Complex history of a zircon aggregate from lunar breccia 73235

Abstract

Results are reported of an investigation of the age and origin of the exceptional zircon aggregate in an anorthositic clast from lunar breccia 73235. Cathodoluminescence and birefringence images show that the aggregate consists of numerous angular fragments of sector zoned primary zircon in a matrix of secondary zircon with an overall texture that resembles a pseudotachylite. SIMS U-Pb analyses of the primary fragments and the matrix yielded two clearly defined ages, an age of 4.315 ± 0.015 Ga and initial Th/U ratio of 0.21-0.35 for the primary zircon and an age of 4.187 ± 0.011 Ga and Th/U of 0.04-0.17, for the secondary zircon matrix. A Raman spectroscopic study the secondary matrix zircon was undertaken to investigate its structure. Results showed that the matrix has a zircon structure but there is also evidence for the presence of an amorphous component. Implications of the structural and U-Pb age data are discussed in terms of the origin and evolution of the aggregate and the history of lunar events. It is proposed that an original single, millimetresized, sector zoned zircon, formed at 4.31 Ga, was subjected to a severe shock event at 4.18 Ga. This event resulted in the fracturing of the zircon, the displacement and rotation of fragments, the compression of the aggregate to a lensoid shape, and the shock reduction of zircon to sub-micron-sized and amorphous granules in crush zones in the mosaic of fractures. Volatilisation loss of Pb and the addition of U to the secondary zircon is attributed to processes activated by the extreme thermal pulse which accompanied the 4.18 Ga shock event. Shock effects are seen in some of the primary fragments but Raman spectra of the primary and secondary zircon show no evidence for pressure-induced transformation of zircon to a scheelite structure. The zircon U-Pb system has not been affected by the ca. 3.95 Ga thermal pulse that accompanied formation of the host breccia although this event has largely reset the K-Ar systems.

Thermal history recorded by the Apollo 17 impact melt breccia 73217

“…In particular, zircon from Apollo 17 breccias shows no evidence of events younger than 4186 ± 7 Ma (Nemchin et al, 2008). This age was determined for the impact-related reset part of a complex zircon found in an anorthosite clast in section 73235,82 (Pidgeon et al, 2007). The size of the zircon and its location within the anorthosite clast suggest excavation from substantial depth and that the 4186 ± 7 Ma impact was a significant event in the lunar history.…”

Timing and dynamics of the juxtaposition of the Eastern Ghats Mobile Belt against the Bhandara Craton, India: A structural and zircon U‐Pb SHRIMP study of the fold‐thrust belt and associated nepheline syenite plutons

“…Zircon of all shapes and sizes were hand picked and mounted in epoxy resin together with zircon standard BR266 [Stern, 2001], TEMORA-2 [Black et al, 2003[Black et al, , 2004 and CZ3 [Pidgeon et al, 1994]. The mount was polished to expose the zircons mid-section, and coated with carbon in preparation to imaging on a JEOL-6400 scanning electron microscope (SEM) fitted with a cathodoluminescence (CL) detector.…”

“…Analysis procedure of the SHRIMP follows methods similar to those described in detail by Claoué-Long [1994] [Stern, 2001]. TEMORA-2 and CZ3 were used as control standards and yielded 206 Pb/ 238 U ages within error of those reported for them [Pidgeon et al, 1994;Black et al, 2003Black et al, , 2004. Common Pb correction is based on measured non-radiogenic 204 Pb isotope, and a common Pb composition applied following the Pb-evolution model of Stacey and Kramers [1975].…”

The comparative behavior of apatite‐zircon U‐Pb systems in Apollo 14 breccias: Implications for the thermal history of the Fra Mauro Formation

“…Zircon U-Pb geochronology has already made an important contribution to the understanding of lunar evolution (Compston et al 1984a;Meyer et al 1996;Pidgeon et al 2007;Nemchin et al 2008). However, the U-Pb system of lunar apatite has rarely been used despite its potential for dating lunar events, especially as apatite (together with merrillite) is widely available in lunar rocks compared to zircon, which appears to be restricted to rocks that carry a KREEP signature.…”