Lithium-7 is now generally accepted to originate in the hot Big Bang nucleosynthesis (BBN), with a primordial abundance (7Li/H)⋍ 10-10 in excellent agreement with the observed uniformity of the Li abundance in very metal deficient Pop II stars.
During the galactic evolution, both Li isotopes are created by spallation reactions of galactic cosmic rays (GCR) interacting with the ISM, that yield (7Li/H)⋍ 2 × 10-10 in 10 Gyrs, with a ratio (7Li/6Li)GCR=1.4. The major problem is then to explain the observed Pop I Li abundance, (7Li/H)
∼ 10-9, of which only 30% is accounted for by BBN and GCR spallation, as well as the high 7Li/6Li ratio measured in meteorites, representative of the solar system formation epoch 4.6 Gyrs ago, (7Li/6Li)⊙=12.3, whereas the above mechanisms predict a ratio around 2.
The existence of an extra stellar source of Li has been suggested. GCR spallation alone tends to decrease the 7Li/6Li ratio with time, and one should observe today an interstellar ratio ≃5-6 without production of Li in stars, or ≳6 with a steuar production. Measuring this ISM ratio thus provides a key test for the models of lithium evolution. If it is found to be ≲5, then another scenario would have to be considered.