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From exploratory studies and theoretical expectations it is known thatsimplifying approximations in spectroscopic analysis (LTE, 1D) lead tosystematic biases of stellar parameters and abundances. These biases dependstrongly on surface gravity, temperature, and, in particular, for LTE vs.non-LTE (NLTE) on metallicity of the stars. Here we analyse the [Mg/Fe] and[Fe/H] plane of a sample of 326 stars, comparing LTE and NLTE results obtainedusing 1D hydrostatic models and averaged <3D> models. We show that compared tothe <3D>NLTE benchmark, all other three methods display increasing biasestowards lower metallicities, resulting in false trends of [Mg/Fe] against[Fe/H], which have profound implications for interpretations by chemicalevolution models. In our best <3D> NLTE model, the halo and disc stars show aclearer behaviour in the [Mg/Fe] - [Fe/H] plane, from the knee in abundancespace down to the lowest metallicities. Our sample has a large fraction ofthick disc stars and this population extends down to at least [Fe/H] ~ -1.6dex, further than previously proven. The thick disc stars display a constant[Mg/Fe] ~ 0.3 dex, with a small intrinsic dispersion in [Mg/Fe] that suggeststhat a fast SN Ia channel is not relevant for the disc formation. The halostars reach higher [Mg/Fe] ratios and display a net trend of [Mg/Fe] at lowmetallicities, paired with a large dispersion in [Mg/Fe]. These indicate thediverse origin of halo stars from accreted low-mass systems tostochastic/inhomogeneous chemical evolution in the Galactic halo.

Non-local thermodynamic equilibrium stellar spectroscopy with 1D and 3D models - II. Chemical properties of the Galactic metal-poor disc and the halo