DIAGENETIC TRANSFORMATION OF CLINOPTILOLITE TO ANALCIME IN SILICIC TUFFS OF HOKKAIDO, JAPAN

Diagenetic clinoptilolite and analcime are present in the diagenesis o f Tertiary marine silicic tuffs in the MITI-Toyokoro borehole, Hokkaid o, japan. Chemical analysis, SEM and petrographic observations show th e mechanism of clinoptilolite to analcime transformation. Na-K clinopt ilolite (Na:K = 6:4), an alteration product of volcanic glass, is dete cted at depths of 1230 to 2710 m. A more Na-rich clinoptilolite (Na:K = 9:1) discovered at depths between 2590 and 2710 m is a result of a c ation exchange mechanism. The formation of analcime at greater depths is due to a microdissolution-precipitation mechanism, that is Na-rich clinoptilolite dissolved and analcime precipitated as a result of incr easing burial. At 2710 m analcime occurs next to the Na-rich clinoptil olite and it is assumed that the Na-rich clinoptilolite is an intermed iate phase.