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

We present undercooling (Delta 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 degrees C, H2Omelt = 0-3 wt%, and fO(2) = Ni-NiO + 1.5 buffer. An initial stage of supersaturation was imposed to the melt under nominally anhydrous (Delta T-anh = 10-150 degrees C) and hydrous (Delta T-hyd = 25-125 degrees 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 Delta T-anh = 10 degrees C and dominates the phase assemblage as the degree of undercooling increases. Conversely, clinopyroxene and spinel co-saturate the melt at Delta T-hyd = 25 degrees C, followed by the subordinate formation of plagioclase. At Delta T-anh/hyd <= 50 degrees 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 Delta T-anh/hyd >= 75 degrees 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 Delta T-anh/hyd = 95 degrees 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, Al-T and Ti-M1 cations are progressively incorporated in the lattice site of clinopyroxene at the expense of Si-T and Mg-M1 cations. Because of the effect of H2Omelt on the liquidus depression and melt depolymerization, crystals obtained at Delta T-hyd are also more enriched in Al-T and Ti-M1 and depleted in Si-T and Mg-M1 than those growing at Delta T-anh. 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.