X-Ray Tomography of the Most Diamondiferous Peridotite: A Unique Xenolith from Udachnaya, Siberia

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.