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Imbrium provenance for the Apollo 16 Descartes terrain: Argon ages and geochemistry of lunar breccias 67016 and 67455

Norman, Marc; Duncan, Robert A.; Huard, John J.

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In order to improve our understanding of impact history and surface geology on the Moon, we obtained40Ar-39Ar incremental heating age data and major + trace element compositions of anorthositic and melt breccia clasts from Apollo 16 feldspathic fragmental breccias 67016 and 67455. These breccias represent the Descartes terrain, a regional unit often proposed to be ejecta from the nearby Nectaris basin. The goal of this work is to better constrain the emplacement age and provenance of the...[Show more]

dc.contributor.authorNorman, Marc
dc.contributor.authorDuncan, Robert A.
dc.contributor.authorHuard, John J.
dc.date.accessioned2015-12-10T21:56:47Z
dc.identifier.issn1872-9533
dc.identifier.urihttp://hdl.handle.net/1885/39599
dc.description.abstractIn order to improve our understanding of impact history and surface geology on the Moon, we obtained40Ar-39Ar incremental heating age data and major + trace element compositions of anorthositic and melt breccia clasts from Apollo 16 feldspathic fragmental breccias 67016 and 67455. These breccias represent the Descartes terrain, a regional unit often proposed to be ejecta from the nearby Nectaris basin. The goal of this work is to better constrain the emplacement age and provenance of the Descartes breccias. Four anorthositic clasts from 67016 yielded well-defined40Ar-39Ar plateau ages ranging from 3842 ± 19 to 3875 ± 20 Ma. Replicate analyses of these clasts all agree within measurement error, with only slight evidence for either inheritance or younger disturbance. In contrast, fragment-laden melt breccia clasts from 67016 yielded apparent plateau ages of 4.0-4.2 Ga with indications of even older material (to 4.5 Ga) in the high-T fractions. Argon release spectra of the 67455 clasts are more variable with evidence for reheating at 2.0-2.5 Ga. We obtained plateau ages of 3801 ± 29 to 4012 ± 21 Ma for three anorthositic clasts, and 3987 ± 21 Ma for one melt breccia clast. The anorthositic clasts from these breccias and fragments extracted from North Ray crater regolith (Maurer et al., 1978) define a combined age of 3866 ± 9 Ma, which we interpret as the assembly age of the feldspathic fragmental breccia unit sampled at North Ray crater. Systematic variations in diagnostic trace element ratios (Sr/Ba, Ti/Sm, Sc/Sm) with incompatible element abundances show that ferroan anorthositic rocks and KREEP-bearing lithologies contributed to the clast population. The Descartes breccias likely were deposited as a coherent lithologic unit in a single event. Their regional distribution suggests emplacement as basin ejecta. An assembly age of 3866 ± 9 Ma would be identical with the accepted age of the Imbrium basin, and trace element compositions are consistent with a provenance in the Procellarum-KREEP Terrane. The combination of age and provenance constraints points toward deposition of the Descartes breccias as ejecta from the Imbrium basin rather than Nectaris. Diffusion modeling shows that the older apparent plateau ages of the melt brecia clasts plausibly result from incomplete degassing of ancient crust during emplacement of the Descartes breccias. Heating steps in the melt breccia clasts that approach the primary crystallization ages of lunar anorthosites show that earlier impact events did not completely outgas the upper crust.
dc.publisherPergamon-Elsevier Ltd
dc.sourceGeochimica et Cosmochimica Acta
dc.subjectKeywords: breccia; chemical composition; degassing; emplacement; lunar crust; provenance; trace element
dc.titleImbrium provenance for the Apollo 16 Descartes terrain: Argon ages and geochemistry of lunar breccias 67016 and 67455
dc.typeJournal article
local.description.notesImported from ARIES
local.identifier.citationvolume74
dc.date.issued2010
local.identifier.absfor040303 - Geochronology
local.identifier.ariespublicationu4027924xPUB180
local.type.statusPublished Version
local.contributor.affiliationNorman, Marc, College of Physical and Mathematical Sciences, ANU
local.contributor.affiliationDuncan, Robert A., Oregon State University
local.contributor.affiliationHuard, John J., Oregon State University
local.description.embargo2037-12-31
local.bibliographicCitation.startpage763
local.bibliographicCitation.lastpage783
local.identifier.doi10.1016/j.gca.2009.10.024
local.identifier.absseo970104 - Expanding Knowledge in the Earth Sciences
dc.date.updated2016-02-24T10:28:34Z
local.identifier.scopusID2-s2.0-70649104689
local.identifier.thomsonID000273325500024
CollectionsANU Research Publications

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