Mineralogy Petrology And Geochemistry Genesis And Composition Characteristics Of Basalts Of CA Seamounts In The Central Pacific Ocean

The CA seamounts in Central Pacific ocean is a key area where the cobalt-rich crust broadly distributed.Research in geochemistry and mineralogy of basalts from seamounts helps gaining insight into the source area characteristics,formation mechanism of seamount and the impact of basalts on cobalt-rich crust's formation and growth.CA seamounts located in the south-central Central Pacific Ocean seamount group and¹400n miles in the west by south of island Honolulu.Flight DY105-11,DY105-12 and DY105-14 of China have made a reconnaissance of geography and incrustation as well as other mineral resources of CA seamounts.Geochemistry and mineralogy of CA seamounts basalts and coblat-rich crust collected by flight DY105-11,based on electronic probe,ICP-MS and X-ray diffraction have been evaluated and the recognition is as follows:Results in petrology show that the rocks possess characteristics of high value of Al,K and Ti,and low Mg and NaO/K₂O.Compared with CM seamount basalts in adjacent region,the rocks shows characteristics of high Si,Al,K,Na and Ti,and low Ca and Mg.Trace elements distribution is similar to oceanic island basalt?OIB?,and enrich large ion lithophile element?LILE?and incomparible high field strong elements?HFSE?.The REE distribution patterns of rocks present characteristics of right slant and enrich LREE.LREE/HREE=4.87⁵.06.La_N/Yb_N=6.81¹2.20 indicates relatively high degree light-heavy RE fractionation.Negative anomaly of Ce is probably related to phosphate rocks and volcaniclastic rocks in seamounts area.The REE features show negative abnormality of Eu.Geochemistry characteristics of CA seamounts indicates that the rocks are basalt and trachybasalt,belonging to alkaline series.Evolutionary procession of basalts is dominated by partialing melting.The ratios of incompatible elements of trace elements indicate the basalt magma is mainly from EM and HIUM type mantle.Component of EM type mantle can't directly enter the source area of HIMU type mantle is widely recognized,therefore,it's probably tha EM and HIMU type mantles come together into a small mantle plume in the procession of rising and form the CA seamounts.Furthermore,the CA seamounts are inferentially effected by hot spots in the formation.Data of electron probe analysis shows that the mainly basalt rock-forming minerals are plagioclase and pyroxene.Zone texture developed in phenocryst of plagioclase with labradorite distributed in edge and bytownite in centre,indicating a normal zone formed in magmatic evolution.The average evaluated temperatures of the centre?1281??and the edge?1198??of phenocryst and crystallite?1071??of plagioclase are respectively represent the temperatures of the source of magma,magma chamber and eruption.All the phenocrysts of pyroxene are clinopyroxene,present no transformation and zone texture.Crystallization temperatures of clinopyroxene evaluated by two thermobarometers are1143?and 1168?,with pressures in the area of 0.78×10²⁷.43×10²MPa.The corresponding forming depth relation to pressure is 2.57²4.53km.Compared with average thickness of oceanic crust?5⁶km?,crystallization depth of pyroxene is obviously greater than oceanic crust,indicating deep source characteristic.Analysis results show that vesicles in basalts are mainly filled by ilmenite,apatite,phillipsite,opal and other minerals,indicating the basalts are affected by seawater hydrolysis and phosphatization,Compared with adjancent area,the rocks have a weak connection with carbonatation.Based on X-Ray diffraction,the mainly minerals of alterated basalt are composed of boiling apatite,phillipsite and smectite.Analysis results from incrustation samples show that,Co/Mn?0.021⁰.050?,Mn/Fe?1.107¹.925?and Ni/Mn?0.014⁰.023?are less than the same element ratio in seawater,indicating the key metallogenic elements are from seawater.Hence,CA seamounts basalts are OIB basalts,associating with the origin of mantle plume.The hot spots contribute to the formation and the alteration of basalts as well as the transformation of matter.Manganese,iron,cobalt,nickel and other metallogenic elements of pyroxene and other minerals migrate to seawater by means of vesicles and other path on the effect of hot spots and halmyrolysis of seawater.Iron,cobalt and nickel hydroxides are colloid of positive charge,well manganese hydroxides are colloid of negative charge.Iron and manganese oxides&hydroxides such as FeOOH and?-MnO₂ are the mainly composition of cobalt-rich crust.Clay mineral smectite in alterated are negatively charged and phillipsite are positively charged.Phillipsite are relatively less in vesicles,and two forming mechanisms of cobalt-rich crust deduced are as follows:negatively charged smectite adsorb positively charged colloid of iron,cobalt and nickel hydroxides,and then,positively charged colloid of iron,cobalt and nickel hydroxides adsorb negatively charged manganese hydroxides,the cyclic adsorption cause the rhythmic changed?iron&manganese content?incrustation;another one is that smectite adsorb colloid of iron,cobalt and nickel hydroxides,positively charged phillipsite in vesicles absorb negatively charged manganese hydroxides in the same time,thus,the initial crust formed and surround around the alterated basalts,and iron,cobalt and nickel hydroxides in the initial crust adsorb negatively charged manganese hydroxides in seawater,and the continual growth form the cobalt-rich crust with parallel laminated structure inside ultimately.