A New Finding on the Crustal Formation of the Lunar Highland 2
Updated : 2009.09.18
Observation date :

This is an example of places where large blue squares are plotted in previous page. Blue portions in the rock-type color image are highly pure anorthosite, and their distribution suggests that highly pure anorthosite rocks are exposed only in highly sloped areas.

(Formation mechanism of crater central peaks.)

A rock that had been conventionally believed to be a sample of a typical highland crust based on the results of examining small portions of the Moon by Apollo missions shown in here. Because the anorthite content of this sample is about 90 percent, this is a sample having a lesser amount of anorthite than the highly pure anorthosite found to be widely distributed by the MI this time. Conventionally, the lunar highland crust was estimated to be made of anorthosite consisting of 90 percent anorthite and 10 percent other minerals such as in this Apollo sample. In that case, it was believed that when anorthite crystallized from a magma ocean, not only anorthite but also pyroxene and olivine also crystallized from the magma ocean. However, this finding has clarified that a new formation mechanism of the lunar highland crust must be constructed which can efficiently and selectively collect only anorthite in order to form a crustal material consisting of only anorthite.
As a mechanism to form a crust consisting of highly pure anorthosite, while it is believed that when anorthite crystallizes from a magma ocean, magma trapped in between crystals is pushed out by other anorthite crystals floating up from below, there is another possibility that it was formed by deformation similar to the mechanism that pure anorthosite rocks are formed on Earth.
Note that it is believed that the reason why such relatively low-purity anorthosites are found among the lunar rock samples returned by Apollo missions is because such rocks may be samples of lunar surface layers mixed by meteorite bombardments or that low-purity anorthosites exist in some parts of the crust.

The data was processed by LISM/MI team.