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

A suite of submarine volcanic rocks from the Southern Chile Ridge has been examined in order to investigate the early stages of low temperature altera tion. 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 pyro xene were sometimes found to be slightly altered along internal fissures. S econdary 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 we re Fe-oxyhydroxides, celadonite/Fe-oxyhydroxide (a mixture between celadoni te and Fe-oxyhydroxide) and saponite. The formation of secondary minerals i s controlled by the oxidation state which is a direct result of seawater to rock ratio. Fe-oxyhydroxides and celadonite/Fe-oxyhydroxide form during ox idative 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 c onsistently 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 a nd Cs were documented, the enrichments are consistently in the order Cs > R b > K. During initial stages of alteration the behavior of some trace eleme nts such as rare-earth elements (REE), Ba, Zr, Hf, Ta, Nb, and Mo are solel y controlled by the precipitation of Mn-rich Fe-oxyhydroxides. The preferre d incorporation of Ce into Mn-rich Fe-oxyhydroxides may be a principal fact or explaining the Ce depletion in seawater. We conclude that the earliest s tages of submarine weathering are controlled by Eh and pH gradients between the rock and seawater. In the absence of a buffer, oxidation of ferrous ir on causes a decrease in solution pH. (C) 1999 Elsevier Science B.V. All rig hts reserved.