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

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