U-Pb Zircon geochronology, Hf isotope and trace element geochemistry of a unique lower crustal - upper mantle section of a dying slow-spreading mid-ocean ridge (Macquarie Island, Southern Ocean)

Macquarie Island, located in the Southern Ocean, is a section of oceanic crust formed at the now extinct proto-Macquarie, slow-spreading, mid-ocean ridge. The northernmost part of Macquarie Island is composed primarily of lower crustal gabbro and upper mantle peridotite. The mantle sequence consists of harzburgite intruded by a variety of gabbro dikes/dikelettes and zircon and phlogopite-bearing veins. Here, we report integrated Pb/U zircon ages and Hf isotopic and trace element data from six samples of the lower crust-upper mantle sequence. Samples consist of two lower crustal gabbros, and three gabbro dikes/dikelettes and one phlogopite-bearing vein from the upper mantle sequence. 206Pb/238U SHRIMP-RG zircon ages for the gabbros range from 8.7 ± 0.3 Ma to 9.0 ± 0.2 Ma, whereas gabbro dikes/dikelettes yield overlapping ages of 8.7 ± 0.2 Ma, 8.7 ± 0.3 Ma, and 8.9 ± 0.2 Ma (all errors 2s). The phlogopitic vein yielded a slightly younger age of 8.5 ± 0.1 Ma. Initial epsilon Hf results for zircons from the same samples show a broad distribution ranging from +9.5 to +13.3 for the lower crustal gabbro (N=28), +7.0 to +16.4 for the gabbro dikes/diklettes (N=24), and +8.4 to +12.2 for the phlogopite vein (N=12). The wide range in values (particularly from the gabbro dikes/diklettes) is consistent with a heterogeneous source region composed of a range of relatively long-lived depleted- and enriched mantle sources. Zircon trace element concentrations also support a heterogeneous source, displaying enrichment in U/Yb relative to N-MORB zircons from the Mid-Atlantic and Southwest Indian Ridge systems. We interpret these results to indicate that magmatic construction of Macquarie Island occurred from 8.7 to 9.0 Ma and involved sampling of at least two distinct mantle sources.