tokens: were likely formed in-situ at the Galactic halo (in black). Interestingly, it seems here that the stars from Gaia Enceladus are the ones closer to the extension of the linear relation, while those that deviate the most are in-situ stars. Fig. 5. Beryllium abundances as a function of metallicity. The points are the same as in the left panel of Fig. 2 , except that we colour the four extremely metal-poor stars found to likely be in-situ halo stars in Fig. 4 in black. In particular, we should remark here the position of stars BPS BS 16968 and CD-33 1173, for which we attempted to derive Be abundances for the first time. These stars are the two open blue triangles plotted over each other at about [Fe/H] ∼ −3.0 and log(Be/H) ∼ −13.8 in Fig. 5 . They are the most important points that demonstrate how stars from Gaia -Enceladus remain closer to the linear relation in a wide range of metallicities. We believe any extended discussion about Be abundances in stellar sub-components of this regime is for now precluded by our small sample. Nevertheless, two main results can be obtained from our analysis: (i) the deviation from the linear relation at [Fe/H] ∼−3.0 is real, although not universal; and (ii) the scatter we observed at these metallicities seems related to the inhomogeneous nature of the early halo. 4.3. Inhomogeneous star formation in the early Galaxy Having established our observational results, we now searched for a possible explanation to the increased scatter of points in the log(Be/H) versus [Fe/H] at the extremely metal-poor regime. To do that, we summarised results from an extensive body of investigations that have looked at the details of the first phases of Galactic chemical enrichment. The interpretation we would like to advance is that the main drivers of the observed scatter are the abundances of Fe and not those of Be. Or, in other words, that at the time of the formation of the early halo, Fe abundances (in a Galactic context) were more inhomogeneous than the Be abundances. 4.3.1. Early enrichment in beryllium The idea that abundances of Be (and of other cosmic-ray spallation products) can have smaller scatter when compared to abundances of elements produced by stellar nucleosynthesis was first suggested by the results of Suzuki et al. (1999) and Suzuki Yoshii (2001) . These authors computed chemical evolution models of an inhomogeneous early Galactic halo. Based on these models, Be abundances were suggested to be a good cosmochronometer for the early stages of the Galaxy ( Beers et al. 2000 ; Suzuki Yoshii 2001 ). This suggestion was tested by Pasquini et al. (2004 , 2007) who derived Be abundances in turn-off stars of the globular clusters NGC 6397 and NGC 6752. These measurements were very challenging, but ages obtained comparing the Be abundances with predictions of chemical evolution models (from Valle et al. 2002 ) were shown to be in excellent agreement with ages derived using isochrones. Suzuki Yoshii (2001) seem to have
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