Composition and origin of the Dewar geochemical anomaly

Samuel J. Lawrence; B. Ray Hawke; Jeffrey J. Gillis-Davis; G. Jeffrey Taylor; David J. Lawrence; Joshua T. Cahill; Justin Hagerty; Paul G. Lucey; Gregory A. Smith; Klaus Keil

doi:10.1029/2007JE002904

Composition and origin of the Dewar geochemical anomaly

Journal of Geophysical Research E: Planets

By: Samuel J. Lawrence, B. Ray Hawke, Jeffrey J. Gillis-Davis, G. Jeffrey Taylor, David J. Lawrence, Joshua T. Cahill, Justin Hagerty, Paul G. Lucey, Gregory A. Smith, and Klaus Keil

https://doi.org/10.1029/2007JE002904

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Abstract

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.

Additional publication details
Publication type	Article
Publication Subtype	Journal Article
Title	Composition and origin of the Dewar geochemical anomaly
Series title	Journal of Geophysical Research E: Planets
DOI	10.1029/2007JE002904
Volume	113
Issue	2
Year Published	2008
Language	English
Publisher	American Geophysical Union
Publisher location	Washington, D.C.
Contributing office(s)	Astrogeology Science Center
Description	13 p.
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