title: A Nitrogen and Carbon Stable Isotope Study of some Western Australian Diamonds
Creators: name: Heerden, Leon Andrê
Publisher: name: The Open University
Year: 1994

description: 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 ẟ<sup>13</sup>C and ẟ<sup>15</sup>N values. The widest range in Western Australian magmatic diamonds occurs in Ellendale 9 samples which have -22.1‰ ≤ ẟ<sup>13</sup>C ≤ 0.0‰ and -9.3‰ ≤ ẟ<sup>15</sup>N ≤ -9.9‰. Diamonds from placer deposits have a wider range of ẟ<sup>15</sup>N and ẟ<sup>13</sup>C values however (-14.8‰ ≤ ẟ<sup>13</sup>C ≤ +1.9‰ &amp; -28.4‰ ≤ ẟ<sup>15</sup>N ≤+9.7‰ in Northern Queensland samples). This may be due to placers representing multiple primary sources. <br><br> A relationship between diamond paragenesis and ẟ<sup>13</sup>C value is shown. Eclogitic diamonds worldwide have a mean ẟ<sup>13</sup>C value of -8.6 ± 5.8‰ and range from ẟ<sup>13</sup>C = -34.5‰ to +1.5‰. In contrast, peridotitic paragenesis diamonds have a mean ẟ<sup>13</sup>C value of -5.4 ± 2.6‰, and their range in ẟ<sup>13</sup>C values is from -23.5‰ to +0. 1‰. ẟ<sup>15</sup>N 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 ẟ<sup>15</sup>N value of 4.9‰ ±4.6‰ whereas lherzolitic diamonds have a mean ẟ<sup>15</sup>N value of 4.1‰ ±4.9‰. These parageneses have different mean ẟ<sup>15</sup>N values when compared to the mean ẟ<sup>15</sup>N 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. <br><br> Three Argyle diamonds and a single Ellendale diamond are zoned in ẟ<sup>13</sup>C and ẟ<sup>15</sup>N 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. <br><br> All available diamond ẟ<sup>15</sup>N data are combined and it is concluded that ẟ<sup>15</sup>N = 0‰ ± 6‰ is a characteristic of the sub-continental lithospheric upper mantle. Heterogeneities within this are however evident when ẟ<sup>13</sup>C and ẟ<sup>15</sup>N 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 ẟ<sup>15</sup>N 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.