Context. The detailed chemical abundances of extremely metal-poor (EMP) stars are key guides to understanding the early chemical evolution of the Galaxy. Most existing data, however, treat giant stars that may have experienced internal mixing later. Aims. We aim to compare the results for giants with new, accurate abundances for all observable elements in 18EMP turnoff stars. Methods. VLT/UVES spectra at and 130perpixel (330-1000nm) are analysed with OSMARCS model atmospheres and the TURBOSPECTRUM code to derive abundances for C, Mg, Si, Ca, Sc, Ti, Cr, Mn, Co, Ni, Zn, Sr, andBa. Results. For Ca, Ni, Sr, and Ba, we find excellent consistency with our earlier sample of EMPgiants, at all metallicities. However, our abundances of C, Sc, Ti, Cr, Mn and Co are 0.2dex larger than in giants of similar metallicity. Mg and Siabundances are 0.2dex lower (the giant [Mg/Fe]values are slightly revised), while Zn is again 0.4dex higher than in giants of similar[Fe/H] (6stars only). Conclusions. For C, the dwarf/giant discrepancy could possibly have an astrophysical cause, but for the other elements it must arise from shortcomings in the analysis. Approximate computations of granulation (3D)effects yield smaller corrections for giants than for dwarfs, but suggest that this is an unlikely explanation, except perhaps for C, Cr, and Mn. NLTEcomputations for Na and Al provide consistent abundances between dwarfs and giants, unlike the LTEresults, and would be highly desirable for the other discrepant elements as well. Meanwhile, we recommend using the giant abundances as reference data for Galactic chemical evolution models. Key words: Galaxy: abundances - Galaxy: halo - Galaxy: evolution - stars: abundances - stars: population II - stars: supernovae: general
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