Skip to main content
U.S. flag

An official website of the United States government

Composition and origin of the Dewar geochemical anomaly

January 1, 2008

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.

Citation Information

Publication Year 2008
Title Composition and origin of the Dewar geochemical anomaly
DOI 10.1029/2007JE002904
Authors Samuel J. Lawrence, B. Ray Hawke, Jeffrey J. Gillis-Davis, G. Jeffrey Taylor, David J. Lawrence, Joshua T. Cahill, Justin J. Hagerty, Paul G. Lucey, Gregory A. Smith, Klaus Keil
Publication Type Article
Publication Subtype Journal Article
Series Title Journal of Geophysical Research E: Planets
Index ID 70031856
Record Source USGS Publications Warehouse
USGS Organization Astrogeology Science Center