Fe-Mg Isotopes Trace the Mechanism of Crustal Recycling and Arc Magmatic Processes in the Neo-Tethys Subduction Zone

There has been intense debate on the mechanism of crustal recycling in subduction zones, and Fe-Mg isotopes may provide new constraints on this issue. This study reports Fe-Mg isotope data for mafic plutonic rocks from the eastern and central Gangdese arc, along with their associated trench sediments in southern Tibet. The d(26)Mg values of the eastern Gangdese arc rocks show negative correlations with (Sr-87/Sr-86)(i) and (Pb-206/Pb-204)(i) values, but positive correlations with e(Nd)(t) and e(Hf)(t) values. Conversely, the d(56)Fe values of the eastern Gangdese arc rocks show positive correlations with (Sr-87/Sr-86)(i) and (Pb-206/Pb-204)(i) values, but negative correlations with e(Nd)(t) and e(Hf)(t) values. The Mg and Fe isotopic compositions of the central Gangdese arc rocks are comparable with those of the eastern ones, but they do not correlate with Sr-Pb-Nd-Hf isotopic compositions. Notably, the Fe-Mg isotopic compositions of most arc rocks fall between those of local trench sediments and the mantle wedge. Combined qualitative analyses and quantitative simulations suggest that: (a) the Fe-Mg isotope variations observed in the eastern Gangdese arc rocks highlight the important role of source mixing between sediment-derived melts and peridotite, whereas (b) the Fe-Mg isotope variations observed in the central Gangdese arc rocks reflect the superposition of carbonated serpentinite-derived Mg-rich fluids-peridotite source mixing and source melting. The strong correlations between Fe-Mg isotope ratios and traditional geochemical tracers provide further evidence for the recycling of crustal materials in subduction zones via various types of slab-derived fluids and melts.