Geology Reference
In-Depth Information
Helium Evolution
Neon Evolution
Argon Evolution
15
8.5
15
14.5
8
14.5
14
7.5
14
13.5
7
13.5
13
6.5
13
0
2
4
0
2
4
0
2
4
11
10
11
10.5
9.5
10.5
10
9
10
9.5
8.5
9.5
9
8
9
0
2
4
0
2
4
0
2
4
100
30
0.07
20
0.06
50
0.05
10
0.04
0
0.03
0
0
2
4
0
2
4
0
2
4
Earth time (Gyr)
Earth time (Gyr)
Earth time (Gyr)
Figure 10.25. An indicative calculated evolution of He, Ne and Ar isotope concen-
trations and ratios for the 'Medium gas' case of Table 10.5. MORB source: solid
curves; OIB source: dotted curves; radiogenic production: dashed curves. The
calculation includes faster degassing in the first 0.5 Gyr (see text). From Davies
[227]. Copyright American Geophysical Union.
material of essentially all ages is present in the D accumulations; Figure 10.23.)
The resulting present values are given in Table 10.6(a) and ratios in Table 10.6(b).
The ratios are representative of the mid-range of observed OIB values.
Another OIB calculation was run assuming a degassing rate 40% of the MORB
rate. This would correspond to a component of OIB material with a final age of
about 3.75 Ga. The resulting present values are also given in Table 10.6(a) and (b),
and the evolutions are included in Figure 10.25 (dotted curves). The present ratios
are representative of the most unradiogenic OIB noble gas values.
Thus the model is able to span the observed range from MORB to the most
unradiogenic OIB ratios by adjusting the residence time to be consistent with
results from numerical models, as hypothesised in Section 10.8.1. The proposal
that noble gases are carried in a hybrid pyroxenite component of the mantle is thus
able to account straightforwardly and quantitatively for He, Ne and Ar in the mantle
(except for the argon abundance, to be discussed shortly). Particularly notable is
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