Multiple-stage diamond formation in the Yubileinaya pipe of the Yakutian kimberlite province

Rubanova E.V., Garanin V.K.

Lomonosov Moscow State University

The Yubileinaya deposit is located 15 km northwest of the Aikhal village, in the Alakit-Markha field of the Daldyn-Alakit region, Yakutian kimberlite province (Kharkiv et al., 1998).

Its diamonds are typically colourless and form octahedral and octahedral-dodecahedral (40%), laminar dodecahedral (20-25%), and rounded (10%) crystals (Zinchuk and Koptil, 2003). Many diamonds of  IV, VIII, IX, and, more rarely, I groups contain graphite inclusions, which give them grey and black color. Yellow, yellow-green, grey-green crystals of IV group account for 4 % of all diamonds and their percentage grows with increasing grain size (Zinchuk and Koptil, 2003). The Yubileinaya pipe is also characterized by high concentration of twins and intergrowths (30%) (Zinchuk and Koptil, 2003). 

A large number of crystals (25%) from this deposit display resorption features (matting, cavities, etching). Matting was caused by diamond oxidation during hydrothermal-pneumatolytic activity related to diatreme formation and by the thermal effect of trap emplacement (Zinchuk and Koptil, 2003).

The studied collection contains 12 large diamond crystals varying in weight from 4,98 to 10,75 ct. Due to large size they have long history of diamond growth and their inner structure can provide full information about conditions and stages of diamond formation in this pipe. The crystals are octahedral, cubic, and octahedral intergrowths in shape; one sample is represented by two intergrown cubes. All samples are non- and semitransparent due to strong resorption, graphitization, matting or a large amount of graphite inclusions. After detailed description of the diamond surface, all crystals were cut through the center in order to study their inner structure and staged diamond formation.

Most crystals are zoned and contain nontransparent zones, which are supposedly saturated  in graphite. These zones differ in width, homogeneity and represent either single zone or several alternating zones.

Two intergrown coated octahedrons of different sizes were absolutely nontransparent, but after cutting zoning was found in this sample. Four zones can be defined visually based on inner structure (figs. 1, 2).

The central part of the largest crystal is transparent and doesn’t have any defects and inhomogeneities (first zone). Only this zone has light blue photoluminescence. Very thin zoning identified in it using cathodoluminescence (alternating zones of light and dark blue luminescence) points to quiet, stable and homogeneous diamond growth conditions. This transparent crystal is surrounded by second zone, which is narrower and subtransparent due to high concentration of inclusions in it. This zone cuts across growth zoning of the first zone and experienced partial resorption, which  results in the oval morphology of the crystal. Subsequent surface regeneration led to the capture of numerous inclusions. The third zone consists of small (from tenth to 200 microns) intergrown microcrystals of different sizes. This zone is nontransparent due to its structure. The undulating boundary between these zones also suggests another dissolution stage. Third zone has weaker blue cathodoluminescence. Small crystals that build up this zone have distinct outlines. The presence of these crystals results from oversaturation of material and formation of a lot of crystallization centres. This moment can be defined as a new population formation. All except first (transparent) zone are also observed in the smaller crystal of this intergrowth. These crystals are linked by third fine-grained zone.

Fourth zone is a coat consisting of parallel-columnar crystals. It has pistachio-yellow colour and grows on joined crystals. Its growth is related to the formation of numerous diamond centres on the sample surface and oversaturation of crystals’ environment during the growth. However, this oversatutation was lower as compared to that during the 3^rd zone formation. The coat has variable thickness, which can result from different oversaturation in the crystallization material. The fact that this zone shows no cathodoluminescence also indicates the change in crystallization conditions.

IR-spectroscopy revealed heterogeneous distribution of the impurity defects, which supports the repeated changes of crystallization conditions in the Yubileinaya pipe. The measurements were done by ,G.K. Khachatryan, the senior scientist of Central Institute of Geological Exploration for Base and Precious Metals (TsNIGRI). The diamonds are characterized by low concentration of nitrogen defects in 1^st zone (N_A=160 ppm, N_B=189), absence of hydrogen defects, and high concenration of “platelets” (P) (12.6 cm^-1). The second zone has the same concentrations of A, B, P defects, but low concentration of hydrogen defects (H=0.3 cm^-1). The outer zone (coat) lacks B and H defects, while A and C defects account for, respectively, 525 ppm and 424 ppm. This zone contains water (3400cm^-1) and carbonates (1400 cm^-1), which is a common feature of all columnar diamond coats.

The alternation of sharply different zones is the main evidence for significant changes in crystallization substrate and multiple-stage diamond formation in the Yubileinaya pipe.

Inclusions from the 3^rd zone were analyzed on electron microprobe at the Geological Department of MSU. Most analyses were made only qualitatively due to their heterogeneity and destructions during cutting. There are only epigenetic inclusions such as tetra-ferri-phlogopite, chlorite, biotite and ilmenite. Ilmenites contain low MgO concentration (less than 3 wt.%). There are a lot of fractures, which provided easy penetration of epigenetic solutions to primary inclusions during postcrystallization alteration (fig. 3).

Two transparent octahedrons are crowed with inclusions. Cathodoluminescence revealed no any heterogeneities in growth. Many fluid inclusions (fig. 4) in these samples were formed during crystallization and post-crystallization processes. The graphite was identified by Raman spectroscopy at the boundary of fluid inclusions and diamonds. Graphitization was presumably caused by high-temperature post-crystallization processes and, therefore, occurs at the contact between fluid inclusions and diamond, the most distorted part of the crystal (fig. 5). Moreover the graphite was found in black and grey zones of non-transparent crystals. A peak at 1582-1586 cm^-1 marks the presence of crystalline graphite (fig. 6). Several samples display a weak peak of X-ray amorphous graphite at 1360 cm^-1.

References:

Zinchuk N.N., Koptil V.I. Diamond typomorphism  os Siberia. Moscow, Nedra, 2003, p. 188-200 (in Russian).

Kharkiv A.D., Zinchuk N.N., Kruchkov A.I. Primary diamond deposits of the World. Moscow, Nedra, 1998 (in Russian).