Abstract The pāhoehoe—‘a‘ā morphological transition involves a change in the rheological response of the magmatic suspension from pure viscous to complex rheological behavior, resulting in the development of tear‐apart features. Here, we present a suite of concentric cylinder experiments aimed at studying the effects of cooling and shear rates on the rheology of a phonotephrite melt in response to crystallization. Experiments were conducted at: (a) isothermal subliquidus temperatures of 1,167–1,189°C and shear rates of 1–3 s −1 ; (b) constant cooling rates of 1–10°C/min and shear rates of 1–20 s −1 . We defined the viscosity‐temperature‐time window of lava solidification, as well as the transition from coherent flow to shear localization and physical separation (i.e., viscous rupture). Through this approach, we mapped the processes and timescales affecting pāhoehoe—‘a‘ā transition in natural lavas at variable cooling and shear rates. Under disequilibrium conditions, as the cooling rate increases, both crystallization onset and viscous rupture occur at lower temperature and earlier in time. Moreover, the time to reach the crystallization onset and viscous rupture also decreases with increasing shear rate. Both increasing cooling and shear rate reduces the critical crystallinity required for viscous rupture, a consequence of the non‐linear interplay between temperature, crystallization kinetics, and melt viscosity. This outcome expands our knowledge on compositional, thermal, and rheological changes in phonotephritic systems. In addition to shear rate and apparent viscosity, comparison with previous measurements on basaltic systems indicates that the pāhoehoe—‘a‘ā transition is sensitive to the composition and cooling path of lavas.
Intraplate basaltic systems, often occurring as fields of small monogenetic volcanoes, are dominated by eruption of alkaline basaltic rocks, ranging from nephelinite/basanite to transitional/subalkaline. Their generally primitive erupted compositions imply limited crustal modification, and hence they provide an important probe into deep, lithospheric mantle and partial melting processes. Partial melting and magmatic ascent processes can be investigated using the composition of crystals, glass, and whole rock, although a combination of these is preferable. The whole-rock chemical variability within single eruptions or over the temporal and spatial extent of a volcanic field is controlled by the characteristics of the primary melting source, as well as near-source percolative/reaction processes. Coupled crystal- and -whole-rock detailed investigations are most promising to constrain the processes that modify primary melts into the primitive magmas that accumulate before ascent. Complex crystal textures and chemistry have so far demonstrated that basaltic magmas are principally processed and modified within the lithospheric mantle with minor modification en route through the crust. Fractional crystallization and magma mixing modify melts throughout ascent, and can imprint secondary chemical intra-eruptive variability. Quantifiable temperature and pressure parameters based on crystal-melt compositions constrain the depth of formation, and hence provide information about the role of different mineral phases in deep versus shallow chemical evolution. Volatile components in the melt (e.g., H2O and CO2) can be quantified on glass and melt inclusions. These analyses, coupled with solubility models, may help to reconstruct initial dissolved volatile content to further constrain the source characteristics and magmatic ascent dynamics. Integrated studies of crystals and melt paint a picture of extended lithospheric mantle to minor crustal processing resulting from the complex deep plumbing of monogenetic basaltic systems. This highlights the need for improved resolution to characterize true primary signatures and hence elucidate the formation of intraplate alkaline basalts.
Abstract Magma ascending through Earth’s crust undergoes complex chemical and physical changes that may induce crystallization, a process that contributes to lead the magmatic system toward a thermodynamic state of equilibrium. The diverse cooling and deformative regimes suffered by magmas heavily influence crystallization rates, solidification timescales, and consequently, the rheological evolution of magma. This, in turn, significantly impacts the dynamics of volcanic plumbing systems and the associated eruptive styles. Here, we investigate the rheological changes in Stromboli magma (Italy) during disequilibrium crystallization under non-isothermal subliquidus conditions. By systematically varying the cooling rate (1-10 °C/min) and the shear rate (1-10 s −1 ), we find that cooling rates significantly influence the solidification path of the basalt, whereas shear rates have a subordinate effect. By comparing our results with literature data on basalts from Mt. Etna (Italy), characterized by higher TiO 2 and FeO tot contents, we observed distinct timescales and rates of solidification, contributing to unique eruptive dynamics in these volcanic plumbing systems.
We present undercooling (∆T) experiments aimed at investigating the effect of growth kinetics on the textural and compositional evolution of clinopyroxene crystals growing from a high-K basalt erupted during the 2003 paroxysm of Stromboli volcano (Italy). The experiments were performed at P = 350 MPa, T = 1050-1210 °C, H2Omelt = 0-3 wt.%, and fO2 = Ni-NiO+1.5 buffer. An initial stage of supersaturation was imposed to the melt under nominally anhydrous (∆Tanh= 10-150 °C) and hydrous (∆Thyd = 15-115 °C) conditions. Afterwards, this supersaturation state was mitigated by melt relaxation phenomena over an annealing time of 24 h. Results show that plagioclase is the liquidus mineral phase of the high-K basalt at ∆Tanh = 10 °C and dominates the phase assemblage as the degree of undercooling increases. Conversely, clinopyroxene and spinel co-saturate the melt at ∆Thyd = 15 °C, followed by the subordinate formation of plagioclase. At ∆Tanh/hyd £ 50 °C, the textural maturation of clinopyroxene produces polyhedral crystals with {-111} (hourglass) and {h k 0} (prism) sectors typical of a layer-by-layer growth mechanism governed by an interface-controlled crystallization regime. At ∆Tanh/hyd ³ 65 °C, the attainment of dendritic and skeletal morphologies testifies to the establishment of diffusion-limited reactions at the crystal-melt interface. 3D reconstructions of synchrotron radiation X-ray microtomographic data reveal a composite growth history for clinopyroxene crystals obtained at ∆Tanh/hyd > 100 °C. The early stage of melt supersaturation produces rosette-like structures composed of dendritic branches of clinopyroxene radiating from a common spinel grain acting as surface for heterogeneous nucleation. As diffusive relaxation phenomena progress over the annealing time, the elongate dendrites that constitute the inner crystal domain are partially infilled by the melt and develop skeletal overgrowths in the outer domain. With the increasing degree of undercooling, TAl and M1Ti cations are progressively incorporated in the lattice site of clinopyroxene at the expense of TSi and M1Mg cations. Because of the effect of H2O on the liquidus depression and melt depolymerization, crystals obtained at ∆Thyd are also more enriched in TAl and M1Ti and depleted in TSi and M1Mg than those growing at ∆Tanh. The emerging picture is that the morphological and geochemical evolution of clinopyroxene is mutually controlled by the combined effect of melt supersaturation and relaxation phenomena. A new empirical relationship based on the cation exchange reactions in the lattice site of clinopyroxene is finally proposed to estimate the degree of undercooling governing the crystallization of augitic phenocrysts erupted during normal and violent explosions at Stromboli.
