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

作者：Smart Katie A^*; Chacko Thomas; Stachel Thomas; Muehlenbachs Karlis; Stern Richard A; Heaman Larry M

来源：Geochimica et Cosmochimica Acta, 2011, 75(20): 6027-6047.

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 delta C-13 values. While diamonds from high-MgO eclogites have low nitrogen contents (5-82 ppm) and extremely low delta C-13 values clustering at similar to-40 parts per thousand, diamonds from the low-MgO eclogites have high nitrogen contents (>1200 ppm) and delta C-13 values from -3.5 parts per thousand to -5.3 parts per thousand.
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 delta C-13 values and nitrogen contents. Some of these diamonds have secondary rims with increasing delta C-13 values from -40 parts per thousand to similar to-34 parts per thousand, which suggests secondary diamond growth occurred from an oxidized growth medium. The extreme negative delta C-13 values of the high-MgO eclogite diamonds cannot be produced by Rayleigh isotopic fractionation of average mantle-derived carbon (-5 parts per thousand) or carbon derived from typical organic matter (similar to-25 parts per thousand). However, excursions in delta C-13 values to -60 parts per thousand 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 delta C-13 values coupled with decreases in nitrogen contents, and a rim with pronounced alternating growth zones. The coupled delta C-13-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 delta C-13-N yielded a partition coefficient (K-N) value of 5, indicating that nitrogen is strongly compatible in diamond relative to the growth medium. delta C-13 values of diamond cores ( 4 parts per thousand) dictate the growth medium had higher delta C-13 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.

• 出版日期2011-10-15