ION-EXCHANGE EXPERIMENTS AND RB SR DATING ON CELADONITES FROM THE TROODOS OPHIOLITE, CYPRUS/

Rb/Sr dating of celadonites from several geographic/stratigraphic loca tions from the Troodos ophiolite, Cyprus, yield crystallization ages v arying between 57.9 +/- 3.9 and 89.0 +/- 1.9 Ma. The oldest celadonite age (89.0 +/- 1.9 Ma) agrees well with the crystallization age of the Troodos igneous complex suggesting that low-temperature chemical exch ange with seawater started immediately after formation of the ophiolit e. Drillcore samples suggest a duration of low-temperature alteration of ca. 10 Ma, whereas outcrop samples yield much longer durations of g reater than or equal to 30 Ma. The exact reason for this discrepancy i s not fully understood, but may represent the difference in localized downweliing in different parts of the ophiolite, or may be related to the extensive weathering of the upper extrusive suite. Additionally, i n nearly all cases Rb/Sr age estimates are older than K/Ar age data fr om the same samples. To evaluate the potential disturbances of Rb/Sr i sotope systematics from exchangeable interlayer and basal surface posi tions, a series of ion exchange experiments was carried out. These dat a show that significant amounts of Rb and Sr can be in exchangeable po sitions, and therefore a fraction of the parent and daughter inventory of these clays is vulnerable to late-stage chemical exchange, However , the overall agreement between the data of the whole mineral, and ion -exchanged clay separates suggests that the influence of this exchange on the Rb/Sr isotopic system is minor. The post-crystallization chemi cal behavior of exchangeable K and Ar was not evaluated, but the loss of radiogenic argon from exchangeable sites could result in K/Ar ages younger than Rb/Sr ages of the same sample.