K/Ar dating and 87Rb/86Sr isochrons of two celadonite veins from the Troodos ophiolite, Cyprus, consistently yield precipitation ages of 77–80 and 87–92 Ma, respectively. Our K/Ar bulk-rock age data (84.7 ± 23 Ma), combined with the oldest celadonite age and recently published radiolarian ages, suggest that the Troodos ophiolite was formed between 85 and 92 Ma, providing an estimate for the duration of celadonite formation of at least 7 m.y., but possibly up to 15 m.y. These data are consistent with data from Deep Sea Drilling Project holes, suggesting that precipitation of the most significant portion of vein minerals in the oceanic crust occurs within about 12 m.y., but possibly up to 19 m.y. after crustal formation. Leaching experiments suggest that K, Ar, Rb, and Sr in exchangeable and nonexchangeable sites of the structure of stoichiometric celadonite maintain their isotope characteristics and are resistant to change from diagenetic processes or surface alteration.