Nanoscale silicate melt textures determine volcanic ash surface chemistry

Abstract

Explosive volcanic eruptions produce vast quantities of silicate ash, whose surfaces are subsequently altered during atmospheric transit. These altered surfaces mediate environmental interactions, including atmospheric ice nucleation, and toxic effects in biota. A lack of knowledge of the initial, pre-altered ash surface has required previous studies to assume that the ash surface composition created during magmatic fragmentation is equivalent to the bulk particle assemblage. Here we examine ash particles generated by controlled fragmentation of andesite and find that fragmentation generates ash particles with substantial differences in surface chemistry. We attribute this disparity to observations of nanoscale melt heterogeneities, in which Fe-rich nanophases in the magmatic melt deflect and blunt fractures, thereby focusing fracture propagation within aureoles of single-phase melt formed during diffusion-limited growth of crystals. In this manner, we argue that commonly observed pre-eruptive microtextures caused by disequilibrium crystallisation and/or melt unmixing can modify fracture propagation and generate primary discrepancies in ash surface chemistry, an essential consideration for understanding the cascading consequences of reactive ash surfaces in various environments.

Fig. 1. Macro-to-micro textures of starting materials.

a Photo of the scoriaceous bomb collected from PDC deposits of the 16–17^th August 2006 eruption of Tungurahua on a 1 cm grid. The top surface was cut flat, and cores were drilled perpendicular to the cut surface. b, c Orthogonal side views of the starting block, showing the pore texture along the axis of the drilled cores. d Microphotograph in transmitted light and (e) SEM-BSE image, showing pore structure (in white in (b) and in black in (c)), microlite rich matrix glass (mGl) and crystals of plagioclase (Pl) and pyroxene (Px).

Fig. 2. Surface and bulk composition of the fragmented clasts.

a The quotient of the elemental abundance at the nanoscale surface of experimental clasts measured by XPS with the average bulk composition measured by XRF. b Relative variation in the major phases between the bulk and the micron-scale surface of particles as measured by QEMSCAN. Positive values indicate phase enrichment at the surface. The average phase composition from all samples is shown in the inset. c The quotient of the nanoscale surface chemistry measured by XPS and the microscale surface chemistry calculated from EPMA and QEMSCAN data (calculated μm-scale surface). The coloured bars show ±2× standard error for all panels and mean absolute error is shown in the inset to (b). In the legend, RT and HT indicate room temperature or high temperature (850 °C), respectively, 10 and 30 refer to confining pressure (in MPa) in shock tube experiments, and the suffix letter refers to samples produced by crushing (C) or decompression fragmentation (F).

Fig. 3. Textural and petrological characterisation of fragmented clasts.

SEM-BSE image (a) and overlain QEMSCAN phase map (b) for polished clasts from a crushed block, with phases given in the key. c SEM-BSE image showing high-magnification matrix texture, including euhedral plagioclase feldspar (Pl) and pyroxene microlites (PxM) hosted in a glass matrix (mGl) containing numerous rounded, nm-scale bright features that may represent Fe-rich nanolites or immiscible globules. This phase is absent in a ~1 μm thick zone surrounding pyroxene microlites marked by a dashed line; the zone has decreasing BSE intensity toward the crystal surface, suggesting a compositional boundary-layer depleted in Fe relative to the matrix glass. d SEM-BSE image of the matrix in a natural volcanic ash particle from the August 2006 eruption of Tungurahua, showing the same nanoscale textural features described in (c). e SEM-SE image of the unpolished surface of an ash grain from the August 2006 eruption of Tungurahua showing bright nanoscale speckles. SEM-EDX analysis (f, g) with dashed box showing area in (e) shows that the speckles are enriched in Fe (f) and Mg (g).

Fig. 4. Model for scale-dependent volcanic ash surface chemistry via fragmentation through a heterogenous melt.

a Volcanic ash particle surfaces produced by magma fragmentation have a similar composition to the bulk at the micron-scale but can show significant variations from the bulk at the nanoscale. Chemical analysis results from experimentally fragmented Tungurahua ash are shown for selected elements for the non-shaded volumes. b Diffusion-limited growth of mafic microlites during pre-eruptive magma mixing, ascent and storage causes boundary-layer formation within the melt. Subsequent nucleation and/or unmixing in the matrix melt produces nanoscale Fe-rich phases whose size and number density is sensitive to the local melt chemistry; they reduce in size toward the outer extreme of the Fe- and Mg-depleted boundary-layer melt and become absent within ca. 1 um of the mafic crystal boundary. All phase labels are the same as in Fig. 2. c The interstitial melt bears nanoscale Fe-rich phases and contains Mg + Fe-depleted boundary-layer melt around mafic crystals. During magma fragmentation, fractures preferentially propagate through the single-phase glass in boundary layers (<1 μm from mafic crystal faces) and may deflect around the Fe-rich nanolites or immiscible globules in the unmixed melt.