Geology Reference
In-Depth Information
the fact that the hitherto enigmatic occurrence of unradiogenic helium in OIBs has
been accounted for in a straightforward way using numerical models as a guide.
10.8.6 The
40
Ar budget
The
40
Ar results require additional discussion. The
40
Ar abundance in the MORB
source yielded by the evolution model, 12
10
13
atoms/g, is only 55% of that
×
10
13
atoms/g) to balance the usual estimate of the Earth's
40
Ar
required (22
×
10
13
atoms/g). The inferred concentration in D
is a little higher, but
D
and its related superpiles are equivalent to only about 2% of the mass of the
mantle, so it can hold only perhaps 3%. With about 50% in the atmosphere and
27% in the MORB source, this seems to leave about 20% still unaccounted for.
However, this deficit is much smaller than has been conventionally inferred [225],
and we should consider whether it is significant given the uncertainties involved.
One uncertainty is in the K/U ratio of the mantle, and there is some debate about
whether it might be lower [235, 236] or higher [237] than the value of 13 000 [184]
commonly used. Another possibility is that there is less K in the continental crust
than is usually estimated. Taylor and McLennan [238] estimated the K
2
O content
of the continents to be 1.1%, compared with Rudnick and Fountain's 1.9% [142].
The continental crust is so heterogeneous that it is doubtful any estimates have
great accuracy. If, as an illustration, we sum the mass of K in the crust using Taylor
and McLennan's estimate (crustal mass 2.6
budget (50
×
10
25
g; K mass 2.38
10
23
g), D
×
×
10
25
g; K concentration 650 μg/g and K mass 0.55
10
23
(mass 8.5
×
×
g) and
10
25
g; K mass 5.2
10
23
g), we get a total of 8.1
MORB source (mass 390
×
10
23
g of K and a bulk silicate Earth concentration of 203 μg/g. This compares with
the more usual estimate of around 240 μg/g [140], but the total K content of the
Earth has significant uncertainty because of the volatility of K during the planetary
accretion process (Section 10.2).
This reduced K content (203 μg/g) would yield 169
×
×
10
40
atoms of
40
Ar over
×
10
40
atoms in the MORB
4.5 Gyr. The inventory for the present model is 48
×
10
40
atoms in D
. The amount in the continental crust is not well
determined but estimated to be about 7
source and 0.7
×
10
40
, and the amount in the atmosphere
×
10
40
atoms. An evolution based on the
'High gas' case of Table 10.5 yields a final total of 170
10
40
[225, 229]. These sum to 155
is 99
×
×
10
40
atoms, so between
them the models span the bulk silicate amount just estimated. Thus a lower crustal
abundance of K, and a consequent lower global abundance, is consistent with the
present models.
Finally, we should not really assume that the Earth has retained all of its volatiles,
because some of the atmosphere may have been blasted off by very large impacts
during the late stages of accretion, or by the late heavy bombardment if that was a
×
