7 Sm-Nd Isotope Geochemistry of Argillaceous Sediments 187
3=
cJ
10
d.=,
|
Ioo
..: 80
o
>
~p 60
o
~0
(O
20
__f
I
t.=
,
I
0.10
0.11
0.12
I~7Sm / 1~Nd
A
0
~D
60 "~-80
Fig. 7.2. Chlonte content and organic carbon concentration from the Gunflint-clays as a
function of their Sm/Nd ratios. (Stille and Clauer 1986).
Stifle and Clauer (1986) selected clay minerals for their Sm-Nd age dating, mostly
illites and chlorites. The illite crystallinities and the common presence of pure
chlorite implied a primary, diagenetic origin for these mineral assemblages. The
clays contained up to 9% organic carbon. Investigations showed that the organic
matter consisted largely of pristane, phytane and paraffin. Therefore, these were
characteristic chemical sediments, with oil, primary diagenetic minerals and
extremely high contents of biogenic carbon. Clay minerals of this type could have
possessed Sr and Nd isotopic compositions at the time of their formation that were
fully equilibrated with seawater. Samples for isotopic analysis were chosen with
various chlorite/illite ratios and organic carbon contents in order to obtain the
largest possible variation in Rb/Sr and Sm/Nd ratios.
The K-Ar ages of these samples were scattered between 1.57 and 1.38 Ga. The
gradient of the Rb-Sr isochron indicated an age of 1.46 Ga. Similar variations in
the K-Ar and Rb-Sr ages were obtained from stratigraphically neighbouring, very
brown, oxidised clays (Peterman 1966). Peterman assumed that the sediments had
been altered by hydrothermal weathering some 1.46 Ga ago.