Magnesium isotopic compositions of altered oceanic basalts and gabbros from IODP site 1256 at the East Pacific Rise

•δ26Mg of carbonate-barren altered oceanic crust (AOC) in the EPR were determined.•The carbonate-barren AOC has a mantle-like Mg isotopic composition.•The offset in δ26Mg between seawater and global runoff results from dolomite output.•Recycling of carbonate-barren AOC cannot cause mantle Mg isotope heterogeneity.

To investigate the behaviour of Mg isotopes during alteration of oceanic crust and constrain the Mg isotopic compositions of the altered oceanic crust (AOC), high-precision Mg isotope analyses have been conducted on forty-four altered basalts and gabbros recovered from IODP site 1256, which represent the carbonate-barren AOC formed at the East Pacific Rise (EPR). These samples were altered by interaction of seawater-derived fluids with oceanic crust at different temperatures and water/rock ratios. With the exception of one sample that has a slightly heavier Mg isotopic composition (δ26Mg = 0.01 ± 0.08‰), all the other samples have relatively homogenous and mantle-like Mg isotopic compositions, with δ26Mg ranging from − 0.36 to − 0.14‰ (an average value of − 0.25 ± 0.11‰, 2SD, n = 43). This suggests that limited Mg isotope fractionation occurred during alteration of oceanic crust at the EPR at bulk rock scale, irrespective of highly variable alteration temperatures and variable water/rock ratios. Thus, our study suggests that the offset of δ26Mg values between seawater and global runoff dominantly results from the formation of marine dolomite as a sink for Mg. The mantle-like Mg isotopic composition further indicates that recycling of carbonate-barren AOC would not result in Mg isotope heterogeneity of the mantle at global scale. Consequently, the light Mg isotopic compositions of the mantle at local scale must result from incorporation of recycled Mg isotopically light carbonates.