Geoscience Reference
In-Depth Information
Table 4.13 Nd isotopic data of the basaltic and granitic
end-members
147 Sm/ 144 Nd
143 Nd/ 144 Nd
[Nd] (ppm)
Rhyolite
40
0.11
0.5115
Basalt
15
0.17
0.5128
basalt-rhyolite mixtures. Plot the mixing line with its end-members in the isochron
diagram and “age” the samples by 1 Gy and 2 Gy. Calculate the apparent ages of the
alignment for 0, 2, and 3 Gy into the future. Devise a strategy to decide whether an
alignment in the isochron diagram is a mixing line or a true isochron.
16. Sketch out the evolution of the Bulk Silicate Earth ( 147 Sm/ 144 Nd
=
0.1967 and
143 Nd/ 144 Nd
0.512638 today) in a 143 Nd/ 144 Nd vs. t evolution diagram between
2 Ga and the present and the evolution of a crustal rock formed at 2 Ga from the
primitive mantle with a 147 Sm/ 144 Nd ratio of 0.11. What is the modern
=
ε Nd of this
rock?
17. Compare the evolution diagram 143 Nd/ 144 Nd vs. t and the isochron diagram
143 Nd/ 144 Nd vs. 147 Sm/ 144 Nd. Plot the points representing the modern depleted man-
tle ( 147 Sm/ 144 Nd
0.222 and 143 Nd/ 144 Nd
=
=
0.51312) and a crustal sample with
147 Sm/ 144 Nd
0.12 and 143 Nd/ 144 Nd
0.5111. First, find graphically in each dia-
gram the model age at which the protolith of this crustal sample was extracted from
the depleted mantle. Second, derive a common expression that will let you calculate
this model age with precision.
=
=
References
Abouchami, W., Boher, M., Michard, A., and Albarède, F. (1990) A major 2.1 Ga old event
of mafic magmatism in West Africa: an early stage of crustal accretion. J. Geophys. Res. ,
95 , 17 605-17 629.
Arnold, J.R. and Libby, W. F. (1949) Age determination by radiocarbon content: Checks
with samples of known ages. Science , 110 , 678-680.
Bateman, H. (1910) Solution of a system of differential equations occurring in the theory
of radioactive transformations. Proc. Cambridge Phil. Soc. , 15 , 423-427.
Blichert-Toft, J., Albarède, Rosing, M., Frei, R., and Bridgwater, D. (1999) The Nd
and Hf isotopic evolution of the mantle through the Archean. Results from the Isua
supracrustals, West Greenland, and from the Birimian terranes of West Africa. Geochim.
Cosmochim. Acta , 63 , 3901-3914.
Heath, E., Turner, S. P., Macdonald, R., Hawkesworth, C. J., and van Calsteren, P. (1998)
Long magma residence times at an island arc volcano (Soufriére, St. Vincent) in the
Lesser Antilles: evidence from 238 U- 230 Th isochron dating. Earth Planet. Sci. Letters ,
160 , 49-63.
 
 
 
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