Fig. 5: Diffusion impact and constraints on diamond 3He/4He model ages.
From: Helium in diamonds unravels over a billion years of craton metasomatism
3He/4He as a function of age and He diffusivity for a ON-DBP-332 (red diamond symbol in Fig. 2b) and b ON-FCH-349 (yellow circle in Fig. 2b), calculated based on the measured 3He/4He and the 4He, 3He, U, and Th concentrations in these diamonds, and assuming HDFs initial Ra values between 5 and 9 (gray horizontal shading in (a); representing common values for MORB, the CLM and subducted components). The 3He/4He model ages increase with increasing diffusivity. For ON-DBP-332 (a) the ages are between 320–340, 420–460, and 500–540 Ma (turquoise, gray, and red vertical shading, respectively). The potential range 3He/4He model ages for diamond ON-FCH-349 (b) can be constrained by considering the oldest diamond ages in the Kaapvaal craton19,71 and the distribution of Kaapvaal peridotite Re-depletion ages70 (TRD model age; relative probability of n = 228; gray shading in b). As HDF-bearing diamonds are unlikely to survive the thermal and tectonic history prior to craton stabilization at ~2.6 Ga72,73,74, this translates to a maximum diffusivity of He in HDF-bearing diamonds of <1.8 × 10−19 cm2 s−1. Thus, the timing of diamond formation and CLM metasomatism based on (U-Th)/He dating of HDF-bearing diamonds is constrained to be between 3He/4He model ages for D = 1 × 10−21 and 1.8 × 10−19 cm2 s−1. Additional details are given in the text.
