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We study zCOSMOS-bright optical spectra for 609 Spitzer/MIPS 24micron-selected galaxies with S(24um)> 0.30 mJy and I<22.5 (AB mag) over 1.5sq. deg. of the COSMOS field. From emission-line diagnostics we find that: 1)star-formation rates (SFR) derived from the observed Halpha and Hbeta linesunderestimate, on average, the total SFR by factors ~5 and 10, respectively; 2)both the Calzetti et al. and the Milky Way reddening laws are suitable todescribe the extinction observed in infrared (IR) sources in most cases; 3)some IR galaxies at z<0.3 have low abundances, but many others with similar IRluminosities and redshifts are chemically enriched; 4) The average [OIII]/Hbetaratios of nuLnu(24um)>10^11 Lsun galaxies at 0.6<z<0.7 are ~0.6 dex higher thanthe average ratio of all zCOSMOS galaxies at similar redshifts. Massive starformation and active galactic nuclei (AGN) could simultaneously be present inthose galaxies with the highest ionising fluxes; 5) ~1/3 of the galaxies withmetallicity measurements at 0.5<z<0.7 lie below the general mass-metallicityrelation at the corresponding redshifts. The strengths of the 4000 Angstrombreak and the Hdelta EW of our galaxies show that secondary bursts of starformation are needed to explain the spectral properties of most IR sources. TheLIRG and ULIRG phases occur, on average, between 10^7 and 10^8 years after theonset of a starburst on top of underlying older stellar populations. Theseresults are valid for galaxies of different IR luminosities at 0.6<z<1.0 andseem independent of the mechanisms triggering star formation.

The optical spectra of 24 micron galaxies in the COSMOS field: I. Spitzer/MIPS bright sources in the zCOSMOS-bright 10k catalogue