Excess argon evolution in HP–LT rocks: a UVLAMP study of phengite and K-free minerals, NW Turkey

Excess argon is a common problem in high-pressure metamorphic rocks. Here, we present 40Ar/39Ar data from metabasite, metachert and metapelite lithologies and a range of minerals. The results allow us to discuss the origin of the excess argon and processes leading to its incorporation during high-pressure metamorphism. Samples were derived from blueschist and low-temperature eclogite facies rocks from the Tavsanli Zone of NW Turkey. Samples were analysed using the 40Ar/39Ar Ultra-Violet Laser Ablation Microprobe (UVLAMP) and yield ages ranging from 72±3 to 154±7 Ma. Detailed intra-grain profiling of phengites in situ, and analyses of sodic-amphibole, lawsonite, quartz and garnet, present a detailed picture of the spatial distribution of excess argon within different minerals in the same hand samples, revealing that they acted as near closed systems where excess argon was preferentially partitioned into phengite. Whole rock XRF data indicates a scattered relationship between bulk K2O content of the rock and the concentration of excess argon in phengite. We conclude that excess argon was derived in situ in a relatively closed, fluid-absent system, rather than introduced by fluids.