Major and trace element composition of surface sediments from the Southwest Indian Ridge: evidence for the incorporation
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Major and trace element composition of surface sediments from the Southwest Indian Ridge: evidence for the incorporation of a hydrothermal component LI Zhenggang1, 2, CHU Fengyou1, 2*, JIN Lu1, 2, LI Xiaohu1, DONG Yanhui1, CHEN Ling1, 2, ZHU Jihao1 1 Key Laboratory of Submarine Geoscience, Second Institute of Oceanography, State Oceanic Administration (SOA),
Hangzhou 310012, China 2 Department of Earth Sciences, Zhejiang University, Hangzhou 310027, China
Received 5 December 2014; accepted 31 March 2015 ©The Chinese Society of Oceanography and Springer-Verlag Berlin Heidelberg 2016
Abstract Hydrothermal materials in deep-sea sediments provide a robust tracer to the localized hydrothermal activity at mid-ocean ridges. Major, trace and rare earth element (REE) data for surface sediments collected from the ultraslow spreading Southwest Indian Ridge are presented to examine the existence of hydrothermal component. Biogenic carbonate oozes dominate all the sediment samples, with CaO content varying from 85.5% to 89.9% on a volatile-free basis. The leaching residue of bulk sediments by ~5% HCl is compositionally comparable to the Upper Continental Crust (UCC) in SiO2, Al2O3, CaO, MgO, alkali elements (Rb, Cs) and high field strength elements (Nb, Ta, Zr, Hf, Ti). These detritus-hosted elements are inferred to be prominently derived from the Australian continent by means of eolian dust, while the contribution of local volcaniclastics is insignificant. In addition, the residual fraction shows a clear enrichment in Fe, Mn, and Ba compared with the UCC. Combining the positive Eu anomaly of residual fraction which is opposed to the UCC but the characteristic of hydrothermal fluids and associated precipitates occurred at mid-ocean ridges, the incorporation of localized hydrothermal component can be constrained. REE mixing calculations indicate that more than half REE within the residual fraction (~55%–60%) are derived from a hydrothermal component, which is inferred to be resulted from a diffuse fluid mineralization. The low-temperature diffuse flow may be widely distributed along the slow-ultraslow spreading ridges where crustal faults and fissures abound, and probably have a great mineralization potential. Key words: deep-sea sediments, hydrothermal component, diffuse fluid, eolian dust, Southwest Indian Ridge Citation: Li Zhenggang, Chu Fengyou, Jin Lu, Li Xiaohu, Dong Yanhui, Chen Ling, Zhu Jihao. 2016. Major and trace element composition of surface sediments from the Southwest Indian Ridge: evidence for the incorporation of a hydrothermal component. Acta Oceanologica Sinica, 35(2): 101–108, doi: 10.1007/s13131-015-0678-8
1 Introduction Seafloor hydrothermal activity along the mid-ocean ridges is an important contributor to the ocean heat flux (Elderfield and Schultz, 1996). The incidence of hydrothermal plumes was proposed to be a linear function of the spreading rate on the basis of investigation on the intermediate-fast spreading mid-ocean ridges (e.g., Baker et al., 1996; Baker and German, 2004). This model predicts
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