Figure 5. ⁴⁰Ar/³⁹Ar incremental-heating release spectra of the Chikwawa basalt and cross-cutting dike. ‘%⁴⁰Ar*’ represents the percentage of radiogenic argon released. ‘Int. Age’ is the integrated age, calculated by isotopic recombination of all experimental steps. ‘Plateaus’ are formed from those steps that are: 1) contiguous, 2) constitute greater than 50% of the total ³⁹Ar released, 3) have an acceptable mean square of weighted residuals (MSWD, probability >95%), and 4) not intolerably different from the plateau mean age if an outermost step (>1.4 times the error in age of the step). (A) and (B): Dike cross-cutting Chikwawa basalt, sample LJ-3; (C) and (D), Chikwawa basalt flow, sample LJ-4.