Physics
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p51c1441o&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #P51C-1441
Physics
[3672] Mineralogy And Petrology / Planetary Mineralogy And Petrology, [5455] Planetary Sciences: Solid Surface Planets / Origin And Evolution, [5464] Planetary Sciences: Solid Surface Planets / Remote Sensing, [6250] Planetary Sciences: Solar System Objects / Moon
Scientific paper
The magma ocean hypothesis has been the most widely accepted mechanism explaining the generation of the lunar highland crust. This hypothesis is based on analyses of returned samples [1] and an assumption that Fe-bearing, plagioclase-rich rocks exist globally as the major component of the lunar crust. However, no crystalline plagioclase had been detected by remote sensing before SELENE [2], except for some ambiguous or indirect indications of the existence of plagioclase. Subsequently, a global distribution of rocks of extremely high plagioclase abundance (approaching 100 vol%; called purest anorthosite (PAN)) was reported using an unambiguous plagioclase absorption band around 1250 nm found by the SELENE Multiband Imager (MI) [3]. The estimated plagioclase abundance is significantly higher than previous estimates of 82 to 92 vol% [1], providing a valuable constraint on models for lunar magma ocean evolution. Further study using continuous reflectance spectra derived by the SELENE Spectral Profiler (SP) [4] revealed a global and common distribution of the PAN over the entire lunar surface, supporting the high abundance of PAN rocks within the upper crust. In this study, we investigated a vertical compositional (modal abundance and/or mineral composition) trend of the PAN rocks within the crust using their reflectance spectra derived from SP and MI. Knowing the compositional trend of the lunar upper crust may enable us to understand the mechanism of the lunar crustal growth. All of the SP data observed throughout SELENE mission periods were used in this study (about 7,000 orbits and roughly 10,000 spectra for each orbit). The absorption depth at each wavelength was calculated after a linear continuum was removed. Spectra with the deepest absorption depth, around 1250 nm, which is caused by a minor amount of Fe2+ (in the order of 0.1 wt% FeO) contained in the plagioclase, were selected to detect the PAN rocks. The original burial depth of each PAN rock outcrop was estimated from a crater scaling law using the crater diameter of each outcrop observed in MI data. Results indicate that the majority of the derived absorption depths (strengths) of the detected PAN rock spectra around 1250 nm appear to form a trend which increases as their estimated original burial depths increase within the crust (the trend is observed up to 30 km of the original burial depth). Although understanding the actual cause of this trend requires further studies, such a trend may indicate a decrease in the mafic mineral abundance within the already very mafic-poor rock and/or an increase in the Fe2+ content of plagioclase with depth. References: [1] Warren P. H. (1990) Am. Mineral., 75, 46-58. [2] Matsunaga T. et al. (2008) Geophys. Res. Let., 35, L23201, doi:10.1029/2008GL035868. [3] Ohtake M. et al. (2009) Nature, 461, doi:10.1038. [4] Ohtake et al. (2010) Lunar Planet. Sci. Conf. XXXXI, 1628.
Haruyama Junji
Hiroi Takahiro
Matsunaga Takahiro
Moroda T.
Nakamura Riou
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