  We analyse the gas-phase metallicity properties of a sample of low stellar
mass (log M*/M_sun <= 9) galaxies at 3 < z < 10, observed with JWST/NIRSpec as
part of the JADES programme in its deep GOODS-S tier. By combining this sample
with more massive galaxies at similar redshifts from other programmes, we study
the scaling relations between stellar mass, oxygen abundance (O/H), and
star-formation rate (SFR) for 146 galaxies, spanning across three orders of
magnitude in stellar mass and out to the epoch of early galaxy assembly. We
find evidence for a shallower slope at the low-mass-end of the mass-metallicity
relation (MZR), with 12 + log(O/H) = (7.72+-0.02) + (0.17+-0.03) log(M* / 10^8
M_sun), in good agreement with the MZR probed by local analogues of
high-redshift systems like 'Green Pea' and 'Blueberry' galaxies. The inferred
slope is well matched by models including 'momentum-driven' SNe winds,
suggesting that feedback mechanisms in dwarf galaxies (and at high-z) might be
different from those in place at higher masses. The evolution in the
normalisation is observed to be relatively mild compared to previous
determinations of the MZR at z~3 (~ 0.1 - 0.2 dex across the explored mass
regime). We observe a deviation from the local fundamental metallicity relation
(FMR) for our sample at high redshift, especially at z > 6, with galaxies
significantly less enriched (with a median offset in log(O/H) of ~ 0.5 dex,
significant at ~ 5 sigma) than predicted given their M* and SFR. These
observations are consistent with an enhanced stochasticity in the
star-formation history, and/or with an increased efficiency in metal removals
by outflows, prompting us to reconsider the nature of the relationship between
M*, O/H, and SFR in the early Universe.
