Central peak composition
Central peak composition(glossary entry)
Central peaks form by the rebound of the impact point, bringing previously buried rocks up into view. Central peaks excavate rocks from depths of 5-30 km (the deep ones from basin rings) and thus are probes of the lunar subsurface. Multi-spectral studies of light reflected from peaks can provide composition information about the peak (and hence rocks at depth). The-Moon Wiki includes compositions of 109 lunar central peaks based on spectral measures from Clementine data published by Tompkins & Pieters in 1999.
The chief difficulty in understanding this compositional information is the complexity of names for lunar rocks. For highland rocks, as these peaks dominantly are, the main classification is based upon the abundance of the bright rock anorthosite (main mineral plagioclase feldspar which contains the elements Ca, Al, Si, O), and then subdivided by the amounts of pyroxenes (Fe, Mg, Ca, Si, O). Tompkins & Pieters include diagrams that illustrate how rock compositional names relate to observed characteristics and inferred minerals. Here is a beginner's guide to the rock types.
A (one of 11 codes used by Tompkins & Pieters) is for Anorthosites - these are rocks that formed when the early Moon melted, and the low density mineral plagioclase floated to the surface. Rocks are considered pure anorthosite if they have 90% or more of plagioclase.
The other minerals included in highland rocks can be considered "mafic" - a term used to describe rocks that include dark dense minerals such as iron and magnesium. They sank in the magma ocean, and later were somehow mixed with the plagioclase to form the varieties listed below.
GNTA stands for rocks that contain the spectral signatures of gabbro, norite, troctolite and anorthosite. If the plagioclase level is between 85 and 90% they are labelled with a 1, and if it is 80-85% with a 2. Notice that these are still largely plagioclase - the lunar crust is dominantly left over magma ocean scum mixed with later melted mafic materials.
AN is anorthositic norite
AGN is anorthositic gabbronorite
AG is anorthositic gabbro
AT is anorthositic troctolite
N is norite (60% plagioclase, 35% pyroxene, 5% olivine)
GN is gabbronorite
G is gabbro
and T is troctolite (60% plagioclase, 5% pyroxene, 35% olivine). T contains a large amount of olivine (Fe, Mg, Si, O). The most famous olivine-rich mountains on the Moon are the central peaks of Copernicus.
40% of central peaks contain multiple rock types/compositions. Additionally, many of the compositions occur in craters of different diameters, meaning that various rock types occur at different depths in the lunar crust. Tompkins & Pieters conclude, based on their central peak studies, that the upper crust of the Moon is dominated by A and GNTA1 (plagioclase-rich rocks).
Mafic rocks are thought to occur as pods of material that melted and rose up into the plagioclase-rich crust. Craters that excavate down into a pod will have mafic peaks, perhaps mixed in with non-mafic rocks. Tycho is the most mafic peak on the Moon.
- tychocrater Jul 22, 2007
- Yue, z. et al (2013). Projectile remnants in central peaks of lunar impact craters – Nature Geoscience | Letter, doi:10.1038/ngeo1828. Published online 26 May, 2013.