Geochronology and geochemistry of low temperature hydrothermal alteration in oceanic crust: An investigation of celadonite in the Troodos ophiolite, Cyprus.

摘要

Celadonite, a low-temperature hydrothermal alteration mineral, sampled from drill cores and outcrops within the extrusive units of the Troodos ophiolite, Cyprus has been studied with geochronological and geochemical methods to provide new constraints on the timing and extent of chemical exchange between seawater and oceanic crust. Potassium-argon age determinations of 54 celadonite samples yield ages ranging from {dollar}90.9pm 1.0{dollar} to {dollar}49.8pm 0.5.{dollar} The oldest age is in close agreement with the estimated 91-92 Ma crystallization age of Troodos igneous rocks. The youngest age indicates that low temperature chemical exchange continued for at least 40 m.y. after crustal formation. This represents a 100 percent increase over previous estimates based on radiometric dating of secondary alteration minerals from Troodos. Extrapolation of these new data from Troodos, to contemporary in situ oceanic crust, partially resolves a discrepancy between geochronologic data from DSDP/ODP samples and geophysical data of heat flow and seismic wave velocities.; Major element compositions of Troodos celadonites are consistent with those analyzed from DSDP/ODP cores. They are defined by mixing between three components: pure celadonite, nontronite, and saponite. Fluorine and chlorine within Troodos celadonites demonstrate very different geochemical affinities. Both elements demonstrate concentration maxima in some samples in which octahedrally coordinated cations sum to approximately 4. However, approximately 5 percent of all samples have at least 1.0 percent fluorine, with the highest having 4.8 percent. No sample contains greater than 0.25 percent chlorine. These data suggest that celadonite precipitation may act as a significant sink for fluorine and that it may fractionate halide elements.; Rare earth element (REE) concentrations were measured for 27 celadonites and the REE patterns of 24 samples mimic those of Troodos lava compositions. REE mineral/fluid partition coefficients were calculated using hypothetical fluids determined from a "single pass" dissolution/dilution model. These partition coefficients indicate that celadonite may act as a significant sink for REE in hydrothermal solutions. Three celadonites yielded anomalous REE concentrations and patterns that could be reproduced only from a different, light REE enriched source.; Alkali element concentrations and ratios indicate that celadonite is similar to other smectites and fractionates these elements. The Cs/Rb signatures of the celadonites are significantly higher than from those found along a mixing path between Troodos glasses and seawater for appropriate water/rock ratios.