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The Mafic Component of the Lunar Crust
| Content Provider | Semantic Scholar |
|---|---|
| Author | Crites, Sarah T. Lucey, Paul G. Norman, Jessica A. Taylor, G. Jeffrey Hawke, Bernard Ray Lemelin, Myriam |
| Copyright Year | 2014 |
| Abstract | Introduction: The lunar magma ocean hypothesis is well supported by both samples (e.g. [1][2][3]) and remote sensing (e.g. [4][5][6][7][8]). However, remote sensing observations by the Lunar Prospector gamma ray spectrometer [5] reveal that the typical lunar highlands surface contains 4-5 wt% FeO, equivalent to 15% or more mafic minerals, more mafic than strictly defined anorthosites. This raises the question of the origin of the mafic component in the lunar highlands crust. We offer three hypotheses for the surface iron enrichment: 1) pure lunar anorthosites are contaminated by impact gardening of local mafic igneous intrusions and ancient volcanism [9]; 2) the highlands regolith is dominated globally by ejecta of very large basins, which includes mafic lower crust or upper mantle material [10] or 3) the lunar anorthosites themselves are inherently more mafic than the strict definition as suggested by Warren [2], with pure plagioclase detections the exception and not the rule. We use new global mineral mineral maps based on Clementine spectra [11] and reconciled with Lunar Prospector neutron and gamma ray data as inputs to mixing models to examine the plausibility of the three sources of mafic material. We use a global spectral survey of of 4506 immature craters with diameters less than 1 km using near-IR data from the Kaguya Spectral Profiler as a constraint on mantle composition assuming the uppermost regolith sampled by these small craters is dominated by large basin ejecta [12]. Methods: We calculated 19 mixing models assuming different combinations of post-magma ocean igneous activity, mantle material excavated by large basins, and inherent mafic content of magma ocean anorthosites to account for the mafic minerals present in our Clementine and Lunar Prospector-based mineral maps. Table 1 summarizes the values for the variables used in our models: anorthosite mafic content; the maximum amount of mantle material excavated; and the presence of clinopyroxene in the mantle. For each combination of variables we used the mineral abundances to calculate the distribution of lunar rock types representative of each mafic source: anorthosite, the major crustal magma ocean product; norite, troctolite, and gabbro or mare basalt, used to represent post-magma ocean igneous intrusive or extrusive volcanism and mare basalt contamination; and dunite and pyroxenite, ultramafic rock types representing the lunar mantle. We followed the method of Spudis [13] to calculate the total volume of ejecta and the proportion of this ejecta derived from the lunar mantle for each of the 42 largest lunar basins identified by [13] as well as the South Pole-Aitken Basin (SPA). In this method the excavation cavity is modeled as a sphere intersecting the sphere of the Moon, with the circle of intersection between the two spheres defined by the transient crater diameter. The depth excavated, or the depth to diameter ratio, can be changed by varying the radius of the excavating sphere and the distance of its center from the center of the Moon. Our initial calculations used a depth to diameter ratio of 1/10 [13] and using crustal thicknesses of 34 and 43 km [14] we obtained estimates of 30 to 40 vol% mantle that could be present in the uppermost regolith. Results and implications: The results of our mix- |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | https://www.hou.usra.edu/meetings/lpsc2014/pdf/2126.pdf |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
