NEWS & EVENTS

Magmatic oxygen fugacity is an emerging area of interest in igneous petrological study with fundamental implication to the origin of continental crust. The reliable determination and interpretation of the oxygen fugacities of felsic rocks, which are the main constituents of the continental crust, are hampered by a lack of both accurate oxybarometers and a thorough understanding of redox evolution during magmatic differentiation. This study has calibrated a new amphibole oxybarometer and provided a major correction considering pressure to a previous zircon oxybarometer. Numerical modeling and examination of natural samples suggest limited oxidation during high pressure fractional crystallization and partial melting. Therefore, oxygen fugacity can be a useful fingerprint to track igneous sources. The corrected zircon oxybarometer is applied to the Archean TTG zircons, which suggests high oxygen fugacities comparable to those of modern arc magmas. Compiled Archean TTG Fe3+/ΣFe ratios are also comparable with those of modern arc magmas. The oxidized Archean TTG magmas seem not to be generated by subduction because 1.9 Ga TTG rocks, which were formed by subduction, show lower Fe3+/ΣFe ratios than the Archean TTG rocks. It is proposed here that the Archean TTG rocks were derived from the differentiation of mafic material that generated by the partial melting of lower mantle material, which is demonstrated by recent experimental studies to have a higher oxidation state than the upper mantle. The recognition of lower mantle material in the petrogenesis of the Archean TTG rocks offers a new perspective on the origin of continental crust.

For additional information, please contact Ruining Yao, geochem@connect.hku.hk.