Art:
Hypothetical geothermal temperature gradients can be found in many
references – you might try Ernst's "The Dynamic Planet" (Columbia University
Press, 1990) for starters. For a more detailed discussion of heat flow,
geothermal gradients, and deep earth structure, see "The Inaccessible Earth" by
Brown and Mussett (George Allen & Unwin Ltd., 1981).
Be advised that temperature curves for the deep earth are necessarily
estimates based on near-surface heat flow measurements, geophysical parameters,
and assumptions about the chemical composition of the mantle & core (among
other things). The temperature curves one might see in published references as
listed above are *not* taken from direct measurements.
Hope that this helps.
Cheers,
Scott M.