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dc.contributor.authorGrange, Marion
dc.contributor.authorNemchin, Alexander
dc.contributor.authorPidgeon, Robert
dc.contributor.authorTimms, Nicholas Eric
dc.contributor.authorMuhling, J.
dc.contributor.authorKennedy, Allen
dc.identifier.citationGrange, Marion and Nemchin, Alexander and Pidgeon, Robert and Timms, Nicholas and Muhling, Janet and Kennedy, Allen. 2009. Thermal history recorded by the Apollo 17 impact melt breccia 73217. Geochimica et Cosmochimica Acta. 73 (10): pp. 3093-3107.

Lunar breccia 73217 is composed of plagioclase and pyroxene clasts originating from a single gabbronorite intrusion, mixed with a silica-rich glass interpreted to represent an impact melt. A study of accessory minerals in a thin section from this breccia (73217,52) identified three different types of zircon and anhedral grains of apatite which represent distinct generations of accessory phases and provide a unique opportunity to investigate the thermal history of the sample. Equant, anhedral zircon grains that probably formed in the gabbronorite, referred to as type-1, have consistent U?Pb ages of 4332 7 Ma. A similar age of 4335 5 Ma was obtained from acicular zircon (type-2) grains interpreted to have formed from impact melt. A polycrystalline zircon aggregate (type-3) occurs as a rim around a baddeleyite grain and has a much younger age of 3929 10 Ma, similar to the 3936 17 Ma age of apatite grains found in the thin section. A combined apatite-type-3 zircon age of 3934 12 Ma is proposed as the age of the Serenitatis impact event and associated thermal pulse. X-ray mapping andelectron probe analyses showed that Ti is inhomogeneous in the zircon grains on the sub-micrometer scale. However, model temperatures estimated from SHRIMP analyses of Ti-concentration in the 10 lm diameter spots on the polished surfaces of type-1 and type-2 zircons range between about 1300 and 900 C respectively, whereas Ti-concentrations determined for the type-3 zircon are higher at about 1400?1500 C. A combination of U?Pb ages, Ti-concentration data and detailed imaging and petrographic studies of the zircon grains shows that the gabbronorite parent of the zircon clasts formed shortly before the 4335 5 Ma impact, which mixed the clasts and the felsic melt and projected the sample closer to the surface where fast cooling resulted in the crystallization of acicular zircon (type-2). The 3934 12 Ma Serenitatis event resulted in partial remelting of the glass and formation of polycrystalline zircon (type-3). This event also reset the U?Pb system of apatite, formed merrillite coronas around some apatite grains, and probably re-equilibrated some pyroxenes in the clasts. Although there have been arguments for pre-3.9 Ga impacts based on other types of samples, the age of the acicular zircon at 4335 5 Ma provides the first evidence of impact melt significantly predating the lunar cataclysm. Our data, combined with other chronological results, demonstrate the occurrence of pre-3.9 Ga impacts on the Moon and suggest that the lunar impact history consisted of a series of intense bombardment episodes interspersed with relatively calm periods of low impact flux.

dc.publisherPergamon-Elsevier Science Ltd
dc.subjectLunar breccia
dc.subjectzircon U-Pb ages
dc.subjectSerenitatis impact event
dc.subjectthermal history
dc.titleThermal history recorded by the Apollo 17 impact melt breccia 73217
dc.typeJournal Article
dcterms.source.titleGeochimica et Cosmochimica Acta

The link to the journal’s home page is: Copyright © 2009 Elsevier B.V. All rights reserved

curtin.accessStatusOpen access
curtin.facultyDepartment of Applied Geology
curtin.facultyFaculty of Science and Engineering
curtin.facultyWA School of Mines

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