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Obvious problems in lunar petrogenesis and new perspectives

By
Michael J. O'Hara
Michael J. O'Hara
Institute of Geography and Earth Sciences, Aberystwyth University, Aberystwyth SY23 3DB, UK
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Yaoling Niu
Yaoling Niu
Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China, and Department of Earth Sciences, Durham University, Durham DH1 3LE, UK
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Published:
October 01, 2015

Deceased

Forty-six years ago saw the first manned landing on the Moon and the return of the first lunar samples. Since then a vast database has been accumulated with many ideas published on lunar petrogenesis, yet important problems recognized in early days remain under-addressed. In this paper, we first review these problems and emphasize that these problems need resolving before genuine progress can be made. We then discuss that contrary to the prevalent view, the available data do not show the presence of a strong positive Eu anomaly (Eu/Eu* > 1) in the lunar highland crust, but a weak negative one (Eu/Eu* < 1) if any. This observation weakens the plagioclase flotation hypothesis, which is the very foundation of the prevailing lunar magma ocean (LMO) hypothesis. Recent success in the determination of abundant water in lunar glasses and minerals confirms the prediction in the early days of lunar research that the Moon may have been a water-rich planet and may still be so in its interior, which disfavors the dry Moon hypothesis, weakens the LMO hypothesis, and questions many related lunar petrogenesis interpretations. Volatilization (into the vacuum-like lunar “atmosphere”) of lunar magmatism during its early history could have further facilitated plagioclase crystallization and feldspathic crustal formation. The important role and effect of plagioclase crystallization are best manifested by the significant correlation (R 2 = 0.983 for N = 21) of Eu/Eu* (0.24–1.10) with Sr/ Sr* (0.10–1.12) defined by the lunar samples. Although the anorthositic lunar highlands are expected to have large positive Eu (Eu/Eu* > 1; ~1.99) and Sr (Sr/Sr* > 1; ~2.56) anomalies, their absence inferred from the global remote sensing data is best explained by the widespread but areally and volumetrically insignificant KREEP-like material that is enriched in K, rare earth elements, and P (hence, KREEP) as well as all other incompatible elements with very strong negative Eu (Eu/Eu* << 1; as low as 0.24) and Sr (Sr/Sr* << 1; as low as 0.10) anomalies. The KREEP-like material may have been produced through fractional crystallization enrichment equivalent to processes in advancing, periodically replenished, periodically tapped, continuously fractionated magma chambers. Compared with magmatic rocks on the Earth, lunar rocks are depleted in moderately volatile elements like P, Na, K, Rb, Cs, etc., probably associated with volatilization during the early history of the lunar magmatism. Further work is needed toward an improved understanding of the origin and evolution of the Moon and its magmatism.

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Contents

GSA Special Papers

The Interdisciplinary Earth: A Volume in Honor of Don L. Anderson

Gillian R. Foulger
Gillian R. Foulger
Department of Earth Sciences, Durham University, Durham DH1 3LE, UK
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Michele Lustrino
Michele Lustrino
Dipartimento di Scienze della Terra, Universita` degli Studi di Roma La Sapienza, P.le A. Moro, 5, 00185 Roma, Italy
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Scott D. King
Scott D. King
Department of Geosciences, Virginia Tech, Blacksburg, Virginia 24061, USA
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Geological Society of America
Volume
514
ISBN print:
9780813725147
Publication date:
October 01, 2015

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