Do orbicules and comb layers record the transient passage of decompressing hydrous melts in shallow subvolcanic conduits?

Date & time

12.30–1.30pm 20 April 2018


Ringwood Room, J4, RSES


Anders McCarthy (University of Bristol)


 Michael Anenburg

Understanding the dynamics of volcanic eruptions requires constraints on the processes of melt extraction in shallow magma reservoirs. In this presentation we will discuss the formation of comb layers and orbicules from the Sierra Nevada, a particular type of igneous layering formed at the rims of small plutons and which are characterised by elongated, feather-shaped minerals growing perpendicular to the layering. Though texturally diverse, the majority of layers are formed by the initiation of plagioclase nucleation on pre-existing pluton-walls and xenoliths followed by interstitial growth of oxides, pyroxenes and hornblende. The combination of simple phase diagrams and the detailed study of invidiual layers indicate that orbicules and comb layers represent crystallization fronts resulting from in situ crystallization and extraction of evolved melt fractions during decompression-driven crystallization of volatile-saturated mafic melts, which forms a plagioclase-dominated cumulate at the mm- to m-scale. The conditions necessary for comb layering and orbicule growth requires the near-adiabatic ascent of volatile-undersaturated melts through the crust, leading to the resorption of crystals and the formation of superheated melts, thereby allowing for heterogenous nucleation of plagioclase on wall-rocks and locally remobilized xenoliths upon decompression at shallow depth. Comb layering and orbicules might form on very short timescales and might record variations in P-T conditions within subvolcanic conduits during the transient passage of decompressing melts to shallow depth.

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