Origin of Zircon Megacrysts from Cenozoic Basalts in Northeastern Cambodia: Evidence from U-Pb Age, Hf-O Isotopes, and Inclusions

Zircon megacrysts found in basalts may provide a window into metasomatic processes and element transport in the upper mantle. However, little is known about the formation conditions of these zircons. Here we report U-Pb ages, Hf-O isotope compositions, trace element data, and mineral inclusion analyses for six large fragments of zircon megacrysts from the Gem village in northeastern Cambodia. The zircons have low contents of U, Th, Y, P, and rare earth elements (REEs; 130-288 ppm). The normalized REE pattern is characterized by a steeply rising slope from light REEs to heavy REEs and positive Ce anomalies. Secondary ion mass spectrometer isotopic analyses on the zircon megacrysts yield a mean U-Pb age of 0.98 +/- 0.04 Ma (2 sigma) and homogeneous oxygen isotopic composition with a mean O-18 value of 5.0 parts per thousand +/- 0.18 parts per thousand (2 sigma), identical to the O-18 mantle value. The Hf-176/Hf-177 values range from 0.282970 +/- 0.000024 (2 sigma) to 0.283040 +/- 0.000016 (2 sigma), with epsilon(Hf) (0.98 Ma) values of 6.6-9.0, obtained by the laser ablation-multicollector-inductively coupled plasma mass spectrometry technique, further points to the origin of the grains in the depleted mantle. The zircon megacrysts contain apatite inclusions and a reaction rim composed of baddeleyite and Al-Fe-Zr-silicate glass. This leads us to conclude that mantle zircon crystallized during a metasomatic event from phosphate-rich fluids and/or silicate melts enriched in zirconium.