NanoSIMS and EPMA dating of lunar zirconolite

Nian Wang,Yangting Lin,Ting Zhang,Qian Mao,Jialong Hao

doi:10.1186/s40645-021-00446-3

Abstract

Zirconolite is a common Zr-rich accessary mineral in mafic rocks. It is also an ideal U–Pb/Pb–Pb chronometer because it commonly contains high U content (mostly 0.1–10 wt%) and negligible initial Pb. However, zirconolite is usually very small (e.g., ~ 1 μm in width) in lunar rocks, requiring a high spatial resolution analysis. We analyzed a single, large (25 μm × 20 μm) grain of zirconolite in lunar meteorite NWA 4485 using Pb–Pb dating by NanoSIMS and U–Th–Pb dating by EPMA. The resultant U–Th–Pb age is 4540 ± 340 Ma (2σ) with a spatial resolution of 1.3 μm. The Pb–Pb age by NanoSIMS is 4348.5 ± 4.8 Ma (2σ) with a spatial resolution of ~ 2 μm, consistent with the age of 4352 ± 10 Ma and 4344 ± 14 Ma reported in the same meteorite and its paired meteorite NWA 4472. Although U–Th–Pb age is somewhat older, it still includes the NanoSIMS results within the analytical uncertainty. This work demonstrates the potential application of the combined EPMA dating and REE analysis of lunar zirconolite, with the benefits of high spatial resolution, non-destructive, and readily accessibility of the instrument. The precision of the EPMA dating (7.6%, 2σ) can be improved by increasing the counting time for Pb, U and Th. We expect to apply this EPMA technique for a quick and non-destructive age survey and geochemical study of zirconolite grains from the lunar mare basalts newly returned by Chang’E-5 mission which landed on a very young (1.2–2.0 Ga by crater-counting chronology) basalt unit in Procellarum KREEP Terrain.

Highlights

Zirconolite (CaZrTi2O7) has been recognized as a fairly common accessory mineral first in terrestrial rocks (Williams and Giere 1996) and in lunar basaltic rocks from Apollo 11 and 12 (Lovering and Wark 1971; Wark et al 1973)
According to the U, Th, and Pb composition reported in previous literatures and our study, it is evidenced that zirconolite is a good candidate for electron probe microanalyzer (EPMA) chemical dating
As for analytical uncertainty, the 2σ relative deviation of chemical age is 7.6%, which is consistent with 7.2% of Apollo zirconolite chemical age reported by Seddio et al (2013), but significantly poor than 2.3% in Rajesh et al (2006)

Summary

Introduction

Zirconolite (CaZrTi2O7) has been recognized as a fairly common accessory mineral first in terrestrial rocks (Williams and Giere 1996) and in lunar basaltic rocks from Apollo 11 and 12 (Lovering and Wark 1971; Wark et al 1973). Lunar zirconolite was commonly reported as a ubiquitous trace mineral in the late stage mesostasis of lunar mare basalt (Day et al 2006; Rasmussen et al 2008; Wark et al 1973; Zeigler et al 2005) It usually contains lower U and Th contents than terrestrial zirconolite and less degree of metamictization and Pb-loss caused by alpha-decay damage. Most lunar zirconolite grains are always fine-grained or elongated “strings”, ranging in shapes from laths to tubular rods (Rasmussen et al 2008; Seddio et al 2013) They rarely grow up to 10 μm in size and frequently appear as needle-like strings with length up to tens of μm but width less than 2 μm (Rasmussen and Fletcher 2004; Seddio et al 2013). This typical morphology of zirconolite makes most occurrences too small for traditional analyses like SIMS

Methods

Results

Conclusion