Diamond growth from oxidized carbon sources beneath the Northern Slave Craton, Canada: A δ 13C–N study of eclogite-hosted diamonds from the Jericho kimberlite

• Published in: Geochimica et cosmochimica acta Vol. 75; no. 20; pp. 6027 - 6047
• Main Authors: Smart, Katie A., Chacko, Thomas, Stachel, Thomas, Muehlenbachs, Karlis, Stern, Richard A., Heaman, Larry M.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 2011
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/j.gca.2011.07.028

Diamonds from high- and low-MgO groups of eclogite xenoliths from the Jericho kimberlite, Slave Craton, Canada were analyzed for carbon isotope compositions and nitrogen contents. Diamonds extracted from the two groups show remarkably different nitrogen abundances and δ 13C values. While diamonds from high-MgO eclogites have low nitrogen contents (5–82 ppm) and extremely low δ 13C values clustering at ∼−40‰, diamonds from the low-MgO eclogites have high nitrogen contents (>1200 ppm) and δ 13C values from −3.5‰ to −5.3‰. Coupled cathodoluminescence (CL) imaging and SIMS analysis of the Jericho diamonds provides insight into diamond growth processes. Diamonds from the high-MgO eclogites display little CL structure and generally have constant δ 13C values and nitrogen contents. Some of these diamonds have secondary rims with increasing δ 13C values from −40‰ to ∼−34‰, which suggests secondary diamond growth occurred from an oxidized growth medium. The extreme negative δ 13C values of the high-MgO eclogite diamonds cannot be produced by Rayleigh isotopic fractionation of average mantle-derived carbon (−5‰) or carbon derived from typical organic matter (∼−25‰). However, excursions in δ 13C values to −60‰ are known in the organic sedimentary record at ca. 2.7 and 2.0 Ga, such that diamonds from the high-MgO eclogites could have formed from similar organic matter brought into the Slave lithospheric mantle by subduction. SIMS analyses of a diamond from a low-MgO eclogite show an outer core with systematic rimwards increases in δ 13C values coupled with decreases in nitrogen contents, and a rim with pronounced alternating growth zones. The coupled δ 13C-nitrogen data suggest that the diamond precipitated during fractional crystallization from an oxidized fluid/melt from which nitrogen was progressively depleted during growth. Model calculations of the co-variation of δ 13C–N yielded a partition coefficient ( K N) value of 5, indicating that nitrogen is strongly compatible in diamond relative to the growth medium. δ 13C values of diamond cores (−4‰) dictate the growth medium had higher δ 13C values than primary mantle-derived carbon. Therefore, possible carbon sources for the low-MgO eclogite diamonds include oxidized mantle-derived (e.g. protokimberlite or carbonatite) fluids/melts that underwent some fractionation during migration or, devolatilized subducted carbonates.