Mg-Fe isotopes trace the mechanism of crustal recycling and arc magmatic
processes in the Neo-Tethys subduction zone
Abstract
The mechanism of crustal recycling in subduction zones has been a heated
debate, and Mg–Fe isotopes may provide new constraints for this debate.
This study reported the Fe–Mg isotope data for mafic plutonic rocks
from the eastern and central Gangdese arc and their associated trench
sediments in southern Tibet. The δ26Mg (–0.32 to –0.20‰) and δ56Fe
(0.04 to 0.12‰) values of the eastern Gangdese arc rocks show negative
and positive correlations with (87Sr/86Sr)i and (206Pb/204Pb)i values,
but positive and negative correlations with εNd(t) and εHf(t) values,
respectively. The Mg and Fe isotopic compositions (δ26Mg = –0.28 to
–0.15‰; δ56Fe = 0.02 to 0.12‰) of the central Gangdese arc rocks are
comparable with the eastern ones, but they are not covariant with
Sr–Pb–Nd–Hf isotopes. More importantly, the Fe–Mg isotopes for most
of the arc rocks fall in between local trench sediments (δ26Mg = –0.61
to –0.30‰; δ56Fe = 0.00 to 0.17‰) and the normal mantle. Integrated
qualitative analyses and quantitative simulations suggest that while the
Mg–Fe isotope variations in the eastern Gangdese arc rocks revealed the
important role of source mixing between sediment-derived melts and
peridotite, their variations in the central Gangdese arc rocks reflected
the controlling effects of source mixing between carbonated
serpentinite-derived Mg-rich fluid and peridotite and source melting.
The good covariant relationships between Mg–Fe isotope 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.