  We have conducted a survey of a sample of infrared-dark clouds (IRDCs) with
the Spitzer Space Telescope in order to explore their mass distribution. We
present a method for tracing mass using dust absorption against the bright
Galactic background at 8 microns. The IRDCs in this sample are comprised of
tens of clumps, ranging in sizes from 0.02 to 0.3 pc in diameter and masses
from 0.5 to a few 10 Msun, the broadest dynamic range in any clump mass
spectrum study to date. Structure with this range in scales confirms that IRDCs
are the the precursors to stellar clusters in an early phase of fragmentation.
Young stars are distributed in the vicinity of the IRDCs, but the clumps are
typically not associated with stars and appear pre-stellar in nature. We find
an IRDC clump mass spectrum with a slope of 1.76 +/- 0.05 for masses from 30 to
3000 Msun. This slope is consistent with numerous studies, culled from a
variety of observational techniques, of massive star formation regions and is
close to the mass function of Galactic stellar clusters and star clusters in
other galaxies. We assert that the shape of the mass function is an intrinsic
and universal feature of massive star formation regions, that are the birth
sites of stellar clusters. As these clouds evolve and their constituent clumps
fragment, the mass spectrum will steepen and eventually assume the form of the
core mass function that is observed locally.
