Rare earth element compositions of chlorite-rich hydrothermal sediments in the middle Okinawa Trough, East China Sea

Author(s): Shao, H.; Yang, S.; Humphris, S. E.; Cai, D.; Wang, Q.
Author Affiliation(s): Primary:
Tongji University, Shanghai, China
Other:
Woods Hole Oceanographic Institution, Woods Hole, MA, United States
University of Tokyo, Atmosphere and Ocean Research Institute, Tokyo, Japan
Volume Title: AGU 2015 fall meeting
Source: American Geophysical Union Fall Meeting, Vol.2015; American Geophysical Union 2015 fall meeting, San Francisco, CA, Dec. 14-18, 2015. Publisher: American Geophysical Union, Washington, DC, United States
Note: In English
Summary: Rare earth elements (REEs) have been used as powerful tracers in the study of hot fluid-rock reaction in hydrothermal system. However, the behavior of the REEs during interaction of hydrothermal solution with rocks remains to be clarified more quantitatively. The Okinawa Trough (OT), located in the East Asian continental margin, is characterized by thick terrigenous sediment and ubiquitous volcanic-hydrothermal activities. In this study, the sediments collected during IODP Expedition 331 to the mid-OT were determined for mineralogical and REEs as well as Nd isotopic compositions, aiming to investigate the geochemical behavior of REEs during hydrothermal processes. All samples were separated into bulk and clay-size (< 2 µm) fractions and pretreated by 1N HCl to remove carbonate and other unstable components. The hydrothermal sediments in the mid-OT is dominated by Mg-rich chlorite based on the XRD analysis, especially the clay-size fraction comprising pure chlorite. The clay-size samples bear different mineralogical and geochemical compositions between the upper and lower parts in this hydrothermal area. All hydrothermal samples are relatively enriched in HREEs in the residues. The upper clays have higher values of δEu and (La/Yb)N as well as LREEs concentration than the lower part, while the bulk samples have weak REE differentiation. In the 1N HCl leachates, the concentrations of REEs in the bulk samples are higher than in the clays in the lower part but LREEs are obviously enriched in the upper clays. We infer that the grain size may be an important factor controlling the behavior of REEs in the mid-OT hydrothermal system. Both bulk and clay samples show negative Eu anomalies relative to chondrite, similar as the detrital sediments and volcanic rocks. This study confirms the hypothesis that HREEs patterns are constant throughout the system compatible with higher LREEs mobility as chlorine ion complexes in acidic solutions (Mills and Elderfield 1995; Douville et al. 1999). The Nd isotopic compositions (143Nd/144Nd and (open e)Nd) reveal the origin of hydrothermal fluids with the values close to hosted rocks. Our study also suggests variable REEs compositions of hydrothermal deposits between the sediment-covered OT and sediment-starved mid-Ocean Ridge, diagnostic of their different geological settings and origins.
Year of Publication: 2015
Research Program: IODP Integrated Ocean Drilling Program
Key Words: 02 Geochemistry; Cores; Expedition 331; Integrated Ocean Drilling Program; Marine sediments; Metals; North Pacific; Northwest Pacific; Okinawa Trough; Pacific Ocean; Rare earths; Sediments; West Pacific
Coordinates: N274700 N274800 E1265500 E1265300
Record ID: 2016109998
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