TRACE ELEMENT CHEMISTRY OF ZIRCONS FROM OCEANIC CRUST: A METHOD FOR DISTINGUISHING ZIRCON PROVENANCE

Zircon, not traditionally considered an abundant accessory mineral in mafic rocks, has been recognized in oceanic gabbroic rocks exposed at the fast-spreading East Pacific Rise, the slow and ultra-slow spreading Mid-Atlantic and SW Indian ridges, and in plutonic rocks from ophiolites. It's recognition in rocks from all tectonic settings, and in both continental and oceanic lithosphere further complicates interpretations of provenance.

Here we compare the trace element chemistry from a large suite of zircon derived from modern in situ oceanic crust (Mid-Atlantic and Southwest Indian Ridges) with zircon derived from both island- and continental arcs. Rare earth element (REE) patterns for zircon recovered from modern oceanic crust and continental granitoids, as well as Hadean detrital grains show complete overlap on REE diagrams. In contrast, plots of U versus Yb and U/Yb versus Hf or Y discriminate zircons crystallized in oceanic crust from zircon crystallized in continental crust. Characteristically low U/Yb ratios (generally < 0.2) enable approximately 80% of the modern ocean crust zircons to be distinguished from a field of more than 1700 continental zircons from Archean and Phanerozoic samples. These discrimination diagrams provide a new tool for fingerprinting zircons derived from MORB and continental reservoirs in both modern and ancient detrital and metamorphic zircon populations. Hadean detrital zircons previously reported from the Acasta Gneiss, Canada, and the Narryer Gneiss terrane, Western Australia, plot in the continental granitoid field, supporting hypotheses that at least some Hadean detrital zircons crystallized in continental crust forming magmas and not from a reservoir like modern MORB.