142 Peter Sdlle and Graham Shields
20
I0
"O 0
C:
O
~
LU
-20
t'fOR8
|
-30
,
L
-2
-1
O Pacific Ocean
13 Atlantic Ocean
9 Drake Passage
9
Fyne
. - o -
0
1
2
Epsilon Ce
Fig. 5.36. Nd and Ce isotope results for l'enomanganese nodules (Amakawa et al. 1996)
5 . 5 0 s Isotopic Composition of Seawater: Stratigraphic Tool or
Tracer of Ocean Circulation Patterns?
Rhenium (Re) has just two naturally occurring isotopes: 185Re and IS7Re.
Osmium (Os) has seven naturally occurring isotopes: 184Os, 186Os, 187Os, 18SOs,
ISUOs, 19~ and 192Os. lSTRe undergoes beta decay with a half-life of 45.6 Ga ('t
= 1.52 x 10 "ll yr "1) to form 187Os in a similar fashion to 87Rb and 87Sr. The
present day 187Os/186Os ratio depends on the age of the system, its initial
1870.r 186Os ratio and its initial 187Re/Ig6Os ratio analogous to the Sr isotope
system.
Rhenium and osmium have very different compatibilities (preference for fluid or
solid phase during crystallization) during magmatic processes. Rhenium is
incompatible during crust-mantle differentiation processes and so becomes