A Nitrogen and Carbon Stable Isotope Study of some Western Australian Diamonds

Heerden, Leon Andrê (1994). A Nitrogen and Carbon Stable Isotope Study of some Western Australian Diamonds. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000fb72

Abstract

The carbon and nitrogen stable isotope ratios, nitrogen content and aggregation state have been measured in diamonds from Australian magmatic and alluvial sources and from a Chinese placer deposit. There are no consistent relationships between any of these properties, and there are no consistent relationships with diamond shape or colour. Both individual diamonds and diamond sources are shown to be heterogeneous and have a range of ẟ¹³C and ẟ¹⁵N values. The widest range in Western Australian magmatic diamonds occurs in Ellendale 9 samples which have -22.1‰ ≤ ẟ¹³C ≤ 0.0‰ and -9.3‰ ≤ ẟ¹⁵N ≤ -9.9‰. Diamonds from placer deposits have a wider range of ẟ¹⁵N and ẟ¹³C values however (-14.8‰ ≤ ẟ¹³C ≤ +1.9‰ & -28.4‰ ≤ ẟ¹⁵N ≤+9.7‰ in Northern Queensland samples). This may be due to placers representing multiple primary sources.

A relationship between diamond paragenesis and ẟ¹³C value is shown. Eclogitic diamonds worldwide have a mean ẟ¹³C value of -8.6 ± 5.8‰ and range from ẟ¹³C = -34.5‰ to +1.5‰. In contrast, peridotitic paragenesis diamonds have a mean ẟ¹³C value of -5.4 ± 2.6‰, and their range in ẟ¹³C values is from -23.5‰ to +0. 1‰. ẟ¹⁵N data from diamonds of known provenance are scarce, but there are no significant differences between eclogitic and lherzolitic diamonds from Western Australia. Eclogitic diamonds have a mean ẟ¹⁵N value of 4.9‰ ±4.6‰ whereas lherzolitic diamonds have a mean ẟ¹⁵N value of 4.1‰ ±4.9‰. These parageneses have different mean ẟ¹⁵N values when compared to the mean ẟ¹⁵N value of samples thought to be of the harzburgitic paragenesis. It is provisionally suggested that carbon and nitrogen stable isotope ratios may discriminate eclogitic, lherzolitic and harzburgitic paragenesis samples.

Three Argyle diamonds and a single Ellendale diamond are zoned in ẟ¹³C and ẟ¹⁵N values, nitrogen content and aggregation state. Models for these zonation profiles are consistent with diamond formation by fractional crystallization, and fractionation factors of α ≤ 1.001 for carbon and α = 1.004 for nitrogen during diamond growth are suggested. These samples are also zoned in age and in the most extreme case a difference of 1.2 Ga between the core and rim of a diamond 5 mm in diameter is found.

All available diamond ẟ¹⁵N data are combined and it is concluded that ẟ¹⁵N = 0‰ ± 6‰ is a characteristic of the sub-continental lithospheric upper mantle. Heterogeneities within this are however evident when ẟ¹³C and ẟ¹⁵N covariations are considered. It is suggested that the subduction of oceanic sediment, carbonates and peridotite into the mantle, combined with isotope fractionation processes are responsible for the stable isotope characteristics of the diamond source region. A possible trend of increasing ẟ¹⁵N value in diamonds as their age decreases is noted, and it is speculated that this may be a result of a kinetic fractionation during mantle degassing and atmosphere formation.