Atom probe tomography reveals nanoscale immiscibility as the cause of rare blue coloration of basaltic glass

Liquid immiscibility is commonly invoked to explain differentiation between mafic and silicic melts, but its physical effects are rarely visible at macroscopic scales. In this study, we show that the distinctive blue coloration of the Blue Dragon pahoehoe lava flow in Craters of the Moon National Monument and Preserve (Idaho, USA) results from Rayleigh scattering caused by nanoscale Fe-rich glass domains dispersed within a Si-rich glass matrix. Atom probe tomography and electron beam imaging demonstrate that these domains occur as spherical or tubular structures ∼10–50 nm in diameter. Droplet-like and bicontinuous intergrowths result from binodal and spinodal decomposition, respectively. The presence of these textures only in the near-surface glass layer of the Blue Dragon flow suggests that their development is controlled by cooling and melt compositional variations at crystal boundaries. The Blue Dragon nanoscale structures are analogous to those in synthetic opaline glasses and rare natural examples such as blue fulgurites and impact glasses. Nanoemulsions of immiscible liquids also form in other quenched lavas, but Blue Dragon represents the only documented case of naturally occurring blue volcanic glass. Its rarity reflects the need for (1) melt compositions sufficiently enriched in Fe to enter the miscibility gap upon cooling, (2) quenching rates that promote phase separation yet inhibit microlite formation, and (3) preservation of near-surface flow layers. Although eruptions of Fe-rich lavas such as Blue Dragon are uncommon for terrestrial volcanic systems, basaltic melts may commonly enter the immiscibility gap upon magma differentiation.