Geochemical Characteristics Of Basalt And Effect Of Cobalt-rich Crusts Growth In The Mid-pacific CNW Seamount

In this paper, basalt and cobalt-rich crust samples were obtained in DY105-11voyage.Based on the microscope observation of the CNW seamounts basalt and overlyingcobalt-rich crust, we analysed the main elements, ICP-MS, electronic probe and X-raydiffraction and studied the element geochemistry characteristics and singlemineral(pyroxene) of different degree alteration samples. We discussed the relationsbetween basalt and cobalt-rich crust, get to know the following.According to the data analysis of major elements known: the basalt is tephrite，belongto alkaline series. SiO2changes in the size of the area between43.14~43.56wt%, with anaverage of43.38wt%. P2O5content is about1.9wt%, indicating that the phosphorizationin basalt is relatively small. Basalts standardized trace element distribution patterns similarto OIB, enrichment in large ion lithophile elements (LILE) and incompatible high fieldstrength elements (HFSE); REE distribution pattern are right-slop type, LREE/HREEratio in the range of8.93-9.04; enriched in LREE，indicating that there is a higher degreeof LREE fractionation and the low degree of partial melting of mantle source. CNWseamount basalt belongs Oceanic island basalt.It can be determined that CNW seamount basalts are mostly the product of originalalkaline basaltic magma occurred random RTF evolutionary process by the "random RTFmagma chamber" theory. By the incompatible element ratios range analysis in the trace element of sample, Itcan be drawn the area of basalt magma mainly related to the enriched mantle EM and highU/Pb ratio of the mantle (HIMU). And they may be exposed at the surface by formingsmall plumes during rising process, it can be inferred CNW seamount formation related tothe role of hot activities.Electron probe data analysis showed that the plagioclase in basalt is mainlylabradorite, while the pyroxene are mostly augite and diopside. Pyroxene were erosionedinto montmorillonite. In the process of alteration, Ca, Mn, Cu decreased out of the mineralwhile Fe increased into the mineral.There are two types of vesicle in the CNW seamount basalt samples, S+C-type andC-type, the edge of S+C-type vesicle filling composition rich in Si, Fe, K, Mg, Al and theinterior is filled with carbonates; C-type vesicle filling composition mainly is calcitecrystal.,which may be related with ocean water material deposition. S-type vesicle fillingcomposition is similar to the composition of alkali basalt. some elements like K, Fe, Mg ofCNW seamount basalts reduced more,It can explain the source of S-type filling materialmay be relevant of basalt dissolution. Si, Al, Fe, Mg, K of S+C-type vesicle contents werehigher than the crust, and the curve between SiO2and MgO, K2O can be drawn: basaltindirect supplies material for crust.Through the X-ray diffraction characteristics of the study on the cobalt-rich crust,components of crust including pyrochroite, quartz, anorthite, apatite and calcite. Theexistence of pyrochroite indicate a reductive environment.In the major elements of the cobalt-rich crusts: the content of Fe2O3was14.95~18.05%, indicating that the cobalt-rich crusts of the area mainly is non-phosphate typecrust. From the trace element distribution graph, all samples have similar distributionpatterns; The REE contenare is high and∑REE range is1299.65×10-6~1677.00×10-6. Eunegative anomaly; the cobalt-rich crusts are all light rare earth element enrichment,ΣLREE/ΣHREE values vary between6.71~7.45. Mn, Fe has strong positive relationship, shows vernadite and iron hydroxide have important influence on the enrichment of rareearth and trace elements.The ratio of manganese, iron, cobalt and nickel in cobalt-rich crusts: Co/Mn=0.033-0.069, Ni/Mn=0.0180-0.036，Mn/Fe=0.596-1.243,the characteristics of major main metalcontent in crust is not overrun major main metal element in seawater, It explain portionelement in crust from seawater.The study shows that the basalt is oceanic island basalt(OIB). Because of the hotspots and halmyrolysis, elements such as Mn, Fe, Co, Ni were emigrated from pyroxeneminerals, they entered water through the stoma of basalt or other channel, part of ironprecipitated in the hole wall which fomed a high iron ring, causing a local enrichment ofiron in altered basalt, while other elements entered sea water, where Mn formed intoMn-hydroxide, negative colloid, Fe/Co/Ni-hydroxide are positive colloid. Pyroxene,plagioclase erosioned into montmorillonite, kaolin and other caly minerals, incluingnegative charge. Montmorillonite clay minerals firstly adsorbed positive colloid carryingFe/Co/Ni-hydroxide and Fe/Co/Ni-hydroxide positive colloid secondly adsorbed negativecolloid carrying Mn-hydroxide, and constantly forming a crust in which iron-manganesecontent had a rhythmic change.