  We present the initial results of a study of the surface lithium abundance in
a sample of low-mass members (M~0.4-1.0 Msun) of the Orion Nebula Cluster (ONC)
that provide an independent clock to estimate stellar ages. We report discovery
of significant depletion of lithium in four stars with estimated mass of ~0.4
Msun and age ~10Myr. Comparison with the predictions of numerical and
analytical models shows excellent agreement between the isochronal age and
lithium depletion time scale for two objects, the first case in lithium-poor
pre-main sequence stars. Our results bear on the issue of the real age spread
in the ONC and hence on the overall duration of the star formation process,
indicating that the stellar population did not came into existence in a single,
rapid burst.
