Ti zircon thermometry applied to metamorphic and igneous systems

摘要: Ti concentration within zircon is seemingly controlled by temperature (Watson and Harrison, 2005). This has the potential to yield valuable information as to the thermal evolution of metamorphic and igneous rocks. We present Ti zircon thermometry results for two Archean samples from southwest Greenland. Data were acquired on the SHRIMP II multi-collector at ANU by simultaneous measurement of 49Tiand 28Si16O. Referencing the minor Ti isotope to SiO rather than Zr allows the cross-calibration between NIST SRM glasses and SL13 zircon standard, without isobaric interference of 48Ca on the major 48Ti peak, or uncertainty caused by zircon Hf concentration. G03-38 is a localized (< 1 m3)‘granitic’partial melt, segregated from an amphibolite unit during 2.7 Ga regional amphibolite facies metamorphism. Metamorphic zircons from the partial melt are in equilibrium with garnet, clinopyroxene and ilmenite in a confined system. Temperatures determined from the zircons (mean 677±12 C; n= 11) lie within uncertainty of independent garnet—clinopyroxene thermometry. In contrast, G97-18 is a 3.8 Ga, weakly deformed and homogeneous, large tonalitic intrusion. Oscillatory zoned igneous zircons are in equilibrium with titanite. Whole-rock geochemistry indicates Zr undersaturation (121 ppm)(Nutman et al., 1999). Ti in zircon temperatures (mean 681±29 C; n= 34) probably underestimate that of the parental melt (800–900 C). Composite analyses reveal complex Ti distributions within single zircon crystals, possibly related to magmatic evolution or trace element incorporation during growth. Variations in apparent crystallization temperatures between …