Geology Reference
In-Depth Information
Figure 10.16. Histograms of isotopic ratios observed in MORB (dark grey) and
OIB (light grey). Helium is normalised to the atmosphere. Dashed lines mark one
standard deviation about the medians (solid lines).
3
He/
4
He values for MORB and
OIB are 7.92
5.82 R
A
, respectively;
87
Sr/
86
Sr values are
±
1.06 R
A
and 11.62
±
0.0010; and
206
Pb/
204
Pb values are 18.32
0.7026
±
0.0004 and 0.7036
±
±
0.39
and 19.16
±
0.77. After Ito and Mahoney [213]. Copyright American Geophysical
Union.
10.6.3 Focus on the depleted MORB mantle
The influence of the two-reservoir picture has persisted in a more subtle but quite
misleading way, in the concept of the
depleted MORB mantle
. The idea that the
MORB source has a relatively tightly defined composition that can be clearly sep-
arated from other putative components of the mantle has become so entrenched
that it is difficult to find illustrations of the real distributions of data. One such
illustration is shown in Figure 10.16. The populations of MORBs and OIBs com-
pletely overlap in all three systems. The MORB populations for Sr and Pb are rather
skewed, with a fat tail extending to the enriched side of the distribution, though
there are not enough data in this example to make this really clear.
Although, strictly, the depleted reservoir of a two-reservoir mantle ought to
be more depleted than any observed value, some heterogeneity of the MORB
source is conceded, and the depleted MORB source is taken to be represented
by something called
normal MORB
(NMORB). NMORB is taken to be the most
