Geothermometry using minor and trace elements in igneous and hydrothermal magnetite

The temperature (T) and oxygen fugacity (fO(2)) dependence of minor and trace elements (Mg, Al, Mn, Cr, Ni) in magnetite (Mag) in equilibrium with water-bearing silicate liquids varying in composition from basalt to rhyolite (49-77 wt% SiO2) is examined in a compilation of experimental data (n = 299) from 700 to 1050 degrees C, 50-900 MPa and five order of magnitude range in fO(2). The XMg (=Mg/(Mg + Fe-tot)) of Mag is strongly dependent on T, and can be formulated into an empirical geothermometer: TMg-mag (degrees C) = - 8344(+/- 320)/[lnX(Mg) - 4.1(+/- 0.28)] - 273 r(2) = 0.83 that reproduces 72% of the experimentas to +/- 50 degrees C, and conventional Fe-Ti oxide thermometry in tuffs and vitrophyres to better than +/- 60 degrees C for > 85% of the samples. The T-Mg Mag thermometer may have potential application in igneous or hydrothermally altered rocks where conventional thermometry or oxybarometry is not possible. Application to natural Mag (n = 476) compiled from several settings shows igneous rocks mostly record > 700 degrees C but with some lower temperatures in plutonic rocks due to slow cooling. Hydrothermal samples (skarns, porphyry deposits) show consistently subsolidus conditions (TMg-mag of 700-350 degrees C) consistent with other mineral and O isotopic equilibria. The element correlations in natural Mag from various settings are explored using multivariate statistical analysis, to identify which minor or trace elements most critically depend on T, fO(2) or fluid composition.