Magmatic fluids immiscible with silicate melts: examples from inclusions in phenocrysts and glasses, and implications for magma evolution and metal transport

Abstract

The first occurrence of immiscibility in magmas appears to be most important in the magmatic-hydrothermal transition, and thus studies of magmatic immiscibility should be primarily directed towards recognition of coexisting silicate melt and essentially non-silicate liquids and fluids (aqueous, carbonic and sulphide). However, immiscible phase separation during decompression, cooling and crystallization of magmas is an inherently fugitive phenomenon. The only remaining evidence of this process and the closest approximation of natural immiscible magmatic liquids and vapours can be provided by melt and fluid inclusions trapped in silicate glasses and magmatic phenocrysts. Such inclusions are often used as a natural experimental laboratory to model the process of exsolution and the compositions of volatile-rich phases from a wide range of terrestrial magmas. In this paper several examples from recent research on melt and fluid inclusions are used to demonstrate the significance of naturally occurring immiscibility in understanding some large-scale magma chamber processes, such as degassing and partitioning of metals.

Magma chamber–scale liquid immiscibility in the Siberian Traps represented by melt pools in native iron

“…in common silicate magmas proved to be diffi cult, largely because of the transient nature of the phases involved. The existing record of the silicate-silicate liquid immiscibility is supported by observations in matrix glasses and melt inclusions (Charlier et al, 2011;De, 1974;Jakobsen et al, 2005;Kamenetsky and Kamenetsky, 2010;McBirney and Nakamura, 1974;Philpotts, 1978Philpotts, , 1981Philpotts, , 1982Roedder and Weiblen, 1970). However, the compositional evolution of immiscible melts with cooling is unknown and the small size (<10 µm) and disequilibrium state of coexisting immiscible liquid and crystal phases render much of the evidence controversial.…”

“…This process results in major petro-and geochemical differentiation, because the compositional divergence between unmixed phases can be extreme (e.g., Charlier et al, 2013;Kamenetsky and Kamenetsky, 2010;Roedder, 1992;Thompson et al, 2007;Veksler et al, 2007). Even though the physical amounts of one of the immiscible phases may be small, its separation and transport can be important, especially with respect to formation of some rock types and orthomagmatic ore deposits (e.g., Charlier et al, 2013;De, 1974;Philpotts, 1967;Roedder, 1992;Thompson et al, 2007;Veksler, 2004;Veksler et al, 2007).…”

Bubbles matter: An assessment of the contribution of vapor bubbles to melt inclusion volatile budgets

“…The occurrence of carbonates (and other minerals) on the bubble wall has been observed in MI from various volcanic settings (Kamenetsky and Kamenetsky, 2010 and references therein). However, such phases are usually not reported in studies that focus on volcanic degassing behavior (e.g., Kamenetsky et al, 2007).…”

Magmatic–hydrothermal evolution of highly fractionated granites: Evidence from the Kuiqi miarolite in Fujian province, SE China

“…Exsolution of volatile phases from the magmatic melts takes place in or between the orthomagmatic-and hydrothermal regimes during magma cooling and crystallization (Lu, 2011). This process is difficult to document, largely because of the transient and reactive nature of the volatile phases released during magmatic cooling (Kamenetsky and Kamenetsky, 2010). It is well accepted that the magmatic-hydrothermal ore deposits are formed through a complex sequence of events starting from the generation of hydrous melts in the crust and ending in the ore metals precipitation from the hydrothermal fluids (e.g., Barnes, 1997;Gruen et al, 2014;Mogdisov and Williams-Jones, 2013).…”