126 Peter Stille and Graham Shields
0.710
\ - -
0708
0.706
0.704
0.702
0.700 Phanerozoie
~
P r e c a m b r i a n
+
J
|
Time (10 3Mo)
,IP
Fig. 5.Z4. The Sr isotopic composition of Archean-Proterozoic seawater. {after Velzer and
Compston 1974: 1976; Burke r al. 1982)
5.2.3 Nd Isotopic Evolution in Seawater
Precambrian and the Paleozoic
There are few available data for the reconstruction of isotopic evolution of Nd in
the Precambrian. There are some data from iron-rich, Precambrian sediments
which have been placed under the general title of "Banded Iron Formations" or
"BIFs". These sediments were deposited widely during most of the Precambrian
and can be found today in rocks from the Americas, Australia, Africa and northern
Europe. It has generally been assumed that these sediments are of marine origin
and it has been maintained that these sediments have incorporated the Nd isotopic
composition of coeval seawater. Ivliller and O'Nions (1985), Stille and Clauer
(t986), and Jacobsen and Pimentel-Klose (1988a,1988b) were the first to
investigate these sediments isotopically.
They report strong variations in initial ENd values of more than 6e units. Such
strong variations in initial Nd isotopic composition, found in samples of about the
same age could be the result of the presence of detritus. This seems all the more
likely if we consider that there are no mineralogic or X-ray diffraction data for
many of these samples. The initial eNd rise begins in the region of mantle values
at about 2500 Ma. This trend implies that early Precambrian seawater, in
particular that of the Archean, was more strongly influenced by the mantle than
0.710
\ - -
0708
0.706
0.704
0.702
0.700 Phanerozoie
~
P r e c a m b r i a n
+
J
|
Time (10 3Mo)
,IP
Fig. 5.Z4. The Sr isotopic composition of Archean-Proterozoic seawater. {after Velzer and
Compston 1974: 1976; Burke r al. 1982)
5.2.3 Nd Isotopic Evolution in Seawater
Precambrian and the Paleozoic
There are few available data for the reconstruction of isotopic evolution of Nd in
the Precambrian. There are some data from iron-rich, Precambrian sediments
which have been placed under the general title of "Banded Iron Formations" or
"BIFs". These sediments were deposited widely during most of the Precambrian
and can be found today in rocks from the Americas, Australia, Africa and northern
Europe. It has generally been assumed that these sediments are of marine origin
and it has been maintained that these sediments have incorporated the Nd isotopic
composition of coeval seawater. Ivliller and O'Nions (1985), Stille and Clauer
(t986), and Jacobsen and Pimentel-Klose (1988a,1988b) were the first to
investigate these sediments isotopically.
They report strong variations in initial ENd values of more than 6e units. Such
strong variations in initial Nd isotopic composition, found in samples of about the
same age could be the result of the presence of detritus. This seems all the more
likely if we consider that there are no mineralogic or X-ray diffraction data for
many of these samples. The initial eNd rise begins in the region of mantle values
at about 2500 Ma. This trend implies that early Precambrian seawater, in
particular that of the Archean, was more strongly influenced by the mantle than