The physical and chemical evolution of magmatic fluids in near-solidus silicic magma reservoirs: Implications for the formation of pegmatites

Abstract

As ascending magmas undergo cooling and crystallization, water and fluid-mobile elements (e.g. Li, B, C, F, S, Cl) become increasingly enriched in the residual melt, until fluid saturation is reached. The consequential exsolution of a fluid phase dominated by H 2 O (magmatic volatile phase or MVP) is predicted to occur early in the evolution of long-lived crystal-rich "mushy" magma reservoirs, and can be simulated by tracking the chemical and physical evolution of these reservoirs in thermomechanical numerical models. Pegmatites are commonly interpreted as the products of crystallization of late-stage volatile-rich liquids sourced from granitic igneous bodies. However, little is known about the timing and mechanism of extraction of pegmatitic liquids from their source. In this study, we review findings from thermomechanical models on the physical and chemical evolution of melt and MVP in near-solidus magma reservoirs, and apply these to textural and chemical observations from pegmatites. As an example, we use a three-phase compaction model of a section of a mushy reservoir, and couple this to fluid-melt and mineral-melt partition coefficients of volatiles trace elements (Li, Cl, S, F, B). We track various physical parameters of melt, crystals and MVP, such as volume fractions, densities, velocities, as well as the content in the volatile trace elements mentioned above. The results suggest that typical pegmatite-like compositions (i.e. enriched in incompatible elements) require high crystallinities (>70-75 vol% crystals) in the magma reservoir, at which MVP is efficiently trapped in the crystal network. Fluid-mobile trace elements can become enriched beyond contents expected from closed-system equilibrium crystallization This is the peer-reviewed, final accepted version for American Mineralogist, published by the Mineralogical Society of America.The published version is subject to change. Cite as Authors (Year) Title. American Mineralogist, in press.

Biotite as a recorder of an exsolved Li-rich volatile phase in upper-crustal silicic magma reservoirs

“…The record of Li-rich fluids trapped in pristine magmatic biotites from unmineralized, young volcanic deposits has important implications for the evolution of magmatic systems in the shallow crust and the transition from magmatic to hydrothermal conditions. First, the partitioning of Li into a MVP provides a mechanism to concentrate Li, and the fate of this MVP may be an important control on the generation of economic Li deposits as either brine deposits (Munk et al, 2016) or pegmatites (Troch et al, 2021). Second, the potential for biotites to contain "invisible" fluid inclusions between crystal layers may play a role in the occurrence of "too-old" 40 Ar/ 39 Ar ages that have previously been reported both from the Kos Plateau Tuff (Bachmann et al, 2010) and elsewhere (Hora et al, 2010).…”

Magmatic to hydrothermal conditions in the transition from the A-type Pikes Peak granite (Colorado) to its related pegmatite

“…The presence of primary fluorite in the granite indicates that this magma was already enriched in F in early stages of the crystallization of the granitic magma, leading to higher concentrations towards the late stages near fluid saturation and hence sliding the solidus to lower temperatures than TISol. While complete miscibility between hydrous silicate melt and solute-rich aqueous fluid has been claimed for several pegmatite and mineralized granite systems Thomas and Davidson, 2013), it remains unclear whether this is a general phenomenon during pegmatite genesis (Troch et al, 2021). Incomplete miscibility between silicate melt and solute-rich fluid close to the critical point could explain the distinct differences between quartz compositions in the pegmatite wall zone and those in the intermediate zone (Figure 7).…”

“…Therefore, for any considered case, the system must have been strongly fluid-saturated near the traditionally inferred 660 °C solidus temperature, when pegmatite extraction occurred, and wall zone precipitation commenced (Figure 11). This large amount of exsolved fluid can result in over-pressurization (Parmigiani et al, 2017) that may lead to pegmatite extraction (Troch et al, 2021). This fluid abundance, coupled with the flux enrichment, also reduces the overall viscosity of the liquids present in the system, allowing for injection of what would otherwise be an extremely viscous rhyolitic melt (with viscosities likely > 10 5 Pas; Scaillet et al, 1998) into small dykes and pods.…”

“…All magmas contain some dissolved volatile elements, which will ultimately exsolve into a magmatic volatile phase (MVP) as the magma cools towards its solidus and crystallizes. Hence, all systems could show textural, mineralogical, and/or geochemical evidence for fluid exsolution from the silicate melt, with this transition from a melt to a fluid-dominated environment ("magmatichydrothermal transition") commonly documented by metasomatic bodies (e.g., Audétat et al, 2000;Campos et al, 2006;Peterková and Dolejš, 2019;Troch et al, 2021).…”

Geometry of Freezing Impacts Ice Composition: Implications for Icy Satellites

“…While these phases would likely comprise only a small volumetric fraction of the ice shell, concentrated in regions associated with solidified/saturated hydrologic features, they would constitute unique cryopetrologic features containing deposits rich in non-ice geochemical species. Here we draw a potential analog to terrestrial pegmatites, the crystallization product of late-stage volatile-rich granitic magma [Troch et al, 2021]. These intrusive pockets, dikes, and sills contain significant chemical zonation as well as some of the rarest Earth elements due to their unique enrichment in incompatible trace elements and late-stage solidification [Thomas et al, 2006;Troch et al, 2021].…”