268-7 Coarse Crystals Facilitating Efficient Magma Mixing: The Case of an Andesite-Dacite Mixed Lava in the Neogene of Iceland

Session: Old and the New, Long and the Short: Perspectives on Integration of Timescales of Magmatic Processes: Special Session Related to MGPV Awards to Madison Myers and Anita Grunder (Posters)

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Magma mixing, the spectrum from magma mingling to hybridization, is an important petrologic process for the generation of intermediate magmas in a variety of tectonic settings. Many studies have characterized intrusive and volcanic products of magma mixing and developed fluid dynamic and thermodynamic models for this process. Among the factors that can influence the efficiency of mixing is the presence of crystals.

A ~7.8 Ma sequence of petrologically related volcanic units in the mountains Laxárdalsfjöll and Langadalsfjall in north-central Iceland includes a >3 km³ mixed andesite-dacite lava (59-69% wt.% SiO₂), overlain by a discontinuous dacite to low-silica rhyolite tuff, and a high-silica rhyolite lava. The mixed lava possesses varying abundances (<1-20%) of distinct coarse (up to 3 cm) calcic plagioclase (An_{81- 89}) megacrysts, which are also found in the tuff. In some areas the lava is hybrdidized, while in others mingling is evident at the outcrop scale. Texturally and compositionally identical plagioclase occurs in a plagioclase ultraphyric basalt lava mapped in the nearby mountains Svínadalsfjall and Vatnsdalsfjall. A previously developed petrologic model (e.g., Jordan et al., 2019, GSA Field Trip Guide no. 54) links these units via the following processes: (1) an evolved basaltic magma (~6.7 wt.% MgO) picked up the coarse calcic plagioclase from an anorthositic mush (making them xenocrysts); (2) this magma injected into a zoned dacite to high-silica rhyolite magma chamber; (3) magma mixing produced the andesite-dacite lava and overlying tuff; and (4) the thermal and dynamic perturbation of the silicic magma chamber led to the eruption of the high-silica rhyolite lava.

In this scenario, the plagioclase megacrysts may have impacted mixing processes. Regarding thermal-temporal processes, plagioclase has a higher thermal conductivity than basaltic magma and thus could have helped resist quenching against cooler silicic magmas and transferred heat during crystal transfer. More significant is the role of crystals in the fluid dynamics of mixing. The presence of coarse crystals, when below a fraction that forms a network, promotes mixing by creating local velocity gradients and increasing shear. The plagioclase megacrysts in the basalt thus may have improved the efficiency of physical mixing processes and facilitated the formation of a relatively large volume of variably mixed products at relatively low proportion of mafic input.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7053

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