Quantitative analysis of trace element concentrations in some gem-quality diamonds

Abstract

The geochemical signature of diamond-forming fluids can be used to unraveldiamond-forming processes and is of potential use in the detection of so-called ‘conflict’diamonds. While fluid-rich fibrous diamonds can be analyzed by a variety of techniques,very few data have been published for fluid-poor, gem-quality diamonds because of theirvery low impurity levels. Here we present a new ICPMS-based (ICPMS: inductivelycoupled plasma mass spectrometry) method for the analysis of trace element concentrationswithin fluid-poor, gem-quality diamonds. The method employs a closed-system laserablation cell. Diamonds are ablated and the products trapped for later pre-concentrationinto solutions that are analyzed by sector-field ICPMS. We show that our limits ofquantification for a wide range of elements are at the sub-pg to low pg level. The method isapplied to a suite of 10 diamonds from the Cullinan Mine (previously known as Premier),South Africa, along with other diamonds from Siberia (Mir and Udachnaya) andVenezuela. The concentrations of a wide range of elements for all the samples(expressed by weight in the solid) are very low, with rare earth elements alongwith Y, Nb, Cs ranging from 0.01 to 2 ppb. Large ion lithophile elements (LILE)such as Rb and Ba vary from 1 to 30 ppb. Ti ranges from ppb levels up to 2 ppm.From the combined, currently small data set we observe two kinds of diamond-formingfluids within gem diamonds. One group has enrichments in LILE over Nb, whereas a secondgroup has normalized LILE abundances more similar to those of Nb. These two groupsbear some similarity to different groups of fluid-rich diamonds, providing some supportingevidence of a link between the parental fluids for both fluid-inclusion-rich and gemdiamonds.