Rare earth element geochemistry of hydrothermal deposits from Southwest Indian Ridge
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Rare earth element geochemistry of hydrothermal deposits from Southwest Indian Ridge CAO Zhimin1 , CAO Hong1,2∗ , TAO Chunhui3 , LI Jun2 , YU Zenghui1 , SHU Liping1,4 1
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Key Laboratory of Submarine Geosciences and Technology, Ministry of Education, Department of Marine Geoscience, Ocean University of China, Qingdao 266100, China Key Laboratory of Marine Hydrocaobon Resources and Environment Geology MLR, Qingdao Institute of Marine Geology, Qingdao 266071, China Laboratory of Submarine GeoScience, Second Institute Oceanography, State Oceanic Administration, Hangzhou 310012, China Petrology of the Oceanic Crust, Geosciences Department, University of Bremen, Bremen 28359, Germany
Received 6 May 2011; accepted 20 October 2011 © The Chinese Society of Oceanography and Springer-Verlag Berlin Heidelberg 2012
Abstract The REE compositions of hydrothermal deposits and basalt samples from the Southwest Indian Ridge (SWIR) were determined with ICP-MS. The results show that there are significant differences between different types of samples although all samples show relative LREE enrichment. The contents of REE in hydrothermal sulfides and alterated rocks samples are lower (from 7.036×10−6 to 23.660×10−6 ), while those in the white chimney deposits are relatively higher (ranging from 84.496×10−6 to 103.511×10−6 ). Both of them are lower than basalts. Chondrite-normalized REE distribution patterns show that sulfides and alterated rocks samples are characterized by significant positive Eu anomalies. On the contrary, white chimney deposits have obvious negative Eu anomalies, which may be caused by abundant calcite existing in the white chimney samples. Both the content and distribution pattern of REE in sulfides suggest that REE most possibly is originally derived from hydrothermal fluids, but influenced by the submarine reducing ore-forming environment, seawater convection, mineral compositions as well as the constraint of mineral crystallizations. Key words: rare earth element, hydrothermal deposits, Southwest Indian Ridge
Yokosuka investigation ship in 1998, relict hydrothermal sulfides, hydrothermal chimneys and hydrothermal mounds were found in the east of the Melville fracture zone (27◦ 51 S, 63◦ 56 E), whose ages were determined varing between 70 000 and 13 000 years (M¨ unch et al., 2001). This discovery proves that high-temperature sulfide chimneys can also develop at ultra-slow spreading ridge such as the Southwest Indian Ridge. Furthermore, the sulfide samples from this field show great differences in their mineralogical, as well as chemical compositions, when compared with sulfide samples from rapid spreading ridge environment (M¨ unch et al., 2001). So, this discovery provides a new chance to better understanding the global mid-ocean ridge hydrothermal system, thus greatly enriching the hydrothermal mineralization theory. The special tectonic environment of ultra-slow spreading segment on Southwest Indian Ocean Ridge
1 Introduction After the discovery of high-temperature black smokers on the East Pacific Rise (EPR) 2
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