EGU2020-1169, updated on 12 Jun 2020
https://doi.org/10.5194/egusphere-egu2020-1169
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Lunar Phosphates Record Impact Cratering Events at Micro to Nano Scales

Ana Cernok1,2,3, Lee White1,2, Kimberly Tait1,2, Mahesh Anand3,4, Sandra Kamo1, Martin Whitehouse5, and James Darling6
Ana Cernok et al.
  • 1Department of Earth Sciences, University of Toronto, Toronto, Canada.
  • 2Centre for Applied Planetary Mineralogy, Department of Natural History, Royal Ontario Museum, Toronto, Canada.
  • 3School of Physical Sciences, The Open University, Walton Hall, Milton Keynes, United Kingdom.
  • 4Department of Earth Sciences, The Natural History Museum, London, United Kingdom.
  • 5Department of Geosciences, Swedish Museum of Natural History, Stockholm, Sweden.
  • 6University of Portsmouth, School of the Environment, Geography and Geosciences, Burnaby Road, Portsmouth, United Kingdom.

The field of planetary mineralogy has greatly benefited from recent studies of accessory minerals that utilise µm- and nm-scale analytical techniques such as EBSD, APT, TEM and SIMS. Apatite and merrillite have been of particular interest, as they record vital information on the volatile content, U-Pb ages and trace-element composition of various planetary materials. However, the extent to which shock-deformation, pervasive among all planetary materials, affects the distribution of these valuable geochemical tracers is still poorly understood. Here we focus on exploring the U-Pb and Pb-Pb ages of apatite and merrillite in a set of variably shocked lunar rocks, building on previous nanostructural analyses of the phosphates.

We carried out U-Pb and Pb-Pb analyses of phosphates in Apollo 17 samples of the Mg-suite rocks (76535, 76335, 76255, 72255, 78235 and 78236) using the CAMECA 1280 ion microprobe at the NordSIMS facility (Stockholm). In addition, we applied a recently developed approach of conducting high-precision U-Pb and Pb-Pb analyses by ID-TIMS of extracted phosphate grains (Jack Satterly Lab, University of Toronto). For this purpose, individual ~50x50x30 µm crystals of apatite and merrillite were extracted directly from thin sections using a Xe+ plasma FIB.

As determined by SIMS, 207Pb/206Pb systematics of the unshocked or weakly shocked apatite in 76535 and 76335 is undisturbed, implying cooling of the rock below the closure temperature of Pb diffusion in apatite (~450°C) at ~4.2 Ga, ~100 Ma younger than what is interpreted as the rock’s crystallization age. Phosphates that experienced similar levels of deformation but were in proximity or in direct contact with the impact melt in samples 76255 and 72255 show almost complete age resetting (~3.92 Ga). The SIMS determined age of 16 phosphates in sample 76255 is 3922.2 ± 6.7 Ma (2σ) and agrees with the previously published 207Pb/206Pb phosphate ages of impact melt breccias found within the same boulder and was interpreted as the timing of the Imbrium impact. These recrystallized phosphates yield comparable TIMS Pb-Pb ages (3917.8 ± 1.8 Ma and 3921.0 ± 1.3 Ma, 2σ) with significantly lower internal uncertainties than that of the individual SIMS measurements and may represent multiple impact-events close to the Imbrium event.

SIMS U-Pb analyses of highly shocked phosphates (78235 and 78236) reveal a discordia line with an upper intercept of ~4.2 Ga and a lower intercept of ~0.5 Ga. We interpret this new, younger age as a minor thermal event that reactivated existing shock-induced nm-scale grain boundaries, as visualised by APT, within the apatite population to allow for Pb-loss at ~0.5 Ga. We propose a small crater located near the Apollo 17 landing site as a possible source of this sample.

By correlating micro- to nanostructural characterization with in-situ age systematics we show that apatite and merrillite are powerful thermochronometers that provide a new approach to dating which has the potential to discriminate between temporally similar events. This can greatly aid in unravelling the bombardment record of solar system and be helpful when dealing with samples of limited availability (e.g. space return missions).

How to cite: Cernok, A., White, L., Tait, K., Anand, M., Kamo, S., Whitehouse, M., and Darling, J.: Lunar Phosphates Record Impact Cratering Events at Micro to Nano Scales , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1169, https://doi.org/10.5194/egusphere-egu2020-1169, 2019

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