Geology Reference
In-Depth Information
Refractories
Moderately Volatiles
Volatiles
Mg
1
Planetary volatility
trend @ 1AU
Li
Si
Na
Mn
K
B
F
0.1
Lithophile
Rb
Elements
Zn
Cs
(
abundances relative to CI chondrite and Mg-normalized)
0.01
3.2
3.1
3.0
2.9
2.8
2.7
log 50% condensation Temperature (K) at 10
-4
atms
Figure 10.6. Abundances of lithophile elements as a function of condensation
temperature in the solar nebula, normalised to chondritic meteorite abundances.
The more volatile elements are systematically depleted. From McDonough [183].
Copyright Elsevier Science. Reprinted with permission.
Neon is inferred to have started in the mantle with a composition similar to solar
wind (SW) or meteoritic (Ne-B) neon. Nucleogenic
21
Ne is assumed to have been
progressively added (Figure 10.5(c)). The arrays of data evident in Figures 10.5(a)
and (b) are inferred to result from contamination with atmospheric neon during
eruption at the Earth's surface. The atmospheric composition is believed to have
resulted from mass fractionation during early loss from the top of the atmosphere,
which would move it along the 'mfl' line, followed by a small addition of
21
Ne
from the mantle.
It is clear that OIBs tend to fall along a less nucleogenic trend than MORBs
(Figures 10.5(a) and (b)). The mantle sources are inferred to lie at the inter-
sections of these trends, with the mantle line evolving by nucleogenic addition
(Figure 10.5(c)). Thus the OIB sources are inferred to be less nucleogenic than
the MORB source. These relationships are similar to those of helium isotopes,
reflecting the fact that additions come from U and Th for both elements.
10.2 Global budgets
Studies of meteorites have been used to estimate the total amounts of some elements
that were incorporated into the Earth [140, 182]. Elements that are refractory in
the environment of the early solar nebula are believed to have been added to the
Earth in unmodified relative proportions, whereas more volatile elements were
blown out of the Earth's formation zone by the solar wind, so that the more volatile
elements are systematically depleted within the Earth. Figure 10.6 shows estimates
of normalised abundances of lithophile elements (i.e. those that prefer the silicate
