Temporal And Spatial Enrichment Of Metallogenic Elements In Polymetallic Crusts From Central And Western Pacific And Its Paleoceanographic Significance

Polymetallic crust is a type of hydrogenetic mineral resources which grows in layers on the sea floor.Its growth process records the evolution history of paleoceanographic environment which on the other way round controls its growth conditions.Based on the regional applicable Co-empirical formula and the Os isotope stratigraphy,as well as by referring to the evidence of structural characteristics,phosphatization stages and the microfossil chronology,the regional age frame of the crusts from the central and western Pacific has been obtained.Based on the results of plate circuit model,plate migration geometry method,apparent pole wandering method and hot spot tracking method,the paleolatitude and longitude of seamounts where crusts located were reconstructed.By applying the model of thermal evolution of the lithosphere,the paleodepth in the process of crust mineralization was reconstructed.Based on the results of the age frame and the 3D-migration-subsidence history,the temporal and spatial variation characteristics of the main ore-forming elements were studied,and their paleoceanographic significance were discussed.The following understandings have been obtained:1)The regional applicable Co-formula for the crusts in each seamounts of the study area were found.The reason for the regional applicability of the formulas is related to the heterogeneity of Co flux into the sea.This study summarized the growing and hiatus periods of the crusts in the central and western Pacific,in which the growing stages corresponds to the periods of strong chemical weathering,high carbonate dissolution rate and high continental dust flux during geological history,while the hiatus corresponds to the large-scale phosphatization events.2)4 specific features of the Os isotopic composition evolution curve recorded in the crusts have been found.They could be treated as the time anchor in Co-Os dating system,so as to improve the accuracy and reliability.The Os isotopic composition curves of crusts from Marcus-Wake and Line seamounts are higher than curve of sea water since 7 Ma and before 33 Ma,which relates to the process that the crusts drifting in and out of the equatorial trade wind and the prevailing westerlies belt following the Pacific plate.3)The crusts have a high growth rate in the period of high carbonate dissolution rate and high continental dust flux in history,and during which,the crusts received sufficient ore-forming materials.The crusts from Line seamounts have the relatively higher growth rate.There are two interpretations for that:1)the samples are located to the east of the study area,so that they have been directly nourishing by the current from the east of the Pacific,receiving the continental metallogenic materials from the American continent;2)The crusts in this area were generally growing in deeper water,which makes them grow in a more suitable metallogenic environment beneath the OMZ.4)The completeness of each structural layer of crusts in different regions is different and its growing duration and thickness depend on the existence and absence of the sublayers.The corresponding age of the structural layers of the crusts in the west and east of the study area is relatively older than that in the northeast,which reflects that the growth of the crusts of the former took the lead in reaching the appropriate conditions for the development of the corresponding layer in longitude,latitude and depth.5)The content of the main ore-forming elements in the crust has a regulation through time or region.Some profiles of the elements correlate to mineralization age,for example,P and Ca are controlled by phosphatization,while Co,Mg,Ni,Cu and Ba,Pb,Mo,V,Ti,REE are separately related to seawater oxidation and cumulative adsorption.Some of the profiles correlate to latitude and longitude,for instance,K,Si,Al,Fe,¹⁸⁷Os and Cu,Zn are separately controlled by the evolution of dust flux and paleoproductivity of seawater.