Detailed petrology and geochemistry of a rare corundum eclogite xenolith from Obnazhennaya pipe (Yakutia)

A large (6 kg), rare, corundum eclogite xenolith from the Obnazhennaya kimberlite, Yakutia, was divided into seven portions for detailed petrologic and geochemical studies. The variations observed in minerals, especially in granular garnets, are remarkable. The CaO content of a large garnet grain (similar to 10 mm) increases from core (8.41%) to rim (19.2%), and MgO and FeO decrease from core (17.4% and 8.41%, respectively) to rim (10.6% and 5.70%, respectively). The garnet core was equilibrated with a precursor clinopyroxene at about 1400 degrees C; the garnet rim and adjacent clinopyroxene rim, as well as garnet exsolution lamellae and adjacent host clinopyroxene were last equilibrated at about 1050 degrees C. The zonation preserved in these garnets requires a rapid cooling rate from 1050 degrees C to a final ''frozen'' temperature. REE concentrations of clinopyroxene and garnet are much lower compared to other normal eclogites from Obnazhennaya. High Al2O3 and CaO contents in the xenolith and positive Eu anomalies in garnets indicate that the protolith of this eclogite was a noritic anorthosite, possibly apart of the ancient oceanic crust. However, delta(18)O values of garnet and clinopyroxene separates falls within the normal mantle range. This, combined with a low jadeite component in clinopyroxene, indicates that seawater alteration was not severe. Conversely, later rehomogenization processes in the upper mantle may have significantly changed its low-temperature alteration signatures.