  The initial conditions found in infrared dark clouds (IRDCs) provide insights
on how high-mass stars and stellar clusters form. We have conducted
high-angular resolution and high-sensitivity observations toward thirty-nine
massive IRDC clumps, which have been mosaicked using the 12m and 7m arrays from
the Atacama Large Millimeter/submillimeter Array (ALMA). The targets are 70
$\mu$m dark massive (220-4900 $M_\odot$), dense ($>$10$^4$ cm$^{-3}$), and cold
($\sim$10-20K) clumps located at distances between 2 and 6 kpc. We identify an
unprecedented number of 839 cores, with masses between 0.05 and 81 $M_\odot$
using 1.3 mm dust continuum emission. About 55% of the cores are low-mass ($<$1
$M_\odot$), whereas $\lesssim$1% (7/839) are high-mass ($\gtrsim$27 $M_\odot$).
We detect no high-mass prestellar cores. The most massive cores (MMC)
identified within individual clumps lack sufficient mass to form high-mass
stars without additional mass feeding. We find that the mass of the MMCs is
correlated with the clump surface density, implying denser clumps produce more
massive cores and a larger number of cores. There is no significant mass
segregation except for a few tentative detections. In contrast, most clumps
show segregation once the clump density is considered instead of mass. Although
the dust continuum emission resolves clumps in a network of filaments, some of
which consist of hub-filament systems, the majority of the MMCs are not found
in the hubs. Our analysis shows that high-mass cores and MMCs have no preferred
location with respect to low-mass cores at the earliest stages of high-mass
star formation.
