18. VIVISECTION AND AUTOPSY OF ACTIVE AND FOSSIL HYDROTHERMAL ALTERATIONS OF BASALT BENEATH AND WITHIN THE TAG HYDROTHERMAL MOUND1José J. Honnorez,2 Jeffrey C. Alt,3 and Susan E. Humphris4 |
ABSTRACTThe TAG (Trans-Atlantic Geotraverse) active hydrothermal mound is 30-40 m thick and consists 4 to 5 x 106 tons of pyrite-, quartz-, and anhydrite-rich breccias. A vertically and laterally zoned stockwork, approximately 100 m in diameter, extends at least 125 m below the mound surface. Petrographic and mineralogic studies of core drilled during ODP Leg 158 were used to reconstruct the sequence of alteration processes of crustal rocks beneath and within the mound. The first stage of hydrothermal alteration is the chloritization of the basaltic basement beneath the mound as a result of interaction with Mg-bearing hydrothermal solutions, which are different from the "zero" Mg fluids presently discharged by the Black Smoker Complex, near the center of the TAG mound. In the deepest part of the stockwork underlying the central part of the mound, the basalt is almost completely chloritized (with minor quartz and pyrite) and Cr-spinel is the only primary mineral left. Beneath the mound margins, the chloritization process is restricted to millimeter- to centimeter-thick halos parallel to the exposed surfaces of basalt fragments. Chloritization occurred at temperatures ranging from 250°-370°C. Chlorite under the southeast mound margin is Mg-rich and formed via reaction with heated seawater (Alt and Teagle, Chap. 21, this volume; Teagle et al., Chap. 22, this volume). Chlorite under the northwest margin of the mound and in the deep chloritized stockwork is poorer in Mg and formed from black smoker-like hydrothermal fluids mixed with small amounts of seawater. The second stage of hydrothermal alteration is the replacement of basalt and chloritized basalt by paragonite + quartz + pyrite. At this stage, all of the primary minerals are replaced. Paragonite formed via replacement of chlorite in the stockwork samples and as a result of direct precipitation from hydrothermal fluids in interstitial spaces of the quartz + pyrite breccias. Paragonite is often Cr-rich. Na-rich micas reflect the high Na/K ratio of the present day hydrothermal fluids discharged by the TAG black smokers at temperatures of up to 360°C. As a result of the recurrent silicification and pyritization, basaltic minerals and textures are progressively obliterated and completely replaced by mixtures of "dirty" quartz + pyrite with interstitial paragonite, which is extremely difficult to distinguish from the directly precipitated quartz + pyrite + paragonite assemblage. The last stage of hydrothermal mineralization consists of anhydrite precipitation in open spaces such as veins and voids in the breccias. Finally, the basement under the mound margins that are already displaying the first stage of chloritization is affected by low-temperature alteration by cold oxygenated seawater. Intensely altered basalt clasts are found within the mound breccias at much shallower depths above than that expected for the top of the basement. Three mechanisms are tentatively proposed to explain this observation: (1) high velocity entrainment in hydrothermal fluid, (2) volume expansion during alteration and mineralization in a process analogous to "frost jacking" and "frost heave"; the volume expansion process could proceed via the formation of quartz + pyrite, or by repeated precipitation and dissolution of anhydrite in veins and pore spaces, or (3) nearly complete replacement of a volcanic mound by hydrothermal alteration products. The third mechanism implies that the hydrothermal mound was superimposed upon and replaced a volcanic edifice. |
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