21. PROBING THE TAG HYDROTHERMAL MOUND AND STOCKWORK: OXYGEN-ISOTOPIC PROFILES FROM DEEP OCEAN DRILLING1

Jeffrey C. Alt2 and Damon A.H. Teagle2

ABSTRACT

Oxygen isotope analyses carried out on quartz separates and whole rocks from the Trans-Atlantic Geotraverse (TAG) hydrothermal mound and the underlying stockwork zone document temperatures and processes within and beneath the mound. The mound is 25-40 m thick and consists of pyrite-rich, anhydrite-rich, and siliceous breccias, which nearly all contain quartz. The underlying pervasively quartz-veined stockwork is zoned, with quartz + paragonite + pyrite assemblages down to 101 mbsf, followed by paragonite + quartz + pyrite to 111 mbsf, and then chlorite + quartz + pyrite assemblage to the maximum depth penetrated, 125.7 mbsf.

Basement beneath the eastern and western margins of the mound ranges from fresh basalt (delta18O = 5.9‰), to basalt affected by low-temperature smectitic alteration (delta18O = 6.4‰-6.8‰), and partly to totally chloritized basalts (delta18O = 2.1‰-4.5‰). Whole-rocks from the deep chloritic stockwork have delta18O values of 2.1‰-5.7‰, and chlorites have inferred values of 1‰-3‰. Chloritization of basalt at the margins of the mound and at depth proceeded via reaction of basalt with variable mixtures of seawater and hydrothermal fluids, at temperatures of ~250-350C and at integrated water/rock mass ratios of at least 30-300.

Chloritization by early Mg-bearing hydrothermal fluids was followed by reaction of the chloritized rocks with Mg-depleted, alkali-enriched hydrothermal fluids having elevated Na/K ratios to produce the paragonite + quartz + pyrite assemblage characteristic of the main TAG stockwork. Stockwork quartz has delta18O = 7.9‰-11.7‰, indicating temperatures of 260-360C in equilibrium with hydrothermal fluid (1.7‰). The maximum temperatures calculated for quartz yield temperatures identical to those of the vent fluids (360-365C). Scatter to higher delta18O and lower calculated temperatures are the results of precipitation of quartz during cooling of hydrothermal fluids and mixing with seawater in the subsurface, plus local 18O-enrichments of fluids via interactions with wallrocks. Generally higher delta18O values of quartz from the mound compared to the stockwork (12.3‰ 2.7‰ vs. 8.9‰ 0.9‰, respectively) reflect lower temperatures in the mound as the result of cooling of the mound by entrained seawater.

1Herzig, P.M., Humphris, S.E., Miller, D.J., and Zierenberg, R.A. (Eds.), 1998. Proc. ODP, Sci. Results, 158: College Station, TX (Ocean Drilling Program).
2Department of Geological Sciences, 2534 C.C. Little Building, The University of Michigan, Ann Arbor, MI 48109-1063, U.S.A. jalt@umich.edu