Geoscience Reference
In-Depth Information
During the Archean and the early Proterozoic, the slope of the isotopic evolution curve
for basalt Nd, and so the
147
Sm/
144
Nd ratio of the mantle, remained fairly low and then,
at around 2 Ga, progressively attained the modern value. Interpretation of this observation
is complex and controversial because much continental crust seems to have been formed
around 2.7 billion years ago. Two important points are involved in the explanation of this
observation: (1) the Earth underwent substantial primordial differentiation that plate tec-
tonics took one or two billion years to erase; and (2) because of the cooling of the Earth,
the significance of violent convective instabilities in the form of hot spots diminished in
favor of slower and more regular heat loss through ridges. The fast cooling of the core
also acted to stabilize mantle convection with respect to plume generation. We will show
in the next chapter that the
142
Nd anomalies left by the extinct
146
Sm radioactivity in
the 3.8 Ga old rocks from Isua, west Greenland require the existence of an early magma
ocean and therefore support hypothesis (1). However, simply because heat production by
radioactivity has declined by a factor of three over the last four billion years, the overall
strength of mantle convection must be less today than in the Archean. This simple argu-
ment implies a reduction over the Earth's history of the juvenile magmatic contribution
to crustal growth from hot spots. It is probable that if recycling of ancient crust is per-
ceived as so important today, as for example in the Alps, the Himalayas, the Appalachians,
or the Hercynian belt of Western Europe, it is because plumes have become much more
infrequent.
References
Abouchami, W., Boher, M., Michard, A., and Albarède, F. (1990) A major 2.1 Ga event of
mafic magmatism in West Africa on early stage of crustal accretion.
J. Geophys
.
Res
.,
95
, 17 605-17 629.
Albarède, F. and van der Hilst, R. (2002) Zoned mantle convection.
Phil. Trans. R. Soc.
,
A360
, 2569-2592.
♠
Allègre, C. J. (1982) Chemical geodynamics.
Tectonophys.
,
81
, 109-132.
♠
Gastil, G. (1960) The distribution of mineral dates in space and time.
Amer. J. Sci
.,
258
,
1-35.
♠
Hurley, P. M., Hughes, H., Faure, G., Fairbairn, H. W., and Pinson, W. H. (1962) Radio-
genic strontium 87 model of continent formation.
J. Geophys. Res.
,
67
, 5315-5333.
♠
Hofmann, A. W. and White, W. M. (1982) Mantle plumes from ancient oceanic crust.
Earth Planet. Sci. Lett.
,
57
, 421-436.
Iizuka, T.
et al.
(2008) Detrital zircon evidence for Hf isotopic evolution of granitic crust
and continental growth. Submitted.
Lassiter, J. C. and Hauri, E. H. (1998) Osmium-isotope variations in the Hawaiian lavas:
evidence for recycled oceanic lithosphere in the Hawaiian plume.
Earth Planet. Sci.
Lett.
,
164
, 483-496.
McBirney, A. R. (1992)
Igneous Petrology
. San Francisco, Freeman-Cooper.
