The nature and timescale of crust recycling process in the Pacific mantle: isotopic evidence from quartz-garnet clinopyroxenite xenolith, Malaita, Solomon Islands

The role of recycled eclogitic/pyroxenitic sources in the formation of OIB magmas has received great attention for understanding the nature of mantle heterogeneity. However, the origin and timescale of the recycled reservoir are still poorly constrained because the erupted magmas can only tell us chemical signatures of their mantle sources due to variable effects of melting, mixing and crystal fractionation. We present Sr-Nd-Hf-Pb isotope compositions of a quartz-garnet clinopyroxenite xenolith from Malaita, Solomon Islands, as a convincing best reference of recycled material from within Pacific convective mantle. This argument is founded on three main lines of evidence: (1) the xenolith was brought up essentially in an oceanic setting at 34 Ma; (2) thermobarometric analyses reveal the xenolith entrainment occurred at the base of ca. 160 Ma Pacific MORB lithosphere (110-125 km in depth), showing recent isolation from the convective mantle; (3) normative quartz-rich composition excludes derivation of the xenolith as a high-pressure cumulate or melt from normal peridotitic mantle. Sr-Nd-Hf isotopic data with their parent/daughter ratios of clinopyroxene and garnet yield two-point inter-mineral isochrons corresponding to an age of host eruption, indicative of isotopic homogenization due to continuous isotopic exchange equilibria. The initial Sr-Nd isotope ratios of reconstructed bulk adjusted to 160-34 Ma display enriched signatures relative to BSE, whereas the Hf isotope ratio possesses strong depleted signature (87Sr/86Sr = 0.707, ɛNd = -6, ɛHf = +15 at 120 Ma). The decoupled Hf-Nd isotopic behavior implies the involvement of pelagic sediment component. This can also be inferred from Pb isotope compositions characterized by a good agreement with ca. 500 Ma points of Stacey-Kramers Pb evolution lines. Thus, isotopic signatures observed in the xenolith strongly support that the ancient oceanic crust polluted by pelagic sediment is truly resided in the Pacific mantle as enriched eclogitic/pyorxenitic reservoir. If the ca. 500 Ma age is plausible as a protolith formation, the timescale of recycling process in Pacific region may be significantly shorter than the 1 to 2 Ga often assumed.