Geochronology of pallasite meteorites: First in situ Lu–Hf ages and Pu fission track methodology refinements

Abstract

Pallasite meteorites are stony-iron meteorites that are generally accepted to have originated from the core-mantle boundary of a planetesimal. With only 141 pallasite meteorites discovered on Earth, it is uncertain if they were sourced from a single planetary body or multiple planetoids. Constraining absolute ages would aid our understanding of this, yet few have been determined. This is due to historical age methodologies involving time consuming, challenging analytical processes that usually result in whole-rock ages and complete sample destruction. This study develops an analytical workflow using micro-X-ray fluorescence spectrometry, scanning electron microscopes, electron probe micro-analysis and laser ablation inductively coupled plasma-mass spectrometry to identify and date phosphate minerals in pallasites. The main advantages of the approach are: (1) no destructive sample preparation requirements, (2) high spatial resolution, and (3) large quantities of data can be collected in a short time span. Using this workflow, the first in situ Lu–Hf ages and Pu fission track analyses for phosphate crystals in pallasites are presented. The Lu-Hf ages for Springwater (4564±91Ma) and Seymchan (4560±67Ma) pallasites are in good agreement with the timing of solar system formation. The Pu fission track data demonstrate the initial 244Pu/238U ratio used in published work (0.015) is flawed and a more appropriate minimum initial ratio of 0.019-0.029 was calculated for stanfieldite phosphates within Springwater, Sericho and NWA10023. These new methods have wide application to rapidly obtain age constraints for the formation and thermal evolution of not only pallasites, but potentially any phosphate-bearing meteorites, enabling new insights into the evolution of the solar system.