Cooling history of a dike as revealed by mineral chemistry: A case study from Mt. Etna volcano

Seven rock samples were systematically collected from innermost to the outermost portion of a dike outcropping at Mt. Etna volcano. Results show that, from dike core-to-rim, plagioclase, clinopyroxene and titanomagnetite show compositional variations due to increasing cooling rate. Plagioclase is progressively enriched in An from innermost to the outermost part of the dike. Similarly, clinopyroxene components En + CaTs + CaFeTs increase, whereas Di + Hd decrease. The Usp content in titanomagnetite also systematically decrease from dike core-to-rim. Partition coefficients and thermometers based on the crystal-liquid exchange reaction indicate that, due to rapid cooling rates at the dike outer portions, early-formed crystal nuclei do not re-equilibrate with the melt. The chemistry of minerals progressively deviates from that of equilibrium; consequently, from dike core-to-rim, mineral compositions resemble those of high-temperature formation. The chemical variations of clinopyroxene and plagioclase in dike samples mirror those obtained from cooling experiments carried out on alkaline basalts. Accordingly, we used an experimental equation based on clinopyroxene compositional variation as a function of cooling rate to determine the cooling conditions experienced by the crystals during dike emplacement. The estimated cooling rates are comparable to those predicted by thermal modeling based on an explicit finite-difference scheme.