Excess 40Ar transport scale and mechanism in high-pressure phengites: A case study from an eclogitized metabasite of the Dora-Maira nappe, western Alps

Abstract

Application of the bulk-separate furnace heating, laser step-heating, and in situ laser agemapping 40Ar/ 39Ar techniques reveals that high-pressure phengites from an Alpine eclogite of the Dora-Maira basement nappe, western Alps, were pervasively contaminated with excess 40Ar to yield ages > 130 Ma. Spot-fusion dating of individual grains with the 40Ar/ 39Ar laserprobe reveals concentric radial patterns with core ages as high as 320 Ma, showing that this excess component was trapped at an early stage and subsequently remobilized by volume diffusion during a late greenschist thermal pulse. The single grains and population from this hand-sample retain major differences in apparent ages indicating that the extent of the excess 40Ar contamination varies from grain to grain. Attempt to model these differences in terms of grain-size-dependent fractional loss falls short and demonstrates that strong cm-scale argon isotopic gradients locally prevailed during syn-kinematic high-pressure metamorphism. On petrostructural grounds, a model of internal isotopic buffering is outlined, whereby the excess 40Ar is argued to have been internally derived from within the eclogite protolith itself. Low intergranular Ar diffusion rates due to reduced water activity (buffered below unity by the phengite assemblage) resulted in the local segregation and partitioning of the excess 40Ar along grain boundaries during ongoing ductile deformation. Supporting evidence for this model comes from the recent literature on fluid and stable isotope behaviour in Alpine eclogites and very-high-pressure metasediments, which shows that volatile recycling during high-pressure metamorphism may occur on a very limited scale.