UBC Theses and Dissertations
A new method for diagnosing and distinguishing magma mixing and overpressure events using chemical variations in plagioclase Wilson, Heather Anne
Striking textural and petrologic evidence for mixing of basalt into silicic melts occurs in volcanic arc settings worldwide. These textures (e.g. mingled/banded pumices, inclusions), along with chemical signatures of mixing (e.g. zoning in solid solution minerals) found in many eruption products, support a popular hypothesis that magma mixing can trigger explosive eruptions through a variety of possible mechanisms. However, rigorous observational constraints on the nature of the underlying thermal and mechanical processes remain elusive. A method is developed based on Nomarski differential interferometry to image and quantitatively characterise quasi-periodic zoning in plagioclase at very high spatial resolution. Applied to individual crystals, variations in zoning with crystallographic direction confirm experimental measurements of anisotropic diffusivities for Ca²⁺Al³⁺ and Na⁺Si⁴⁺. When applied to the AD 1315 Kaharoa eruption of Tarawera Volcano, Taupo Volcanic Zone, New Zealand, periodic zoning at scale lengths of 3.1–12.4 μm is consistent with quasi-periodic convective motions within the magma chamber acting on timescales of 39 days to 1.8 years prior to eruption. The structure of the zoning is inconsistent with a single large basalt injection causing eruption. Rather, this eruptive episode may have been preceded by several small volume basaltic inputs, consistent with observations from mafic-silicic layered intrusions.
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