1 Introduction
5
corresponds to the increase in the value of (eXt-1). From the gradient of this
straight line we can work out the time at which the system was in isotopic
equilibrium assuming that the system has remained closed in the meantime.
Subsequent hydrothermal events, metamorphosis or diagenesis can lead to the
reopening of the system and to a rehomogenization of the isotopic ratios. When
the newly opened system recloses after such an event, our clock starts again, this
time with a higher, initial isotopic ratio.
1.2.2 The U-Th-Pb Method
The parent isotopes ~8 U (Uranium), n5 U and 232Th (Thorium) undergo decay by
way of a long sequence of half-way houses to form the stable lead isotopes z~
:~
and 2~
respectively. The 2~
isotope is not produced through
radioactive decay. The time-dependent evolution of lead isotopes can be
represented by the following equations:
('-~176 Pb) ..... =('-~176 Pb),+('-~SUI '-~ Pb)(ea"-l)
('-~176 Pb). .... =('-~176
Pb)i+(z~'U/:~ Pb)(ea'-'-l)
O0
(xI)
(2~176 Pb) ...... =('-ospb/z~ pb) +('-.;'-Th/2~ pb)(ea3,_l)
(XlI)
where ~.l, L~ und L3 are the radioactive decay constants for --~sU, -'35U und z3-'Th.
~.1 (_'.',s U)= 1.55125* 10-1~ M('_35 U)= 9.8485* 10-m/y; L3(L~2Th)=4.9475,10-I J/y
= '-~~U(e a'' - 1)
(XIII)
with a normal isotopic
reducing equations X und XI gives:
("~176 Pbl,,,,, -(:~176 Pb)i
el,),
where the -"~sU/'28U ratio for terrestrial material
composition is constant (I/137,88).
1.2.3 The Sm-Nd Method
Samarium and neodymium belong to the group of elements called the rare earth
elements (REE) and both possess 7 isotopes (ta4Sm, 147Sm, I~Sm, 149Sm, 15~
tSZSm, Ls-LSm; 14_" Nd, I.*~Nd. , t~Nd, 145Nd, I~Nd ' I~Nd ' 15o Nd). b*7Sm decays