Depositional age and geochemistry of the 2.44–2.32 Ga Granular Iron Formation in the Songshan Group, North China Craton: Tracing the effects of atmospheric oxygenation on continental weathering and seawater environment
Depositional age and geochemistry of the 2.44–2.32 Ga Granular Iron Formation in the Songshan Group, North China Craton: Tracing the effects of atmospheric oxygenation on continental weathering and seawater environment
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DOI：
10.1016/j.precamres.2021.106142
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发表时间：
2021-06
影响因子：
3.8
通讯作者：
Caiyun Lan;X. Long;M. Zhai;Jingyu Wang
中科院分区：
文献类型：
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作者：
Caiyun Lan;X. Long;M. Zhai;Jingyu Wang
由于缺乏同时代的海洋化学沉淀物，海洋化学和氧化还原状态对全球大氧化事件期间（2.45-2.10 Ga）大气氧化的响应仍然受到很大的限制，铁阵。本文首次报道了华北地区松山群中的一个粒铁建造，并将其沉积时代限定在2.44- 2.32Ga。在GIF中，赤铁矿是主要的铁氧化物，它的出现表明2.44- 2.32Ga的浅水海水是含氧的，这与松山GIF上下同时代碳酸盐的正δ 13 C偏移相一致。氧化还原敏感的微量元素（Cr，V，Mo和U）和Al 2 O3含量之间没有显着的相关性排除陆源碎屑的来源，这表明这些元素在沉积过程中富集，并已被富铁的化学沉淀物吸附。GIF和自生赤铁矿的高Cr（高达2000 ppm）和B（高达300 ppm）含量进一步表明化学风化和随后输入海水的增加。结合GIF中V（高达1500 ppm）和P（高达7000 ppm）的高含量，可以推断2.44-2.32 Ga期间大陆氧化风化作用强烈。松山GIF及其自生赤铁矿的Cr含量和Cr/Ti比值的富集表明，由于土壤或地壳中富Cr副矿物中的Cr（III）被氧化，Cr（VI）以可溶态形式通过河流向海洋迁移。Cr、V在赤铁矿中的富集过程可能是由下降流表层沃茨提供的可溶性Cr（VI）、V（V）在缺氧-缺氧底层沃茨中转化为Cr（III）、V（III），然后通过赤铁矿的自生埋藏作用从海水中去除，这是由于三价阳离子可在赤铁矿结构中进行置换。因此，我们提出了一个新的沉积情景松山GIF，降水发生由于上升流的深层，缺氧，还原Cr-V富铁沃茨到一个含氧，高生产力浅水设置。
The responses of oceanic chemistry and redox state to the oxygenation of atmosphere at the period (2.45–2.10 Ga) of the global Great Oxidation Event remain poorly constrained due to the lacking of coeval marine chemical precipitates, e.g., Iron Formation. Here, we report a Granular Iron Formation (GIF) in the Songshan Group of the North China Craton and its depositional age was constrained at 2.44–2.32 Ga at the first time. In the GIF, hematite is the primary iron oxide and its occurrence indicates that the shallow seawater at 2.44–2.32 Ga was oxygenated, which is in agreement with the positiveδ13C excursion of the coeval carbonates up and below the Songshan GIF. No significant correlations between redox-sensitive trace elements (Cr, V, Mo and U) and Al2O3contents exclude terrigenous clastic origin, suggesting that these elements are enriched during the deposition and have been adsorbed by the Fe-rich chemical precipitates. The high Cr (up to 2000 ppm) and B (up to 300 ppm) contents of the GIF and the authigenic hematites further indicate an increase of chemical weathering and subsequent input into the seawater. Combined with high V (up to 1500 ppm) and P (up to 7000 ppm) contents in the GIF, it can be inferred that continental oxidative weathering was intensive during 2.44–2.32 Ga. The enrichments of Cr contents and Cr/Ti ratios of the Songshan GIF and its authigenic hematites suggest that Cr was transported to the oceans as soluble Cr (VI) species by rivers because of the oxidative Cr(III) in Cr-rich accessory minerals in soil or crust. The enrichment processes of Cr and V in the hematites are likely to be that soluble Cr (VI) and V (V) supplying by the downwelling surface waters transform into Cr (III) and V (III) in the suboxic-anoxic bottom waters and then removed from seawater via authigenic burial of hematite because trivalent cations can substitute in the hematite structure. Based on above, we therefore suggest a new depositional scenario for the Songshan GIF, in which precipitation occurred due to upwelling of deep, anoxic, reduced Cr-V-rich ferruginous waters into an oxygenated, high productivity shallow-water setting.