Elsevier

Geochimica et Cosmochimica Acta

Volume 101, 15 January 2013, Pages 112-132
Geochimica et Cosmochimica Acta

Interpreting U–Pb data from primary and secondary features in lunar zircon

https://doi.org/10.1016/j.gca.2012.10.013Get rights and content

Abstract

In this paper, we describe primary and secondary microstructures and textural characteristics found in lunar zircon and discuss the relationships between these features and the zircon U–Pb isotopic systems and the significance of these features for understanding lunar processes. Lunar zircons can be classified according to: (i) textural relationships between zircon and surrounding minerals in the host breccias, (ii) the internal microstructures of the zircon grains as identified by optical microscopy, cathodoluminescence (CL) imaging and electron backscattered diffraction (EBSD) mapping and (iii) results of in situ ion microprobe analyses of the Th–U–Pb isotopic systems. Primary zircon can occur as part of a cogenetic mineral assemblage (lithic clast) or as an individual mineral clast and is unzoned, or has sector and/or oscillatory zoning. The age of primary zircon is obtained when multiple ion microprobe analyses across the polished surface of the grain give reproducible and essentially concordant data. A secondary set of microstructures, superimposed on primary zircon, include localised recrystallised domains, localised amorphous domains, crystal–plastic deformation, planar deformation features and fractures, and are associated with impact processes. The first two secondary microstructures often yield internally consistent and close to concordant U–Pb ages that we interpret as dating impact events. Others secondary microstructures such as planar deformation features, crystal–plastic deformation and micro-fractures can provide channels for Pb diffusion and result in partial resetting of the U–Pb isotopic systems.

Introduction

The first high precision U–Pb measurements on lunar zircon were determined using the newly developed SHRIMP I (Sensitive High Resolution Ion Micro Probe) at the Australian National University (Compston et al., 1984). Early results (Meyer et al., 1985, Meyer et al., 1989, Meyer et al., 1996) demonstrated that lunar zircons retained their primary U–Pb ages and that magmatic activity on the Moon lasted from ∼4.37 to ∼3.90 Ga. However, complexities in the U–Pb ages and internal structures were found in lunar zircons by Pidgeon et al., 2007, Nemchin et al., 2009, Grange et al., 2009, Grange et al., 2011 and Timms et al. (2012), and attributed to disturbance of the U–Pb systems during impact. The presence of variably isotopically disturbed zircon presents problems in interpreting U–Pb ages which require distinguishing between primary and secondary microstructures. In this contribution we describe primary and secondary features in lunar zircons and link these with the behaviour of the U–Pb system.

Section snippets

Analytical techniques

Analytical methods applied to determine the origin of zircon grains in lunar samples include investigation of (i) spatial relationships between zircon grains and surrounding minerals using optical microscopy and scanning electron microscopy (SEM) with secondary and backscattered electron detectors (SE and BSE, respectively), and energy-dispersive X-ray spectroscopy (EDS); (ii) internal compositional domains/zoning of zircon grains using BSE and cathodoluminescence (CL) imaging; (iii) fractures

Characteristics of lunar zircons and their U–Pb systems

Nemchin et al. (2008) proposed a classification of lunar zircons according to their morphology and recognised a range of morphological types such as euhedral to rounded zircon grains in breccia matrices and related these features to a relative history of mechanical abrasion. This classification is useful for answering questions on the late history of a matrix zircon grain but is not sufficient for addressing more fundamental questions such as the meaning of zircon U–Pb ages. To provide a basis

Origin of U–Pb system disturbance in lunar zircon

Lunar breccias contain a record of multiple impact events which have repeatedly shocked, pulverised, metamorphosed and mixed materials such that the largest surviving fragments of igneous rock rarely exceed 1 cm in size and adjacent apparently identical rock fragments and mineral grains can have different origins and ages. This introduces serious problems of sample selection for geochronological methods that analyse composite mineral samples. In addition, the vulnerability of systems such as

Summary and conclusions

In this report we demonstrate the importance of microstructures in lunar zircons for the interpretation of U–Pb ages and for providing information on the nature of lunar processes. Where zircons have secondary microstructures like amorphisation or recrystallisation overprinting primary structures, it is possible to obtain the age of both primary zircon and secondary impact event within the same grain. Where zircons are individual clasts in the breccia matrix, identification of internal

Acknowledgements

We thank NASA, and especially the crew of Apollo 14 and Apollo 17 for the provision of the samples. We are also grateful to E. Gnos who provides us with image of zircon grain from the meteorite SaU169. M.G. and N.T. acknowledge funding from a Curtin Internal Research Grant. A.N. and M.G. acknowledge ARC Discovery Project DP120102457. M.G. wants to thank the ARC Centre of Excellence CCFS for funding. Rob Hart is thanked for his assistance with scanning electron microscopy. We would also like to

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