Page 324 - Inorganic Mass Spectrometry - Fundamentals and Applications

Multiple-Colle~tor ICP-MS 309

of
merely highlight three very different areas research that illustrate the power of
this method.

MC-ICP-MS opens up a variety of new fields relating to the basic ability to meas-
ure precisely isotopic compositions of elements with high first ionization poten-
tial at high sensitivity. A good example of this is found in the element tungsten
(W). The time scales over which inner solar system planets and planetesimals ac-
creted and differentiated are unclear because the isotopic systems of many mete-
orites are disturbed and the more robust long-lived chronometers (such as U-Pb)
on the order
provide inadequate temporal resolution. Radionuclides with half-lives
of 106-108 years can provide unparalleled insights into the earliest history of the
of
solar system and the nature the nucleosynthetic events that contributed material
to the molecular cloud that collapsed to form the solar nebula. Variations in iso-
topic abundances in daughter elements in many meteorites are a function of the
paren~daughter element ratio, the time at which the object formed, and the abun-
dance of the radionuclide at the start of the solar system (Reynolds, 1960; Lee et
al., 1977; Kelly and Wasserburg, 1978; Birck and All&gre, 1988).
= 9 Myr) is particularly powerful for
The 182Hf-182W chronometer (half-life
of
constraining the time scales accretion and metausilicate differentiation (such as
core formation) in planets and planetesimals (Harper al., 1991 a; Lee and Hall-
et
iday, 199Sb, 1996, 1997; Halliday et al., 1996) for the following reasons: Both Hf
and W are highly refractory elements and thus are expected to occur in average
solar system (= chondritic) proportions in much of the solar system. The effects of
the strong heating within the circumstellar disk from friction, accretion, or the T-
tauri stage of the sun, all of which may have driven off the volatile elements from
the inner solar system, should be negligible. Therefore, we can make reasonable
assumptions about the approximate paren~daughter refractory element ratios of
planets. However, Hf substitutes into silicates, whereas W prefers to substitute into
metals and metallic liquids. So the HfN ratio of silicate phases is much higher
than that of coexisting metals. If such segregation of metal from silicate occurred
during the lifetime of 182Hf, the isotopic abundance of 182W would eventually be
greater in the silicate with high HfN but be low in the metal with low HfN, rel-
ative to that found in undi~erentiated chondritic material. The mag~tude of such
an effect in terrestrial W can be used to place constraints on the age of the earth's
core (Lee and Halliday, 199Sb; Halliday et al., 1996; Harper and Jacobsen, 1996;
Jacobsen and Harper, 1996). In a similar manner, the W isotopic compositions of
meteorites thought to be derived from the asteroid belt, Martian meteorites, and
lunar samples can be used to constrain when their parent planets and planetesimals
accreted, melted, and differentiated into silicate/metallic portions (Halliday et al.,
1996; Lee and Halliday, 1996, 1997; Lee et al., 1997).