Mineralogy and oxygen isotope values of silicified claystone and porcelanite from DSDP Hole 35-323

Discrepancies between predicted and observed interstitial water profiles for sites 322 and 323, Leg 35, Bellingshausen Abyssal Plain, were used to identify sites of reaction for further mineralogical and chemical investigations. Two major reaction sites were identified at site 323:(1) In the silicification zone between 410 and 505 m depth, where dissolution of biogenic (opaline) silica, plagioclase and a few coccoliths and the formation of opal-CT, Mg-rich smectite and K-feldspar are responsible for the observed silica, Ca[2+], Mg[2+], and K[+] interstitial water gradients. Dissolution of biogenic silica provided most of the silica for the porcelanites.(2) In the basalt, weathering of pyroxene and plagioclase and the formation of celadonite, smectite, calcite, and goethite are probably responsible for the observed Ca[2+], Mg[2+], and K[+] interstitial water gradients below the silicification zone.The chemistry of authigenic smectites reflects the composition of their precursor(s).