Unraveling the solidification path of a pahoehoe "cicirara" lava from Mount Etna volcano

The solidified surface of a lava flow reflects the
viscosity of its molten fraction and the crystal content during
flow; crystal-poor basaltic lavas produce pahoehoe fields,
whereas crystal-rich ones solidify with aa carapaces. At
Mount Etna, volcano aa morphologies are very common,
whereas pahoehoe lavas are rare. The latter are locally
named “cicirara” due to the presence of centimeter-sized
plagioclase phenocrysts much more abundant than in aa
lavas. The phenocryst content of “cicirara” lavas contrasts
with the low viscosity generally associated with pahoehoe
morphology. Therefore, to reconcile the discrepancy between
textural and volcanic observations, we have studied
the most primitive pahoehoe “cicirara” lava sampled until
now. Two samples at 0.5 and 1 m from the bottom of the 2-
m thick lava flow were investigated on the basis of their
mineral compositional variations and textural features, i.e.,
size frequency and crystal size distribution (CSD). Results
coupled with rheological models indicate that only large
phenocrysts of plagioclase (>1 mm) and clinopyroxene have
grown before eruption. Thermobarometric models and petrological
computations based on the composition of plagioclase
and clinopyroxene phenocryst cores highlight that
only a small amount (10–15 vol.%) of crystals equilibrated
at 12 km of depth. Cumulative size frequency and CSD data
also indicate that plagioclase and clinopyroxene phenocryst
rims grew heterogeneously and coalesced around their cores
at depths <1 km, before eruption. In this view, the “cicirara”
lava was erupted with a low crystalline content that favoured
the formation of its pahoehoe surface; however, crystals
with a size <1 mm (~75 vol.%) solidified at post-eruptive
conditions. Our findings underline that the emplacement of
high-viscosity aa or low-viscosity pahoehoe lavas is driven
by the degree of undercooling imposed by the volatile
exsolution rate in the shallowest portion of the Etnean
plumbing system. A slow magma ascent rate promotes
significant intratelluric degassing and widespread nucleation;
consequently, the viscosity of the suspension significantly
increases leading to an aa morphology. In contrast,
pahoehoe “cicirara” lavas are associated with a rapid rise to
the surface of poorly degassed, undercooled magmas.