Geology Reference
In-Depth Information
Table 10.2.
Average MORB composition estimates.
Nb
Th
U
(μg/g)
(ng/g)
(ng/g)
'All MOR'
a
5.95
400
240
NMORB
b
2.33
120
47
EPR average
c
3.79
200
80
EMORB
b
8.3
600
180
MARK EMORB
c
16.05
1100
305
a
Raw averages from PetDB database, www.petdb.org/
petdbWeb/.
b
Sun and McDonough [188].
c
Donnelly
et al.
[189].
the source regions should be characterised by a mean and spread, rather than by
hypothetical end-members for which there is no good evidence.
This is most telling for the OIBs, particularly for helium, for which the two-
reservoir model is still widely held. Thus there is held to be an 'undegassed'
reservoir with high
3
He/
4
He [214], which must be at least 35 R
A
(atmospheric
ratio) to accommodate the highest value in Figure 10.16. However, if the OIB
distribution simply represents the values in a heterogeneous source, then the mean
value is the most appropriate way to characterise the source. The mean value is
only 11.6 R
A
, less than twice the mean of MORBs, 7.9 R
A
. The main distinction
between the helium of MORBs and OIBs is not the difference in mean values but
the difference in the spread,
6R
A
for OIBs.
Not only are the mean and spread a more accurate way to characterise the obser-
vations, but also the mean is the quantity required in mass balances and in con-
sidering mantle heat sources. Unfortunately, the means are not easy to estimate at
present, partly because the mean may be strongly affected by relatively uncommon
enriched samples, and partly because so little serious effort has been made to com-
pile the required data. For example, the mean La/Sm value of Figure 10.17 should
be calculated by weighting the individual values with their La content, which varies
by two orders of magnitude. The mean will be strongly skewed to the high end.
A crude indication of the effect of including most MORB values is shown in
Table 10.2, which lists abundances of Nb, Th and U. The first row shows simple
raw averages from the category 'All mid-ocean ridges' from the PetDB database.
The other rows show comparison values for two estimates of 'normal MORB'
and two 'enriched' MORBs. In the all-MORB average U is 3-5 times larger than
previous normal MORB estimates. It may be that U values are less reliable because
U concentrations are so low, so Th and Nb values have been included. Typical
±
1R
A
for MORBs and
±
