Constraints on the pre-eruption thermal and fO2 conditions in the magma reservoir of Ciomadul (SE Carpathians, Romania) based on Fe-Ti oxide geochemistry

Ciomadul is the southernmost eruptive centre of the post-collisional Călimani-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 (“PAMS” volcano) beneath Ciomadul, the pre-eruptive conditions of the upper crustal magma reservoir (including temperature, oxygen fugacity and TiO₂ 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).

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 (640–780 °C) than those obtained by the geothermometers of Andersen and Lindsley (1985, 1988), showing 750–830 and 710–790°C temperatures, respectively. On the other hand, andesitic lava dome rocks of Dealul Mare show higher, 800–900 °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.

In case of oxygen fugacity, the Ghiorso and Evans (2008) and Andersen and Lindsley (1985) methods showed fairly similar values (fO₂=0.9–1.8) whereas the Andersen and Lindsley (1988) calculations gave higher oxygen fugacity (fO₂=1.1–2.5). Nevertheless, these results, irrespective the applied calculation methods, suggest relatively oxidized conditions (ΔNNO>1) what is comparable with many other andesitic to dacitic volcanic systems (e.g. Mount St. Helens, Mount Unzen, Santorini). Values of TiO₂ activity was calculated and obtained a range between 0.76 and 0.98 what is consistent with the common presence of titanite.

 

This study was financed by NKFIH K135179 project.

 

Andersen, D.J. & Lindsley, D.H. (1985). EOS Transactions of the American Geophysical Union, 66, 416.

Andersen, D.J. & Lindsley, D.H. (1988). Amer Miner 73:714–726.

Ghiorso, M.S. & Evans, B.W. (2008). Amer J Sci 308:957–1039.