Mo isotopes archive oceanic sediments in post-orogenic lithospheric mantle

The super-chondritic Mo isotopic composition of the continental crust has been attributed to contributions from subducted oceanic slabs and/or sediments, which introduce isotopically light Mo to the mantle. It has been hypothesized that dehydrated oceanic slabs subduct into the deep mantle and contribute to the genesis of ocean island basalts (OIBs). However, OIBs have similar average delta Mo-98/95 values (-0.22 parts per thousand +/- 0.04 parts per thousand ; 2SE; n = 87) to the depleted mantle. Subducted oceanic sediments may create a light-delta Mo-98/95 reservoir in the subcontinental lithospheric mantle, but this has not been investigated in detail. Understanding the Mo isotopic systematics of the continental crust and mantle requires an understanding of the Mo isotopic composition of lithospheric mantle-derived volcanic rocks. The litho-sphere in the Tibetan region has experienced multiple stages of oceanic subduction and is thus ideal for studying magmas derived from sediment-modified lithospheric mantle. Mafic ultrapotassic volcanic rocks (UPVRs) from the Sailipu area on the southern Tibetan Plateau have high Mg# values (67.1-72.5) and enriched Sr-Nd isotopic compositions and are primitive magmas derived from the lithospheric mantle. These rocks have higher Ce/Mo ratios (225-422) and lighter delta Mo-98/95 values (-0.45 parts per thousand to-0.13 parts per thousand) than the depleted mantle, mid-ocean ridge basalts (MORBs), and OIBs, as well as high epsilon(Hf)(t) values at given epsilon(Nd)(t) values compared with the terrestrial array (delta epsilon(Hf) = 4.59-5.73). The involvement of Ca-poor pelagic sediments rather than the lower continental crust and hydrated oceanic slab best accounts for the high Ce/Mo ratios and light delta Mo-98/95 values of the studied rocks. The Sailipu UPVRs also have higher Nb/Ta (18.9-20.8) and Th/La (2.6-3.4) ratios than arc-related igneous rocks and MORBs, suggesting that their source was phlogopite-bearing lithospheric mantle with a metasomatic vein component (rutile + monazite +/- lawsonite). We propose a two-stage geodynamic process to explain the formation of the Sailipu UPVRs: (1) a metasomatic vein component with high Th/La and Ce/Mo ratios formed in the litho -spheric mantle during multiple stages of oceanic subduction; (2) high mantle temperatures related to convective thinning of the lithosphere resulted in melting of the vein component and the ambient man-tle, which produced hybrid melts of various mantle domains during the post-orogenic stage. The Sailipu UPVRs have enriched Sr-Nd isotopic compositions and high Th/La and Ce/Mo ratios, similar to lamproites that are widespread in the Tethyan orogenic realm from the Mediterranean to the southern Tibetan Plateau. Given that UPVRs with light delta Mo-98/95 values have also been identified in the Western Alps, we propose that the isotopically light Mo component is widespread in Tethyan lithospheric mantle. This iso-topically light Mo in the lithospheric mantle may be a complementary end-member to the isotopically heavier continental crust. The recycling of subducted oceanic sediments has a key role in the deep recy-cling of Mo.(c) 2022 Elsevier Ltd. All rights reserved.

Automated summary

The super-chondritic Mo isotopic composition of the continental crust has been attributed to contributions from subducted oceanic slabs and/or sediments. The involvement of Ca-poor pelagic sediments rather than the lower continental crust and hydrated oceanic slab best accounts for the high Ce/Mo ratios and light delta Mo-98/95 values of the studied rocks. The recycling of subducted oceanic sediments has a key role in the deep recycling of Mo. [