The Earth in the Solar System - Geochemistry: An Introduction - page 281

Table 12.2 182 Hf- 182 W results for two whole-rock chondrites
and their silicate and metal phases (Yin et al. , 2002 )

180 Hf/ 183 W 182 W/ 183 W

CHUR 2.84

Bulk Silicate Earth 36.87 1.8513

Moon 53.88 1.8515

Dhurmsala, Silicate 16.96 1.8524

Dhurmsala, Metal 0.10 1.8507

Dhurmsala, Whole-rock 3.52 1.8511

Dalgety Downs Silicate 6.30 1.8513

Dalgety Downs Metal 0.02 1.8507

Dalgety Downs Whole-rock 2.84 1.8510

equilibration of W isotopes took place while 182 Hf was still extant; 183 W and 180 Hf are
stable non-radiogenic nuclides used for normalization.
3. The data listed in Table 12.2 give the 180 Hf/ 183 W and 182 W/ 183 W ratios measured
in two whole-rock chondrites (Dhurmsala and Dalgety Downs) and their silicate and
metal phases. Examination of the 180 Hf/ 183 W ratios shows that Hf is lithophile and W
siderophile.
a. From the slope of the isochron formed by the six points, calculate the ( 182 Hf/ 180 Hf) t
ratio of the chondritic material at the time its constitutive phases last equilibrated
with each other.
b. Calculate the ( 182 W/ 183 W) t ratio of the mean chondritic reservoir (CHUR) at that
time, given its mean 180 Hf/ 183 W listed in the table.
c. Using these values as those of the parent reservoir, calculate the ( 182 Hf/ 180 Hf) t ratio
of the Earth at the time the core and silicate material last equilibrated.
d. Calculate the apparent age of core segregation from the Bulk Silicate Earth.
e. Do the same calculation for the Moon.
4. Define similar isochron diagrams for other extinct radioactivities: 26 Al- 26 Mg (normal-
ize to 24 Mg and 27 Al), 53 Mn- 53 Cr (normalize to 52 Cr and 55 Mn), 60 Fe- 60 Ni (normalize
to 58 Ni and 56 Fe), 146 Sm- 142 Nd (normalize to 144 Nd and 144 Sm). Refer to Table 4.1
for the decay constants. From your knowledge of the geochemical properties of the
elements of the parent and daughter isotopes and from the half-life of the radioactive
nuclide, discuss some potential geochronological applications of each system.
5. Dating the Universe I.
a. Explain why the radioactive nuclides 238 U and 232 Th must have been produced by
the r process.
b. The present-day 232 Th/ 238 U of the Solar System is 3.7. Calculate the value of this
ratio 4.56 Gy ago. Refer to Table 4.1 for the decay constants.
c. Let us assume that these two nuclides have been created at a constant rate since the
formation of the Universe and call p i their production rates ( i

238 Uor 232 Th).

=