Effect of undercooling on clinopyroxene crystallization in a high K basalt. Implications for magma dynamics at Stromboli volcano

We present undercooling (ΔT) 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 (ΔTanh = 10–150 ◦C) and hydrous (ΔThyd = 25–125 ◦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 ΔTanh = 10 ◦C and dominates the phase assemblage as the degree of undercooling increases. Conversely, clinopyroxene and spinel co-saturate the melt at ΔThyd = 25 ◦C, followed by the subordinate formation of plagioclase. At ΔTanh/hyd ≤ 50 ◦C, the textural maturation of clinopyroxene produces polyhedral crystals with {- 111} (hourglass) and {hk0} (prism) sectors typical of a layer-by-layer growth mechanism governed by an interface-controlled crystallization regime. At ΔTanh/hyd ≥ 75 ◦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 ΔTanh/hyd ≥ 95 ◦C. The early stage of melt supersaturation produces rosette-like structures composed of dendritic branches of clinopyroxene radiating from a common spinel grain, which acts 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 H2Omelt on the liquidus depression and melt depolymerization, crystals obtained at ΔThyd are also more enriched in TAl and M1Ti and depleted in TSi and M1Mg than those growing at ΔTanh. The emerging picture is that the morphological and geochemical evolution of clinopyroxene is mutually controlled by the combined effects 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.