CHAPTER 4: MELT INCLUSION RECORD OF MAGMATIC IMMISCIBILITY IN CRUSTAL AND MANTLE MAGMAS Vadim S. Kamenetsky School of Earth Sciences and ARC Centre of Excellence in Ore Deposits, University of Tasmania, Private Bag 79, Hobart, Tasmania 7001, Australia E-mail: [email protected] INTRODUCTION process. If separation of immiscible phases was Immiscibility (unmixing of melts and fluids) efficient, the residual magma should be significantly should be almost inevitable at some point in the depleted in incompatible volatiles and metals evolution of most mantle and crustal magmas relative to the parental magma, and reconstructing during cooling and crystallization. Formation of two the original metal and volatile content is extremely immiscible phases “results in a major geochemical difficult. One rapidly developing approach to this fractionation – all chemical species present, the problem is the use of melt and fluid inclusions elements (and their isotopes), and their various trapped and preserved in magmatic minerals (e.g., compounds, become distributed between these two Roedder 1992, De Vivo & Frezzotti 1994, Bodnar phases…the compositional divergence between the 1995, Lowenstern 1995, Student & Bodnar 1999, two phases is…extreme” (Roedder 2003). The Frezzotti 2001, Kamenetsky et al. 2003, variations in compositions of melts undergoing Lowenstern 2003 and references therein). Such immiscibility, and physical parameters of their inclusions provide the closest approximation to evolution in the plutonic environments, mean that samples of continuously evolving (and thus each intrusion should be expected to show ephemeral) melts and magmatic fluids. Many differences in the processes of exsolution and studies of magmatic inclusions have made possible compositions of exsolving phases. One of the the recognition of several types of magmatic immiscible phases is universally volatile-rich, and immiscibility (e.g., between silicate melts, sulfide this has important consequences for further magma melts, aqueous and carbonic liquids and vapors, evolution and geological processes related to it. hydrosaline liquids and various combinations of More specifically, volatile-rich phases generally these). For brevity, in this work only those have significant density and viscosity contrasts with examples from plutonic systems of which the author parental silicate magmas, and thus rapid separation has first-hand experience will be presented and of the newly exsolved phases is expected. Further, discussed in detail. the exsolution of volatile-rich phases exerts major controls on the chemistry of a magmatic system, TYPES OF MAGMATIC IMMISCIBILITY particularly on metal partitioning between Immiscible silicate melts immiscible melts and fluids, so the volatile phase is Among the reports on silicate-silicate melt highly efficient at sequestering the metals (e.g., immiscibility (e.g., Roedder & Weiblen 1970, De Candela 1989, Candela & Piccoli 1995, Williams et 1974, Dixon & Rutherford 1979, McBirney 1980, al. 1995, Heinrich et al. 1999, Webster 2004). Philpotts 1982, Jakobsen et al. 2005) only a few are Magmatic immiscibility and the related formation of related to mineralized magmatic systems (e.g., volatile-rich melts and fluids are prerequisites for pegmatites of the Ehrenfriedersdorf Sn–W deposit, the origin of mineralized hydrothermal solutions Germany, Thomas et al. 2000, the La Copa felsic that may transport metals to a suitable depositional intrusions within the Rio Blanco Cu–Mo deposit, site. Immiscible separation, however, is not Chile, Davidson & Kamenetsky 2001, Davidson et restricted to magmas that form mineralized rocks. al. 2005). The fugitive nature of magmatic immiscibility Quartz-hosted silicate melt inclusions from involves problems in unraveling physical and the La Copa complex, coexisting within the same chemical characteristics of this fundamental grains and growth planes, belong to two contrasting Mineralogical Association of Canada Short Course 36, Montreal, Quebec, p. 1-xx 81 V.S. KAMENETSKY types: clear rhyolitic glass with one or several mapping, is elevated metal (Fe, Mn, Cu, Zn, Pb) shrinkage bubbles, and dark fine-grained crystalline and alkali element abundances in a volatile-rich aggregates of feldspar, mica and quartz (Fig. 4-1, silicate melt (Fig. 4-2). Davidson & Kamenetsky 2001, Davidson et al. Davidson & Kamenetsky (2001) 2005). Dark inclusions may also contain daughter emphasized that dark silicate melt inclusions, crystals of sphalerite, chalcopyrite, Fe oxides and enriched in volatiles and metals, are important to hydroxides, halite, carbonates and unidentified the understanding of late-stage magmatic phases. They are also characterized by a significant immiscibility and suggested revisiting the results of and variable amount of fluid components, identified other studies on similar inclusions. For example, the by laser Raman spectroscopy as vapor and liquid melt inclusions from the Lower Bandelier Tuff H2O. A common feature of dark inclusions is an (Dunbar & Hervig 1992) can be “strongly associated halo of aqueous vapor-dominated devitrified” and “darker in color…and some contain bubbles (< 5 µm) forming a discontinuous rim, clots of crystals”, and also contain up to 8 wt.% which probably formed as a result of post- H2O, in contrast to clear glass inclusions (3–5 wt.% crystallization decrepitation of inclusions. H2O). It is possible that crystallization of trapped Decrepitation of seemingly intact large dark melts was promoted by high H2O abundances. The inclusions at T~600°C is responsible for their interpretation of coexisting volatile-poor, and failure to melt and homogenize during heating volatile-rich, silicate melts (melt inclusions) as experiments. Some smaller inclusions show melting immiscible liquids at La Copa (Davidson & at 750–800°C, although complete homogenization Kamenetsky 2001) was partly based on the phase (bubble disappearance) was not achieved even at diagram derived from homogenization experiments higher temperatures. with melt inclusions from the Ehrenfriedersdorf Clear evidence of silicate-silicate melt pegmatite (Thomas et al. 2000). According to their immiscibility in the La Copa samples is observed in ground-breaking study, the original H2O- inclusions that comprise both clear glass and undersaturated felsic magma separates at a certain rounded, commonly spherical, crystalline and temperature into two immiscible liquids, a volatile- amorphous dark masses of dominantly silicate poor silicate melt and a volatile-rich silicate melt (or phases with interstitial aqueous fluid (Fig. 4-2). The a silicate-bearing fluid). The latter inevitably relative proportions of trapped immiscible melts in evolves via one or more consecutive unmixing such composite inclusions are highly variable, and events into aqueous metal-, salt-rich their compositions are distinctly different (Fig. 4-2). (“hydrothermal”) fluids with a potential for The main difference, as indicated by PIXE economic mineralization (see below). FIG. 4-1. A growth plane in a quartz phenocryst from La Copa complex, Chile (Davidson & Kamenetsky 2006), containing coexisting crystallized silicate melt inclusion (1), glass inclusions (2) and two-phase aqueous fluid inclusions (3). The two- phase aqueous inclusions show Brownian motion of the vapor bubbles, and freeze at low temperature. 82 MELT INCLUSION RECORD OF MAGMATIC IMMISCIBILITY IN CRUSTAL AND MANTLE MAGMAS FIG. 4-2. Optical images and proton-induced X-ray emission (PIXE) element maps of quartz-hosted composite inclusions from La Copa complex, Chile (Davidson et al. 2005). The inclusions are composed of clear silicate glass and a globule of crystallized volatile-rich melt (dark). Outlines on element maps mark boundaries of inclusions and their volatile-rich silicate globules. Scale bars are 15 µm. Immiscible silicate melt and aqueous saline fluids trapped in the same growth plane, is provided in Quartz phenocrysts in felsic intrusive rocks Fig. 4-1 (after Davidson & Kamenetsky 2006). commonly contain numerous aqueous fluid Notably, the dark silicate melt inclusion in Fig. 4-1 inclusions, but in most cases they are clearly later belongs to the volatile-rich type described above. than the silicate melt inclusions (if present) in the Another example of explicit immiscibility between same grains. Trails of secondary aqueous inclusions the silicate melt and magmatic aqueous fluid prior commonly obliterate any evidence of primary to or simultaneous with quartz crystallization is aqueous inclusions, at least presenting a recorded by Davidson & Kamenetsky (2006) as considerable challenge to their confident composite inclusions containing very large numbers identification. An unambiguous example of silicate of one- or two-phase aqueous bubbles in a silicate melt and aqueous two-phase fluid inclusions, co- glass (microemulsion, Fig. 4-3). 83 V.S. KAMENETSKY FIG. 4-3. Optical images and proton-induced X-ray emission (PIXE) element maps of quartz-hosted composite inclusions from La Copa complex, Chile (Davidson et al. 2005). The inclusions are composed of clear silicate glass and numerous bubbles of aqueous fluid (vapor- and liquid-rich). Outlines on element maps mark boundaries of inclusions and aqueous bubbles (on B). Scale bars are 25 µm. On the other hand, in cases where post-entrapment exsolution of the aqueous fluid. crystallization occurs deeper than the magma Partly crystallized silicate melt inclusions saturation in H2O, primary aqueous fluid inclusions from both localities are