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Behaviour of radiogenic Pb in zircon during ultrahigh‑temperature metamorphism: an ion imaging and ion tomography case study from the Kerala Khondalite Belt,southern India
Naturhistoriska riksmuseet, Enheten för geovetenskap. (Nordsim)ORCID-id: 0000-0003-2227-577X
2014 (engelsk)Inngår i: Contributions to Mineralogy and Petrology, ISSN 0010-7999, E-ISSN 1432-0967, Vol. 168Artikkel i tidsskrift (Fagfellevurdert) Published
Abstract [en]

Zircon crystals from a locally charnockitizedPaleoproterozoic high-K metagranite from the KeralaKhondalite Belt (KKB) of southern India have been investigatedby high-spatial resolution secondary ion mass spectrometryanalysis of U–Th–Pb and rare earth elements(REE), together with scanning ion imaging and scanningion tomography (depth-profiled ion imaging). The spotanalyses constrain the magmatic crystallization age of themetagranite to ca. 1,850 Ma, with ultrahigh-temperature(UHT) metamorphism occurring at ca. 570 Ma and superimposedcharnockite formation at ca. 520–510 Ma, whilethe ion imaging reveals a patchy distribution of radiogenicPb throughout the zircon cores. Middle- to heavy-REEdepletion in ca. 570 Ma zircon rims suggests that thesegrew in equilibrium with garnet and therefore date theUHT metamorphism in the KKB. The maximum apparent207Pb/206Pb age obtained from the unsupported radiogenicPb concentrations is also consistent with formationof the Pb patches during this event. The superimposed charnockitization event appears to have caused additionalPb-loss in the cores and recrystallization of the rims. Theresults of depth-profiling of the scanning ion tomographyimage stack show that the Pb-rich domains range in sizefrom <5 nm to several 10 nm (diameter if assumed to bespherical). The occurrence of such patchy Pb has previouslybeen documented only from UHT metamorphiczircon, where it likely results from annealing of radiation-damaged zircon. The formation of a discrete, heterogeneouslydistributed and subsequently immobile Pb phaseeffectively arrests the normal Pb-loss process seen at lowergrades of metamorphism.

sted, utgiver, år, opplag, sider
2014. Vol. 168
Emneord [en]
Secondary ion mass spectrometry · Zircon · Ion imaging · Ion tomography · Ultrahigh-temperature metamorphism · Kerala Khondalite Belt
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URN: urn:nbn:se:nrm:diva-824DOI: 10.1007/s00410-014-1042-2OAI: oai:DiVA.org:nrm-824DiVA, id: diva2:757021
Forskningsfinansiär
Swedish Research Council, G5103-20006231/2000Tilgjengelig fra: 2014-10-20 Laget: 2014-10-20 Sist oppdatert: 2017-12-05bibliografisk kontrollert

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