# Origin and Composition of Ferromanganese Deposits of New Caledonia Exclusive Economic Zone

Paul Staszak, Julien Collot, Pierre Josso, Ewan Pelleter, Samuel Etienne, Martin Patriat, Sandrine Cheron, Audrey Boissier, Yaël Guyomard

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Paul Staszak, Julien Collot, Pierre Josso, Ewan Pelleter, Samuel Etienne, et al. Origin and Composition of Ferromanganese Deposits of New Caledonia Exclusive Economic Zone. Minerals, 2022, 12 (2), 255 (28p.). 10.3390/min12020255. [https://hal-04203740](https://hal.science/hal-04203740)

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# Origin and Composition of Ferromanganese Deposits of New Caledonia Exclusive Economic Zone

Paul Staszak, Julien Collot, Pierre Josso, Ewan Pelleter, Samuel Etienne, Martin Patriat, Sandrine Cheron, Audrey Boissier, Yaël Guyomard

► To cite this version:

Paul Staszak, Julien Collot, Pierre Josso, Ewan Pelleter, Samuel Etienne, et al. Origin and Composition of Ferromanganese Deposits of New Caledonia Exclusive Economic Zone. Minerals, 2022, 12 (2), 255 (28p.). 10.3390/min12020255. [https://doi.org/10.3390/min12020255]

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https://hal.science/hal-04203740v1

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entific research documents, whether they are pub-
lished or not. The documents may come from
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Article

# Origin and Composition of Ferromanganese Deposits of New Caledonia Exclusive Economic Zone

Paul Staszak <fnref n="1" />,<fnref n="2" />,* , Julien Collot <fnref n="1" />, Pierre Josso <fnref n="3" />, Ewan Pelleter <fnref n="2" />,*, Samuel Etienne <fnref n="1" />, Martin Patriat <fnref n="2" />, Sandrine Cheron <fnref n="2" />, Audrey Boissier <fnref n="2" /> and Yaël Guyomard <fnref n="1" />

<figure><img src="image_3.png" /></figure>

check for
updates

**Citation:** Staszak, P.; Collot, J.; Josso, P.; Pelleter, E.; Etienne, S.; Patriat, M.; Cheron, S.; Boissier, A.; Guyomard, Y. *Origin and Composition of Ferromanganese Deposits of New Caledonia Exclusive Economic Zone*. *Minerals* **2022**, *12*, 255. https://doi.org/10.3390/min12020255

Academic Editor: Francisco J. González

Received: 29 January 2022

Accepted: 10 February 2022

Published: 16 February 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

<figure><img src="image_4.png" /></figure>

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

**Abstract:** Located in the South-West Pacific, at the northern extremity of the mostly submerged Zealandia continent, the New Caledonian Exclusive Economic Zone (EEZ) covers 1,470,000 km<fnref n="2" /> and includes basins, ridges and seamounts where abundant ferromanganese crusts have been observed. Several investigations have been conducted since the 1970s on the nature and composition of ferromanganese crusts from New Caledonia's seamounts and ridges, but none have covered the entire EEZ. We present data from 104 ferromanganese crusts collected in New Caledonia's EEZ during twelve oceanographic cruises between 1974 and 2019. Samples were analysed for mineralogy, geochemical compositions, growth rates, and through a statistical approach using correlation coefficients and factor analysis. Crust thicknesses range from 1 mm to 115 mm, with growth rates between 0.45 mm/Ma and 102 mm/Ma. Based on textures, structures, discrimination plots, and growth rates, we distinguish a group of hydrogenetic crusts containing the highest mean contents of Co (0.42 wt%), Ni (0.31 wt%), and high contents of Mo, V, W, Pb, Zn, Nb, from a group of hydrothermal and/or diagenetic deposits showing high mean contents of Mn (38.17 wt%), Ba (0.56 wt%) and low contents of other trace metals. Several samples from this later group have exceptionally high content of Ni (0.7 wt%). The data shows that crusts from the southern part of the EEZ, notably seamounts of the Loyalty Ridge and the Lord Howe Rise, present high mineral potential for prospectivity owing to high contents of valuable metals, and constitute a great target for further investigation.

**Keywords:** ferromanganese crusts; New Caledonia; hydrogenetic; hydrothermal and diagenetic deposits

## 1. Introduction

Hydrogenetic ferromanganese (Fe-Mn) oxide deposits are known to be distributed widely in all oceans of the planet, the largest known fields being located in the Pacific Ocean [1]. They occur as crusts on sediment free surfaces like seamount flanks and summits, ridges, or any topographic reliefs located between 400 and 7000 m water depths [2,3]. Ferromanganese crusts grow by precipitation of metals from ambient cold seawater and accumulation on the seafloor, forming layers of Mn oxides and Fe-oxyhydroxides. Their thickness ranges from less than a millimeter up to 25 cm [4]. The hydrogenetic accumulation of Mn oxides and Fe-oxyhydroxides requires stable conditions over long periods of time (million years) to form thick crusts [5]. Their distribution, textures and composition are impacted by several parameters, such as surface bioproductivity, depth of the

<footnote n="1">¹ Geological Survey of New Caledonia, Direction de l'Industrie, des Mines et de l'Énergie de Nouvelle-Calédonie, BP M2, Nouméa 98845, New Caledonia; julien.collot@gouv.nc (J.C.); samuel.etienne@gouv.nc (S.E.); yael.guyomard@gouv.nc (Y.G.)</footnote>

<footnote n="2">² Institut Français de Recherche pour l'Exploitation de la Mer (IFREMER), Unité Géosciences Marines, 29280 Plouzané, France; martin.patriat@ifremer.fr (M.P.); sandrine.cheron@ifremer.fr (S.C.); audrey.boissier@ifremer.fr (A.B.)</footnote>

<footnote n="3">³ British Geological Survey, Environmental Science Centre, Nottingham NG12 5GG, UK; piesso@bgs.ac.uk</footnote>

<footnote>* Correspondence: pl.staszak@laposte.net (P.S.); ewan.pelleter@ifremer.fr (E.P.)</footnote>
<figure><img src="image_2.png" /></figure>

Article

# Origin and Composition of Ferromanganese Deposits of New Caledonia Exclusive Economic Zone

Paul Staszak <fnref n="1" />,<fnref n="2" />,* , Julien Collot <fnref n="1" />, Pierre Josso <fnref n="3" />, Ewan Pelleter <fnref n="2" />,*, Samuel Etienne <fnref n="1" />, Martin Patriat <fnref n="2" />, Sandrine Cheron <fnref n="2" />, Audrey Boissier <fnref n="2" /> and Yaël Guyomard <fnref n="1" />

<figure><img src="image_3.png" /></figure>

check for
updates

**Citation:** Staszak, P.; Collot, J.; Josso, P.; Pelleter, E.; Etienne, S.; Patriat, M.; Cheron, S.; Boissier, A.; Guyomard, Y. *Origin and Composition of Ferromanganese Deposits of New Caledonia Exclusive Economic Zone*. *Minerals* **2022**, *12*, 255. https://doi.org/10.3390/min12020255

Academic Editor: Francisco J. González

Received: 29 January 2022

Accepted: 10 February 2022

Published: 16 February 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

<figure><img src="image_4.png" /></figure>

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

**Abstract:** Located in the South-West Pacific, at the northern extremity of the mostly submerged Zealandia continent, the New Caledonian Exclusive Economic Zone (EEZ) covers 1,470,000 km<fnref n="2" /> and includes basins, ridges and seamounts where abundant ferromanganese crusts have been observed. Several investigations have been conducted since the 1970s on the nature and composition of ferromanganese crusts from New Caledonia's seamounts and ridges, but none have covered the entire EEZ. We present data from 104 ferromanganese crusts collected in New Caledonia's EEZ during twelve oceanographic cruises between 1974 and 2019. Samples were analysed for mineralogy, geochemical compositions, growth rates, and through a statistical approach using correlation coefficients and factor analysis. Crust thicknesses range from 1 mm to 115 mm, with growth rates between 0.45 mm/Ma and 102 mm/Ma. Based on textures, structures, discrimination plots, and growth rates, we distinguish a group of hydrogenetic crusts containing the highest mean contents of Co (0.42 wt%), Ni (0.31 wt%), and high contents of Mo, V, W, Pb, Zn, Nb, from a group of hydrothermal and/or diagenetic deposits showing high mean contents of Mn (38.17 wt%), Ba (0.56 wt%) and low contents of other trace metals. Several samples from this later group have exceptionally high content of Ni (0.7 wt%). The data shows that crusts from the southern part of the EEZ, notably seamounts of the Loyalty Ridge and the Lord Howe Rise, present high mineral potential for prospectivity owing to high contents of valuable metals, and constitute a great target for further investigation.

**Keywords:** ferromanganese crusts; New Caledonia; hydrogenetic; hydrothermal and diagenetic deposits

## 1. Introduction

Hydrogenetic ferromanganese (Fe-Mn) oxide deposits are known to be distributed widely in all oceans of the planet, the largest known fields being located in the Pacific Ocean [1]. They occur as crusts on sediment free surfaces like seamount flanks and summits, ridges, or any topographic reliefs located between 400 and 7000 m water depths [2,3]. Ferromanganese crusts grow by precipitation of metals from ambient cold seawater and accumulation on the seafloor, forming layers of Mn oxides and Fe-oxyhydroxides. Their thickness ranges from less than a millimeter up to 25 cm [4]. The hydrogenetic accumulation of Mn oxides and Fe-oxyhydroxides requires stable conditions over long periods of time (million years) to form thick crusts [5]. Their distribution, textures and composition are impacted by several parameters, such as surface bioproductivity, depth of the

<footnote n="1">¹ Geological Survey of New Caledonia, Direction de l'Industrie, des Mines et de l'Énergie de Nouvelle-Calédonie, BP M2, Nouméa 98845, New Caledonia; julien.collot@gouv.nc (J.C.); samuel.etienne@gouv.nc (S.E.); yael.guyomard@gouv.nc (Y.G.)</footnote>

<footnote n="2">² Institut Français de Recherche pour l'Exploitation de la Mer (IFREMER), Unité Géosciences Marines, 29280 Plouzané, France; martin.patriat@ifremer.fr (M.P.); sandrine.cheron@ifremer.fr (S.C.); audrey.boissier@ifremer.fr (A.B.)</footnote>

<footnote n="3">³ British Geological Survey, Environmental Science Centre, Nottingham NG12 5GG, UK; piesso@bgs.ac.uk</footnote>

<footnote>* Correspondence: pl.staszak@laposte.net (P.S.); ewan.pelleter@ifremer.fr (E.P.)</footnote>
oxygen-minimum zone (OMZ), metal partitioning along the water column, bottom currents, proximity to land masses, and sedimentation rates [1]. The hydrogenetic accretion is believed to involve an inorganic colloidal-chemical mechanism, coupled to a surface-chemical mechanism [2]. Mn and Fe, under normal physicochemical properties of seawater (Eh > 0.5 V and pH ± 8, [6]), are present in their oxidized form (Mn⁴⁺O₂ and Fe³⁺OOH, respectively), and tends to form hydrated colloids [7,8]. These colloids generally have a positive or a negative surface that interacts with other colloids and dissolved hydrous metals ions [2]. Colloidal hydrous Mn oxide particles, with negatively charged surfaces, attract hydrated cations such as Co, Cu, Zn, Ni, Tl, Ce or Y, whilst slightly positively charged hydrous Fe-oxyhydroxide particles attract hydrated anions such as Ti, Zr, Mo, V, Pb, U, Nb or rare-earth elements (REE) [2,6]. Hydrogenetic Fe-Mn crusts precipitate at very slow rates (between 1 to 10 mm/Ma), allowing the adsorption and concentration of large quantities of metals on crust surfaces via the continuous interaction between oxides and seawater [1,2]. Compared to Earth's lithosphere, hydrogenetic Fe-Mn crusts are significantly enriched in several critical and rare metals such as Bi, Co, Mo, Nb, Pt, REE, Te, W, Y, Zr, which are critical to ensure the transition from a fossil-fuel-based energy system to a zero-carbon and renewable energy system [9]. Contrastingly, hydrothermal Fe-Mn-oxyhydroxide crusts form in the vicinity of high temperature hydrothermal systems, with supply from fall-out or precipitating directly from diffuse low temperature systems, showing starkly different morphologies and strongly fractionated compositions [10–14].

The first ferromanganese crusts collected in the vicinity of New Caledonia have been sampled between 1974 and 1976 during the GEORSTOM and EVA cruises. These crusts dredged from the flanks of New Caledonia's main island, Grande Terre, and the Loyalty Islands have been previously analysed and attest to their hydrogenetic origin with a mean Co content of 0.5% and a detrital enrichment [15–18]. Samples from the Loyalty Ridge seamounts were analysed in 2004, revealing hydrothermal deposits associated to a Miocene volcanic activity [19].

The present work provides a new set of data, combining analyses of deposits from recent and older cruises across the Exclusive Economic Zone (EEZ) of New Caledonia [20–30]. We report mineralogical and geochemical studies of 104 selected samples using analytical methods such as X-ray powder diffraction (XRD), X-ray fluorescence (XRF) and inductively-coupled plasma mass spectrometry (ICP-MS), and discuss the nature, age and mineral associations of those Fe-Mn deposits. Factors controlling Fe-Mn crusts' composition and distribution within the New Caledonian EEZ are discussed. Finally, we conclude by presenting the most promising zone for further investigations in the vicinity of New Caledonia.

## 2. Regional Settings

New Caledonia is a French overseas territory located in the South-West Pacific Ocean, 2000 km east of Australia. Its EEZ and extended continental shelf cover an area of 1,470,000 km², and includes islands, continental and volcanic submarine ridges, an active subduction zone, ancient hotspots, and one of the largest ophiolitic complex on Earth [31–33]. The present-day physiography of the South-West Pacific results from a complex geological history, involving successive basin opening and closing since the Mesozoic fragmentation of the Gondwana supercontinent [34–36]. From West to East, several continental ridges are encountered: the Dampier Ridge, Lord Howe Rise, Fairway Ridge, and Norfolk Ridge (Figure 1). These are separated by the Fairway Basin, New Caledonia Trough, and South Loyalty Basin, all being part of the mostly submerged Zealandia continent (Figure 1A). The Pre-Mesozoic to Early Cretaceous rocks known onshore in New Caledonia, Australia and New Zealand attest for the presence of an ancient Andean-like subduction zone along Gondwana's eastern margin during much of the 260–110 Ma interval [37–39]. A change in tectonic regime, from convergence to extension, along Gondwana's eastern margin occurred at the end of the Early Cretaceous (110–100 Ma) and led to the break-up of Zealandia and Gondwana [40–42]. This extension phase ended during the Early Eocene and is marked by the end of oceanic seafloor spreading in the Tasman Sea and the development of a regional event known as TECTA (Tectonic Event
oxygen-minimum zone (OMZ), metal partitioning along the water column, bottom currents, proximity to land masses, and sedimentation rates [1]. The hydrogenetic accretion is believed to involve an inorganic colloidal-chemical mechanism, coupled to a surface-chemical mechanism [2]. Mn and Fe, under normal physicochemical properties of seawater (Eh > 0.5 V and pH ± 8, [6]), are present in their oxidized form (Mn⁴⁺O₂ and Fe³⁺OOH, respectively), and tends to form hydrated colloids [7,8]. These colloids generally have a positive or a negative surface that interacts with other colloids and dissolved hydrous metals ions [2]. Colloidal hydrous Mn oxide particles, with negatively charged surfaces, attract hydrated cations such as Co, Cu, Zn, Ni, Tl, Ce or Y, whilst slightly positively charged hydrous Fe-oxyhydroxide particles attract hydrated anions such as Ti, Zr, Mo, V, Pb, U, Nb or rare-earth elements (REE) [2,6]. Hydrogenetic Fe-Mn crusts precipitate at very slow rates (between 1 to 10 mm/Ma), allowing the adsorption and concentration of large quantities of metals on crust surfaces via the continuous interaction between oxides and seawater [1,2]. Compared to Earth's lithosphere, hydrogenetic Fe-Mn crusts are significantly enriched in several critical and rare metals such as Bi, Co, Mo, Nb, Pt, REE, Te, W, Y, Zr, which are critical to ensure the transition from a fossil-fuel-based energy system to a zero-carbon and renewable energy system [9]. Contrastingly, hydrothermal Fe-Mn-oxyhydroxide crusts form in the vicinity of high temperature hydrothermal systems, with supply from fall-out or precipitating directly from diffuse low temperature systems, showing starkly different morphologies and strongly fractionated compositions [10–14].

The first ferromanganese crusts collected in the vicinity of New Caledonia have been sampled between 1974 and 1976 during the GEORSTOM and EVA cruises. These crusts dredged from the flanks of New Caledonia's main island, Grande Terre, and the Loyalty Islands have been previously analysed and attest to their hydrogenetic origin with a mean Co content of 0.5% and a detrital enrichment [15–18]. Samples from the Loyalty Ridge seamounts were analysed in 2004, revealing hydrothermal deposits associated to a Miocene volcanic activity [19].

The present work provides a new set of data, combining analyses of deposits from recent and older cruises across the Exclusive Economic Zone (EEZ) of New Caledonia [20–30]. We report mineralogical and geochemical studies of 104 selected samples using analytical methods such as X-ray powder diffraction (XRD), X-ray fluorescence (XRF) and inductively-coupled plasma mass spectrometry (ICP-MS), and discuss the nature, age and mineral associations of those Fe-Mn deposits. Factors controlling Fe-Mn crusts' composition and distribution within the New Caledonian EEZ are discussed. Finally, we conclude by presenting the most promising zone for further investigations in the vicinity of New Caledonia.

## 2. Regional Settings

New Caledonia is a French overseas territory located in the South-West Pacific Ocean, 2000 km east of Australia. Its EEZ and extended continental shelf cover an area of 1,470,000 km², and includes islands, continental and volcanic submarine ridges, an active subduction zone, ancient hotspots, and one of the largest ophiolitic complex on Earth [31–33]. The present-day physiography of the South-West Pacific results from a complex geological history, involving successive basin opening and closing since the Mesozoic fragmentation of the Gondwana supercontinent [34–36]. From West to East, several continental ridges are encountered: the Dampier Ridge, Lord Howe Rise, Fairway Ridge, and Norfolk Ridge (Figure 1). These are separated by the Fairway Basin, New Caledonia Trough, and South Loyalty Basin, all being part of the mostly submerged Zealandia continent (Figure 1A). The Pre-Mesozoic to Early Cretaceous rocks known onshore in New Caledonia, Australia and New Zealand attest for the presence of an ancient Andean-like subduction zone along Gondwana's eastern margin during much of the 260–110 Ma interval [37–39]. A change in tectonic regime, from convergence to extension, along Gondwana's eastern margin occurred at the end of the Early Cretaceous (110–100 Ma) and led to the break-up of Zealandia and Gondwana [40–42]. This extension phase ended during the Early Eocene and is marked by the end of oceanic seafloor spreading in the Tasman Sea and the development of a regional event known as TECTA (Tectonic Event
of the Cenozoic in the Tasman Area) lasting until the Oligocene [43–45]. It is characterised by contractional deformation, basin formation and volcanism. It strongly impacted northern Zealandia, forming much of its present-day physiography, and ended with the obduction of sedimentary, mafic and ultramafic nappes in New Caledonia [31,46,47]. The origin of this event is likely related to a subduction initiation along Zealandia’s eastern margin, of which the vergence and detailed chronology are debated [36]. Because of the lack of basement samples, the nature of the Loyalty Ridge remains unclear but is thought to be an Eocene to Miocene volcanic arc associated with this new subduction [48]. Other structures around New Caledonia, such as the Pines Ridge and the New Caledonia Trough, were also formed during this event [31,49,50]. The Neogene is characterised by extensional tectonics, causing the opening of the back-arc Norfolk and North Fiji basins [51]. Intense intraplate volcanism also affected the region during this period leading to the formation of the Lord Howe Seamounts Chain [52] (Figure 1B) and of numerous isolated guyots and seamounts [53–56]. The Middle Miocene is also the time when the North-East dipping Vanuatu subduction zone initiated along the Vitiaz Lineament and rolled back to its present-day position [36,57]. The subduction of the Australian plate under the Vanuatu Arch induces a flexure of the downgoing lithosphere and hence uplift and extensional tectonics as far as southern Grande Terre [58,59].

<figure><img src="image_5.png" /><figcaption>**Figure 1.** (A) Nature of basement of the South-West Pacific (modified after [33]); (B) Age of basement formation of the South-West Pacific (modified after [33]).</figcaption></figure>
of the Cenozoic in the Tasman Area) lasting until the Oligocene [43–45]. It is characterised by contractional deformation, basin formation and volcanism. It strongly impacted northern Zealandia, forming much of its present-day physiography, and ended with the obduction of sedimentary, mafic and ultramafic nappes in New Caledonia [31,46,47]. The origin of this event is likely related to a subduction initiation along Zealandia’s eastern margin, of which the vergence and detailed chronology are debated [36]. Because of the lack of basement samples, the nature of the Loyalty Ridge remains unclear but is thought to be an Eocene to Miocene volcanic arc associated with this new subduction [48]. Other structures around New Caledonia, such as the Pines Ridge and the New Caledonia Trough, were also formed during this event [31,49,50]. The Neogene is characterised by extensional tectonics, causing the opening of the back-arc Norfolk and North Fiji basins [51]. Intense intraplate volcanism also affected the region during this period leading to the formation of the Lord Howe Seamounts Chain [52] (Figure 1B) and of numerous isolated guyots and seamounts [53–56]. The Middle Miocene is also the time when the North-East dipping Vanuatu subduction zone initiated along the Vitiaz Lineament and rolled back to its present-day position [36,57]. The subduction of the Australian plate under the Vanuatu Arch induces a flexure of the downgoing lithosphere and hence uplift and extensional tectonics as far as southern Grande Terre [58,59].

<figure><img src="image_5.png" /><figcaption>**Figure 1.** (A) Nature of basement of the South-West Pacific (modified after [33]); (B) Age of basement formation of the South-West Pacific (modified after [33]).</figcaption></figure>
# 3. Material and Methods

## 3.1. Sample Collection

A total of 104 samples of Fe-Mn deposits were selected from dredge material collected during multiple oceanographic cruises between 1974 and 2019 (Table 1). Samples were collected at different water depths ranging from 430 m to 4677 m (Figure 2). Samples from the Lord Howe seamount chain, Lord Howe Rise, Fairway Ridge, Norfolk Ridge, D'Entrecasteaux Basin, North D'Entrecasteaux Ridge and Loyalty Ridge were recovered in diverse settings with regard to depositional environments, age of structures, and nature of substrate rocks (Figure 1). For all crusts, a representative bulk sample of the whole stratigraphy has been selected. If any macroscopic boundaries were observed within the crust, representative sub-samples of the macro-layers were taken, so that all the crust thickness was sampled. In such context, sub-samples are sorted in stratigraphic order. Table 1 shows the sampling type, thickness, location, depth, cruise and substrate rock (vacuolar to amygdaloidal basalt, andesite, hyaloclastite breccia, shoshonite, polygenic breccia, bioclastic limestone, or mudstone). A large proportion of crusts were lacking substrate rocks.

<table><caption>**Table 1.** Sample information for selected deposits (n = 104).</caption><thead><tr><th>Samples</th><th>Type</th><th>Avg Crusts Thickness (mm)</th><th>Latitude (S)</th><th>Longitude (E)</th><th>Water Depth (m)</th><th>Cruise</th><th>Simplified Substrate</th></tr></thead><tbody><tr><td>DW772</td><td>Bulk</td><td>5</td><td>26°46′59.88″</td><td>170°22′1.21″</td><td>902</td><td>BATHUS3</td><td>-</td></tr><tr><td>DW774</td><td>Bulk</td><td>2</td><td>26°48′</td><td>170°22′1.21″</td><td>925</td><td>BATHUS3</td><td>Volcanics</td></tr><tr><td>DW778B</td><td>Bulk</td><td>-</td><td>25°16′59.88″</td><td>170°7′1.21″</td><td>755</td><td>BATHUS3</td><td>-</td></tr><tr><td>DW778D</td><td>Bulk</td><td>-</td><td>25°16′59.88″</td><td>170°7′1.21″</td><td>755</td><td>BATHUS3</td><td>-</td></tr><tr><td>DW778D2</td><td>Bulk</td><td>-</td><td>25°16′59.88″</td><td>170°7′1.21″</td><td>755</td><td>BATHUS3</td><td>-</td></tr><tr><td>DW2482</td><td>Bulk</td><td>-</td><td>24°8′39.01″</td><td>161°43′5.99″</td><td>430</td><td>EBISCO</td><td>-</td></tr><tr><td>DR11Ai</td><td>Bulk</td><td>97</td><td>18°0′58.68″</td><td>160°43′42.6″</td><td>2375</td><td>ECOSAT</td><td>-</td></tr><tr><td>DR11Ai-1</td><td>Layers</td><td>0–40</td><td>18°0′58.68″</td><td>160°43′42.6″</td><td>2375</td><td>ECOSAT</td><td>-</td></tr><tr><td>DR11Ai-2</td><td>Layers</td><td>40–50</td><td>18°0′58.68″</td><td>160°43′42.6″</td><td>2375</td><td>ECOSAT</td><td>-</td></tr><tr><td>DR11Ai-3</td><td>Layers</td><td>50–80</td><td>18°0′58.68″</td><td>160°43′42.6″</td><td>2375</td><td>ECOSAT</td><td>-</td></tr><tr><td>DR11Ai-4</td><td>Layers</td><td>80–97</td><td>18°0′58.68″</td><td>160°43′42.6″</td><td>2375</td><td>ECOSAT</td><td>-</td></tr><tr><td>DR13Bii</td><td>Bulk</td><td>30</td><td>17°21′37.44″</td><td>158°50′8.16″</td><td>1765</td><td>ECOSAT</td><td>-</td></tr><tr><td>DR14H</td><td>Bulk</td><td>10</td><td>18°55′28.2″</td><td>159°11′56.76″</td><td>2900</td><td>ECOSAT</td><td>Volcanics</td></tr><tr><td>DR15A</td><td>Bulk</td><td>2</td><td>18°18′36.72″</td><td>158°28′17.04″</td><td>2225</td><td>ECOSAT</td><td>Limestone</td></tr><tr><td>DR18F</td><td>Bulk</td><td>14</td><td>20°5′3.12″</td><td>160°11′32.64″</td><td>1150</td><td>ECOSAT</td><td>-</td></tr><tr><td>E-DR08B</td><td>Bulk</td><td>60</td><td>18°25′8.04″</td><td>164°0′28.44″</td><td>1400</td><td>ECOSAT</td><td>-</td></tr><tr><td>E-DR08B-1</td><td>Layers</td><td>0–20</td><td>18°25′8.04″</td><td>164°0′28.44″</td><td>1400</td><td>ECOSAT</td><td>-</td></tr><tr><td>E-DR08B-2</td><td>Layers</td><td>20–40</td><td>18°25′8.04″</td><td>164°0′28.44″</td><td>1400</td><td>ECOSAT</td><td>-</td></tr><tr><td>E-DR08B-3</td><td>Layers</td><td>40–60</td><td>18°25′8.04″</td><td>164°0′28.44″</td><td>1400</td><td>ECOSAT</td><td>-</td></tr><tr><td>DR48-021</td><td>Bulk</td><td>44</td><td>17°53′38″</td><td>159°15′11″</td><td>2060</td><td>ECOSAT3</td><td>-</td></tr><tr><td>DR53-008</td><td>Bulk</td><td>31</td><td>23°14′31″</td><td>159°46′11″</td><td>1390</td><td>ECOSAT3</td><td>-</td></tr><tr><td>DR54-009</td><td>Bulk</td><td>8</td><td>24°39′6″</td><td>159°42′57″</td><td>1350</td><td>ECOSAT3</td><td>Volcanics</td></tr><tr><td>102D</td><td>Bulk</td><td>-</td><td>18°38′24″</td><td>163°31′29.97″</td><td>2644</td><td>EVA</td><td>-</td></tr><tr><td>105D</td><td>Bulk</td><td>-</td><td>18°20′6″</td><td>163°58′1.18″</td><td>1467</td><td>EVA</td><td>-</td></tr><tr><td>108D</td><td>Bulk</td><td>-</td><td>19°31′45.48″</td><td>164°11′56.42″</td><td>2954</td><td>EVA</td><td>-</td></tr><tr><td>109D</td><td>Bulk</td><td>-</td><td>20°30′18″</td><td>165°13′37.22″</td><td>2245</td><td>EVA</td><td>-</td></tr><tr><td>110D</td><td>Bulk</td><td>34</td><td>20°33′18″</td><td>165°19′8.4″</td><td>2100</td><td>EVA</td><td>-</td></tr><tr><td>113D</td><td>Bulk</td><td>11</td><td>21°22′14.88″</td><td>166°50′31.18″</td><td>1200</td><td>EVA</td><td>Breccia</td></tr><tr><td>114D</td><td>Bulk</td><td>3</td><td>21°36′11.88″</td><td>166°51′50.39″</td><td>1185</td><td>EVA</td><td>Limestone</td></tr><tr><td>116D</td><td>Bulk</td><td>25</td><td>21°14′6″</td><td>167°29′49.18″</td><td>2210</td><td>EVA</td><td>Breccia</td></tr><tr><td>117D</td><td>Bulk</td><td>30</td><td>24°43′59.88″</td><td>169°25′30.01″</td><td>1800</td><td>EVA</td><td>Limestone</td></tr><tr><td>GO14D</td><td>Bulk</td><td>14</td><td>24°28′0.12″</td><td>168°49′58.77″</td><td>1450</td><td>GEORSTOM1</td><td>Breccia</td></tr><tr><td>GO15D</td><td>Bulk</td><td>5</td><td>24°27′42.12″</td><td>168°51′25.18″</td><td>1325</td><td>GEORSTOM1</td><td>Mudstone</td></tr><tr><td>GO16D</td><td>Bulk</td><td>6</td><td>24°22′59.88″</td><td>168°50′31.18″</td><td>1240</td><td>GEORSTOM1</td><td>Mudstone</td></tr><tr><td>GO18D</td><td>Bulk</td><td>16</td><td>24°18′54″</td><td>168°14′16.82″</td><td>585</td><td>GEORSTOM1</td><td>Limestone</td></tr><tr><td>GO20D</td><td>Bulk</td><td>2</td><td>25°55′23.88″</td><td>168°0′28.78″</td><td>1220</td><td>GEORSTOM1</td><td>Sandstone</td></tr><tr><td>GO3D</td><td>Bulk</td><td>38</td><td>23°28′0.12″</td><td>167°58′4.81″</td><td>2100</td><td>GEORSTOM1</td><td>Breccia</td></tr><tr><td>GO202</td><td>Bulk</td><td>57</td><td>19°56′53.88″</td><td>160°48′10.77″</td><td>2425</td><td>GEORSTOM2</td><td>-</td></tr></tbody></table>
# 3. Material and Methods

## 3.1. Sample Collection

A total of 104 samples of Fe-Mn deposits were selected from dredge material collected during multiple oceanographic cruises between 1974 and 2019 (Table 1). Samples were collected at different water depths ranging from 430 m to 4677 m (Figure 2). Samples from the Lord Howe seamount chain, Lord Howe Rise, Fairway Ridge, Norfolk Ridge, D'Entrecasteaux Basin, North D'Entrecasteaux Ridge and Loyalty Ridge were recovered in diverse settings with regard to depositional environments, age of structures, and nature of substrate rocks (Figure 1). For all crusts, a representative bulk sample of the whole stratigraphy has been selected. If any macroscopic boundaries were observed within the crust, representative sub-samples of the macro-layers were taken, so that all the crust thickness was sampled. In such context, sub-samples are sorted in stratigraphic order. Table 1 shows the sampling type, thickness, location, depth, cruise and substrate rock (vacuolar to amygdaloidal basalt, andesite, hyaloclastite breccia, shoshonite, polygenic breccia, bioclastic limestone, or mudstone). A large proportion of crusts were lacking substrate rocks.

<table><caption>**Table 1.** Sample information for selected deposits (n = 104).</caption><thead><tr><th>Samples</th><th>Type</th><th>Avg Crusts Thickness (mm)</th><th>Latitude (S)</th><th>Longitude (E)</th><th>Water Depth (m)</th><th>Cruise</th><th>Simplified Substrate</th></tr></thead><tbody><tr><td>DW772</td><td>Bulk</td><td>5</td><td>26°46′59.88″</td><td>170°22′1.21″</td><td>902</td><td>BATHUS3</td><td>-</td></tr><tr><td>DW774</td><td>Bulk</td><td>2</td><td>26°48′</td><td>170°22′1.21″</td><td>925</td><td>BATHUS3</td><td>Volcanics</td></tr><tr><td>DW778B</td><td>Bulk</td><td>-</td><td>25°16′59.88″</td><td>170°7′1.21″</td><td>755</td><td>BATHUS3</td><td>-</td></tr><tr><td>DW778D</td><td>Bulk</td><td>-</td><td>25°16′59.88″</td><td>170°7′1.21″</td><td>755</td><td>BATHUS3</td><td>-</td></tr><tr><td>DW778D2</td><td>Bulk</td><td>-</td><td>25°16′59.88″</td><td>170°7′1.21″</td><td>755</td><td>BATHUS3</td><td>-</td></tr><tr><td>DW2482</td><td>Bulk</td><td>-</td><td>24°8′39.01″</td><td>161°43′5.99″</td><td>430</td><td>EBISCO</td><td>-</td></tr><tr><td>DR11Ai</td><td>Bulk</td><td>97</td><td>18°0′58.68″</td><td>160°43′42.6″</td><td>2375</td><td>ECOSAT</td><td>-</td></tr><tr><td>DR11Ai-1</td><td>Layers</td><td>0–40</td><td>18°0′58.68″</td><td>160°43′42.6″</td><td>2375</td><td>ECOSAT</td><td>-</td></tr><tr><td>DR11Ai-2</td><td>Layers</td><td>40–50</td><td>18°0′58.68″</td><td>160°43′42.6″</td><td>2375</td><td>ECOSAT</td><td>-</td></tr><tr><td>DR11Ai-3</td><td>Layers</td><td>50–80</td><td>18°0′58.68″</td><td>160°43′42.6″</td><td>2375</td><td>ECOSAT</td><td>-</td></tr><tr><td>DR11Ai-4</td><td>Layers</td><td>80–97</td><td>18°0′58.68″</td><td>160°43′42.6″</td><td>2375</td><td>ECOSAT</td><td>-</td></tr><tr><td>DR13Bii</td><td>Bulk</td><td>30</td><td>17°21′37.44″</td><td>158°50′8.16″</td><td>1765</td><td>ECOSAT</td><td>-</td></tr><tr><td>DR14H</td><td>Bulk</td><td>10</td><td>18°55′28.2″</td><td>159°11′56.76″</td><td>2900</td><td>ECOSAT</td><td>Volcanics</td></tr><tr><td>DR15A</td><td>Bulk</td><td>2</td><td>18°18′36.72″</td><td>158°28′17.04″</td><td>2225</td><td>ECOSAT</td><td>Limestone</td></tr><tr><td>DR18F</td><td>Bulk</td><td>14</td><td>20°5′3.12″</td><td>160°11′32.64″</td><td>1150</td><td>ECOSAT</td><td>-</td></tr><tr><td>E-DR08B</td><td>Bulk</td><td>60</td><td>18°25′8.04″</td><td>164°0′28.44″</td><td>1400</td><td>ECOSAT</td><td>-</td></tr><tr><td>E-DR08B-1</td><td>Layers</td><td>0–20</td><td>18°25′8.04″</td><td>164°0′28.44″</td><td>1400</td><td>ECOSAT</td><td>-</td></tr><tr><td>E-DR08B-2</td><td>Layers</td><td>20–40</td><td>18°25′8.04″</td><td>164°0′28.44″</td><td>1400</td><td>ECOSAT</td><td>-</td></tr><tr><td>E-DR08B-3</td><td>Layers</td><td>40–60</td><td>18°25′8.04″</td><td>164°0′28.44″</td><td>1400</td><td>ECOSAT</td><td>-</td></tr><tr><td>DR48-021</td><td>Bulk</td><td>44</td><td>17°53′38″</td><td>159°15′11″</td><td>2060</td><td>ECOSAT3</td><td>-</td></tr><tr><td>DR53-008</td><td>Bulk</td><td>31</td><td>23°14′31″</td><td>159°46′11″</td><td>1390</td><td>ECOSAT3</td><td>-</td></tr><tr><td>DR54-009</td><td>Bulk</td><td>8</td><td>24°39′6″</td><td>159°42′57″</td><td>1350</td><td>ECOSAT3</td><td>Volcanics</td></tr><tr><td>102D</td><td>Bulk</td><td>-</td><td>18°38′24″</td><td>163°31′29.97″</td><td>2644</td><td>EVA</td><td>-</td></tr><tr><td>105D</td><td>Bulk</td><td>-</td><td>18°20′6″</td><td>163°58′1.18″</td><td>1467</td><td>EVA</td><td>-</td></tr><tr><td>108D</td><td>Bulk</td><td>-</td><td>19°31′45.48″</td><td>164°11′56.42″</td><td>2954</td><td>EVA</td><td>-</td></tr><tr><td>109D</td><td>Bulk</td><td>-</td><td>20°30′18″</td><td>165°13′37.22″</td><td>2245</td><td>EVA</td><td>-</td></tr><tr><td>110D</td><td>Bulk</td><td>34</td><td>20°33′18″</td><td>165°19′8.4″</td><td>2100</td><td>EVA</td><td>-</td></tr><tr><td>113D</td><td>Bulk</td><td>11</td><td>21°22′14.88″</td><td>166°50′31.18″</td><td>1200</td><td>EVA</td><td>Breccia</td></tr><tr><td>114D</td><td>Bulk</td><td>3</td><td>21°36′11.88″</td><td>166°51′50.39″</td><td>1185</td><td>EVA</td><td>Limestone</td></tr><tr><td>116D</td><td>Bulk</td><td>25</td><td>21°14′6″</td><td>167°29′49.18″</td><td>2210</td><td>EVA</td><td>Breccia</td></tr><tr><td>117D</td><td>Bulk</td><td>30</td><td>24°43′59.88″</td><td>169°25′30.01″</td><td>1800</td><td>EVA</td><td>Limestone</td></tr><tr><td>GO14D</td><td>Bulk</td><td>14</td><td>24°28′0.12″</td><td>168°49′58.77″</td><td>1450</td><td>GEORSTOM1</td><td>Breccia</td></tr><tr><td>GO15D</td><td>Bulk</td><td>5</td><td>24°27′42.12″</td><td>168°51′25.18″</td><td>1325</td><td>GEORSTOM1</td><td>Mudstone</td></tr><tr><td>GO16D</td><td>Bulk</td><td>6</td><td>24°22′59.88″</td><td>168°50′31.18″</td><td>1240</td><td>GEORSTOM1</td><td>Mudstone</td></tr><tr><td>GO18D</td><td>Bulk</td><td>16</td><td>24°18′54″</td><td>168°14′16.82″</td><td>585</td><td>GEORSTOM1</td><td>Limestone</td></tr><tr><td>GO20D</td><td>Bulk</td><td>2</td><td>25°55′23.88″</td><td>168°0′28.78″</td><td>1220</td><td>GEORSTOM1</td><td>Sandstone</td></tr><tr><td>GO3D</td><td>Bulk</td><td>38</td><td>23°28′0.12″</td><td>167°58′4.81″</td><td>2100</td><td>GEORSTOM1</td><td>Breccia</td></tr><tr><td>GO202</td><td>Bulk</td><td>57</td><td>19°56′53.88″</td><td>160°48′10.77″</td><td>2425</td><td>GEORSTOM2</td><td>-</td></tr></tbody></table>
<table><caption>Table 1. *Cont.*</caption><thead><tr><th>Samples</th><th>Type</th><th>Avg Crusts Thickness (mm)</th><th>Latitude (S)</th><th>Longitude (E)</th><th>Water Depth (m)</th><th>Cruise</th><th>Simplified Substrate</th></tr></thead><tbody><tr><td>GO209</td><td>Bulk</td><td>42</td><td>18°31′59.88″</td><td>163°37′58.78″</td><td>1310</td><td>GEORSTOM2</td><td>-</td></tr><tr><td>GO302D</td><td>Bulk</td><td>40</td><td>13°57′14.4″</td><td>158°13′40.8″</td><td>2190</td><td>GEORSTOM3</td><td>-</td></tr><tr><td>GO327D</td><td>Bulk</td><td>115</td><td>20°5′48.12″</td><td>164°45′25.21″</td><td>1820</td><td>GEORSTOM3</td><td>-</td></tr><tr><td>GO302D-1</td><td>Layers</td><td>0–10</td><td>13°57′14.4″</td><td>158°13′40.8″</td><td>2190</td><td>GEORSTOM3</td><td>-</td></tr><tr><td>GO302D-2</td><td>Layers</td><td>10–40</td><td>13°57′14.4″</td><td>158°13′40.8″</td><td>déc-05</td><td>GEORSTOM3</td><td>-</td></tr><tr><td>GO310</td><td>Bulk</td><td>43</td><td>13°58′18.48″</td><td>162°43′35.76″</td><td>3375</td><td>GEORSTOM3</td><td>-</td></tr><tr><td>GO314D10</td><td>Bulk</td><td>-</td><td>16°27′</td><td>165°1′19.21″</td><td>3513</td><td>GEORSTOM3</td><td>-</td></tr><tr><td>GO316D31</td><td>Bulk</td><td>-</td><td>16°34′0.12″</td><td>164°33′10.77″</td><td>3147</td><td>GEORSTOM3</td><td>-</td></tr><tr><td>GO350-D6</td><td>Bulk</td><td>33</td><td>33°38′52″</td><td>169°8′5″</td><td>2500</td><td>GEORSTOM3</td><td>Sandstone</td></tr><tr><td>GO317D10</td><td>Bulk</td><td>-</td><td>16°26′35.88″</td><td>164°25′47.97″</td><td>3311</td><td>GEORSTOM3</td><td>-</td></tr><tr><td>GO320</td><td>Bulk</td><td>6</td><td>16°7′54.12″</td><td>163°20′49.2″</td><td>4150</td><td>GEORSTOM3</td><td>-</td></tr><tr><td>GO321D2</td><td>Bulk</td><td>-</td><td>17°36′29.88″</td><td>163°17′41.99″</td><td>4677</td><td>GEORSTOM3</td><td>-</td></tr><tr><td>GO322D4</td><td>Bulk</td><td>-</td><td>17°46′30″</td><td>163°8′52.78″</td><td>4336</td><td>GEORSTOM3</td><td>-</td></tr><tr><td>GO323D4</td><td>Bulk</td><td>-</td><td>16°4′30″</td><td>163°24′7.17″</td><td>4493</td><td>GEORSTOM3</td><td>-</td></tr><tr><td>GO325D2</td><td>Bulk</td><td>-</td><td>18°54′29.88″</td><td>163°1′22.78″</td><td>4019</td><td>GEORSTOM3</td><td>-</td></tr><tr><td>GO327D-1</td><td>Layers</td><td>0–25</td><td>20°5′48.12″</td><td>164°45′25.21″</td><td>1820</td><td>GEORSTOM3</td><td>-</td></tr><tr><td>GO327D-2</td><td>Layers</td><td>25–65</td><td>20°5′48.12″</td><td>164°45′25.21″</td><td>1820</td><td>GEORSTOM3</td><td>-</td></tr><tr><td>GO327D-3</td><td>Layers</td><td>65–105</td><td>20°5′48.12″</td><td>164°45′25.21″</td><td>1820</td><td>GEORSTOM3</td><td>-</td></tr><tr><td>GO327D-4</td><td>Layers</td><td>105–115</td><td>20°5′48.12″</td><td>164°45′25.21″</td><td>1820</td><td>GEORSTOM3</td><td>-</td></tr><tr><td>GO338D2</td><td>Bulk</td><td>-</td><td>23°57′42.12″</td><td>167°18′0.01″</td><td>226,500</td><td>GEORSTOM3</td><td>-</td></tr><tr><td>GO347D</td><td>Bulk</td><td>-</td><td>31°31′55″</td><td>168°5′4″</td><td>2416</td><td>GEORSTOM3</td><td>-</td></tr><tr><td>GO348D6</td><td>Bulk</td><td>10</td><td>31°52′58″</td><td>167°29′8″</td><td>1150</td><td>GEORSTOM3</td><td>Sandstone</td></tr><tr><td>GO350-D6-1</td><td>Layers</td><td>0–20</td><td>33°38′52″</td><td>169°8′5″</td><td>2500</td><td>GEORSTOM3</td><td>Sandstone</td></tr><tr><td>GO350-D6-2</td><td>Layers</td><td>20–30</td><td>33°38′52″</td><td>169°8′5″</td><td>2500</td><td>GEORSTOM3</td><td>Sandstone</td></tr><tr><td>GO350-D6-3</td><td>Layers</td><td>30–33</td><td>33°38′52″</td><td>169°8′5″</td><td>2500</td><td>GEORSTOM3</td><td>Sandstone</td></tr><tr><td>DR06B</td><td>Bulk</td><td>4</td><td>22°33′11.76″</td><td>164°55′23.22″</td><td>878</td><td>IPOD</td><td>Limestone</td></tr><tr><td>DW4998D</td><td>Bulk</td><td>-</td><td>24°10′24″</td><td>161°43′24″</td><td>650</td><td>KANADEEP</td><td>-</td></tr><tr><td>DW4998E</td><td>Bulk</td><td>-</td><td>24°10′24″</td><td>161°43′24″</td><td>650</td><td>KANADEEP</td><td>-</td></tr><tr><td>CP5069</td><td>Bulk</td><td>9</td><td>24°22′15.38″</td><td>169°35′20.65″</td><td>1118</td><td>KANADEEP2</td><td>Limestone</td></tr><tr><td>DN5064</td><td>Bulk</td><td>14</td><td>24°49′1.92″</td><td>169°24′59.58″</td><td>1023</td><td>KANADEEP2</td><td>Breccia</td></tr><tr><td>DN5079</td><td>Bulk</td><td>47</td><td>25°31′51.31″</td><td>169°9′28.58″</td><td>2038</td><td>KANADEEP2</td><td>Sandstone</td></tr><tr><td>DN5080B</td><td>Bulk</td><td>9</td><td>25°32′21.19″</td><td>169°1′55.56″</td><td>1591</td><td>KANADEEP2</td><td>Breccia</td></tr><tr><td>DN5085A</td><td>Bulk</td><td>23</td><td>25°38′18.28″</td><td>168°21′22.61″</td><td>1606</td><td>KANADEEP2</td><td>-</td></tr><tr><td>DW5067B</td><td>Bulk</td><td>28</td><td>24°28′8.15″</td><td>169°36′48.96″</td><td>864</td><td>KANADEEP2</td><td>Limestone</td></tr><tr><td>DW5070A</td><td>Bulk</td><td>19</td><td>24°15′57.67″</td><td>169°37′45.98″</td><td>1709</td><td>KANADEEP2</td><td>Volcanics</td></tr><tr><td>DW5073</td><td>Bulk</td><td>12</td><td>24°16′33.6″</td><td>169°51′45.14″</td><td>796</td><td>KANADEEP2</td><td>Limestone</td></tr><tr><td>DW5086B</td><td>Bulk</td><td>22</td><td>25°38′57.73″</td><td>168°22′26.87″</td><td>1540</td><td>KANADEEP2</td><td>-</td></tr><tr><td>DW5087</td><td>Bulk</td><td>17</td><td>25°38′26.34″</td><td>168°25′3.61″</td><td>1680</td><td>KANADEEP2</td><td>Limestone</td></tr><tr><td>DW5089B</td><td>Bulk</td><td>8</td><td>24°24′37.12″</td><td>168°50′47.54″</td><td>1393</td><td>KANADEEP2</td><td>Volcanics</td></tr><tr><td>DW5090B</td><td>Bulk</td><td>3</td><td>24°25′8.76″</td><td>168°48′14.47″</td><td>1328</td><td>KANADEEP2</td><td>Limestone</td></tr><tr><td>DW5091A</td><td>Bulk</td><td>6</td><td>24°26′54.2″</td><td>168°50′59.1″</td><td>1582</td><td>KANADEEP2</td><td>Sandstone</td></tr><tr><td>DR01A</td><td>Bulk</td><td>4</td><td>25°47′17.52″</td><td>166°58′14.52″</td><td>1115</td><td>VESPA</td><td>Volcanics</td></tr><tr><td>DR04C</td><td>Bulk</td><td>54</td><td>28°23′29.04″</td><td>167°8′52.44″</td><td>2321</td><td>VESPA</td><td>-</td></tr><tr><td>DR07B</td><td>Bulk</td><td>27</td><td>29°41′53.52″</td><td>167°14′26.52″</td><td>1569</td><td>VESPA</td><td>Breccia</td></tr><tr><td>DR08C</td><td>Bulk</td><td>55</td><td>25°3′42.84″</td><td>170°19′34.68″</td><td>2271</td><td>VESPA</td><td>Volcanics</td></tr><tr><td>DR10B</td><td>Bulk</td><td>40</td><td>26°50′55.32″</td><td>170°19′9.12″</td><td>1523</td><td>VESPA</td><td>Limestone</td></tr><tr><td>DR14F</td><td>Bulk</td><td>60</td><td>26°25′17.04″</td><td>169°41′48.12″</td><td>2112</td><td>VESPA</td><td>Breccia</td></tr><tr><td>DR13Ci</td><td>Bulk</td><td>24</td><td>26°19′54.84″</td><td>169°32′55.68″</td><td>2731</td><td>VESPA</td><td>Breccia</td></tr><tr><td>DR13D</td><td>Bulk</td><td>15</td><td>26°19′54.84″</td><td>169°32′55.68″</td><td>2731</td><td>VESPA</td><td>Breccia</td></tr><tr><td>DR14F-1</td><td>Layers</td><td>0–35</td><td>26°25′17.04″</td><td>169°41′48.12″</td><td>2112</td><td>VESPA</td><td>Breccia</td></tr><tr><td>DR19K</td><td>Bulk</td><td>58</td><td>27°50′49.56″</td><td>170°28′5.88″</td><td>3028</td><td>VESPA</td><td>Breccia</td></tr><tr><td>DR14F-2</td><td>Layers</td><td>35–60</td><td>26°25′17.04″</td><td>169°41′48.12″</td><td>2112</td><td>VESPA</td><td>Breccia</td></tr><tr><td>DR21F</td><td>Bulk</td><td>50</td><td>27°29′52.44″</td><td>171°25′42.24″</td><td>3385</td><td>VESPA</td><td>Breccia</td></tr><tr><td>DR19K-1</td><td>Layers</td><td>0–25</td><td>27°50′49.56″</td><td>170°28′5.88″</td><td>3028</td><td>VESPA</td><td>Breccia</td></tr><tr><td>DR19K-2</td><td>Layers</td><td>25–60</td><td>27°50′49.56″</td><td>170°28′5.88″</td><td>3028</td><td>VESPA</td><td>Volcanics</td></tr><tr><td>DR21Biii</td><td>Bulk</td><td>16</td><td>27°29′52.44″</td><td>171°25′42.24″</td><td>3385</td><td>VESPA</td><td>-</td></tr></tbody></table>
<table><caption>Table 1. *Cont.*</caption><thead><tr><th>Samples</th><th>Type</th><th>Avg Crusts Thickness (mm)</th><th>Latitude (S)</th><th>Longitude (E)</th><th>Water Depth (m)</th><th>Cruise</th><th>Simplified Substrate</th></tr></thead><tbody><tr><td>GO209</td><td>Bulk</td><td>42</td><td>18°31′59.88″</td><td>163°37′58.78″</td><td>1310</td><td>GEORSTOM2</td><td>-</td></tr><tr><td>GO302D</td><td>Bulk</td><td>40</td><td>13°57′14.4″</td><td>158°13′40.8″</td><td>2190</td><td>GEORSTOM3</td><td>-</td></tr><tr><td>GO327D</td><td>Bulk</td><td>115</td><td>20°5′48.12″</td><td>164°45′25.21″</td><td>1820</td><td>GEORSTOM3</td><td>-</td></tr><tr><td>GO302D-1</td><td>Layers</td><td>0–10</td><td>13°57′14.4″</td><td>158°13′40.8″</td><td>2190</td><td>GEORSTOM3</td><td>-</td></tr><tr><td>GO302D-2</td><td>Layers</td><td>10–40</td><td>13°57′14.4″</td><td>158°13′40.8″</td><td>déc-05</td><td>GEORSTOM3</td><td>-</td></tr><tr><td>GO310</td><td>Bulk</td><td>43</td><td>13°58′18.48″</td><td>162°43′35.76″</td><td>3375</td><td>GEORSTOM3</td><td>-</td></tr><tr><td>GO314D10</td><td>Bulk</td><td>-</td><td>16°27′</td><td>165°1′19.21″</td><td>3513</td><td>GEORSTOM3</td><td>-</td></tr><tr><td>GO316D31</td><td>Bulk</td><td>-</td><td>16°34′0.12″</td><td>164°33′10.77″</td><td>3147</td><td>GEORSTOM3</td><td>-</td></tr><tr><td>GO350-D6</td><td>Bulk</td><td>33</td><td>33°38′52″</td><td>169°8′5″</td><td>2500</td><td>GEORSTOM3</td><td>Sandstone</td></tr><tr><td>GO317D10</td><td>Bulk</td><td>-</td><td>16°26′35.88″</td><td>164°25′47.97″</td><td>3311</td><td>GEORSTOM3</td><td>-</td></tr><tr><td>GO320</td><td>Bulk</td><td>6</td><td>16°7′54.12″</td><td>163°20′49.2″</td><td>4150</td><td>GEORSTOM3</td><td>-</td></tr><tr><td>GO321D2</td><td>Bulk</td><td>-</td><td>17°36′29.88″</td><td>163°17′41.99″</td><td>4677</td><td>GEORSTOM3</td><td>-</td></tr><tr><td>GO322D4</td><td>Bulk</td><td>-</td><td>17°46′30″</td><td>163°8′52.78″</td><td>4336</td><td>GEORSTOM3</td><td>-</td></tr><tr><td>GO323D4</td><td>Bulk</td><td>-</td><td>16°4′30″</td><td>163°24′7.17″</td><td>4493</td><td>GEORSTOM3</td><td>-</td></tr><tr><td>GO325D2</td><td>Bulk</td><td>-</td><td>18°54′29.88″</td><td>163°1′22.78″</td><td>4019</td><td>GEORSTOM3</td><td>-</td></tr><tr><td>GO327D-1</td><td>Layers</td><td>0–25</td><td>20°5′48.12″</td><td>164°45′25.21″</td><td>1820</td><td>GEORSTOM3</td><td>-</td></tr><tr><td>GO327D-2</td><td>Layers</td><td>25–65</td><td>20°5′48.12″</td><td>164°45′25.21″</td><td>1820</td><td>GEORSTOM3</td><td>-</td></tr><tr><td>GO327D-3</td><td>Layers</td><td>65–105</td><td>20°5′48.12″</td><td>164°45′25.21″</td><td>1820</td><td>GEORSTOM3</td><td>-</td></tr><tr><td>GO327D-4</td><td>Layers</td><td>105–115</td><td>20°5′48.12″</td><td>164°45′25.21″</td><td>1820</td><td>GEORSTOM3</td><td>-</td></tr><tr><td>GO338D2</td><td>Bulk</td><td>-</td><td>23°57′42.12″</td><td>167°18′0.01″</td><td>226,500</td><td>GEORSTOM3</td><td>-</td></tr><tr><td>GO347D</td><td>Bulk</td><td>-</td><td>31°31′55″</td><td>168°5′4″</td><td>2416</td><td>GEORSTOM3</td><td>-</td></tr><tr><td>GO348D6</td><td>Bulk</td><td>10</td><td>31°52′58″</td><td>167°29′8″</td><td>1150</td><td>GEORSTOM3</td><td>Sandstone</td></tr><tr><td>GO350-D6-1</td><td>Layers</td><td>0–20</td><td>33°38′52″</td><td>169°8′5″</td><td>2500</td><td>GEORSTOM3</td><td>Sandstone</td></tr><tr><td>GO350-D6-2</td><td>Layers</td><td>20–30</td><td>33°38′52″</td><td>169°8′5″</td><td>2500</td><td>GEORSTOM3</td><td>Sandstone</td></tr><tr><td>GO350-D6-3</td><td>Layers</td><td>30–33</td><td>33°38′52″</td><td>169°8′5″</td><td>2500</td><td>GEORSTOM3</td><td>Sandstone</td></tr><tr><td>DR06B</td><td>Bulk</td><td>4</td><td>22°33′11.76″</td><td>164°55′23.22″</td><td>878</td><td>IPOD</td><td>Limestone</td></tr><tr><td>DW4998D</td><td>Bulk</td><td>-</td><td>24°10′24″</td><td>161°43′24″</td><td>650</td><td>KANADEEP</td><td>-</td></tr><tr><td>DW4998E</td><td>Bulk</td><td>-</td><td>24°10′24″</td><td>161°43′24″</td><td>650</td><td>KANADEEP</td><td>-</td></tr><tr><td>CP5069</td><td>Bulk</td><td>9</td><td>24°22′15.38″</td><td>169°35′20.65″</td><td>1118</td><td>KANADEEP2</td><td>Limestone</td></tr><tr><td>DN5064</td><td>Bulk</td><td>14</td><td>24°49′1.92″</td><td>169°24′59.58″</td><td>1023</td><td>KANADEEP2</td><td>Breccia</td></tr><tr><td>DN5079</td><td>Bulk</td><td>47</td><td>25°31′51.31″</td><td>169°9′28.58″</td><td>2038</td><td>KANADEEP2</td><td>Sandstone</td></tr><tr><td>DN5080B</td><td>Bulk</td><td>9</td><td>25°32′21.19″</td><td>169°1′55.56″</td><td>1591</td><td>KANADEEP2</td><td>Breccia</td></tr><tr><td>DN5085A</td><td>Bulk</td><td>23</td><td>25°38′18.28″</td><td>168°21′22.61″</td><td>1606</td><td>KANADEEP2</td><td>-</td></tr><tr><td>DW5067B</td><td>Bulk</td><td>28</td><td>24°28′8.15″</td><td>169°36′48.96″</td><td>864</td><td>KANADEEP2</td><td>Limestone</td></tr><tr><td>DW5070A</td><td>Bulk</td><td>19</td><td>24°15′57.67″</td><td>169°37′45.98″</td><td>1709</td><td>KANADEEP2</td><td>Volcanics</td></tr><tr><td>DW5073</td><td>Bulk</td><td>12</td><td>24°16′33.6″</td><td>169°51′45.14″</td><td>796</td><td>KANADEEP2</td><td>Limestone</td></tr><tr><td>DW5086B</td><td>Bulk</td><td>22</td><td>25°38′57.73″</td><td>168°22′26.87″</td><td>1540</td><td>KANADEEP2</td><td>-</td></tr><tr><td>DW5087</td><td>Bulk</td><td>17</td><td>25°38′26.34″</td><td>168°25′3.61″</td><td>1680</td><td>KANADEEP2</td><td>Limestone</td></tr><tr><td>DW5089B</td><td>Bulk</td><td>8</td><td>24°24′37.12″</td><td>168°50′47.54″</td><td>1393</td><td>KANADEEP2</td><td>Volcanics</td></tr><tr><td>DW5090B</td><td>Bulk</td><td>3</td><td>24°25′8.76″</td><td>168°48′14.47″</td><td>1328</td><td>KANADEEP2</td><td>Limestone</td></tr><tr><td>DW5091A</td><td>Bulk</td><td>6</td><td>24°26′54.2″</td><td>168°50′59.1″</td><td>1582</td><td>KANADEEP2</td><td>Sandstone</td></tr><tr><td>DR01A</td><td>Bulk</td><td>4</td><td>25°47′17.52″</td><td>166°58′14.52″</td><td>1115</td><td>VESPA</td><td>Volcanics</td></tr><tr><td>DR04C</td><td>Bulk</td><td>54</td><td>28°23′29.04″</td><td>167°8′52.44″</td><td>2321</td><td>VESPA</td><td>-</td></tr><tr><td>DR07B</td><td>Bulk</td><td>27</td><td>29°41′53.52″</td><td>167°14′26.52″</td><td>1569</td><td>VESPA</td><td>Breccia</td></tr><tr><td>DR08C</td><td>Bulk</td><td>55</td><td>25°3′42.84″</td><td>170°19′34.68″</td><td>2271</td><td>VESPA</td><td>Volcanics</td></tr><tr><td>DR10B</td><td>Bulk</td><td>40</td><td>26°50′55.32″</td><td>170°19′9.12″</td><td>1523</td><td>VESPA</td><td>Limestone</td></tr><tr><td>DR14F</td><td>Bulk</td><td>60</td><td>26°25′17.04″</td><td>169°41′48.12″</td><td>2112</td><td>VESPA</td><td>Breccia</td></tr><tr><td>DR13Ci</td><td>Bulk</td><td>24</td><td>26°19′54.84″</td><td>169°32′55.68″</td><td>2731</td><td>VESPA</td><td>Breccia</td></tr><tr><td>DR13D</td><td>Bulk</td><td>15</td><td>26°19′54.84″</td><td>169°32′55.68″</td><td>2731</td><td>VESPA</td><td>Breccia</td></tr><tr><td>DR14F-1</td><td>Layers</td><td>0–35</td><td>26°25′17.04″</td><td>169°41′48.12″</td><td>2112</td><td>VESPA</td><td>Breccia</td></tr><tr><td>DR19K</td><td>Bulk</td><td>58</td><td>27°50′49.56″</td><td>170°28′5.88″</td><td>3028</td><td>VESPA</td><td>Breccia</td></tr><tr><td>DR14F-2</td><td>Layers</td><td>35–60</td><td>26°25′17.04″</td><td>169°41′48.12″</td><td>2112</td><td>VESPA</td><td>Breccia</td></tr><tr><td>DR21F</td><td>Bulk</td><td>50</td><td>27°29′52.44″</td><td>171°25′42.24″</td><td>3385</td><td>VESPA</td><td>Breccia</td></tr><tr><td>DR19K-1</td><td>Layers</td><td>0–25</td><td>27°50′49.56″</td><td>170°28′5.88″</td><td>3028</td><td>VESPA</td><td>Breccia</td></tr><tr><td>DR19K-2</td><td>Layers</td><td>25–60</td><td>27°50′49.56″</td><td>170°28′5.88″</td><td>3028</td><td>VESPA</td><td>Volcanics</td></tr><tr><td>DR21Biii</td><td>Bulk</td><td>16</td><td>27°29′52.44″</td><td>171°25′42.24″</td><td>3385</td><td>VESPA</td><td>-</td></tr></tbody></table>
<table><caption>**Table 1.** *Cont.*</caption><thead><tr><th>Samples</th><th>Type</th><th>Avg Crusts Thickness (mm)</th><th>Latitude (S)</th><th>Longitude (E)</th><th>Water Depth (m)</th><th>Cruise</th><th>Simplified Substrate</th></tr></thead><tbody><tr><td>DR38C</td><td>Bulk</td><td>35</td><td>28°33′56.16″</td><td>172°43′21″</td><td>2072</td><td>VESPA</td><td>-</td></tr><tr><td>DR21F-1</td><td>Layers</td><td>0–30</td><td>27°29′52.44″</td><td>171°25′42.24″</td><td>3385</td><td>VESPA</td><td>-</td></tr><tr><td>DR21F-2</td><td>Layers</td><td>30–50</td><td>27°29′52.44″</td><td>171°25′42.24″</td><td>3385</td><td>VESPA</td><td>Volcanics</td></tr><tr><td>DR22A</td><td>Bulk</td><td>13</td><td>27°18′48.6″</td><td>171°55′37.56″</td><td>2849</td><td>VESPA</td><td>-</td></tr><tr><td>DR29F</td><td>Bulk</td><td>27</td><td>28°38′18.6″</td><td>172°2′0.96″</td><td>2145</td><td>VESPA</td><td>-</td></tr><tr><td>DR38C-1</td><td>Layers</td><td>0–25</td><td>28°33′56.16″</td><td>172°43′21″</td><td>2072</td><td>VESPA</td><td>-</td></tr><tr><td>DR38C-2</td><td>Layers</td><td>25–35</td><td>28°33′56.16″</td><td>172°43′21″</td><td>2072</td><td>VESPA</td><td>-</td></tr><tr><td>DR41Ai</td><td>Bulk</td><td>8</td><td>25°44′1.32″</td><td>170°4′0.84″</td><td>2861</td><td>VESPA</td><td>Volcanics</td></tr><tr><td>DR42B</td><td>Bulk</td><td>20</td><td>24°12′37.08″</td><td>167°8′24.36″</td><td>1145</td><td>VESPA</td><td>Limestone</td></tr><tr><td>V-DR08B</td><td>Bulk</td><td>52</td><td>25°3′42.84″</td><td>170°19′34.68″</td><td>2271</td><td>VESPA</td><td>Hyaloclastite</td></tr></tbody></table>

Symbol “-” means that there is no thickness information for the sample; Crusts with no substrate are marked as “-”.

<figure><img src="image_6.png" /><figcaption>**Figure 2.** (A) Bathymetric map of the South-West Pacific; (B) Bathymetric map of southern New Caledonia.</figcaption></figure>

## 3.2. Mineralogical Analyses

X-ray diffraction (XRD) analyses were conducted with a BRUKER AXS D8 Advance diffractometer. Samples were top loaded into 2.5 cm diameter circular cavity holders, and all analyses were run between 5° and 70° 2θ, with 0.01° 2θ step at 1 s/step (monochromatic Cu Kα radiation, 40 kV, 30 mA). Minerals were identified using the Diffrac.Suite EVA software. This methodology allows the quick identification of most minerals (e.g., silicates, carbonates, well-crystallised manganates, well-crystallised iron oxyhydroxides). δ-MnO₂ is barely visible on diffractograms even when it constitutes the main crystalline phase of a sample. Estimation of the proportion of δ-MnO₂ from other crystalline phases is made on the basis of a qualitative analysis of the diffractograms, i.e., ratio between δ-MnO₂ visible peaks (±37° and ±66° 2θ; 2.45 Å and 1.42 Å) and peak signals of other well-crystallised minerals.

Scanning electron microscopy (SEM) imaging was done with an FEI Quanta 200 SEM on C-coated polished thin sections. Backscatter images were acquired for textural charac-
<table><caption>Table 1. *Cont.*</caption><thead><tr><th>Samples</th><th>Type</th><th>Avg Crusts Thickness (mm)</th><th>Latitude (S)</th><th>Longitude (E)</th><th>Water Depth (m)</th><th>Cruise</th><th>Simplified Substrate</th></tr></thead><tbody><tr><td>DR38C</td><td>Bulk</td><td>35</td><td>28°33′56.16″</td><td>172°43′21″</td><td>2072</td><td>VESPA</td><td>-</td></tr><tr><td>DR21F-1</td><td>Layers</td><td>0–30</td><td>27°29′52.44″</td><td>171°25′42.24″</td><td>3385</td><td>VESPA</td><td>-</td></tr><tr><td>DR21F-2</td><td>Layers</td><td>30–50</td><td>27°29′52.44″</td><td>171°25′42.24″</td><td>3385</td><td>VESPA</td><td>Volcanics</td></tr><tr><td>DR22A</td><td>Bulk</td><td>13</td><td>27°18′48.6″</td><td>171°55′37.56″</td><td>2849</td><td>VESPA</td><td>-</td></tr><tr><td>DR29F</td><td>Bulk</td><td>27</td><td>28°38′18.6″</td><td>172°2′0.96″</td><td>2145</td><td>VESPA</td><td>-</td></tr><tr><td>DR38C-1</td><td>Layers</td><td>0–25</td><td>28°33′56.16″</td><td>172°43′21″</td><td>2072</td><td>VESPA</td><td>-</td></tr><tr><td>DR38C-2</td><td>Layers</td><td>25–35</td><td>28°33′56.16″</td><td>172°43′21″</td><td>2072</td><td>VESPA</td><td>-</td></tr><tr><td>DR41Ai</td><td>Bulk</td><td>8</td><td>25°44′1.32″</td><td>170°4′0.84″</td><td>2861</td><td>VESPA</td><td>Volcanics</td></tr><tr><td>DR42B</td><td>Bulk</td><td>20</td><td>24°12′37.08″</td><td>167°8′24.36″</td><td>1145</td><td>VESPA</td><td>Limestone</td></tr><tr><td>V-DR08B</td><td>Bulk</td><td>52</td><td>25°3′42.84″</td><td>170°19′34.68″</td><td>2271</td><td>VESPA</td><td>Hyaloclastite</td></tr></tbody></table>

Symbol “-” means that there is no thickness information for the sample; Crusts with no substrate are marked as “-”.

<figure><img src="image_6.png" /><figcaption>**Figure 2.** (A) Bathymetric map of the South-West Pacific; (B) Bathymetric map of southern New Caledonia.</figcaption></figure>

## 3.2. Mineralogical Analyses

X-ray diffraction (XRD) analyses were conducted with a BRUKER AXS D8 Advance diffractometer. Samples were top loaded into 2.5 cm diameter circular cavity holders, and all analyses were run between 5° and 70° 2θ, with 0.01° 2θ step at 1 s/step (monochromatic Cu Kα radiation, 40 kV, 30 mA). Minerals were identified using the Diffrac.Suite EVA software. This methodology allows the quick identification of most minerals (e.g., silicates, carbonates, well-crystallised manganates, well-crystallised iron oxyhydroxides). δ-MnO₂ is barely visible on diffractograms even when it constitutes the main crystalline phase of a sample. Estimation of the proportion of δ-MnO₂ from other crystalline phases is made on the basis of a qualitative analysis of the diffractograms, i.e., ratio between δ-MnO₂ visible peaks (±37° and ±66° 2θ; 2.45 Å and 1.42 Å) and peak signals of other well-crystallised minerals.

Scanning electron microscopy (SEM) imaging was done with an FEI Quanta 200 SEM on C-coated polished thin sections. Backscatter images were acquired for textural charac-
terisation of the Fe-Mn oxyhydroxides. Energy Dispersive Spectroscopy (EDS) analysis was performed with and OXFORD X-MAXN Silicon Drift Detector (detector size: 80 mm²).

### 3.3. *Geochemical Analyses*

X-ray fluorescence analyses were conducted with a wavelength dispersive X-ray fluo-
rescence spectrometer (WD-XRF; BRUKER AXS S8 TIGER) on fusion beads or compressed
pellets (for major and trace elements, respectively). After data acquisition, measured net
peak intensities corrected from inter-element interferences were converted into concentra-
tions using calibration curves generated from the analysis of certified reference material
powders (using BHVO-2 [60]), measured under identical analytical conditions.

Additional trace elements (Sr, Y, Zr, Nb, Th, REE) were analysed for 17 hydrogenetic crusts from the Loyalty, Norfolk and D'Entrecasteaux ridges (Figure 2) by inductively coupled plasma mass spectrometry using an ELEMENT II magnetic field ICP-MS at Institut Universitaire Européen de la Mer (IUEM) in Brest. The dissolution procedure was as follows; 0.1 g of sample powder was digested in a Teflon bottle with 4 mL of 6 mol/L hydrochloric acid for 24 h on a hot plate (120 °C) with a Tm spike [61,62]. If present, the residual phase composed of mostly silicates and refractory minerals was extracted by centrifuge and digested by a mixture of hydrofluoric and hydrochloric acid (3:1) for 48 h on hot plate (120 °C), evaporated and then remixed with the previously digested phase. Then, 0.5 µL of the solution was evaporated on a hot plate and the residue was made up to 10 mL with a 2% nitric and 0.05% hydrofluoric acid solution for trace element analysis by ICP-MS. Samples were corrected using internal calibrations, BHVO-2 reference material, and a Tm spike correction [62]. Every concentration later in the text expressed as % represents weight %.

The Co-chronometer method considers that the supply of Co in the ocean is constant over time and that Fe and Mn oxides are the main scavengers of this element [63,64]. Considering these hypotheses, a proportional relationship can be established to estimate growth rates. Using crusts' thicknesses, it is possible to derive minimum crusts ages from growth rates. However, this method cannot account for post-depositional events like phosphatisation, dissolution, or erosional events that are known to affect Co concentration, preservation of the stratigraphic record, and could therefore alter calculated ages [1,65]. The minimal age of crusts was determined using the empirically derived cobalt chronometer method defined as: GR = 0.68/(Coⁿ)¹.⁶⁷ [66], where GR is the growth rate in mm/Ma, and Coⁿ = Co × (50/Fe + Mn) with elements in wt.%. The equation of [64] was not considered to compute the growth rate as several samples exhibit Co content lower than 0.24%, which is the threshold needed to apply this method.

Several methods were used to examine statistically significant variations in major and minor elements concentrations for selected crusts samples. A Pearson correlation coefficient matrix was computed using chemical data to evaluate the strength of linear dependence between variables. To investigate possible chemical factor variations and biases in the data set, a matrix was produced using hydrogenetic macro-layers and bulk samples (n = 89). Bulk samples that have been subsampled were not considered to avoid duplicating data. All correlation coefficients in bold are significant at the 99% confidence level (CL). Factor analysis of the major and minor elements data was also run on the same data set (n = 89) to study element relationships and to determine groups of elements with the same behaviour. Using X-ray diffraction mineralogy and correlation coefficient matrices, each resultant factor of this analysis can be interpreted as a specific mineral or group of minerals in the Fe-Mn crusts and elements correlated with those factors to be part of the mineral group or mineral.

## 4. Results

### 4.1. *Sample Description*

Two types of samples can be distinguished from the macroscopic study: (1) Brown to black Fe-Mn encrustations which are referred here as Fe-Mn crusts, and (2) Grey to dark and rarely brownish grey Mn-rich (±Ca-Fe) samples. Fe-Mn crusts show a large diversity of surface and layered textures (Figure 3A–D). The surface can be smooth, granular and
terisation of the Fe-Mn oxyhydroxides. Energy Dispersive Spectroscopy (EDS) analysis was performed with and OXFORD X-MAXN Silicon Drift Detector (detector size: 80 mm²).

### 3.3. *Geochemical Analyses*

X-ray fluorescence analyses were conducted with a wavelength dispersive X-ray fluo-
rescence spectrometer (WD-XRF; BRUKER AXS S8 TIGER) on fusion beads or compressed
pellets (for major and trace elements, respectively). After data acquisition, measured net
peak intensities corrected from inter-element interferences were converted into concentra-
tions using calibration curves generated from the analysis of certified reference material
powders (using BHVO-2 [60]), measured under identical analytical conditions.

Additional trace elements (Sr, Y, Zr, Nb, Th, REE) were analysed for 17 hydrogenetic crusts from the Loyalty, Norfolk and D'Entrecasteaux ridges (Figure 2) by inductively coupled plasma mass spectrometry using an ELEMENT II magnetic field ICP-MS at Institut Universitaire Européen de la Mer (IUEM) in Brest. The dissolution procedure was as follows; 0.1 g of sample powder was digested in a Teflon bottle with 4 mL of 6 mol/L hydrochloric acid for 24 h on a hot plate (120 °C) with a Tm spike [61,62]. If present, the residual phase composed of mostly silicates and refractory minerals was extracted by centrifuge and digested by a mixture of hydrofluoric and hydrochloric acid (3:1) for 48 h on hot plate (120 °C), evaporated and then remixed with the previously digested phase. Then, 0.5 µL of the solution was evaporated on a hot plate and the residue was made up to 10 mL with a 2% nitric and 0.05% hydrofluoric acid solution for trace element analysis by ICP-MS. Samples were corrected using internal calibrations, BHVO-2 reference material, and a Tm spike correction [62]. Every concentration later in the text expressed as % represents weight %.

The Co-chronometer method considers that the supply of Co in the ocean is constant over time and that Fe and Mn oxides are the main scavengers of this element [63,64]. Considering these hypotheses, a proportional relationship can be established to estimate growth rates. Using crusts' thicknesses, it is possible to derive minimum crusts ages from growth rates. However, this method cannot account for post-depositional events like phosphatisation, dissolution, or erosional events that are known to affect Co concentration, preservation of the stratigraphic record, and could therefore alter calculated ages [1,65]. The minimal age of crusts was determined using the empirically derived cobalt chronometer method defined as: GR = 0.68/(Coⁿ)¹.⁶⁷ [66], where GR is the growth rate in mm/Ma, and Coⁿ = Co × (50/Fe + Mn) with elements in wt.%. The equation of [64] was not considered to compute the growth rate as several samples exhibit Co content lower than 0.24%, which is the threshold needed to apply this method.

Several methods were used to examine statistically significant variations in major and minor elements concentrations for selected crusts samples. A Pearson correlation coefficient matrix was computed using chemical data to evaluate the strength of linear dependence between variables. To investigate possible chemical factor variations and biases in the data set, a matrix was produced using hydrogenetic macro-layers and bulk samples (n = 89). Bulk samples that have been subsampled were not considered to avoid duplicating data. All correlation coefficients in bold are significant at the 99% confidence level (CL). Factor analysis of the major and minor elements data was also run on the same data set (n = 89) to study element relationships and to determine groups of elements with the same behaviour. Using X-ray diffraction mineralogy and correlation coefficient matrices, each resultant factor of this analysis can be interpreted as a specific mineral or group of minerals in the Fe-Mn crusts and elements correlated with those factors to be part of the mineral group or mineral.

## 4. Results

### 4.1. *Sample Description*

Two types of samples can be distinguished from the macroscopic study: (1) Brown to black Fe-Mn encrustations which are referred here as Fe-Mn crusts, and (2) Grey to dark and rarely brownish grey Mn-rich (±Ca-Fe) samples. Fe-Mn crusts show a large diversity of surface and layered textures (Figure 3A–D). The surface can be smooth, granular and
botryoidal, with botryoids ranging from millimeters to centimeters. Layers can be well separated from others with interstitial sediment and porous, columnar, dendritic, or very dense and well laminated. The thickness of Fe-Mn crusts vary from 2 mm to 115 mm, with a mean thickness of 27 mm (from 61 hydrogenetic bulk samples). The thickest crusts can contain up to four distinct macroscopic layers, but no uniform sequence of texture has been found between these crusts. Mn-rich (±Ca-Fe) samples (DW778B, DW778D, DW778D2, DW4998E, DW4998D, and DW2482, Figure 3E–H) present different morphologies and textures. These samples are denser and harder than Fe-Mn crusts. Some samples (Figure 3F,H) display a strong imbrication of a metallic black zone and a pale white/reddish zone, showing colloform to dendritic-like growth textures in parts of the sample. Other samples (Figure 3E,G) present comparable macro-layers with metallic black and blue/grey units, as well as colors ranging from pale grey/blue and white/reddish to metallic black. Alternation of macro-layers is visible in Figure 3E, with an innermost imbrication of pale grey, blue and white layers, presenting in some areas a more or less porous and colloform texture, followed by a black metallic layer present on both sides of the sample. Whilst most previous analysis reports samples of a hydrogenetic nature, these morphological and visual characteristics match criterion proposed by [11] of a hydrothermal nature or influence.

<figure><img src="image_7.png" /><figcaption>**Figure 3.** Fe-Mn deposits photographs with centimetre scale bars. (A) Cross-section of DR08C showing a columnar/porous texture with interstitial sediments filling pores, deposited on a vacuolar basalt; (B) Cross section of the thickest crust sample of the data set GO327D, four layers are identified and sub-sampled; (C) Current-polished centimetre-scale botryoidal surface of sample 110D; (D) V-DR08B showing massive/laminated textures without any sub-layers deposited on a hyaloclastite breccia; (E) Hydrothermal deposit from sample DW778D; (F) Imbrication of Mn-rich and Ca-rich zones in sample DW2482; (G) Sample DW4998E composed primarily of 10 Å manganates and calcite on a strongly altered hyaloclastite breccia impregnated with Fe-oxyhydroxides; (H) Sample DW4998D showing a metallic grey Mn-rich zone and fluorapatite/Mg-calcite/calcite area.</figcaption></figure>
botryoidal, with botryoids ranging from millimeters to centimeters. Layers can be well separated from others with interstitial sediment and porous, columnar, dendritic, or very dense and well laminated. The thickness of Fe-Mn crusts vary from 2 mm to 115 mm, with a mean thickness of 27 mm (from 61 hydrogenetic bulk samples). The thickest crusts can contain up to four distinct macroscopic layers, but no uniform sequence of texture has been found between these crusts. Mn-rich (±Ca-Fe) samples (DW778B, DW778D, DW778D2, DW4998E, DW4998D, and DW2482, Figure 3E–H) present different morphologies and textures. These samples are denser and harder than Fe-Mn crusts. Some samples (Figure 3F,H) display a strong imbrication of a metallic black zone and a pale white/reddish zone, showing colloform to dendritic-like growth textures in parts of the sample. Other samples (Figure 3E,G) present comparable macro-layers with metallic black and blue/grey units, as well as colors ranging from pale grey/blue and white/reddish to metallic black. Alternation of macro-layers is visible in Figure 3E, with an innermost imbrication of pale grey, blue and white layers, presenting in some areas a more or less porous and colloform texture, followed by a black metallic layer present on both sides of the sample. Whilst most previous analysis reports samples of a hydrogenetic nature, these morphological and visual characteristics match criterion proposed by [11] of a hydrothermal nature or influence.

<figure><img src="image_7.png" /><figcaption>**Figure 3.** Fe-Mn deposits photographs with centimetre scale bars. (A) Cross-section of DR08C showing a columnar/porous texture with interstitial sediments filling pores, deposited on a vacuolar basalt; (B) Cross section of the thickest crust sample of the data set GO327D, four layers are identified and sub-sampled; (C) Current-polished centimetre-scale botryoidal surface of sample 110D; (D) V-DR08B showing massive/laminated textures without any sub-layers deposited on a hyaloclastite breccia; (E) Hydrothermal deposit from sample DW778D; (F) Imbrication of Mn-rich and Ca-rich zones in sample DW2482; (G) Sample DW4998E composed primarily of 10 Å manganates and calcite on a strongly altered hyaloclastite breccia impregnated with Fe-oxyhydroxides; (H) Sample DW4998D showing a metallic grey Mn-rich zone and fluorapatite/Mg-calcite/calcite area.</figcaption></figure>
## 4.2. XRD and SEM Mineralogy

XRD mineralogical analyses are reported in Table 2. δ-MnO₂ is the most dominant phase detected in Fe-Mn crusts. Given no Fe mineral was identified on XRD, we conclude that most of the Fe is in the form of X-ray amorphous Fe-oxyhydroxides [1,2] and/or Fe-rich δ-MnO₂. In most crusts, quartz, feldspar, calcite, and Mg-calcite represent the main detrital components. Other detrital phases include mica, clay minerals (DR19K, DR19K-1, DR21Biii, GO310 and DR08C), gypsum and amphibole (114D and GO310). Samples where the only Mn phase is δ-MnO₂ are characteristic of hydrogenetic ferromanganese crusts. They generally show complex internal structures including laminated layers of Fe-Mn oxides with varying porosities, cuspate texture (i.e., more porous and chaotic structure) or massive jointed columnar texture with only small amounts of detrital minerals (Figure 4A–C). Only one Fe-Mn crust sample (DN5080B) contains detectable amounts of fluorapatite.

<table><caption>**Table 2.** XRD and SEM mineralogy of ferromanganese deposits (n = 104) from New Caledonia.</caption><thead><tr><th>Sample</th><th>Major</th><th>Moderate</th><th>Minor</th></tr></thead><tbody><tr><td>102D</td><td>-</td><td>-</td><td>-</td></tr><tr><td>105D</td><td>-</td><td>-</td><td>-</td></tr><tr><td>108D</td><td>-</td><td>-</td><td>-</td></tr><tr><td>109D</td><td>-</td><td>-</td><td>-</td></tr><tr><td>110D</td><td>δ-MnO2, Quartz, Plagioclase</td><td>-</td><td>Calcite</td></tr><tr><td>113D</td><td>δ-MnO2</td><td>-</td><td>Quartz, Plagioclase</td></tr><tr><td>114D</td><td>δ-MnO2</td><td>Plagioclase, Calcite</td><td>Quartz, Gypsum</td></tr><tr><td>116D</td><td>δ-MnO2</td><td>Quartz, Plagioclase</td><td>-</td></tr><tr><td>117D</td><td>δ-MnO2</td><td>-</td><td>Quartz</td></tr><tr><td>CP5069</td><td>δ-MnO2</td><td>Fluorapatite, Calcite</td><td>10 Å manganates</td></tr><tr><td>DN5064</td><td>δ-MnO2</td><td>-</td><td>Calcite</td></tr><tr><td>DN5079</td><td>δ-MnO2</td><td>Quartz</td><td>Plagioclase</td></tr><tr><td>DN5080B</td><td>δ-MnO2</td><td>-</td><td>Fluorapatite, Quartz</td></tr><tr><td>DN5085A</td><td>δ-MnO2, Quartz</td><td>-</td><td>Plagioclase</td></tr><tr><td>DR01A</td><td>δ-MnO2</td><td>-</td><td>-</td></tr><tr><td>DR04C</td><td>δ-MnO2, Quartz</td><td>-</td><td>Plagioclase</td></tr><tr><td>DR06B</td><td>δ-MnO2</td><td>Quartz</td><td>Mg-Calcite, Plagioclase</td></tr><tr><td>DR07B</td><td>δ-MnO2</td><td>-</td><td>-</td></tr><tr><td>DR08C</td><td>δ-MnO2, Calcite</td><td>Clays/Micas, Quartz</td><td>Plagioclase</td></tr><tr><td>DR10B</td><td>δ-MnO2</td><td>-</td><td>Quartz</td></tr><tr><td>DR11Ai</td><td>δ-MnO2, Quartz</td><td>Plagioclase</td><td>-</td></tr><tr><td>DR11Ai-1</td><td>Quartz, Plagioclase</td><td>δ-MnO2</td><td>-</td></tr><tr><td>DR11Ai-2</td><td>δ-MnO2, Quartz</td><td>Plagioclase</td><td>-</td></tr><tr><td>DR11Ai-3</td><td>δ-MnO2, Quartz</td><td>Plagioclase</td><td>-</td></tr><tr><td>DR11Ai-4</td><td>δ-MnO2, Quartz</td><td>Plagioclase</td><td>-</td></tr><tr><td>DR13Bii</td><td>δ-MnO2, Quartz</td><td>Calcite</td><td>-</td></tr><tr><td>DR13Ci</td><td>δ-MnO2</td><td>Calcite</td><td>Quartz</td></tr><tr><td>DR13D</td><td>δ-MnO2</td><td>Quartz</td><td>Calcite, Plagioclase</td></tr><tr><td>DR14F</td><td>δ-MnO2</td><td>Quartz</td><td>-</td></tr><tr><td>DR14F-1</td><td>δ-MnO2</td><td>Quartz</td><td>Calcite, Plagioclase</td></tr><tr><td>DR14F-2</td><td>δ-MnO2</td><td>Quartz</td><td>-</td></tr><tr><td>DR14H</td><td>δ-MnO2, Quartz</td><td>Plagioclase</td><td>-</td></tr><tr><td>DR15A</td><td>δ-MnO2</td><td>Quartz, Plagioclase</td><td>-</td></tr><tr><td>DR18F</td><td>δ-MnO2</td><td>-</td><td>Plagioclase</td></tr><tr><td>DR19K</td><td>δ-MnO2, Clays/Micas</td><td>-</td><td>Quartz, Calcite</td></tr><tr><td>DR19K-1</td><td>δ-MnO2, Clays/Micas</td><td>Calcite</td><td>Quartz, Plagioclase</td></tr><tr><td>DR19K-2</td><td>δ-MnO2</td><td>Quartz</td><td>-</td></tr><tr><td>DR21Biii</td><td>δ-MnO2, Clays/Micas</td><td>Quartz, Plagioclase</td><td>-</td></tr><tr><td>DR21F</td><td>δ-MnO2</td><td>Quartz</td><td>Plagioclase</td></tr><tr><td>DR21F-1</td><td>δ-MnO2</td><td>Quartz</td><td>Plagioclase</td></tr><tr><td>DR21F-2</td><td>δ-MnO2</td><td>Quartz</td><td>Plagioclase</td></tr></tbody></table>
## 4.2. XRD and SEM Mineralogy

XRD mineralogical analyses are reported in Table 2. δ-MnO₂ is the most dominant phase detected in Fe-Mn crusts. Given no Fe mineral was identified on XRD, we conclude that most of the Fe is in the form of X-ray amorphous Fe-oxyhydroxides [1,2] and/or Fe-rich δ-MnO₂. In most crusts, quartz, feldspar, calcite, and Mg-calcite represent the main detrital components. Other detrital phases include mica, clay minerals (DR19K, DR19K-1, DR21Biii, GO310 and DR08C), gypsum and amphibole (114D and GO310). Samples where the only Mn phase is δ-MnO₂ are characteristic of hydrogenetic ferromanganese crusts. They generally show complex internal structures including laminated layers of Fe-Mn oxides with varying porosities, cuspate texture (i.e., more porous and chaotic structure) or massive jointed columnar texture with only small amounts of detrital minerals (Figure 4A–C). Only one Fe-Mn crust sample (DN5080B) contains detectable amounts of fluorapatite.

<table><caption>**Table 2.** XRD and SEM mineralogy of ferromanganese deposits (n = 104) from New Caledonia.</caption><thead><tr><th>Sample</th><th>Major</th><th>Moderate</th><th>Minor</th></tr></thead><tbody><tr><td>102D</td><td>-</td><td>-</td><td>-</td></tr><tr><td>105D</td><td>-</td><td>-</td><td>-</td></tr><tr><td>108D</td><td>-</td><td>-</td><td>-</td></tr><tr><td>109D</td><td>-</td><td>-</td><td>-</td></tr><tr><td>110D</td><td>δ-MnO2, Quartz, Plagioclase</td><td>-</td><td>Calcite</td></tr><tr><td>113D</td><td>δ-MnO2</td><td>-</td><td>Quartz, Plagioclase</td></tr><tr><td>114D</td><td>δ-MnO2</td><td>Plagioclase, Calcite</td><td>Quartz, Gypsum</td></tr><tr><td>116D</td><td>δ-MnO2</td><td>Quartz, Plagioclase</td><td>-</td></tr><tr><td>117D</td><td>δ-MnO2</td><td>-</td><td>Quartz</td></tr><tr><td>CP5069</td><td>δ-MnO2</td><td>Fluorapatite, Calcite</td><td>10 Å manganates</td></tr><tr><td>DN5064</td><td>δ-MnO2</td><td>-</td><td>Calcite</td></tr><tr><td>DN5079</td><td>δ-MnO2</td><td>Quartz</td><td>Plagioclase</td></tr><tr><td>DN5080B</td><td>δ-MnO2</td><td>-</td><td>Fluorapatite, Quartz</td></tr><tr><td>DN5085A</td><td>δ-MnO2, Quartz</td><td>-</td><td>Plagioclase</td></tr><tr><td>DR01A</td><td>δ-MnO2</td><td>-</td><td>-</td></tr><tr><td>DR04C</td><td>δ-MnO2, Quartz</td><td>-</td><td>Plagioclase</td></tr><tr><td>DR06B</td><td>δ-MnO2</td><td>Quartz</td><td>Mg-Calcite, Plagioclase</td></tr><tr><td>DR07B</td><td>δ-MnO2</td><td>-</td><td>-</td></tr><tr><td>DR08C</td><td>δ-MnO2, Calcite</td><td>Clays/Micas, Quartz</td><td>Plagioclase</td></tr><tr><td>DR10B</td><td>δ-MnO2</td><td>-</td><td>Quartz</td></tr><tr><td>DR11Ai</td><td>δ-MnO2, Quartz</td><td>Plagioclase</td><td>-</td></tr><tr><td>DR11Ai-1</td><td>Quartz, Plagioclase</td><td>δ-MnO2</td><td>-</td></tr><tr><td>DR11Ai-2</td><td>δ-MnO2, Quartz</td><td>Plagioclase</td><td>-</td></tr><tr><td>DR11Ai-3</td><td>δ-MnO2, Quartz</td><td>Plagioclase</td><td>-</td></tr><tr><td>DR11Ai-4</td><td>δ-MnO2, Quartz</td><td>Plagioclase</td><td>-</td></tr><tr><td>DR13Bii</td><td>δ-MnO2, Quartz</td><td>Calcite</td><td>-</td></tr><tr><td>DR13Ci</td><td>δ-MnO2</td><td>Calcite</td><td>Quartz</td></tr><tr><td>DR13D</td><td>δ-MnO2</td><td>Quartz</td><td>Calcite, Plagioclase</td></tr><tr><td>DR14F</td><td>δ-MnO2</td><td>Quartz</td><td>-</td></tr><tr><td>DR14F-1</td><td>δ-MnO2</td><td>Quartz</td><td>Calcite, Plagioclase</td></tr><tr><td>DR14F-2</td><td>δ-MnO2</td><td>Quartz</td><td>-</td></tr><tr><td>DR14H</td><td>δ-MnO2, Quartz</td><td>Plagioclase</td><td>-</td></tr><tr><td>DR15A</td><td>δ-MnO2</td><td>Quartz, Plagioclase</td><td>-</td></tr><tr><td>DR18F</td><td>δ-MnO2</td><td>-</td><td>Plagioclase</td></tr><tr><td>DR19K</td><td>δ-MnO2, Clays/Micas</td><td>-</td><td>Quartz, Calcite</td></tr><tr><td>DR19K-1</td><td>δ-MnO2, Clays/Micas</td><td>Calcite</td><td>Quartz, Plagioclase</td></tr><tr><td>DR19K-2</td><td>δ-MnO2</td><td>Quartz</td><td>-</td></tr><tr><td>DR21Biii</td><td>δ-MnO2, Clays/Micas</td><td>Quartz, Plagioclase</td><td>-</td></tr><tr><td>DR21F</td><td>δ-MnO2</td><td>Quartz</td><td>Plagioclase</td></tr><tr><td>DR21F-1</td><td>δ-MnO2</td><td>Quartz</td><td>Plagioclase</td></tr><tr><td>DR21F-2</td><td>δ-MnO2</td><td>Quartz</td><td>Plagioclase</td></tr></tbody></table>
<table><caption>Table 2. *Cont.*</caption><thead><tr><th>Sample</th><th>Major</th><th>Moderate</th><th>Minor</th></tr></thead><tbody><tr><td>DR22A</td><td>δ-MnO2</td><td>Quartz</td><td>Plagioclase</td></tr><tr><td>DR29F</td><td>δ-MnO2</td><td>Quartz</td><td>Quartz</td></tr><tr><td>DR38C</td><td>δ-MnO2</td><td>-</td><td>Quartz, Calcite</td></tr><tr><td>DR38C-1</td><td>δ-MnO2</td><td>-</td><td>Quartz</td></tr><tr><td>DR38C-2</td><td>δ-MnO2</td><td>Calcite</td><td>Quartz</td></tr><tr><td>DR41Ai</td><td>δ-MnO2, Plagioclase</td><td>Quartz</td><td>-</td></tr><tr><td>DR42B</td><td>δ-MnO2</td><td>-</td><td>-</td></tr><tr><td>DR48-021</td><td>δ-MnO2, Quartz</td><td>-</td><td>Plagioclase</td></tr><tr><td>DR53-008</td><td>δ-MnO2</td><td>-</td><td>Quartz</td></tr><tr><td>DR54-009</td><td>δ-MnO2</td><td>-</td><td>Quartz, Plagioclase</td></tr><tr><td>DW4998D</td><td>Pyrolusite</td><td>Mg-Calcite</td><td>Calcite, Fluorapatite</td></tr><tr><td>DW4998E</td><td>Fe-oxyhydroxides, Calcite</td><td>Fluorapatite, 10 Å<br>manganates</td><td></td></tr><tr><td>DW5067B</td><td>δ-MnO2</td><td>Calcite</td><td>-</td></tr><tr><td>DW5070A</td><td>δ-MnO2</td><td>-</td><td>Quartz, Calcite</td></tr><tr><td>DW5073</td><td>δ-MnO2</td><td>-</td><td>Calcite</td></tr><tr><td>DW5086B</td><td>δ-MnO2</td><td>Quartz</td><td>Plagioclase</td></tr><tr><td>DW5087</td><td>δ-MnO2</td><td>Quartz</td><td>Plagioclase</td></tr><tr><td>DW5089B</td><td>δ-MnO2</td><td>-</td><td>Quartz</td></tr><tr><td>DW5090B</td><td>δ-MnO2</td><td>-</td><td>Quartz</td></tr><tr><td>DW5091A</td><td>δ-MnO2</td><td>Quartz</td><td>Calcite</td></tr><tr><td>DW772</td><td>δ-MnO2</td><td>-</td><td>-</td></tr><tr><td>DW774</td><td>δ-MnO2</td><td>-</td><td>Calcite</td></tr><tr><td>DW778B</td><td>Pyrolusite</td><td>Mg-Calcite</td><td>-</td></tr><tr><td>DW778D</td><td>Pyrolusite</td><td>Mg-Calcite</td><td>-</td></tr><tr><td>DW778D2</td><td>Pyrolusite</td><td>Mg-Calcite</td><td>-</td></tr><tr><td>DW2482</td><td>10 Å manganates</td><td>Mg-Calcite</td><td>Fluorapatite</td></tr><tr><td>E-DR08B</td><td>δ-MnO2, Quartz</td><td>Plagioclase</td><td>-</td></tr><tr><td>E-DR08B-1</td><td>δ-MnO2, Quartz</td><td>-</td><td>Plagioclase</td></tr><tr><td>E-DR08B-2</td><td>δ-MnO2, Quartz</td><td>-</td><td>-</td></tr><tr><td>E-DR08B-3</td><td>δ-MnO2, Quartz</td><td>Plagioclase</td><td>-</td></tr><tr><td>GO14D</td><td>δ-MnO2</td><td>-</td><td>-</td></tr><tr><td>GO15D</td><td>δ-MnO2</td><td>Calcite</td><td>Quartz, Plagioclase</td></tr><tr><td>GO16D</td><td>δ-MnO2</td><td>Quartz</td><td>Plagioclase, Calcite</td></tr><tr><td>GO18D</td><td>δ-MnO2</td><td>-</td><td>Quartz, Plagioclase</td></tr><tr><td>GO202</td><td>δ-MnO2, Quartz</td><td>-</td><td>Plagioclase</td></tr><tr><td>GO209</td><td>δ-MnO2</td><td>Quartz</td><td>Plagioclase</td></tr><tr><td>GO20D</td><td>δ-MnO2</td><td>-</td><td>Quartz</td></tr><tr><td>GO302D</td><td>δ-MnO2, Plagioclase</td><td>Quartz</td><td>-</td></tr><tr><td>GO302D-1</td><td>δ-MnO2, Plagioclase</td><td>Quartz</td><td>-</td></tr><tr><td>GO302D-2</td><td>δ-MnO2, Plagioclase</td><td>Quartz</td><td>-</td></tr><tr><td>GO310</td><td>δ-MnO2, Clays/Micas</td><td>Amphibole</td><td>Quartz, Calcite,<br>Plagioclase</td></tr><tr><td>GO314D10</td><td>-</td><td>-</td><td>-</td></tr><tr><td>GO316D31</td><td>-</td><td>-</td><td>-</td></tr><tr><td>GO317D10</td><td>-</td><td>-</td><td>-</td></tr><tr><td>GO320</td><td>δ-MnO2</td><td>-</td><td>Plagioclase, Quartz</td></tr><tr><td>GO321D2</td><td>-</td><td>-</td><td>-</td></tr><tr><td>GO322D4</td><td>-</td><td>-</td><td>-</td></tr><tr><td>GO323D2</td><td>-</td><td>-</td><td>-</td></tr><tr><td>GO325D2</td><td>-</td><td>-</td><td>-</td></tr><tr><td>GO327D</td><td>δ-MnO2, Quartz</td><td>Plagioclase</td><td>-</td></tr><tr><td>GO327D-1</td><td>δ-MnO2, Quartz</td><td>-</td><td>Plagioclase</td></tr><tr><td>GO327D-2</td><td>δ-MnO2, Quartz</td><td>-</td><td>Plagioclase</td></tr><tr><td>GO327D-3</td><td>δ-MnO2, Quartz</td><td>-</td><td>Plagioclase</td></tr><tr><td>GO327D-4</td><td>δ-MnO2, Quartz</td><td>-</td><td>Plagioclase</td></tr><tr><td>GO338D2</td><td>-</td><td>-</td><td>-</td></tr><tr><td>GO347D</td><td>-</td><td>-</td><td>-</td></tr><tr><td>GO348D6</td><td>δ-MnO2</td><td>-</td><td>Quartz</td></tr></tbody></table>
<table><caption>Table 2. *Cont.*</caption><thead><tr><th>Sample</th><th>Major</th><th>Moderate</th><th>Minor</th></tr></thead><tbody><tr><td>DR22A</td><td>δ-MnO2</td><td>Quartz</td><td>Plagioclase</td></tr><tr><td>DR29F</td><td>δ-MnO2</td><td>Quartz</td><td>Quartz</td></tr><tr><td>DR38C</td><td>δ-MnO2</td><td>-</td><td>Quartz, Calcite</td></tr><tr><td>DR38C-1</td><td>δ-MnO2</td><td>-</td><td>Quartz</td></tr><tr><td>DR38C-2</td><td>δ-MnO2</td><td>Calcite</td><td>Quartz</td></tr><tr><td>DR41Ai</td><td>δ-MnO2, Plagioclase</td><td>Quartz</td><td>-</td></tr><tr><td>DR42B</td><td>δ-MnO2</td><td>-</td><td>-</td></tr><tr><td>DR48-021</td><td>δ-MnO2, Quartz</td><td>-</td><td>Plagioclase</td></tr><tr><td>DR53-008</td><td>δ-MnO2</td><td>-</td><td>Quartz</td></tr><tr><td>DR54-009</td><td>δ-MnO2</td><td>-</td><td>Quartz, Plagioclase</td></tr><tr><td>DW4998D</td><td>Pyrolusite</td><td>Mg-Calcite</td><td>Calcite, Fluorapatite</td></tr><tr><td>DW4998E</td><td>Fe-oxyhydroxides, Calcite</td><td>Fluorapatite, 10 Å<br>manganates</td><td></td></tr><tr><td>DW5067B</td><td>δ-MnO2</td><td>Calcite</td><td>-</td></tr><tr><td>DW5070A</td><td>δ-MnO2</td><td>-</td><td>Quartz, Calcite</td></tr><tr><td>DW5073</td><td>δ-MnO2</td><td>-</td><td>Calcite</td></tr><tr><td>DW5086B</td><td>δ-MnO2</td><td>Quartz</td><td>Plagioclase</td></tr><tr><td>DW5087</td><td>δ-MnO2</td><td>Quartz</td><td>Plagioclase</td></tr><tr><td>DW5089B</td><td>δ-MnO2</td><td>-</td><td>Quartz</td></tr><tr><td>DW5090B</td><td>δ-MnO2</td><td>-</td><td>Quartz</td></tr><tr><td>DW5091A</td><td>δ-MnO2</td><td>Quartz</td><td>Calcite</td></tr><tr><td>DW772</td><td>δ-MnO2</td><td>-</td><td>-</td></tr><tr><td>DW774</td><td>δ-MnO2</td><td>-</td><td>Calcite</td></tr><tr><td>DW778B</td><td>Pyrolusite</td><td>Mg-Calcite</td><td>-</td></tr><tr><td>DW778D</td><td>Pyrolusite</td><td>Mg-Calcite</td><td>-</td></tr><tr><td>DW778D2</td><td>Pyrolusite</td><td>Mg-Calcite</td><td>-</td></tr><tr><td>DW2482</td><td>10 Å manganates</td><td>Mg-Calcite</td><td>Fluorapatite</td></tr><tr><td>E-DR08B</td><td>δ-MnO2, Quartz</td><td>Plagioclase</td><td>-</td></tr><tr><td>E-DR08B-1</td><td>δ-MnO2, Quartz</td><td>-</td><td>Plagioclase</td></tr><tr><td>E-DR08B-2</td><td>δ-MnO2, Quartz</td><td>-</td><td>-</td></tr><tr><td>E-DR08B-3</td><td>δ-MnO2, Quartz</td><td>Plagioclase</td><td>-</td></tr><tr><td>GO14D</td><td>δ-MnO2</td><td>-</td><td>-</td></tr><tr><td>GO15D</td><td>δ-MnO2</td><td>Calcite</td><td>Quartz, Plagioclase</td></tr><tr><td>GO16D</td><td>δ-MnO2</td><td>Quartz</td><td>Plagioclase, Calcite</td></tr><tr><td>GO18D</td><td>δ-MnO2</td><td>-</td><td>Quartz, Plagioclase</td></tr><tr><td>GO202</td><td>δ-MnO2, Quartz</td><td>-</td><td>Plagioclase</td></tr><tr><td>GO209</td><td>δ-MnO2</td><td>Quartz</td><td>Plagioclase</td></tr><tr><td>GO20D</td><td>δ-MnO2</td><td>-</td><td>Quartz</td></tr><tr><td>GO302D</td><td>δ-MnO2, Plagioclase</td><td>Quartz</td><td>-</td></tr><tr><td>GO302D-1</td><td>δ-MnO2, Plagioclase</td><td>Quartz</td><td>-</td></tr><tr><td>GO302D-2</td><td>δ-MnO2, Plagioclase</td><td>Quartz</td><td>-</td></tr><tr><td>GO310</td><td>δ-MnO2, Clays/Micas</td><td>Amphibole</td><td>Quartz, Calcite,<br>Plagioclase</td></tr><tr><td>GO314D10</td><td>-</td><td>-</td><td>-</td></tr><tr><td>GO316D31</td><td>-</td><td>-</td><td>-</td></tr><tr><td>GO317D10</td><td>-</td><td>-</td><td>-</td></tr><tr><td>GO320</td><td>δ-MnO2</td><td>-</td><td>Plagioclase, Quartz</td></tr><tr><td>GO321D2</td><td>-</td><td>-</td><td>-</td></tr><tr><td>GO322D4</td><td>-</td><td>-</td><td>-</td></tr><tr><td>GO323D2</td><td>-</td><td>-</td><td>-</td></tr><tr><td>GO325D2</td><td>-</td><td>-</td><td>-</td></tr><tr><td>GO327D</td><td>δ-MnO2, Quartz</td><td>Plagioclase</td><td>-</td></tr><tr><td>GO327D-1</td><td>δ-MnO2, Quartz</td><td>-</td><td>Plagioclase</td></tr><tr><td>GO327D-2</td><td>δ-MnO2, Quartz</td><td>-</td><td>Plagioclase</td></tr><tr><td>GO327D-3</td><td>δ-MnO2, Quartz</td><td>-</td><td>Plagioclase</td></tr><tr><td>GO327D-4</td><td>δ-MnO2, Quartz</td><td>-</td><td>Plagioclase</td></tr><tr><td>GO338D2</td><td>-</td><td>-</td><td>-</td></tr><tr><td>GO347D</td><td>-</td><td>-</td><td>-</td></tr><tr><td>GO348D6</td><td>δ-MnO2</td><td>-</td><td>Quartz</td></tr></tbody></table>
<table><caption>**Table 2.** *Cont.*</caption><thead><tr><th>Sample</th><th>Major</th><th>Moderate</th><th>Minor</th></tr></thead><tbody><tr><td>GO350D6</td><td>δ-MnO2</td><td>Quartz, Plagioclase</td><td>-</td></tr><tr><td>GO350D6-1</td><td>δ-MnO2</td><td>Plagioclase, Quartz</td><td>-</td></tr><tr><td>GO350D6-2</td><td>δ-MnO2</td><td>Quartz, Plagioclase</td><td>-</td></tr><tr><td>GO350D6-3</td><td>δ-MnO2</td><td>-</td><td>Quartz, Plagioclase</td></tr><tr><td>GO3D</td><td>δ-MnO2</td><td>Quartz</td><td>Plagioclase</td></tr><tr><td>V-DR08B</td><td>δ-MnO2, Quartz</td><td>Plagioclase</td><td>Calcite</td></tr></tbody></table>

<figure><img src="image_8.png" /><figcaption>**Figure 4.** BSE images of selected Fe-Mn deposits samples. (A–D): Fe-Mn crusts; (E–H): Hydrothermal Mn + Ca ± Fe deposits; (A) Large columnar structure with quartz infill; (B) Laminar structure (bottom) evolving upwards to columnar structure; (C) Massive columnar texture formed by dense pillar structure with little amount of carbonate infill; (D) Well-crystallised needle-like Mn-oxyhydroxides with overgrowth or crosscut by Ca-phosphate (fluorapatite); (E) Well-crystallised needle-like Mn-oxyhydroxides infilling cavity and surrounded by pyrolusite; (F) Pyrolusite and ± calcite replacing areas composed of alternating layers of amorphous crystalline and microcrystalline Mn-oxyhydroxides; (G) Radially-oriented and spherulitic structure showing alternating layers of amorphous crystalline, microcrystalline and crystalline Mn-oxyhydroxides cemented by a massive microcrystalline Mn-oxyhydroxides; (H) Microcrystalline and crystalline Mn-oxyhydroxides observed between hydrothermally altered hyaloclastite clasts replaced by clays and Fe-oxyhydroxides; a subsequent carbonate and Ca-phosphate (fluorapatite) infilling event is observed.</figcaption></figure>
<table><caption>**Table 2.** *Cont.*</caption><thead><tr><th>Sample</th><th>Major</th><th>Moderate</th><th>Minor</th></tr></thead><tbody><tr><td>GO350D6</td><td>δ-MnO2</td><td>Quartz, Plagioclase</td><td>-</td></tr><tr><td>GO350D6-1</td><td>δ-MnO2</td><td>Plagioclase, Quartz</td><td>-</td></tr><tr><td>GO350D6-2</td><td>δ-MnO2</td><td>Quartz, Plagioclase</td><td>-</td></tr><tr><td>GO350D6-3</td><td>δ-MnO2</td><td>-</td><td>Quartz, Plagioclase</td></tr><tr><td>GO3D</td><td>δ-MnO2</td><td>Quartz</td><td>Plagioclase</td></tr><tr><td>V-DR08B</td><td>δ-MnO2, Quartz</td><td>Plagioclase</td><td>Calcite</td></tr></tbody></table>

<figure><img src="image_8.png" /><figcaption>**Figure 4.** BSE images of selected Fe-Mn deposits samples. (A–D): Fe-Mn crusts; (E–H): Hydrothermal Mn + Ca ± Fe deposits; (A) Large columnar structure with quartz infill; (B) Laminar structure (bottom) evolving upwards to columnar structure; (C) Massive columnar texture formed by dense pillar structure with little amount of carbonate infill; (D) Well-crystallised needle-like Mn-oxyhydroxides with overgrowth or crosscut by Ca-phosphate (fluorapatite); (E) Well-crystallised needle-like Mn-oxyhydroxides infilling cavity and surrounded by pyrolusite; (F) Pyrolusite and ± calcite replacing areas composed of alternating layers of amorphous crystalline and microcrystalline Mn-oxyhydroxides; (G) Radially-oriented and spherulitic structure showing alternating layers of amorphous crystalline, microcrystalline and crystalline Mn-oxyhydroxides cemented by a massive microcrystalline Mn-oxyhydroxides; (H) Microcrystalline and crystalline Mn-oxyhydroxides observed between hydrothermally altered hyaloclastite clasts replaced by clays and Fe-oxyhydroxides; a subsequent carbonate and Ca-phosphate (fluorapatite) infilling event is observed.</figcaption></figure>
Mn-rich (±Ca-Fe) samples (DW4998D, DW4998E, DW778B, DW778D, DW778D2 and DW2482) contain significant amounts of 10 Å manganates and/or pyrolusite, which reflect a diagenetic and/or hydrothermal contribution [65]. These samples might contain a significant amount of δ-MnO₂, but its presence (based on 2.46 Å and 1.42 Å XRD reflections) is difficult to determine when 10 Å manganates and/or pyrolusite are detected on diffractograms. All samples containing 10 Å manganates also show moderate to minor amounts of Ca-phosphate (fluorapatite) (Figure 4D). Pyrolusite is abundant in four samples (DW4998D, DW778B, DW778D and DW778D2). Pyrolusite is a manganese oxide (MnO₂) encountered in hydrothermal deposits with a high oxidation state or forming following oxidation of primary todorokite (10 Å manganates) during diagenetic processes [67–69]. In DW778D, pyrolusite associated with calcite progressively replaces first-stage Mn minerals composed of an alternation of amorphous crystalline and microcrystalline Mn-oxyhydroxides (Figure 4E). In some places, acicular/fibrous Ba-rich Mn oxyhydroxides are observed in pore spaces between pyrolusite minerals (Figure 4F). Spherulitic structure characterised by alternating amorphous, microcrystalline and crystalline Mn-oxyhydroxides were also observed in the hydrothermal/diagenetic samples (Figure 4G). Sample DW4998E presents characteristics of a strongly hydrothermally-altered hyaloclastite (Figure 3G), mainly composed of calcite and amorphous Fe-oxyhydroxides, with minor clays and Mn-oxyhydroxides (Figure 4H).

### 4.3. Geochemistry

#### 4.3.1. Fe-Mn Samples Classification

A ternary plot of Fe, Mn and (Co + Cu + Ni) × 10 [70] (Figure 5A) shows that the majority of our samples (including DW4998E sample) fall within the hydrogenetic field [71]. This is in good agreement with macroscopic and mineralogical results, which point out that most of the samples (n = 98/104) can be referred to as hydrogenetic Fe-Mn crusts. However, one sample (i.e., DW4998E) with hydrothermal macroscopic and mineralogical characteristics is plot in the hydrogenetic field due to high Co, Ni and Cu concentrations. The last five samples (DW2482, DW788B, DW778D, DW778D2 and DW4998D) fall within the overlap of the diagenetic and the hydrothermal part of the diagram, consistent with the observed morphologies and mineralogy. The second diagram [72] (Figure 5B) plots (Fe + Mn)/4, 100 × (Zr + Y + Ce), 15 × (Cu + Ni) and shows a major clustering of samples in the lower part of the hydrogenetic area as well. Two samples (DN5085A and GO16D) plot slightly below the hydrogenetic field, due to high contents of Cu and Ni and relatively low Zr, Ce and Y concentrations. Four samples (DW778D, DW778D2, DW778B and DW4998D) are located within the hydrothermal fall-out crusts and impregnations, strengthening our interpretation of a hydrothermal origin. DW4998E and DW2482 are distributed between the diagenetic and the hydrothermal field.

<figure><img src="image_9.png" /><figcaption>**Figure 5.** Ternary classification schemes of Fe-Mn samples from New Caledonia's EEZ (n = 104) after (A) [70] and (B) [72].</figcaption></figure>

The classification of [73] requires full REE determination which was only produced on 17 of the 104 samples of this set. Nonetheless, the 17 analysed samples plot in the
Mn-rich (±Ca-Fe) samples (DW4998D, DW4998E, DW778B, DW778D, DW778D2 and DW2482) contain significant amounts of 10 Å manganates and/or pyrolusite, which reflect a diagenetic and/or hydrothermal contribution [65]. These samples might contain a significant amount of δ-MnO₂, but its presence (based on 2.46 Å and 1.42 Å XRD reflections) is difficult to determine when 10 Å manganates and/or pyrolusite are detected on diffractograms. All samples containing 10 Å manganates also show moderate to minor amounts of Ca-phosphate (fluorapatite) (Figure 4D). Pyrolusite is abundant in four samples (DW4998D, DW778B, DW778D and DW778D2). Pyrolusite is a manganese oxide (MnO₂) encountered in hydrothermal deposits with a high oxidation state or forming following oxidation of primary todorokite (10 Å manganates) during diagenetic processes [67–69]. In DW778D, pyrolusite associated with calcite progressively replaces first-stage Mn minerals composed of an alternation of amorphous crystalline and microcrystalline Mn-oxyhydroxides (Figure 4E). In some places, acicular/fibrous Ba-rich Mn oxyhydroxides are observed in pore spaces between pyrolusite minerals (Figure 4F). Spherulitic structure characterised by alternating amorphous, microcrystalline and crystalline Mn-oxyhydroxides were also observed in the hydrothermal/diagenetic samples (Figure 4G). Sample DW4998E presents characteristics of a strongly hydrothermally-altered hyaloclastite (Figure 3G), mainly composed of calcite and amorphous Fe-oxyhydroxides, with minor clays and Mn-oxyhydroxides (Figure 4H).

## 4.3. Geochemistry

### 4.3.1. Fe-Mn Samples Classification

A ternary plot of Fe, Mn and (Co + Cu + Ni) × 10 [70] (Figure 5A) shows that the majority of our samples (including DW4998E sample) fall within the hydrogenetic field [71]. This is in good agreement with macroscopic and mineralogical results, which point out that most of the samples (n = 98/104) can be referred to as hydrogenetic Fe-Mn crusts. However, one sample (i.e., DW4998E) with hydrothermal macroscopic and mineralogical characteristics is plot in the hydrogenetic field due to high Co, Ni and Cu concentrations. The last five samples (DW2482, DW788B, DW778D, DW778D2 and DW4998D) fall within the overlap of the diagenetic and the hydrothermal part of the diagram, consistent with the observed morphologies and mineralogy. The second diagram [72] (Figure 5B) plots (Fe + Mn)/4, 100 × (Zr + Y + Ce), 15 × (Cu + Ni) and shows a major clustering of samples in the lower part of the hydrogenetic area as well. Two samples (DN5085A and GO16D) plot slightly below the hydrogenetic field, due to high contents of Cu and Ni and relatively low Zr, Ce and Y concentrations. Four samples (DW778D, DW778D2, DW778B and DW4998D) are located within the hydrothermal fall-out crusts and impregnations, strengthening our interpretation of a hydrothermal origin. DW4998E and DW2482 are distributed between the diagenetic and the hydrothermal field.

<figure><img src="image_9.png" /><figcaption>**Figure 5.** Ternary classification schemes of Fe-Mn samples from New Caledonia's EEZ (n = 104) after (A) [70] and (B) [72].</figcaption></figure>

The classification of [73] requires full REE determination which was only produced on 17 of the 104 samples of this set. Nonetheless, the 17 analysed samples plot in the
hydrogenetic field are in good agreement with other classification for this subset (Figure 6).

<figure><img src="image_10.png" /><figcaption>Figure 6. Genetic discrimination plot of the selected hydrogenetic crusts after [73].</figcaption></figure>

### 4.3.2. Hydrogenetic Fe-Mn Crusts

For the combined hydrogenetic data set (n = 98), chemical composition of bulk and macro-layers is presented as mean ± 2σ% (Table 3; Supplementary Material Table S1). The mean Fe/Mn ratio for the combined data set is 1.26 ± 0.66 and the mean Si/Al ratio is 3.76 ± 2.37. The highest values of Si/Al ratio are found in layers of the thickest crusts (e.g., GO327D-4, GO327D-3 and DR11Ai-3). The mean combined Cu + Ni + Co (%) concentration is 0.81 ± 0.56%. This wide variation in metals with the greatest economic interest is mainly controlled by cobalt concentrations ranging from 0.16% to 1.02%. However, few Fe-Mn crusts samples (e.g., DN5085A, GO16D) are dominated by nickel enrichment with concentrations up to 0.61%.

<table><caption>Table 3. Statistics for the geochemical data sets normalised to 0% H₂O⁻.</caption><thead><tr><th rowspan="2">Element</th><th rowspan="2">N</th><th colspan="2">Hydrogenetic Bulk and Macro-Layers Crusts</th><th colspan="6">Hydrothermal/Diagenetic Deposits</th></tr><tr><th>Mean</th><th>±2σ</th><th>DW778B</th><th>DW778D</th><th>DW778D2</th><th>DW4998D</th><th>DW4998E</th><th>DW2482</th></tr></thead><tbody><tr><td>(H₂O⁻<sup>a</sup>) (%)</td><td>85</td><td>10.26</td><td>11.93</td><td>0.42</td><td>0.36</td><td>0.82</td><td>0.6</td><td>2.61</td><td>1.59</td></tr><tr><td>LOI<sup>b</sup></td><td>85</td><td>17.98</td><td>4.6</td><td>17.34</td><td>17.76</td><td>17.36</td><td>21.46</td><td>21.35</td><td>18.91</td></tr><tr><td>Mn</td><td>98</td><td>17.37</td><td>7.18</td><td>50.38</td><td>48.37</td><td>48.82</td><td>28.96</td><td>14.03</td><td>38.44</td></tr><tr><td>Fe</td><td>98</td><td>20.79</td><td>3.57</td><td>1.27</td><td>1.59</td><td>1.28</td><td>1.82</td><td>13.67</td><td>2.8</td></tr><tr><td>Si</td><td>98</td><td>6.15</td><td>5.98</td><td>0.49</td><td>0.53</td><td>0.51</td><td>0.35</td><td>1.07</td><td>0.29</td></tr><tr><td>Al</td><td>98</td><td>1.63</td><td>1.5</td><td>0.25</td><td>0.27</td><td>0.23</td><td>0.28</td><td>1.99</td><td>0.57</td></tr><tr><td>Mg</td><td>98</td><td>1.28</td><td>0.69</td><td>0.99</td><td>0.92</td><td>0.8</td><td>1.25</td><td>3.58</td><td>2.31</td></tr><tr><td>Na</td><td>98</td><td>1.56</td><td>0.42</td><td>0.42</td><td>0.32</td><td>0.21</td><td>0.3</td><td>0.42</td><td>0.84</td></tr><tr><td>Ca</td><td>98</td><td>2.61</td><td>1.69</td><td>6.21</td><td>7.54</td><td>8.14</td><td>17.79</td><td>13.52</td><td>8.67</td></tr><tr><td>K</td><td>98</td><td>0.42</td><td>0.29</td><td>0.2</td><td>0.13</td><td>0.11</td><td>0.13</td><td>0.2</td><td>0.37</td></tr><tr><td>Ti</td><td>98</td><td>0.9</td><td>0.36</td><td>0.05</td><td>0.08</td><td>0.06</td><td>0.05</td><td>0.17</td><td>0.1</td></tr><tr><td>P</td><td>98</td><td>0.57</td><td>0.31</td><td>0.53</td><td>0.41</td><td>0.37</td><td>1.88</td><td>1.97</td><td>1</td></tr><tr><td>As (ppm)</td><td>98</td><td>275</td><td>136</td><td>71</td><td>72</td><td>59</td><td>63</td><td>260</td><td>89</td></tr><tr><td>Ba</td><td>98</td><td>1500</td><td>560</td><td>8460</td><td>4946</td><td>3339</td><td>4151</td><td>869</td><td>11,719</td></tr><tr><td>Ce</td><td>98</td><td>711</td><td>384</td><td>148</td><td>112</td><td>81</td><td>86</td><td>221</td><td>275</td></tr><tr><td>Co</td><td>98</td><td>4188</td><td>3762</td><td>514</td><td>898</td><td>655</td><td>405</td><td>1409</td><td>1148</td></tr><tr><td>Cr</td><td>85</td><td>45</td><td>78</td><td>10</td><td>10</td><td>10</td><td>10</td><td>35</td><td>12</td></tr><tr><td>Cu</td><td>98</td><td>775</td><td>611</td><td>283</td><td>242</td><td>187</td><td>108</td><td>655</td><td>295</td></tr><tr><td>La</td><td>98</td><td>239</td><td>88</td><td>88</td><td>105</td><td>96</td><td>74</td><td>80</td><td>88</td></tr><tr><td>Mo</td><td>98</td><td>430</td><td>268</td><td>168</td><td>141</td><td>128</td><td>82</td><td>410</td><td>228</td></tr><tr><td>Nb</td><td>98</td><td>60</td><td>26</td><td>5</td><td>10</td><td>7</td><td>7</td><td>18</td><td>15</td></tr></tbody></table>
hydrogenetic field are in good agreement with other classification for this subset (Figure 6).

<figure><img src="image_10.png" /><figcaption>Figure 6. Genetic discrimination plot of the selected hydrogenetic crusts after [73].</figcaption></figure>

### 4.3.2. Hydrogenetic Fe-Mn Crusts

For the combined hydrogenetic data set (n = 98), chemical composition of bulk and macro-layers is presented as mean ± 2σ% (Table 3; Supplementary Material Table S1). The mean Fe/Mn ratio for the combined data set is 1.26 ± 0.66 and the mean Si/Al ratio is 3.76 ± 2.37. The highest values of Si/Al ratio are found in layers of the thickest crusts (e.g., GO327D-4, GO327D-3 and DR11Ai-3). The mean combined Cu + Ni + Co (%) concentration is 0.81 ± 0.56%. This wide variation in metals with the greatest economic interest is mainly controlled by cobalt concentrations ranging from 0.16% to 1.02%. However, few Fe-Mn crusts samples (e.g., DN5085A, GO16D) are dominated by nickel enrichment with concentrations up to 0.61%.

<table><caption>Table 3. Statistics for the geochemical data sets normalised to 0% H₂O⁻.</caption><thead><tr><th rowspan="2">Element</th><th rowspan="2">N</th><th colspan="2">Hydrogenetic Bulk and Macro-Layers Crusts</th><th colspan="6">Hydrothermal/Diagenetic Deposits</th></tr><tr><th>Mean</th><th>±2σ</th><th>DW778B</th><th>DW778D</th><th>DW778D2</th><th>DW4998D</th><th>DW4998E</th><th>DW2482</th></tr></thead><tbody><tr><td>(H₂O⁻<sup>a</sup>) (%)</td><td>85</td><td>10.26</td><td>11.93</td><td>0.42</td><td>0.36</td><td>0.82</td><td>0.6</td><td>2.61</td><td>1.59</td></tr><tr><td>LOI<sup>b</sup></td><td>85</td><td>17.98</td><td>4.6</td><td>17.34</td><td>17.76</td><td>17.36</td><td>21.46</td><td>21.35</td><td>18.91</td></tr><tr><td>Mn</td><td>98</td><td>17.37</td><td>7.18</td><td>50.38</td><td>48.37</td><td>48.82</td><td>28.96</td><td>14.03</td><td>38.44</td></tr><tr><td>Fe</td><td>98</td><td>20.79</td><td>3.57</td><td>1.27</td><td>1.59</td><td>1.28</td><td>1.82</td><td>13.67</td><td>2.8</td></tr><tr><td>Si</td><td>98</td><td>6.15</td><td>5.98</td><td>0.49</td><td>0.53</td><td>0.51</td><td>0.35</td><td>1.07</td><td>0.29</td></tr><tr><td>Al</td><td>98</td><td>1.63</td><td>1.5</td><td>0.25</td><td>0.27</td><td>0.23</td><td>0.28</td><td>1.99</td><td>0.57</td></tr><tr><td>Mg</td><td>98</td><td>1.28</td><td>0.69</td><td>0.99</td><td>0.92</td><td>0.8</td><td>1.25</td><td>3.58</td><td>2.31</td></tr><tr><td>Na</td><td>98</td><td>1.56</td><td>0.42</td><td>0.42</td><td>0.32</td><td>0.21</td><td>0.3</td><td>0.42</td><td>0.84</td></tr><tr><td>Ca</td><td>98</td><td>2.61</td><td>1.69</td><td>6.21</td><td>7.54</td><td>8.14</td><td>17.79</td><td>13.52</td><td>8.67</td></tr><tr><td>K</td><td>98</td><td>0.42</td><td>0.29</td><td>0.2</td><td>0.13</td><td>0.11</td><td>0.13</td><td>0.2</td><td>0.37</td></tr><tr><td>Ti</td><td>98</td><td>0.9</td><td>0.36</td><td>0.05</td><td>0.08</td><td>0.06</td><td>0.05</td><td>0.17</td><td>0.1</td></tr><tr><td>P</td><td>98</td><td>0.57</td><td>0.31</td><td>0.53</td><td>0.41</td><td>0.37</td><td>1.88</td><td>1.97</td><td>1</td></tr><tr><td>As (ppm)</td><td>98</td><td>275</td><td>136</td><td>71</td><td>72</td><td>59</td><td>63</td><td>260</td><td>89</td></tr><tr><td>Ba</td><td>98</td><td>1500</td><td>560</td><td>8460</td><td>4946</td><td>3339</td><td>4151</td><td>869</td><td>11,719</td></tr><tr><td>Ce</td><td>98</td><td>711</td><td>384</td><td>148</td><td>112</td><td>81</td><td>86</td><td>221</td><td>275</td></tr><tr><td>Co</td><td>98</td><td>4188</td><td>3762</td><td>514</td><td>898</td><td>655</td><td>405</td><td>1409</td><td>1148</td></tr><tr><td>Cr</td><td>85</td><td>45</td><td>78</td><td>10</td><td>10</td><td>10</td><td>10</td><td>35</td><td>12</td></tr><tr><td>Cu</td><td>98</td><td>775</td><td>611</td><td>283</td><td>242</td><td>187</td><td>108</td><td>655</td><td>295</td></tr><tr><td>La</td><td>98</td><td>239</td><td>88</td><td>88</td><td>105</td><td>96</td><td>74</td><td>80</td><td>88</td></tr><tr><td>Mo</td><td>98</td><td>430</td><td>268</td><td>168</td><td>141</td><td>128</td><td>82</td><td>410</td><td>228</td></tr><tr><td>Nb</td><td>98</td><td>60</td><td>26</td><td>5</td><td>10</td><td>7</td><td>7</td><td>18</td><td>15</td></tr></tbody></table>
<table><caption>Table 3. *Cont.*</caption><thead><tr><th rowspan="2">Element</th><th colspan="3">Hydrogenetic Bulk and Macro-Layers Crusts</th><th colspan="7">Hydrothermal/Diagenetic Deposits</th></tr><tr><th>N</th><th>Mean</th><th>±2σ</th><th>DW778B</th><th>DW778D</th><th>DW778D2</th><th>DW4998D</th><th>DW4998E</th><th>DW2482</th></tr></thead><tbody><tr><td>Nd</td><td>98</td><td>150</td><td>66</td><td>30</td><td>30</td><td>30</td><td>30</td><td>30</td><td>30</td></tr><tr><td>Ni</td><td>98</td><td>3100</td><td>2185</td><td>1095</td><td>1432</td><td>1178</td><td>736</td><td>6936</td><td>4138</td></tr><tr><td>Pb</td><td>98</td><td>1544</td><td>1176</td><td>219</td><td>359</td><td>257</td><td>171</td><td>322</td><td>349</td></tr><tr><td>Pt <sup>c</sup></td><td>46</td><td>0.66</td><td>0.8</td><td>&lt;0.5</td><td>&lt;0.5</td><td>&lt;0.5</td><td>&lt;0.5</td><td>&lt;0.5</td><td>&lt;0.5</td></tr><tr><td>Rb</td><td>98</td><td>&lt;5</td><td>&lt;5</td><td>&lt;5</td><td>&lt;5</td><td>&lt;5</td><td>&lt;5</td><td>7</td><td>&lt;5</td></tr><tr><td>Sr</td><td>98</td><td>1482</td><td>550</td><td>1858</td><td>1440</td><td>1261</td><td>1119</td><td>719</td><td>2656</td></tr><tr><td>Tl</td><td>98</td><td>100</td><td>112</td><td>77</td><td>75</td><td>61</td><td>43</td><td>78</td><td>141</td></tr><tr><td>V</td><td>98</td><td>751</td><td>336</td><td>271</td><td>263</td><td>214</td><td>289</td><td>796</td><td>525</td></tr><tr><td>Y</td><td>98</td><td>165</td><td>66</td><td>62</td><td>65</td><td>61</td><td>68</td><td>74</td><td>41</td></tr><tr><td>Zn</td><td>98</td><td>665</td><td>235</td><td>182</td><td>201</td><td>111</td><td>161</td><td>459</td><td>589</td></tr><tr><td>Zr</td><td>98</td><td>535</td><td>170</td><td>73</td><td>73</td><td>61</td><td>39</td><td>91</td><td>73</td></tr><tr><td>Co + Cu + Ni (%)</td><td>98</td><td>0.81</td><td>0.56</td><td>0.19</td><td>0.26</td><td>0.2</td><td>0.12</td><td>0.9</td><td>0.56</td></tr><tr><td>Si/Al</td><td>98</td><td>3.76</td><td>2.37</td><td>1.96</td><td>1.96</td><td>2.22</td><td>1.25</td><td>0.54</td><td>0.51</td></tr><tr><td>Fe/Mn</td><td>98</td><td>1.26</td><td>0.66</td><td>0.03</td><td>0.03</td><td>0.03</td><td>0.06</td><td>0.97</td><td>0.07</td></tr></tbody></table>

<sup>a</sup> Humidity value at 80 °C; <sup>b</sup> Loss on ignition at 950 °C; <sup>c</sup> Mean value for Pt was obtained from 46 samples. The rest of the samples exhibit values below the detection limit of 0.5 ppm.

Rare earth elements (REE) concentrations of the 17 selected hydrogenetic crusts are compiled in Table 4 and PAAS-normalised [74] trends presented in Figure 7. REE data are consistent with typical hydrogenetic crusts [73]. The PAAS-normalised patterns are characterised by a two- to ten-fold enrichment in REE compared to average PAAS and present a positive Ce anomaly, and HREE<sub>PAAS</sub> and/or MREE<sub>PAAS</sub> enrichment (Figure 7. Total REE (ΣREE in Table 4) varies from 919 ppm to 1705 ppm, with a mean value of 1307 ± 426 ppm. Percentage of heavy REE (HREE; Eu-Lu + Y in Table 4) varies between 14% and 27%, with a mean value of 22 ± 7%).

<table><caption>Table 4. Rare earth elements (REE) compositions of the 17 samples analysed with ICP-MS.</caption><thead><tr><th></th><th>DR102</th><th>DR105</th><th>DR108</th><th>DR117</th><th>GO14D</th><th>GO202</th><th>GO209</th><th>GO314D10</th><th>GO316</th><th>GO317D10</th><th>GO321D1</th><th>GO322</th><th>GO323</th><th>GO325</th><th>GO338D2</th><th>GO347</th><th>Mean</th><th>2σ</th></tr></thead><tbody><tr><td>Water depth (m)</td><td>2525</td><td>2498</td><td>2080</td><td>1800</td><td>1450</td><td>2425</td><td>1310</td><td>3120</td><td>3200</td><td>3200</td><td>4190</td><td>3680</td><td>4180</td><td>4000</td><td>1600</td><td>2070</td><td>2751</td><td>1945</td></tr><tr><td>La</td><td>169</td><td>187</td><td>192</td><td>216</td><td>289</td><td>205</td><td>210</td><td>138</td><td>148</td><td>151</td><td>135</td><td>199</td><td>166</td><td>177</td><td>202</td><td>181</td><td>186</td><td>75</td></tr><tr><td>Ce</td><td>557</td><td>612</td><td>690</td><td>545</td><td>862</td><td>741</td><td>523</td><td>392</td><td>411</td><td>522</td><td>535</td><td>763</td><td>972</td><td>809</td><td>715</td><td>638</td><td>643</td><td>323</td></tr><tr><td>Pr</td><td>26</td><td>32</td><td>34</td><td>34</td><td>45</td><td>37</td><td>34</td><td>24</td><td>26</td><td>32</td><td>25</td><td>40</td><td>29</td><td>38</td><td>30</td><td>26</td><td>32</td><td>12</td></tr><tr><td>Nd</td><td>117</td><td>133</td><td>142</td><td>142</td><td>173</td><td>149</td><td>141</td><td>110</td><td>114</td><td>135</td><td>111</td><td>161</td><td>125</td><td>152</td><td>136</td><td>120</td><td>136</td><td>36</td></tr><tr><td>Sm</td><td>24</td><td>27</td><td>29</td><td>28</td><td>35</td><td>31</td><td>28</td><td>23</td><td>24</td><td>30</td><td>24</td><td>35</td><td>26</td><td>34</td><td>27</td><td>23</td><td>28</td><td>8</td></tr><tr><td>Eu</td><td>6</td><td>7</td><td>8</td><td>7</td><td>9</td><td>8</td><td>7</td><td>6</td><td>6</td><td>8</td><td>6</td><td>9</td><td>6</td><td>8</td><td>7</td><td>6</td><td>7</td><td>2</td></tr><tr><td>Gd</td><td>29</td><td>31</td><td>35</td><td>35</td><td>38</td><td>35</td><td>34</td><td>29</td><td>29</td><td>35</td><td>28</td><td>37</td><td>28</td><td>36</td><td>33</td><td>29</td><td>33</td><td>7</td></tr><tr><td>Tb</td><td>4</td><td>5</td><td>5</td><td>5</td><td>6</td><td>5</td><td>5</td><td>4</td><td>4</td><td>5</td><td>4</td><td>6</td><td>4</td><td>6</td><td>5</td><td>4</td><td>5</td><td>1</td></tr><tr><td>Dy</td><td>27</td><td>28</td><td>33</td><td>34</td><td>35</td><td>33</td><td>33</td><td>26</td><td>27</td><td>32</td><td>27</td><td>33</td><td>25</td><td>33</td><td>32</td><td>29</td><td>31</td><td>7</td></tr><tr><td>Y</td><td>149</td><td>140</td><td>164</td><td>215</td><td>162</td><td>163</td><td>206</td><td>125</td><td>142</td><td>153</td><td>135</td><td>153</td><td>114</td><td>138</td><td>186</td><td>191</td><td>156</td><td>57</td></tr><tr><td>Ho</td><td>6</td><td>6</td><td>7</td><td>8</td><td>7</td><td>7</td><td>8</td><td>6</td><td>6</td><td>7</td><td>6</td><td>7</td><td>5</td><td>7</td><td>7</td><td>7</td><td>7</td><td>1</td></tr><tr><td>Er</td><td>17</td><td>17</td><td>20</td><td>22</td><td>20</td><td>21</td><td>23</td><td>16</td><td>17</td><td>19</td><td>17</td><td>19</td><td>14</td><td>19</td><td>21</td><td>20</td><td>19</td><td>5</td></tr><tr><td>Yb</td><td>16</td><td>16</td><td>19</td><td>21</td><td>18</td><td>19</td><td>21</td><td>15</td><td>16</td><td>17</td><td>16</td><td>17</td><td>13</td><td>18</td><td>19</td><td>18</td><td>17</td><td>4</td></tr><tr><td>Lu</td><td>3</td><td>2</td><td>3</td><td>3</td><td>3</td><td>3</td><td>3</td><td>2</td><td>2</td><td>3</td><td>3</td><td>3</td><td>2</td><td>3</td><td>3</td><td>3</td><td>3</td><td>1</td></tr><tr><td>ΣREE</td><td>1155</td><td>1248</td><td>1385</td><td>1319</td><td>1705</td><td>1461</td><td>1281</td><td>919</td><td>975</td><td>1154</td><td>1077</td><td>1485</td><td>1535</td><td>1482</td><td>1426</td><td>1299</td><td>1307</td><td>426</td></tr><tr><td>%HREE</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>(Eu-Lu + Y)</td><td>23</td><td>20</td><td>21</td><td>27</td><td>18</td><td>20</td><td>27</td><td>25</td><td>26</td><td>25</td><td>23</td><td>19</td><td>14</td><td>18</td><td>22</td><td>24</td><td>22</td><td>7</td></tr></tbody></table>

### 4.3.3. Non-Hydrogenetic Mn-Rich (±Ca-Fe) Samples Deposits

The six samples characterised by non-hydrogenetic morphologies and mineralogy (DW778B, DW778D, DW778D2, DW4998D, DW4998E and DW2482) fall in either the hydrothermal or diagenetic field of common classification schemes [72,73]. These samples have very low Fe/Mn ratios (<0.07), except for sample DW4998E (0.97) where the analysis incorporates both Mn-dominated mineralization and Fe-Ca-rich, hydrothermally-altered hyaloclastite. They have low content of elements characteristic of the aluminosilicate phase (Si, Al, K and Na), but higher Ca concentrations compared to samples of hydrogenetic origin. Phosphorus is high and can reach a max of up to 2% related to the presence of fluorapatite. Cobalt is relatively low (<0.15%) whereas Ni, Ba and Sr reach concentrations
<table><caption>Table 3. *Cont.*</caption><thead><tr><th rowspan="2">Element</th><th colspan="3">Hydrogenetic Bulk and Macro-Layers Crusts</th><th colspan="6">Hydrothermal/Diagenetic Deposits</th></tr><tr><th>N</th><th>Mean</th><th>±2σ</th><th>DW778B</th><th>DW778D</th><th>DW778D2</th><th>DW4998D</th><th>DW4998E</th><th>DW2482</th></tr></thead><tbody><tr><td>Nd</td><td>98</td><td>150</td><td>66</td><td>30</td><td>30</td><td>30</td><td>30</td><td>30</td><td>30</td></tr><tr><td>Ni</td><td>98</td><td>3100</td><td>2185</td><td>1095</td><td>1432</td><td>1178</td><td>736</td><td>6936</td><td>4138</td></tr><tr><td>Pb</td><td>98</td><td>1544</td><td>1176</td><td>219</td><td>359</td><td>257</td><td>171</td><td>322</td><td>349</td></tr><tr><td>Pt <sup>c</sup></td><td>46</td><td>0.66</td><td>0.8</td><td>&lt;0.5</td><td>&lt;0.5</td><td>&lt;0.5</td><td>&lt;0.5</td><td>&lt;0.5</td><td>&lt;0.5</td></tr><tr><td>Rb</td><td>98</td><td>&lt;5</td><td>&lt;5</td><td>&lt;5</td><td>&lt;5</td><td>&lt;5</td><td>&lt;5</td><td>7</td><td>&lt;5</td></tr><tr><td>Sr</td><td>98</td><td>1482</td><td>550</td><td>1858</td><td>1440</td><td>1261</td><td>1119</td><td>719</td><td>2656</td></tr><tr><td>Tl</td><td>98</td><td>100</td><td>112</td><td>77</td><td>75</td><td>61</td><td>43</td><td>78</td><td>141</td></tr><tr><td>V</td><td>98</td><td>751</td><td>336</td><td>271</td><td>263</td><td>214</td><td>289</td><td>796</td><td>525</td></tr><tr><td>Y</td><td>98</td><td>165</td><td>66</td><td>62</td><td>65</td><td>61</td><td>68</td><td>74</td><td>41</td></tr><tr><td>Zn</td><td>98</td><td>665</td><td>235</td><td>182</td><td>201</td><td>111</td><td>161</td><td>459</td><td>589</td></tr><tr><td>Zr</td><td>98</td><td>535</td><td>170</td><td>73</td><td>73</td><td>61</td><td>39</td><td>91</td><td>73</td></tr><tr><td>Co + Cu + Ni (%)</td><td>98</td><td>0.81</td><td>0.56</td><td>0.19</td><td>0.26</td><td>0.2</td><td>0.12</td><td>0.9</td><td>0.56</td></tr><tr><td>Si/Al</td><td>98</td><td>3.76</td><td>2.37</td><td>1.96</td><td>1.96</td><td>2.22</td><td>1.25</td><td>0.54</td><td>0.51</td></tr><tr><td>Fe/Mn</td><td>98</td><td>1.26</td><td>0.66</td><td>0.03</td><td>0.03</td><td>0.03</td><td>0.06</td><td>0.97</td><td>0.07</td></tr></tbody></table>

<sup>a</sup> Humidity value at 80 °C; <sup>b</sup> Loss on ignition at 950 °C; <sup>c</sup> Mean value for Pt was obtained from 46 samples. The rest of the samples exhibit values below the detection limit of 0.5 ppm.

Rare earth elements (REE) concentrations of the 17 selected hydrogenetic crusts are compiled in Table 4 and PAAS-normalised [74] trends presented in Figure 7. REE data are consistent with typical hydrogenetic crusts [73]. The PAAS-normalised patterns are characterised by a two- to ten-fold enrichment in REE compared to average PAAS and present a positive Ce anomaly, and HREE<sub>PAAS</sub> and/or MREE<sub>PAAS</sub> enrichment (Figure 7. Total REE (ΣREE in Table 4) varies from 919 ppm to 1705 ppm, with a mean value of 1307 ± 426 ppm. Percentage of heavy REE (HREE; Eu-Lu + Y in Table 4) varies between 14% and 27%, with a mean value of 22 ± 7%.

<table><caption>Table 4. Rare earth elements (REE) compositions of the 17 samples analysed with ICP-MS.</caption><thead><tr><th></th><th>DR102</th><th>DR105</th><th>DR108</th><th>DR117</th><th>GO14D</th><th>GO202</th><th>GO209</th><th>GO314D10</th><th>GO316</th><th>GO317D10</th><th>GO321D1</th><th>GO322</th><th>GO323</th><th>GO325</th><th>GO338D2</th><th>GO347</th><th>Mean</th><th>2σ</th></tr></thead><tbody><tr><td>Water depth (m)</td><td>2525</td><td>2498</td><td>2080</td><td>1800</td><td>1450</td><td>2425</td><td>1310</td><td>3120</td><td>3200</td><td>3200</td><td>4190</td><td>3680</td><td>4180</td><td>4000</td><td>1600</td><td>2070</td><td>2751</td><td>1945</td></tr><tr><td>La</td><td>169</td><td>187</td><td>192</td><td>216</td><td>289</td><td>205</td><td>210</td><td>138</td><td>148</td><td>151</td><td>135</td><td>199</td><td>166</td><td>177</td><td>202</td><td>181</td><td>186</td><td>75</td></tr><tr><td>Ce</td><td>557</td><td>612</td><td>690</td><td>545</td><td>862</td><td>741</td><td>523</td><td>392</td><td>411</td><td>522</td><td>535</td><td>763</td><td>972</td><td>809</td><td>715</td><td>638</td><td>643</td><td>323</td></tr><tr><td>Pr</td><td>26</td><td>32</td><td>34</td><td>34</td><td>45</td><td>37</td><td>34</td><td>24</td><td>26</td><td>32</td><td>25</td><td>40</td><td>29</td><td>38</td><td>30</td><td>26</td><td>32</td><td>12</td></tr><tr><td>Nd</td><td>117</td><td>133</td><td>142</td><td>142</td><td>173</td><td>149</td><td>141</td><td>110</td><td>114</td><td>135</td><td>111</td><td>161</td><td>125</td><td>152</td><td>136</td><td>120</td><td>136</td><td>36</td></tr><tr><td>Sm</td><td>24</td><td>27</td><td>29</td><td>28</td><td>35</td><td>31</td><td>28</td><td>23</td><td>24</td><td>30</td><td>24</td><td>35</td><td>26</td><td>34</td><td>27</td><td>23</td><td>28</td><td>8</td></tr><tr><td>Eu</td><td>6</td><td>7</td><td>8</td><td>7</td><td>9</td><td>8</td><td>7</td><td>6</td><td>6</td><td>8</td><td>6</td><td>9</td><td>6</td><td>8</td><td>7</td><td>6</td><td>7</td><td>2</td></tr><tr><td>Gd</td><td>29</td><td>31</td><td>35</td><td>35</td><td>38</td><td>35</td><td>34</td><td>29</td><td>29</td><td>35</td><td>28</td><td>37</td><td>28</td><td>36</td><td>33</td><td>29</td><td>33</td><td>7</td></tr><tr><td>Tb</td><td>4</td><td>5</td><td>5</td><td>5</td><td>6</td><td>5</td><td>5</td><td>4</td><td>4</td><td>5</td><td>4</td><td>6</td><td>4</td><td>6</td><td>5</td><td>4</td><td>5</td><td>1</td></tr><tr><td>Dy</td><td>27</td><td>28</td><td>33</td><td>34</td><td>35</td><td>33</td><td>33</td><td>26</td><td>27</td><td>32</td><td>27</td><td>33</td><td>25</td><td>33</td><td>32</td><td>29</td><td>31</td><td>7</td></tr><tr><td>Y</td><td>149</td><td>140</td><td>164</td><td>215</td><td>162</td><td>163</td><td>206</td><td>125</td><td>142</td><td>153</td><td>135</td><td>153</td><td>114</td><td>138</td><td>186</td><td>191</td><td>156</td><td>57</td></tr><tr><td>Ho</td><td>6</td><td>6</td><td>7</td><td>8</td><td>7</td><td>7</td><td>8</td><td>6</td><td>6</td><td>7</td><td>6</td><td>7</td><td>5</td><td>7</td><td>7</td><td>7</td><td>7</td><td>1</td></tr><tr><td>Er</td><td>17</td><td>17</td><td>20</td><td>22</td><td>20</td><td>21</td><td>23</td><td>16</td><td>17</td><td>19</td><td>17</td><td>19</td><td>14</td><td>19</td><td>21</td><td>20</td><td>19</td><td>5</td></tr><tr><td>Yb</td><td>16</td><td>16</td><td>19</td><td>21</td><td>18</td><td>19</td><td>21</td><td>15</td><td>16</td><td>17</td><td>16</td><td>17</td><td>13</td><td>18</td><td>19</td><td>18</td><td>17</td><td>4</td></tr><tr><td>Lu</td><td>3</td><td>2</td><td>3</td><td>3</td><td>3</td><td>3</td><td>3</td><td>2</td><td>2</td><td>3</td><td>3</td><td>3</td><td>2</td><td>3</td><td>3</td><td>3</td><td>3</td><td>1</td></tr><tr><td>ΣREE</td><td>1155</td><td>1248</td><td>1385</td><td>1319</td><td>1705</td><td>1461</td><td>1281</td><td>919</td><td>975</td><td>1154</td><td>1077</td><td>1485</td><td>1535</td><td>1482</td><td>1426</td><td>1299</td><td>1307</td><td>426</td></tr><tr><td>%HREE</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>(Eu-Lu + Y)</td><td>23</td><td>20</td><td>21</td><td>27</td><td>18</td><td>20</td><td>27</td><td>25</td><td>26</td><td>25</td><td>23</td><td>19</td><td>14</td><td>18</td><td>22</td><td>24</td><td>22</td><td>7</td></tr></tbody></table>

### 4.3.3. Non-Hydrogenetic Mn-Rich (±Ca-Fe) Samples Deposits

The six samples characterised by non-hydrogenetic morphologies and mineralogy (DW778B, DW778D, DW778D2, DW4998D, DW4998E and DW2482) fall in either the hydrothermal or diagenetic field of common classification schemes [72,73]. These samples have very low Fe/Mn ratios (<0.07), except for sample DW4998E (0.97) where the analysis incorporates both Mn-dominated mineralization and Fe-Ca-rich, hydrothermally-altered hyaloclastite. They have low content of elements characteristic of the aluminosilicate phase (Si, Al, K and Na), but higher Ca concentrations compared to samples of hydrogenetic origin. Phosphorus is high and can reach a max of up to 2% related to the presence of fluorapatite. Cobalt is relatively low (<0.15%) whereas Ni, Ba and Sr reach concentrations
up to 0.69%, 1.17% and 0.26%, respectively. Other trace elements such as As, Ce, Cr, Mo, Pb, Nb, and Zr are depleted compared to usual hydrogenetic content.

<figure><img src="image_11.png" /><figcaption>**Figure 7.** PAAS-normalised REE plots for selected hydrogenetic crusts.</figcaption></figure>

4.4. *Growth Rates and Ages*

Estimated growth rates have been calculated using an empirical Co chronometer [66]. Growth rates vary from 0.45 to 102 mm/Ma (Figure 8) and show no geographic correlations. Bulk hydrogenetic crusts (n = 74) have a mean growth rate of 2.2 ± 2.5 mm/Ma. Growth rates for macro-layers sub-samples (n = 24) are on average higher, at 3.1 ± 2.9 mm/Ma. Considering stratigraphic variations (Figure 8B), most samples (DR14F, DR21F, DR38C, GO302D) present an increasing growth rate towards the most recent period, whereas only one crust exhibits a decreasing growth rate (DR19K) towards its top two layers. These trends highlight why bulk samples, which could not be divided in macro-layers, have a lower average growth rate as they might only represent the most recent growth period. Hydrothermal/diagenetic deposits are characterised by higher growth rates than typical hydrogenetic crusts [75]. Our data set reveals values of 18.3, 38, 64, 64.7, and 102 mm/Ma, for samples DW2482, DW778D, DW778D2, DW4998D, and DW778B, respectively consistent with a hydrothermally or diagenetically influenced growth. Considering an apparent mean thickness of crust samples, it is possible to extrapolate a period of oxide accumulation, which represents the time it would have taken the crusts to form assuming no hiatuses, and could be assimilated with great care as a minimal age of initiation of growth. Assuming the surface of each sample represents present-day, the minimal age of initiation of growth ranges from 0.79 to 34.3 Ma. It was not possible to obtain ages for samples showing no signs of stratigraphic polarity without substrate.

<figure><img src="image_12.png" /><figcaption>**Figure 8.** (A) Histogram of growth rates of New Caledonia's EEZ crusts samples using [66] equation; (B) Graph of the growth rate variation versus crust thickness of the samples with one or more sub-samples (n = 9).</figcaption></figure>
up to 0.69%, 1.17% and 0.26%, respectively. Other trace elements such as As, Ce, Cr, Mo, Pb, Nb, and Zr are depleted compared to usual hydrogenetic content.

<figure><img src="image_11.png" /><figcaption>**Figure 7.** PAAS-normalised REE plots for selected hydrogenetic crusts.</figcaption></figure>

4.4. *Growth Rates and Ages*

Estimated growth rates have been calculated using an empirical Co chronometer [66]. Growth rates vary from 0.45 to 102 mm/Ma (Figure 8) and show no geographic correlations. Bulk hydrogenetic crusts (n = 74) have a mean growth rate of 2.2 ± 2.5 mm/Ma. Growth rates for macro-layers sub-samples (n = 24) are on average higher, at 3.1 ± 2.9 mm/Ma. Considering stratigraphic variations (Figure 8B), most samples (DR14F, DR21F, DR38C, GO302D) present an increasing growth rate towards the most recent period, whereas only one crust exhibits a decreasing growth rate (DR19K) towards its top two layers. These trends highlight why bulk samples, which could not be divided in macro-layers, have a lower average growth rate as they might only represent the most recent growth period. Hydrothermal/diagenetic deposits are characterised by higher growth rates than typical hydrogenetic crusts [75]. Our data set reveals values of 18.3, 38, 64, 64.7, and 102 mm/Ma, for samples DW2482, DW778D, DW778D2, DW4998D, and DW778B, respectively consistent with a hydrothermally or diagenetically influenced growth. Considering an apparent mean thickness of crust samples, it is possible to extrapolate a period of oxide accumulation, which represents the time it would have taken the crusts to form assuming no hiatuses, and could be assimilated with great care as a minimal age of initiation of growth. Assuming the surface of each sample represents present-day, the minimal age of initiation of growth ranges from 0.79 to 34.3 Ma. It was not possible to obtain ages for samples showing no signs of stratigraphic polarity without substrate.

<figure><img src="image_12.png" /><figcaption>**Figure 8.** (A) Histogram of growth rates of New Caledonia's EEZ crusts samples using [66] equation; (B) Graph of the growth rate variation versus crust thickness of the samples with one or more sub-samples (n = 9).</figcaption></figure>
## 4.5. Element Correlations

A Pearson correlation coefficient matrix was calculated for the hydrogen genetic macro-layers and bulk crusts (n = 89). Bulk samples that have been subsampled were not considered in the data set. In addition to 27 elements, the analysis contains growth rates and the Fe/Mn ratios (Table 5). Based on statistically significant (CL > 99%) correlations and identified mineralogical phases by XRD, the statistical analysis reflects the major distribution of elements between four phases. A Mn oxide phase (δ-MnO₂) contains Mn, Mo, Sr, Tl, Pb, Co, La, Ni, V, As, Y, Nd, Nb, Zn, and an Fe-oxyhydroxide phase with Fe and Zr, whilst a biogenic phase accounts for Ba, Zn and Ce. Presence of calcite and fluorapatite identified in SEM images and XRD is consistent with the correlation of Ca, P and Pb. Aluminosilicate elements Si, Al, Na, K, Cu and Zr are negatively correlated with the δ-MnO₂ and Fe-oxyhydroxide phases, which is a common observation in other areas of the Pacific Ocean [1,67]. The distinction between a fluorapatite phase and a residual biogenic phase is not obvious. Only samples CP5069 and DN5080B contain fluorapatite, but no correlation has been found between Ca and principal biogenic elements (Ba, Ce and Zn). The weak correlation between Ni, P and Ca could be explained by the presence of 10 Å manganates associated with fluorapatite in CP5069.

<table><caption>Table 5. Pearson correlation coefficients matrix for the hydrogen genetic macro-layers and bulk crusts (n = 89) a.</caption><thead><tr><th></th><th>Mn</th><th>Fe</th><th>Si</th><th>Al</th><th>Mg</th><th>Na</th><th>Ca</th><th>K</th><th>Ti</th><th>P</th><th>As</th><th>Ba</th><th>Ce</th><th>Co</th><th>Cu</th><th>La</th></tr></thead><tbody><tr><td>Mn</td><td>1</td><td>-0.364</td><td>-0.916</td><td>-0.831</td><td>0.09</td><td>-0.507</td><td>0.335</td><td>-0.643</td><td>0.258</td><td>0.458</td><td>0.61</td><td>0.355</td><td>0.344</td><td>0.832</td><td>-0.177</td><td>0.702</td></tr><tr><td>Fe</td><td>-0.364</td><td>1</td><td>0.233</td><td>-0.013</td><td>-0.552</td><td>0.007</td><td>-0.519</td><td>-0.079</td><td>-0.135</td><td>-0.246</td><td>0.048</td><td>0.015</td><td>0.121</td><td>-0.37</td><td>-0.271</td><td>0.057</td></tr><tr><td>Si</td><td>-0.916</td><td>0.233</td><td>1</td><td>0.801</td><td>-0.127</td><td>0.485</td><td>-0.494</td><td>0.675</td><td>-0.21</td><td>-0.54</td><td>-0.699</td><td>-0.268</td><td>-0.269</td><td>-0.797</td><td>0.264</td><td>-0.661</td></tr><tr><td>Al</td><td>-0.831</td><td>-0.013</td><td>0.801</td><td>1</td><td>0.127</td><td>0.578</td><td>-0.174</td><td>0.833</td><td>-0.1</td><td>-0.441</td><td>-0.713</td><td>-0.433</td><td>-0.487</td><td>-0.68</td><td>0.345</td><td>-0.778</td></tr><tr><td>Mg</td><td>0.09</td><td>-0.552</td><td>-0.127</td><td>1</td><td>0.127</td><td>0.07</td><td>-0.077</td><td>0.42</td><td>0.061</td><td>0.109</td><td>0.083</td><td>-0.149</td><td>-0.24</td><td>-0.197</td><td>0.075</td><td>-0.267</td></tr><tr><td>Na</td><td>-0.507</td><td>0.007</td><td>0.485</td><td>0.578</td><td>-0.077</td><td>1</td><td>-0.228</td><td>0.523</td><td>0.103</td><td>-0.35</td><td>-0.636</td><td>-0.387</td><td>-0.409</td><td>-0.566</td><td>0.338</td><td>-0.379</td></tr><tr><td>Ca</td><td>0.335</td><td>-0.519</td><td>-0.494</td><td>-0.174</td><td>0.42</td><td>-0.228</td><td>1</td><td>-0.291</td><td>-0.075</td><td>0.691</td><td>0.469</td><td>-0.03</td><td>-0.109</td><td>0.377</td><td>0.221</td><td>0.055</td></tr><tr><td>K</td><td>-0.643</td><td>-0.079</td><td>0.675</td><td>0.833</td><td>0.061</td><td>0.523</td><td>-0.291</td><td>1</td><td>0.07</td><td>-0.434</td><td>-0.768</td><td>-0.186</td><td>-0.307</td><td>-0.583</td><td>0.554</td><td>-0.62</td></tr><tr><td>Ti</td><td>0.258</td><td>-0.135</td><td>-0.21</td><td>-0.1</td><td>-0.148</td><td>0.013</td><td>-0.075</td><td>0.07</td><td>1</td><td>-0.211</td><td>-0.307</td><td>0.324</td><td>0.373</td><td>0.347</td><td>0.255</td><td>0.44</td></tr><tr><td>P</td><td>0.458</td><td>-0.246</td><td>-0.54</td><td>-0.441</td><td>0.109</td><td>-0.35</td><td>0.691</td><td>-0.434</td><td>-0.211</td><td>1</td><td>0.63</td><td>0.269</td><td>0.1</td><td>0.342</td><td>-0.243</td><td>0.175</td></tr><tr><td>As</td><td>0.61</td><td>0.048</td><td>-0.699</td><td>-0.713</td><td>0.083</td><td>-0.636</td><td>0.469</td><td>-0.768</td><td>-0.307</td><td>0.63</td><td>1</td><td>0.189</td><td>0.25</td><td>0.602</td><td>-0.633</td><td>0.457</td></tr><tr><td>Ba</td><td>0.355</td><td>0.015</td><td>-0.268</td><td>-0.433</td><td>-0.149</td><td>-0.387</td><td>-0.03</td><td>-0.186</td><td>0.324</td><td>0.269</td><td>0.189</td><td>1</td><td>0.754</td><td>0.163</td><td>0.113</td><td>0.484</td></tr><tr><td>Ce</td><td>0.344</td><td>0.121</td><td>-0.269</td><td>-0.487</td><td>-0.24</td><td>-0.409</td><td>-0.109</td><td>-0.307</td><td>0.373</td><td>0.1</td><td>0.25</td><td>0.754</td><td>1</td><td>0.341</td><td>-0.012</td><td>0.619</td></tr><tr><td>Co</td><td>0.832</td><td>-0.37</td><td>-0.797</td><td>-0.68</td><td>0.197</td><td>-0.566</td><td>0.377</td><td>-0.583</td><td>0.347</td><td>0.342</td><td>0.602</td><td>0.163</td><td>0.341</td><td>1</td><td>-0.229</td><td>0.582</td></tr><tr><td>Cu</td><td>-0.177</td><td>-0.271</td><td>0.264</td><td>0.345</td><td>0.075</td><td>0.338</td><td>-0.221</td><td>0.554</td><td>0.255</td><td>-0.243</td><td>-0.633</td><td>0.113</td><td>-0.012</td><td>-0.229</td><td>-0.332</td><td>1</td></tr><tr><td>La</td><td>0.702</td><td>0.037</td><td>-0.661</td><td>-0.778</td><td>-0.267</td><td>-0.379</td><td>0.055</td><td>-0.62</td><td>0.44</td><td>0.175</td><td>0.457</td><td>0.484</td><td>0.619</td><td>0.582</td><td>-0.332</td><td>0.774</td></tr><tr><td>Mo</td><td>0.829</td><td>-0.11</td><td>-0.732</td><td>-0.855</td><td>-0.053</td><td>-0.552</td><td>0.154</td><td>-0.701</td><td>0.02</td><td>0.364</td><td>0.688</td><td>0.509</td><td>0.551</td><td>0.619</td><td>-0.366</td><td>0.508</td></tr><tr><td>Nb</td><td>0.48</td><td>-0.165</td><td>-0.485</td><td>-0.453</td><td>-0.011</td><td>-0.37</td><td>0.166</td><td>-0.324</td><td>0.697</td><td>0.108</td><td>0.187</td><td>0.329</td><td>0.398</td><td>0.68</td><td>0.042</td><td>0.308</td></tr><tr><td>Nd</td><td>0.403</td><td>0.159</td><td>-0.32</td><td>-0.526</td><td>-0.236</td><td>-0.26</td><td>-0.109</td><td>-0.447</td><td>0.28</td><td>0.023</td><td>0.28</td><td>0.401</td><td>0.611</td><td>0.293</td><td>-0.263</td><td>0.837</td></tr><tr><td>Ni</td><td>0.805</td><td>-0.64</td><td>-0.716</td><td>-0.549</td><td>0.486</td><td>-0.433</td><td>0.396</td><td>-0.399</td><td>0.091</td><td>0.395</td><td>0.462</td><td>0.206</td><td>0.173</td><td>0.754</td><td>0.073</td><td>0.302</td></tr><tr><td>Pb</td><td>0.82</td><td>-0.255</td><td>-0.804</td><td>-0.759</td><td>0.177</td><td>-0.624</td><td>0.366</td><td>-0.68</td><td>0.132</td><td>0.401</td><td>0.736</td><td>0.322</td><td>0.307</td><td>0.815</td><td>-0.359</td><td>0.596</td></tr><tr><td>Sr</td><td>0.891</td><td>-0.102</td><td>-0.894</td><td>-0.872</td><td>-0.08</td><td>-0.5</td><td>0.314</td><td>-0.754</td><td>0.186</td><td>0.435</td><td>0.736</td><td>0.404</td><td>0.393</td><td>0.75</td><td>-0.142</td><td>0.793</td></tr><tr><td>Tl</td><td>0.853</td><td>-0.529</td><td>-0.788</td><td>-0.625</td><td>0.238</td><td>-0.531</td><td>0.44</td><td>-0.721</td><td>0.332</td><td>0.445</td><td>0.506</td><td>0.417</td><td>0.403</td><td>0.819</td><td>-0.015</td><td>0.535</td></tr><tr><td>V</td><td>0.657</td><td>0.041</td><td>-0.681</td><td>-0.757</td><td>0.059</td><td>-0.608</td><td>0.247</td><td>-0.518</td><td>-0.205</td><td>-0.308</td><td>0.875</td><td>0.399</td><td>0.467</td><td>0.579</td><td>-0.46</td><td>0.563</td></tr><tr><td>Y</td><td>0.533</td><td>0.03</td><td>-0.605</td><td>-0.562</td><td>0.195</td><td>-0.308</td><td>0.195</td><td>-0.518</td><td>0.083</td><td>0.343</td><td>0.515</td><td>-0.057</td><td>-0.033</td><td>0.484</td><td>-0.561</td><td>0.542</td></tr><tr><td>Zn</td><td>0.514</td><td>-0.274</td><td>-0.472</td><td>-0.505</td><td>0.209</td><td>-0.387</td><td>0.175</td><td>-0.28</td><td>0.11</td><td>0.404</td><td>0.325</td><td>0.675</td><td>0.434</td><td>0.364</td><td>0.117</td><td>0.344</td></tr><tr><td>Zr</td><td>-0.426</td><td>0.455</td><td>0.461</td><td>0.259</td><td>-0.429</td><td>0.352</td><td>-0.589</td><td>0.35</td><td>0.339</td><td>-0.451</td><td>-0.636</td><td>0.103</td><td>0.055</td><td>-0.501</td><td>0.316</td><td>-0.019</td></tr><tr><td>Fe/Mn</td><td>-0.935</td><td>0.553</td><td>0.842</td><td>0.735</td><td>-0.182</td><td>0.357</td><td>-0.37</td><td>0.5</td><td>-0.298</td><td>-0.436</td><td>-0.439</td><td>-0.336</td><td>-0.274</td><td>-0.758</td><td>-0.027</td><td>-0.591</td></tr><tr><td>Growth rate</td><td>-0.616</td><td>0.33</td><td>0.57</td><td>0.519</td><td>-0.194</td><td>0.554</td><td>-0.297</td><td>0.489</td><td>-0.339</td><td>-0.233</td><td>-0.422</td><td>-0.116</td><td>-0.215</td><td>-0.794</td><td>0.134</td><td>-0.368</td></tr><tr><td>Mn</td><td>0.829</td><td>0.48</td><td>0.403</td><td>0.805</td><td>0.82</td><td>0.891</td><td>0.853</td><td>0.657</td><td>0.533</td><td>0.514</td><td>-0.426</td><td>-0.935</td><td>-0.935</td><td>-0.616</td><td>0.33</td><td>-0.616</td></tr><tr><td>Fe</td><td>-0.11</td><td>-0.165</td><td>0.159</td><td>-0.64</td><td>-0.255</td><td>-0.102</td><td>-0.529</td><td>0.041</td><td>0.03</td><td>-0.274</td><td>-0.455</td><td>0.553</td><td>0.455</td><td>0.553</td><td>0.57</td><td>0.57</td></tr><tr><td>Si</td><td>-0.732</td><td>-0.485</td><td>-0.32</td><td>-0.716</td><td>-0.804</td><td>-0.894</td><td>-0.788</td><td>-0.681</td><td>-0.757</td><td>-0.605</td><td>-0.472</td><td>0.461</td><td>0.842</td><td>0.461</td><td>0.57</td><td>0.57</td></tr><tr><td>Al</td><td>-0.855</td><td>-0.453</td><td>-0.526</td><td>-0.549</td><td>-0.759</td><td>-0.872</td><td>-0.625</td><td>-0.569</td><td>-0.562</td><td>-0.505</td><td>-0.259</td><td>0.735</td><td>0.735</td><td>0.735</td><td>0.519</td><td>0.519</td></tr><tr><td>Mg</td><td>-0.053</td><td>-0.011</td><td>-0.236</td><td>0.486</td><td>0.177</td><td>-0.08</td><td>0.238</td><td>0.059</td><td>0.058</td><td>0.209</td><td>-0.429</td><td>-0.182</td><td>-0.194</td><td>-0.194</td><td>-0.194</td><td>-0.194</td></tr><tr><td>Na</td><td>-0.552</td><td>-0.37</td><td>-0.26</td><td>-0.433</td><td>-0.624</td><td>-0.5</td><td>-0.531</td><td>-0.608</td><td>-0.308</td><td>-0.387</td><td>0.352</td><td>0.357</td><td>-0.357</td><td>-0.46</td><td>-0.554</td><td>-0.554</td></tr><tr><td>Ca</td><td>0.154</td><td>0.166</td><td>-0.109</td><td>0.396</td><td>0.366</td><td>0.314</td><td>0.44</td><td>0.247</td><td>0.195</td><td>0.175</td><td>-0.589</td><td>-0.37</td><td>-0.297</td><td>-0.37</td><td>-0.297</td><td>-0.297</td></tr><tr><td>K</td><td>-0.701</td><td>-0.324</td><td>-0.447</td><td>-0.399</td><td>-0.68</td><td>-0.754</td><td>-0.457</td><td>-0.721</td><td>-0.518</td><td>-0.28</td><td>0.35</td><td>0.5</td><td>0.489</td><td>0.489</td><td>0.399</td><td>0.489</td></tr><tr><td>Ti</td><td>0.02</td><td>0.697</td><td>0.28</td><td>0.091</td><td>0.132</td><td>0.186</td><td>0.332</td><td>-0.205</td><td>0.083</td><td>0.11</td><td>0.339</td><td>-0.298</td><td>-0.298</td><td>-0.339</td><td>-0.233</td><td>-0.233</td></tr><tr><td>P</td><td>0.364</td><td>0.108</td><td>0.023</td><td>0.395</td><td>0.401</td><td>0.435</td><td>0.445</td><td>0.419</td><td>0.343</td><td>0.404</td><td>0.404</td><td>-0.451</td><td>-0.436</td><td>-0.436</td><td>-0.436</td><td>-0.436</td></tr><tr><td>As</td><td>0.688</td><td>0.187</td><td>0.28</td><td>0.462</td><td>0.736</td><td>0.736</td><td>0.506</td><td>0.875</td><td>0.515</td><td>0.325</td><td>0.636</td><td>-0.439</td><td>-0.439</td><td>-0.439</td><td>-0.422</td><td>-0.422</td></tr><tr><td>Ba</td><td>0.509</td><td>0.329</td><td>0.401</td><td>0.206</td><td>0.322</td><td>0.404</td><td>0.417</td><td>0.399</td><td>-0.057</td><td>0.675</td><td>0.103</td><td>0.336</td><td>0.103</td><td>0.336</td><td>-0.116</td><td>-0.116</td></tr><tr><td>Ce</td><td>0.551</td><td>0.398</td><td>0.611</td><td>0.173</td><td>0.307</td><td>0.393</td><td>0.403</td><td>0.467</td><td>-0.033</td><td>0.434</td><td>0.055</td><td>-0.274</td><td>-0.274</td><td>-0.274</td><td>-0.215</td><td>-0.215</td></tr><tr><td>Co</td><td>0.619</td><td>0.68</td><td>0.293</td><td>0.754</td><td>0.815</td><td>0.75</td><td>0.819</td><td>0.579</td><td>0.484</td><td>0.364</td><td>-0.501</td><td>-0.758</td><td>-0.758</td><td>-0.758</td><td>-0.794</td><td>-0.794</td></tr><tr><td>Cu</td><td>-0.366</td><td>0.042</td><td>-0.263</td><td>0.073</td><td>-0.359</td><td>-0.42</td><td>-0.015</td><td>-0.46</td><td>-0.561</td><td>0.117</td><td>0.316</td><td>-0.027</td><td>-0.027</td><td>-0.027</td><td>0.134</td><td>0.134</td></tr><tr><td>La</td><td>0.774</td><td>0.508</td><td>0.837</td><td>0.073</td><td>0.596</td><td>0.793</td><td>0.535</td><td>0.563</td><td>0.542</td><td>0.344</td><td>-0.019</td><td>-0.591</td><td>-0.591</td><td>-0.591</td><td>-0.638</td><td>-0.638</td></tr><tr><td>Mo</td><td>1</td><td>0.247</td><td>0.594</td><td>0.568</td><td>0.763</td><td>0.871</td><td>0.674</td><td>0.841</td><td>0.446</td><td>0.52</td><td>-0.399</td><td>-0.702</td><td>-0.702</td><td>-0.399</td><td>-0.399</td><td>-0.399</td></tr><tr><td>Nb</td><td>0.247</td><td>1</td><td>0.243</td><td>0.366</td><td>0.554</td><td>0.469</td><td>0.554</td><td>0.197</td><td>0.295</td><td>0.356</td><td>0.045</td><td>-0.489</td><td>-0.489</td><td>-0.489</td><td>-0.595</td><td>-0.595</td></tr><tr><td>Nd</td><td>0.594</td><td>0.243</td><td>1</td><td>0.07</td><td>0.293</td><td>0.455</td><td>0.248</td><td>0.388</td><td>0.298</td><td>0.146</td><td>0.125</td><td>-0.295</td><td>-0.295</td><td>-0.185</td><td>-0.185</td><td>-0.185</td></tr><tr><td>Ni</td><td>0.568</td><td>0.366</td><td>0.07</td><td>1</td><td>0.684</td><td>0.584</td><td>0.844</td><td>0.53</td><td>0.242</td><td>0.581</td><td>-0.581</td><td>-0.581</td><td>-0.859</td><td>-0.859</td><td>-0.586</td><td>-0.586</td></tr><tr><td>Pb</td><td>0.763</td><td>0.554</td><td>0.293</td><td>0.684</td><td>1</td><td>0.866</td><td>0.775</td><td>0.793</td><td>0.492</td><td>0.515</td><td>0.578</td><td>-0.378</td><td>-0.71</td><td>-0.609</td><td>-0.609</td><td>-0.609</td></tr><tr><td>Sr</td><td>0.871</td><td>0.469</td><td>0.455</td><td>0.584</td><td>0.866</td><td>1</td><td>0.709</td><td>0.789</td><td>0.624</td><td>0.488</td><td>-0.394</td><td>-0.77</td><td>-0.77</td><td>-0.651</td><td>-0.651</td><td>-0.651</td></tr><tr><td>Tl</td><td>0.674</td><td>0.554</td><td>0.248</td><td>0.844</td><td>0.775</td><td>0.709</td><td>1</td><td>0.587</td><td>0.252</td><td>0.572</td><td>-0.556</td><td>-0.84</td><td>-0.84</td><td>-0.84</td><td>-0.84</td><td>-0.84</td></tr><tr><td>V</td><td>0.841</td><td>0.197</td><td>0.388</td><td>0.53</td><td>0.793</td><td>0.492</td><td>0.789</td><td>0.587</td><td>0.371</td><td>0.487</td><td>-0.581</td><td>-0.581</td><td>-0.581</td><td>-0.581</td><td>-0.581</td><td>-0.581</td></tr><tr><td>Y</td><td>0.446</td><td>0.295</td><td>0.298</td><td>0.242</td><td>0.624</td><td>0.624</td><td>0.252</td><td>1</td><td>1</td><td>1</td><td>-0.133</td><td>-0.43</td><td>-0.133</td><td>-0.133</td><td>-0.302</td><td>-0.302</td></tr><tr><td>Zn</td><td>0.52</td><td>0.356</td><td>0.581</td><td>0.515</td><td>0.488</td><td>0.572</td><td>0.572</td><td>0.487</td><td>0.098</td><td>0.192</td><td>-0.192</td><td>-0.554</td><td>-0.554</td><td>-0.554</td><td>-0.554</td><td>-0.554</td></tr><tr><td>Zr</td><td>-0.399</td><td>0.045</td><td>0.125</td><td>-0.581</td><td>-0.578</td><td>-0.394</td><td>-0.556</td><td>-0.581</td><td>-0.133</td><td>-0.192</td><td>1</td><td>0.401</td><td>1</td><td>0.401</td><td>0.356</td><td>0.356</td></tr><tr><td>Fe/Mn</td><td>-0.702</td><td>-0.489</td><td>-0.295</td><td>-0.859</td><td>-0.71</td><td>-0.77</td><td>-0.84</td><td>-0.506</td><td>-0.43</td><td>-0.554</td><td>0.401</td><td>1</td><td>1</td><td>0.401</td><td>0.579</td><td>0.579</td></tr><tr><td>Growth rate</td><td>-0.399</td><td>-0.595</td><td>-0.185</td><td>-0.586</td><td>-0.609</td><td>-0.511</td><td>-0.651</td><td>-0.351</td><td>-0.351</td><td>-0.351</td><td>-0.351</td><td>-0.351</td><td>-0.351</td><td>-0.351</td><td>-0.351</td><td>-0.351</td></tr></tbody></table>

<footnote>a Bold values represent correlations at the 99% confidence level.</footnote>
## 4.5. Element Correlations

A Pearson correlation coefficient matrix was calculated for the hydrogen genetic macro-layers and bulk crusts (n = 89). Bulk samples that have been subsampled were not considered in the data set. In addition to 27 elements, the analysis contains growth rates and the Fe/Mn ratios (Table 5). Based on statistically significant (CL > 99%) correlations and identified mineralogical phases by XRD, the statistical analysis reflects the major distribution of elements between four phases. A Mn oxide phase (δ-MnO₂) contains Mn, Mo, Sr, Tl, Pb, Co, La, Ni, V, As, Y, Nd, Nb, Zn, and an Fe-oxyhydroxide phase with Fe and Zr, whilst a biogenic phase accounts for Ba, Zn and Ce. Presence of calcite and fluorapatite identified in SEM images and XRD is consistent with the correlation of Ca, P and Pb. Aluminosilicate elements Si, Al, Na, K, Cu and Zr are negatively correlated with the δ-MnO₂ and Fe-oxyhydroxide phases, which is a common observation in other areas of the Pacific Ocean [1,67]. The distinction between a fluorapatite phase and a residual biogenic phase is not obvious. Only samples CP5069 and DN5080B contain fluorapatite, but no correlation has been found between Ca and principal biogenic elements (Ba, Ce and Zn). The weak correlation between Ni, P and Ca could be explained by the presence of 10 Å manganates associated with fluorapatite in CP5069.

<table><caption>Table 5. Pearson correlation coefficients matrix for the hydrogen genetic macro-layers and bulk crusts (n = 89) a.</caption><thead><tr><th></th><th>Mn</th><th>Fe</th><th>Si</th><th>Al</th><th>Mg</th><th>Na</th><th>Ca</th><th>K</th><th>Ti</th><th>P</th><th>As</th><th>Ba</th><th>Ce</th><th>Co</th><th>Cu</th><th>La</th></tr></thead><tbody><tr><td>Mn</td><td>1</td><td>-0.364</td><td>-0.916</td><td>-0.831</td><td>0.09</td><td>-0.507</td><td>0.335</td><td>-0.643</td><td>0.258</td><td>0.458</td><td>0.61</td><td>0.355</td><td>0.344</td><td>0.832</td><td>-0.177</td><td>0.702</td></tr><tr><td>Fe</td><td>-0.364</td><td>1</td><td>0.233</td><td>-0.013</td><td>-0.552</td><td>0.007</td><td>-0.519</td><td>-0.079</td><td>-0.135</td><td>-0.246</td><td>0.048</td><td>0.015</td><td>0.121</td><td>-0.37</td><td>-0.271</td><td>0.057</td></tr><tr><td>Si</td><td>-0.916</td><td>0.233</td><td>1</td><td>0.801</td><td>-0.127</td><td>0.485</td><td>-0.494</td><td>0.675</td><td>-0.21</td><td>-0.54</td><td>-0.699</td><td>-0.268</td><td>-0.269</td><td>-0.797</td><td>0.264</td><td>-0.661</td></tr><tr><td>Al</td><td>-0.831</td><td>-0.013</td><td>0.801</td><td>1</td><td>0.127</td><td>0.578</td><td>-0.174</td><td>0.833</td><td>-0.1</td><td>-0.441</td><td>-0.713</td><td>-0.433</td><td>-0.487</td><td>-0.68</td><td>0.345</td><td>-0.778</td></tr><tr><td>Mg</td><td>0.09</td><td>-0.552</td><td>-0.127</td><td>1</td><td>0.127</td><td>0.07</td><td>-0.07</td><td>0.42</td><td>0.061</td><td>0.109</td><td>0.083</td><td>-0.149</td><td>-0.24</td><td>-0.197</td><td>0.075</td><td>-0.267</td></tr><tr><td>Na</td><td>-0.507</td><td>0.007</td><td>0.485</td><td>0.578</td><td>-0.077</td><td>1</td><td>-0.228</td><td>0.523</td><td>0.013</td><td>-0.35</td><td>-0.636</td><td>-0.387</td><td>-0.409</td><td>-0.566</td><td>0.338</td><td>-0.379</td></tr><tr><td>Ca</td><td>0.335</td><td>-0.519</td><td>-0.494</td><td>-0.174</td><td>0.42</td><td>-0.228</td><td>1</td><td>-0.291</td><td>-0.075</td><td>0.691</td><td>0.469</td><td>-0.03</td><td>-0.109</td><td>0.377</td><td>0.221</td><td>0.055</td></tr><tr><td>K</td><td>-0.643</td><td>-0.079</td><td>0.675</td><td>0.833</td><td>0.061</td><td>0.523</td><td>-0.291</td><td>1</td><td>0.07</td><td>-0.434</td><td>-0.768</td><td>-0.186</td><td>-0.307</td><td>-0.583</td><td>0.554</td><td>-0.62</td></tr><tr><td>Ti</td><td>0.258</td><td>-0.135</td><td>-0.21</td><td>-0.1</td><td>-0.148</td><td>0.013</td><td>-0.075</td><td>0.07</td><td>1</td><td>-0.211</td><td>-0.307</td><td>0.324</td><td>0.373</td><td>0.347</td><td>0.255</td><td>0.44</td></tr><tr><td>P</td><td>0.458</td><td>-0.246</td><td>-0.54</td><td>-0.441</td><td>0.109</td><td>-0.35</td><td>0.691</td><td>-0.434</td><td>-0.211</td><td>1</td><td>0.63</td><td>0.269</td><td>0.1</td><td>0.342</td><td>-0.243</td><td>0.175</td></tr><tr><td>As</td><td>0.61</td><td>0.048</td><td>-0.699</td><td>-0.713</td><td>0.083</td><td>-0.636</td><td>0.469</td><td>-0.768</td><td>-0.307</td><td>0.63</td><td>1</td><td>0.189</td><td>0.25</td><td>0.602</td><td>-0.633</td><td>0.457</td></tr><tr><td>Ba</td><td>0.355</td><td>0.015</td><td>-0.268</td><td>-0.433</td><td>-0.149</td><td>-0.387</td><td>-0.03</td><td>-0.186</td><td>0.324</td><td>0.269</td><td>0.189</td><td>1</td><td>0.754</td><td>0.163</td><td>0.113</td><td>0.484</td></tr><tr><td>Ce</td><td>0.344</td><td>0.121</td><td>-0.269</td><td>-0.487</td><td>-0.24</td><td>-0.409</td><td>-0.109</td><td>-0.307</td><td>0.373</td><td>0.1</td><td>0.25</td><td>0.754</td><td>1</td><td>0.341</td><td>-0.012</td><td>0.619</td></tr><tr><td>Co</td><td>0.832</td><td>-0.37</td><td>-0.797</td><td>-0.68</td><td>0.197</td><td>-0.566</td><td>0.377</td><td>-0.583</td><td>0.347</td><td>0.342</td><td>0.602</td><td>0.163</td><td>0.341</td><td>1</td><td>-0.229</td><td>0.582</td></tr><tr><td>Cu</td><td>-0.177</td><td>-0.271</td><td>0.264</td><td>0.345</td><td>0.075</td><td>0.338</td><td>-0.221</td><td>0.554</td><td>0.255</td><td>-0.243</td><td>-0.633</td><td>0.113</td><td>-0.012</td><td>-0.229</td><td>-0.332</td><td>1</td></tr><tr><td>La</td><td>0.702</td><td>0.037</td><td>-0.661</td><td>-0.778</td><td>-0.267</td><td>-0.379</td><td>0.055</td><td>-0.62</td><td>0.44</td><td>0.175</td><td>0.457</td><td>0.484</td><td>0.619</td><td>0.582</td><td>-0.332</td><td>0.774</td></tr><tr><td>Mo</td><td>0.829</td><td>-0.11</td><td>-0.732</td><td>-0.855</td><td>-0.053</td><td>-0.552</td><td>0.154</td><td>-0.701</td><td>0.02</td><td>0.364</td><td>0.688</td><td>0.509</td><td>0.551</td><td>0.619</td><td>-0.366</td><td>0.508</td></tr><tr><td>Nb</td><td>0.48</td><td>-0.165</td><td>-0.485</td><td>-0.453</td><td>-0.011</td><td>-0.37</td><td>0.166</td><td>-0.324</td><td>0.697</td><td>0.108</td><td>0.187</td><td>0.329</td><td>0.398</td><td>0.68</td><td>0.042</td><td>0.308</td></tr><tr><td>Nd</td><td>0.403</td><td>0.159</td><td>-0.32</td><td>-0.526</td><td>-0.236</td><td>-0.26</td><td>-0.109</td><td>-0.447</td><td>0.28</td><td>0.023</td><td>0.28</td><td>0.401</td><td>0.611</td><td>0.293</td><td>-0.263</td><td>0.837</td></tr><tr><td>Ni</td><td>0.805</td><td>-0.64</td><td>-0.716</td><td>-0.549</td><td>0.486</td><td>-0.433</td><td>0.396</td><td>-0.399</td><td>0.091</td><td>0.395</td><td>0.462</td><td>0.206</td><td>0.173</td><td>0.754</td><td>0.073</td><td>0.302</td></tr><tr><td>Pb</td><td>0.82</td><td>-0.255</td><td>-0.804</td><td>-0.759</td><td>0.177</td><td>-0.624</td><td>0.366</td><td>-0.68</td><td>0.132</td><td>0.401</td><td>0.736</td><td>0.322</td><td>0.307</td><td>0.815</td><td>-0.359</td><td>0.596</td></tr><tr><td>Sr</td><td>0.891</td><td>-0.102</td><td>-0.894</td><td>-0.872</td><td>-0.08</td><td>-0.5</td><td>0.314</td><td>-0.754</td><td>0.186</td><td>0.435</td><td>0.736</td><td>0.404</td><td>0.393</td><td>0.75</td><td>-0.12</td><td>0.793</td></tr><tr><td>Tl</td><td>0.853</td><td>-0.529</td><td>-0.788</td><td>-0.625</td><td>0.238</td><td>-0.531</td><td>0.44</td><td>-0.721</td><td>0.332</td><td>0.445</td><td>0.506</td><td>0.417</td><td>0.403</td><td>0.819</td><td>-0.015</td><td>0.535</td></tr><tr><td>V</td><td>0.657</td><td>0.041</td><td>-0.681</td><td>-0.757</td><td>0.059</td><td>-0.608</td><td>0.247</td><td>-0.518</td><td>-0.205</td><td>-0.308</td><td>0.875</td><td>0.399</td><td>0.467</td><td>0.579</td><td>-0.46</td><td>0.563</td></tr><tr><td>Y</td><td>0.533</td><td>0.03</td><td>-0.605</td><td>-0.562</td><td>0.195</td><td>-0.308</td><td>0.195</td><td>-0.518</td><td>0.083</td><td>0.343</td><td>0.515</td><td>-0.057</td><td>-0.033</td><td>0.484</td><td>-0.561</td><td>0.542</td></tr><tr><td>Zn</td><td>0.514</td><td>-0.274</td><td>-0.472</td><td>-0.505</td><td>0.209</td><td>-0.387</td><td>0.175</td><td>-0.28</td><td>0.11</td><td>0.404</td><td>0.325</td><td>0.675</td><td>0.434</td><td>0.364</td><td>0.344</td><td>0.344</td></tr><tr><td>Zr</td><td>-0.426</td><td>0.455</td><td>0.461</td><td>0.259</td><td>-0.429</td><td>0.352</td><td>-0.589</td><td>0.35</td><td>0.339</td><td>-0.451</td><td>-0.636</td><td>0.103</td><td>0.055</td><td>-0.501</td><td>0.316</td><td>-0.019</td></tr><tr><td>Fe/Mn</td><td>-0.935</td><td>0.553</td><td>0.842</td><td>0.735</td><td>-0.182</td><td>0.357</td><td>-0.37</td><td>0.5</td><td>-0.298</td><td>-0.436</td><td>-0.439</td><td>-0.336</td><td>-0.274</td><td>-0.758</td><td>-0.027</td><td>-0.591</td></tr><tr><td>Growth rate</td><td>-0.616</td><td>0.33</td><td>0.57</td><td>0.519</td><td>-0.194</td><td>0.554</td><td>-0.297</td><td>0.489</td><td>-0.339</td><td>-0.233</td><td>-0.422</td><td>-0.116</td><td>-0.215</td><td>-0.794</td><td>0.134</td><td>-0.368</td></tr><tr><td>Mn</td><td>0.829</td><td>0.48</td><td>0.403</td><td>0.805</td><td>0.82</td><td>0.891</td><td>0.853</td><td>0.657</td><td>0.533</td><td>0.514</td><td>-0.426</td><td>-0.935</td><td>-0.19</td><td>-0.935</td><td>-0.616</td><td>0.33</td></tr><tr><td>Fe</td><td>-0.11</td><td>-0.165</td><td>0.159</td><td>-0.64</td><td>-0.255</td><td>-0.102</td><td>-0.529</td><td>0.041</td><td>0.03</td><td>-0.274</td><td>-0.455</td><td>0.553</td><td>-0.455</td><td>-0.455</td><td>-0.616</td><td>0.57</td></tr><tr><td>Si</td><td>-0.732</td><td>-0.485</td><td>-0.32</td><td>-0.716</td><td>-0.804</td><td>-0.894</td><td>-0.788</td><td>-0.681</td><td>-0.681</td><td>-0.605</td><td>-0.472</td><td>0.842</td><td>-0.461</td><td>-0.461</td><td>-0.461</td><td>0.57</td></tr><tr><td>Al</td><td>-0.855</td><td>-0.453</td><td>-0.526</td><td>-0.549</td><td>-0.759</td><td>-0.872</td><td>-0.625</td><td>-0.757</td><td>-0.562</td><td>-0.505</td><td>-0.505</td><td>0.259</td><td>0.735</td><td>-0.182</td><td>-0.194</td><td>0.519</td></tr><tr><td>Mg</td><td>-0.053</td><td>-0.011</td><td>-0.236</td><td>0.486</td><td>0.177</td><td>-0.08</td><td>0.238</td><td>0.059</td><td>0.058</td><td>0.209</td><td>-0.429</td><td>-0.352</td><td>-0.352</td><td>-0.357</td><td>-0.554</td><td>0.554</td></tr><tr><td>Na</td><td>-0.552</td><td>-0.37</td><td>-0.26</td><td>-0.433</td><td>-0.624</td><td>-0.5</td><td>-0.531</td><td>-0.608</td><td>-0.308</td><td>-0.387</td><td>-0.387</td><td>-0.387</td><td>-0.387</td><td>-0.387</td><td>-0.387</td><td>0.535</td></tr><tr><td>Ca</td><td>0.154</td><td>0.166</td><td>-0.109</td><td>0.396</td><td>0.366</td><td>0.314</td><td>0.44</td><td>0.247</td><td>0.195</td><td>0.175</td><td>-0.589</td><td>-0.37</td><td>-0.012</td><td>-0.297</td><td>-0.297</td><td>0.489</td></tr><tr><td>K</td><td>-0.701</td><td>-0.324</td><td>-0.447</td><td>-0.399</td><td>-0.68</td><td>-0.754</td><td>-0.457</td><td>-0.721</td><td>-0.518</td><td>-0.28</td><td>-0.35</td><td>0.5</td><td>0.35</td><td>0.5</td><td>0.35</td><td>0.35</td></tr><tr><td>Ti</td><td>0.02</td><td>0.697</td><td>0.28</td><td>0.091</td><td>0.132</td><td>0.186</td><td>0.332</td><td>-0.205</td><td>0.083</td><td>0.11</td><td>0.339</td><td>-0.298</td><td>-0.298</td><td>-0.298</td><td>-0.339</td><td>-0.339</td></tr><tr><td>P</td><td>0.364</td><td>0.108</td><td>0.023</td><td>0.395</td><td>0.401</td><td>0.435</td><td>0.445</td><td>0.419</td><td>0.343</td><td>0.404</td><td>0.404</td><td>-0.451</td><td>-0.436</td><td>-0.436</td><td>-0.233</td><td>-0.233</td></tr><tr><td>As</td><td>0.688</td><td>0.187</td><td>0.28</td><td>0.462</td><td>0.736</td><td>0.736</td><td>0.506</td><td>0.875</td><td>0.515</td><td>0.325</td><td>0.636</td><td>-0.439</td><td>-0.439</td><td>-0.439</td><td>-0.422</td><td>-0.422</td></tr><tr><td>Ba</td><td>0.509</td><td>0.329</td><td>0.401</td><td>0.206</td><td>0.322</td><td>0.404</td><td>0.417</td><td>0.399</td><td>-0.057</td><td>0.675</td><td>0.103</td><td>-0.336</td><td>-0.336</td><td>-0.336</td><td>-0.116</td><td>-0.116</td></tr><tr><td>Ce</td><td>0.551</td><td>0.398</td><td>0.611</td><td>0.173</td><td>0.307</td><td>0.393</td><td>0.403</td><td>0.467</td><td>-0.033</td><td>0.434</td><td>0.055</td><td>-0.274</td><td>-0.274</td><td>-0.274</td><td>-0.215</td><td>-0.215</td></tr><tr><td>Co</td><td>0.619</td><td>0.68</td><td>0.293</td><td>0.754</td><td>0.815</td><td>0.75</td><td>0.819</td><td>0.579</td><td>0.484</td><td>0.364</td><td>-0.501</td><td>-0.758</td><td>-0.758</td><td>-0.758</td><td>-0.794</td><td>-0.794</td></tr><tr><td>Cu</td><td>-0.366</td><td>0.042</td><td>-0.263</td><td>0.073</td><td>-0.359</td><td>-0.42</td><td>-0.015</td><td>-0.46</td><td>-0.561</td><td>0.117</td><td>0.316</td><td>-0.027</td><td>-0.027</td><td>-0.027</td><td>-0.134</td><td>-0.134</td></tr><tr><td>La</td><td>0.774</td><td>0.508</td><td>0.837</td><td>0.073</td><td>0.596</td><td>0.793</td><td>0.535</td><td>0.563</td><td>0.542</td><td>0.344</td><td>-0.019</td><td>-0.591</td><td>-0.591</td><td>-0.591</td><td>-0.638</td><td>0.591</td></tr><tr><td>Mo</td><td>1</td><td>0.247</td><td>0.594</td><td>0.568</td><td>0.763</td><td>0.871</td><td>0.674</td><td>0.841</td><td>0.446</td><td>0.52</td><td>-0.399</td><td>-0.702</td><td>-0.702</td><td>-0.399</td><td>-0.399</td><td>0.399</td></tr><tr><td>Nb</td><td>0.247</td><td>1</td><td>0.243</td><td>0.366</td><td>0.554</td><td>0.469</td><td>0.554</td><td>0.197</td><td>0.295</td><td>0.356</td><td>0.045</td><td>-0.489</td><td>-0.489</td><td>-0.489</td><td>-0.595</td><td>-0.595</td></tr><tr><td>Nd</td><td>0.594</td><td>0.243</td><td>1</td><td>0.07</td><td>0.293</td><td>0.455</td><td>0.248</td><td>0.388</td><td>0.298</td><td>0.146</td><td>0.125</td><td>-0.295</td><td>-0.295</td><td>-0.295</td><td>-0.185</td><td>-0.185</td></tr><tr><td>Ni</td><td>0.568</td><td>0.366</td><td>0.07</td><td>1</td><td>0.684</td><td>0.584</td><td>0.844</td><td>0.53</td><td>0.242</td><td>0.581</td><td>-0.581</td><td>-0.581</td><td>-0.859</td><td>-0.859</td><td>-0.586</td><td>-0.586</td></tr><tr><td>Pb</td><td>0.763</td><td>0.554</td><td>0.293</td><td>0.684</td><td>1</td><td>0.866</td><td>0.775</td><td>0.793</td><td>0.492</td><td>0.515</td><td>0.578</td><td>-0.378</td><td>-0.71</td><td>-0.71</td><td>-0.609</td><td>-0.609</td></tr><tr><td>Sr</td><td>0.871</td><td>0.469</td><td>0.455</td><td>0.584</td><td>0.866</td><td>1</td><td>0.709</td><td>0.789</td><td>0.624</td><td>0.488</td><td>-0.394</td><td>-0.77</td><td>-0.77</td><td>-0.77</td><td>-0.511</td><td>-0.511</td></tr><tr><td>Tl</td><td>0.674</td><td>0.554</td><td>0.248</td><td>0.844</td><td>0.775</td><td>0.709</td><td>1</td><td>0.587</td><td>0.252</td><td>0.572</td><td>-0.556</td><td>-0.84</td><td>-0.84</td><td>-0.84</td><td>-0.651</td><td>-0.651</td></tr><tr><td>V</td><td>0.841</td><td>0.197</td><td>0.388</td><td>0.53</td><td>0.793</td><td>0.492</td><td>0.789</td><td>0.587</td><td>0.371</td><td>0.487</td><td>-0.581</td><td>-0.581</td><td>-0.581</td><td>-0.581</td><td>-0.581</td><td>-0.581</td></tr><tr><td>Y</td><td>0.446</td><td>0.295</td><td>0.298</td><td>0.242</td><td>0.624</td><td>0.624</td><td>0.252</td><td>1</td><td>0.1</td><td>0.098</td><td>-0.133</td><td>-0.133</td><td>-0.133</td><td>-0.133</td><td>-0.133</td><td>-0.133</td></tr><tr><td>Zn</td><td>0.52</td><td>0.356</td><td>0.581</td><td>0.515</td><td>0.488</td><td>0.572</td><td>0.572</td><td>0.487</td><td>0.098</td><td>1</td><td>-0.192</td><td>-0.192</td><td>-0.192</td><td>-0.192</td><td>-0.192</td><td>-0.192</td></tr><tr><td>Zr</td><td>-0.399</td><td>0.045</td><td>0.125</td><td>-0.581</td><td>-0.578</td><td>-0.394</td><td>-0.556</td><td>-0.581</td><td>-0.133</td><td>-0.192</td><td>1</td><td>0.401</td><td>1</td><td>0.401</td><td>0.356</td><td>0.356</td></tr><tr><td>Fe/Mn</td><td>-0.702</td><td>-0.489</td><td>-0.295</td><td>-0.859</td><td>-0.71</td><td>-0.77</td><td>-0.84</td><td>-0.506</td><td>-0.43</td><td>-0.554</td><td>0.401</td><td>1</td><td>1</td><td>0.401</td><td>0.579</td><td>0.579</td></tr><tr><td>Growth rate</td><td>-0.399</td><td>-0.595</td><td>-0.185</td><td>-0.586</td><td>-0.609</td><td>-0.511</td><td>-0.651</td><td>-0.351</td><td>-0.351</td><td>-0.351</td><td>-0.351</td><td>-0.351</td><td>-0.351</td><td>-0.351</td><td>-0.351</td><td>-0.351</td></tr></tbody></table>

<footnote>a Bold values represent correlations at the 99% confidence level.</footnote>
## 4.6. Factor Analysis

A factor analysis was performed for the 89 hydrogenetic macro-layers and bulk crusts. Four significant factors explain 75% of the variance in the data set (Figure 9). Factor 1 is interpreted as δ-MnO₂ and accounts for 45.2% of the variance, factor 2 as Fe-oxyhydroxides and accounts for 12.8%, factor 3 as a Fe (+As) dominated phase accounting for 10.3%, and factor 4 as a Ti phase accounting for 5.9%. For each factor, elements with the highest scores are: δ-MnO₂: Sr, Mn, Pb, Mo, Co, Tl, V, As, La, Ni, Zn, Y, P, Nb; Fe-oxyhydroxides: Zr, Ce, Fe, Nd; Fe (+As) phase: negatively correlated to Cu and Ti; Ti phase: negatively correlated to Ba.

<figure><img src="image_13.png" /><figcaption>**Figure 9.** Histograms of factor scores for the four factors (computed for the hydrogenetic bulk and macro-layers samples, *n* = 89).</figcaption></figure>

Mineral associations and phases determined using factor analysis present important discrepancies compared to phases obtained with correlation matrices (Table 5). A dominant δ-MnO₂ phase is found, presenting associations with common Mn-associated elements (Mn, Ba, Co, Mo, Ni, Zn) and elements from Fe-oxyhydroxides (Pb, As, Nb, Y), and includes elements partitioned between both groups: Tl, La, Nd, Sr and V. This phase is also characterised by a strong opposition to aluminosilicate elements such as Si, Al, Mg, Na, K, Cu and Zr. The Fe-oxyhydroxides phase is different; Zr and Ce are the main elements and Nd is correlated to this factor. Negative correlations are found with likely biogenic related elements, Ca, Mg and Ni. Using factor analysis, no fluorapatite or residual biogenic phases were detected. Fe (+As) phase shows stark anti-correlations with Ti and Cu, and weaker ones with Mn-associated elements. The last factor is difficult to identify because its only significant correlation is an anti-correlation with Ba. Weaker anti-correlations with Ce
## 4.6. Factor Analysis

A factor analysis was performed for the 89 hydrogenetic macro-layers and bulk crusts. Four significant factors explain 75% of the variance in the data set (Figure 9). Factor 1 is interpreted as δ-MnO₂ and accounts for 45.2% of the variance, factor 2 as Fe-oxyhydroxides and accounts for 12.8%, factor 3 as a Fe (+As) dominated phase accounting for 10.3%, and factor 4 as a Ti phase accounting for 5.9%. For each factor, elements with the highest scores are: δ-MnO₂: Sr, Mn, Pb, Mo, Co, Tl, V, As, La, Ni, Zn, Y, P, Nb; Fe-oxyhydroxides: Zr, Ce, Fe, Nd; Fe (+As) phase: negatively correlated to Cu and Ti; Ti phase: negatively correlated to Ba.

<figure><img src="image_13.png" /><figcaption>**Figure 9.** Histograms of factor scores for the four factors (computed for the hydrogenetic bulk and macro-layers samples, *n* = 89).</figcaption></figure>

Mineral associations and phases determined using factor analysis present important discrepancies compared to phases obtained with correlation matrices (Table 5). A dominant δ-MnO₂ phase is found, presenting associations with common Mn-associated elements (Mn, Ba, Co, Mo, Ni, Zn) and elements from Fe-oxyhydroxides (Pb, As, Nb, Y), and includes elements partitioned between both groups: Tl, La, Nd, Sr and V. This phase is also characterised by a strong opposition to aluminosilicate elements such as Si, Al, Mg, Na, K, Cu and Zr. The Fe-oxyhydroxides phase is different; Zr and Ce are the main elements and Nd is correlated to this factor. Negative correlations are found with likely biogenic related elements, Ca, Mg and Ni. Using factor analysis, no fluorapatite or residual biogenic phases were detected. Fe (+As) phase shows stark anti-correlations with Ti and Cu, and weaker ones with Mn-associated elements. The last factor is difficult to identify because its only significant correlation is an anti-correlation with Ba. Weaker anti-correlations with Ce
and Zn are also found, possibly indicating an opposition to a biogenic phase composed of Ba, Ce and Zn [1,4]. As Y is already significantly correlated to δ-MnO₂, the main element positively correlated to this factor is Ti.

**5. Discussion**

According to their macroscopic features, as well as mineralogical and geochemical compositions, we can distinguish a group of hydrogenetic Fe-Mn crusts (98 of 104 samples) from a group of deposits presenting hydrothermal characteristics (six samples).

**5.1. Comparison of New Caledonia's Fe-Mn Crusts with Other Oceans Deposits**

Fe-Mn deposits are found in all oceans, covering different types of geomorphological settings and environments, reflecting a large panel of chemical compositions and morphologies. The physiography within the New Caledonian EEZ is complex and contains several ridges and seamounts where crusts are expected to be found. In order to compare New Caledonia's Fe-Mn crusts composition with crusts from elsewhere in the global ocean, Figure 10 compiles compositions of Fe-M crusts and nodules from several oceans (after [1,76,77]) compared to New Caledonia's Fe-Mn crusts (this study). New Caledonian crusts' concentrations were normalised to other oceans' crusts' concentrations, and presented as ratios.

The New Caledonian crusts show Fe concentrations close to that of Fe-Mn crusts from the Atlantic (0.99), Indian (0.93) and Non-Prime North Pacific (0.92) oceans, whilst crusts from the North-Pacific Prime Zone (PCZ, 1.22) and the South Pacific Ocean (1.14) exhibit enriched concentrations. Mn is higher for crusts from New Caledonia compared to the Atlantic Ocean (1.20), but lower than crusts from the CA margin (0.89), the South-Pacific (0.80), the Non-Prime North Pacific (0.74) and the PCZ (0.76) oceans. Concentrations of elements of economic interest Co and Ni are higher than in crusts of the CA margin (1.34 and 1.37, respectively) and Atlantic (1.16 and 1.20), Indian (1.27 and 1.21) and North-Pacific (only Co with 1.12) oceans, but lower than crusts from the PCZ (0.63 and 0.74) and the South-Pacific (0.68 and 0.67) oceans. Detrital elements Si and Al have higher concentrations in New Caledonia's crusts than the South-Pacific (1.30 and 1.27, respectively) and PCZ (1.52 and 1.61) oceans; higher concentrations are found in the CA margin (only Si with 0.60) and the Atlantic Ocean (only Al with 0.74), whilst the rest of the concentrations are close to that of New Caledonia's crusts (within the 0.9–1.1 range). Elements that present a strong concentration in New Caledonia's crusts compared to other crusts are Pb, Sr, V and Zn, whilst concentrations of K, Ba, and Cu are low in New Caledonia's crusts.

New Caledonian crusts mostly resemble crusts from the Indian and Atlantic oceans, showing a mean Fe/Mn ratio greater than 1.2 that is generally suggesting a mixed hydrogenetic and hydrothermal, or continental margin hydrogenetic origin [1]. Studies on Atlantic crusts pointed a significant enrichment in terrigenous component (Fe, Pb, Al, and Si) compared to the Pacific Ocean crusts due to fluvial and eolian input [78]. Similar enrichments are observed in New Caledonian crusts and could be also associated with strong terrigenous components. Low K concentrations could reflect the nature of terrigenous elements coming from New Caledonia since the Eocene obduction of mafic and ultramafic nappes [28,43,44].

Considering the 17 hydrogenetic Fe-Mn crusts samples analysed with ICP-MS (Table 4), the mean ΣREE in New Caledonia's crusts is 1307 ppm, whilst it is ranging between 2352 ppm and 2541 ppm for the Atlantic, Indian and North Pacific oceans (South Pacific value is closer with 1634 ppm) [12,70]. The mean percentage of HREE is slightly higher than the global oceans, with a value of 22% compared to 16% to 21%.

Compared to polymetallic nodules from the Clarion-Clipperton Zone (CCZ), the Peru Basin and the Indian Ocean, Fe, Ca, Ti, P, As, Co, Cr, Pb, Sr, V, Y, Zr, La, Ce and a majority of REE concentrations are higher in New Caledonia's crusts. This observation confirmed that the Fe-Mn crusts are dominantly hydrogenetic, contrasting with nodules where diagenetic processes can lead to higher concentrations in Cu, Ni, Zn, Al, K and Cd [1].
and Zn are also found, possibly indicating an opposition to a biogenic phase composed of Ba, Ce and Zn [1,4]. As Y is already significantly correlated to δ-MnO₂, the main element positively correlated to this factor is Ti.

**5. Discussion**

According to their macroscopic features, as well as mineralogical and geochemical compositions, we can distinguish a group of hydrogenetic Fe-Mn crusts (98 of 104 samples) from a group of deposits presenting hydrothermal characteristics (six samples).

**5.1. Comparison of New Caledonia's Fe-Mn Crusts with Other Oceans Deposits**

Fe-Mn deposits are found in all oceans, covering different types of geomorphological settings and environments, reflecting a large panel of chemical compositions and morphologies. The physiography within the New Caledonian EEZ is complex and contains several ridges and seamounts where crusts are expected to be found. In order to compare New Caledonia's Fe-Mn crusts composition with crusts from elsewhere in the global ocean, Figure 10 compiles compositions of Fe-M crusts and nodules from several oceans (after [1,76,77]) compared to New Caledonia's Fe-Mn crusts (this study). New Caledonian crusts' concentrations were normalised to other oceans' crusts' concentrations, and presented as ratios.

The New Caledonian crusts show Fe concentrations close to that of Fe-Mn crusts from the Atlantic (0.99), Indian (0.93) and Non-Prime North Pacific (0.92) oceans, whilst crusts from the North-Pacific Prime Zone (PCZ, 1.22) and the South Pacific Ocean (1.14) exhibit enriched concentrations. Mn is higher for crusts from New Caledonia compared to the Atlantic Ocean (1.20), but lower than crusts from the CA margin (0.89), the South-Pacific (0.80), the Non-Prime North Pacific (0.74) and the PCZ (0.76) oceans. Concentrations of elements of economic interest Co and Ni are higher than in crusts of the CA margin (1.34 and 1.37, respectively) and Atlantic (1.16 and 1.20), Indian (1.27 and 1.21) and North-Pacific (only Co with 1.12) oceans, but lower than crusts from the PCZ (0.63 and 0.74) and the South-Pacific (0.68 and 0.67) oceans. Detrital elements Si and Al have higher concentrations in New Caledonia's crusts than the South-Pacific (1.30 and 1.27, respectively) and PCZ (1.52 and 1.61) oceans; higher concentrations are found in the CA margin (only Si with 0.60) and the Atlantic Ocean (only Al with 0.74), whilst the rest of the concentrations are close to that of New Caledonia's crusts (within the 0.9–1.1 range). Elements that present a strong concentration in New Caledonia's crusts compared to other crusts are Pb, Sr, V and Zn, whilst concentrations of K, Ba, and Cu are low in New Caledonia's crusts.

New Caledonian crusts mostly resemble crusts from the Indian and Atlantic oceans, showing a mean Fe/Mn ratio greater than 1.2 that is generally suggesting a mixed hydrogenetic and hydrothermal, or continental margin hydrogenetic origin [1]. Studies on Atlantic crusts pointed a significant enrichment in terrigenous component (Fe, Pb, Al, and Si) compared to the Pacific Ocean crusts due to fluvial and eolian input [78]. Similar enrichments are observed in New Caledonian crusts and could be also associated with strong terrigenous components. Low K concentrations could reflect the nature of terrigenous elements coming from New Caledonia since the Eocene obduction of mafic and ultramafic nappes [28,43,44].

Considering the 17 hydrogenetic Fe-Mn crusts samples analysed with ICP-MS (Table 4), the mean ΣREE in New Caledonia's crusts is 1307 ppm, whilst it is ranging between 2352 ppm and 2541 ppm for the Atlantic, Indian and North Pacific oceans (South Pacific value is closer with 1634 ppm) [12,70]. The mean percentage of HREE is slightly higher than the global oceans, with a value of 22% compared to 16% to 21%.

Compared to polymetallic nodules from the Clarion-Clipperton Zone (CCZ), the Peru Basin and the Indian Ocean, Fe, Ca, Ti, P, As, Co, Cr, Pb, Sr, V, Y, Zr, La, Ce and a majority of REE concentrations are higher in New Caledonia's crusts. This observation confirmed that the Fe-Mn crusts are dominantly hydrogenetic, contrasting with nodules where diagenetic processes can lead to higher concentrations in Cu, Ni, Zn, Al, K and Cd [1].
<figure><img src="image_14.png" /><figcaption>**Figure 10.** Element enrichment of New Caledonian crusts compared to (A) the California margin (CA), Atlantic and Indian oceans' Fe-Mn crusts, (B) the Pacific Ocean Fe-Mn crusts, and (C) poly-metallic nodules from the Clarion-Clipperton Zone (CCZ), the Peru Basin and the Indian Ocean (after [1,76,77]). Values greater than 1 are enriched compared to other oceans crusts, whereas values lower than 1 are depleted. Fe/Mn* and Si/Al* ratios are calculated using mean ocean values.</figcaption></figure>

## 5.2. Crusts Chemical Changes with Water Depth

Chemical changes in Fe-Mn crusts with water depth are a common phenomenon that has been identified in several studies [2,64,79–83]. Fe-Mn crusts selected for this study range from 430 m to 4677 m and allow us to observe changes in chemical composition with water depth (Figure 11). Non-hydrogenetic deposits are illustrated on the graphs but are not considered for this analysis. Manganese shows a large range of values at shallow depths and decreases with depth. On the contrary, Fe is more stable along the water column, with values close to 20%. As a result, the Fe/Mn ratio increases from 1 to 1.5 between 1000 m and 3000 m, which is phenomenon observed in several other locations [81,83]. Silicon, Al, K and Na exhibit a net increase with depth, emphasising an increase of the aluminosilicate fraction in Fe-Mn crusts with depth. Increasing Fe, Si, Al, K and Na contents in crusts with depth are also observed in other oceans and are generally explained by an increased supply
<figure><img src="image_14.png" /><figcaption>**Figure 10.** Element enrichment of New Caledonian crusts compared to (A) the California margin (CA), Atlantic and Indian oceans' Fe-Mn crusts, (B) the Pacific Ocean Fe-Mn crusts, and (C) poly-metallic nodules from the Clarion-Clipperton Zone (CCZ), the Peru Basin and the Indian Ocean (after [1,76,77]). Values greater than 1 are enriched compared to other oceans crusts, whereas values lower than 1 are depleted. Fe/Mn* and Si/Al* ratios are calculated using mean ocean values.</figcaption></figure>

## 5.2. Crusts Chemical Changes with Water Depth

Chemical changes in Fe-Mn crusts with water depth are a common phenomenon that has been identified in several studies [2,64,79–83]. Fe-Mn crusts selected for this study range from 430 m to 4677 m and allow us to observe changes in chemical composition with water depth (Figure 11). Non-hydrogenetic deposits are illustrated on the graphs but are not considered for this analysis. Manganese shows a large range of values at shallow depths and decreases with depth. On the contrary, Fe is more stable along the water column, with values close to 20%. As a result, the Fe/Mn ratio increases from 1 to 1.5 between 1000 m and 3000 m, which is phenomenon observed in several other locations [81,83]. Silicon, Al, K and Na exhibit a net increase with depth, emphasising an increase of the aluminosilicate fraction in Fe-Mn crusts with depth. Increasing Fe, Si, Al, K and Na contents in crusts with depth are also observed in other oceans and are generally explained by an increased supply
of detrital phases, and/or a weaker input of Mn due to the distance with the Mn-rich OMZ, whilst Fe increases with the dissolution of biogenic calcite [1,81]. Phosphorus and Ca in crusts decrease with water depth, possibly also representing the effect of the lysocline on carbonates and their continuous dissolution with increasing pressure at depth. Elements of economic interest Ni and Co show decreasing concentrations with depth, ranging from 7000 ppm and 5000 ppm at 1000 m, respectively, to 4000 ppm and 3000 ppm at 3000 m. This trend is correlated to the changing concentrations of the dissolved metals along the water column, with higher values around 1000 m and a marked decrease that tends to reduce with increasing depth below 2000 m [2]. This reflects the relationships of some trace metals with Mn, which is explained by an enhanced supply of dissolved Mn²⁺ near the OMZ [1]. Other elements presenting decreasing trends with increasing water depth are Mo, Pb, Zn, As, Sr, Tl and V. Contrarily to other metals, Cu shows a slight increase with depth, with values ranging from 500 ppm at 1000 m, to 1200 ppm at 3000 m water depth. This increase in Cu content can be explained by its role in biogeochemical cycles, depleting its dissolve form in shallower waters whilst sinking organic particles progressively release it at depth [82]. Other elements such as Ti, Nd, Ba, Nb, Ce, Cr, Y and La show no particular trends, suggesting they are neither especially enriched nor depleted with depths in New Caledonian crusts.

<figure><img src="image_15.png" /><figcaption>**Figure 11.** Repartition of selected elements with water depth. Black dots are the bulk crusts (n = 74), red dots are the macro-layers crusts (n = 24), and blue dots are the non-hydrogenetic deposits. Graph of crust thickness only considers bulk crusts with a measurable/known thickness.</figcaption></figure>
of detrital phases, and/or a weaker input of Mn due to the distance with the Mn-rich OMZ, whilst Fe increases with the dissolution of biogenic calcite [1,81]. Phosphorus and Ca in crusts decrease with water depth, possibly also representing the effect of the lysocline on carbonates and their continuous dissolution with increasing pressure at depth. Elements of economic interest Ni and Co show decreasing concentrations with depth, ranging from 7000 ppm and 5000 ppm at 1000 m, respectively, to 4000 ppm and 3000 ppm at 3000 m. This trend is correlated to the changing concentrations of the dissolved metals along the water column, with higher values around 1000 m and a marked decrease that tends to reduce with increasing depth below 2000 m [2]. This reflects the relationships of some trace metals with Mn, which is explained by an enhanced supply of dissolved Mn²⁺ near the OMZ [1]. Other elements presenting decreasing trends with increasing water depth are Mo, Pb, Zn, As, Sr, Tl and V. Contrarily to other metals, Cu shows a slight increase with depth, with values ranging from 500 ppm at 1000 m, to 1200 ppm at 3000 m water depth. This increase in Cu content can be explained by its role in biogeochemical cycles, depleting its dissolve form in shallower waters whilst sinking organic particles progressively release it at depth [82]. Other elements such as Ti, Nd, Ba, Nb, Ce, Cr, Y and La show no particular trends, suggesting they are neither especially enriched nor depleted with depths in New Caledonian crusts.

<figure><img src="image_15.png" /><figcaption>**Figure 11.** Repartition of selected elements with water depth. Black dots are the bulk crusts (n = 74), red dots are the macro-layers crusts (n = 24), and blue dots are the non-hydrogenetic deposits. Graph of crust thickness only considers bulk crusts with a measurable/known thickness.</figcaption></figure>
Crust thicknesses have been measured for every sample where the right-way up was evident and for crusts with or without substrate. Final values are a mean of six measurements performed along crusts widths. The distribution of measured thicknesses highlights the presence of thicker crusts below 2000 m despite higher sampling density at shallower depth. The low number of valid thickness measurements for samples below 3000 m in our data set prevents definitive interpretation of the trend as continuously increasing, constant or decreasing. It is important to note that all samples were dredged during cruises that were not dedicated to study crusts and that sample recovery is strongly impacted by seabed and outcrop morphology. Fe-Mn crusts recovered by ROV along depth transects usually show no variation in crust thicknesses [84,85]. The two thickest crusts (GO327: 115 mm, DR11Ai: 97 mm) are situated respectively at 1820 m and 2375 m.

### 5.3. *Nature of Non-Hydrogenetic Deposits*

Based on macroscopic, mineralogical and geochemical characteristics, six samples from the data set are considered as non-hydrogenetic. DW2482, DW4998D and DW4998E were dredged on the top and flanks of an Oligo-Miocene intraplate volcanic edifice on the Lord Howe Rise, part of a North/North-West oriented cluster of several seamounts (Figure 2) [19,53]. DW778D, DW778D2 and DW778B were dredged on the summit of Mount K, a volcanic edifice of the Loyalty Ridge that is likely subduction related (Figure 2B). The presence of both amorphous and crystalline 10 Å manganate (±pyrolusite), and the pseudo-layered structure observed at the macro and microscopic scales in the DW778 samples, are consistent with other oceans hydrothermal deposits [11–13,86]. Hydrothermal deposits originating from ascending fluids are known to form in distal parts of the venting site in several geomorphological environments: back-arc basins [12,13], arc systems [11,69,87,88], or hot spot volcanoes [67,89,90]. They usually present a different mineralogy compared to hydrogenetic crusts, a strong partitioning between Fe and Mn, depleted trace metal contents (however, some deposits can exhibit notable enrichments in specific trace metals), and high growth rates [64].

Such specific characteristics are found in samples DW2482 and DW4998E that present high growth rates and unusual enrichments in Ni (up to 0.7%). Ni enrichment in hydrothermal deposits have been found in several places, such as the Yap volcanic arc [64], the submarine rift zones near Hawaii [91], or in the Wallis and Futuna back-arc system [9]. Same observations have also been pointed out by [19] in samples from the same group of volcanic seamounts on the Lord Howe Rise (samples DR01 were dredged from the same seamount as DW2482, DW4998D and DW4998E). In this study, Fe-Mn encrusted hyaloclastites and foraminifer-rich chalks are hydrothermally influenced, show strong Mn concentrations (up to 42.7%), low Fe concentrations (down to 0.53%), Ni enrichment (up to 0.65%), and globally depleted Co contents. These concentrations are similar to the ones we reported for DW2482, DW4998D and DW4998E, suggesting that they could possibly share the same origin.

Trace metal enrichments in these type of deposits are controlled by several parameters, such as the volume and type of leached rocks and sediments, the precipitation of sulfides at depth, the degree of partitioning between Mn and Fe, the amount of mixing with seawater and the distance from the vent sites [11,92]. It is likely that these parameters have influenced the formation of these samples, as notable differences in Ni, Ba, Zn and Cu concentrations between close dredging sites are observed. Based only on macroscopic description and XRD/XRF/SEM analyses of these six samples, it is not possible to decipher the origin of the Ni enrichment.

### 5.4. *Resource Considerations*

Hydrogenetic Fe-Mn crusts can be strongly enriched in rare and critical metals, such as Co, Te, Mo, Bi, Pt, W, Zr, Nb, Y and REE [12]. These concentrations make Fe-Mn crusts potential resources for metals used in high and green technology [93]. As mentioned in Section 5.2, New Caledonian Fe-Mn crusts show typical metal compositions of deposits
Crust thicknesses have been measured for every sample where the right-way up was evident and for crusts with or without substrate. Final values are a mean of six measurements performed along crusts widths. The distribution of measured thicknesses highlights the presence of thicker crusts below 2000 m despite higher sampling density at shallower depth. The low number of valid thickness measurements for samples below 3000 m in our data set prevents definitive interpretation of the trend as continuously increasing, constant or decreasing. It is important to note that all samples were dredged during cruises that were not dedicated to study crusts and that sample recovery is strongly impacted by seabed and outcrop morphology. Fe-Mn crusts recovered by ROV along depth transects usually show no variation in crust thicknesses [84,85]. The two thickest crusts (GO327: 115 mm, DR11Ai: 97 mm) are situated respectively at 1820 m and 2375 m.

### 5.3. *Nature of Non-Hydrogenetic Deposits*

Based on macroscopic, mineralogical and geochemical characteristics, six samples from the data set are considered as non-hydrogenetic. DW2482, DW4998D and DW4998E were dredged on the top and flanks of an Oligo-Miocene intraplate volcanic edifice on the Lord Howe Rise, part of a North/North-West oriented cluster of several seamounts (Figure 2) [19,53]. DW778D, DW778D2 and DW778B were dredged on the summit of Mount K, a volcanic edifice of the Loyalty Ridge that is likely subduction related (Figure 2B). The presence of both amorphous and crystalline 10 Å manganate (±pyrolusite), and the pseudo-layered structure observed at the macro and microscopic scales in the DW778 samples, are consistent with other oceans hydrothermal deposits [11–13,86]. Hydrothermal deposits originating from ascending fluids are known to form in distal parts of the venting site in several geomorphological environments: back-arc basins [12,13], arc systems [11,69,87,88], or hot spot volcanoes [67,89,90]. They usually present a different mineralogy compared to hydrogenetic crusts, a strong partitioning between Fe and Mn, depleted trace metal contents (however, some deposits can exhibit notable enrichments in specific trace metals), and high growth rates [64].

Such specific characteristics are found in samples DW2482 and DW4998E that present high growth rates and unusual enrichments in Ni (up to 0.7%). Ni enrichment in hydrothermal deposits have been found in several places, such as the Yap volcanic arc [64], the submarine rift zones near Hawaii [91], or in the Wallis and Futuna back-arc system [9]. Same observations have also been pointed out by [19] in samples from the same group of volcanic seamounts on the Lord Howe Rise (samples DR01 were dredged from the same seamount as DW2482, DW4998D and DW4998E). In this study, Fe-Mn encrusted hyaloclastites and foraminifer-rich chalks are hydrothermally influenced, show strong Mn concentrations (up to 42.7%), low Fe concentrations (down to 0.53%), Ni enrichment (up to 0.65%), and globally depleted Co contents. These concentrations are similar to the ones we reported for DW2482, DW4998D and DW4998E, suggesting that they could possibly share the same origin.

Trace metal enrichments in these type of deposits are controlled by several parameters, such as the volume and type of leached rocks and sediments, the precipitation of sulfides at depth, the degree of partitioning between Mn and Fe, the amount of mixing with seawater and the distance from the vent sites [11,92]. It is likely that these parameters have influenced the formation of these samples, as notable differences in Ni, Ba, Zn and Cu concentrations between close dredging sites are observed. Based only on macroscopic description and XRD/XRF/SEM analyses of these six samples, it is not possible to decipher the origin of the Ni enrichment.

### 5.4. *Resource Considerations*

Hydrogenetic Fe-Mn crusts can be strongly enriched in rare and critical metals, such as Co, Te, Mo, Bi, Pt, W, Zr, Nb, Y and REE [12]. These concentrations make Fe-Mn crusts potential resources for metals used in high and green technology [93]. As mentioned in Section 5.2, New Caledonian Fe-Mn crusts show typical metal compositions of deposits
formed in the vicinity of a continental mass with moderate enrichment in metals of eco-
nomic interest compared to values of PCZ and higher detrital content. Cobalt and Ni,
considered of greatest economic interest [12,71], are in the range of crusts from the Atlantic
or Indian oceans, but less concentrated than in North and South-Pacific crusts. Data from
this study indicates that the highest Co and Ni concentrations are located at water depth
ranging from 1000 m to 2000 m, whilst crust thickness tends to be the highest around
2000 m (Figure 11). Correlation analysis showed that Co and Ni are mostly bound to the
δ-MnO₂ phase, which also displays significant correlations with elements of economic
interest like Mo and Nb (see part 4.6). The Co + Ni + Cu (%) content can reach high values
in New Caledonia in sites close to the ridges and seamounts of the southern part of the
EEZ. There, clusters of crust samples exhibit values reaching up to 1.62% (Figure 12). The
physiography of this area is particularly favourable to Fe-Mn crust exploration; (i) several
seamounts of the Loyalty and Norfolk ridges combine acceptable exploration and mine-site
parameters, such as a seamount area larger than 400 km²; (ii) water depth above 2500 m;
and (iii) large areas with slope values between 0° and 20° [2,94].

<figure><img src="image_16.png" /><figcaption>**Figure 12.** Map of hydrogenetic sample's Co + Cu + Ni (%) concentration, focused on the Southern part of New Caledonia EEZ, with indications of slope values and surface area (contour lines = 500 m).</figcaption></figure>
formed in the vicinity of a continental mass with moderate enrichment in metals of eco-
nomic interest compared to values of PCZ and higher detrital content. Cobalt and Ni,
considered of greatest economic interest [12,71], are in the range of crusts from the Atlantic
or Indian oceans, but less concentrated than in North and South-Pacific crusts. Data from
this study indicates that the highest Co and Ni concentrations are located at water depth
ranging from 1000 m to 2000 m, whilst crust thickness tends to be the highest around
2000 m (Figure 11). Correlation analysis showed that Co and Ni are mostly bound to the
δ-MnO₂ phase, which also displays significant correlations with elements of economic
interest like Mo and Nb (see part 4.6). The Co + Ni + Cu (%) content can reach high values
in New Caledonia in sites close to the ridges and seamounts of the southern part of the
EEZ. There, clusters of crust samples exhibit values reaching up to 1.62% (Figure 12). The
physiography of this area is particularly favourable to Fe-Mn crust exploration; (i) several
seamounts of the Loyalty and Norfolk ridges combine acceptable exploration and mine-site
parameters, such as a seamount area larger than 400 km²; (ii) water depth above 2500 m;
and (iii) large areas with slope values between 0° and 20° [2,94].

<figure><img src="image_16.png" /><figcaption>**Figure 12.** Map of hydrogenetic sample's Co + Cu + Ni (%) concentration, focused on the Southern part of New Caledonia EEZ, with indications of slope values and surface area (contour lines = 500 m).</figcaption></figure>
A summary of Fe-Mn crusts resource assessment is proposed in Figure 12 using criteria cited above and samples analysed in this study. Surface areas and slopes were calculated using ArcMap's 3D analyst, ArcGIS®, from a 100 m scale bathymetric map [95]. Polygons of seamounts and ridges above 2500 m and presenting one or more samples were hand-drawn following bathymetric and slopes variations. This map underlines several zones of the Loyalty and Lord Howe ridges with large areas (up to 2045 km²), a relatively flat top (slopes < 20°) and samples with noticeable enrichment in Co + Ni + Cu (%). However, crusts used in this study are the result of opportunistic sampling during scientific cruises of various origins (mostly biology or geology). This makes it difficult to go beyond a first-order resource assessment. To further characterise this potential, exploration should be conducted, notably through a detailed sampling strategy, seamount-scale bathymetric and backscatter mappings, and the study of physicochemical properties and motion of water masses.

**6. Summary and Conclusions**

(1) Several deposit styles were identified within the EEZ: a group of hydrogenetic crusts with chemical, textural and mineralogical characteristics similar to other hydroge- netic deposits found elsewhere in the ocean, and two groups of hydrothermal and diagenetic deposits located on the Loyalty and the Lord Howe ridges.

(2) Hydrogenetic crusts started to grow about 34 Ma ago, at a rate of 2.2–3.1 mm/Ma, leading to a maximum crust thickness of 115 mm.

(3) The hydrothermal/diagenetic samples from the Lord Howe Rise and the Loyalty Ridge exhibit wider chemical and mineralogical compositions (10 Å manganates ± pyrolusite), as well as a significant enrichment in Ni for two samples.

(4) New Caledonia's hydrogenetic crust compositions are in the range of typical hydroge- netic Fe-Mn crusts. The mean combined concentration of metals with high economic potential Co + Ni + Cu is 0.81%, which is higher than Indian and Atlantic oceans, but lower than the Pacific Prime Crust Zone and the South Pacific Ocean. Several seamounts in the Southern part of the EEZ present clusters of Co + Ni + Cu values above 1%.

(5) Further investigations will be needed to constrain more precisely the depositional settings of the hydrothermal/diagenetic samples, and the economic potential of hydrogenetic Fe-Mn crusts inside New Caledonia's EEZ.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/ 10.3390/min12020255/s1: Table S1: Normalised to 0% H₂O⁻ XRF chemical compositions of 104 Fe-Mn samples.

**Author Contributions:** Conceptualization, P.S. and J.C.; methodology, P.S., P.J. and E.P.; validation, J.C., P.J. and E.P.; formal analysis, P.S., P.J. and E.P.; resources, S.C., A.B. and Y.G.; writing—original draft preparation, P.S.; writing—review and editing, J.C., P.J., E.P., S.E. and M.P.; supervision, J.C., S.E. and E.P.; project administration, J.C., S.E. and E.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research has been funded by a research collaboration between Ifremer and the Gou- vernment of New Caledonia.

**Data Availability Statement:** The data presented in this study are available in the main body of the paper and in Supplementary Materials.

**Acknowledgments:** P.S. thanks the Direction de l'Industrie des Mines et de l'Energie de Nouvelle- Calédonie (DIMENC), the Agence de Développement Économique de la Nouvelle-Calédonie (ADE- CAL), and the Institut Français pour l'Exploitation de la Mer (IFREMER) for hosting whilst working on the manuscript. P.J. publishes with the permission of the Executive Director, British Geological Sur- vey (UKRI). We thank three anonymous reviewers for relevant suggestions and editors for handling of this publication.

**Conflicts of Interest:** The authors declare no conflict of interest.