Combustion and Pyrolysis EA-IRMS Techniques to Determine the δ²H of Diamonds

Diamonds are generally considered to be metasomatic minerals originating from the Earth's mantle. They formed through the interaction of carbon-bearing fluids or melts with the surrounding deep lithology. Most knowledge about the formation of diamonds comes from studying their mineral inclusions or stable isotopes. Furthermore, diamonds are the only geological samples that provide insight into the volatile element composition of the Earth's mantle. While hydrogen has been known since the early 1970s to be a common impurity in natural diamonds, only a few studies have provided satisfactory quantitative measurements of its concentration and isotopic composition.

Here, we compare the results obtained using two different techniques involving EA-IRMS. The first analytical procedure involves H-pyrolysis and is a modification from existing techniques. The second innovative setup, which has not yet been described, involves diamond combustion followed by hydrogen reduction.

As diamond test samples are rare, we first analysed analogous samples (graphite) to validate the method, before analysing polycrystalline diamonds (from the Orapa mine in Botswana) with both techniques. After assessing the consistency between the two procedures, we analysed three natural diamonds. The results obtained using both techniques were highly comparable. This confirmed the validity of the H-Pyrolysis method, which is slightly more sensitive.

We designed and validated a novel methodology to incorporate an additional isotopic record into diamonds, namely, carbon, oxygen and nitrogen, with the objective of tracing the formation process of natural diamonds. Diamond in situ analyses might enhance our comprehension of the deep water cycle. Those measurements require precisely calibrated materials. This technique could provide such references.