Controls on metal enrichment in ferromanganese crusts: Temporal changes in oceanic metal flux or phosphatisation?

Pierre Josso,Paul Lusty,Simon Chenery,Bramley Murton

doi:10.1016/j.gca.2021.06.002

Pierre Josso, Paul Lusty + Show 2 more

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https://doi.org/10.1016/j.gca.2021.06.002

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Abstract

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.

Highlights

This study provides a comparison of accurately dated geochemical profiles for two Fe-Mn crust samples recovered at 1100 and 3000 mbsl on Tropic Seamount
The variations in texture, mineralogy and major element geochemistry of summit sample 085_004 are discussed by Josso et al (2020b) in the context of their importance to paleoceanographic reconstructions. This previous study, and a number of others (Koschinsky et al, 1996; Marino et al, 2017), indicate that the Fe-Mn crust samples from Tropic Seamount are hydrogenetic, with average compositions typical of Fe-Mn crusts occurring proximal to continental margins
This study provides a comparison of accurately dated geochemical profiles for two Fe-Mn crusts recovered at 1100 and 3000 mbsl from Tropic Seamount

Summary

Introduction

Fe-Mn crusts are of particular interest because of their high concentrations in a range of metals of increasing economic importance (Mn, Co, Te, REY, Pt, Ni and Cu) (Hein et al, 2013; Lusty et al, 2018) Enrichment in these metals is attributed to the physio-chemical properties of Fe and Mn oxyhydroxides, and their extremely slow accumulation rates on the seafloor (a few mm/Ma), which provides an efficient and continuous scavenging mechanism for. During periods of major oceanic circulation overturning, upwelling of nutrient-rich deep waters results in rapid increases in biological productivity, resulting in oxygen depletion in the upper water column (Halbach and Puteanus, 1984) This causes the formation of a phosphatesaturated oxygen minimum zone (OMZ) extending downwards in the water column. Strong suboxic conditions can notably lead to the partialdissolution of Fe and Mn oxyhydroxides and precipitation of more stable secondary mineral phases such as todorokite with varying metal accommodation mechanisms (Halbach et al, 1989a, 1989b; Koschinsky et al, 1997)

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