Distribution and significance of extraneous argon in UHP eclogite (Sulu terrain, China): insight from in situ 40Ar/39Ar UV-laser ablation analysis

In situ 40Ar/39Ar UV-laser ablation analyses of garnet, omphacite, epidote, phengite, barroisite and symplectite in a sample of UHP eclogite from Qinglongshan (Sulu terrain, China) reveal extremely heterogeneous argon isotopic ratios. Results from the 220–240 Ma eclogite indicate a significant and variable extraneous argon signature that is restricted to the early-formed phengite and epidote and later-formed barroisite. Individual, compositionally zoned, phengite grains record a large variation in 40Ar/39Ar dates (300–1100 Ma) that are positively correlated with celadonite content. These data are consistent with retrograde chemical exchange and argon isotopic repartitioning in response to decreasing partial pressures of argon during decompression. Consistent with the observation of atmospheric argon ratios in garnet, omphacite and symplectite these data support an interpretation that the extraneous Ar was incorporated early in the crystallization history of the eclogite, and not from a late infiltrating fluid. The ultimate source of the extraneous argon remains speculative, however on the basis of supporting stable oxygen and hydrogen isotope data from this locality, which support a closed isotopic system, it is inferred to have been inherited from the protolith of the UHP eclogite.