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Magma mixing drastically enhanced by fragmentation and sintering

Yan Lavallée1, Jonathan M. Castro2, Fabian B. Wadsworth1, Paul Wallace1, Marize Muniz da Silva1, Jackie E. Kendrick1, Anthony Lamur1, Janine Birnbaum1, Jérémie Vasseur1, Honor James1, Kai-Uwe Hess1

  • Affiliations: 1Earth and Environmental Sciences, Ludwig-Maximilians-Universität, 80333 Munich, Germany 2Institute of Geosciences, Johannes Gutenberg University, Mainz, Germany

  • Presentation type: Poster

  • Presentation time: Tuesday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 219

  • Programme No: 3.2.42

  • Theme 3 > Session 2

Abstract

Magma mixing results from diffusive exchange following mingling of two magma endmembers. Such exchange requires either long equilibration timescales or small lengthscales. It is often presumed that magmas with starkly contrasting viscosities would not readily mingle via viscous flow; also, the widespread observation of transfer of xenocrysts from one endmember into another endmember (and far from the contact between the two) raises questions as to how crystals transfer across magmas with high viscosities. Such observations are commonplace at Glass Creek dome (Long Valley Caldera, California, USA) which is believed to have formed from the mingling of crystal-poor and crystal-rich rhyolitic endmembers (erupted at Obsidian Dome and Deadman Creek, respectively) during magma transport in the shallow feeder dyke. Here we present a wide range of field observations that suggest that mixing took place in a fragmental state rather than by viscous flow. Thermal analyses of the obsidian indicate multiple glass transition temperatures, commensurate with spatially heterogeneous chemistries, despite common cooling histories over short lengthscales. The observations are congruent with recent models indicating that silicic lavas are the products of agglutination and sintering of pyroclasts in shallow conduits. The diffusion lengthscales, and thus mixing timescales, between mingled pyroclasts would be much shorter than those of larger coherent magma schlieren. We advance that the efficiency of mixing can drastically increased when mingling takes place in a pyroclastic state, which could explain the observation of chemically homogeneous lavas despite the near ubiquity of magma mixing episodes in shallow magmas.