Progress in linking accessory mineral growth and breakdown to major mineral evolution in metamorphic rocks: A thermodynamic approach in the Na_2O-CaO-K_2O-FeO-MgO-Al_2O_3-SiO_2-H_2O-TiO_2-ZrO_2 system

摘要

Activity-composition (a-x) models have been generated for zirconium-bearing haplogranitic silicate melt and garnet from experimental data on zircon dissolution and natural rock data, respectively. Additionally including the recently proposed a-x model for Zr-bearing rutile [Tomkins et al., Journal of Metamorphic Geology25 (2007) 401], calculated phase diagrams that explicitly include ZrO_2 in the bulk composition predict the growth and dissolution of zircon at sub- and supra-solidus conditions. This occurs within the context of the evolution of major metamorphic minerals and mineral assemblages in pressure-temperature-composition space for a metapelitic rock composition. The stability of zircon is a function of the bulk ZrO_2 content. Garnet contains insufficient Zr to affect the stability of zircon whereas rutile does contain sufficient Zr that zircon stability can be curtailed in rocks with significant rutile. Silicate melt contains appreciable Zr and zircon abundance varies inversely with melt abundance. Thermometers based on the Zr-content of rutile (and potentially garnet) can be graphically portrayed as compositional contours in mineral assemblage fields on the phase diagrams, thereby potentially adding to the utilization of such thermometers. The ability to calculate phase diagrams explicitly including Zr is a major step towards more systematically linking zircon growth - and zircon geochronology - and accessory phase thermometry in a readily adaptable way to the metamorphic evolution of major silicate minerals in a wide range of rocks.