Fluid characteristics from shallow magmatic environments: A contribution to danburite bearing Luc Yen pegmatites, northern Vietnam

Abstract

Danburite as a member of the Luc Yen pegmatite mineral assemblage has been studied using fluid inclusion microthermometry and Raman spectroscopy. Data characterize well-preserved fluid inclusions which originate from primary large tubular inclusions as result of necking down. Same modifications underwent a second inclusion generation that evolved during healing of a later crack. Both generations of fluid inclusions show the same chemistry (H₂O-CO₂) characterizing 3-phase inclusions with additional solids (calcite, sassolite and danburite). Inclusions consist of pure CO₂ and H₂O with additional NaCl ± KCl comprising a salinity of about 4.5 mass%. Internal fluid inclusion pressures as well as bulk inclusion densities have been calculated using the fermi diad split method of pure CO₂ at clathrate melting temperatures of the system and total homogenization temperatures, respectively. Mean internal pressures of ca. 4.5 MPa as well as a bulk density around 0.60 g/cm³ represent a low-dense fluid with XH₂O~0.86 and XCO₂~0.14 in composition that was present during formation of danburite. Data characterize danburite as a late stage crystallization member of the pegmatite in a shallow magmatic environment.

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