Trace elements in New Zealand ferromanganese nodules: implications for deep sea environments

<p>Ferromanganese nodules are authigenic marine sediments that form over millions of years from the precipitation of Fe oxyhydroxides and Mn oxides from seawater (hydrogenetic-type growth) and sediment pore-water (diagenetic-type growth). Fe-Mn (oxyhydr)oxides grow in layers about nuclei, effectively scavenging minor metals such as Ni, Cu and Co from the waters they grow in. The uptake of different elements into the ferromanganese nodules reflects their environment and mechanism of growth, and these deposits are of interest both as a potential source of metals of economic interest, and as records of changing ocean conditions. This study investigates the composition of 77 ferromanganese nodules from the seafloor around New Zealand. Samples analysed come from locations several thousand kilometres apart under the same water mass (Lower Circumpolar Deep Water – LCDW), but with varying depth, current velocity, and sediment type. The outermost 1 mm rim of each nodule, representing near-modern growth, was sampled to compare with modern environmental parameters including substrate sediment composition and chemical and physical oceanography. Major, minor, and trace element analysis of nodule rims were undertaken, and the authigenic and detrital components examined via leaching experiments to evaluate their relative influence on growth mechanisms. Overall, New Zealand ferromanganese nodules are hydrogenetic in origin. However, there are systematic variations in composition that reflect variable diagenetic influence. Hydrogenetic endmember compositions are defined by samples from two localities in the Southern Ocean that have no evidence for diagenetic influence. Diagenetic influence on nodule composition is exemplified by samples from the two locations in the Tasman Sea, but also include nodules from the Campbell nodule field. Nodules from the Campbell nodule field come from two transects perpendicular to the Campbell Plateau, and the Deep Western Boundary Current (DWBC). Both sediment composition and nodule rim chemistry vary systematically across both transects. Areas closest to the slope have sediment profiles indicating high energy, erosive environments, continental-sourced sand components, and are dominated by nodules with hydrogenetic chemical characteristics similar to those of the Southern Ocean. Further from the slope, the sediment profiles indicate silt dominated sediments of a more oceanic crustal provenance, lower energy environment, and increased influence of oxic diagenetic processes on the major, minor and trace element profiles of the nodules. No hydrothermal contribution was identified in the chemistry of any of the nodules analysed. The physical and chemical properties of the sediment, along with current velocities, were found to be the key influences in diagenetic enrichment in the nodules. The influence of seawater chemistry was difficult to determine due to the lack of direct analyses in the area. Ferromanganese nodule chemistry is a function of the nodule environment, including water body, sediment composition and depth. The authigenic components of nodules can therefore be used to investigate the deep-sea environment. The redox conditions of sediments and the productivity of the overlying water will affect the trace metal constituents of the pore-waters of a sediment (Kuhn et al., 2017). Sediments with a larger fraction of labile organic matter may result in trace element enrichment of the pore-water. Sediments below the CCD will be higher in trace elements than sediments below the CCD (U1413, U1406B, U1402, U1398, U1398, and U1378) due to carbonate matter acting as a dilutant that can limit the supply of trace elements mobilised in the pore-water during diagenesis (Glasby, 2006). Terrigenous clasts such as quartz (Chester, 1990), will also reduce trace element enrichment in the pore-water due to their low reactivity, e.g. for the sediment U1406B, which has a high lithic component (Table 3.2). Sediments with a higher biogenic silica component (such as U1373, U1374, and U1378) (Table 3.2, Table 3.4) are predicted to produce nodules with higher trace element contents (ISA, 2010). In contrast to both the CCZ and Indian Ocean nodules, the Campbell nodule field samples formed above the CCD, and hence in sediments that include a significant carbonate component. This minimises the trace element pore-water enrichment and can account for the lower Cu+Ni+Co contents observed in the Campbell nodule field nodules compared with those that formed below the CCD (CCZ and Indian Ocean).</p>

Trace elements in New Zealand ferromanganese nodules: implications for deep sea environments

<p>Ferromanganese nodules are authigenic marine sediments that form over millions of years from the precipitation of Fe oxyhydroxides and Mn oxides from seawater (hydrogenetic-type growth) and sediment pore-water (diagenetic-type growth). Fe-Mn (oxyhydr)oxides grow in layers about nuclei, effectively scavenging minor metals such as Ni, Cu and Co from the waters they grow in. The uptake of different elements into the ferromanganese nodules reflects their environment and mechanism of growth, and these deposits are of interest both as a potential source of metals of economic interest, and as records of changing ocean conditions. This study investigates the composition of 77 ferromanganese nodules from the seafloor around New Zealand. Samples analysed come from locations several thousand kilometres apart under the same water mass (Lower Circumpolar Deep Water – LCDW), but with varying depth, current velocity, and sediment type. The outermost 1 mm rim of each nodule, representing near-modern growth, was sampled to compare with modern environmental parameters including substrate sediment composition and chemical and physical oceanography. Major, minor, and trace element analysis of nodule rims were undertaken, and the authigenic and detrital components examined via leaching experiments to evaluate their relative influence on growth mechanisms. Overall, New Zealand ferromanganese nodules are hydrogenetic in origin. However, there are systematic variations in composition that reflect variable diagenetic influence. Hydrogenetic endmember compositions are defined by samples from two localities in the Southern Ocean that have no evidence for diagenetic influence. Diagenetic influence on nodule composition is exemplified by samples from the two locations in the Tasman Sea, but also include nodules from the Campbell nodule field. Nodules from the Campbell nodule field come from two transects perpendicular to the Campbell Plateau, and the Deep Western Boundary Current (DWBC). Both sediment composition and nodule rim chemistry vary systematically across both transects. Areas closest to the slope have sediment profiles indicating high energy, erosive environments, continental-sourced sand components, and are dominated by nodules with hydrogenetic chemical characteristics similar to those of the Southern Ocean. Further from the slope, the sediment profiles indicate silt dominated sediments of a more oceanic crustal provenance, lower energy environment, and increased influence of oxic diagenetic processes on the major, minor and trace element profiles of the nodules. No hydrothermal contribution was identified in the chemistry of any of the nodules analysed. The physical and chemical properties of the sediment, along with current velocities, were found to be the key influences in diagenetic enrichment in the nodules. The influence of seawater chemistry was difficult to determine due to the lack of direct analyses in the area. Ferromanganese nodule chemistry is a function of the nodule environment, including water body, sediment composition and depth. The authigenic components of nodules can therefore be used to investigate the deep-sea environment. The redox conditions of sediments and the productivity of the overlying water will affect the trace metal constituents of the pore-waters of a sediment (Kuhn et al., 2017). Sediments with a larger fraction of labile organic matter may result in trace element enrichment of the pore-water. Sediments below the CCD will be higher in trace elements than sediments below the CCD (U1413, U1406B, U1402, U1398, U1398, and U1378) due to carbonate matter acting as a dilutant that can limit the supply of trace elements mobilised in the pore-water during diagenesis (Glasby, 2006). Terrigenous clasts such as quartz (Chester, 1990), will also reduce trace element enrichment in the pore-water due to their low reactivity, e.g. for the sediment U1406B, which has a high lithic component (Table 3.2). Sediments with a higher biogenic silica component (such as U1373, U1374, and U1378) (Table 3.2, Table 3.4) are predicted to produce nodules with higher trace element contents (ISA, 2010). In contrast to both the CCZ and Indian Ocean nodules, the Campbell nodule field samples formed above the CCD, and hence in sediments that include a significant carbonate component. This minimises the trace element pore-water enrichment and can account for the lower Cu+Ni+Co contents observed in the Campbell nodule field nodules compared with those that formed below the CCD (CCZ and Indian Ocean).</p>

The Geochemistry and Ecotoxicity of Offshore New Zealand Phosphorites

<p>The New Zealand offshore seabed hosts diverse resources including phosphate rich rocks. Phosphate rock deposits on the Chatham Rise have been the focus of previous investigations into their composition and mining potential; however, the diversity of the geochemistry of phosphate deposits, including their wider distribution beyond the Chatham Rise, their trace metal budget, and potential for ecotoxicity, remain poorly characterised. This study addresses some of these gaps by presenting a geochemical investigation, including trace metals, for a range of phosphate nodules from across the Chatham Rise, Bollons Seamount and offshore southeastern South Island. Elutriate and reconnaissance bioaccumulation experiments provide insights into the potential for ecotoxic trace metal release and effects on biota should sediment disturbance through mining activities occur. The bulk chemistry of Bollons Seamount phosphorite nodules have been characterised for the first time, and show significant enrichment in first row transition metals; Co, Ni, Cu, Zn, in addition to Sr, Y, Mo, U, MnO, CaO and P2O5, and depletion in TiO2, Al2O3, MgO, K2O, FeO, SiO2, Sc, Cr, Ga, Rb, Cs, Hf, and Th relative to average upper continental crust. The cores of these nodules are dominated by apatite, quartz and anorthoclase phases, which are cross cut by Mn rich dendrites. The abundant presence of these minerals results in the significant differences in chemistry observed relative to Chatham Rise phosphorite nodules. The nodules also contain a secondary authigenic apatite phase, with a Mn crust rim. Significant rare earth element enrichment (REE) is most likely due to efficient scavenging by the Mn crust, resulting in seawater REE patterns characterised by negative Ce and Eu anomalies and heavy rare earth element enrichment. The bulk geochemistry of the Chatham Rise and offshore South Island phosphorite nodules is characterised by enrichment in CaO, P2O5, Sr, U, Y, Mo and depletion in TiO2, Al2O3, MnO, MgO, FeO, K2O, Sc, Cr, Cu, Ga, Rb, Cs, Ba, Hf, Ta, Pb and Th relative to average upper continental crust. The low concentrations of Cd in Chatham Rise, offshore South Island, and Bollons Seamount phosphorites make them potentially suitable sources for direct application fertilizers. The New Zealand marine phosphorite nodule deposits formed by repeated cycles of erosive bottom currents and phosphogenesis, resulting in the winnowing and concentration of the deposits. The iron pump model is proposed as a mechanism for the formation of apatite and associated mineral phases, giving the nodules their characteristic concentric zoning. The migration of the nodules through the oxic, suboxic, and anoxic zones of the sediment profile led to the formation of glaucony, apatite (suboxic zone), goethite (oxic zone), and pyrite with associated U enriched (anoxic zone) minerals. Rare earth elements (REE) in the Chatham Rise phosphorite nodules are associated with the glaucony rim minerals, and indicate that since the formation of the rims, very little diagenesis has occurred, preserving seawater REE patterns characterised by negative Ce and Eu anomalies and heavy REE enrichment. Site specific enrichments in trace elements Ba, V, Co, Ni, Cu, Zn, Y, Cd and Pb are attributed to either differences in incorporation of material into precursor carbonate e.g. volcanic materials, or higher fluxes of organic matter, delivering high concentrations of essential metals from biota, especially Cu and Zn. Direct pore water measurements from surficial sediment of the Chatham Rise show high concentrations of dissolved Fe and Mn, along with Cu, indicating suboxic conditions. High Cu concentrations measured in sediment pore water suggest that Cu release requires monitoring should seafloor surficial sediments on the Chatham Rise be disturbed. However, the elutriate experiments were not able to resolve if Cu release by sediment disturbance would exceed Australian and New Zealand Environment Conservation Council (2000) environmental guideline trigger values. The surrogate amphipod species Chaetocorophium c.f. lucasi shows promise as a biomonitor for disturbed marine sediments. Elements enriched in surficial sediments and phosphorite nodules, Hg, Pb, Fe, U and V, were not observed to bioaccumulate. Site specific differences in chemistry were observed, specifically in the different total relative bioaccumulation of Mo between amphipods exposed to sediments from two different sites. This suggests that future monitoring of chemical release during marine sediment disturbance requires the full geochemical characterisation of the substrate. Furthermore, fresh sediment and deep water should be used for future elutriate experiments, as storage of material by freeze-thawing and/or refrigeration causes mobilisation of some key trace metals such as U, V, Mo, Mn.</p>

The Geochemistry and Ecotoxicity of Offshore New Zealand Phosphorites

<p>The New Zealand offshore seabed hosts diverse resources including phosphate rich rocks. Phosphate rock deposits on the Chatham Rise have been the focus of previous investigations into their composition and mining potential; however, the diversity of the geochemistry of phosphate deposits, including their wider distribution beyond the Chatham Rise, their trace metal budget, and potential for ecotoxicity, remain poorly characterised. This study addresses some of these gaps by presenting a geochemical investigation, including trace metals, for a range of phosphate nodules from across the Chatham Rise, Bollons Seamount and offshore southeastern South Island. Elutriate and reconnaissance bioaccumulation experiments provide insights into the potential for ecotoxic trace metal release and effects on biota should sediment disturbance through mining activities occur. The bulk chemistry of Bollons Seamount phosphorite nodules have been characterised for the first time, and show significant enrichment in first row transition metals; Co, Ni, Cu, Zn, in addition to Sr, Y, Mo, U, MnO, CaO and P2O5, and depletion in TiO2, Al2O3, MgO, K2O, FeO, SiO2, Sc, Cr, Ga, Rb, Cs, Hf, and Th relative to average upper continental crust. The cores of these nodules are dominated by apatite, quartz and anorthoclase phases, which are cross cut by Mn rich dendrites. The abundant presence of these minerals results in the significant differences in chemistry observed relative to Chatham Rise phosphorite nodules. The nodules also contain a secondary authigenic apatite phase, with a Mn crust rim. Significant rare earth element enrichment (REE) is most likely due to efficient scavenging by the Mn crust, resulting in seawater REE patterns characterised by negative Ce and Eu anomalies and heavy rare earth element enrichment. The bulk geochemistry of the Chatham Rise and offshore South Island phosphorite nodules is characterised by enrichment in CaO, P2O5, Sr, U, Y, Mo and depletion in TiO2, Al2O3, MnO, MgO, FeO, K2O, Sc, Cr, Cu, Ga, Rb, Cs, Ba, Hf, Ta, Pb and Th relative to average upper continental crust. The low concentrations of Cd in Chatham Rise, offshore South Island, and Bollons Seamount phosphorites make them potentially suitable sources for direct application fertilizers. The New Zealand marine phosphorite nodule deposits formed by repeated cycles of erosive bottom currents and phosphogenesis, resulting in the winnowing and concentration of the deposits. The iron pump model is proposed as a mechanism for the formation of apatite and associated mineral phases, giving the nodules their characteristic concentric zoning. The migration of the nodules through the oxic, suboxic, and anoxic zones of the sediment profile led to the formation of glaucony, apatite (suboxic zone), goethite (oxic zone), and pyrite with associated U enriched (anoxic zone) minerals. Rare earth elements (REE) in the Chatham Rise phosphorite nodules are associated with the glaucony rim minerals, and indicate that since the formation of the rims, very little diagenesis has occurred, preserving seawater REE patterns characterised by negative Ce and Eu anomalies and heavy REE enrichment. Site specific enrichments in trace elements Ba, V, Co, Ni, Cu, Zn, Y, Cd and Pb are attributed to either differences in incorporation of material into precursor carbonate e.g. volcanic materials, or higher fluxes of organic matter, delivering high concentrations of essential metals from biota, especially Cu and Zn. Direct pore water measurements from surficial sediment of the Chatham Rise show high concentrations of dissolved Fe and Mn, along with Cu, indicating suboxic conditions. High Cu concentrations measured in sediment pore water suggest that Cu release requires monitoring should seafloor surficial sediments on the Chatham Rise be disturbed. However, the elutriate experiments were not able to resolve if Cu release by sediment disturbance would exceed Australian and New Zealand Environment Conservation Council (2000) environmental guideline trigger values. The surrogate amphipod species Chaetocorophium c.f. lucasi shows promise as a biomonitor for disturbed marine sediments. Elements enriched in surficial sediments and phosphorite nodules, Hg, Pb, Fe, U and V, were not observed to bioaccumulate. Site specific differences in chemistry were observed, specifically in the different total relative bioaccumulation of Mo between amphipods exposed to sediments from two different sites. This suggests that future monitoring of chemical release during marine sediment disturbance requires the full geochemical characterisation of the substrate. Furthermore, fresh sediment and deep water should be used for future elutriate experiments, as storage of material by freeze-thawing and/or refrigeration causes mobilisation of some key trace metals such as U, V, Mo, Mn.</p>

Characteristics of polymetallic enrichment on oceanic red bed in Matano Formation, Baturubei, Central Sulawesi, Indonesia

The Matano Formation on Sulawesi Island has Cretaceous oceanic red bed (CORB) facies, but there is no further research on the geochemical characteristics of the sedimentary rocks. This study aims to classify the CORB of the Matano Formation and reveal the polymetallic element enrichment in CORB by using the X-ray fluorescence (XRF) analysis method. Matano Formation in the research area consists of two different facies: radiolarites facies and red clay facies. Radiolarites facies grouped as Si-ORB, dominated by siliceous pelagic bioclastic and deposited under oxic environment. The red clay facies grouped as Al-ORB, the components dominated by hemipelagic material with a small amount of silica shelled pelagically and deposited under reduction environment. Red clay facies have higher enrichment elements Mn, V, Ni, Zn and La than radiolarites facies. Oceanic red bed facies of Matano Formation deposited in basin plain environment below carbonate compensation depth in a passive margin tectonic setting.

Various sources of rare earth element enrichment at Manamas volcanic rock, Timor Island

Manamas volcanic rock formed due to crustal thinning in fore arc setting. This research aims to provide information and the enrichment process of rare earth elements in Manamas Formation on the Timor Island and their tectonic implication. Manamas volcanic rock exposed in Bihati River, Baun, Timor consists of two different types of basalts, namely alkaline basalt and sub alkaline basalt. Analysis using ICP-MS method shows enrichment in large ion lithophile element and high field strength element. Subalkaline basalt has N-MORB patterns and alkaline basalt have OIB patterns. The Nb element is relatively impoverished that indicates influence of subduction activities. Thorium and uranium elements also show significant enrichment, due to sedimentary rocks contamination or continental crust or directly from the asthenosphere due to magma upwelling. The two distinctive patterns interpreted due to slab tear phenomenon beneath Timor Island during Australia oceanic plate subduction and recycled oceanic crust beneath Banda Arc.

A new kind of invisible gold in pyrite hosted in deformation-related dislocations

Mining of “invisible gold” associated with sulfides in gold ores represents a significant proportion of gold production worldwide. Gold hosted in sulfide minerals has been proposed to be structurally bound in the crystal lattice as a sulfide-gold alloy and/or to occur as discrete metallic nanoparticles. Using a combination of microstructural quantification and nanoscale geochemical analyses on a pyrite crystal from an orogenic gold deposit, we show that dislocations hosted in a deformation low-angle boundary can be enriched in Ni, Cu, As, Pb, Sb, Bi, and Au. The cumulative trace-element enrichment in the dislocations is 3.2 at% higher compared to the bulk crystal. We propose that trace elements were segregated during the migration of the dislocation following the dislocation-impurity pair model. The gold hosted in nanoscale dislocations represents a new style of invisible gold.

The composition of subduction zone fluids and the origin of the trace element enrichment in arc magmas

The partitioning of major and trace elements between eclogite and aqueous fluids with variable salinity was studied at 700–800 °C and 4–6 GPa in piston cylinder and multi anvil experiments. Fluid compositions were determined using the diamond trap technique combined with laser ablation ICP-MS measurements in the frozen state. In addition to NaCl, SiO2 is the main solute in the fluids. The fluid/eclogite partition coefficients of the large ion lithophile elements (LILE), such as Rb, Cs, Sr, and Ba as well as those of the light rare earths (LREE), of Pb, and of U increase by up to three orders of magnitude with salinity. These elements will therefore be efficiently transported by saline fluids. On the other hand, typical high field strength elements, such as Ti, Nb, and Ta, are not mobilized even at high salinities. Increasing temperature and pressure gradually increases the partitioning into the fluid. In particular, Th is mobilized by silica-rich fluids at 6 GPa already at low salinities. We show that we can fully reproduce the trace element enrichment pattern of primitive arc basalts by adding a few percent of saline fluid (with 5–10 wt% Cl) released from the basaltic slab to the zone of melting in the mantle wedge. Assuming 2 wt% of rutile in the eclogite equilibrated with the saline fluid produces a negative Nb Ta anomaly that is larger than in most primitive arc basalts. Therefore, we conclude that the rutile fraction in the subducted eclogite below most arcs is likely < 1 wt%. In fact, saline fluids would even produce a noticeable negative Nb Ta anomaly without any rutile in the eclogite residue. Metasomatism by sediment melts alone, on the other hand, is unable to produce the enrichment pattern seen in arc basalts. We, therefore, conclude that at least for primitive arc basalts, the release of hydrous fluids from the basaltic part of the subducted slab is the trigger for melting and the main agent of trace element enrichment. The contribution of sediment melts to the petrogenesis of these magmas is likely negligible. In the supplementary material, we provide a “Subduction Calculator” in Excel format, which allows the calculation of the trace element abundance pattern in primitive arc basalts as function of fluid salinity, the amount of fluid released from the basaltic part of the subducted slab, the fluid fraction added to the source, and the degree of melting.