In recent (historically dated) lava flows, lava which hardened in air may date to zero. It may also date up to 1 million years old. The reason for this discrepancy is not clear. It has been suggested that some of the age may be correlated with the original makeup of the argon in the basalt before it erupted. Parts which cooled under water may date much older, even over 40 million years, because of "premature" solidification (quenching; hardening before all the argon came out). So different parts of the same recent lava flow have different dates.
The straightforward conclusion from this data is that extreme caution should be used when interpreting basalt dates under 1 million years old, or under 40 million years old for deposits made under water. The problem may be worse than it appears, as these may be minimum figures.
2010 Arthur V. Chadwick, Ph.D.