The study of mantle diamonds offers the possibility to investigate the geodynamic carbon cycle with an unrivalled spatial and temporal scope. Impurities in diamond, of which nitrogen is the most common, provide a window into the cycling of mantle volatiles. The isotopic compositions of carbon and nitrogen for crustal and mantle derived samples are, on average, distinct, making the carbon and nitrogen isotopic systems potentially useful indicators of interactions between crustal and mantle volatiles. For peridotitic diamonds in particular there is a very large range in δ 15N (−40 to +15 ‰) with only a small range in δ 13C (−5 ± 3 ‰). These variations in diamonds can be attributed to multistage growth from isotopically distinct reservoirs or to fractionation processes operating during single growth steps, or perhaps both. In this thesis, data from mixed-habit diamonds shows that large kinetic fractionations are unlikely to occur in nature. Data from sub-lithospheric diamonds from Juina, and diamondites from Orapa show evidence of recycling of crustal fluids as well as evidence of the involvement of primordial nitrogen during diamond formation. These data lead to the creation of a preliminary model for the deep nitrogen cycle.