Research On The Growth Layer Formation Conditions And Metallogenic Mechanism Of Polymetallic Nodules In CCZ Of East Pacific Ocean

The deep sea contains abundant valuable mineral resources,prominently featuring polymetallic nodules that are widespread throughout vast deep-sea basins.These nodules contain vital deposits of strategically important metals,such as copper,nickel,and cobalt.Polymetallic nodules grow slowly and have experienced intricate and long-lasting changes in a paleoenvironment,resulting in various growth characteristics and complex mineralization patterns of nodules.Nevertheless,there are limited studies on the pattern of change of the nodule growth layer,especially the difficulties and deficiencies in the study of diagenetic nodule growth layers with high growth rates and notable Ni and Cu concentrations,thus limiting our understanding of the mineralization mechanism reflected in the growth layer.This study focuses on investigating the polymetallic nodules in the western,central,and eastern parts of the Clarion–Clipperton Fault Zone（CCZ）within the eastern Pacific Ocean.Information on the structure,mineralogical and geochemical compositions,isotope geochemistry,and ages of different nodule layers are obtained through X-ray diffraction,electron microprobe analysis,laser ablation inductively coupled plasma mass spectrometry（LA–ICP–MS）,and Sr–Nd–Pb–Hf radioisotope and ¹⁰Be isotope dating.Further,we study the growth layer characteristics of polymetallic nodules in different areas of the CCZ,revealing the differences in the nodule growth layers and controlling factors for the enrichment of metallogenic elements.Through combining with the ¹⁰Be isotope dating framework and spatial nodule distribution,we also perform a comprehensive investigation of the main controlling factors for the spatial and temporal distributions of the nodule mineralization.Thus,a mineralization model for the nodule is established within the study area.The main conclusions and insights gained from this study are summarized below:1.The chronological framework of polymetallic nodules in the CCZ of the eastern Pacific Ocean is established through ¹⁰Be isotope dating,revealing the major growth periods and growth hiatuses of the nodules.The nodules in the western,central,and eastern parts of the CCZ have similar growth intervals,with Layer I forming around 4.7 Ma,Layer II growing between 4.5 and 3 Ma,and Layer III growing after 3 Ma.This phenomenon indicates that the Pliocene was an important developmental period for polymetallic nodule formation in the CCZ of the eastern Pacific Ocean.A clear growth hiatus of approximately 3 Ma exists between Layers II and III in the western,central,and eastern nodules.The growth rate varies from high to low（i.e.,from an average of 9.62 mm/Ma to an average of 1.3 mm/Ma）before and after this hiatus,which may have been influenced by considerable global events,including the rapid cooling of global climate and Arctic ice sheet formation during this period.2.Discrepancies in the chemical composition between the inner and outer nodule layers within the CCZ reveal variations in the suboxic and oxic diagenetic environments.Our analysis confirms that the Mn/Fe ratios（>10）within the inner layers of diagenetic nodules（Layers I and II）are consistently higher than those（5-10）observed in the outer layers of the nodules（Layer III）in the eastern and central parts of the CCZ.Notably,the mineral composition is primarily characterized by todorokite dominance.The fluxes of Mn and Fe elements in the nodules show that the differences in the Mn/Fe ratios are mainly caused by changes in the Mn supply from pore water.During the suboxic diagenetic phase of early diagenesis,pore water contains a significant amount of Mn²⁺,which consequently leads to the development of nodule growth layers,characterized by high Mn flux and Mn/Fe ratios.In contrast,in the oxic diagenetic stage,pore water has low Mn²⁺content;hence,the influx of Mn into the nodules is relatively low,yielding a relatively small Mn/Fe ratio within the nodules.According to the Mn flux characteristics,the nodule growth environment in the CCZ changes from an inner layer of the suboxic diagenetic environment to an outer layer of the oxic diagenetic environment.3.The controlling factors of the Ni and Cu enrichments are discussed.The inner layers of the diagenetic nodules within the CCZ typically exhibit Ni deficit with respect to Cu.Analyses on the elemental fluxes reveal that the primary driver for this phenomenon is the growth rate of the nodules.Additionally,high growth rates can considerably dilute the Ni enrichment,but exert a constrained influence on the Cu enrichment.This phenomenon could potentially arise from the different enrichment mechanisms governing Ni and Cu within the pore water.Ni is mainly released from manganese minerals through the reduction of manganese oxides.When the organic matter flux in the sediments is too high,the Ni release flux does not increase.Cu is mainly released from organic matter as a result of its decomposition.Hence,Cu enrichment is positively correlated with the organic matter flux.4.The diversity of the diagenetic nodule layers and their possible formation mechanism are explored.The polymetallic nodule growth layer data were extracted through LA–ICP–MS mapping,demonstrating the diagenetic layer diversity.Combined with the two-endmember mixing model,the diversity in the nodule formation was found to be caused by changes in the diagenetic endmember.In other words,different nodule growth layers were formed due to variations in the pore water compositions in the sediments.Three special layers were identified with a composition that is markedly different from the geochemical characteristics of the other growth layers.Their possible formation mechanisms are as follows:（1）The formation of diagenetic layers with low Ni（<1 wt.%）and Cu（<1.5 wt.%）and high Co（0.4–0.8 wt.%）contents in the nodules in the central part of the CCZ may be attributed to the possible incorporation of metallic elements（e.g.,Co and Ce）,which were released from the buried-nodule and micronodule dissolution under reducing conditions,into the pore water.This process can increase the metallic element concentration in the pore water,such that under the effect of the concentration gradient,the deep pore water can migrate upward over a greater distance to the nodule growth location,consequently supplying additional Co and Ce to the nodules.（2）The Co-poor（0.05 wt.%）diagenetic nodule layers in the central part of the CCZ,characterized by high growth rate of 18 mm/Ma,was probably formed in a locally confined suboxic environment.This environment may have been formed by the extensive biogenic dissolution in the sediments,gathering large amounts of suboxic pore water,which provided sufficient amounts of Mn to nodules and ensured that they had the highest growth rate.（3）The formation of Ni-poor（0.5 wt.%）,Cu-poor（0.5 wt.%）and Co-poor（0.05 wt.%）diagenetic nodule layers in the eastern part of the CCZ may have been caused by pore water with certain hydrothermal properties,including Ni,Cu,and Co depletion,very low Ce anomalies,and Li,Mo,and Zn enrichment,which were likely to have originated from the mid-oceanic ridges of the eastern Pacific Ocean or from the regional intraplate volcanic activity.5.A model of the nodule mineralization in the CCZ of the eastern Pacific Ocean is established,elucidating how the primary productivity and the Antarctic Bottom Water（AABW）control the nodule mineralization by altering the sediment deposition rates and redox conditions and elucidating the temporal and spatial differences in the nodule mineralization in the CCZ under the action of different controlling factors.Temporally,in Stage I of nodule mineralization（Layers I and II）,the biological productivity is high,the AABW is weak,the nodule growth is dominated by suboxic diagenesis,and nodule mineralization is characterized by Mn and Cu enrichment.In Stage II of nodule mineralization（Layer III）,the biological productivity decreases,the AABW is stronger,the nodule growth is dominated more by oxic–suboxic mixing,the growth rate decreases,and nodule mineralization is characterized by Ni enrichment.Spatially,the western part of the CCZ is affected by the lowest primary productivities and strongest AABW.The nodules in this part are dominated by a hydrogenetic process and enriched in Co.The southern and eastern parts of the CCZ have high biological productivities;therefore,the nodules in these areas are more enriched in Mn and Cu,supplied by the suboxic diagenetic process.By contrast,in the case of Ni,the biological productivities are too high and nodules are growing rapidly,which is detrimental to Ni enrichment.The mineralizing element Ni is more enriched during stage II of nodule mineralization,which is mainly developed in areas exhibiting medium biological productivity,including the nodules in the north-central and west-central parts of the CCZ.This study provides a systematic analysis and discussion on the mineralogy,geochemistry,and isotope geochemistry of the nodule growth layer within the CCZ.This comprehensive exploration effectively supplements existing research on nodule growth layers,which is relatively limited.The findings hold substantial significance for comprehending the attributes of the nodule growth layers in the CCZ and enriching the nodule mineralization theory,offering valuable insights that will guide nodule exploration efforts.