Composition and origin of the Dewar geochemical anomaly

doi:doi:10.1029/2007JE002904

Abstract

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Abstract

Composition and origin of the Dewar geochemical anomaly

Samuel J. Lawrence

Hawaii Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, Hawaii, USA

B. Ray Hawke

Hawaii Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, Hawaii, USA

Jeffrey J. Gillis-Davis

Hawaii Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, Hawaii, USA

G. Jeffrey Taylor

Hawaii Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, Hawaii, USA

David J. Lawrence

Los Alamos National Laboratory, Los Alamos, New Mexico, USA

Joshua T. Cahill

Hawaii Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, Hawaii, USA

Justin J. Hagerty

Los Alamos National Laboratory, Los Alamos, New Mexico, USA

Paul G. Lucey

Hawaii Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, Hawaii, USA

Gregory A. Smith

Hawaii Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, Hawaii, USA

Klaus Keil

Hawaii Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, Hawaii, USA

Dewar crater is a 50-km diameter impact structure located in the highlands northwest of the South Pole–Aitken basin on the lunar farside. A low-albedo area with enhanced Th and Sm values is centered east-northeast of Dewar crater. This area also exhibits elevated FeO abundances (9.0–16.6 wt %) and TiO₂ values (0.6–2 wt %). The range of FeO and TiO₂ abundances determined for the darkest portions of the geochemical anomaly overlap the range of FeO and TiO₂ values determined for nearside mare basalt deposits. Analysis of Clementine spectra obtained from the darkest portions of the Dewar geochemical anomaly indicates that the low-albedo materials contain large amounts of high-Ca clinopyroxene consistent with the presence of major amounts of mare basalt. Cryptomare deposits have played an important role in the formation of the Dewar geochemical anomaly. The evidence indicates that buried basalt, or cryptomare, was excavated from depth during impact events that formed dark-haloed craters in the region. We show that an early Imbrian- or Nectarian-age, low-TiO₂ mare basalt deposit with enhanced Th concentrations (6–7 μg/g) exists in the Dewar region. This ancient mare unit was buried by ejecta from Dewar crater, creating a cryptomare. Although most mare units on the central farside of the Moon exhibit low Th abundances, the enhanced Th values associated with the Dewar cryptomare deposit indicate that at least some portions of the underlying lunar interior (mantle and crust) on the farside of the Moon were not Th poor.

Received 12 February 2007; accepted 30 August 2007; published 2 February 2008.

Citation: Lawrence, S. J., B. R. Hawke, J. J. Gillis-Davis, G. J. Taylor, D. J. Lawrence, J. T. Cahill, J. J. Hagerty, P. G. Lucey, G. A. Smith, and K. Keil (2008), Composition and origin of the Dewar geochemical anomaly, J. Geophys. Res., 113, E02001, doi:10.1029/2007JE002904.

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