Solving the Martian meteorite age conundrum using micro-baddeleyite and launch-generated zircon

• Published in: Nature (London) Vol. 499; no. 7459; pp. 454 - 457
• Main Authors: Moser, D. E., Chamberlain, K. R., Tait, K. T., Schmitt, A. K., Darling, J. R., Barker, I. R., Hyde, B. C.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 25.07.2013; Nature Publishing Group
• Subjects: 704/2151/209; 704/445/849; Age; Basalt; Colleges & universities; Cosmic rays; Crystallization; Geochemistry; Grain; Humanities and Social Sciences; Isotopes; letter; Metamorphism; Meteorites; Meteors & meteorites; Methods; Minerals; multidisciplinary; Observations; Properties; Riddles; Science; Systematics; Travel time; Uranium; Zircon; Zoning; United Kingdom
• ISSN: 0028-0836; 1476-4687; 1476-4687
• DOI: 10.1038/nature12341

The age of the representative Martian meteorite NWA 5298 is determined using spatially correlated electron-beam nanostructural and uranium–lead isotopic measurements of microminerals, resolving a paradox of different age interpretations for the evolution of Martian crust. Martian meteorite age mismatch resolved A few of the many meteorites that fall to Earth are of Martian origin. The true age of these rare samples of the Martian surface has been the subject of a decades-long debate, with interpreted ages differing by up to 4 billion years. Desmond Moser and colleagues resolve this problem using a new approach to dating meteorite launch events through nanoscale investigation of crystal growth zoning and structures. Their analysis of the resistant micromineral baddeleyite and host igneous minerals in the highly shocked Martian meteorite Northwest Africa 5298 reveals it as a crystallization product of Martian volcanism of the past 400 million years. Previous estimates of a 4-billion-year age of formation have actually dated a remnant signature of the ancient mantle melting event from which the magma was derived. These findings confirm the presence of an ancient, non-convecting mantle beneath a relatively young volcanic Martian crust. Invaluable records of planetary dynamics and evolution can be recovered from the geochemical systematics of single meteorites 1 . However, the interpreted ages of the ejected igneous crust of Mars differ by up to four billion years 1 , 2 , 3 , 4 , 5 , 6 , a conundrum 7 due in part to the difficulty of using geochemistry alone to distinguish between the ages of formation and the ages of the impact events that launched debris towards Earth. Here we solve the conundrum by combining in situ electron-beam nanostructural analyses and U–Pb (uranium–lead) isotopic measurements of the resistant micromineral baddeleyite (ZrO 2 ) and host igneous minerals in the highly shock-metamorphosed shergottite Northwest Africa 5298 (ref. 8 ), which is a basaltic Martian meteorite. We establish that the micro-baddeleyite grains pre-date the launch event because they are shocked, cogenetic with host igneous minerals, and preserve primary igneous growth zoning. The grains least affected by shock disturbance, and which are rich in radiogenic Pb, date the basalt crystallization near the Martian surface to 187 ± 33 million years before present. Primitive, non-radiogenic Pb isotope compositions of the host minerals, common to most shergottites 1 , 2 , 3 , 4 , do not help us to date the meteorite, instead indicating a magma source region that was fractionated more than four billion years ago 9 , 10 , 11 , 12 to form a persistent reservoir so far unique to Mars 1 , 9 . Local impact melting during ejection from Mars less than 22 ± 2 million years ago caused the growth of unshocked, launch-generated zircon and the partial disturbance of baddeleyite dates. We can thus confirm the presence of ancient, non-convecting mantle beneath young volcanic Mars, place an upper bound on the interplanetary travel time of the ejected Martian crust, and validate a new approach to the geochronology of the inner Solar System.