Keywords:titanomagnetite, partitioning experiments, pre-eruptive magmatic processes
Titanomagnetite (Fe3-XTiXO4, X=0–1) is a common mineral in volcanic rocks. Due to the fast elemental diffusion in this phase and the sensitivity of its composition to temperature and oxygen fugacity, titanomagnetite has been used to probe pre-eruptive magmatic processes. When titanomagnetite coexists with titanohematite (Fe2-XTiXO3, X=0–1), the temperature and oxygen fugacity dependence of titanomagnetite composition is well known, and solid solution composition of the Fe-Ti oxides can be used as a thermo-oxybarometer. However, titanohematite-free magmas are common for mafic to intermediate systems, and the effect of magmatic parameters such as temperature, pressure, oxygen fugacity, and melt composition on titanomagnetite composition in a titanohematite-free system is poorly understood. To expand the applicability of titanomagnetite composition to probe pre-eruptive magmatic processes, it is necessary to determine the titanomagnetite–melt partitioning as a function of the above parameters in a titanohematite-free system. Thus, we conduct high-pressure and high-temperature experiments at 925–1050 °C, 50–150 MPa, and an oxygen fugacity at the Re–ReO2 buffer (~ΔNNO +1.8). Matrix glass of the pumices from the Sakurajima 1914 Plinian eruption (SiO2 ~70 wt%) was used as a starting material, and crushed titanomagnetite phenocrysts from the same eruption (ulvöspinel content 36 mol%) or pure magnetite crystals from Lovers Hole (New York, USA) were added as seed crystals. Experimental charges reproduced both titanohematite-free and titanohematite-bearing systems. When oxygen fugacity is buffered, the ulvöspinel content of titanomagnetite in titanohematite-free charges decreases with increasing temperature. This change is described as an exchange reaction of the magnetite component and the ulvöspinel component between titanomagnetite and silicate melt. The correlation between temperature and ulvöspinel content of titanomagnetite in a titanohematite-free system is opposite to that in a titanohematite-bearing system.