Terrestrial-like zircon in a clast from an Apollo 14 breccia

J.J. Bellucci, A.A. Nemchin, Marion Grange, Katherine Robinson, Gareth Collins, Martin J. Whitehouse, J.F. Snape, Marc Norman, D.A. Kring

Research output: Contribution to JournalArticleAcademicpeer-review

Abstract

A felsite clast in lunar breccia Apollo sample 14321, which has been interpreted as Imbrium ejecta, has petrographic and chemical features that are consistent with formation conditions commonly assigned to both lunar and terrestrial environments. A simple model of Imbrium impact ejecta presented here indicates a pre-impact depth of 30–70 km, i.e. near the base of the lunar crust. Results from Secondary Ion Mass Spectrometry trace element analyses indicate that zircon grains recovered from this clast have positive Ce/Ce⁎ anomalies corresponding to an oxygen fugacity +2 to +4 log units higher than that of the lunar mantle, with crystallization temperatures of 771 ± 88 to 810 ± 37 °C (2σ) that are unusually low for lunar magmas. Additionally, Ti-in-quartz and zircon calculations indicate a pressure of crystallization of 6.9 ± 1.2 kbar, corresponding to a depth of crystallization of 167 ± 27 km on the Moon, contradicting ejecta modelling results. Such low-T, high-fO2, and high-P have not been observed for any other lunar clasts, are not known to exist on the Moon, and are broadly similar to those found in terrestrial magmas.

The terrestrial-like redox conditions inferred for the parental magma of these zircon grains and other accessory minerals in the felsite contrasts with the presence of Fe-metal, bulk clast geochemistry, and the Pb isotope composition of K-feldspar grains within the clast, all of which are consistent with a lunar origin. The dichotomy between redox conditions and the depth of origin inferred from the zircon compositions compared to the ejecta modelling necessitates a multi-stage petrogenesis. Two, currently unresolvable hypotheses for the origin and history of the clast are allowed by these data. The first postulates that the relatively oxidizing conditions were developed in a lunar magma, possibly by fractional crystallization and enrichment of incompatible elements in a fluid-rich, phosphate-saturated magma, at the base of the lunar crust to form the zircon grains and their host felsite. Subsequent excavation by the Imbrium impact introduced more typical lunar features to the clast but preserved primary chemical characteristics in zircon and some other accessory minerals. However, this hypothesis fails to explain the high P of crystallization. Alternatively, the felsite and its zircon crystallized on Earth at a modest depth of 19 ± 3 km in the continental crust where oxidizing, low-T, fluid-rich conditions are common. Subsequently, the clast was ejected from the Earth during a large impact, entrained in the lunar regolith as a terrestrial meteorite with the evidence of reducing conditions introduced during its incorporation into the Imbrium ejecta and host breccia.
Original languageEnglish
Pages (from-to)173-185
Number of pages13
JournalEarth and Planetary Science Letters
Volume510
Early online date29 Jan 2019
DOIs
Publication statusPublished - 15 Mar 2019

Funding

The authors would like to thank the Apollo 14 astronauts for risking their lives to recover the sample studied here. Ryan Zeigler is thanked for lending the original section used in this work. Dr. Justin Simon is thanked for relending think section 14321,933 so quickly. Dr. Jeff Taylor at the University of Hawaii and Dr. Renaud Merle are thanked for thoughtful and helpful discussions. This work benefited from the work of three anonymous reviewers, Dr. Paul Warren, Dr. James Connelly, and the editorial handling of Dr. Frédéric Moynier. This work was funded by grants from the Knut and Alice Wallenberg Foundation ( 2012.0097 ) and the Swedish Research Council ( VR 621-2012-4370 ) to MJW and AAN and (VR 2016-03371) to JJB. This is a product of a consortium with the NASA SSERVI-sponsored Center for Lunar Science and Exploration at the Lunar and Planetary Institute. This is LPI contribution 2146. The NordSIMS ion microprobe facility operates as a Nordic infrastructure. This is NordSIMS publication #584.

FundersFunder number
Horizon 2020 Framework Programme794287
Knut och Alice Wallenbergs Stiftelse2012.0097
VetenskapsrådetVR 621-2012-4370, VR 2016-03371

    Keywords

    • impacts
    • late heavy bombardment
    • lunar zircon
    • Moon
    • terrestrial meteorite
    • terrestrial zircon

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