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

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).