Mineralogy and chemical composition of clay minerals, TAG hydrothermal mound

Abstract

Herein we present preliminary results of a study of the distribution and chemical composition of clay minerals in rocks recovered from the Trans-Atlantic Geotraverse (TAG) hydrothermal mound. This study is part of Leg 158 of the Ocean Drilling Program, an effort to evaluate the subsurface secondary mineral distribution and nature of alteration at the active TAG mound, located at 26°N latitude on the Mid-Atlantic Ridge. X-ray diffraction analyses and petrographic and scanning electron microscopy indicate that the clay minerals include chlorite, smectite, and a mica-like clay. Chemical analyses by flame atomic absorp tion spectrophotometry of clay-mineral separates and inductively coupled plasma-atomic emission spectrophotometry analyses of basalt alteration rims indicate that the chlorite and smectite have similar major-element compositions and that both clay mi nerals have lower SiO2 and Al2O3 and higher Fe2O3 and MgO than bulk basalt. Cu and Zn are elevated above unaltered basalt concentrations in the smectites and chlorites and in bulk-rock alteration rims. Spatial distributions through the mound of SiO 2/ Fe2O3, MgO, and Zn in clay minerals and alteration rims suggest that fluids circulating through the TAG mound originate from two sources: (1) a shallowly circulating fluid that is less evolved and nearer to unaltered seawater chemical composition, and (2) a deeply circulating fluid that is more evolved fluid and changed from unaltered seawater by more extensive chemical reaction with underlying basalt. Clay-mineral compositions suggest that the southeasterly and shallow central portions of the mound are influenced by a greater proportion of the shallowly circulating, less evolved fluid, and the northwesterly and deeper central portions of the mound are influenced by a greater proportion of the deeply circulating, more evolved hydrothermal fluid .