Chemical compositions of altered volcanic rocks in the western Pacific seamount, Uyeda Ridge

Abstract

Mineralogical and geochemical features of low-temperature hydrothermally altered pillow basalts dredged from the western Pacific seamount (Uyeda Ridge) are documented. The altered basalts consist mainly of secondary clay minerals and primary Ca-rich plagioclase with minor K-feldspar and phosphate mineral and rare clinopyroxene relicts. Clay minerals are composed of a mixture of Fe-oxyhydroxide, smectite, and celadonite. Smectite in non-porous samples is Mg-rich, whereas smectite in porous samples is Mg-poor.
Whole-rock chemical compositions reveal that the Uyeda Ridge basalts are oceanic island tholeiite. Compared to average chemical compositions of typical oceanic island tholeiitic basalts, the altered Uyeda Ridge basalts are enriched in K₂O and P₂O₅, and depleted in MnO, MgO, and CaO. Notable enrichment of K is due to the presence of K-rich celadonite and K-feldspar. Phosphorus enrichment is attributed to the presence of phosphate mineral. The altered basalts are mostly depleted in MgO, although the degree of depletion in several non-porous samples is less striking than that in porous samples, probably reflecting the difference of Mg content of smectite. Considerable depletion of Ca is due to the breakdown of clinopyroxene, glass, and plagioclase. The Ca- and Mg-depletion suggest that low-temperature hydrothermal alteration of seamounts is effective for providing Ca and Mg into the ocean.
The minimum flux of Ca leached from the altered Uyeda Ridge basalts through the low-temperature hydrothermal alteration is estimated to be 1.8×10^-11 mol/g/y. It is very likely that the Ca was released into the ocean, and then precipitated as carbonate sediments (CaCO₃) on seafloor. This means that the altered basalts of seamounts play a significant role as a potential sink of CO₂ in the atmosphere-ocean system.