The enigma of crustal zircons in upper-mantle rocks

Zircon, the mineral most widely used to date rocks by the U-Pb method, is common in crustal rocks, but is increasingly being found in rocks from the upper mantle. Several studies have concluded that this reflects the deep subduction of crustal rocks into the mantle.

This paper presents a new explanation for the presence of crustal zircons in the upper mantle rocks. In this case, granitoid-related melts/fluids, injected into already-emplaced mafic-ultramafic rocks, apparently transported pre-existing zircons and possibly crystallized new grains.

This study sends a cautionary message about the interpretation of zircon ages from mafic-ultramafic rocks and emphasizes that integrated isotopic and trace-element information on zircons (not U-Pb age data alone) is critical to the interpretation of events in upper-mantle rocks, including their emplacement and subsequent tectonic history.

A clear understanding of the origin of zircons and their relationship with the host rock increases the probability of correct conclusions about the history of upper mantle rocks, and interpretations of the dynamics of crust-mantle interaction.

Citation: The enigma of crustal zircons in upper-mantle rocks: Clues from the Tumut ophiolite, southeast Australia, Geology, http://dx.doi.org/10.1130/G36231.1.