  Subduction is a crucial part of the long-term water and carbon cycling
between Earth's exosphere and interior. However, there is broad disagreement
over how much water and carbon is liberated from subducting slabs to the mantle
wedge and transported to island-arc volcanoes. In the companion paper Part I,
we parameterize the metamorphic reactions involving H$_2$O and CO$_2$ for
representative subducting lithologies. On this basis, a two-dimensional
reactive transport model is constructed in this Part II. We assess the various
controlling factors of CO$_2$ and H$_2$O release from subducting slabs. Model
results show that up-slab fluid flow directions produce a flux peak of CO$_2$
and H$_2$O at subarc depths. Moreover, infiltration of H$_2$O-rich fluids
sourced from hydrated slab mantle enhances decarbonation or carbonation at
lithological interfaces, increases slab surface fluxes, and redistributes
CO$_2$ from basalt and gabbro layers to the overlying sedimentary layer. As a
result, removal of the cap sediments (by diapirism or off-scraping) leads to
elevated slab surface CO$_2$ and H$_2$O fluxes. The modelled subduction
efficiency (the percentage of initially subducted volatiles retained until
$\sim$200 km deep) of H$_2$O and CO$_2$ is increased by open-system effects due
to fractionation within the interior of lithological layers.
