傅寒晶, 简星, 梁杭海. (2021). 硅酸盐化学风化强度评估的沉积物指标与方法研究进展* [J]. 古地理学报, 23(6): 1192-1209.
Fu Han-Jing, Jian Xing, Liang Hang-Hai. (2021). Research progress of sediment indicators and methods for evaluation of silicate chemical weathering intensity[J]. Journal Of Palaeogeography, 23(6): 1192-1209.   
Doi: 10.7605/gdlxb.2021.06.076
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硅酸盐化学风化强度评估的沉积物指标与方法研究进展*
傅寒晶, 简星, 梁杭海
厦门大学近海海洋环境科学国家重点实验室,海洋与地球学院 福建厦门 361102

通讯作者简介 简星,男,1987年生,副教授,主要从事沉积地质学教学与研究工作。E-mail:xjian@xmu. edu.cn。

第一作者简介 傅寒晶,女,1998年生,硕士研究生,主要从事沉积地质学相关研究。E-mail:1434626361@qq.com。

收稿日期:2021-05-16 修回日期:2021-06-14
基金资助:*国家自然科学基金项目(编号: 41806052,41902126)、福建省自然科学基金项目(编号: 2017J05067)和厦门大学校长基金项目(编号: 20720190097,20720190103)联合资助
摘要

风化作用是源-汇沉积体系中的重要环节,气候、构造、地形、植被和岩性在不同程度上控制着地表硅酸盐化学风化,量化硅酸盐化学风化强度有助于开展全球性的实例研究对比。本研究归纳总结了基于碎屑沉积物的化学风化强度指标,包括岩石学和矿物学指标、元素地球化学指标和非传统稳定同位素指标,并指出了指标在应用中存在的问题。这些指标中:砂质沉积物骨架颗粒组成、粉砂级碎屑的矿物组成和矿物表面结构特征从矿物组成和结构上直观地反映了化学风化强度,运用不多但值得关注;黏土矿物组合和主微量元素指标如CIA、 Rb/Sr、α AlE等在实例研究中运用广泛,同时也显现出易受物源和水动力分选影响等弊端;新近开发的利用锂、硼、钾、镁、硅等同位素示踪化学风化强度的方法具有广阔的应用前景,同位素的分馏机理有待完善。源-汇体系中的其他过程如物源供给、水动力分选、成岩作用、再旋回作用、成土作用、物理淋滤和生物利用等会影响化学风化指标的使用效果,通过多指标的综合运用和相互验证,可以有效提升化学风化强度评估的准确性。

关键词: 化学风化强度; 地球化学; 岩石学; 矿物学; 非传统稳定同位素
中图分类号:P512 文献标志码:A 文章编号:1671-1505(2021)06-1192-18
Research progress of sediment indicators and methods for evaluation of silicate chemical weathering intensity
Fu Han-Jing, Jian Xing, Liang Hang-Hai
State Key Laboratory of Marine Environmental Science,College of Ocean and Earth Sciences, Xiamen University,Fujian Xiamen 361102,China

About the corresponding author Jian Xing,born in 1987,is an associate professor in College of Ocean and Earth Sciences,Xiamen University. He is mainly engaged in teaching and research on sedimentary geology. E-mail:xjian@xmu.edu.cn.

About the first author Fu Han-Jing,born in 1998,is a master degree candidate in College of Ocean and Earth Sciences,Xiamen University, and is mainly engaged in sedimentary geology. E-mail:1434626361@qq.com.

Fund:National Natural Science Foundation of China(Nos. 41806052,41902126),the Natural Science Foundation of Fujian Province(No.2017J05067)and the Xiamen University Fundamental Research Funds for the Central Universities(Nos. 20720190097,20720190103)
Abstract

Chemical weathering is a key process in source-to-sink depositional systems,which is controlled by climate,tectonics,topography,vegetation and lithology. Quantifying the intensity of silicate chemical weathering is beneficial to carry out global case study comparison. This paper aims to summarize the sediment chemical weathering indicators,including petrological and mineralogical,element geochemical and non-traditional stable isotope proxies,and point out the potential problems in application. Among these proxies,the composition of sandy sediment framework grains,mineral composition of silty sediments and textural characteristics of mineral surface can clearly indicate chemical weathering intensity,which have been overlooked in most studies and deserve more attention. Clay mineral assemblages and element geochemical indicators,such as CIA,Rb/Sr and αAlE,are most widely used for quantitative analysis of chemical weathering intensity. However,these proxies are easily affected by the sediment source and hydrodynamic sorting. The newly developed indexes of Li,B,K,Mg and Si isotopes show high potentials to evaluate chemical weathering intensity and need further study on their fractionation mechanism. It's important to note that all indicators can be affected by other geological processes from source to sink,e.g., source,hydrodynamic sorting,diagenesis,sediment recycling,pedogenesis,leaching and biological utilization. It is highly suggested to comprehensively use multiple indexes for evaluating silicate chemical weathering intensity,which can effectively improve the accuracy of the analytic results.

Key words: chemical weathering intensity; geochemistry; petrology; mineralogy; non-traditional stable isotopes

开放科学(资源服务)标识码(OSID)

1 概述

化学风化是在地球表生环境下通过一系列化学作用对母岩进行分解改造的过程, 长时间尺度的全球气候调控主要依靠硅酸盐化学风化和大气CO2浓度的负反馈机制(Kump et al., 2000; Maher and Chamberlain, 2014)。大陆硅酸盐化学风化在全球碳循环过程中作为净碳汇, 对维持地球长期宜居的自然环境具有重要意义(Oliva et al., 2003)。当前全球气候变化形势空前紧迫, 开展硅酸盐化学风化相关研究可为这一焦点议题提供理论依据。

陆表化学风化的作用对象包括出露的岩石和沉积物, 碎屑沉积物的成分和结构会记录相应的化学风化信息。岩石的风化作用早在20世纪初就受到关注, 随后表征化学风化强度的指标被接连提出并不断发展。按研究技术和手段可将化学风化指标分为3类: 岩石学和矿物学指标、元素地球化学指标、非传统稳定同位素指标(林春明等, 2021)。岩石学和矿物学指标包括沉积物骨架颗粒组成、粉砂级细粒全岩矿物、矿物表面结构特征、黏土矿物组成和晶体结构, 其记录了气候主导的化学风化过程的关键信息, 能够有效示踪化学风化强度演变(杨作升等, 2008; Kamp, 2010; Clift et al., 2014; Hessler and Lowe, 2017)。基于元素迁移性差异的地球化学指标如K/Al、Rb/Sr、α AlNa、化学蚀变指数(CIA)等是接受度最高、运用最广泛的一类指标(Dinis et al., 2020)。新兴同位素指标如δ 7Li、δ 11B、δ 41K、 δ 26Mg、δ 30Si等的开发和应用是近年来化学风化强度评估的热门研究方向, 具有广阔的应用前景, 有待进一步探究(Millot et al., 2010; Ma et al., 2015; Wei et al., 2015; Teng et al., 2020)。

众多实例研究表明各类化学风化指标在应用中存在一定问题, 在应用过程中需综合考虑选用指标的参数配置和外源性因素如物源供给、水动力分选、成岩蚀变、成土作用、生物利用等对分析结果的干扰(Buggle et al., 2011; 李银川等, 2020)。不同类型指标的影响因素和适用条件不同, 多指标综合运用、相互验证已经成为化学风化研究的主流趋势(Garzanti et al., 2014; 郭望等, 2020)。本研究旨在阐明地表化学风化过程的控制因素, 归纳表征硅酸盐化学风化强度的常用指标, 强调各指标在实际应用中常见的问题, 以促进硅酸盐化学风化研究不断向前发展。

2 地表化学风化的影响因素

化学风化强度和化学风化速率是化学风化研究的主要内容, 化学风化强度由化学风化速率与风化持续时间共同定义, 体现的是一段时间内区域综合的化学风化程度。地表化学风化过程的控制因素是多元的, 主要包括气候、构造、地形、水文特征、植被和岩性(图 1), 上述因素对化学风化的影响程度具有地域性差异。岩石化学风化是在水、酸、空气等介质的作用下将新鲜矿物转化为溶解质和次生矿物的过程。从风化机制上, 可将化学风化分为供应限制型和动力限制型。供应限制型化学风化多发生在水热条件良好、土壤层厚度大的地区, 化学风化速率的改变主要受控于新鲜物质供给(物理剥蚀)速率(Riebe et al., 2004), 构造活动增强或地形坡度增大能够加速新鲜岩石的剥蚀, 进而增强化学风化作用。但在构造十分活跃的高山地区, 物理剥蚀速率高, 新鲜物质供给充足, 同时, 强烈的构造变形缩短硅酸盐矿物与水、酸的反应时间, 化学风化由供应限制型转变为动力限制型。此时化学风化速率主要受控于气候因素, 增加温度和降水量能够有效增大化学风化强度(Gabet and Mudd, 2009; Dixon et al., 2012; Riebe et al., 2017)。植被生长是加速化学风化溶蚀基岩的关键生物因素, 植物的根系分泌有机酸、蒸腾作用延长水岩接触时间, 均能有效促进硅酸盐化学风化(Berner, 1997)。

图 1 硅酸盐化学风化机制及其影响因素
A— 供应限制型化学风化, 该类化学风化受限于新鲜物质供应速率, 多发育在构造背景相对稳定、地形坡度较小、气候温暖湿润、植被茂密的地区, 此处的原位风化剖面所受化学风化作用强, 往往发育较厚的土壤层; B— 动力限制型化学风化, 该类化学风化主要受限于温度和降水, 多发育在构造活跃、地形坡度大、气候寒冷干旱、植被较少的高山地区, 此处的原位风化剖面所受化学风化作用弱, 基岩蚀变程度相对较低Fig.1 Mechanisms and influencing factors of silicate chemical weathering

除外在的环境因素外, 源岩的结构和矿物组成是影响化学风化作用的重要内因。火成岩中缺乏晶体结构的玄武岩、安山岩等喷出岩比结晶的花岗岩更易风化(解晨骥等, 2012; 孙明照等, 2018)。硅酸盐矿物的抗风化能力与岩浆结晶的先后顺序有关, 常见造岩矿物的化学风化顺序与鲍文反应序列相反(Zakharova et al., 2007)。同等气候条件下, 新鲜硅酸盐矿物的风化先后顺序如下: 橄榄石> 角闪石/辉石> 斜长石(钙长石> 钠长石)> 钾长石, 结构相似的云母族矿物中白云母的抗风化能力强于黑云母, 石英抗风化能力最强(Nesbitt et al., 1996; White and Buss, 2014)。除新鲜火成岩外, 沉积岩和变质岩会保留前期风化改造的影响, 岩石本身在矿物组成和化学组成上呈现较高的成熟度, 沉积旋回带来的继承性风化效应和最新的化学风化改造叠加, 这在一定程度上会影响化学风化强度的评估(Chesworth, 1973)。

3 岩石学矿物学指标
3.1 骨架颗粒组成

单旋回硅质碎屑沉积物的矿物组成, 尤其是骨架颗粒组成(石英Q, 长石F, 岩屑L)常被用于气候主导的原位或者小流域的化学风化强度评估(Nesbitt and Markovics, 1997)。Kamp(2010)对全球不同气候、构造、地形条件下由长英质结晶基底发育的现代非海相、单旋回沉积物的矿物和化学成分进行定量分析, 数据显示各气候区化学风化强度不同且对应着独特的砂质、泥质沉积物组成。干冷环境中, 物理风化作用强, 沉积物的矿物组成和化学成分与源岩相似(Hessler and Lowe, 2017)。而在湿热气候条件下, 长石风化殆尽, 主要产生富石英的砂质沉积物和富高岭石的泥质沉积物(Nesbitt et al., 1996)。介于2个端元之间的气候带具有不同的沉积碎屑组合, 随着温度和降水量的增加、化学风化增强, 砂岩类型从以斜长石为主的长石砂岩(冰川、干旱— 半干旱)过渡到以钾长石为主的次长石砂岩(温带、亚热带、热带)(图 2-A)。随化学风化强度的增加, 沉积物的Q/(Q+F)值逐渐升高(冰川0.3, 温带/亚热带0.5~0.8, 赤道0.8~1.0), P/F值逐渐降低(0.5, 0.1~0.6, 0~0.2)。

图 2 沉积物骨架颗粒组成示踪化学风化强度
A— 不同气候条件下的砂质沉积物骨架颗粒(Q-F-L)组成(修改自Kamp, 2010), 图中显示随着气候条件变湿热, 化学风化强度增大, 产生的砂质沉积物中长石含量逐渐减小并不断富集石英; B— 矿物蚀变指数MIA配合Q-P-K图解用于表征化学风化强度(修改自Roy et al., 2013), 沉积物样品骨架颗粒组分投点越靠近石英端元, 斜长石、钾长石含量越小, 石英含量越大, MIA指数越大, 化学风化强度增强Fig.2 Framework compositions of sandy sediments for tracing chemical weathering intensity

因此, 有学者使用矿物蚀变指数(MIA=Q/(Q+P+K)× 100)并结合Q-P-K图解(斜长石P, 钾长石K)来表征化学风化强度(图 2-B), 研究表明MIA值与化学风化强度正相关, 能有效示踪砂质沉积物的化学风化强度演变(Rieu et al., 2007a; Roy et al., 2013; Hessler et al., 2017; Perri, 2018; Dinis et al., 2020)。然而, 碎屑沉积物的矿物组成并非总由化学风化主控, 物源也对其有很大的影响。除此之外, 水动力分选、成岩改造、再旋回等过程都有可能对使用沉积物骨架颗粒指标进行化学风化强度评估造成干扰, 需通过结合物源指标、粒度指标来减轻其他因素的干扰(Rieu et al., 2007a, 2007b)。

3.2 粉砂级矿物组成

同一来源不同粒级沉积物的矿物组成受水动力分选的影响会产生一定的差异, 采用沉积物全岩矿物组成来评估化学风化强度可能会模糊掉粒级间的风化信息差异。杨作升等(2008)用X射线衍射法(XRD)对长江、黄河入海沉积物全岩和分粒级石英和长石的相对含量进行半定量测定, 计算了长石与石英(F/Q)、钾长石与石英(K/Q)、斜长石与石英(P/Q)的比值。研究表明长江沉积物的化学风化程度是高于黄河沉积物的, 但在粗粒沉积物中两河的F/Q值相近, 无法体现化学风化差异。只有当沉积物粒级细化到一定区间后(< 32μ m), 长江、黄河的F/Q比值才出现较大差异, 这表明细粒沉积物对化学风化强度区分更为敏感(杨作升等, 2008)。相较于砂级和黏土级的沉积物, 粉砂级矿物组成与区域化学风化强度的相关性更好(Tanaka and Watanabe, 2015; Hatano et al., 2019), 合理利用粉砂级沉积物全岩矿物组成评估化学风化强度可以减小粒度效应, 提高分析结果准确性。

3.3 矿物表面结构特征

矿物在源汇过程中会经历多次物理作用和化学作用改造, 颗粒表面结构能够记录环境变化信息和沉积物风化形成过程(Velbel, 2007)。矿物的风化稳定性、晶体结构和所处环境都会影响其表面结构。研究显示新鲜基岩结构致密, 矿物表面未经蚀变、光滑均匀, 晶体边界无破裂; 随风化作用增强, 岩石内部结构变得松散, 沿斜长石双晶面、晶体边界和矿物核心出现黏土和氧化物蚀变(Hessler and Lowe, 2017)。在化学风化程度高的湿热地区, 强烈的溶解作用会对稳定矿物乃至超稳定矿物(石英、电石气和锆石等)产生影响。因此, 碎屑重矿物的表面结构能够定性反映化学风化强度(Andò et al., 2012; Li et al., 2015a)。

常见的研究矿物表面结构的方法是使用偏光显微镜或扫描电镜观察晶体表面特征(Garzanti et al., 2013a; Yue et al., 2019)。Andò 等(2012)提出鉴定砂质沉积物重矿物颗粒表面结构特征的图版, 定义了5个连续的风化阶段(未风化、侵蚀、蚀刻、深蚀、骨架), 每个风化阶段又根据侵蚀程度分为4个渐进的等级(图 3)。相对而言, 较不稳定的重矿物的结构特征区分更明显, 链状硅酸盐矿物(辉石和闪石)在低温水解作用下会出现锯齿状边缘、蚀刻坑、凹陷等(Velbel, 2007)。在对具有相同风化和搬运过程的复合颗粒(由2种及以上矿物组成)的结构特征进行鉴定时可以观察到明显的选择性风化。同一样品中的同种矿物若出现风化纹理的多峰分布则可能指示着多个风化源, 需结合其他信息才能准确再现风化过程。除化学风化外, 侵蚀、搬运、沉积和成岩作用, 会进一步改变风化颗粒的表面特征, 叠加新的纹理, 并带着已有的结构特征进入下一个沉积旋回(Velbel and Losiak, 2010)。这为使用矿物表面特征重建化学风化历史增添困难, 但加强对矿物表面结构的特征鉴定无疑对化学风化强度评估具有良好的辅助作用。

图 3 赤道不同流域河砂中角闪石的表面结构特征指示化学风化强度(据Andò et al., 2012)
图中矿物颗粒均使用偏光显微镜进行鉴定并获取图像, 单偏光, 照片中线段比例尺均代表长度63μ mFig.3 Visual classification of surface textures in hornblende grains from equatorial river sands in various drainages for indicating chemical weathering intensity (after Andò et al., 2012)

3.4 黏土矿物

黏土矿物是化学风化作用的典型产物, 主要包括高岭石(Ka)、绿泥石(Chl)、伊利石(Ill)和蒙脱石(Sm)。高岭石是在潮湿气候酸性介质中由长石、云母和辉石受强烈淋滤分解形成(Garzanti et al., 2014; Dinis et al., 2020)。绿泥石是在化学风化受抑制、弱淋滤作用的碱性环境中形成, 主要来源于低级变质岩和铁镁质岩石, 通常在冰川或干旱地区的土壤和沉积物中富集(汤艳杰等, 2002)。伊利石是在干旱寒冷的弱淋滤、弱碱性环境下, 由长石、云母等铝硅酸盐矿物风化脱钾形成(Galá n and Ferrell, 2013; 靳华龙等, 2019)。伊利石和绿泥石往往指示较差的水热条件(低温干旱)和较强的物理风化(方谦等, 2018)。蒙脱石是由变质岩中的富铁镁钙矿物在化学风化早期形成, 季节性干湿交替的气候、中等化学风化程度以及排水不畅的盆地环境都有利于生成蒙脱石。此外, 火山物质在碱性介质中也会蚀变为蒙脱石(Garzanti et al., 2013a; 曾蒙秀等, 2014; Pang et al., 2018)。

沉积物中黏土矿物相对含量、黏土矿物比值(如Ka/Ill、Ka/Chl、Ka/(Ill+Chl)、Sm/(Ill+Chl))以及与其他矿物的比值(Ill/Q, Chl/Q)、黏土矿物结晶学特征, 都可用于反映古气候和古化学风化演变(Liu et al., 2005; Clift et al., 2014)。需注意的是, 当物源混杂或发生转变, 以及黏土矿物在搬运过程中出现差异性絮凝或分选作用时都有可能改变沉积物中的黏土矿物组成并影响后续的解释。黏土矿物晶体学特征如伊利石结晶度、伊利石化学指数等受沉积分异作用影响较小, 能稳定保存源区气候信息(Wang and Yang, 2013)。伊利石结晶度指数(Kü bler指数)是以乙二醇饱和曲线上伊利石10Å (001)衍射峰的半高宽(FWHM)表示, 分为4个等级: < 0.4(结晶极好), 0.4~0.6(结晶好), 0.6~0.8(结晶中等), > 0.8(结晶差)(Ehrmann, 1998)。伊利石结晶度可以反映矿物水解程度, 化学风化增强会使结晶度变差, 伊利石结晶度指数增大(Alizai et al., 2012)。伊利石化学指数是通过计算乙二醇饱和曲线上5Å 和10Å 的衍射峰面积比值, 比值小于0.4代表物理风化主导的富铁镁伊利石; 比值大于0.4代表化学风化主导的富铝伊利石(Esquevin, 1969)。

沉积物中的黏土矿物按成因可分为碎屑黏土和自生黏土, 碎屑成因的黏土矿物在经历多个源汇过程后汇集在沉积区, 其矿物组成与含量变化记录的是流域的整体风化情况, 是源区、搬运途径和沉积区风化信号的综合, 但自生黏土矿物(湖相、海相)多反映沉积区原位的气候变动(Liu et al., 2010)。当研究区物源输入对黏土矿物的影响大于气候时, 黏土矿物指标指示化学风化强度的可靠性会降低(方谦等, 2018)。此外, 黏土矿物如蒙脱石和高岭石在成岩过程中会转变为伊利石和绿泥石, 随埋藏深度的增加, 高岭石、蒙脱石、伊蒙混层矿物会逐渐减少, 蒙脱石的成岩伊利石化要求埋藏深度大于1500m, 转变压力900~920kg/cm2, 转变温度100~140℃(Song et al., 2018), 在成岩过程中发生的变质作用也会影响伊利石结晶度和化学指数(靳华龙等, 2019)。因此, 确定化学风化强度需要综合运用多种黏土矿物指标并充分考虑物源、水动力分选以及成岩作用的影响。

4 元素地球化学指标
4.1 主量元素

基于沉积物主量元素地球化学的双元素指标(如Al2O3/SiO2、K/Al、Na/Al)和多元素指标是化学风化强度定量分析最常用的指标。早期的主量元素化学风化指标(见表 1, 7~16号指标)适用范围较窄, 在近年来的风化研究应用中已较为少见(Gupta and Rao, 2001; Duzgoren-Aydin, 2002; Price and Velbel, 2003)。鉴于该类指标数量众多, 仅具体阐述部分应用广泛的指标(各指标公式详见表 1)。长石作为重要的造岩矿物, 占据上地壳体积60%以上, 硅酸盐风化过程主要由长石的蚀变程度来表征。化学蚀变指数(CIA)、斜长石蚀变指数(PIA)和化学风化指数(CIW), 这3个指标都是基于长石风化蚀变过程中易迁移的碱金属元素氧化物K2O、CaO、Na2O和残留物中难迁移的Al2O3的摩尔百分数比值来表征化学风化强度。

表 1 主量元素化学风化指标综合表 Table1 Comprehensive summary of major elemental indicators of chemical weathering intensity

Nesbitt 和 Young(1982)定义了CIA, 公式(表 1)中CaO* 为硅酸盐矿物中的CaO, 常采用酸溶法或使用McLennan(1993)提出的CaO-Na2O转换计算法排除非硅酸盐矿物(碳酸盐、磷酸盐)中的CaO获取CaO* 值(李徐生等, 2007; Shao and Yang, 2012)。CIA值与化学风化强度正相关, 结合A-CN-K图解(Al2O3-CaO* +Na2O-K2O)(图 4-A), 可以有效观测化学风化趋势, 校正钾交代引起的CIA值偏差, 反映源岩组成, 是表征硅酸盐化学风化程度的理想指标(Nesbitt and Young, 1984; 冯连君等, 2003)。考虑到钾离子在化学风化过程中地化行为不一致, 强风化时多淋滤迁移, 弱风化时多吸附保留, Harnois(1988)在CIA的公式基础上删除钾元素并提出了CIW。

图 4 利用A-CN-K图解表征化学风化强度(A)和消除粒度效应的校正方法(B)
A— CIA指数及 A(Al2O3)-CN(CaO* +Na2O)-K(K2O)图解, 据Nesbitt和Young, 1984; Fedo等, 1995。线段①是理想化学风化趋势线, ②③表示发生钾交代作用, ②代表斜长石转变为钾长石, 对CIA计算值不产生影响, ③是黏土矿物的伊利石化, 会导致CIA计算值明显降低, ④表示经过钾校正之后CIA值回升。红点及黑点分别代表正常风化过程及发生钾交代时的元素特征。B— 利用A-CN-K图解校正水动力分选导致的粒度效应(修改自Jian et al., 2013), 受水动力分选影响, 同一剖面相邻采样点的泥岩较砂岩普遍具有更高的CIA值。通常研究中会使用泥岩的CIA值来评估源区风化条件而非砂岩, 因此需对砂岩结果进行校正, 将砂岩样品的投点和A端的连线与理想风化趋势线交点处的CIA值作为校正后的CIA值(图中除粒度影响外, 也存在钾交代, 一并校正)。Ka: 高岭石, Chl: 绿泥石, Gi: 三水铝石, Sm: 蒙脱石, Ill: 伊利石, Mu: 白云母, Pl: 斜长石, Kfs: 钾长石Fig.4 Application of A-CN-K plot in evaluating chemical weathering intensity(A) and calibration method to eliminate grain size effects(B)

但CIW并不适合钾长石含量高的样品, 富钾岩石不论风化与否都会产生较高的CIW值。于是, Fedo 等(1995)在CIA的基础上再次修改并提出PIA, 用于表征斜长石的化学风化蚀变程度。后续还提出了蚀变化学指数(CPA)(Buggle et al., 2011)和CIX指数(Garzanti et al., 2014)用于降低钾交代和碳酸盐矿物给化学风化评估带来的不确定性。上述化学风化指标都是在花岗岩基底的风化剖面研究中提炼出来, Babechuk等(2014)提出了铁镁质蚀变指数(MIA)用于量化铁镁质基底风化剖面的化学风化强度, 并将其分为适用于氧化环境的MIA(O)和适用于还原环境的MIA(R), MIA可以结合Al-Fe-Mg-Ca-Na-K图解使用。

WIP、CIA、CIW、PIA等虽然具有高风化敏感性, 在应用时仍存在一定限制。首先是指标中元素的地球化学行为和赋存矿物类型造成的限制, 如使用碳酸盐相关元素Ca、Mg和Sr时, 需要消除碳酸盐动力学对硅酸盐化学风化强度评估的干扰; Rb、Ba和K等离子半径大的元素易溶也易吸附于黏土矿物, 使用时需注意元素在整个化学风化过程中行为是否一致(Buggle et al., 2011)。其次, 当剖面经历的化学风化程度非常高时, 沉积物中可能会含有大量氧化物(sesquioxides), 改变风化剖面中Al2O3、Fe2O3、MgO、K2O和TiO2的理想分布规律, 在没有野外调查和岩石学手段辅助的情况下使用大部分指标得到的结果都会导致误判(Duzgoren-Aydin et al., 2002; Price and Velbel, 2003)。河流和湖泊沉积物CIA指示的化学风化强度通常是长时间尺度流域综合的风化情况(原位风化研究除外)(Shao and Yang, 2012)。使用这些指标无法进行实时的化学风化强度监测, 并且在使用过程中要排除源岩、粒度差异和成岩钾交代等的影响(徐小涛和邵龙义, 2018)。

4.2 微量元素

微量元素指标与主量元素指标的应用原理相似, 采取在风化过程中迁移性差异大的微量元素来反映化学风化程度。最常用的微量元素指标是Rb/Sr, Rb主要赋存于含K矿物中(钾长石、黑云母和白云母), 而Sr主要赋存在含Ca矿物中(斜长石、辉石、角闪石和碳酸盐)。富Sr矿物比富Rb矿物更不耐风化, Sr在化学风化过程倾向于迁移亏损, Rb/Sr与化学风化强度正相关, 类似指标还有Ba/Sr(陈骏等, 1996; Jin et al., 2001; Hossain et al., 2017)。碱金属和碱土金属元素离子半径越大, 越易被黏土矿物吸附残留, 因此研究中多将Cs、Rb和Ba视为不易迁移元素, K视为易迁移元素, 使用Cs/K、Ba/K、Rb/K、K/Zr、K/Ti、Cs/Ti和Rb/Ti等指标能有效示踪中高强度化学风化, 但对弱化学风化蚀变不太敏感(Yan et al., 2007; Buggle et al., 2011; Clift et al., 2014)。另一常用指标α E是由单一易迁移元素(Mg、Ca、Na、Sr、K、Ba)和不易迁移元素(Al、Ti、Sm、Nd、Th)在样品中与在UCC(大陆上地壳)中的比值标准化表示(Gaillardet et al., 1999), 例如:

α Na=(Sm/Na)sample/(Sm/Na)UCC (1)

使用α E评估化学风化程度可以减小源岩、沉积旋回的影响。考虑到Ti、Sm、Nd、Th等元素主要赋存在重矿物(独居石、钛铁矿、金红石)中, 而重矿物优先富集在砂质沉积物中, 为减少水动力分选的干扰, Garzanti 等(2013b)选择Al元素作为不易迁移元素, 重新定义了α AlE:

α AlE=(Al/E)sample/(Al/E)UCC (2)

α AlE> 1表示元素E相对于UCC是亏损的, α AlE< 1则表示富集。单元素α AlE值变化可反映化学风化强度, 不同元素α AlE值大小可反映风化过程中各元素的迁移顺序。

沉积岩中的Th/U值可用于反演长时间尺度(亿年)的化学风化强度, 由于多旋回的化学风化作用导致老克拉通沉积物中的U大量损失, Th保持稳定, Th/U值会缓慢增大。Th/U值的变化也可能与稀土元素分异相关, 一般认为Th/U> 4(上地壳的Th/U≈ 4)即可认为与风化历史有关, 但该指标不适合在U富集的缺氧环境或含碳酸盐岩的情况下使用(McLennan et al., 1995; Gu et al., 2002; Carpentier et al., 2013)。由于高岭石富集Th, 伊利石、伊蒙混层矿物富集K, 在贫有机质(< 2%wt)的泥质沉积物中Th/K与高岭石/伊利石比例有很强的相关性, 因此Th/K也可以用来反映化学风化强度(Deconinck et al., 2003)。强烈化学风化作用会改变风化产物中稀土元素的配分模式, 残留物中的重稀土比轻稀土更易亏损, Sm比La更易被清除, 因此La/Sm值可以反映源区是否发生过强烈的化学风化(Wei et al., 2006)。

5 非传统稳定同位素

近年来, 随着地球化学分析设备的快速发展, 化学风化研究中基于水体和沉积物的非传统稳定同位素指标逐渐兴起, 文中关注沉积物同位素指标的应用。碎屑沉积物的Sr-Nd同位素组成(87Sr/86Sr和ε Nd)常被用于探讨大流域范围的化学风化强度变化和全球性气候事件(Miriyala et al., 2017), 实例研究背景宏大, 分辨率不高, 故在此不作具体展开。着重关注δ 7Li、δ 11B 、δ 41K、 δ 26Mg和 δ 30Si 的发展和应用, 指标具体信息见表 2。

表 2 稳定同位素化学风化指标公式 Table2 Formulas of stable isotopic chemical weathering indicators
5.1 锂同位素δ 7Li

锂主要赋存在硅酸盐矿物(黑云母、长石、角闪石、磷灰石)中(浓度大于50 μ g/g; Millot et al., 2010), 锂同位素分馏受控于硅酸盐化学风化, 物理作用、生物利用、氧化还原对其无影响, 因此沉积物中的δ 7Li常被用于示踪地表化学风化强度变化(Li et al., 2015b)。硅酸盐风化过程中原生矿物的破碎溶解不改变δ 7Li, 但次生矿物的形成会以表面吸附和晶格结合的方式富集轻锂同位素6Li, 释放重锂同位素7Li至水体中(Dellinger et al., 2017; Weynell et al., 2021)。次生矿物的类型会影响分馏程度(分馏量: 三水铝石大于10‰ , 高岭石7‰ , 绿泥石1‰ ~3‰ , 蒙脱石、伊利石小于1‰ )(Pistiner and Henderson, 2003; Li and Liu, 2020)。使用 δ 7Li 评估化学风化强度需要注意源岩(沉积岩δ 7Li值-0.5%± 1‰ 、火成岩3.5%± 1.5‰ )和粒度(细粒沉积物δ 7Li值更低)的影响(Weynell et al., 2017; Millot et al., 2019)。热带地区花岗岩遭受强烈化学风化时, 风化产物中会富集石英(富7Li), 石英含量的改变会显著影响δ 7Li的正常分布规律, 从而干扰化学风化强度的解释(Zhang et al., 2021)。原位风化剖面研究需注意外源物质输入(雨水和风尘)以及锂同位素垂向淋滤迁移等问题。

5.2 硼同位素 δ 11B

硼是易溶元素, 也是植物生长所需的微量营养元素, 具有2个稳定同位素10B和11B, 大幅度的硼同位素分馏主要发生在地球表生过程中如低温水岩作用和植物循环利用中(Cividini et al., 2010; Louvat et al., 2011; Lemarchand et al., 2012; Noireaux et al., 2021)。硼无价态变化, 分馏不受氧化还原条件影响(肖军, 2012)。硼常以B(OH)3和B(OH)4-形式在自然条件下存在, 通常重同位素 11B 倾向以三角配位富集在B(OH)3中, 轻同位素 10B 通过四面体配位富集在B(OH)4-中(Muttik et al., 2011)。硅酸盐化学风化过程中形成的具有四面体结构的黏土矿物可以通过表面吸附、层间吸附和晶格结合优先富集轻同位素10B, 将重同位素 11B 释放到溶液中(Williams et al., 2001; Romer et al., 2014; Ercolani et al., 2019)。硼同位素具备作为大陆化学风化指标的潜力, 在黄土— 古土壤风化剖面化学风化研究中已有相关应用(赵志琦等, 2002; 肖军, 2012; Wei et al., 2015), 有关硼同位素的具体分馏机制及在化学风化领域的应用仍有待加强。

5.3 钾同位素δ 41K

钾是组成上地壳岩石的主要元素(1.81%wt), 主要赋存在硅酸盐矿物如钾长石、白云母、伊利石中(Rudnick and Gao, 2014), 具备分布范围广、化学活性强等特点。钾同位素在低温水岩作用即可显著分馏, 是示踪硅酸盐化学风化过程的理想之选(Li et al., 2019a; Teng et al., 2020)。在硅酸盐风化过程中, 由于钾在液相和矿相中的化学键能差异, 轻钾同位素39K倾向于以表面吸附或晶格结合的形式保留在次生矿物中(伊利石δ 41K值比钾长石低0.3‰ ), 而重钾同位素41K则释放到水溶液中(Santiago Ramos et al., 2018; Li et al., 2019b; Zeng et al., 2019)。风化剖面钾同位素分馏程度主要由化学风化强度和物质形成过程共同控制(黄土剖面: -0.58‰ ~-0.35‰ ; 页岩剖面: -0.69‰ ~-0.08‰ ), 黄土主要由物理剥蚀和风力搬运堆积形成, 页岩则要经过化学蚀变、搬运、沉积成岩等更为复杂的过程(Huang et al., 2020; 王昆等, 2020)。对于典型的受化学风化主导的花岗岩风化剖面, 其δ 41K值变化范围大并与黏土矿物含量耦合(Teng et al., 2020)。需要注意的是, 钾是生物敏感元素, 钾的生物利用会改变土壤或沉积剖面中的同位素分馏, 在使用δ 41K表征化学风化强度时要考虑生物因素的影响(Li et al., 2016; Chen et al., 2020a)。关于钾同位素的分馏机制, 目前仍有较多不确定性亟待完善。

5.4 镁同位素δ 26Mg

镁广泛分布在水圈、岩石圈和生物圈中, 其同位素在低温水岩作用中即可发生明显的质量分馏, 镁无价态变化, 分馏不受氧化还原条件影响(闫雅妮等, 2021)。硅酸盐化学风化会导致较大的镁同位素分馏, 在风化过程中基岩向流体释放轻镁同位素24Mg, 保留重镁同位素26Mg(Liu et al., 2014)。风化残留物的δ 26Mg值与化学风化强度正相关(Chen et al., 2020b)。成岩和低级变质作用对碎屑沉积物的δ 26Mg值影响较小(Huang et al., 2016)。镁以结构态和交换态2种形式赋存在矿物中, 结构态镁(26Mg)位于矿物晶体的八面体结构中, 交换态镁(24Mg)则以离子形式吸附在黏土矿物层间或表面(闫雅妮等, 2021)。化学风化过程中原生矿物(辉石、角闪石、黑云母)溶解释放24Mg进入水体, 次生矿物对风化残留物δ 26Mg值的影响取决于矿物的种类, 伊利石、蒙脱石和绿泥石倾向于保留26Mg, 而蛭石和高岭石倾向于保留24Mg(Opfergelt et al., 2012; Ma et al., 2015)。除黏土矿物种类外, 应用镁同位素示踪化学风化强度时, 也需要注意物源输入和搬运途径的影响, 当物源掺入风成沙等弱风化物质, 或表层沉积物的风尘搬运和水流运移优先带走富26Mg的黏土时, 都会导致δ 26Mg值偏小(Hu et al., 2017; Brewer et al., 2018)。δ 26Mg在黄土— 古土壤剖面的应用需格外注意富24Mg的自生碳酸盐矿物对化学风化强度评估的干扰(Wimpenny et al., 2014)。镁同位素会在植物体内分馏, 植物根系富集26Mg, 叶片和枝条富集24Mg, 与植被生长相关的镁循环也会改变表层土壤中的δ 26Mg值(Ma et al., 2015; 刘金科和韩贵琳, 2019)。诸多研究表明镁同位素对硅酸盐化学风化具有高敏感性, 后续仍需加强次生矿物种类和植物生长过程对镁同位素分馏影响的研究, 进一步完善其作为化学风化强度指标的机理。

5.5 硅同位素 δ 30Si

硅是构成造岩矿物的关键元素, 硅同位素分馏主要发生在岩石形成、水岩反应和生物过程中(Opfergelt and Delmelle, 2012)。粉砂级沉积物中的 δ 30Si 值与上地壳硅同位素组成相似, 黏土中的 δ 30Si 值变化范围较广, 与研究区气候条件尤其是温度具有良好相关性, 常被用于示踪流域硅酸盐化学风化(Bayon et al., 2018)。化学风化形成次生黏土矿物和铁氧化物时优先结合轻硅同位素28Si, 将重硅同位素30Si排放到水溶液中(Hughes et al., 2013)。在应用 δ 30Si 指示化学风化强度时, 需要注意以下几点: (1)碎屑沉积物中石英和原生硅酸盐矿物与次生黏土矿物的比例会影响 δ 30Si 的值(Opfergelt and Delmelle, 2012)。(2)在动力限制型化学风化主导的高山地区, 硅同位素的分馏程度微弱, 干冷缺氧环境导致次生矿物30Si偏正, 难以利用硅同位素示踪化学风化强度(Bayon et al., 2018)。(3)源岩类型和沉积旋回会改变细粒沉积物的硅同位素组成(Ding et al., 2011)。(4)热带高风化地区土壤中的硅同位素组成也会受到生物硅循环的影响, 如含硅生物及组成(硅藻、海绵骨针、硅鞭藻、放射虫)的生物矿化作用、高等植物利用水体中的溶解硅并富集轻硅同位素形成植硅体等(Baronas et al., 2020)。

6 沉积物化学风化强度指标应用的外源干扰因素

化学风化作为沉积源汇体系中的重要一环, 与源岩剥蚀、搬运、沉积成岩和再旋回等过程密切相关。不考虑指标本身的参数设置优劣, 文中介绍的基于沉积物岩石学、矿物学、地球化学的指标在实例应用中都会受到外源信号的干扰。

1)岩性。源岩性质对特定化学风化指标应用的影响主要来源于结晶岩的原生矿物种类和含量差异、沉积岩或变沉积岩的风化继承性、多源供给引起的碎屑矿物组分的混合和改变(Li et al., 2012; Garzanti and Resentini, 2016; Amireh, 2020)。

2)水动力分选。在碎屑颗粒搬运过程中, 黏土矿物及云母类矿物富集于泥级沉积物中以悬浮态搬运, 石英、长石和岩屑倾向于在砂级沉积物中以底负载形式搬运, 锆石等重矿物多赋存于粉砂— 细砂级沉积物中(Su et al., 2017; 杨江海和马严, 2017)。矿物在不同粒级沉积物中的富集导致化学元素也呈现相似规律, 大部分主微量元素(如Al、Fe、Mn、Mg、Ca、K、P、Rb、Ni、V、Sc、Ga、Pb、Cu、Y)趋于在细颗粒沉积物中富集, 赋存在石英和重矿物中的Si和高场强元素(U、Th、Zr、Hf)则相反(邵菁清和杨守业, 2012; Pang et al., 2018)。因此, 使用主微量元素和同位素指标对具有相同化学风化背景的沉积物进行分粒级测试得到的化学风化强度会因粒度差异出现不一致(Xiong et al., 2010)。

3)成岩作用和再旋回作用。成岩过程中黏土矿物的溶解蚀变(高岭石和蒙脱石的伊利石化)和晶体结构转变, 会削弱黏土矿物指标的风化表征意义(Fedo et al., 1995; 曾蒙秀等, 2014)。再旋回的沉积物会保留前期风化形成的矿物组合和颗粒表面结构特征, 伴随着石英稀释效应, 增加了运用骨架颗粒、矿物表面结构特征和地化指标示踪近期化学风化强度的难度(Guo et al., 2018)。

4)元素垂向淋滤、成土作用和生物利用。这三者对化学风化指标应用的影响主要体现在原位风化剖面研究中(Mei et al., 2021)。活跃、复杂的生物地球化学交互作用发生在剖面表层的沉积物和土壤中, 包括元素和同位素的生物利用过程、物理淋滤、黏土矿物的吸附解析、不同类型氧化物的生成等, 给利用地化指标表征化学风化强度带来更多不确定性。

图 5 外源因素影响化学风化强度指标应用图示Fig.5 Diagram showing external factors affect application of chemical weathering intensity indicators

针对上述问题也有相应的解决方案和改进方法。以常见指标CIA为例, 样品选择粒度相近的细粒沉积物, 并配合粒度敏感指标如Al/Si、Ti/Al、 Zr/Rb和Zr/Al2O3等共同使用, 可以有效地降低水动力分选带来的影响(Liang et al., 2013; Pang et al., 2018; Greber and Dauphas, 2019)。结合物源分析方法如岩石薄片、重矿物分析、Q-F-L物源判别图、微量元素二元图(如Zr/Sc-Th/Sc图解)、REE特征、成分变异指数(ICV)等可以获取源岩改变以及再旋回作用的信号(Cox et al., 1995; 徐亚军等, 2007)。当CIA指标指示的化学风化强度结果已经出现粒度效应时, 可以尝试运用A-CN-K图解进行校正(图 4-B; Nesbitt et al., 1996; Jian et al., 2013)。成岩作用的影响可以通过观察矿物的微观特征、A-CN-K图解和黏土矿物随埋深的变化进行判断(Fedo et al., 1995)。只有结合多种化学风化强度指标并综合运用其他领域的研究手段才能有效提升化学风化强度评估的准确度。此外, 还应强调的是, 岩石学和矿物学指标, 如砂质沉积物的骨架颗粒组成、重矿物结构特征, 多用于定性分析; 基于XRD分析的黏土矿物和粉砂级细粒全样矿物的相对含量属于半定量测定。由于定性和半定量指标较宽泛的误差值范围, 在运用过程中应更多地关注变化趋势而非绝对数值。除了改进传统化学风化强度指标外, 加快开发稳定同位素指标并完善其分馏机理也有望推进化学风化强度的准确量化发展。

7 总结与展望

系统总结了基于沉积物的示踪硅酸盐化学风化强度的各类指标, 除常用的主微量元素和黏土矿物指标外, 砂质沉积物骨架颗粒组成和重矿物表面特征能直观有效地反映沉积物经化学风化改造后在成分和结构方面的变化特征, 在今后的研究中值得更多关注。同位素在化学风化领域的应用尚处于探索阶段, 对特定同位素分馏机制的探讨仍有较大的发展空间。考虑到全球范围内实例研究的复杂性以及源汇系统中多种过程的影响, 综合运用多指标评估化学风化强度已成主流趋势。从岩石学、矿物学和地球化学角度综合评估化学风化强度可将宏观与微观有机结合, 有效提升化学风化强度评估的准确度。今后仍需加强对化学风化强度示踪过程中常见干扰的规避和校正方法的研究。

致谢 两位审稿专家在文章评审过程中提出了宝贵的建设性修改意见, 在此向他们致以衷心的感谢!

(责任编辑 李新坡; 英文审校 徐 杰)

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