6 Isotope Geochemistry of Clay Minerals 177
C~
0.73
0.72
0.71
\
t
I
I
l
\
{3
I
|
E
ca.
Q.
v
O'3
E
EX
CL
X3
Z
60
t.O
20
60
~.0
20
{:313
13
C1
t
I
I
I
O
i1"1
I
z~
r=s
I=1
9
I
!
I
I
I
t
2
3
4
5
7
$
K+ 0 (%1
Fig. 6.19. 87Sr/S6Sr ratios and Sr concentration values of glaucony residues (squares).
muddy clays (triangle) and untreated glauconies (filled circles) relative to their K20
contents. (Stille and Clauer 1994)
Armed with this information, we are now in a position to develop a model of
glauconitization. In the early stages of glauconite genesis (K20 <4%), Sr and Nd
are released from the glauconite. Its isotopic compositions are identical to those of
the Sr and Nd remaining in the crystal lattice and with the Sr and Nd isotopic
compositions of the clay mud in which the worm lived. This is made clearer in
Fig. 6.19 in which the Sr and Nd concentrations and Sr isotopic ratios are shown
relative to K20 contents. This first stage of glauconite formation is therefore
dominated by the chemical and isotopic signature of the glauconite precursor