X-Ray Tomography of the Most Diamondiferous Peridotite: A Unique Xenolith from Udachnaya, Siberia
Lawrence A Taylor1, Geoffrey Hamilton Howarth1, Alla M Logvinova2, V. Reustsy2, E. Fedorova2, Richard A Ketcham3, Richard Wirth4 and Nikolai V Sobolev2, (1)University of Tennessee, Knoxville, TN, United States, (2)Siberian Brch Russian Acad, Novosibrisk, Russia, (3)University of Texas at Austin, Jackson School of Geosciences, Austin, TX, United States, (4)Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences, Potsdam, Germany
Abstract:
The Udachnaya kimberlite pipe is dominated by ultramafic xenoliths; however, it is the diamondiferous eclogites that are most recovered, with precious few diamondiferous peridotites. A truly unique diamondiferous garnet-peridotite xenolith was recovered, consisting of equal amounts of enstatite (Mg# 92.7) and lherzolitic garnet (Mg# 83.8), with lesser unaltered olivine (Mg# 92-95).
High-Resolution X-ray Computerized Tomography (HRXCT) of this strange xenolith produced numerous 2-D and 3-D images, revealing detailed relationship between the diamonds and the host minerals. The modal abundances of the minerals are: diamonds (9.5 vol%); enstatite (38 vol%); pyrope (35 vol%); sulfides (4 vol%; pyrrhotite, pentlandite, and chalcopyrite); and the remainder being mainly alteration products, consisting of “spinach” – probably serpentine – mostly in the interior of the peridotite, along with the majority of the diamonds. This 10.5 g peridotite contains >30,000 colorless, all euhedral, octahedral micro-diamonds, with sizes of 100 to 700 mm, many occurring in clusters. This is the absolute highest-yield of diamonds ever in a mantle xenolith – 106 cts/tonne – but tiny Ds.
The δ13C isotopic composition in these crystals has a value of -22 ‰, not typical for diamonds of ultramafic paragenesis, which are usually in the narrow range in δ13C (-2 to -8 ‰), characteristic of the mantle (Cartigny, Elements, 2005). Diamonds with such light-isotopic carbon, as low as -44 ‰ typical for eclogitic diamonds, are considered as indicative of subduction of oceanic crust (Sobolev & Sobolev, Russ. G & G, 1980). The majority of diamonds are Type IIa; others are Type IaB. Nitrogen impurity is of the B form indicating a high-aggregation state, considered evidence for long mantle residence times at high temperatures.
In some Ds, nano-size inclusions were discovered using TEM techniques and composed of Mg-Al silicate-rich phases, a Ca-carbonate phase, graphite, and fluid. These micro-inclusions represent mantle fluid that had been captured in the diamond growth process, as described by Logvinova (Eur. J Min, 2008). The non-mantle δ13C values to the myriad of diamonds, combined with their nitrogen-aggregation states, are considered as supporting evidence for the formation of the diamonds from metasomatic fluids with crustal signatures.
