Controls on metal enrichment in ferromanganese crusts: temporal changes in oceanic metal flux or phosphatisation?
Controls on metal enrichment in ferromanganese crusts: temporal changes in oceanic metal flux or phosphatisation?
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DOI：
10.1016/j.gca.2021.06.002
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发表时间：
2021-06
期刊：
影响因子：
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通讯作者：
P. Josso;P. Lusty;Simon Chenery;B. Murton
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文献类型：
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作者：
P. Josso;P. Lusty;Simon Chenery;B. Murton
海洋氢化铁锰（Fe-Mn）结壳是许多金属的主要储存库，例如钴、镍、铜、铂、碲和稀土元素，这些金属对于运输和能源系统的脱碳至关重要。在磷化过程中发生的次生矿化过程通常会用碳酸盐氟磷灰石（CFA）浸渍较浅的矿床。此类事件期间的低氧海洋条件经常被用来解释较旧的磷化结壳中较低的钴含量和异常高的镍、铜、锌和铂含量。在这里，评估了磷化事件诱导的低氧成岩重结晶作为 Ni、Cu、Zn 和 Pt 富集和 Co 消耗的驱动机制的假设。对来自东北大西洋热带海山的浅层 (1100 mbsl) 磷酸盐样品和较深 (3100 mbsl) 未磷酸盐样品的准确年代的地球化学剖面进行了比较，跨越 75 Ma 的沉积历史。等值线分析可以定量评估传质中的化学增益和损失，从而可以补偿在铁锰结壳中添加 CFA 引起的稀释效应，表明磷化结壳中没有发生 Co 损失，而 Pt、Te、Cu、Ni 和 Zn 相对于年轻的未磷化铁锰结壳而言富集。两种地球化学剖面均显示出一致的趋势和相似的浓度变化幅度。这排除了磷酸化作为金属富集和消耗的驱动机制。两个样品之间金属含量的系统差异，例如较深样品中的铜含量较高和钴含量较低，与水柱中溶解金属浓度的深度剖面一致。在地球化学剖面中观察到的变化与海洋金属通量的时间变化一致，这是由于新生代东北大西洋气候和海洋结构的演变而导致的。结论是，改变金属通量，而不是与磷化相关的次生矿化过程，是对热带海山铁锰结壳矿床原生金属含量的主要控制。
Oceanic hydrogenetic ferromanganese (Fe-Mn) crusts are a major repository for many metals, such as Co, Ni, Cu, Pt, Te and REE, which are essential for decarbonisation of transport and energy systems. Secondary mineralisation processes, occurring during phosphatisation episodes, commonly impregnate the shallower deposits with carbonate fluorapatite (CFA). The suboxic oceanic conditions during such events are frequently invoked to explain the lower Co content and unusually high Ni, Cu, Zn and Pt content of older phosphatised crusts. Here, the hypothesis of suboxic diagenetic recrystallization induced by phosphatisation episodes as a driving mechanism for Ni, Cu, Zn and Pt enrichment and Co depletion is evaluated. Accurately dated geochemical profiles, spanning 75 Ma of depositional history, for a shallow (1100 mbsl) phosphatised sample and a deeper (3100 mbsl) unphosphatised sample from Tropic Seamount in the north-east Atlantic, are compared. An isocon analysis, which allows to quantitively evaluate chemical gains and losses in mass transfer and therefore permits compensation for the dilution effect induced by the addition of CFA in the Fe-Mn crusts, demonstrates that no loss of Co has occurred in the phosphatised crust, whilst Pt, Te, Cu, Ni and Zn are enriched relative to younger, unphosphatised Fe-Mn crust. Both geochemical profiles show sympathetic trends and similar amplitudes of variation in concentration. This excludes phosphatisation as the driving mechanism for the metal enrichment and depletion. Systematic differences in metal content between the two samples, such as higher Cu and lower Co content in the deeper sample, are consistent with the depth profile of dissolved metal concentrations in the water column. The variability observed in the geochemical profiles is consistent with temporal changes in metal fluxes to the ocean, as a result of the evolving climate and oceanographic configuration of the north-east Atlantic Ocean through the Cenozoic. It is concluded that changing metal fluxes, rather than secondary mineralisation process associated with phosphatisation, is the dominant control on the primary metal content in Fe-Mn crust deposits at Tropic Seamount.