geosciences Review Constraints on Martian Chronology from Meteorites Zoltán Váci * and Carl Agee Institute of Meteoritics, University of New Mexico, Albuquerque, NM 87131, USA; [email protected] * Correspondence: [email protected] Received: 16 September 2020; Accepted: 10 November 2020; Published: 12 November 2020 Abstract: Martian meteorites provide the only direct constraints on the timing of Martian accretion, core formation, magmatic differentiation, and ongoing volcanism. While many radiogenic isotope chronometers have been applied to a wide variety of Martian samples, few, if any, techniques are immune to secondary effects from alteration and terrestrial weathering. This short review focuses on the most robust geochronometers that have been used to date Martian meteorites and geochemically model the differentiation of the planet, including 147Sm/143Nd, 146Sm/142Nd, 176Lu/176Hf, 182Hf/182W, and U-Th-Pb systematics. Keywords: Mars; geochemistry; radiogenic isotopes; geochronology 1. Introduction It is generally agreed that the inner solar system planets were formed by the collisional agglomeration of dust particles into cm- or m-sized objects which then grew into planetesimals up to 100 km in size via turbulent accretion triggered by gravitational instability. Gravitational interactions between planetesimals caused them to collide and produce Mars-sized planetary embryos, which then formed terrestrial planets via giant impacts [1,2]. Under this paradigm, Mars is a leftover planetary embryo which escaped the giant impacts that formed the other terrestrial planets [3]. The timescales of the accretion, core formation, and silicate differentiation of Mars are best constrained using the radiometric isotope systematics of the only available samples from the planet, Martian meteorites. Since these meteorites are generally hot or cold desert finds that have spent a considerable amount of time exposed to terrestrial surface conditions, they are often heavily weathered [4]. Isotopic chronometers can be highly sensitive to disruption by weathering, and for this reason this short review only focuses on the most pristine and uncompromised chronological data. Emphasis is placed on recent measurements employing the most robust chronometers. Isotopic systems employed include Sm-Nd, Lu-Hf, Hf-W, Ar-Ar, U-Th-Pb, and various noble gas exposure age chronometers such as cosmogenic 3He, 21Ne, and 38Ar. All ages discussed below are reported with their two sigma uncertainties. 2. Overview of Martian Meteorites The SNC (shergottite, nakhlite, chassignite) meteorite clan has been confirmed as originating from Mars on the basis of trapped noble gas compositions that match that of the Martian atmosphere [5]. These meteorites also plot along their own mass-dependent fractionation line in triple-oxygen isotope space, with a D17O of ~0.3 [6]. There are, to date, 150 meteorite pairing groups of Martian origin. While the traditional classification system divides these meteorites into shergottites (basalts constituting 89% of Martian meteorites [7]), nakhlites (clinopyroxene-rich cumulates), and chassignites (dunites) [8], additional samples have contributed to the variety of the Martian igneous record. Allan Hills (ALH) 84001 is an orthopyroxene cumulate [9], Northwest Africa (NWA) 7034 and its pairs are polymict breccias [10], and NWA 8159 and NWA 7635 are augite-rich shergottites with unique ages [11,12]. Together, the crystallization ages of these meteorites roughly span the age of the planet (Figure1). Geosciences 2020, 10, 455; doi:10.3390/geosciences10110455 www.mdpi.com/journal/geosciences Geosciences 2020, 10, 455 2 of 15 Geosciences 2020, 10, x FOR PEER REVIEW 2 of 15 The shergottites, which represent the overwhelming majority of Martian meteorites, are subdivided into additionaland its pairs categories are polymict based breccias on petrological [10], and NWA and 8159 geochemical and NWA characteristics. 7635 are augite-rich Basaltic shergottites shergottites containwith pyroxene unique ages and [11,12]. plagioclase Together, in the varying crystallization proportions ages of andthese span meteorites grain roughly sizes from span gabbroicthe age to fine-grained.of the planet Olivine-phyric (Figure 1). shergottites are similar to basaltic shergottites but contain olivine phenocrysts,The megacrysts,shergottites, which or xenocrysts, represent andthe poikiliticoverwhelming shergottites majority are of maficMartian to ultramaficmeteorites, plutonicare subdivided into additional categories based on petrological and geochemical characteristics. Basaltic rocks with large-grained olivine and pyroxenes [13]. Geochemically, shergottites are divided into shergottites contain pyroxene and plagioclase in varying proportions and span grain sizes from enriched,gabbroic intermediate, to fine-grained. and Olivine-phyric depleted categories shergottites based are onsimilar the to level basaltic of depletion shergottites of but their contain light rare eartholivine elements phenocrysts, (LREE) andmegacrysts, their Sm-Nd or xenocrysts, and Hf-W and radiogenic poikilitic isotopeshergottites systematics are mafic [ 14to –ultramafic16]. While the majorityplutonic of shergottites rocks with large-grained are young (< olivine600 Ma), and NWApyroxenes 7635 [13]. and Geoche 8159 aremically, 2.4 Ga shergottites exceptions are [ 11divided,12] which extendinto the enriched, age of shergottiteintermediate, volcanism and depleted to encompasscategories ba nearlysed on the the level entire of Amazoniandepletion of their Period light (3.0–0 rare Ga). The nakhlitesearth elements and (LREE) chassignites, and their on Sm-Nd the other and hand,Hf-W radiogenic are geochemically isotope systematics distinct [14–16]. from the While shergottites the and couldmajority represent of shergottites melts are and young cumulates (<600 Ma), from NW aA single 7635 and diff 8159erentiated are 2.4 Ga igneous exceptions body [11,12] [17,18 which] or from severalextend lava the flows age of or shergottite shallow sillsvolcanism associated to encompa withss a nearly single the volcano entire Amazonian [19,20]. Allen Period Hills (3.0–0 84001 Ga). is an orthopyroxeneThe nakhlites cumulate and chassignites, with an on age the of other 4.09 hand, Ga [9 ]are that geochemically gives unique distinct insight from into the the shergottites Noachian and Period could represent melts and cumulates from a single differentiated igneous body [17,18] or from several (4.1–3.7 Ga) of Martian history [9], during which liquid water precipitated secondary carbonate [21]. lava flows or shallow sills associated with a single volcano [19,20]. Allen Hills 84001 is an orthopyroxene Finally,cumulate igneous with clasts an age and of 4.09 zircons Ga [9] in that the gives Martian unique regolith insight into breccias the Noachian NWA 7034 Period and (4.1–3.7 its pairs Ga) of record evidenceMartian of anhistory ancient [9], during crust which that must liquid have water formed precipitated on Mars secondary within carbonate the first [21]. 20 Finally, Ma of igneous the planet’s formationclasts and [22 ].zircons in the Martian regolith breccias NWA 7034 and its pairs record evidence of an ancient Analysescrust that must from have orbital formed and on surface Mars within missions the first have 20 greatly Ma of the expanded planet’s formation the compositional [22]. diversity of Martian igneousAnalyses rocks. from Withorbital the and exception surface missions of NWA have 7034 greatly and its expanded pairs, Martian the compositional meteorites arediversity all basaltic to ultramafic.of Martian Orbital igneous measurements rocks. With the of exception volcanic of provinces NWA 7034 by and Gamma its pairs, Ray Martian Spectroscopy meteorites (GRS) are all aboard Marsbasaltic Odyssey to showultramafic. that theOrbital average measurements Martian crustof volcanic is basaltic provinces to trachybasaltic by Gamma Ray [23 ,Spectroscopy24]. The igneous rocks(GRS) at Gusev aboard Crater Marsexamined Odyssey show by the that Mars the average Exploration Martian Rover crust (MER)is basaltic Spirit to trachybasaltic are roughly [23,24]. basaltic but The igneous rocks at Gusev Crater examined by the Mars Exploration Rover (MER) Spirit are roughly have much higher ranges of total alkalis than the SNC meteorites [25]. The Mars Science Laboratory basaltic but have much higher ranges of total alkalis than the SNC meteorites [25]. The Mars Science (MSL)Laboratory has analyzed (MSL) igneous has analyzed rocks inigneous Gale Craterrocks in and Gale identified Crater and diorites, identified trachytes, diorites, trachyandesites, trachytes, and quartztrachyandesites, diorites [and26]. quartz Such diversediorites [26]. measurements Such diverse suggest measurements that Martian suggest igneous that Martian geology igneous is much moregeology varied is than much is impliedmore varied by thethan SNC is implied meteorites, by the involving SNC meteorites, large degreesinvolving of large differentiation degrees of and fractionaldifferentiation crystallization. and fractional crystallization. FigureFigure 1. Crystallization 1. Crystallization ages ages of of shergottites shergottites (black circle circles),s), nakhlites nakhlites and and chassignites chassignites (white (white squares), squares), NorthwestNorthwest Africa Africa (NWA) (NWA) 7034 7034 (black), (black), AllanAllan Hills (ALH) (ALH) 84001 84001 (black (black diamond), diamond), and andNWA NWA 7034 7034 (black triangle). The age shown for NWA 7034 corresponds to the oldest measured igneous clasts. Data