Very well, but how about the lead from the core? Where can we hope to find a sample of it? It turns out to be easier than you might think. Astronomers believe it highly probable that most meteorites are fragments of a former planet that broke up for reasons that are not entirely clear. It is pretty definite, however, that this protoplanet (or these protoplanets, for there may have been more than one) had an iron core, and this core (or these cores) is the source of the iron meteorites sailing around in space. A large meteorite hit the earth not too long ago (geologically speaking) and caused the Meteor Crater near Canyon Diablo in Arizona.
Examining ocean-bottom sediments obtained by lowering a tube-like instrument that brings up a long rod-shaped “core”, prior to nuclear age determination of the sample.
Many fragments of the meteorite iron have been found around the crater, and it is reasonable to assume that this is the kind of iron we would expect to find in the core of the earth. Like the core iron, it is mixed with a little lead, which can be isolated and analyzed in a mass spectrometer for its isotopic composition. This lead is found to be much less contaminated with radiogenic lead, and hence is much more primitive than the oldest leads found on earth. Thus, meteorites presumably are as close as we can get to true primordial lead—the lead of the time when the earth (and the protoplanet) first formed.
Once these measurements were available, it was easy to write the Houtermans equation for present-day and primordial leads in this way:
| (²⁰⁶Pb²⁰⁴Pb) present - (²⁰⁶Pb²⁰⁴Pb) primordial |
| ²⁰⁶Pb |
| ²⁰⁴Pb |
| ²⁰⁶Pb |
| ²⁰⁴Pb |
| (²⁰⁷Pb²⁰⁴Pb) present - (²⁰⁷Pb²⁰⁴Pb) primordial |
| ²⁰⁷Pb |
| ²⁰⁴Pb |
| ²⁰⁷Pb |
| ²⁰⁴Pb |
= 137.7
| (eλ238t - 1) |
| (eλ235t - 1) |
The present ratio of ²³⁸U to ²³⁵U is 137.7.
e = the base of natural logarithms λ = the [DECAY CONSTANT] of each isotope of uranium t = the age of the earth.