21. PROBING THE TAG HYDROTHERMAL MOUND AND STOCKWORK: OXYGEN-ISOTOPIC PROFILES FROM DEEP OCEAN DRILLING1Jeffrey C. Alt2 and Damon A.H. Teagle2 |
ABSTRACTOxygen 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°-350°C 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°-360°C in equilibrium with hydrothermal fluid (1.7). The maximum temperatures calculated for quartz yield temperatures identical to those of the vent fluids (360°-365°C). 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. |
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