Abstract:
A drilling transect across the eastern flank of the Juan de Fuca Ridge, conducted during Leg 168 of the Ocean Drilling Program, recovered samples of volcanic basement rocks (pillow basalts, massive basalts, and volcanic glass breccias) that exhibit low-temperature hydrothermal alteration. Secondary clays are ubiquitous, with Mg-rich and Fe-rich saponite and celadonitic clays commonly accounting for several percent, and up to 10-20% by volume. Present-day temperatures of the basement sites vary from 15 degrees to 63 degrees C, with the coolest site being about 0.8 Ma old, and the warmest site being about 3.5 Ma old. Whereas clays are present at sites that have been heated to present temperatures of 23 degrees C and higher, the youngest site at 15 degrees C has only a small trace of secondary clay alteration. Alteration increases as temperature increases with the age of the volcanic basement. The chemical compositions of secondary clays were determined by electron microprobe, and additional trace element data were determined by both conventional nebulization ICPMS and laser-ablation ICPMS. Trioctahedral saponite and pyrite are characteristic of the interior of altered rock pieces, indicating formation under conditions of low oxygen fugacity. Dioctahedral celadonite-like clays along with iron (III) oxyhydroxide and Mg-saponite are characteristic of oxidized haloes surrounding the non-oxidized rock interiors. Chemical compositions of these minerals are very similar to those determined from other deep-sea basalts altered at low temperature. The variable Mg:Fe of saponite appears to be a systematic function of both the Mg:Fe of the host rock and the oxidation state during water-rock interaction.