THE TECTONIC SIGNIFICANCE OF (OLD) ZIRCON IN OCEANIC MANTLE PERIDOTITES AND CHROMITITES

Zircon is relatively abundant in oceanic peridotites and chromitites of numerous Phanerozoic ophiolites of different tectonic belts. Individual samples of chromitite, 300-400 kg in weight, may yield >100 zircon grains. Commonly, 20-50% of these are OLD, with Proterozoic and Archean ages. Such grains are typically 100-200 μm across, light brown to colorless and rounded; others have rounded cores with younger, euhedral to irregular overgrowths. Many of the grains contain inclusions of quartz, orthoclase, plagioclase, muscovite, biotite, pyroxene, amphibole, apatite, rutile, titanite or garnet. Individual grains of these silicates and oxides, all much larger than the inclusions, may also occur in the peridotites and chromitites. Trace element compositions of the OLD zircon grains are mostly compatible with a continental origin, and the vast majority of the Precambrian grains have negative Hf isotope values. For example, 53 of 64 Precambrian zircons analyzed from the Paleozoic Sartohay ophiolite, China, have εHF(t) values of -1.2 to -19.9. We interpret these grains as continental crustal detritus collected in oceanic trenches and subducted to the mantle transition zone, where they are mixed with UHP and highly reduced minerals such as diamond, moissanite, native elements and metallic alloys. These peridotites are eventually carried to shallow levels by convection beneath spreading ridges, where some fragments of the shallow oceanic lithosphere are incorporated into ophiolites above suprasubduction zone mantle wedges; others are recycled back into the mantle. We suggest that OLD zircon is a common mineral in the oceanic mantle, an interpretation consistent with its common presence in ophiolites and in Mid-Atlantic Ridge gabbros. Our findings also indicate that zircon can persist for long periods under mantle conditions.