COMPACTION IN SILICIC MUSHES: THE MISSING LINK BETWEEN CRYSTAL-POOR RHYOLITES AND SUBVOLCANIC PLUTONS?

Crystal-poor rhyolites and silicic plutons are known to have occurred simultaneously in numerous magmatic provinces of the globe. Their geochemical affinities and overlapping isotopic ratios strongly suggest a common origin but differing trace elements contents, in particular the Rb/Sr ratio (commonly very high in rhyolites), indicate that volcanic rocks underwent more feldspar-dominated crystal fractionation than their plutonic equivalents. These characteristics seem to require a late-stage crystal-melt segregation event, but relative motion between small crystals and viscous rhyolitic melt is extremely slow, particularly if convective currents, even sluggish, stir the suspension. A potential mechanism of two-phase flow that can reconcile these contradicting observations is compaction in static, high porosity silicic mushes. As silicic magmas stall in the upper crust, they cool and crystallized, but can remain fairly well mixed by convecting currents up to 45-50 % crystals, as illustrated by the eruption of voluminous, crystal-rich (~45 % crystals), and unzoned ignimbrites in continental arcs (the Monotonous Intermediates of Hildreth, 1981). These Monotonous Intermediates also record the presence of vast quantities of high-SiO₂ melt in the upper crust, preserved as interstitial melt in these erupted mushes. Upon reaching their rheological lock-up point (~50 % crystals), these mushes stop convecting, allowing interstitial melt expulsion by compaction to begin. Using hindered settling velocities and compaction rates to constrain a range of possible time scales, formation of the largest known of crystal-poor rhyolite (>500 km³) is shown to occur in 10⁴ to 10⁵ years, within the estimated residence times of mushes in the upper crust (>10⁵ years). This model reconciles numerous observations in the rock record, and provides an explanation for the observed range of ages (up to 200-300 k.y.) recorded by zircons in silicic magmas.

Ref: Hildreth, W., 1981. Gradients in silicic magma chambers: Implications for lithospheric magmatism. Journal of Geophysical Research, 86(B11), 10153-10192.