Geothermometry using minor and trace elements in igneous and hydrothermal magnetite

摘要

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% SiO_2) is examined in a compilation of experimental data (n = 299) from 700 to 1050 °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 (°C) = ? 8344(±320)/ [lnXMg ? 4.1(±0.28)] – 273 r~2 = 0.83 that reproduces 72% of the experimentas to±50 °C, and conventional FeeTi oxide thermometry in tuffs and vitrophyres to better than±60 °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 °C but with some lower temperatures in plutonic rocks due to slow cooling. Hydrothermal samples (skarns, porphyry deposits) show consistently subsolidus conditions (T_(Mgmag) of 700–350 °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.