Evolution of ISM abundances as a function of galactic age t for the Milky Way model (lines): is the mass in the ISM of the element (or elements) referred to by the index i, and is the total mass of the ISM; “metals” denotes the sum on all metals, and “α \ O”, the sum on Ne, Mg, Si, S and Ca. Squares are the corresponding observed abundances of Anders & Grevesse (1989) (shuffled around 13 Gyr for the sake of clarity).