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

Abstract

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 δ26Mg values of the eastern Gangdese arc rocks show negative correlations with (87Sr/86Sr)i and (206Pb/204Pb)i values, but positive correlations with εNd(t) and εHf(t) values. Conversely, the δ56Fe values of the eastern Gangdese arc rocks show positive correlations with (87Sr/86Sr)i and (206Pb/204Pb)i values, but negative correlations with εNd(t) and εHf(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: (1) 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 (2) 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.

Magnesium isotope fractionation processes during seafloor serpentinization and implications for serpentinite subduction

“…Much of this attention has been spurred by the discovery of d 26 Mg (d 26 Mg is the deviation of 26 Mg/ 24 Mg from that of the standard DSM-3 (Galy et al, 2003) in ‰) values slightly higher than the mantle in some arc lavas (Teng et al, 2016;Li et al, 2017). While it has been argued that these high d 26 Mg-values may be accounted for by fractional crystallization and crustal assimilation processes (Brewer et al, 2018), others conclude that they must originate from the sub-arc mantle source region (Teng et al, 2016;Li et al, 2017;Chen et al, 2023b). In this case, the high d 26 Mg values have been challenging to explain via addition of sediment and AOC into arc lava source regions, because crustal rocks have much lower Mg contents than the mantle wedge (Li and Schoonmaker, 2014;Rudnick and Gao, 2003;Mcdonough and Sun, 1995).…”