Ba minerals in clinopyroxene rocks from the Myanmar jadeitite area: implications for Ba recycling in subduction zones

Abstract

Barium (Ba) minerals identified in clinopyroxene rocks from the Myanmar jadeitite area include celsian, hyalophane, and hydrated barium aluminum silicate (an inferred barian zeolite). The hyalophane and hydrated barium aluminum silicate occur as interstitial phases and sometimes crosscut jadeite crystals, indicating they were formed at a later stage than jadeitite. The celsian has two modes of occurrence: (i) in association with jadeite in jadeitite; (ii) as a single-phase mineral forming part of multi-phase pseudomorphs in chromian omphacite rock (omphacitite). The latter rock type was formed predominantly during the same episode as the jadeitite under almost the same P-T conditions. Multi-phase pseudomorphs, mostly showing hexagonal form, contain celsian and kaolinite, with or without quartz, graphite and diaspore. The P-T condition constraints on celsian suggest that a precursor phase, probably cymrite, once existed under high-pressure conditions, and that celsian was formed by decomposition of the phase under decreasing pressure during uplift of the jadeitite. The frequent occurrence of Ba silicates in jadeitites worldwide reflects a Ba-enriched environment for the formation of jadeitite, which is inferred to be related to subducted barite-bearing slab sediments. Therefore, the jadeitite records metasomatism and metamorphism, as well as fluid interactions and phase changes in the BaO-Al 2 O 3 -SiO 2 -H 2 O system. In this way, jadeitite provides information on Ba phase transformations and Ba recycling within the subduction zone. Taken together with previous results, this study further suggests that jadeite-forming fluids are derived from the dehydration of the altered oceanic slab containing deep-sea sediments.