Thermodynamics of Rhombohedral Oxide Solid Solutions and a Revision of the FE-TI Two-Oxide Geothermometer and Oxygen-Barometer

Abstract

A model for the thermodynamic properties of rhombohedral oxide solid solutions in the system Fe 2 O 3 -FeTiO 3 -MgTiO 3 -MnTiO 3 (containing minor amounts of Al 2 O 3 ) is presented. The model accounts for temperature and compositionally dependent long-range cation-order and the related high to low symmetry structural phase transition. The model is calibrated from the cation-ordering data of Harrison and others (2000; Harrison and Redfern, 2001) and experimental data on Fe ؉2 Ti N (Fe ؉3 ) 2 exchange between rhombohedral oxide and spinel from Lattard and others (2005) and Evans and others (2006). Successful calibration require introduction of an energetic contribution attributed to short-range cation-order, which reduces the configurational entropy of the solid solution. The resultant thermodynamic model for the rhombohedral oxides is internally consistent with the model for spinel solid solutions of Sack and Ghiorso (1991a, 1991b) and with the endmember thermodynamic properties database of Berman (1988); a new model equation for the isobaric heat capacity of ulvo ¨spinel (cubic Fe 2 TiO 4 ) is proposed and values of the enthalpy of formation, ؊1490.417 kJ/mol, and third law entropy, 184.199 J/K-mol, at 298.15 K and 10 5 Pa are recommended.The new model forms the basis of a revised FeTi-oxide geothermometer/oxygen barometer, which is applied to a newly compiled dataset of natural two oxide pairs from silicic volcanic rocks. Results are compared to previous formulations with the general conclusion that the new model gives a better estimate of oxidation state for magmas that equilibrated under conditions more oxidizing than the nickel-nickel oxide buffer. Estimates of oxygen fugacity are fairly insensitive to analytical uncertainties in oxide compositions. By contrast, temperature estimates are especially sensitive to analytical error and to the abundances of "minor" constituents. Application of the geothermometer to oxide pairs that grew under conditions where the rhombohedral phase was cation disordered (that is high temperature or at oxygen fugacities greater by about one log 10 unit than the nickel-nickel oxide buffer) results in an uncertainty due solely to analytical error of at least 50°C and sometimes as high as 100 °C. Temperature estimates from the new geothermometer can be made using either the Fe ؉2 Ti N (Fe ؉3 ) 2 exchange or Fe ؉2 N Mg exchange between the two oxides. Comparison of the two temperature estimates provides a means of evaluating the internal consistency of coexisting oxide compositions and assessing the extent of disequilibrium. Temperatures calculated from the new model are found to be consistent with experimental phase relations for the stability of cummingtonite in silicic volcanics. Other petrologic constraints on derived temperatures are examined including limits on the width of the miscibility gap and the development of self-reversed remanent magnetization in the rhombohedral series.Software that implements the new thermodynamic model and the two-oxide geothermometer/oxygen baromete...

The 2011 eruption of Nabro volcano, Eritrea: perspectives on magmatic processes from melt inclusions

“…Putirka (2005) 1098-1108 Ilmenite-magnetite Spencer and Lindsley (1981) 1107 NNO − 0.04 Andersen and Lindsley (1988) 1002 NNO − 0.33 Ghiorso and Evans, (2008) 1013 NNO + 0.04 Cpx-melt Putirka et al (2003) 998-1104 2.3-5.2 Melt inclusion (volatile-saturated) Papale et al (2006) Ghiorso and Gualda (2015) 0.2-4.6 0.2-2.6 should be sulphide-saturated. There are also sulphide globules in the groundmass of some of the samples.…”

“…The models give temperatures ranging from 980 to 1146°C (with two outliers), but most inclusions indicate temperatures around 1080°C with fO 2 between the QFM and NNO buffers. A touching pair of ilmenite and magnetite phenocrysts places fO 2 of NNO + 0.04 (Ghiorso and Evans 2008) or NNO − 0.33 (Andersen and Lindsley 1988). Agreement between different thermometers is generally good (Table 6), with overlap between the melt inclusions and matrix glasses All major elements measured by electron microprobe; trace elements, H 2 O and CO 2 by ion probe.…”

“…According to this model, almost all the inclusions Table 6 Intrinsic variables calculated using a range of models (Andersen and Lindsley 1988;Beattie 1993;Ghiorso and Evans 2008;Harrison and Watson 1984;Papale et al 2006;Putirka 2005;Putirka et al 2003Putirka et al , 2007Sisson and Grove 1993;Spencer and Lindsley 1981) Oxide formulae recalculated according to Stormer (1983). Ilmenite-magnetite was calculated for a single touching pair.…”

Field-trip guide to Mount Hood, Oregon, highlighting eruptive history and hazards

“…Gas geothermonetry based estimates for the depth of magma storage are from Symonds and others (2003) and are based on an empirical gas geothermometer in conjunction with the boiling curve for water. Pressures from the haplogranite projection are based on melt inclusion compositions from Koleszar and others (2012) and follow the method of Blundy and Cashman (2008 (Ridolfi and others, 2010) Plagioclase-hornblende geothermometer (Holland and Blundy, 1994) Fe-Ti oxide geothermometer (Ghiorso and Evans, 2008) Koleszar and others (2012).…”

Petrological investigation of long-term evolution of magma chambers and preparing processes of caldera-forming eruption, Numazawa volcano, NE Japan

“…= trace amounts (less than 0.1 vol.%). (Holland and Blundy, 1994) and magnetite-ilmenite pairs (Ghiorso and Evans, 2008) …”