SPATIAL AND TEMPORAL VARIABILITY IN CRYSTALLIZATION OF CELADONITES WITHIN THE TROODOS OPHIOLITE, CYPRUS - IMPLICATIONS FOR LOW-TEMPERATURE ALTERATION OF THE OCEANIC-CRUST

Celadonite, a low-temperature (<30-degrees-C) hydrothermal alteration mineral, is a predominant fracture and void-filling phase within the volcanic rocks of the Troodos ophiolite. The combined chemical and structural properties of celadonite (i.e., high K2O and Ar retentive), along with its common occurrence, provide a valuable tracer for studying the temporal and spatial variability of low-temperature hydrothermal fluid circulation and alteration within this ancient oceanic crust. Some 54 new K/Ar age determinations of celadonites from various geographic and stratigraphic locations within the extrusive rocks of Troodos yield crystallization ages ranging from 90.9 +/- 1.0 to 49.8 +/- 0.5 Ma, with the oldest in close agreement with the estimated 91-92 Ma crystallization age of Troodos igneous rocks. The youngest age indicates that low-temperature water/rock chemical exchange continued for at least 40 m.y. after crustal formation. This represents a 100% increase over previous estimates, based on limited numbers of K/Ar dates of celadonite, of the duration of low temperature mineral precipitation in Troodos. Correlation between celadonite ages and field relationships suggests the following: (1) There is no apparent relationship between celadonite crystallization and stratigraphic depth. (2) Celadonite crystallization occurs homogeneously on an outcrop scale, with adjacent samples from a single flow unit yielding equivalent K/Ar ages. (3) Flow units with relatively high primary permeabilities, e.g., pillows and breccias, yield celadonites with younger K/Ar ages. (4) Celadonite precipitation at distinct locations occurred rapidly, with little or no age difference between rim and core sections of single large deposits. The above correlations suggest that low-temperature hydrothermal fluid circulation and secondary mineral precipitation are controlled by the local alteration conditions, such as intrinsic permeability, degree of fracturing, and water/rock ratios, and are independent of time and space. Integration of these local heterogeneities over the entire extrusive sequence suggests that the upper oceanic crust undergoes relatively homogeneous closure to hydrothermal fluid circulation. No evidence is found to support a progressive upward sealing of the oceanic crust.