Neoproterozoic amorphous "ekanite" (Ca2Th0.9U0.1Si8O20) from Okkampitiya, Sri Lanka: A metamict gemstone with excellent lead-retention performance

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Authors

NASDALA Lutz CORFU Fernando BLAIMAUER Doris CHANMUANG Chutimun RUSCHEL Katja ŠKODA Radek WILDNER Manfred WIRTH Richard ZEUG Manuela ZOYSA E. Gamini

Year of publication 2017
Type Article in Periodical
Magazine / Source Geology
MU Faculty or unit

Faculty of Science

Citation
Doi http://dx.doi.org/10.1130/G39334.1
Keywords PB ZIRCON AGES; RADIATION-DAMAGE; ALTERED ZIRCON; GEOCHRONOLOGY; DISSOLUTION; METAMORPHISM; MICROPROBE; ACCURACY; URANIUM; SYSTEMS
Description A rare gemstone form of "ekanite" from Okkampitiya, eastern Sri Lanka, has a chemical composition of Ca2Th0.9U0.1Si8O20, corresponding to that of the tetragonal mineral ekanite. The Okkampitiya material, however, has undergone amorphization and is found in a fully metamict state, which is ascribed to the long-term accumulation of high levels of self-irradiation damage (1.38 x 10(20) alpha decay events per gram). The "ekanite" nevertheless does not show any post-growth chemical alteration, and the radiogenic Pb has been retained. The Okkampitiya "ekanite" yielded a Neoproterozoic Pb-207/Pb-206 age of 562.1 +/- 0.8 Ma (uncertainty quoted at the 95% confidence level) that is concordant within the uncertainties of decay constants. The remarkable Pb-retention performance of "ekanite" contradicts the common hypothesis that metamictization in minerals results in U-Pb discordance. The exceptional chemical durability of the heavily radiation-damaged geological material described here has also implications for materials-science research. High radiation resistance, a key criterion in the search for advanced nuclear waste forms, may not be a prerequisite for high aqueous durability of a solid.
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