A Pb isotopic resolution to the Martian meteorite age paradox

Abstract

Determining the chronology and quantifying various geochemical reservoirs on planetary bodies is fundamental to understanding planetary accretion, differentiation, and global mass transfer. The Pb isotope compositions of individual minerals in the Martian meteorite Chassigny have been measured by Secondary Ion Mass Spectrometry (SIMS). These measurements indicate that Chassigny has mixed with a Martian reservoir that evolved with a long-term 238 U/ 204 Pb (μ) value ∼ two times higher than those inferred from studies of all other Martian meteorites except 4.428 Ga clasts in NWA7533. Any significant mixing between this and an unradiogenic reservoir produces ambiguous trends in Pb isotope variation diagrams. The trend defined by our new Chassigny data can be used to calculate a crystallization age for Chassigny of 4.526 ± 0.027 Ga (2σ) that is clearly in error as it conflicts with all other isotope systems, which yield a widely accepted age of 1.39 Ga. Similar, trends have also been observed in the Shergottites and have been used to calculate a >4 Ga age or, alternatively, attributed to terrestrial contamination. Our new Chassigny data, however, argue that the radiogenic component is Martian, mixing occurred on the surface of Mars, and is therefore likely present in virtually every Martian meteorite. The presence of this radiogenic reservoir on Mars resolves the paradox between Pb isotope data and all other radiogenic isotope systems in Martian meteorites. Importantly, Chassigny and the Shergottites are likely derived from the northern hemisphere of Mars, while NWA 7533 originated from the Southern hemisphere, implying that the U-rich reservoir, which most likely represents some form of crust, must be widespread. The significant age difference between SNC meteorites and NWA 7533 is also consistent with an absence of tectonic recycling throughout Martian history.

What Martian Meteorites Reveal About the Interior and Surface of Mars

“…Nevertheless, the isotopic composition (low 147 Sm/ 144 Nd and 176 Lu/ 177 Hf; Figure 10) of some clasts is consistent with an ancient LREE‐enriched crust, which is distinct from the enriched shergottite source (Armytage et al., 2018; Kruijer et al., 2017; Nyquist et al., 2016). In addition, the Pb isotopic compositions of the paired regolith breccias show that a previously unknown enriched reservoir in 207 Pb/ 204 Pb is present in the martian interior and is possibly crustal (Bellucci et al., 2016). Alkali basalt clasts in Northwest Africa 7034 are also highly oxidized compared to all other martian meteorites with ƒO 2 of QFM+3 (calculated from ilmenite‐magnetite pairs, Santos et al., 2015).…”

The Pb isotopic evolution of the Martian mantle constrained by initial Pb in Martian meteorites

“…The enriched shergottites generally have a younger age (~170 Ma) than the depleted shergottites (~300–580 Ma) [e.g., Borg et al ., , , ; Nyquist et al ., ; Moser et al ., ; Symes et al ., ]. Pb isotopic data obtained from solution analysis of shergottite mineral separates, which yield arrays in 207 Pb/ 204 Pb versus 206 Pb /204 Pb diagrams corresponding to apparent >4 Ga ages, are explained either as (1) true crystallization ages resulting from in situ decay of U to Pb [ Bouvier et al ., , , ] or (2) mixing of two end‐members with highly contrasted Pb isotopic compositions either terrestrial or Martian [ Bellucci et al , ; Borg et al ., ; Gaffney et al ., ]. The source reservoir for the enriched shergottites and ALH84001 has been inferred to have formed at 4.513 Ga based on the combined isotopic systematics of 176 Lu‐ 176 Hf, 147 Sm‐ 143 Nd, and 146 Sm‐ 142 Nd [ Borg et al ., ; Lapen et al ., ].…”

Pb‐Pb Dating of Terrestrial and Extraterrestrial Samples Using Resonance Ionization Mass Spectrometry

“…Earth and Space Science understanding of Pb isotopic abundances in planetary materials (e.g., Bellucci et al, 2016Bellucci et al, , 2018Connelly et al, 2017;Snape et al, 2016).…”

“…The Pb‐Pb isotope systematics in Zagami have been the subject of much debate (e.g., Bellucci et al, 2016; Borg et al, 2005; Bouvier et al, 2005). Borg et al (2005) studied U and Pb in Zagami and argued for severe disturbance of the U‐Pb system in the meteorite.…”