Melt inclusions in zircon are powerful petrogenetic indicators and improve zircon thermometry

Melt inclusions in zircon (MIZ) directly reflect the physicochemical state of the magma during zircon growth. However, their potential as geothermometers and petrogenetic indicators is still poorly explored. Therefore, we investigated MIZ from well-characterized mafic and felsic rocks of the Bushveld Complex (South Africa) and acquired a novel dataset of major, trace and volatile element contents. Re-homogenized MIZ of all rock types display rhyolitic compositions (65-78 wt% SiO2) and similar H2O contents (1.6-4.0 wt%). Liquidus temperatures of MIZ obtained from normative Qz-Ab-Or and rhyolite-MELTS modelling indicate melt entrapment at 930–850°C (at 200 MPa), tailing down to 700°C in some samples. For rutile-bearing mafic cumulates of the lower BC (Marginal and Critical Zone), these temperatures overlap with TiO2 saturation temperatures of MIZ as well as with Ti-in-zircon of host crystals using aTiO2=aSiO2=1.0 [1], in accordance with textural associations of zircon+rutile+quartz. In contrast, MIZ in all rutile-free, magnetite-ilmenite-titanite- and quartz-bearing rocks of the upper BC (Upper Zone ferrogabbros, granitic rocks), display strikingly lower Ti contents, but also higher ƩREE and lower Th/U. Cross-calibration of TiO2 saturation (MIZ) and Ti-in-zircon thermometers with MIZ liquidus temperatures suggests that zircon crystallized at highly reduced aTiO2~0.3, significantly below aTiO2~0.6 previously estimated for rutile-free rocks in the literature, usage of which would underestimate zircon crystallization temperatures by 50-100 °C. In summary, MIZ may inherit chemical signatures of host rocks, are powerful zircon geothermometers and provide constraints for aTiO2 in Rt-free rocks. [1] Ferry & Watson (2007) CMP 154, 429–437; [2] Hayden & Watson (2007) EPSL 258, 561-568