  The terrestrial distribution of U, Th, and K abundances governs the thermal
evolution, traces the differentiation, and reflects the bulk composition of the
earth. Comparing the bulk earth composition to chondritic meteorites estimates
the net amounts of these radiogenic heat-producing elements available for
partitioning to the crust, mantle, and core. Core formation enriches the
abundances of refractory lithophile elements, including U and Th, in the
silicate earth by ~1.5. Global removal of volatile elements potentially
increases this enrichment to ~2.8. The K content of the silicate earth follows
from the ratio of K to U. Variable enrichment produces a range of possible
heat-producing element abundances in the silicate earth. A model assesses the
essentially fixed amounts of U, Th, and K in the approximately closed crust
reservoir. Subtracting these sequestered crustal amounts from the variable
amounts in the silicate earth results in a range of possible mantle
allocations, leaving global dynamics and thermal evolution poorly constrained.
Terrestrial antineutrinos from {\beta}-emitting daughter nuclei in the U and Th
decay series traverse the earth with negligible attenuation. The rate at which
large subsurface instruments observe these geo-neutrinos depends on the
distribution of U and Th relative to the detector. Geo-neutrino observations
with sensitivity to U and Th in the mantle are able to estimate silicate earth
enrichment, leading to a more complete understanding of the origin, accretion,
differentiation, and thermal history of the planet.
