Fluid Immiscibility in Metamorphic Rocks

Abstract

INTRODUCTIONEvidence of fluid immiscibility in metamorphic rocks comes, in the best of all worlds, from fluid inclusion observations. In fact, fluid immiscibility should be anticipated over the full range of metamorphic conditions provided that appropriate bulk fluid compositions are present. In a review paper on "Metamorphic fluids: the evidence from fluid inclusions" Crawford and Hollister (1986) summarized: "The role of fluid immiscibility in metamorphism was only recognized after serious study of fluid inclusions in metamorphic rocks was underway in the late 1970s and has yet to attract the attention of more than a handful of petrologists studying metamorphic rocks." Since then, tremendous progress has been made. A huge amount of fluid inclusion studies have convincingly shown that immiscible fluids can be present in rocks of all bulk compositions and at all P-T conditions. Thermodynamic databases of minerals, solid solutions and fluid mixtures along with the development of Gibbs free energy minimization programs now allow for detailed prediction of the evolution of phase assemblages and mineral compositions in P-T space. Significant progress has also been made in understanding reaction kinetics, diffusional and convective mass transport and the interpretation of fluid-rock interaction during metamorphism. Given the vast fluid inclusion evidence of immiscibility it would appear, however, that phase petrology and mass transport in any specific rock where fluids are involved are not correctly interpreted if possible or proven immiscibility is not considered. Relatively few studies directly link mineral reactions with immiscible fluids. Therefore, this review aims at working out the effects of fluid immiscibility on the progress of metamorphic reactions and fluid transport over a wide range of conditions.Questions arise directly from the fluid inclusion record in metamorphic rocks. This chapter, however, is organized the other way round. At first it summarizes phase relations of fluid systems pertinent to metamorphic rocks obtained by laboratory experiments and thermodynamic calculations. It then addresses how calculated mineral 2 reactions are affected by immiscible fluid systems, and how immiscible fluids are produced and evolved when metamorphic mineral reactions proceed. The impure limestone plus H 2 O-CO 2 -salt system is theoretically explored, for which by far the most information is available and which may stand for other fluid-rock systems. A section on the physical behavior of immiscible fluids follows, again mainly based on experimental results. The second part of the paper reviews numerous field examples of contact, regional, and subduction related metamorphic rocks where fluid immiscibility played a significant role during petrogenesis and mass transport. Late events in metamorphic rocks such as formation of hydrothermal systems, late vein formation, ore deposits, etc. where fluid immiscibility and fluid segregation is of paramount importance are not explicitly addressed here. Instead, the review ...

T1 - Immiscible C-H-O fluids formed at subduction zone conditions

“…The occurrence of C-H-O fluid immiscibility in subduction zones is supported by the studies of fluids and fluid inclusions in high-pressure metamorphic rocks from subduction zones (Heinrich, 2007;Frezzotti and Ferrando, 2015). For example, Andersen et al (1993) reported coexistence of CO 2 -N 2 fluid inclusions and H 2 O-rich fluid inclusions with about 30 wt.…”

“…Firstly, C-H-O fluid immiscibility may cause extensive decarbonation in subduction zones. Previous studies show that some low-grade metamorphic reactions may cause extensive decarbonation if these reactions take place at H 2 O-CO 2 immiscibility (Yardley and Bottrell, 1988;Heinrich, 2007). Similarly, for a given decarbonation reaction in subduction zones, if it takes places at H 2 O-CO 2 immiscibility, significant amounts of carbonates may also be consumed.…”

“…Theoretical calculations also indicate the expansion of the fluid miscibility gap with pressure; Churakov and Gottschalk (2003) show that pressures higher than 2 GPa would cause immiscibility in the salt-free H 2 O-CH 4 system at 450 °C. In addition to pressure, the addition of salt such as NaCl, KCl, MgCl 2 , and/or CaCl 2 can also effectively expand the C-H-O fluid miscibility gap (Bowers and Helgeson, 1983;Johnson, 1991;Shmulovich et al, 2004;Heinrich, 2007). Subduction zone fluids may contain 3 to 50 wt.…”

“…It may be worth noting that N 2 is a common component in the C-H-O fluids of high-pressure metamorphism, and the addition of non-polar N 2 usually enlarges the C-H-O fluid immiscibility field (Heinrich, 2007). Recent theoretical calculations also show that depending on the P-T-fO 2 and pH buffered by minerals, carbon could be dissolved in the C-H-O fluids as organic and/or inorganic ionic species (Sverjensky et al, 2014;Galvez et al, 2015).…”

Brines at high pressure and temperature: Thermodynamic, petrologic and geochemical effects

“…The third consequence of H 2 O-CO 2 and H 2 O-NaCl activity relations is an extensive region of immiscibility in H 2 O-CO 2 -salt ternaries at lower crustal conditions (e.g., Heinrich, 2007;Liebscher, 2010;Manning et al, 2013). Most work has been conducted at low pressure (Gehrig et al, 1979;Aranovich et al, 2010;Anovitz et al, 2004), but extension of these results to lower crustal conditions (Frantz et al, 1992;Johnson, 1991;Kotelnikov and Kotelnikova, 1990;Graham, 1999, 2004;Joyce and Holloway, 1993;Gibert et al, 1998) yields a ternary miscibility gap between NaCl-H 2 O-rich brine and CO 2 -H 2 O-rich vapor.…”