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

Abstract

The Pb isotopic compositions of maskelynite and pyroxene grains were measured in ALH84001 and three enriched shergottites (Zagami, Roberts Massif 04262, and Larkman Nunatuk 12011) by secondary ion mass spectrometry. A maskelynite-pyroxene isochron for ALH84001 defines a crystallization age of 4089 ± 73 Ma (2σ). The initial Pb isotopic composition of each meteorite was measured in multiple maskelynite grains. ALH84001 has the least radiogenic initial Pb isotopic composition of any Martian meteorite measured to date (i.e., 206 Pb/ 204 Pb = 10.07 ± 0.17, 2σ and 0.17 Ga, respectively. The model employed here is dependent on a chondritic μ value (~1.2) from 4567 to 4513 Ma, which implies that core segregation had little to no effect on the μ value(s) of the Martian mantle.The proposed Pb isotopic model here can be used to calculate ages that are in agreement with Rb-Sr, Lu-Hf, and Sm-Nd ages previously determined in the meteorites and confirm the young (~170 Ma) ages of the enriched shergottites and ancient, >4 Ga, age of ALH84001.

Hydrogen Isotopic Variations in the Shergottites

“…Bouvier et al [43,44] on the other hand proposed that the depleted shergottites are~4.3 Ga old, whilst the enriched shergottites are as old as the orthopyroxenite ALH 84001 (~4.1 Ga [37]) based on the Pb-Pb isochron. However, most recent in situ Pb-Pb analyses are in good agreement with previous Rb-Sr, Lu-Hf, and Sm-Nd ages, confirming the young ages of the enriched shergottites [38]. Although this review does not rigorously discuss the chronology of the SNC (shergottites, nakhlites and chassignites) Martian meteorites, the epoch of Mars relating to the shergottite parent body formation and evolution will consider their young crystallization ages in this paper.The red planet is thought to have been a warm and wet world in its first hundred million years [45].…”

“…However, the abundance of water and duration of the water-rock interaction recorded in the shergottites could not have been sufficient enough to result in mineral alteration [67]. Deuterium-rich water laths in melt inclusions [68] are examples of the limits of occurrences of most water in the shergottites [83][84][85][86][87][88] representative of the cold and dry paleoclimate encountered by the shergottite lithology on Mars [6,38,48].…”

“…We interpret these observations using an evidence-based approach, considering two particular scenarios: (1) water-rock crustal interactions versus (2) magmatic-based processes. We consider the implications of these measurements and the scope they have for future studies, paying particular attention to future works on H, S, and Cl isotopes in situ, shedding light on the nature of volatiles in the hydrosphere and lithosphere of Mars.The crystallization ages of Martian meteorites vary from: (1)~4.4 Ga for various igneous clasts from the regolith breccia NWA 7034 and its pairings [4,35,36] based on U-Pb dating of zircons, (2)~4.1 Ga for the orthopyroxenite ALH 84001 [37], (3)~1.3 Ga for clinopyroxene nakhlites, and dunitic chassignites [6], and (4)~600-150 Ma for most shergottites based on bulk Rb-Sr and Sm-Nd analyses [6], Pb-Pb isochrons measured in situ [38], and U-Pb dating of baddeleyite and phosphates [39][40][41][42], except for the two recent identified shergottites NWA 8159 and NWA 7635 which are dated at~2.4 Ga [19,20]. Around 80% of Martian meteorites are shergottites (Meteoritical Bulletin).…”

Pb isotopes in the impact melt breccia 66095: Association with the Imbrium basin and the isotopic composition of lithologies at the Apollo 16 landing site

“…The Pb isotope compositions of lunar samples indicate that µ-values (the ratio of 238 U/ 204 Pb extrapolated to the present) in many lunar lithologies and their mantle sources are significantly higher (~100-600; Tatsumoto 1970;Tera and Wasserburg 1974;Gaffney et al 2007a;Nemchin et al 2011;Snape et al 2016) than those determined for the Earth (terrestrial mantle µ-values typically being ~8-10; Kramers and Tolstikhin 1997) and other planetary bodies (e.g. Mars with mantle µ-values of ~1-5; Gaffney et al 2007b;Bellucci et al 2015). Despite evidence of low-µ lunar mantle sources identified in several samples (e.g.…”

“…Following the SEM documentation of the samples and prior to the SIMS analyses, the samples were cleaned with a fine (1 µm) diamond paste and ethanol to remove the carbon coating before adding a 30 nm gold coating. The Pb isotopic compositions of the phases were determined over three analytical 6 sessions using a CAMECA IMS 1280 ion microprobe at the NordSIMS facility in the Swedish Museum of Natural History, Stockholm, using a methodology similar to that outlined in previous studies (Whitehouse et al 2005;Bellucci et al 2015). Apertures in the primary column were used to generate a slightly elliptical O 2 sample probe with dimensions appropriate to the target.…”