Crystal populations of igneous rocks and their implications in genetic mineralogy

Abstract：The new concept of maturity of the magma system emerges from the study of the physical processes of magmatic systems, from which it is recognized that not all of the crystals in igneous rocks are crystallized from their host magma. According to the ways of adding crystals to the magmatic system, the crystals in igneous rocks can be divided into three populations：solid-, melt- and fluid-crystal populations. The solid-crystal population means that the crystals exist in solid state before they are added into the magmatic system, including residual crystal sub-population and xenocryst sub-population. The melt-crystal population consists of the crystals crystallized from a melt, including crystals from the magma chambers at different depths (chamber crystal sub-population), crystals from magma conduits (channel crystal sub-population), crystals that have crystallized from progenitors of the final magma and have been ‘reincorporated’ into the final magma (antecryst sub-population), and crystals that have been crystallized after magma emplacement (matrix crystal sub-population). The fluid-crystal population is used to define crystals separated out from fluids, including crystals from the super-critical fluid (super-critical crystal sub-population), from vapor (condensation crystal sub-population), and from hydrothermal liquid (hydrothermal crystal sub-population). Such a division opens a new window for the future of genetic mineralogy of igneous rocks. Accordingly, an important duty of genetic mineralogy is to clarify the typical characteristics of various crystal populations and their forming conditions. Theoretically, the residual crystal is in thermodynamic equilibrium with the primary magma; the xenocryst is generally in disequilibrium with the host magma; the melt-crystal is in equilibrium with the magma produced at a special stage in the evolution of the magma system; the fluid-crystal is commonly in disequilibrium with magma, but a part of crystals from the super-critical crystal sub-population can be in equilibrium with the host magma. Therefore, the fluid-crystal is occasionally coexisting with the melt-crystals. The preservation of crystal populations in igneous rocks is related to the existing time of the magma system and the resorption rate of crystals. In the magma system where the magma quickly rises up and consolidates, all the crystal populations could be preserved; otherwise, only the matrix crystal sub-population is preserved. Accordingly, the number of population and the crystal size distribution can be used to qualitatively evaluate the existing time of a magma system and its dynamic conditions. Therefore, the quantitative analysis of igneous texture will be an important task in genetic mineralogy.