Crystallization in a saturated magma Part II (Example) A three phase equilibrium approach to the Bishop Tuff
published: Dec 1, 1995
ArtNo. ESP154017001005, Price: 29.00 €
Trace element modelling using three phase (liquid-liquid-solid) equilibrium equations is used in a re-examination of the geochemical information available for the Bishop Tuff (Hildreth 1977, 1979). It is suggested that: (I) the Bishop Tuff originated from a system of two immiscible liquids and a co-existing crystal phases; (2) The proportions of the residual magma, immiscible liquid and crystals are 82.3 %, 4.7 % and 13.0 %, respectively; (3) The rock phases evolved through segregation of crystals and immiscible liquids, generating a zoned magma chamber; (4) Core-rim segregation difference is 67 %, caused by 17 % crystallization under iterative conditions (emulsion); (5) sanidine, biotite, orthopyroxene, clinopyroxene, magnetite and zircon crystallized from the residual magma; plagioclase grew from both the residual magma and the immiscible liquid; allanite and probably quartz crystallized from the immiscible liquid; (6) the residual magma compositional gradients are independent of thermal gradients and crystal-immiscible liquid mix curves. All gradient curves show variations towards the magma chamber core, but the crystal-immiscible liquid curves are strongly affected by convection. The crystal settling process was driven by convection along the side walls, in paths that closely follow the residual magma compositional gradients curves.