Summary

The evolution of the Moon differed radically from that of the Earth. Early lunar magmatism caused irreversible differentiation of the silicate Moon into a light plagioclase-rich crust, a dense olivine-rich mantle and a reservoir highly enriched in incompatible elements that is complementary to these two. The lunar crust/mantle mass ratio exceeds the terrestrial value by at least an order of magnitude. A predominance of plagioclase in the lunar highland crust precludes partial melting as the principal mechanism of crust formation (also in contrast with the Earth; see Chapters 24 and 25). Instead, plagioclase segregation and flotation from a lunar magma ocean is envisaged, after magmas evolved to a high Al2O3 content by Ol + Pyx fractional crystallization and the removal and accumulation of these mafic minerals in a deep dunite layer, the lower lunar mantle. In the course of such differentiation some major elements (Ti, Fe) and trace elements (incompatible in plagioclase) were concentrated further in the melt, the last remnants of which solidified into a reservoir that sourced basalts enormously enriched in K, REE, P, Rb, U and Th, termed KREEP, and never seen in other objects of the solar system. Removal to the crust of elements compatible in plagioclase left specific fingerprints for KREEP, e.g. a negative Eu anomaly. The limited chronological data show that the early crust segregated from the lunar magma ocean 4.45 Gyr ago. The duration of this stage of lunar magmatism was about 100 Myr.

Later on, bombardment of the Moon by accreting planetesimals occurred and was especially intense ~ 3.9 Gyr ago (Elkins-Tanton et al., 2004), indicating that a similar bombardment must have occurred on Earth. Between 4 and 3 Gyr, partial melting of the lunar mantle formed the lunar maria. The compositional features of lunar basalts, e.g. a bimodal Ti distribution, negative Eu anomalies and specific relationships between the Nd- and Hf-isotope compositions, point to sources at different depths in a broadly stratified mantle, which was modified by (partial) overturns causing long-term heterogeneities. KREEP-type matter brought down to the deep mantle in this way could have been a cause of the mare volcanism via radioactive heating; impacts were probably also a trigger.

Since ~ 3 Gyr ago the Moon has been stagnant: only tides, rare impacts and solar wind have affected its history, again in contrast with the still tectonically active Earth. While lunar rocks, interesting and exciting in themselves, provide little knowledge about the structure, composition or evolution of our planet, their study has introduced an invaluable alternative perspective on planetary development.

0 0

Post a comment