| 碳酸盐(岩)的锂同位素组成:一种潜在的古海水pH替代性指标 |
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| 引用本文：赵悦,李延河,胡斌,侯可军,范昌福,高建飞.2020.碳酸盐(岩)的锂同位素组成:一种潜在的古海水pH替代性指标[J].地球学报,41(5):613-622. |
| DOI：10.3975/cagsb.2020.071402 |
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| 作者 | 单位 | E-mail | | 赵悦 | 中国地质科学院矿产资源研究所, 自然资源部成矿作用与资源评价重点实验室,自然资源部同位素地质重点实验室 | zhaoyue0612@hotmail.com | | 李延河 | 中国地质科学院矿产资源研究所, 自然资源部成矿作用与资源评价重点实验室,自然资源部同位素地质重点实验室 | lyh@cei.cn | | 胡斌 | 中国地质科学院矿产资源研究所, 自然资源部成矿作用与资源评价重点实验室,自然资源部同位素地质重点实验室 | | | 侯可军 | 中国地质科学院矿产资源研究所, 自然资源部成矿作用与资源评价重点实验室,自然资源部同位素地质重点实验室 | | | 范昌福 | 中国地质科学院矿产资源研究所, 自然资源部成矿作用与资源评价重点实验室,自然资源部同位素地质重点实验室 | | | 高建飞 | 中国地质科学院矿产资源研究所, 自然资源部成矿作用与资源评价重点实验室,自然资源部同位素地质重点实验室 | |
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| 基金项目:国家重点研发计划(编号: 2017YFC0602705);中国地质科学院矿产资源研究所基本科研业务费(编号: KK1913) |
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| 中文摘要:地球演化历史中海水的pH值发生了明显变化, 海水pH值可能是控制海相碳酸盐岩能否形成及其成分演化的重要因素, 对了解地球早期白云岩的成因和一些矿产的形成等均有重要指示意义。然而, 记录海洋pH值变化的替代性指标非常稀少, 常用的主要是碳酸盐(岩)的硼同位素。古老碳酸盐的硼同位素往往受到后期地质作用的影响, δ11B-pH转化过程中需要基于多种假设, 硼酸和硼酸根之间的分馏系数(αB)、硼酸表观电离常数(pKB*)以及δ11BSW的不确定性, 使硼同位素分析结果具有多解性、不确定性。亟需多个独立指标对海水pH值进行限制, 碳酸盐(岩)锂同位素是一个潜在的替代性指标, Roberts et al.(2018)发现有孔虫碳酸盐壳体的δ7Li与海水pH值呈显著负相关关系, 认为6Li和7Li水合离子在进入碳酸盐晶格时要脱去溶剂水, 这个过程的去溶能与pH值相关, 导致锂离子进入有孔虫方解石壳体的过程中存在显著的同位素分馏。在对蓟县剖面中—新元古代海相碳酸盐岩碳酸盐相的硼、锂同位素进行研究时发现, 纯净原始碳酸盐岩的锂同位素组成(4.9‰～13.4‰, 平均8.03‰)明显低于现代海洋碳酸盐的锂同位素组成, 中元古以来碳酸盐(岩)的锂同位素组成总体呈上升趋势。纯净原始碳酸盐岩的锂同位素组成与硼同位素组成及海水的pH值(δ11Bsw=25‰)呈明显反相关关系; 硅质条带白云岩的硼、锂同位素组成也呈明显反相关关系, 说明碳酸盐(岩)的锂同位素确实有可能成为一种潜在的pH值替代性指标。若碳酸盐(岩)锂同位素可以对海水pH值施加独立约束, 那硼、锂同位素联合研究将对重建古海洋的pH值演化具有重大意义。 |
| 中文关键词:碳酸盐(岩) 锂同位素 硼同位素 pH值 |
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| Lithium Isotopic Composition of Marine Carbonate Rocks:A New Proxy for Paleo-pH Reconstruction |
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| Abstract:The pH value of seawater has changed obviously in the evolution history of the earth. The pH value of seawater may be an important factor controlling the formation of marine carbonate rocks and the evolution of their components, which is of great significance to the genesis of early dolomite and the formation of some minerals. However, there are very few proxies for recording changes in ocean pH, with boron isotopes of carbonate (rocks) being the main proxy. Boron isotopes of ancient carbonates are often affected by diagenetic processes. During the transformation process of δ11B-pH, the uncertainty of αB, pKB* and δ11BSW needs to be based on a variety of assumptions. The analytical results are of multiple solutions and uncertainty. Multiple independent indicators are needed to limit seawater pH value, and lithium isotopic composition of carbonate (rocks) is a potential proxy. Roberts et al. (2018) found that the δ7Li of foraminifera carbonate shells and seawater pH value have significantly negative relationship, and the strength of the 6Li and 7Li hydration spheres (and hence their respective desolvation energy) is pH-dependent, resulting in a significant isotopic fractionation during the incorporation of lithium into foraminifer calcite. The δ7Li values of well preserved pristine carbonate rock range from 4.9‰ to 13.4‰ (averaging 8.03‰), significantly lower than those of the modern marine carbonate. Therefore, the δ7Licarb shows a general increasing trend since the Mesoproterozoic. Preliminary analyses of B and Li isotopes in Mesoproterozoic marine carbonate rocks from Jixian section exhibit a clear anti-correlation trend for δ7Li and δ11B values, also δ7Li and pH (δ11Bsw=25‰). The same pattern is observed in siliceous banded dolomite as well. This implies that the δ7Li of marine carbonate has the potential to be a novel pH proxy, although the mechanism behind requires further investigation. Li isotope may have great implications in the reconstruction of δ11B of seawater, if it can be used as an independent constraint on pH value. |
| keywords:marine carbonate rocks lithium isotopes boron isotopes pH |
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