Laser ablation of ‘diamonds-in-water’ for trace element and isotopic composition analysis

A new laser ablation technique combined with mass spectrometry measurements was applied for trace elements and radiogenic isotopic analyses of high-density fluid (HDF) microinclusion-bearing diamonds. Experiments were conducted using a frequency-doubled Nd:YAG laser (532 nm, 150 mJ per pulse, 7 ns pulse duration, 30 Hz repetition rate) in a closed ultra-clean glass cuvette filled with ultrapure water. Five diamonds were ablated for 1 hour while a single diamond was repeatedly ablated for shorter periods to produce 4 different weights of ablated material. Ablations proceeded at an average rate of 7.8 mg h⁻¹, which is a factor of >10 better than previous studies. ICPMS trace element analyses of the ablated material reveal primitive mantle normalized patterns that are similar in shape to previously analyzed microinclusion-bearing diamonds. Importantly, the new ablation technique produces enough material for quantitative analysis of all rare-earth elements (REEs), even in diamonds of low element abundance levels. The 4 duplicates of a single diamond were analyzed for their Sr, Nd, and Pb isotope compositions by TIMS using 10¹¹ or 10¹³ Ω resistors. The results reveal a relationship between decreasing amounts of analyte and increasing Sr and Pb isotope ratios attributed to blank contribution. No blank influence is detected on Nd isotope ratios. Ablations of a few mg provide sufficient amount of analyte to yield comparable Sr-Nd-Pb isotope values that reflect the composition of the ablated diamond. This result also suggests that HDF microinclusions within individual diamonds are rather homogeneous in their isotopic composition.