There is still some research to do to definitively answer this question. Having said that though, according to research published in the paper "Evidence for geochemical terranes on Mercury: Global mapping
of major elements with MESSENGER’s X-Ray Spectrometer (Weider et al. 2015), the high Mg region (HMR) could be due to the consequences of an impact event - the authors from the article suggest that:
therefore, that the HMR may also be an impact related
feature, and that its high Mg/Si ratio is evidence of
mantle excavation during a large impact event early in Mercury’s
history (>4.1 Ga).
and later, partly resurfaced by subsequent volcanism and smaller impact events.
The authors elaborate further that the mechanism behind it could be
If an ancient large impact at the HMR site penetrated into a
vertically differentiated mantle (with additional lateral variations
possible) and reached a deep, otherwise unsampled layer (i.e., the
lherzolitic source proposed by Charlier et al. 2013*), the HMR composition
would be the signature of high-degree partial melting of
that mantle source.
Additional resource cited:
Charlier et al. 2013, Phase equilibria of ultramafic compositions on Mercury and the origin of the compositional dichotomy, Earth and Planetary Science Letters