Quantitative field constraints on the dynamics of silicic magma chamber rejuvenation and overturn

• Main Author: Bain, Amelia Anne
• Language: English
• Published: University of British Columbia 2010
• Online Access: http://hdl.handle.net/2429/27430

A number of recent papers by Bachmann and co-authors investigate a hypothesis that the catastrophic eruption of large-volume, crystal-rich silicic magmas is a consequence of reheating (so-called rejuvenation) and overturn of partially molten, buoyant silicic material by repeated injection of dense, hot mafic magma. In support of this model, we analyse an extensive suite of kinematic indicators for the buoyant overturn of silicic crystal mush layers of the Coastal Maine Magmatic Province, apparently in response to the injection and cooling of hot, dense mafic magmas. We use spectral analysis, microtextural analysis and scaling theory to identify, characterise and understand the length-scales of deformation along sharp interfaces separating mafic and silicic intrusive layers, from the scale of individual crystals (~1 cm) to in excess of the mafic layer thickness (>100 m). Deformations at the largest scale lengths are comparable to the silicic layer thickness, consistent with Rayleigh-Taylor theory, and support a conjecture that mafic recharge can cause large-scale overturning of silicic magma chambers. By contrast, deformations at the scale of crystals probably record buoyancy effects related to melt percolation and intermediate scales are explained by compaction. The evolution of rejuvenation is investigated and a condition for large-scale overturn of the chamber is proposed. This work provides the first field-based confirmation of the rejuvenation-overturn hypothesis. Additional laboratory experiments addressing the overturn of a particle-rich buoyant fluid layer overlain by a denser fluid layer are outlined in Appendix C.