Melt inclusion record of magmatic immiscibility in crustal and mantle magmas

Immiscibility (unmixing of melts and fluids) should be almost inevitable at some point in the evolution of most mantle and crustal magmas during cooling and crystallization. Formation of two immiscible phases "results in a major geochemical fractionation all chemical species present, the elements (and their isotopes), and their various compounds, become distributed between these two phases... the compositional divergence between the two phases is... extreme" (Roedder 2003). The variations in compositions of melts undergoing immiscibility, and physical parameters of their evolution in the plutonic environments, mean that each intrusion should be expected to show differences in the processes of exsolution and compositions of exsolving phases. One of the immiscible phases is universally volatile-rich, and this has important consequences for further magma evolution and geological processes related to it. More specifically, volatile-rich phases generally have significant density and viscosity contrasts with parental silicate magmas, and thus rapid separation of the newly exsolved phases is expected. Further, the exsolution of volatile-rich phases exerts major controls on the chemistry of a magmatic system, particularly on metal partitioning between immiscible melts and fluids, so the volatile phase is highly efficient at sequestering the metals (e.g., Candela 1989; Candela & Piccoli 1995; Williams et al. 1995; Heinrich et al. 1999; Webster 2004). Magmatic immiscibility and the related formation of volatile-rich melts and fluids are prerequisites for the origin of mineralized hydrothermal solutions that may transport metals to a suitable depositional site. Immiscible separation, however, is not restricted to magmas that form mineralized rocks. The fugitive nature of magmatic immiscibility involves problems in unraveling physical and chemical characteristics of this fundamental process. If separation of immiscible phases was efficient, the residual magma should be significantly depleted in incompatible volatiles and metals relative to the parental magma, and reconstructing the original metal and volatile content is extremely difficult. One rapidly developing approach to this problem is the use of melt and fluid inclusions trapped and preserved in magmatic minerals (e.g., Roedder 1992; De Vivo & Frezzotti 1994; Bodnar 1995; Lowenstern 1995; Student & Bodnar 1999; Frezzotti 2001; Kamenetsky et al. 2003; Lowenstern 2003 and references therein). Such inclusions provide the closest approximation to samples of continuously evolving (and thus ephemeral) melts and magmatic fluids. Many studies of magmatic inclusions have made possible the recognition of several types of magmatic immiscibility (e.g., between silicate melts, sulfide melts, aqueous and carbonic liquids and vapors, hydrosaline liquids and various combinations of these). For brevity, in this work only those examples from plutonic systems, of which the author has first-hand experience, will be presented and discussed in detail.