| The Magellan Seamounts in the Western Pacific is an important contract area for the exploration of ferromanganese crusts,contains abundant potential resources.In this paper,the morphology,mineralogy,geochemistry of 11 top surface ferromanganese crust samples(<1 mm)collected from the Kocebu Guyot were studied by using X-ray diffraction(XRD),inductively coupled plasma-mass spectrometry and atomic emission spectroscopy(ICP-MS,ICP-AES).We analyzed the chemical composition,growth rate characteristics,genesis type of the samples.The oceanographic factors that control the growth of the crusts from Western and Central Pacific were discussed by mathematical statistical methods.Comparative study revealed the environmental influences and mechanism that affect the growth rate and enrichment of rare earth elements of Kocebu crusts.The resource potential of crusts was also evaluated preliminarily.The results show that the crusts were primarily formed by plate-like shape with thickness between 3～12 cm and the substrate rocks are mainly alteration basalts.Laminar structure was observed in the top surface crust layer.XRD patterns show that the mineral component is mainly vernadite,amorphous goethite,and a small amount of quartz.The average contents of the main metal elements Mn,Fe,Co,Ni,Cu,and Zn are 21.7 %,16.0 %,0.62 %,0.54 %,0.05 %,0.06 %,respectively,and the average Mn/Fe ratio is 1.4.The average REY(rare earth elements and yttrium)abundance(∑REY)of the crusts is 1 366 μg/g,which is lower than the previous analysis results in Magellan Seamounts and Marcus-Wake Seamounts.The crusts show enriched LREE(average LREE/HREE ratio 3.73),strongly positive Ce anomalies(mean value 1.45)and negative Y anomalies(mean value 0.51).The average growth rate is 0.96 mm/Myr,which is higher than that of the Western and Central Pacific samples at the same water depth.The mineral component,chemical composition,growth rate characteristics,genetic classification diagram,seawater-crust partition coefficient indicate that all crusts are hydrogenetic origin.The Western and Central Pacific is typical low-nutrient,low-chlorophyll-A region of the ocean,far away from the land and characterized by minor detrital input,welldeveloped oxygen minimum zone(OMZ),relatively strong bottom current activity,low content of nutrient and chlorophyll a in surface water.The Fe-Mn crust samples growing in this area are situated within a variety of oceanographic environments.Statistical analysis methods were employed to determine statistically variance in major and trace element concentrations for 117 Fe-Mn crust samples from 5 different seamount chains along this area from literature.The chemical composition for 23 elements in the Fe-Mn crusts show a large variation,four factors produced by Q-mode factor analysis account for 98.32 % of the variance and is interpreted to represent elements associated with Mn oxide phase,Fe oxyhydroxide phase,debris phase and biogenic phase,respectively.The factor loadings are correlated or anticorrelated with dissolved oxygen concentration,carbonate ion concentration and detrital input in seawater,suggesting these primary parameters as the dominant controlling factor for Fe-Mn crust components in this area.Detrital input,OMZ strength and tectonic position have significant effects on the growth rate of Fe-Mn crusts.The growth rate of surface layer Kocebu crusts is higher than that of other samples at the same water depth from Western and Central Pacific,which is mainly due to the higher OMZ strength,without being affected by detrital minerals and hydrothermal input.Detrital mineral dilution,REY abundance and dissolved oxygen content in seawater are the key factors that affect the REY abundance in surface layer crusts.The low ΣREY in surface layer Kocebu crusts is related to the sampling water depth and affected by lower REY and oxygen content in overlaying water-column,but not observably diluted by detrital minerals.Comparison of grades between Kocebu crusts and PCZ(Pacific Prime Crust Zone)crusts are presented for elements of economic interest.Kocebu crusts have a higher grade for Ni,and a lower grade for Ce than PCZ crusts.Otherwise,concentrations for Mn,Fe,Co,Cu,Zn,and critical metals are roughly comparable between them.The value of the contained metal in one ton of Kocebu crusts is about $1 117 USD,confirm its resource potential and economic value.Kocebu crusts have reached the mining standard.However,the deficient sample amounts and limited water depth by far are still insufficient for quantitative estimation of crust resources or define regions of interest for Fe-Mn crust exploration.Further investigation and research are needed for exploration and mining.Crusts grown in shallow waters or sea areas with higher OMZ intensity have a lower growth rate and a greater potential for critical metals.The crusts at deeper permissive depth and grown in seawater with higher REY concentrations may have higher REY abundance.These inferences need to be verified by further investigation and analysis.Geochemistry research,resource exploration and evaluation of Fe-Mn crusts should take the influence of water depth into further consideration,the analysis of samples from limited water depth may cause large deviations in the research results. |
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