Ferric-ferrous iron oxide ratios: Effect on crystallization pressure of granites estimated by Qtz-geobarometry

This study uses selected granite samples to evaluate the effect of redox conditions expressed as molar ferric-ferrous iron oxide ratio, Fe3+# [=Fe2O3/(FeO + Fe2O3)], on estimation of crystallization pressure (P) of granites using the Qtz-geobarometer. The results indicate that calculated P decreases significantly with increasing Fe3+# for calc-alkaline, metaluminous and peraluminous granites, but much less so for peralkaline and evolved granites. For a calc-alkaline, metaluminous or peraluminous granite sample of a fixed silica content, the calculated P values are a polynomial function of Fe3+#. The difference in P for Fe3+# between 0.1 (reduced) and 1.0 (oxidized) is as high as 437 MPa (or 4.37 kb), much higher than the uncertainty of the geobarometer. However, peralkaline and evolved granite samples display much less difference in P for Fe3+# between 0.1 and 1.0, essentially within the uncertainty. Therefore, redox conditions need to be considered when using Qtz-geobarometry to estimate P of calc-alkaline, metaluminous and peraluminous granites, particularly those bearing hornblende with relatively low silica contents, but such corrections may not be required for peralkaline and highly evolved granites. A practical procedure is proposed to calculate Fe3+# from whole-rock geochemical data to obtain improved P estimation, and a Microsoft Excel spreadsheet program is provided. In addition to constraining the emplacement depth, the program can also be used to evaluate the redox conditions using ferric-ferrous iron oxide ratios, both of which are important for mineralization associated with granite intrusions. •Redox condition (Fe3+#) has notable effect on Qtz-geobarometry for hornblende granite.•Fe3+# has little effect on Qtz-geobarometry for peralkaline and evolved granite.•A new procedure is proposed to calculate granite Fe3+# and crystallization pressure.