The geochemical community has come a long way with respect to the measure- ment and interpretation of siderophile elements in cosmo- and geo-chemical materials. In my 30 years of working with the Re-Os isotope system, I have seen
the precision in the measurement of 187Os/188Os and 186Os/188Os ratios decrease
from about 1 % to about 10 parts per million. The improvements have paved
the way for high precision geochronology, as well as the use of 187Os and 186Os
as sensitive tracers of mantle processes. Similarly, during the past 20 years the Hf-W isotope system has been refined to such an extent that differences in the core segregation ages of asteroids of less than 1 Ma can now be resolved. These are truly exciting times to be working with siderophile elements!
My perspective of what is exciting and important will no doubt differ from those of most other proponents of these elements. I have not been able to cover many prominent topics that involve siderophile elements. For example, until now, I have not mentioned that one of the most famous applications of siderophile elements was the discovery of the Ir anomaly (actually an HSE anomaly) present in deposts resulting from a sizeable impact at the Cretaceous-Tertiary boundary. This discovery led to the conclusion that the major extinction event at this time was caused by an impact, most likely the Chixulub impactor. Nor have I discussed the exciting application of Re-Os dating of inclusions contained within diamonds. I have also only briefly mentioned siderophile elements in komatiites, although at UMd, we devote considerable attention to these rocks, and provide very little overview to applications of siderophile elements to the study of magmatic ores, an area of intense research. To fans of these topics, I apologise.
Here, I have also virtually ignored the application of siderophile elements to low temperature systems due to space and a lack of expertise, not because I believe these applications are unimportant or uninteresting. For example, in the last 15 years we have seen a huge increase in the number of applications of the Re-Os system for dating organic-rich sediments, and even the timing of oil formation. This is an exciting and very practical application of the system. Stable isotopes of Mo are now commonly used to study the redox conditions of aqueous systems at the surface of the Earth. The absolute and relative abundances of siderophile elements in loess and glacial diamictites are valuable tools for studying the chemical evolution and weathering history of the continental crust. However, these and other topics will have to wait for another author to address
in a future Geochemical Perspectives article.
There is much left to do. Here are five examples of “important topics” that I believe bear significant further study:
1. The application of siderophile nucleosynthetic tracers, such as Mo and Ru, to study the genetics of late stages of planetary accretion has only begun. It is logical that these tracers should be applied to terrestrial
and lunar impact sites, to fingerprint the impactors, particularly those impactors involved in what some believe to be a late heavy bombard- ment stage of the Earth and Moon. Further, the Mo and Ru isotopic compositions of Mars, Vesta and the angrite parent body, among other inner solar system bodies, are either unknown or insufficiently well constrained. Hence, it remains important to determine how well these siderophile elements were mixed in the inner solar system by the time of large body formation.
2. The variability in the 182W isotopic composition of terrestrial mantle-
derived samples is an exciting discovery, yet the causes for the hetero- geneities remain poorly constrained. Very few post-Archean rocks have
been analysed to-date, so it remains unknown how long 182W-enriched
domains within the mantle remained isolated from the ambient mantle. High-precision re-measurement of the W isotopic compositions of potential plume-derived materials that may arise from the core-mantle boundary is also warranted. However, identifying the most appropriate materials for this type of study remains a challenge.
3. Estimates for the abundances of siderophile elements in the BSE will require constant updating and re-assessment as the database for these elements continues to grow.
4. Determination of how siderophile elements are distributed in the mantle. Are they homogeneously distributed on a global scale at present, and were they homogeneously distributed in the mantle as far back in time as we can look?
5. Although we know that siderophile elements prefer metal to silicate, we don’t well know the rates of exchange of these elements between metal and silicate that can occur during core formation episodes. Thus, it remains problematic to model the passage of metal through silicate, as must occur during the giant impact stage of planetary growth. Perhaps even more important, we don’t know how rapid isotopic exchange may be relative to elemental exchange. This needs to be much better constrained.