Enriching mantle melts within a dying mid-ocean spreading ridge: Insights from Hf-isotope and trace element patterns in detrital oceanic zircon

Oceanic zircon trace element and Hf-isotope geochemistry offers a means to assess the magmatic evolution of a dying spreading ridge and provides an independent evaluation of the reliability of oceanic zircon as an indicator of mantle melting conditions. The Macquarie Island ophiolite in the Southern Ocean provides a unique testing ground for this approach due to its formation within a mid-ocean ridge that gradually changed into a transform plate boundary. Detrital zircon recovered from the island records this change through a progressive enrichment in incompatible trace elements. Oligocene age (33–27 Ma) paleo-detrital zircon in ophiolitic sandstones and breccias interbedded with pillow basalt have trace element compositions akin to a MORB crustal source, whereas Late Miocene age (8.5 Ma) modern-detrital zircon collected from gabbroic colluvium on the island have highly enriched compositions unlike typical oceanic zircon. This compositional disparity between age populations is not complimented by analytically equivalent εHf data that primarily ranges from 14 to 13 for sandstone and modern-detrital populations. A wider compositional range for the sandstone population reflects a multiple pluton source provenance and is augmented by a single cobble clast with εHf equivalent to the maximum observed composition in the sandstone (~ 17). Similar sandstone and colluvium Hf-isotope signatures indicate inheritance from a similar mantle reservoir that was enriched from the depleted MORB mantle average. The continuity in Hf-isotope signature relative to trace element enrichment in Macquarie Island zircon populations, suggests the latter formed by reduced partial melting linked to spreading-segment shortening and transform lengthening along the dying spreading ridge.

► Zircon trace elements from enriched oceanic crust are similar to continental zircon.
► Hf-isotopes from oceanic zircon agrees with Nd–Sr–Pb isotopes in cogenetic MORB.
► Wide range in detrital zircon Hf-isotopes preserves fine-scale mantle heterogeneity.
► Oceanic zircon Hf-isotope and trace element data records reduced partial melting of enriched DMM.
► Oceanic zircon is a reliable indicator of melting conditions below a mid-ocean ridge.