首页|Iron isotope compositions of subduction-derived rocks: Insights from eclogites and metasediments of the Munchberg Massif (Germany)
Iron isotope compositions of subduction-derived rocks: Insights from eclogites and metasediments of the Munchberg Massif (Germany)
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NSTL
Elsevier
The Fe isotope systematics of subducted lithologies are crucial for the understanding of redox-dependent mass transfer in subducting slabs, with consequences for the compositions of arc magmas and of the deep mantle. We investigated eclogites, metagabbros, and paragneisses from the Variscan Munchberg Massif to unravel whether their Fe isotope compositions are dominated by the igneous/sedimentary protolith signature, by low-temperature seawater alteration, or by later fluid-rock interactions during the subduction-exhumation cycle. Although the eclogites are thought to be derived from a continental rather than oceanic setting (possibly a rift-drift transition stage), they have mid-ocean ridge basalt (MORB)-like major and trace element compositions. They are often moderately oxidized compared to MORB (Fe3+/Sigma Fe = 0.06 to 0.30). Their delta Fe-56 values (+0.00 to +0.17 parts per thousand; mean + 0.08 +/- 0.01 parts per thousand) mostly resemble those of MORB (+0.07 to +0.17 parts per thousand). The metagabbros, which are derived from a more enriched mantle source than the eclogites, yielded heavier delta Fe-56 values (+0.09 to +0.22 parts per thousand) similar to ocean island basalts, whereas those of the paragneisses (+0.03 to +0.10 parts per thousand) are typical for pelitic sediments. It appears that the Fe isotope compositions of the igneous protoliths are largely preserved and little if any Fe was mobilized during the diverse fluid-rock interaction stages. The parental magma of the eclogites was probably somewhat isotopically lighter than similarly differentiated MORB magmas, perhaps due to the presence of metasomatized, isotopically light peridotites in the subcontinental lithospheric mantle (SCLM) source. Although it is possible that delta Fe-56 values were slightly modified during seawater alteration and/or metamorphic fluid-rock interactions in some of the eclogites, the impact of fluid-rock interactions on the major element compositions of the eclogites appears to be small. Furthermore, the scarcity of metamorphic veins in the Munchberg Massif argues against significant Fe mobilization in the slab. We suggest that continental eclogites tend to retain their magmatic delta Fe-56 values throughout the subduction-exhumation cycle, whereas delta Fe-56 values of oceanic eclogites may often be dominated by seafloor alteration with potential local modifications in the slab close to fluid channels. The remarkable robustness of the Fe isotope compositions of continental eclogites suggests that they may be used to reconstruct protolith mantle source properties despite the complex post-magmatic history.
NSTL
Elsevier
