Geochemistry and mineralogy of a silica chimney from an inactive seafloor hydrothermal field (East Pacific Rise, 18°S)

Type Article
Date 2015-11
Language English
Author(s) Dekov V. M.1, Lalonde Stefan2, Kamenov G. D.3, Bayon Germain1, Shanks W. C., Iii4, Fortin D.5, Fouquet Yves1, Moscati R. J.4
Affiliation(s) 1 : IFREMER, Dept Geosci Marines, Lab Geochim & Metallogenie, F-29280 Plouzane, France.
2 : Univ Brest, Lab Domaines Ocean, UMR 6538, CNRS,Inst Univ Europeen Mer, F-29280 Plouzane, France.
3 : Univ Florida, Dept Geol Sci, Gainesville, FL 32611 USA.
4 : US Geol Survey, Denver, CO 80225 USA.
5 : Univ Ottawa, Dept Earth & Environm Sci, Ottawa, ON K1N 6N5, Canada.
Source Chemical Geology (0009-2541) (Elsevier Science Bv), 2015-11 , Vol. 415 , P. 126-140
DOI 10.1016/j.chemgeo.2015.09.017
WOS© Times Cited 10
Keyword(s) Chimney, C-O-Si-Sr-Nd-Pb-Th-U isotopes, Hydrothermal, Seafloor, Silica
Abstract An inactive vent field comprised of dead chimneys was discovered on the ultrafast East Pacific Rise (EPR) at 18°S during the research campaign NAUDUR with the R/V Le Nadir in December 1993. One of these chimneys was sampled, studied and found to be largely composed of silica-mineralized bacterial-like filaments. The filaments are inferred to be the result of microbial activity leading to silica (± Fe-oxyhydroxide) precipitation. The chimney grew from the most external layer (precipitated 226 ± 4 yr. B.P.) towards the central chimney conduit. Hydrothermal activity ceased 154 ± 13 yr. B.P. and the chimney conduit was completely sealed. Mixing between an end-member hydrothermal fluid and seawater explains the Sr–Nd isotopic composition of the chimney. Seawater was the major source of Sr to the chimney, whereas the dominant Nd source was the local mid-ocean ridge basalt (MORB) leached by the hydrothermal fluids. The mixing scenarios point to a dynamic hydrothermal system with fluctuating fluid compositions. The proportion of seawater within the venting fluid responsible for the precipitation of the silica chimney layers varied between 94 and 85%. Pb-isotope data indicates that all of the Pb in the chimney was derived from the underlying MORB. The precipitation temperatures of the chimney layers varied between 55 and 71 °C, and were a function of the seawater/end-member hydrothermal fluid mixing ratio. δ30Si correlates with the temperature of precipitation implying that temperature is one of the major controls of the Si-isotope composition of the chimney. Concentrations of elements across the chimney wall were a function of this mixing ratio and the composition of the end-member hydrothermal fluid. The inward growth of the chimney wall and accompanying decrease in wall permeability resulted in an inward decrease in the seawater/hydrothermal fluid mixing ratio, which in turn exerted a control on the concentrations of the elements supplied mainly by the hydrothermal fluids. The silica chimney is significantly enriched in U, likely a result of bacterial concentration of U from the seawater-dominated vent fluid. The chimney is poor in rare earth elements (REE). It inherited its REE distribution patterns from the parent end-member hydrothermal fluids. The dilution of the hydrothermal fluid with over 85% seawater could not obliterate the particular REE features (positive Eu anomaly) of the hydrothermal fluids.
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