Geochemistry and sulfur-isotopic composition of the TAG hydrothermal mound, Mid-Atlantic Ridge, 26°N

Herzig, Peter, Petersen, Sven and Hannington, Mark D. (1998) Geochemistry and sulfur-isotopic composition of the TAG hydrothermal mound, Mid-Atlantic Ridge, 26°N Proceedings of the Ocean Drilling Program: Scientific Results, 158 . pp. 47-70. DOI 10.2973/odp.proc.sr.158.202.1998.

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Abstract

Eighty-five bulk samples consisting of varying proportions of pyrite, silica, and anhydrite and 82 mineral separates (pyrite,
chalcopyrite) from the TAG hydrothermal mound were analyzed using Neutron Activation Analyses (INAA), Inductively Coupled
Plasma Emission Spectrometry (ICP-ES), Inductively Coupled Plasma Mass Spectrometry (ICP-MS), and sulfur-isotopic
methods. The samples were collected from five different areas of the Trans-Atlantic Geotraverse (TAG) mound during Ocean
Drilling Program Leg 158. The chemistry of the bulk samples is dominated by high Fe (average 30.6 wt%, n = 57) and S concentrations
(average 42.0 wt%, n = 50), reflecting the high amount of pyrite in these rocks. High Ca (up to 11.5 wt%, n = 57)
and SiO2 values (up to 49.8 wt%, n = 50) indicate the presence of anhydrite-rich zones in the center of the mound, and pyritesilica
breccias, silicified wallrock breccias, and paragonitized basalt breccias deeper in the system. The Cu and Zn concentrations
vary from <0.01 to 12.2 wt% Cu (average 2.4 wt%, n = 57) and from <0.01 to 4.1 wt% Zn (average 0.4 wt%, n = 57), with
highest values commonly occurring in the uppermost 20 m of the mound. Most trace-element concentrations are relatively low
compared to other mid-ocean ridge hydrothermal sites and average 0.5 ppm Au, 43 ppm As, 234 ppm Co, 2 ppm Sb, 14 ppm Se
(n = 85), 9 ppm Ag, 11 ppm Cd, and 59 ppm Pb (n = 57). Gold, Ag, Cd, Pb, and Sb behave similarly to Cu and Zn and are
enriched close to the surface of the mound. This is interpreted as evidence for zone refining, a process in which elements that
are mobilized from previously deposited sulfides in the interior of the mound by later hydrothermal fluids are transported to the
surface, where they reprecipitate as a result of mixing with ambient seawater. The trace-element composition of pyrite and chalcopyrite
separates is similar to the bulk geochemistry. However, down to about 50 mbsf, Au, As, Sb, and Mo values in pyrite
separates are generally higher than in bulk samples and chalcopyrite separates. Below this depth, these elements appear to be
enriched in chalcopyrite separates. Cobalt is typically more enriched in pyrite than in chalcopyrite throughout. A major difference
between pyrite and chalcopyrite separates is the strong enrichment of Se in chalcopyrite at the top of the mound, whereas
pyrite separates show a moderate increase of Se with depth. Sulfur-isotopic values for bulk sulfides from the interior of the
TAG mound vary from +4.6‰ to +8.2‰, with an average of +6.4 ‰ d34S (n = 49). These values do not change significantly
downhole, but samples collected from the top of the mound appear to have somewhat lower d34S values than samples from the
interior. The average d34S value for TAG sulfides is about 3‰ higher than for most other sulfides generated at sediment-free
mid-ocean ridges (average 3.2‰, n = 501). This is largely attributed to thermochemical sulfate (anhydrite) reduction by hightemperature
hydrothermal fluids upwelling through the interior of the TAG mound.

Document Type: Article
Research affiliation: OceanRep > GEOMAR
Refereed: Yes
DOI etc.: 10.2973/odp.proc.sr.158.202.1998
ISSN: 1096-7451
Projects: ODP
Date Deposited: 18 Feb 2008 17:24
Last Modified: 15 May 2019 11:27
URI: http://eprints.uni-kiel.de/id/eprint/6622

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