A significant development of the past 4 years has been the renewed effort at determination of Os isotope ratios. Mass spectrometry problems with Os isotopic analysis were largely overcome by development of a new analytical technique [ Creaser et al., 1991], so that very small quantities of Os can be accurately analyzed with relatively minor modification to a conventional mass spectrometer.
Studies using the older analytical techniques had already revealed
much about Os isotope systematics. It appears that the bulk silicate
Earth has a 
Os/
Os of about 1.05--1.10 [e.g., Martin,
1991]. This value is within the range of chondritic meteorites,
a somewhat surprising result since Re and Os have very
different silicate/metal partition coefficients. The depleted
mantle appears to have 
Os/
Os somewhat lower than this,
and oceanic island basalts somewhat higher 
Os/
Os,
consistent with other isotope systematics. Os studies using the
new analytical technique are now revealing the details of Os
isotope variability in the mantle. Os isotope ratios from the
Austral Islands, which have HIMU characteristics, have high and
uniform 
Os/
Os (1.1--1.3) [ Hauri and Hart, 1993;
Reisberg et al., 1993], while Samoa and Tahaa (both EM II) fall
in the range of 1.03--1.16. Samples from Kerguelen and Pitcairn (both EM
I) have 
Os/
Os of 1.36--1.54. Hauri and Hart
[1993] and Reisberg et al. [1993] concluded these results
were consistent with plumes containing a component of
recycled material.
Reisberg et al. [1993], however, raised a warning about
Os isotope ratios in oceanic island basalts. Most basalts from St.
Helena and the Comores (both HIMU type islands) had 
Os/
Os
in the same range as the Australs, but some samples have very much
higher 
Os/
Os, up to 1.95. Reisberg et al.
[1993] suggested these high ratios may reflect assimilation of
Os-rich sediment, such as Mn oxides, during magma ascent. Because
Os concentrations are so low, small amounts of assimilation could shift
the 
Os/
Os ratio without having a detectable effect on
Sr, Nd, and Pb isotope ratios.