Study of diamonds with syngenetic inclusions

Abstract

This thesis has investigated the chemical and physical conditions of natural diamond formation by integrated studies of the geochemistry of mineral inclusions in diamond, the nitrogen aggregation state and the carbon isotopic compositions of diamonds themselves. The diamonds studied were selected from two different sources, namely Bultfontein (Kimberley, South Africa) and Sao Luiz (North East Brazil).

Diamonds from the Bultfontein mine occur in a Cretaceous kimberlite pipe intruded into the Archean Kaapvaal craton. The diamonds are dominated by peridotite paragenesis inclusions, of which chromites are the most numerous. Equilibrium temperatures, indicated by these inclusions, are 930 and 955°C. The high abundance of chromites is interpreted as indicating a relatively shallow depth of origin. Bultfontein diamonds therefore probably formed in a thickened part of the craton where the geothermal gradient was low.

Infra-red studies of the Bultfontein diamonds show a low degree of nitrogen aggregation. There are two populations of diamonds, one population showing high nitrogen content but low aggregation state, the other having a lower nitrogen concentration but more aggregated nitrogen. The geochemical environment of formation for the two populations appears to be the same. The two different populations indicate either two separate formational events separated by a long time period, or, more favoured, two separate environments of formation one with high nitrogen the other with low nitrogen.

The carbon isotope studies of the Bultfontein diamonds show a mean δ ¹³C value of -4.66 ᵒˡₒₒ. The isotope compositions are skewed towards heavier values and this is probably a result of isotope fractionation processes which occurred between diamonds and a C-O-H vapour. The skewness of the carbon isotope values for the Bultfontein diamonds probably results from the precipitation of diamond by CH₄ oxidation reactions. The Bultfontein diamonds probably formed by the introduction of oxidising fluids into a reduced area of the lithosphere.

Cathodoluminescence studies of large diamonds and the study of the variation of δ ¹³C across a diamond by ion microprobe suggest that some of the diamonds formed under conditions of fluctuating oxygen fugacity. This has resulted in the formation of cuboid cores in large diamonds and a variation in δ ¹³C of 4 ᵒˡₒₒ across a single diamond.

Diamonds from the Sao Luiz alluvial mine are believed to be derived from a series of diamondiferous kimberlites intruded into a Proterozoic fold belt in North East Brazil. The diamonds show two principle groups of inclusions, both of which suggest a very deep origin for these diamonds. One group is dominated by garnet, and may be termed eclogitic, the other group is dominated by magnesiowustites.

Garnetiferous inclusions from Sao Luiz are divided into two groups. Group I garnets have normal silica contents and originate at depths shallower than 2001on. Group II have high silica contents than normal garnets with silicon atoms apparently occupying octahedral sites in the garnet structure. The Group II garnets are believed to represent solid solutions of garnet and pyroxene formed at high pressure and probably originated over a range of depths from 200 to 450km. Some of the inclusions are composite and consist of clinopyroxene and garnet phases. In these bimineralic inclusions a range of temperatures of equilibrium from 1297 to 1616°C have been calculated, consistent with estimated geotherms for the asthenosphere. 11 e garnet and pyroxene phases in the bimineralic inclusions are believed to have been separate before incorporation into the diamonds and are not the result of the decomposition of a single high pressure phase. In some of the Group II inclusions the high-silica phases have undergone a limited decomposition which has produced a series of complicated textures that consist of normal garnet and the high-silica phase.

Oxide and other silicate inclusions from the Sao Luiz diamonds suggest an even deeper origin. Magnesiowustites (MgO-FeO) inclusions are probably derived from depths greater than 650km. These inclusions have high Fe/Fe+Mg ratios and thus do not appear to be average products of the perovskite structured (high pressure) mantle. The silicate phases include CaSiO₃ composition phases that indicate a depth of origin below 450km. Olivine, diopside and pure silica phases are also present. Two silicon carbide inclusions were also recovered.

A limited study of the carbon isotope compositions and the infra red characteristics of these diamonds has been completed. The garnet-bearing diamonds show a range of δ ¹³C values from -6.70 to -12.57 ᵒˡₒₒ, values much lighter than the assumed mantle composition. These diamonds may have formed by CO₂ reduction reactions but it is more likely that a number of finite carbon reservoirs of different isotopic composition exist in the asthenosphere. The diamonds which contain the deeper inclusions have δ ¹³C values of -5 ᵒˡₒₒ. The deep diamonds are mostly Type II, that is they contain no nitrogen that is detectable by infra red. The Type I diamonds have high IaB contents and can be shown to indicate rapid aggregation of nitrogen. It is possible that the nitrogen in the deep diamonds aggregated rapidly to form defects that are invisible to infra red as a result of the high temperatures of formation.

The precipitation of diamonds at Sao Luiz that contain garnet inclusions probably occurred at the interface of oxidised areas at the top of the asthenosphere with more reduced areas. The other, deeper diamonds formed under more reduced conditions as shown by the presence of two inclusion of silicon carbide. The deep diamonds probably precipitated from a CH₄-H₂ vapour by H₂ loss, as such the deep Sao Luiz diamonds may have been in redox equilibrium with the earths core.