Ion-exchange experiments and RbSr dating on celadonites from the Troodos ophiolite, Cyprus

RbSr dating of celadonites from several geographic/stratigraphic locations from the Troodos ophiolite, Cyprus, yield crystallization ages varying 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 exchange with seawater started immediately after formation of the ophiolite. Drillcore samples suggest a duration of low-temperature alteration of ca. 10 Ma, whereas outcrop samples yield much longer durations of ≥ 30 Ma. The exact reason for this discrepancy is not fully understood, but may represent the difference in localized downwelling in different parts of the ophiolite, or may be related to the extensive weathering of the upper extrusive suite. Additionally, in nearly all cases RbSr age estimates are older than KAr age data from the same samples. To evaluate the potential disturbances of RbSr isotope systematics from exchangeable interlayer and basal surface positions, a series of ion exchange experiments was carried out. These data show that significant amounts of Rb and Sr can be in exchangeable positions, 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 RbSr isotopic system is minor. The post-crystallization chemical behavior of exchangeable K and Ar was not evaluated, but the loss of radiogenic argon from exchangeable sites could result in KAr ages younger than RbSr ages of the same sample.