Fluid inclusion studies as a guide to the temperature regime within the TAG hydrothermal mound, 26°N, Mid-Atlantic Ridge

Petersen, Sven, Herzig, Peter and Hannington, Mark D. (1998) Fluid inclusion studies as a guide to the temperature regime within the TAG hydrothermal mound, 26°N, Mid-Atlantic Ridge Proceedings of the Ocean Drilling Program: Scientific Results, 158 . pp. 163-178. DOI 10.2973/odp.proc.sr.158.210.1998.

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Abstract

The active Trans-Atlantic Geotraverse (TAG) hydrothermal mound is a mature submarine massive sulfide deposit at the
slow-spreading Mid-Atlantic Ridge at 26°N. Fluid inclusion measurements were conducted on quartz and anhydrite from six
boreholes drilled in different areas of the mound to characterize the fluids responsible for the deposition of sulfide-silica breccias
and anhydrite and to investigate the vertical and horizontal temperature zonation within an actively forming hydrothermal
system. Fluid inclusions in both host minerals are generally two phase liquid/vapor inclusions that homogenize into the liquid
phase. Trapping temperatures for quartz and anhydrite from the TAG mound range from 212° to 390°C. Salinities vary from 1.9
to 6.2 wt% NaCl equivalent for anhydrite and from 4.0 to 6.0 wt% NaCl equivalent for quartz. This salinity variation is probably
best explained by supercritical phase separation at temperatures above 450°C with subsequent remixing of the liquid and
the vapor phase during ascent. A zone of anhydrite-rich precipitates recovered at 20 to 35 mbsf below the central Black Smoker
Complex (TAG-1) is characterized by trapping temperatures averaging 348°C for anhydrite and 358°C for quartz, which is
slightly below the exit temperature of hydrothermal fluids presently venting at the Black Smoker Complex (360°-366°C).
Breccias in the stockwork zone underlying the anhydrite zone were formed at slightly higher temperatures ranging from 327°-
381°C for quartz and from 349° to 384°C for anhydrite. Trapping temperatures vary strongly between different areas of the
mound. Fluid inclusions in quartz and anhydrite from the central Black Smoker Complex are characterized by a narrow range
of trapping temperatures, whereas other areas drilled on the mound were influenced by lower temperature hydrothermal fluids
percolating through the mound or by local entrainment of seawater into the mound. White smokers venting on the southeastern
side of the TAG mound are characterized by exit temperatures of 270°-300°C, (Kremlin area, TAG-2). Fluid inclusion measurements
in quartz and anhydrite from this area give trapping temperatures in the range of 266°-375°C with a distinct peak
around 340°C, only somewhat lower than results for the Black Smoker Complex. Trapping temperatures in anhydrite-hosted
fluid inclusions in this area show a strong vertical temperature increase. The west side of the mound (TAG-4) is characterized
by trapping temperatures ranging from 212° to 390°C showing evidence for seawater entrainment or overprinting by lower
temperature hydrothermal events at the sulfide/basalt interface. Samples from the northern side of the mound (TAG-5) exhibit
trapping temperatures in the range from 258°-383°C with a strong vertical temperature increase, indicating additional hightemperature
upflow at the northern margin of the mound outside the central Black Smoker Complex.

Document Type: Article
Keywords: ODP Leg 158, fluid inclusion, temperature, TAG hydrothermal mound, Mid-Atlantic Ridge
Research affiliation: OceanRep > GEOMAR
Refereed: Yes
DOI etc.: 10.2973/odp.proc.sr.158.210.1998
ISSN: 1096-7451
Projects: ODP
Date Deposited: 18 Feb 2008 17:24
Last Modified: 15 May 2019 11:30
URI: http://eprints.uni-kiel.de/id/eprint/4228

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