Evolution of composition in a superbubble. Top left: ejection rate of ²⁰Ne (dotted) and ²²Ne (solid) in M_⊙/Myr per 1 M_⊙ of stars formed (assuming a Salpeter IMF between 0.1 and 120 M_⊙). Top right: supernova rate per Myr. Middle: total mass of ²⁰Ne (dotted) and ²²Ne (solid) accumulated in the superbubble for no initial gas (M₀ = 0, left) and for an initial mass of gas equal to 1/5 of the formed stars (M₀ = 0.20, right); in the former case ²²Ne dominates by mass early on, whereas in the latter the mass of ²⁰Ne dominates from the very beginning. Bottom: corresponding evolution of the O/H (dashed) and of the (²²Ne/²⁰Ne)_SB ratio (dotted) in the SB gas; the solid curve represents the (²²Ne/²⁰Ne)_CR ratio of particles accelerated up to that time and has to be compared to the observed GCR source ratio (dotted horizontal line).