On the origin of zoned grossular-andradite garnets in hydrothermal systems
Jamtveit, Bjørn; Agnarsdottir, Kristin Vala; Wood, Bernard J.
European Journal of Mineralogy Volume 7 Number 6 (1995), p. 1399 - 1410
published: Dec 27, 1995
manuscript accepted: May 11, 1995
manuscript received: Aug 5, 1994
ArtNo. ESP147050706013, Price: 29.00 €
Abstract Grossular-andradite (grandite) garnets from contact skarns and hydrothermal veins commonly show extensive intracrystalline zonation of major and trace elements. Here, we examine some of the factors that may affect the garnet composition at equilibrium and non-equilibrium conditions. Grandite-solution - aqueoussolution equilibria have been evaluated by using a general form of Lippmann's solutus equation. The grandite composition is very sensitive to variations in temperature and the pH, fo2 and salinity of the hydrothermal solution due to the difference in the aqueous complexation behaviour between Al and Fe. Pressure variation has little effects on the aqueous complexation, but may indirectly affect the garnet composition through pressure-sensitive equilibria that involve other minerals. The effects of variable kinetic dispersion in a hydrothermal system with fluctuations in the fluid flow rate are illustrated though a low-order reaction and transport model. This model indicate that fluid flow rates exceeding 1 μm/s may cause significant dispersion effects on the composition of grandite garnets growing at distances less than about 100 m from the intrusive contact. Finally, the origin of oscillatory zoned garnets from the Oslo rift is discussed