2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 13
Presentation Time: 11:15 AM

The Mineralogy of Deeply Excavating Lunar Impact Crater Central Peaks

CAHILL, Joshua T.S.1, WIECZOREK, Mark A.2, LUCEY, Paul G.1 and SHEARER, Charles K.3, (1)Hawaii Institute of Geophysics and Planetology, 1680 East-West Rd., P.O.S.T. 602b, Honolulu, HI 96822, (2)Institut de Physique du Globe de Paris, 4 Avenue de Neptune, Saint Maur des Fosses Cedex, Paris, 94107, France, (3)Institute of Meteoritics, Univ of New Mexico, Albuquerque, NM 87131, jcahill@higp.hawaii.edu

We present model mineralogy of impact crater central peaks quantitatively determined to have excavated material from deep within the lunar crust. We use multiple quantitative estimates of crustal thickness and crater excavation depth to infer craters with the potential to tap the lower lunar crust and perhaps the mantle. These criteria combined with the presence of a central peak and spectrally immature material cull the number of craters to less than thirty. These remaining peaks are almost exclusively located within South-Pole Aitken basin. A radiative transfer model is used to analyze Clementine visible and visible + near-infrared data sets to place compositional constrains on the material excavated and present on these peaks. In general, peak mineralogy becomes more plagioclase-rich (>60%) as the thickness of the crust increases (>25 km), and material becomes increasing mafic with proximity to the lunar crust-mantle interface. Analyses of visible reflectance spectra show a wide range of clinopyroxene abundances (1-73%) near the calculated boundary. Analyses of visible + near-infrared spectra, calibrated to Earth-based telescopic spectra, show less clinopyroxene (0-41%) but an increase in the abundance of orthopyroxene (4-41%) and olivine (3-46%) within 20 km of the crust-mantle interface. Despite the general increase in mafic mineralogy with depth, several peaks show plagioclase-rich material excavated from significant depth as well. These findings are inconsistent with the accepted vision of a lunar magma ocean induce gradient in feldspar-mafic ratio moving vertically in the crust. Nearly pure anorthosite is not expected near the crust-mantle boundary. These findings are consistent with Spudis et al. [1984] and Hawke et al. [2003] which identified anorthosite in impact basin inner rings and peaks of Orientale, Grimaldi, Humorum, and Nectaris. This suggests anorthosite, though prevalent throughout the upper crust, may also be in stratigraphic layer or pluton form deep in the lunar crust as well.