Titanium isotope heterogeneity in the Earth’s mantle: A case study of the Horoman peridotite massif

Abstract

Titanium isotopes are a promising novel tracer for magmatic processes. The Ti isotope composition of the Earth's mantle is key for using this tracer on a planetary scale, however, recent studies point to potential Ti isotope variations in mantle rocks. This study presents stable Ti isotope data for well-characterised peridotites from the Horoman massif (Japan) as well as for primitive, orogenic (Variscan and Alpine belts) and South African ultrapotassic rocks. The peridotites originate from a continuous section across the layered Horoman peridotite body and comprise highly depleted harzburgites to fertile lherzolites as well as metasomatically overprinted peridotites. Unlike most primitive mantle-derived magmas, which display very limited Ti isotope variations, δ49Ti (deviation of 49Ti/47Ti from the OL-Ti standard) values of the Horoman peridotites vary from –1.523 ± 0.029 to 0.547 ± 0.015‰ (2SD), spanning a total δ49Ti range of 2.07‰. Heavy, continental crust-like Ti isotope compositions are correlated with Nb/Th and are likely related to metasomatism of the mantle wedge above the Hidaka subduction zone. In such a scenario, isotopically heavy Ti was inherited from the subducted terrigenous sediments and might have been further enhanced by Ti isotope fractionation during mobilisation from the slab. Significant δ49Ti differences between coexisting clino- and orthopyroxene (Δ49Tiopx-cpx = 0.16–0.29‰) and notably light Ti isotope compositions (δ49Ti = −1.523 ± 0.029 to −0.677 ± 0.036‰, 2SD) recorded in highly refractory peridotites might be related to kinetic diffusion-driven isotope fractionation during fluid/melt percolation. The analysed ultrapotassic rocks display distinctly high δ49Ti values (0.114 ± 0.040 to 0.290 ± 0.030‰, 2SD) compared to the majority of primitive mantle-derived magmas. The observed enrichment in heavy Ti isotopes likely originates from recycled continental material characterised by high δ49Ti, in line with their derivation from strongly metasomatised mantle sources. Our results demonstrate the presence of significant small-scale Ti isotope heterogeneity within the upper mantle.