Massive layer of pure anorthosite on the Moon

Abstract

[1] We present a new global survey of the purest anorthosite (PAN) rock using the Spectral Profiler onboard Kaguya. We found that PAN rocks are widely distributed over the Moon, including the Feldspathic Highland Terrain and the south and north polar regions. All PAN sites are associated with huge impact structures with diameters larger than 100 km. Based on the global distributions of PAN and olivine-rich sites, we propose the existence of a massive PAN layer with a thickness of $50 km below an uppermost mafic-rich mixed layer with a thickness of $10 km. Below the PAN layer, a lower crustal layer with olivine-rich materials may be present on the nearside, but not on the far side of the Moon. The existence of a PAN layer with a thickness of $50 km suggests an Al 2 O 3 abundance of 33 to 34 wt.% in the lunar crust, which is higher than previous estimates of <32 wt.%. Our data indicate the massive production event of PAN during the early stage of the formation of the Moon, supporting the lunar magma ocean scenario.

The formation of peak rings in large impact craters

“…One of the principal observations used to support a version of the nested melt-cavity hypothesis in Baker et al (7) is that peak rings within basins of all sizes on the Moon contain abundant crystalline anorthosite and must, therefore, originate from the upper crust if indeed the lower crust is noritic. Our results suggest a deeper origin for peak-ring rocks, and thus are more in accordance with alternative models for the composition of a heterogeneous lunar crust in which an anorthositic layer extends regionally to deeper depths (40)(41). The dynamic collapse model and Expedition 364 results predict density reduction by shock and shear fracturing within the uplifted material (33), which is consistent with the recent Gravity…”

The formation of peak-ring basins: Working hypotheses and path forward in using observations to constrain models of impact-basin formation

“…This scenario also implies that peak rings should come from relatively shallow crustal levels, controlled by the geometry of the excavation and melting cavities. These predictions on stratigraphic uplift are consistent with the abundance of pure crystalline anorthosite in many of the peak rings of basins on the Moon (Hawke et al, 2003;Yamamoto et al, 2012;Cheek et al, 2013;Donaldson Hanna et al, 2014;Baker and Head, 2015). However, the hypothesized regions of stratigraphic uplift are distinct from the estimated stratigraphic uplift of central peaks in complex craters, which are commonly inferred to be constrained by the maximum depth of melting (Cintala and Grieve, 1998a,b;Tompkins and Pieters, 1999).…”

“…Furthermore, observations of large outcrops of pure crystalline anorthosite within the lunar peak rings at all sizes (e.g., Hawke et al, 2003;Cheek et al, 2013;Donaldson Hanna et al, 2014;Baker and Head, 2015) suggest that peak rings may be composed of materials uplifted from the upper crust (as opposed to the more noritic lower crust; Wieczorek and Phillips, 1997;Hawke et al, 2003). Alternatively, these results could imply an anorthositic crust to great depth (Yamamoto et al, 2012;Cheek et al, 2013;Donaldson Hanna et al, 2014), in which case lower crustal materials may also be exposed within the inner rings of impact basins (e.g., Kring et al, 2013;Potter et al, 2013a). …”

“…Peak rings on the Moon typically show outcrops of predominantly pure crystalline anorthosite and more noritic lithologies (Hawke et al, 2003;Whitten et al, 2011;Yamamoto et al, 2012;Cheek et al, 2013;Donaldson Hanna et al, 2014;Baker and Head, 2015). An exception is in regions of thinned crust such as the interior of the South Pole-Aitken basin.…”

Spectral mapping of morphological features on the moon with MGM and SAM

“…Reflectance spectra of plagioclase feldspar commonly exhibit an absorption band centered between 1.1 m and 1.3 m that is produced by electronic transitions in Fe 2+ in the plagioclase crystal structure (Adams and Goullaud, 1978;Conel and Nash, 1970). The purest anorthosite (PAN) which consists of over 98 vol.% plagioclase has been observed across the lunar surface including, South Ray, Tycho, Tsiolkovsky, Orientale basin (Ohtake et al, 2009;Cheek et al, 2013), across the FHT and south and north polar regions (Yamamoto et al, 2012).…”