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We investigate the metallicity of the broad line region (BLR) of a sample of 30 quasars in the redshift range 4 z z ~ 6. The inferred metallicity of the BLR gas is so high (several times solar) that metal ejection or mixing with lower metallicity gas in the host galaxy is required to match the metallicities observed in local massive galaxies. On average, the observed metallicity changes neither among quasars in the observed redshift range 4 z z > 6 quasars. The latter result is puzzling, since the minimum enrichment timescale of carbon is about 1 Gyr, i.e. longer than the age of the universe at z ~ 6.
Juárez et al. (Wed,) studied this question.