互作用硼的淋滤量以及热水中硼的实际浓度,评估岩浆流体的补给对硼的贡献比例.
(2)加强地热系统中硼同位素分馏理论的研究.如前所述,目前关于硼同位素分馏理论的研究主要集中于盐湖体系和海洋沉积环境,在地热系统中的研究相对而言还比较零散且缺乏系统性.而硼同位素是追溯地热流体物质来源,分析地热系统中相关水文地球化学过程的一种有效手段,有必要选取代表性地热系统加强硼同位素分馏理论的研究.例如,可选取蒸汽型地热系统(如西藏羊八井),采集地热水和地热蒸汽样品,探讨水‒汽分离过程硼同位素的分馏机制;选取沉积盆地型地热系统(如河北雄安新区),沿径流方向采集地热水样品,探讨粘土矿物的吸附等过程对硼同位素分馏效应的影响;选取钙华广泛分布的地热系统(如西藏曲卓木、卡乌等),采集地热水和钙华样品,探讨碳酸盐类矿物共沉淀过程中硼同位素的分馏机理.
(3)建立富硼地热流体中硼形态的定性识别和定量测试方法.目前,振动光谱是研究和识别水溶液中硼存在形态的主要方法,但是由于检测限的原因,硼形态的特征振动频率只在硼浓度很高的条件下才能获取,因此现阶段对硼赋存形态的研究基本都围绕实验室配置的硼酸盐饱和溶液以及盐湖卤水,鲜有研究涉及其他天然水体(尤其是地热水).高温地热流体具有硼含量很高的特点,并且往往同时富氟,硼的存在形态显然不同于一般水体,如不考虑硼在地热水环境中各种赋存形态的地球化学行为,对于刻画富硼地热水排放的环境效应必然存在片面之处,因此非常有必要建立富硼地热流体中硼形态的定性识别和定量测试方法.可尝试选取硼含量异常高的地热水样品(如西藏色米热泉水中硼浓度可达525.7 mg/L,PHREEQC模拟结果显示多聚硼氧配阴离子百分含量可达36.6%;Liu et al., 2020),开展拉曼光谱或高磁场核磁共振分析,识别高硼地热水中硼的主要赋存形态,并利用峰面积积分法计算硼形态的相对含量.
(4)加强富硼地热水排放的环境效应研究.初步研究表明,富硼地热水的排放将会引发严重的负面环境效应,尤其在当前地热资源正在被大规模开发利用的背景下,地热区周边的环境污染问题也将更为严峻,加强地热来源硼环境效应方面的研究是非常有必要的.可选取受人为活动影响较小的典型地热区(如西藏搭格架),分析泉口沉积物、浅层地下冷水、河水(沿河流上游至下游连续取样)、河床沉积物、土壤等地表环境样品中硼含量变化趋势,探讨富硼热泉水的排放对地热田周边环境介质的污染程度,重点研究热泉水‒泉口沉积物(或河水‒河床沉积物)体系中硼的迁移蓄积过程,确定控制地热来源硼在地表环境中迁移蓄积的主导因素,为地热成因硼污染的治理提供借鉴思路.
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图 1 世界范围内典型高温地热系统地热水中B含量与δ11B值关系
数据来源于Oi et al.(1996)、Aggarwal et al.(2000)、Vengosh et al.(2002, 1991)、Leeman et al.(2005)、Millot et al.(2007, 2012)、Bernard et al.(2011)、吕苑苑等(2014)、Yuan et al.(2014)、Zhang et al.(2015)、Battistel et al.(2016)、Wu et al.(2016)
Fig. 1. Plot of B concentration vs. δ11B value of geothermal waters in typical high temperature geothermal systems worldwide
图 2 西藏地热水中硼富集机制(改自Liu et al., 2019)
Fig. 2. Boron enrichment mechanisms of geothermal waters in Tibet (modified from Liu et al., 2019)
表 1 世界范围内典型高温地热系统地热水中B含量和B同位素特征
Table 1. Boron concentrations and δ11B values of geothermal waters in typical high temperature geothermal systems worldwide
典型地热区 采样温度(℃) B(mg/L) δ11B(‰) 数据来源 新西兰Negwha地热田 38~180 259.2~1 101.6 ‒3.8~‒3.2 Aggarwal et al., 2003 新西兰Taupo火山地热区 205~320 17.5~82.1 ‒6.7~‒1.9 Bégué et al., 2017 法国Limagne盆地 12~73 0.6~6.9 ‒6.3~12.6 Millot et al., 2007 日本Kagoshimadi地热区 29~102 0.6~24.8 2.1~39.4 Oi et al., 1996 意大利Vulcano地热田 21~99 3.0~9.1 ‒7~1 Leeman et al., 2005 意大利Cimino‒Vico火山区 25~62 0.1~1.6 ‒8.4~‒4.1 Battistel et al., 2016 墨西哥Los Humeros地热田 ‒ 214.0~725.0 ‒1.7~0.3 Bernard et al., 2011 以色列Dead Sea地热区 15~39 20.5~31.3 51.7~54.9 Vengosh et al., 1991 土耳其Menderes Massif地热区 35~224 1.1~54.2 ‒2.3~18.7 Vengosh et al., 2002 希腊Milos Island地热区 63~116 1.7~99.0 2.1~40.5 Wu et al., 2016 美国黄石国家公园 32~140 0.4~28.5 ‒9.3~4.4 Palmer et al., 1990 印度尼西亚Java地热区 33.2~102.0 2.7~93.2 ‒2.4~28.7 Purnomo et al., 2016 中国西藏羊八井地热田 86~87 10.2~165.4 ‒13.8~‒8.4 Yuan et al., 2014;Zhang et al., 2015 中国西藏羊易地热田 79~89 38.5~45.7 ‒9.7~‒5.0 Yuan et al., 2014 中国西藏搭格架地热田 37~86 1.1~106.9 ‒16.3~‒11.7 吕苑苑等,2014;Liu et al., 2019 中国西藏曲卓木地热田 57~77 21.9~44.6 ‒11.3~‒7.1 Liu et al., 2019 中国云南热海地热田 56~96 4.6~10.5 ‒6.3~‒4.5 吕苑苑等,2014 中国吉林长白山火山地热区 21~82 0.2~4.4 ‒13.3~35.9 Zhao et al., 2019 -
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