  A large contrast in electrical conductivity has been observed within the
lithospheric mantle between the Tanzanian craton and the Mozambique belt at
depths > 70 km. Such contrasts are typically attributed to changes in volatiles
and/or melt content, with changes in mineralogy deemed insufficient to impact
conductivity. To test this assumption, electrical conductivity measurements
were conducted at pressure-temperature conditions relevant to the Tanzanian
lithosphere (1.5 and 3 GPa; from 400 to > 1500{\deg}C) on xenoliths from
Engorora, Northern Tanzania. I observe that, once garnet becomes stable in
fertile mantle rocks (> 60 km, 1.7 GPa), it nucleates at grain boundaries and
forms the backbone of a conductive network. At 3 GPa, the presence of
garnet-rich networks increases conductivity by a factor of 100 regardless of
temperature. Numerical models demonstrate that low (< 10^{-2} Sm^{-1}) and high
(> 10^{-1} Sm^{-1}) conductivities in the Tanzanian lithospheric mantle are
best explained by low and high degrees of garnet connectivity, respectively.
Such high electrical conductivities in cratonic roots can be explained by the
presence of connected garnet clusters, suggesting mantle refertilization. I
suggest that the garnet network represents the result of plume triggered
metasomatism due to plume impingement, and may represent an unstable structure
that will eventually lead to loss of cratonic lithosphere.
