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