Hydrothermal alteration of oceanic basalts from DSDP hole 504B: An electron microscopic study of microstructures, alteration mechanisms, and variations in mineral chemistry.

Abstract

Transmission and analytical electron microscopy integrated with conventional electron microprobe analysis, X-ray diffraction, scanning electron microscopy, and optical microscopy were used to investigate hydrothermally altered basalts in the transition and sheeted dike zones, Deep Sea Drilling Project (DSDP) hole 504B, Leg 83. Compositions, microstructures, alteration mechanisms, and paragenetic sequences of secondary minerals in the altered basalts were utilized to infer conditions during hydrothermal alteration. Phyllosilicates are the most abundant secondary minerals in the altered basalts. Saponite is dominant only in the uppermost level of the transition zone. Chlorite and corrensite are the major phyllosilicates in the transition zone and upper level of the sheeted dike zone, while talc and chlorite are dominant in the lower level of the sheeted dike zone. Corrensite is inferred to be a unique mineral phase rather than a 1:1 mixed-layer chlorite/smectite. The occurrence of mixed-layer phyllosilicates appears to be mainly controlled by kinetic factors. Parageneses and compositions of phyllosilicates strongly depend upon available chemical components from precursor minerals or fluids. The alteration of other phases such as plagioclase and titanomagnetite provides components for formation of phyllosilicates. The degree of alteration in the basalts is mainly controlled by fluid/rock ratios, which in turn are determined by rock permeability. Alteration of clinopyroxenes gave rise to amphiboles, biopyriboles and a secondary Ca-rich clinopyroxene (magnesian hedenbergite) that formed via dissolution of clinopyroxenes and crystallization of the secondary phases on a submicroscopic scale. Clinopyroxene composition should be used as an indicator of petrogenesis and igneous history for oceanic basalts only with great care. Primary titanomagnetite has been altered by a sequence of processes, including oxidation, exsolution, and hydrothermal alteration, that give rise to end-member magnetite of single magnetic domain size. Magnetization of the sheeted dike basalts is thus contributed from a stable thermoremanent (or chemical) magnetization obtained after (or during) exsolution and modified by chemical remanent magnetization during hydrothermal alteration.