Study On Mineralogy, Geochemistry And Paleoenvironmental Record Of Co-rich Crusts From Western Pacific

The Co-rich crust is rich in a variety of strategic metallic elements, its slow growth rate and being less subjected to outside interference enable it to be a new kind of submarine resource, attracting worldwide attention. In this dissertation,2 Co-rich crusts collected both from Magellan and Marcus-wake seamounts in western pacific have been analyzed for systematic understanding of their mineralogical and geochemical characteristics. By using XRD, ICP-AES(MS) methods, the mineral and geochemical composition, the leaching distribution of major and REE of the crusts were determined, based on which, the genetic classification, material sources of the samples have been discussed. Furthermore, the paleoenvironmental record of the samples have been further explored based on the mineral and element data after determining the growth period of the samples.The crust obtained from Magellan seamounts is layer-shaped with 3 layers from its surface to center, each layer is characterized by different kinds of microstructure under microscope observation, including columnar structure, dendritic structure and taxitic structure, which shows the hydrodynamism in the local area varies a lot raning from one period to another. The crust from Marcus-wake seamounts is nodule-shaped and it is mainly characterized by dendritic structure under microscope observation,The result of mineralogical analysis shows that Maganese minerals and Iron minerals are the main components of the crusts, Vernadite are the main component of manganese oxide with a small amount of todorokite and Birnessite, and amorphous FeOOH is the most abundant component of the ferric mineral and only weakly Goethite peak can be identified under X-ray. Besides, Other minerals such as detrital and clay minerals (including quartz, plagioclase) are identified in some specific layers of the crusts.The average concentration of Mn, Fe, Co, Cu, Ni of the samples equals that of crusts both in central and northwest Pacific, with an average value of 20.08% and 19.01%,16.28% and 16.52,0.68% and 0.64%,0.15% and 0.09%,0.34% and 0.39% respectively, but the Mn/Fe ratio of the samples are lower (ranging from 0.87 to 1.68), showing the crusts are typically hydrogenetic; As for the trace element, the crusts are characterized by high Pb and Mo value under the control of ferric mineral. The REE in crusts are highly enriched, with average ΣREE value of 2182.89 x 10-6 and 1367.29 x 10-6, respectively. The average LREE/HREE values of the crusts are 10.14 and 7.67 respectively, indicating the crusts are relatively enriched in LREE. Also, the REE diagram of the crusts shows the positive anomaly of Ce, Gd, Ho and negative anomaly of Y, suggesting the crusts are both hydrogenetic and seem to be less affected by diagenesis and phosphatization.Phase elemental analysis showed that manganese mineral phase adsorption of trace elements (except Co, Ni addition) is greater than the iron mineral phase; REE in crusts have multiphase origins of which about 90% comes from the iron and manganese phase, which is the main factors causing δCe,δGd and δHo positive anomalies and negative anomalies δY. Based on the results of phase elemental analysis, the elements can be classified as follows:(1) cation exchange and carbonate phase:Ca, Mg, Sr, Rb,;(2) Manganese phase:Mn, Ni, Ba, Co, Cu, Zn, Sr, Ti, REE, Be, Ga, Cd, U;(3) the amorphous iron phases:Fe, Mo, Al, Pb, Bi, Th, Rb, Be, U;(4) crystalline phases of iron:Fe, Al, Hf, W,(5) residues phase:Al, Mg, Rb, Cs.The crusts in our study were classified into hydrogenetic origin. The element composition is controlled orderly by metallogenic adsorption of the Fe-oxyhydroxide fraction, Mn-oxide fraction, and physical mix of biogenic or continental debris..The growth rates of the sample MKD30 and CM2D08 are 2.55 mm/Ma and 1.87 mm/Ma, respectively, and the initial growth ages of them are 19.26 Ma and 7.29Ma, respectively. The former may undergo a growth hiatuse corresponding to the age between 8.05Ma and 11.3Ma.Further studies revealed that the Al/(Fe+Mn), Co/(Ni+Cu) and Fe/Mn records of the sample enable to reflect the evolution of the Asian monsoon, the oxidation degree of benthic seawater and the paleoclimate changes between icehouse and warming periods respectively and the relationship among them.