Constraints on the pre-eruption thermal and fO2 conditions in the magma reservoir of Ciomadul (SE Carpathians, Romania) based on Fe-Ti oxide geochemistry
Authors: Szemerédi Máté , Mészáros Katalin , Lukács Réka , Kovács Zoltán , Harangi Szabolcs 
Published: March 2021

&lt;p&gt;Ciomadul is the southernmost eruptive centre of the post-collisional C&amp;#259;limani-Gurghiu-Harghita andesitic-dacitic volcanic chain (SE Carpathians, Romania) and represents the latest manifestation of the Neogene to Quaternary volcanism in the Carpathian-Pannonian Region. Ciomadul consists of older, peripheral shoshonitic to dacitic lava domes formed episodically between 1 Ma and 300 ka and a voluminous, central volcanic complex developed within the last 200 ka. Although several lines of evidence (based on petrology, geophysics and gas monitoring) suggest a long-lived magmatic plumbing system holding a potentially active magma storage (&amp;#8220;PAMS&amp;#8221; volcano) beneath Ciomadul, the pre-eruptive conditions of the upper crustal magma reservoir (including temperature, oxygen fugacity and TiO&lt;sub&gt;2&lt;/sub&gt; activity) are not completely explored so far. In this study 23 rock samples, representing the whole volcanic activity of Ciomadul in time, were involved. Fe-Ti oxide (magnetite-ilmenite) grains were selected from magnetic heavy minerals, but only a few of the samples contained both magnetite and ilmenite crystals. Equilibrium between Ti-magnetite and ilmenite was tested by their chemical composition (Mg/Mn ratios).&lt;/p&gt;&lt;p&gt;Various geothermobarometer calibrations, including Andersen and Lindsley (1985, 1988) as well as Ghiorso and Evans (2008), were applied to calculate temperature and oxygen fugacity from Fe-Ti oxide compositions. Our results show that, in case of dacitic pyroclastic rocks, temperature values gained by the method of Ghiorso and Evans are significantly lower (640&amp;#8211;780 &amp;#176;C) than those obtained by the geothermometers of Andersen and Lindsley (1985, 1988), showing 750&amp;#8211;830 and 710&amp;#8211;790&amp;#176;C temperatures, respectively. On the other hand, andesitic lava dome rocks of Dealul Mare show higher, 800&amp;#8211;900 &amp;#176;C temperature according to all of these methods. The obtained temperature was compared with amphibole-plagioclase thermometry results and this shows a better agreement with the values yielded by the Andersen and Lindsley (1985) Fe-Ti oxide thermometry, particularly for the pumice samples.&lt;/p&gt;&lt;p&gt;In case of oxygen fugacity, the Ghiorso and Evans (2008) and Andersen and Lindsley (1985) methods showed fairly similar values (fO&lt;sub&gt;2&lt;/sub&gt;=0.9&amp;#8211;1.8) whereas the Andersen and Lindsley (1988) calculations gave higher oxygen fugacity (fO&lt;sub&gt;2&lt;/sub&gt;=1.1&amp;#8211;2.5). Nevertheless, these results, irrespective the applied calculation methods, suggest relatively oxidized conditions (&amp;#916;NNO&gt;1) what is comparable with many other andesitic to dacitic volcanic systems (e.g. Mount St. Helens, Mount Unzen, Santorini). Values of TiO&lt;sub&gt;2&lt;/sub&gt; activity was calculated and obtained a range between 0.76 and 0.98 what is consistent with the common presence of titanite.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;This study was financed by NKFIH K135179 project.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Andersen, D.J. &amp; Lindsley, D.H. (1985). EOS Transactions of the American Geophysical Union, 66, 416.&lt;/p&gt;&lt;p&gt;Andersen, D.J. &amp; Lindsley, D.H. (1988). Amer Miner 73:714&amp;#8211;726.&lt;/p&gt;&lt;p&gt;Ghiorso, M.S. &amp; Evans, B.W. (2008). Amer J Sci 308:957&amp;#8211;1039.&lt;/p&gt;