  The chemical evolution history of the Small Magellanic Cloud (SMC) has been a
matter of debate for decades. The challenges in understanding the SMC chemical
evolution are related to a very slow star formation rate (SFR) combined with
bursts triggered by the multiple interactions between the SMC and the Large
Magellanic Cloud, a significant (~0.5 dex) metallicity dispersion for the SMC
cluster population younger than about 7.5 Gyr, and multiple chemical evolution
models tracing very different paths through the observed age-metallicity
relation of the SMC. There is no doubt that these processes were complex.
Therefore, a step-by-step strategy is required in order to better understand
the SMC chemical evolution. We adopted an existing framework to split the SMC
into regions on the sky, and we focus on the west halo in this work, which
contains the oldest and most metal-poor stellar populations and is moving away
from the SMC, that is, in an opposite motion with respect to the Magellanic
Bridge. We present a sample containing ~60% of all west halo clusters to
represent the region well, and we identify a clear age-metallicity relation
with a tight dispersion that exhibits a 0.5 dex metallicity dip about 6 Gyr
ago. We ran chemical evolution models and discuss possible scenarios to explain
this metallicity dip, the most likely being a major merger accelerating the SFR
after the event. This merger should be combined with inefficient internal gas
mixing within the SMC and different SFRs in different SMC regions because the
same metallicity dip is not seen in the AMR of the SMC combining clusters from
all regions. We try to explain the scenario to better understand the SMC
chemo-dynamical history.
