1 Introduction
7
1.3 P b , S r a n d N d I s o t o p e s as G e o c h e m i c a l " T r a c e r s "
Unlike the stable isotopes, H, C or O, which can undergo isotopic fractionation
depending on temperature or nature of the chemical reaction (see Sect. I. 1), Pb, Sr
and Nd do not appear to show any such behaviour due to their significantly higher
atomic masses. For example, the 87Sr/86Sr ratio of a mineral is solely dependent
upon its Rb/Sr ratio, the time of formation and the initial 87Sr/86Sr ratio (see
Sect.l.2.I).
The inital isotopic ratio which characterizes the beginning of the chronometer
ought to be identical to that of the fluid phase in which the mineral crystallized.
Therefore, this ratio can help us determine the origin, environment of formation
and in certain circumstances the conditions of formation of a mineral. Similarly,
knowledge of the initial isotopic ratio of a granite body can help us identify the
source of the magma. As the mantle and the continental crust have different
isotopic ratios, it is possible to differentiate between granites of mantle and crustal
origin. Such source determinations are only possible because the isotopic ratios of
Pb, Sr and Nd vary sufficiently within the Earth and because these ratios vary
within characteristic end members that define specific, geochemically different
reservoirs. The reasons for these characteristic reservoir isotopic signatures lie in
various geochemical processes of the past which have led to elemental
fractionation, for example, where U-Pb, Rb-Sr und Sm-Nd parent-daughter
fractionation has taken place. The most important elemental fractionation took
place during the development of the mantle-crust. This can be represented using a
simplified Sm-Nd evolution model for the Earth (Fig. 1.2).
It is assumed today that the Earth, together with the other planets of the solar
system, were born out of the same solar nebula and that the stony meteorites that
periodically rain down on the Earth record the isotopic characteristics of this
original nebula. Age dating which has been carried out on these meteorites
(chondntes, achondrites, shergottites) suggests that the solar system orignated
around 4580 million years ago. Because the Earth developed out of the same
nebula, it is assumed that the Earth contained the same initial isotopic ratios as the
meteorites at the time of their formation (A in Fig. 1.2). At this stage, the
"chondritic" Earth ball was still primitive, geochemically speaking, unfractionated
and isotopically homogeneous. Geochemical processes, which led to the
formation of first stable, continental crust may have taken place about 3800
million years ago at the latest.
Enrichment and depletion of certain elements in the newly formed mantle and
crust segments led to fractionation in the Rb-Sr und Sm-Nd parent-daughter
systems (B in Fig. 2.2). Basically, that element of the parent-daughter system
which had the greater ionic radius came to be preferentially enriched in the
continental crust. This means that the parent rubidium became enriched over the
daughter strontium and the daughter neodymium over the parent samarium.
Therefore, the 1~3Nd/l~Nd ratio in the continental crust was not allowed to rise as
7
1.3 P b , S r a n d N d I s o t o p e s as G e o c h e m i c a l " T r a c e r s "
Unlike the stable isotopes, H, C or O, which can undergo isotopic fractionation
depending on temperature or nature of the chemical reaction (see Sect. I. 1), Pb, Sr
and Nd do not appear to show any such behaviour due to their significantly higher
atomic masses. For example, the 87Sr/86Sr ratio of a mineral is solely dependent
upon its Rb/Sr ratio, the time of formation and the initial 87Sr/86Sr ratio (see
Sect.l.2.I).
The inital isotopic ratio which characterizes the beginning of the chronometer
ought to be identical to that of the fluid phase in which the mineral crystallized.
Therefore, this ratio can help us determine the origin, environment of formation
and in certain circumstances the conditions of formation of a mineral. Similarly,
knowledge of the initial isotopic ratio of a granite body can help us identify the
source of the magma. As the mantle and the continental crust have different
isotopic ratios, it is possible to differentiate between granites of mantle and crustal
origin. Such source determinations are only possible because the isotopic ratios of
Pb, Sr and Nd vary sufficiently within the Earth and because these ratios vary
within characteristic end members that define specific, geochemically different
reservoirs. The reasons for these characteristic reservoir isotopic signatures lie in
various geochemical processes of the past which have led to elemental
fractionation, for example, where U-Pb, Rb-Sr und Sm-Nd parent-daughter
fractionation has taken place. The most important elemental fractionation took
place during the development of the mantle-crust. This can be represented using a
simplified Sm-Nd evolution model for the Earth (Fig. 1.2).
It is assumed today that the Earth, together with the other planets of the solar
system, were born out of the same solar nebula and that the stony meteorites that
periodically rain down on the Earth record the isotopic characteristics of this
original nebula. Age dating which has been carried out on these meteorites
(chondntes, achondrites, shergottites) suggests that the solar system orignated
around 4580 million years ago. Because the Earth developed out of the same
nebula, it is assumed that the Earth contained the same initial isotopic ratios as the
meteorites at the time of their formation (A in Fig. 1.2). At this stage, the
"chondritic" Earth ball was still primitive, geochemically speaking, unfractionated
and isotopically homogeneous. Geochemical processes, which led to the
formation of first stable, continental crust may have taken place about 3800
million years ago at the latest.
Enrichment and depletion of certain elements in the newly formed mantle and
crust segments led to fractionation in the Rb-Sr und Sm-Nd parent-daughter
systems (B in Fig. 2.2). Basically, that element of the parent-daughter system
which had the greater ionic radius came to be preferentially enriched in the
continental crust. This means that the parent rubidium became enriched over the
daughter strontium and the daughter neodymium over the parent samarium.
Therefore, the 1~3Nd/l~Nd ratio in the continental crust was not allowed to rise as