Rapid crystallization during recycling of basaltic andesite tephra: timescales determined by reheating experiments

Nicholas Deardorff,Katharine Cashman

doi:10.1038/srep46364

Nicholas Deardorff, Katharine Cashman

Open Access

https://doi.org/10.1038/srep46364

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Abstract

Microcrystalline inclusions within microlite-poor matrix are surprisingly common in low intensity eruptions around the world, yet their origin is poorly understood. Inclusions are commonly interpreted as evidence of crystallization along conduit margins. Alternatively, these clasts may be recycled from low level eruptions where they recrystallize by heating within the vent. We conducted a series of experiments heating basaltic andesite lapilli from temperatures below the glass transition (~690 °C) to above inferred eruption temperatures (>1150 °C) for durations of 2 to >60 minutes. At 690 °C < T < 800 °C, crystallization is evident after heating for ~20 minutes; at T > 800 °C, crystallization occurs in <5 minutes. At T ≥ 900 °C, all samples recrystallize extensively in 2–10 minutes, with pyroxenes, Fe-oxides, and plagioclase. Experimental crystallization textures closely resemble those observed in natural microcrystalline inclusions. Comparison of inclusion textures in lapilli from the active submarine volcano NW Rota-1, Mariana arc and subaerial volcano Stromboli suggest that characteristic signatures of clast recycling are different in the two environments. Specifically, chlorine assimilation provides key evidence of recycling in submarine samples, while bands of oxides bordering microcrystalline inclusions are unique to subaerial environments. Correct identification of recycling at basaltic vents will improve (lower) estimates of mass eruption rate and help to refine interpretations of eruption dynamics.

Highlights

The extent of vent clogging affects the subsequent eruption intensity[12] and grain size distribution[9]
We attempted heating experiments on NW Rota-1 samples, heating the clasts resulted in an odd alteration where the glass ‘inflated’ producing a bubbly, popcorn texture that was very brittle, and we were unable to polish or analyze the clasts
Microcrystalline inclusions are prevalent in NW Rota-1 samples, found in most sideromelane clasts examined, making them less desirable as experimental specimens

Summary

Introduction

The extent of vent clogging affects the subsequent eruption intensity[12] and grain size distribution[9]. The limited experimental data on heating-induced crystallization show that the crystallization kinetics are interface-controlled and depend on oxidation state as well as temperature, and that environments that promote such crystallization include overtopping lava flows in pahoehoe fields[14] and intra-crater pyroclast accumulation[13]. Our experiments complement those of D’Oriano, et al.[13] by exploring shorter timescales (2–64 min) and covering a larger temperature range (

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