Thermal and rheological controls on the formation of mafic enclaves or banded pumice

Abstract

Magma mixing can occur in a fluid manner to produce banded pumice or in a brittle manner to form enclaves. We propose that the critical control on mixing style is a competition between developing networks of crystals in the intruding magma that impart a strength to the magma and melting and disrupting those networks in the host. X-ray computed tomography analysis demonstrates that banded pumice from the 1915 Mt. Lassen eruption lacks crystal networks. In contrast, rhyodacite hosts with mafic enclaves from Chaos Crags contain welldeveloped networks of large crystals. We present a onedimensional conductive cooling model that predicts mixing style, either ductile or brittle, as a function of magma compositions, temperatures, and the size of the intruding dike. Our model relies on three assumptions: (1) Mixing is initiated by the injection of a hot dike into a cooler magma body with a yield strength; (2) when magma crystallinity exceeds a critical value, 13 vol% plagioclase, the magma develops a yield strength; and (3) when total crystallinity exceeds 40 vol%, the magma has a penetrative crystal network and is effectively solid. Importantly, because the two magmas are of different compositions, their crystallinities and viscosities do not have the same variations with temperature. As the intruding magma cools, it crystallizes from the outside in, while simultaneously, host magma temperature near the intruder rises. Mixing of the two magmas begins when the host magma is heated sufficiently to (1) disrupt the crystal network and (2) initiate convection. If the shear stress exerted by the convecting host magma on the dike is greater than the yield strength of the dike margin (and dike crystallinity does not exceed 40 %), then fluid mixing occurs, otherwise enclaves form by brittle deformation of the dike. Application of the model to magma compositions representative of Lassen and Chaos Crags shows that emplacement of dikes \1 m thick should produce enclaves, whereas thicker dikes should generate fluid mixing and form banded pumice within days to weeks of emplacement. Similar relationships apply to other modeled magmatic systems, including Pinatubo, Unzen, and Ksudach/Shtuybel' volcanoes. For all studied systems, the absolute size of the intruding dike, not just its proportion relative to the host, influences mixing style.

A trachyte–syenite core within a basaltic nest: filtering of primitive injections by a multi-stage magma plumbing system (Oki-Dōzen, south-west Japan)

“…The formation of the trachytic enclave within the syenite may be directly related to the emplacement of a new magma batch derived from a deeper trachytic reservoir. Enclave formation follows fragmentation of a cooled dike by convection within the still partially molten syenite body (Andrews and Manga 2014). This may also explain why the enclavehosting syenite sample has high 87 Sr/ 86 Sr, as that portion of syenite would have been affected by magmatic and hydrothermal fluids related to the trachyte dyke emplacement episode.…”

On the conditions of magma mixing and its bearing on andesite production in the crust

“…Typically, magma mixing/mingling occurs during replenishment by a mafic magma of a felsic and mushy reservoir [8][9][10][11][12][13][14] , either deep or shallow, which, in the latter case, may trigger a volcanic eruption 8 . The injection of denser basalt into a lighter reservoir most likely produces viscous gravity currents spreading at the floor, leading to a stratified two-layer system 15 , except when excess momentum is available, which may induce fountaining 16 .…”

“…Ensuing crystallization can lead to density inversion producing either local 17,18 or wholesale 19 overturning and associated mixing/mingling, depending on viscosity contrast 20 . Observations and fluid dynamical considerations [8][9][10][11][12][13][14][15][16][17][18][19][20][21] have shown that, in calcalkaline reservoirs, mixing/mingling most likely starts either from such boundary layer instabilities 17 , with centimetre to decimetre wavelength scale 18 , or from breakup of injected magma dykes 12 . These mechanisms readily explain enclave sizes as observed in plutonic or volcanic rocks 18,21 , that is, mingled magmas.…”

“…Previous work has considered magma mixing using analogical, numerical or experimental approaches [8][9][10][11][12][13][14][15][16][17][18][19][20] , which have all yielded valuable information on the fluid mechanics, leading to efficient stirring. However, the rigorous evaluation of the conditions favourable to intimate magma mixing has been hampered by the difficulty of simulating mixing at high pressure, and by the complex fluid-mechanical behaviour of magmas.…”

Origins of Early Mesozoic granitoids and their enclaves from West Kunlun, NW China: implications for evolving magmatism related to closure of the Paleo-Tethys ocean

“…Vernon 2014; Clemens and Bezuidenhout 2014). The convective rise of the hybrid conduit to form mingled magmas (Snyder and Tait 1996;Clynne 1999;Andrews and Mang 2014) and buoyant rise of hybrid magma blobs that become enclaves suspended within the enclosing host granitic magma at emplacement level (e.g. Vernon 1984;Clynne 1999;Coombs et al 2003).…”