AUTHOR=Giuliani Letizia , Iezzi Gianluca , Hippeli Tyler , Davis Mark , Elbrecht Aubrey , Vetere Francesco , Nazzari Manuela , Mollo Silvio 

TITLE=The Onset and Solidification Path of a Basaltic Melt by in situ Differential Scanning Calorimetry (DSC) and ex situ Investigations

JOURNAL=Frontiers in Earth Science

VOLUME=8

YEAR=2020

URL=https://www.frontiersin.org/journals/earth-science/articles/10.3389/feart.2020.00337

DOI=10.3389/feart.2020.00337

ISSN=2296-6463

ABSTRACT=<p>The <italic>in situ</italic> differential scanning calorimetry (DSC) technique has been applied to investigate the solidification paths of a basaltic liquid. The starting glass was heated up to 1300°C, kept at this <italic>superliquidus</italic> temperature for 2 h and cooled at rates (Δ<italic>T</italic>/Δ<italic>t</italic>) of 7, 60, 180, 1000, and 1800°C/h, down to 800 and 600°C. Glass transition temperature (<italic>T</italic><sub>g</sub>), crystallization temperature (<italic>T</italic><sub>x_HR</sub>) and melting temperature (<italic>T</italic><sub>m</sub>) were measured by <italic>in situ</italic> DSC spectra on heating. <italic>T</italic><sub>x</sub> measured along the cooling paths (<italic>T</italic><sub>x_CR</sub>) shows exothermic peaks that change from a single symmetric shape (7 and 60°C/h) to multi-component patterns (180, 1000, and 1800°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 (<italic>G</italic><sub>max</sub> and <italic>G</italic><sub>CSD</sub>) were also determined. The crystal content slightly increases from 7 to 1800°C/h. Faceted sp are present in all the run products with an amount always &lt;2 area%. Cpx increases from 7 to 1800°C/h, changing its texture from almost faceted to dendritic between 60 and 180°C/h. The area% of mel follows an asymmetric Gaussian trend, while plg nucleates only at 7°C/h with a content &lt;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 Δ<italic>T</italic>/Δ<italic>t</italic> causes an increase of the CSD slope (<italic>m</italic>) and crystal population density per size (<italic>n</italic><sub>0</sub>), 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°C/h suggest multiple nucleation events and crystal growth by coarsening. <italic>G</italic><sub>max</sub> and <italic>G</italic><sub>CSD</sub> for cpx and sp directly measured on the actual crystallization time by DSC spectra, both increase with the increasing of Δ<italic>T</italic>/Δ<italic>t</italic>. The onset temperature of crystallization (<italic>T</italic><sub>xi</sub>) decreases as Δ<italic>T</italic>/Δ<italic>t</italic> increases, 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.</p>