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Moore&Jutzeler High-temperature Oxidation 1886 Tarawera NZ 2022.pdf (5.28 MB)

High‑temperature oxidation of proximal basaltic pyroclasts, 1886 Tarawera, New Zealand

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posted on 2023-05-21, 08:15 authored by Hannah MooreHannah Moore, Rebecca CareyRebecca Carey, Houghton, BF, Martin JutzelerMartin Jutzeler, White, JDL
Microlite crystallization in erupting basalt can occur in the conduit, in flight, or in situ after deposition. Distinguishing the products of primary versus secondary (post-fragmentation) crystallization can be challenging in near-vent environments, but is vital for interpreting shallow conduit conditions from pyroclast textures. Here, we examine pyroclasts of the 1886 basaltic Plinian eruption of Tarawera volcano, New Zealand, to assess the roles of primary versus secondary crystallization of microlites. Lapilli and ash were selected from (a) an ultra-proximal section (T47), < 100 m from vent, which is dominated by pyroclasts derived from the Plinian jet and column margin, and (b) a medial fall deposit section (T43), 2.5 km from the fissure, which contains products from the umbrella cloud. Strong contrasts in pyroclast groundmass crystallinities exist between sections, from near-holocrystalline (90–97% void-free corrected; VFC) in T47 pyroclasts to highly crystalline (77–83% VFC) in T43 pyroclasts. Subhedral-euhedral Fe–Ti microlites (< 3 μm) are ubiquitous and abundant in T47 pyroclasts, whereas they are virtually absent in T43 pyroclasts. Olivine is present in both T47 and T43 pyroclasts but evidence of its subsolidus transformation is only seen in T47 clasts, whereas in T43 clasts olivine is fresh. Near-complete crystallization of the groundmass and subsolidus transformation of olivine in the ultraproximal T47 clasts are evidence that post-depositional modification of primary pyroclast textures occurred in ultra-proximal environments at Tarawera as a response to high residual temperatures and oxidizing conditions, aided by short transport times of relatively coarse ejecta at high accumulation rates, likely supplemented by intense vent-derived heat. Ultra-proximal basaltic eruption products can continue to crystallize after deposition and are thus unlikely to be faithful indictors of shallow conduit magmatic processes; we recommend the use of medial and distal products instead.


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Bulletin of Volcanology



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School of Natural Sciences



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© 2022 The Author(s) This is an open access article under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0) License ( which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made.

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