Large Mg[sbnd]Fe isotope fractionation in volcanic rocks from northeast China: The role of chemical weathering and magma compositional effect

Abstract

This study presents Mg and Fe isotopic data for a suite of volcanic rocks including basalt, trachyte and comendite from Changbaishan, northeast China. Our results show that the Millennium Eruption (ME) trachytes and comendites have δ26Mg ranging from −0.37 to 0.14‰. Combined with literature data, the positive correlation between δ26Mg and 87Sr/86Sr ratios and the almost constant value for Nd isotopes suggest that the heavy Mg isotopic composition most likely derived from involvement of weathered products of basalt instead of the old supracrustal/basement rocks. Iron isotopic composition is significantly varied throughout our suite of samples, with δ56Fe values of 0.17 ± 0.04‰ and 0.16 ± 0.03‰ for basalts, 0.09 to 0.23‰ for cone-construction trachytes, and 0.23 to 0.37‰ for the ME samples. The slightly higher δ56Fe than MORB observed in basalts reflect the role of subducted materials that could elevate source Fe3+/ΣFe ratios via redox reaction. The trachytes from the cone construction, thought to be significantly affected by chemical weathered product, have a restricted Fe isotopic variation, while those ME samples with less weathered product input have overall higher δ56Fe values. This contrasting Fe isotopic signature suggests that the paleo-weathering event likely occurred under oxidized environment, leaving the residue with little Fe isotope fractionation. By contrast, the good correlations between Fe isotopes and indicators of magmatic differentiation (e.g., tFe2O3, SiO2, Mg#) in all rocks reflect that fractional crystallization under an open system most likely accounts for the Fe isotopic variation, which was enhanced by the compositional effect as suggested by the broadly positive relationship between δ56Fe and (Na + K)/(Ca + Mg). This study therefore highlights the potential applications of MgFe isotopes as great tracers of redox condition and ancient climate.