Extreme Chemical Diversity in the Mantle during Eclogitic Diamond Formation: Evidence from 35 Garnet and 5 Pyroxene Inclusions in a Single Diamond

Abstract

We report major- and trace-element variations in 35 garnet inclusions extracted from a single eclogitic diamond from the Mir kimberlite pipe, Yakutia. The range in CaO and Mg# in these garnets is large-from 3.84 to 9.66 wt% and 45 to 56, respectively. These ranges cover nearly half of the total range in eclogitic garnet compositions from diamonds worldwide. An extremely wide range in trace elements such as Y, Zr, and Sr also covers nearly the total range recorded for garnets included in diamonds, as well as that known for rock-forming minerals of diamondiferous eclogite xenoliths-15.1 to 48.9 ppm Y; 2.2 to 40.8 ppm Zr; and 0.5 to 9.1 ppm Sr. The widest ranges in REE between different grains (by an order of magnitude) are noted for LREE and MREE. Such compositional ranges are the first recorded for garnets available as inclusions in a single eclogitic diamond and cover nearly the complete range of compositions known for all Yakutian eclogites. Five clino-pyroxene inclusions exhibit more moderate ranges in both major- and trace-element abundances. Na2O compositions vary from 5.1 to 5.7 wt% and Mg# varies from 72 to 76. Trace-element abundances analyzed from two clinopyroxene inclusions are rather similar to those in eclogite xenoliths from Mir, but REE patterns span the range from LREE-depleted to LREE-enriched. These observations reflect the complex growth history of eclogite diamonds and their xenolith hosts, as well as the extreme chemical diversity in the mantle environment during their growth. The variability in garnet composition is attributed to changing fluid chemistry in successive metasomatic fronts moving through a subducted oceanic slab in the mantle. These metasomatic fluids also could have been the carbon source for the diamond.