Fig. 2: 3He/4He (and 4He/3He) versus nitrogen aggregation, and (U+Th)*/3He in HDF-bearing diamonds from the southwest Kaapvaal craton.
From: Helium in diamonds unravels over a billion years of craton metasomatism
(U + Th)* = (1.291 × 238U + 0.2969 × 232Th) Ga−1 is the 4He production rate. a Relationships between HDF compositional type, 3He/4He ratio, and host diamond nitrogen aggregation indicate formation from different metasomatic events throughout the Kaapvaal CLM history. The %B error bars signify 5% and are a maximum estimate. For diamonds with %B = 0 it is represented by the gray square; the range of 4He/3He ratios also is expanded for clarity. Error bars for 3He/4He and 4He/3He ratios are standard deviations. b 4He/3He vs. (U + Th)*/3He for all the samples; the silicic and carbonatitic samples show both higher 4He production rates and higher 3He/4He than the saline ones. Uncertainties on (U + Th)*/3He are propagated based on the U, Th, and 3He errors (Supplementary Data 148). c Same plot as in (b) showing only the saline HDF-bearing diamonds and the errorchron indicating diamond formation and metasomatism at 90 ± 42 Ma; systematics is in “Methods—‘U-Th-He isochron ages’,” and ages and errors are calculated using IsoplotR87. HDF compositional type and mine symbols are as in Fig. 1, data points with stars in (b, c) are two carbonatitic diamonds with internal microinclusion-bearing and overgrown microinclusion-free zones (“cloudy diamonds”)50 and a saline diamond with unusually high U/Th (i.e., Th/U < 1)49. These diamonds show apparent high (U-Th)*/He ratios and were not used for age determination (additional info is in the section “The significance of microinclusion homogeneity”).
