草莓状黄铁矿与古海洋环境恢复

doi:10.11867/j.issn.1001-8166.2011.05.0475

草莓状黄铁矿是指由等粒度的亚微米级黄铁矿晶体或微晶体紧密堆积而成，形似草莓的黄铁矿球形集合体。它们在氧化和缺氧海洋环境中形成的机理不同，沉积岩中草莓状黄铁矿的粒径分布特征是恢复古海洋的氧化还原状态行之有效的方法之一。然而，草莓状黄铁矿粒径统计仅能区分出氧化和缺氧硫化的环境，不能进一步区分缺氧程度及状态（如次氧化、缺氧含铁等）。桂北泗里口老堡组硅岩沉积于埃迪卡拉纪末期，它们所记录的草莓状黄铁矿粒径分布特征指示了硫化的深水古环境，这与用氧化还原敏感元素等方法获得的深水是缺氧含铁的结论矛盾。这可能是由于当时盆地水体中硫酸盐含量低，细菌硫酸盐还原生成的H₂S少，导致形成了粒径小的黄铁矿。在运用草莓状黄铁矿恢复古海洋环境时，还应与其他地球化学手段，如沉积岩中Fe组分、氧化还原敏感元素、稳定同位素等结合才能获得确切的古海洋氧化还原状态。

关键词: 草莓状黄铁矿, 氧化还原状态, 古海洋

Framboidal pyrites are denselypacked, raspberrylike, spherical aggregates of equigranular, micronsized crystals or microcrysts. Because of the difference principle in framboidal pyrites formation between oxic and euxinic depositional conditions, size distribution of framboids is indicative of oxygen levels for palaeo-ocean water column. However, only oxic and euxinic conditions can be definitely distinguished but suboxic or ferruginous conditions (anoxic and containing dissolved Fe²⁺) using the framboidal pyrites, and framboids formed in ferruginous condition may indicate euxinic environment. Therefore, incompatible results for deepwater redox conditions of the terminal Ediacaran Nanhua Basin have been achieved using framboidal pyrites size statistics and other available methods, respectively. We consider that the euxinic deepwater condition implied by framboids was incorrect, and the small pyrites may be formed due to limited sulfate concentrations in the basin and accordingly little H₂S supply. In order to get credible results, other proxies, such as iron species, redox sensitive trace elements and stable isotope methods should be applied simultaneously when using framboidal pyrites to reconstruct palaeoocean environments.

Key words: Framboidal pyrite, Redox conditions, Palaeo ocean

中图分类号:

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摘要

Pyrite Framboids and Palaeo ocean Redox Condition Reconstruction