Low-temperature alteration of dredged volcanics from the Southern Chile Ridge: additional information about early stages of seafloor weathering

Abstract

A suite of submarine volcanic rocks from the Southern Chile Ridge has been examined in order to investigate the early stages of low temperature alteration. Alteration in these samples proceeded as follows: (1) Fe-staining on sample surface and along fractures, (2) filling of vesicles with secondary material, (3) breakdown of glassy matrix, (4) breakdown of microcrystalline matrix, and (5) breakdown and replacement of olivine. Plagioclase and pyroxene were sometimes found to be slightly altered along internal fissures. Secondary or alteration phases generally showed high K (3–5 wt.%), Fe (30–70 wt.%) and low Al (<2 wt.%) contents. The most common secondary minerals were Fe-oxyhydroxides, celadonite/Fe-oxyhydroxide (a mixture between celadonite and Fe-oxyhydroxide) and saponite. The formation of secondary minerals is controlled by the oxidation state which is a direct result of seawater to rock ratio. Fe-oxyhydroxides and celadonite/Fe-oxyhydroxide form during oxidative diagenesis, whereas saponite forms during non-oxidative diagenesis. The final alteration products, however, are controlled by external factors, e.g., sedimentation rate or formation of a manganese crust. Ca has been consistently removed, Mg has been added to some rocks and lost from others, and Si has either been removed or remained unaffected. Fe and H2O increases are accompanied by an increase in Fe3+/Fe2+. Wherever increases in K, Rb and Cs were documented, the enrichments are consistently in the order Cs>Rb>K. During initial stages of alteration the behavior of some trace elements such as rare-earth elements (REE), Ba, Zr, Hf, Ta, Nb, and Mo are solely controlled by the precipitation of Mn-rich Fe-oxyhydroxides. The preferred incorporation of Ce into Mn-rich Fe-oxyhydroxides may be a principal factor explaining the Ce depletion in seawater. We conclude that the earliest stages of submarine weathering are controlled by Eh and pH gradients between the rock and seawater. In the absence of a buffer, oxidation of ferrous iron causes a decrease in solution pH.