Fig. 4: Model for scale-dependent volcanic ash surface chemistry via fragmentation through a heterogenous melt.
From: Nanoscale silicate melt textures determine volcanic ash surface chemistry
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.
