Microdiamonds from the Zeleny Yar Neogene Placer (the Ros'-Tikych Megablock of the Ukrainian Shield)

Abstract

We studied a collection of microdiamonds from the Zeleny Yar Neogene placer on the Ros’-Tikych megablock of the Ukrainian Shield, which consists of 102 microdiamonds, with crystal sizes 0.2-0.5 mm, using crystal morphology, photoluminescence, infrared and Raman spectroscopy, isotopic composition of carbon and helium of microdiamonds. Crystal forms are dominantly octahedra, others are cubes, combinational crystals {111}+{110}+{100}, cube-octahedra, and transitional forms {111}+{110}; rhombic dodecahedrons are rare. Many microdiamonds are damaged crystals and their fragments. Up to 20% of the studied microdiamonds are yellow, green, pink-purple, brown, gray, and black. The most colored crystals are among the cubes. Up to 70% of the studied microdiamonds are blue, green, yellow and orange in ultraviolet light. Many of the microdiamonds, especially cubes and crystals of the combinational form {111}+{110}+{100}, have an orange photoluminescence (center at 575 nm). 50% of the crystals are nitrogen-free crystals of type IIa. Nitrogen crystals of type Ia amount to 45% (among them 29 % of subtype IaA, 11% of subtype IaAB and 5% of subtype Iab) and 5% of type Ib. The content of nitrogen ranges from 57 to 612 at. ppm with an average nitrogen content of 224 at. ppm. The centers B2 (lines 1358-1380 cm–1) and hydrogen centers (3107 cm–1, 1405 cm–1) were recorded in IR spectra of some microdiamonds. The Raman line ranges from 1331.25 cm–1 to 1331.75 cm–1 with FWHM from 2.04 cm–1 to 4.47 cm–1. The carbon isotopic composition of microdiamonds shows a very wide range from –26.74 to –3.55‰ δ13C PDB. The content of helium isotopes for the studied crystal is 73.42 × 10–12 cm3/g of 3He and 0.265 × 10–6 cm3/g of 4He. Given the wide range of values of the isotopic composition of carbon δ13C, relatively high nitrogen content and cubic form of many crystals, it is possible to predict the eclogite association as the mantle crystallization medium of the Zeleny Yar microdiamonds. The location of the parent rocks of the microdiamonds is assumed to be the Berdychiv uplift of the Podolia block of the Ukrainian Shield.