Mathematics – Logic
Scientific paper
Dec 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufm.p23a0232c&link_type=abstract
American Geophysical Union, Fall Meeting 2004, abstract #P23A-0232
Mathematics
Logic
5410 Composition, 5464 Remote Sensing, 5494 Instruments And Techniques
Scientific paper
The global mapping of the lunar surface using the petrological parameter Mg-number (Mg*) was undertaken because Mg*, or the ratio of Mg to the sum of Mg and Fe on an atomic basis, is an important disciminator in defining and understanding lunar rocks. The dominant lunar rock types, ferroan anorthosites (FAN), high-magnisium suite (HMS), and high-alkali suite (HAS) rocks all vary in Mg* depending upon the petrologic scenario that formed them. Of particular interest are FAN mineralogy and chemistry, which varies from high-Mg# (~70) troctolites to low-Mg# norites (~50) and for some time has been considered to represent a single magma frac-tionation trend. However, recent studies have also shown that the crystallization of FAN rocks may have been more complicated than originally thought. James et al. [1] found that instead of one simple fractionation trend for ferroan anorthosites, there may have been four. Studies by [2], [3], and [4] using Apollo and lunar meteorites for analysis have also eluded to the possibility that FAN rocks may have evolved from a more complex source or process. Therefore a global assessment of lithologies and corresponding Mg* is of great value for lunar petrology. In a remote sensing context, Mg* is the most important control on the spectral properties of lunar mafic silicates. For stoichiometric orthopyroxene and olivine, Mg* is mathematically linked to the Fe content that controls the overall reflectance and intensity of absorption. The changes in band centers and shape that accompany the structural changes as Fe substitutes for Mg along the solid solution series have long been recognized; these changes are highly correlated with Mg*. In clinopyroxene, the strong effect of Ca on structure makes this element important, but Mg* has the dominant effect on reflectance and a comparable effect on spectral shape. In this study, the lunar surface is quantitatively mapped using a theoretical treatment of mineralogic spectra and the effect of environmental space exposure on the optical properties of these minerals. This was accomplished using Clementine ultraviolet and visible (UVVIS) data and a Hapke radiative transfer mixing model. The major features evident in these maps are the strong distinction between mare and highland regions, the former showing low Mg* and the latter generally higher; a large northern highlands unit with low Mg*, and an Mg* high north of South Pole-Aitken basin. Mare units are not universally low, mare Frigoris in particular has elevated Mg* relative to other mare. The strongest variations in the highlands occur in plagioclase rich, low FeO units, that exhibit values ranging near 50 to near 100 in coherent units. The craters Tycho and Aristarchus also exhibit high Mg*; these gabbroic anomalies may indicate more extensive Mg-rich material at depth. Deposits within SPA are unremarkable relative to surroundings, and share the intermediate Mg* of most of the highlands. 1. James, O.B. et al. PLPSC. 1989.; 2.Bersch, M.G., et al., GRL, 1991.; 3. Floss, C., et al., GCA, 1998.; 4. Korotev, R.L., et al., GCA, 2003.
Cahill Joshua T.
Gillis Jeffery J.
Lucey Paul G.
Steutel Donovan
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