  The disc structure of the Milky Way is marked by a chemical dichotomy, with
high-alpha and low-alpha abundance sequences, traditionally identified with the
geometric thick and thin discs. This identification is aided by the old ages of
the high-alpha stars, and lower average ages of the low-alpha ones. Recent
large scale surveys such as APOGEE have provided a wealth of data on this
chemical structure, including showing that an identification of chemical and
geometric thick discs is not exact, but the origin of the chemical dichotomy
has remained unclear. Here we demonstrate that a dichotomy arises naturally if
the early gas-rich disc fragments, leading to some fraction of the star
formation occurring in clumps of the type observed in high-redshift galaxies.
These clumps have high star formation rate density. They, therefore, enrich
rapidly, moving from the low-alpha to the high-alpha sequence, while more
distributed star formation produces the low-alpha sequence. We demonstrate that
this model produces a chemically-defined thick disc that has many of the
properties of the Milky Way's thick disc. Because clump formation is common in
high redshift galaxies, we predict that chemical bimodalities are common in
massive galaxies.
