A CONTINENTAL-SCALE PROCELLARUM IMPACT’S POTENTIAL RELEVANCE TO MANY UNRESOLVED ISSUES OF LUNAR AND TERRESTRIAL HISTORY

The Procellarum Basin is approximately 3200 km in diameter and consists of a broad area of lunar crust as thin as about 25 km. To the west, between Procellarum and another continental scale basin, South Pole-Aitken, there exists a region of lunar crust as thick as about 60 km. In addition to explaining this contrast in crustal thicknesses (Wieczorek, et al, 2013), if Procellarum were formed by a very large impact, a number of unresolved issues in lunar and terrestrial histories may be resolved. With respect to the Moon, these issues include, overturn of the lunar mantle (Wang, et al., 2015; Schmitt, 2016), origin of PKT (Schmitt, 2003), concentration of Mg-Suite Ages at about 4.35 Ga (Borg, et al., 2015), relative young ages of Procellarum mare basalts (Schmitt, 2003), and possible shift in positions of lunar poles (Siegler, 2015). Comparable scale impacts on Earth and differentiation of their melt-sheets may explain both ~4.4 Ga zircons and seeding the first continents (Schmitt, 2003).

Lunar mantle overturn, at least under Procellarum, is suggested by the iron isotopic composition of dunite 72415 and that dunite’s pyroxene-spinel symplectites in that dunite and in troctolite 76535. An impact induced reduction of lithostatic pressure may have caused migration of globally distributed residual magma ocean into that region as well as having caused melting of magma ocean cumulates to produce those Mg-suite magmas that crystallized at about 4.35 Ga. Impact removal of insulating crust also would have delayed the later re-melting of the mantle, producing relatively young mare basalts. Finally, removal of mass from the region may have caused a reorientation of the Moon’s spin access.

On Earth, melt-sheets from impacts like Procellarum in the 4.4 Ga time-frame could be expected to differentiate into a silicic crust that ultimately would include a small amount to zircon. A differentiated melt-sheet also could act as a seed for the formation of even larger continental crust.