The onset and solidification path of a basaltic melt by in situ differential scanning calorimetry (DSC) and ex situ investigations

Thein situdifferential scanning calorimetry (DSC) technique has been applied to investigate the solidification paths of a basaltic liquid. The starting glass was heated up to 1300 degrees C, kept at thissuperliquidustemperature for 2 h and cooled at rates (Delta T/Delta t) of 7, 60, 180, 1000, and 1800 degrees C/h, down to 800 and 600 degrees C. Glass transition temperature (T-g), crystallization temperature (T-x_HR) and melting temperature (T-m) were measured byin situDSC spectra on heating.T(x)measured along the cooling paths (T-x_CR) shows exothermic peaks that change from a single symmetric shape (7 and 60 degrees C/h) to multi-component patterns (180, 1000, and 1800 degrees C/h). The recovered products characterized by field emission gun source of the scanning electron microscopy and electron probe micro-analyzer-wavelength dispersive spectrometers show a phase assemblage of spinel (sp), clinopyroxene (cpx), melilite (mel), plagioclase (plg), and glass. Moreover, crystal size distributions (CSDs) and growth rates (G(max)andG(CSD)) were also determined. The crystal content slightly increases from 7 to 1800 degrees C/h. Faceted sp are present in all the run products with an amount always <2 area%. Cpx increases from 7 to 1800 degrees C/h, changing its texture from almost faceted to dendritic between 60 and 180 degrees C/h. The area% of mel follows an asymmetric Gaussian trend, while plg nucleates only at 7 degrees C/h with a content <2 area%. The coupling of DSC and SEM outcomes indicate that sp nucleate first, followed by cpx and mel (and/or plg). The increment of Delta T/Delta tcauses an increase of the CSD slope (m) and crystal population density per size (n(0)), as well as a decrease of the crystal size, for both cpx and sp. The log-linear CSD segments with different slopes at 7 and 60 degrees C/h suggest multiple nucleation events and crystal growth by coarsening.G(max)andG(CSD)for cpx and sp directly measured on the actual crystallization time by DSC spectra, both increase with the increasing of Delta T/Delta t. The onset temperature of crystallization (T-xi) decreases as Delta T/Delta tincreases, following an exponential trend that defines the uppermost portion of a time-transformation-temperature-like curve. This analytical model allows us to quantitatively model the kinetic crystallization paths of dry basalts.