Magnesium isotope fractionation during granite weathering

The Mg isotope compositions of weathering profiles developed on granite and granodiorite bedrock have been analyzed to investigate Mg isotope behavior during the weathering of felsic rocks in a continental environment. delta Mg-26 generally exhibits a small increase from the bedrock values, which range from -0.17 +/- 0.06% to +0.07 +/- 0.06%, until the upper two meters in which a decrease is observed, producing near surface values between -0.14 +/- 0.06% and +0.09 +/- 0.06%. The small observed increase in isotopic composition below 2 m is likely due to Mg loss to the hydrosphere during biotite weathering and in situ illite production, while the decrease at depths < 2 m is likely due to illite weathering near the surface, with a possible minor contribution from downslope deposition of less weathered material. One hydrothermally altered profile shows more extreme fractionation, reaching delta Mg-26 values as heavy as +0.55 +/- 0.06% in the near surface component of the section. The fractionation associated with Mg loss during weathering can be modeled by Rayleigh distillation with fractionation factors between the residual rock and fluid ranging from alpha = 1.00001 to 1.00040. The Mg isotope composition is primarily controlled by the behavior of illite in the weathering profile, as Mg isotope composition is positively correlated with the illite fraction of major Mg-bearing minerals. A thorough understanding of Mg isotope systematics in these complex clay minerals may permit the use of Mg isotopes in paleosols as paleoclimate indicators.