XENOLITH COLLECTIONS AT THE SMITHSONIAN: SCOPE, HISTORY, AND OUTLOOK FOR RESEARCH

The top three reasons for curating museum collections of xenoliths are: 1) Most mantle and crustal xenoliths arrive at Earth’s surface due to unusual, even rare, geologic phenomena; 2) thoroughly harvested xenolith localities do not regenerate; and 3) the samples contain information about the lower crust and mantle that cannot be obtained from any other Earth materials. Xenolith localities have vanished due to land development, remediation of mine sites, and former crusher-reject dumps being processed to recover eclogitic and peridotitic diamonds. Some xenoliths are found with diamonds, which are mined under restricted access. Diamonds are of longstanding geological as well as economic interest. Indeed, collections of “ultramafic nodules” and their host “diamond conglomerates” at the Smithsonian’s National Museum of Natural History date to the late 19th Century. A major 1904 donation by G.F. Williams preserves the rock types of the De Beers mines at Kimberley. The Williams Collection became the nucleus of the Museum’s Ultramafic Xenolith collections. These now consist of approximately 25,000 specimens from 22 countries in the Americas, Europe, Africa, Asia and Australia. Xenocrysts, megacrysts, and host rocks are grouped with the xenoliths. Most have been studied with results published prior to donation. The donor’s numbering system is preserved, so that the geochemical and petrologic literature can be used to request specimens. Individual donor contributions are kept together; major components include the career research collections of E.D. Jackson, H.O.A. Meyer, H.G. Wilshire, B.C. Hearn, Jr., H.S. Yoder and F.R. Boyd. The collection offers unparalleled opportunities for broad-based comparative studies of single rock types, mineral associations, xenolith provinces, deformation mechanisms, magma sources, and continental evolution. Specimens from massif-type peridotite bodies also allow for comparisons relating to emplacement mechanisms. Previous data can guide users who wish to establish new geochemical methods; for example, radiometric dating and isotopic tracer methods have been tested using the Museum’s Rock and Ore Collections. The Museum’s catalog is now in an Emu database system, which means that soon, users will be able to browse these collections via the internet.