Group of lunar meteorites suggest a new scenario for the origin of lunar basalts
Home/ Group of lunar meteorites suggest a new scenario for the origin of lunar basalts
Feb 16, 2023
Moon's dark regions, visible to the naked eye, known as the 'mare', are remnants of a violent history of the Solar System. There are no records of these violent events on our dynamic Earth. Moon, having changed very little in the last billions of years, provides us a window to ponder over the past. The large mare regions on the near side of the Moon that we always see from Earth, mainly consists of basalts which are volcanic rocks. These regions hold the key to how the Moon cooled and evolved and what were the sources of heat that melted and crystallized the material to the present day rocks.
The Apollo, Luna, and Chang’E-5 missions have brought to Earth an extensive collection of mare basalts. Apollo mare basalts date back to the age of 3.8–3.3 Ga (Ga = one Billion years) and were collected from a region unusually rich in potassium (K), rare Earth elements (REE), and phosphorous (P) (together called as KREEP), known as Procellarum KREEP Terrane (PKT). These are rich in radioactive elements that provided the heat to melt rocks resulting in KREEP rich basalts. Are there alternate ways for melting on the Moon?
A team of scientists from Physical Research Laboratory (PRL), Ahmedabad, India, USA and Japan have found a unique group of ancient lunar basaltic meteorites with very low abundance of KREEP. This suggests that these meteorites must have come from a region different from PKT (Figure 1). The samples studied in this work are: (i) Lunar meteorite Asuka-881757 found in 1988 at Antartica, collected by National Institute of Polar Research, Japan, (ii) Lunar meteorite Kalahari 009 found in 1999 at Kalahari Desert in South Africa, (iii) Samples collected by Russian Luna-24 mission (location shown on the map).
Figure 1. Surface map of the Moon from Chandrayaan-1 mission data, showing two distinct regions. The confined yellow dashed line region marks the Procellarum KREEP Terrane (PKT) at the nearside. Most of the returned lunar mare basalts, which are from the PKT of the Moon, were formed due to mantle melting by radioactive heating. The samples studied in this research are KREEP-free basalts, originated away from the PKT and the meteorites from the unknown locations (away from PKT) on the surface of the Moon. These samples were not formed through the radioactive heating mechanism.
The calculations show that these basalts must be a result of low-pressure melting in the Moon, similar to those in other terrestrial bodies, such as Earth and Mars. Further they show that these basalts originated from a cool, shallow, and compositionally distinct part of the lunar interior (Figure 2).
Figure 2. A cartoon diagram depicting difference in formation mechanisms of KREEP-free basalts away from the PKT and KREEP-rich basalts from the PKT. While PKT basalts are formed by mixing of radioactive heating at a great depth, the studied lunar meteorites and samples were formed by low pressure (shallow) decompression melting. This study gives global perspective on the thermal evolution of the Moon in comparison to the regional Procellarum KREEP Terrane.
This finding suggests that the Moon's interior melted in the form of basalt magmatism from as early as 4.3–3.9 Ga globally to a more localized scenario in the PKT region later (3.8–3.0 Ga). Fundamentally these new results challenge currently proposed scenarios for the generation of basalts and propose an additional new regime that might be more common on the Moon globally.
Srivastava, Y., Basu Sarbadhikari, A., Day, J.M.D., Yamaguchi, A., and Takenouchi, A. (2022), A changing thermal regime revealed from shallow to deep basalt source melting in the Moon, Nature Communications, 13, 7594. https://doi.org/10.1038/s41467-022-35260-y