| Polymetallic nodules not only have great potential economic value,but also can document the changes of paleoenvironment.In this study,polymetallic nodules from Parece Vela Basin,East Philippine Sea,are studied by bulk geochemical analysis,X-ray diffraction analysis（XRD）and microscopic petrographic observation.And the mineral morphology is observed by scanning electron microscopy（SEM）.The elements distribution of nodule is analyzed by micro X-ray fluorescence spectroscopy（i XRF）.Fine scale occurrence characteristics of elements in nodules are analyzed by electron probe microanalysis（EPMA）and laser ablation inductively coupled plasma mass spectrometry（LA-ICP-MS）.Chronological framework of nodule is constructed by in-situ fine scale paleomagnetic dating and cobalt chronology.Based on these,the origin,enrichment of elements and paleoenvironmental records of the nodules are discussed below.Bulk geochemical analysis shows that,the average Fe content of nodules in study area is 19.2%,Mn content is 10.2%,Mn/Fe ratio is only 0.54 and（Co+Ni+Cu）%is0.27%.AverageΣREY（Rare earth elements and yttrium）is 967μg/g in nodules,and Ge,Se,Cd,In and Tl are relatively enriched.The main manganese minerals in nodules areδ-MnO2,but also contains a small amount of todorokite（or buserite）.Nodules in the study area have positive Ce anomaly,negative Y anomaly(YSN/Ho SN<1)and high Nd contents（>100μg/g）,which indicate that the nodules are generally hydrogenetic nodules,mainly formed in an oxidizing environment.Fine scale analysis shows that,the coats of nodule is mainly composed of hydrogenetic Fe-rich layers ofδ-MnO2,whose average FeO%is 28.7%,MnO%is21.9%,Mn/Fe ratio is 0.93 and（Co+Ni+Cu）%is 0.60;but there is a thin layer（<100μm）diagenetic Mn-rich layer at the edge,whose average FeO%is 3.41%,MnO%is52.6%,Mn/Fe ratio is 17.5 and（Co+Ni+Cu）%is 3.49%.The Mn-rich layer may have formed in suboxic bottom water environment since the last glacial maximum（20～30ka）.This study also found that there are diagenetic Mn-rich minerals within the porous rock debris at the nodule core,with MnO contents of 22.5%～64.7%,FeO contents of0.46%～7.05%and Mn/Fe ratios of 3.96～126.The core Mn-rich minerals change from laminar to fibrous,and Co,Ni and Cu tend to remain in the laminar Mn-rich minerals.The Mn-rich minerals in the core may be formed by the diagenetic activation and migration from coat of nodule.Besides,there is a"inflection point"in the enrichment of Co,Ni,Cu and REY in the nodules.The averageΣREY of nodule is 2066μg/g analyzed by LA-ICP-MS,which is significantly higher than that of the whole rock.REY（especially HREE）in nodules are mainly enriched in manganese minerals.Based on in-situ fine scale paleomagnetic dating and cobalt chronology,this paper interprets the records on marine paleoenvironment and paleoclimate evolution in nodules.The nodules in the study area grew since the early Quaternary（2.5～2.8 Ma）,with a growth rate of 1.5～8.2 mm/Ma.From the inside to the outside,the nodule coat changes from laminar to columnar.The inner laminar structure has high contents of Mo（250μg/g）,but lowδCe（1.51）and LREE/HREE ratios（7.94）,whose 10（?）manganese minerals are mainly unstable buserite I,reflecting relatively weak AABW activity and lower bottom water oxidizability;While the outer columnar structure has lower contents of Mo（123μg/g）,but higherδCe（2.07）and LREE/HREE（9.54）ratios,whose10（?）manganese minerals are mainly stable todorokite（or buserite II）,which reflects that AABW is relatively active and bottom water has higher oxidizability.The age at the boundary between the two structures is 1.1～1.5 Ma.Since～1.2 Ma,there is a good corresponding relationship between Fe/Mn ratios of nodules andδ18O of benthic foraminifera,which shows a transition from～40 ka to～100 ka,corresponding to the climate transition of the"Mid-Pleistocene Revolution". |
