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The accumulation of non-formula elements in zircons during weathering: Ancient zircons from the Jack Hills, Western Australia
Swedish Museum of Natural History, Department of Geology. (Nordsim)ORCID iD: 0000-0003-2227-577X
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2019 (English)In: Chemical Geology, ISSN 0009-2541, E-ISSN 1872-6836, Vol. 530Article in journal (Refereed) Published
Abstract [en]

In this contribution we describe the influx of non-formula elements (Fe, Ca, Al, Y, U and Th) into fractures and selected zone lamellae in zircons from Jack Hills during recent weathering and discuss the effects of this on overlapping SIMS U-Th-Pb and oxygen isotope analyses. Previous research has recognised the importance of fractures in the generation of anomalous U-Th-Pb and oxygen isotope systems. In this report we show that besides fractures specific zones in euhedrally zoned zircon can act as pathways for the influx of weathering solutions and contain a similar range of trace element materials as do the fractures. Whereas zero-age discordant U-Pb systems of Jack Hills zircons have been explained by many authors in terms of Pb loss, present results confirm conclusions of our previous study that the main discordance mechanism of Jack Hills zircons is U-Th gain, due to overlap of SIMS analyses with mineralized fractures and zone lamellae with excess weathering-fluid-deposited U and Th. We explain the anomalously light and heavy oxygen isotopes and significant OH in SIMS analyses that overlap fractures and mineralized zones as due to the presence in the fractures of Ca, Fe, Al oxides and hydroxides with complexly fractionated oxygen isotopic systems. There is a suggestion in some of the elemental maps that there has been minor dispersion of trace elements away from fractures. But SIMS U-Th-Pb and oxygen isotope analyses on parts of the zircon away from fractures and mineralized zones show no evidence of interaction with weathering-fluid, indicating that penetration of weathering fluids into the body of the zircon at the location of the SIMS spots has not occurred. Results of this study have implications for other SIMS U-Th-Pb and oxygen isotope studies of zircons from rocks that have been subjected to weathering and also for early TIMS U-Pb measurements of bulk zircon samples that show zero Ma U-Pb discordance.

Place, publisher, year, edition, pages
2019. Vol. 530
Keywords [en]
Jack Hills, Weathering of zircons, Non-formula elements in zircon, Discordant U-Th-Pb ags, Oxygen isotopes and OH in zircon, Zircon fractures
National Category
Geochemistry
Research subject
The changing Earth
Identifiers
URN: urn:nbn:se:nrm:diva-3582OAI: oai:DiVA.org:nrm-3582DiVA, id: diva2:1375366
Funder
Knut and Alice Wallenberg Foundation, 2012.0097Available from: 2019-12-04 Created: 2019-12-04 Last updated: 2019-12-05Bibliographically approved

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