An image-based technique to determine the freezing temperature Tf of vesicle volumes in decompressed, synthetic melt samples
Authors: Eul Dennis , Allabar Anja , Nowak Marcus 
Published: March 2021

&lt;p&gt;The non-in-situ analysis of H&lt;sub&gt;2&lt;/sub&gt;O degassing of silicate melt at high temperature and pressure is conducted using synthetic, decompressed melt samples quenched to glass. Interpretations regarding the degassing behavior are based on the number of H&lt;sub&gt;2&lt;/sub&gt;O filled vesicles and the porosity of the vitrified samples. These properties of the glass samples may not represent the vesiculation at experiment temperature &lt;em&gt;T&lt;/em&gt;&lt;sub&gt;exp&lt;/sub&gt; and target pressure &lt;em&gt;P&lt;/em&gt;&lt;sub&gt;final&lt;/sub&gt;. Even at high quench rates &lt;em&gt;q&lt;/em&gt;, a decrease of vesicle volumes during cooling occurs, facilitated by resorption of H&lt;sub&gt;2&lt;/sub&gt;O fluid back into the melt (McIntosh et al., 2014) and by the decrease of molar volume of H&lt;sub&gt;2&lt;/sub&gt;O (Marxer et al., 2015) in the vesicles. This vesicle shrinkage introduces uncertainty regarding the true &lt;em&gt;q&lt;/em&gt;-dependent &amp;#8220;freezing&amp;#8221; temperature &lt;em&gt;T&lt;/em&gt;&lt;sub&gt;f&lt;/sub&gt;, at which shrinkage stops, represented by the vesiculated glass sample. While often neglected, knowledge of &lt;em&gt;T&lt;/em&gt;&lt;sub&gt;f&lt;/sub&gt; is useful for improved sample interpretation.&lt;/p&gt;&lt;p&gt;McIntosh et al. (2015) developed a computer tomography (CT) based method to determine &lt;em&gt;T&lt;/em&gt;&lt;sub&gt;f&lt;/sub&gt;. This approach infers &lt;em&gt;T&lt;/em&gt;&lt;sub&gt;f &lt;/sub&gt;from the volume fraction of liquid H&lt;sub&gt;2&lt;/sub&gt;O in vesicles (whose volumes are comprised of a liquid and a gaseous H&lt;sub&gt;2&lt;/sub&gt;O phase) which decreases for increasing &lt;em&gt;T&lt;/em&gt;&lt;sub&gt;f&lt;/sub&gt;.&lt;/p&gt;&lt;p&gt;Using their theoretical foundations, we developed a simple, transmitted light microscopy (TLM) image-based approach for the determination of this intra-vesicle phase ratio, applying two different model calculations: 1) Approximation of phase boundaries using polynomial functions. 2) Calculation of total vesicle and gas-phase volumes from ellipsoid axes measurements, approximating the vesicle and gas-phase volumes with symmetrical spheroids. In our analyzed hydrous, haplogranitic samples, we found mean &lt;em&gt;T&lt;/em&gt;&lt;sub&gt;f&lt;/sub&gt;&amp;#8217;s up to ~250 to ~300 K lower than &lt;em&gt;T&lt;/em&gt;&lt;sub&gt;exp&lt;/sub&gt;, at which quench was initiated, for &lt;em&gt;q&lt;/em&gt;&amp;#8217;s of ~40 and ~90 K/s. These values are close to the estimated &lt;em&gt;T&lt;/em&gt;&lt;sub&gt;f&lt;/sub&gt;&amp;#8217;s obtained using an independent glass porosity equation (Gardner et al., 1999). The large scatter of volume fractions and thus &lt;em&gt;T&lt;/em&gt;&lt;sub&gt;f&lt;/sub&gt; for individual vesicles cannot be attributed to our image-based approach as data obtained from phonolitic samples using the CT method (Allabar et al., 2020) depict a similar scatter. At present, no correlation of &lt;em&gt;T&lt;/em&gt;&lt;sub&gt;f&lt;/sub&gt; with vesicle size or position within the sample could be made. The method is, for the range of vesicle sizes investigated (20 to 50 &amp;#181;m in diameter), limited to liquid volume fractions larger than ~10 vol% as a distinction between phases is limited by optical resolution.&lt;/p&gt;&lt;p&gt;Nevertheless, our TLM based approach provides a simple, readily available method to constrain &lt;em&gt;T&lt;/em&gt;&lt;sub&gt;f&lt;/sub&gt; of vitrified vesiculated samples which significantly improves the quality and comparability of derived interpretations. Our method uses standard polished sections for FTIR, making it even applicable to already existing samples.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Allabar, A. et al. (2020), Contrib. Mineral. Petrol, 175, 21, 1-19&lt;/p&gt;&lt;p&gt;Gardner, J.E., Hilton, M. and Carroll, M.R. (1999), Earth Planet. Sci. Lett, 168, 201-218&lt;/p&gt;&lt;p&gt;Marxer, H., Bellucci, P. and Nowak, M. (2015), J. Volcanol. Geotherm. Res, 297, 109-124&lt;/p&gt;&lt;p&gt;McIntosh, I.M. et al. (2014), Earth Planet. Sci. Lett, 401, 1-11&lt;/p&gt;&lt;p&gt;McIntosh et al. (2015): &amp;#8216;Practical&amp;#8217; glass transition temperatures of vesicular glasses: a combined FTIR-XRCT approach. Abstract, 10th Silicate melt workshop. La Petite Pierre, France&lt;/p&gt;