Origin and age of the earliest Martian crust from meteorite NWA7533

Journal name:
Nature
Volume:
503,
Pages:
513–516
Date published:
DOI:
doi:10.1038/nature12764
Received
Accepted
Published online

The ancient cratered terrain of the southern highlands of Mars is thought to hold clues to the planet’s early differentiation1, 2, but until now no meteoritic regolith breccias have been recovered from Mars. Here we show that the meteorite Northwest Africa (NWA)7533 (paired with meteorite NWA70343) is a polymict breccia consisting of a fine-grained interclast matrix containing clasts of igneous-textured rocks and fine-grained clast-laden impact melt rocks. High abundances of meteoritic siderophiles (for example nickel and iridium) found throughout the rock reach a level in the fine-grained portions equivalent to 5 per cent CI chondritic input, which is comparable to the highest levels found in lunar breccias. Furthermore, analyses of three leucocratic monzonite clasts show a correlation between nickel, iridium and magnesium consistent with differentiation from impact melts. Compositionally, all the fine-grained material is alkalic basalt, chemically identical (except for sulphur, chlorine and zinc) to soils from Gusev crater. Thus, we propose that NWA7533 is a Martian regolith breccia. It contains zircons for which we measured an age of 4,428±25 million years, which were later disturbed 1,712±85 million years ago. This evidence for early crustal differentiation implies that the Martian crust, and its volatile inventory4, formed in about the first 100 million years of Martian history, coeval with earliest crust formation on the Moon5 and the Earth6. In addition, incompatible element abundances in clast-laden impact melt rocks and interclast matrix provide a geochemical estimate of the average thickness of the Martian crust (50 kilometres) comparable to that estimated geophysically2, 7.

At a glance

Figures

  1. Backscattered-electron image of NWA[thinsp]7533 section[thinsp]1.
    Figure 1: Backscattered-electron image of NWA7533 section1.

    The breccia contains many large bodies of clast-laden impact melt rock (light or medium grey), some outlined with dot–dash lines, in fine-grained interclast crystalline matrix. Solid ellipses show crystal and lithic fragments, close-ups of which (lettered) are shown in Supplementary Information. Pyroxene (pxn; light or medium grey), feldspar (dark grey) and pyroxene–feldspar rock fragments are found in both melt rocks and matrix. Bright grey minerals include chlorapatite and Fe-rich oxides and oxyhydroxides.

  2. Siderophile-element abundances in NWA[thinsp]7533.
    Figure 2: Siderophile-element abundances in NWA7533.

    a, Ni versus Mg, comparing abundances in NWA7533 components with those in Gusev rocks and soils12, 13, other Martian meteorites (SNCs23 and ALH8400124, 25), Apollo 15–17 breccias26, 27, 28 and lunar meteorites8, 9, and a lunar felsite, 14321, c4 (ref. 29). b, Ir versus Mg for the same samples (excluding Gusev rocks and soils, for which Ir data are not available). For literature sources, see above. Some of the in situ analyses from NWA7533 are higher in Ir than any of the lunar breccias, owing to the influence of Ir-rich nuggets.

  3. Gusev rock and soil analyses have systematically higher Zn abundances than both Martian meteorites and NWA[thinsp]7533.
    Figure 3: Gusev rock and soil analyses13 have systematically higher Zn abundances than both Martian meteorites and NWA7533.

    Pyroxene-rich nakhlites and ALH84001 are higher in Zn than are olivine-rich chassignites, but none of the known nakhlites is as Fe-rich as some of the igneous-textured clasts from NWA7533, which extend beyond the SNC field to higher Fe. Together with S and Cl, Zn abundances are systematically enriched in modern soils relative to NWA7533, presumably because of the lack of liquid water on modern Mars.

  4. REE patterns for the representative components of NWA[thinsp]7533 including the fine-grained ICM and CLIMR.
    Figure 4: REE patterns for the representative components of NWA7533 including the fine-grained ICM and CLIMR.

    The previously reported bulk REE analysis of NWA70343 (purple) represents a mixture between the ICM or CLIMR and clasts such as monzonite clastII (green). Earth’s upper continental crust30 (UCC) is shown for comparison. The blue curves depict model results: a 4% partial melt of primitive Martian mantle (PM) and the complementary residue termed the depleted Martian source (DM); a higher degree melt (15%) of the DM source; and Tissint19, a depleted shergottite.

  5. Concordia plot for SHRIMP analysis of five zircon grains from NWA[thinsp]7533 section[thinsp]4 defines a discordia line.
    Figure 5: Concordia plot for SHRIMP analysis of five zircon grains from NWA7533 section4 defines a discordia line.

    Data error ellipses are 2σ. Analyses from three zircons plot on the upper intercept (Z1, Z14, Z15), and the analysis from one grain plots on the lower intercept (Z3). MSWD, mean squared weighted deviation.

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Author information

Affiliations

  1. Department of Earth, Ocean and Atmospheric Science, and National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32310, USA

    • M. Humayun
  2. Department of Applied Geology, Curtin University, Perth, Western Australia 6845, Australia

    • A. Nemchin &
    • M. Grange
  3. Laboratoire de Minéralogie et Cosmochimie du Muséum, CNRS and Muséum National d’Histoire Naturelle, 75005 Paris, France

    • B. Zanda,
    • R. H. Hewins,
    • C. Fieni &
    • S. Pont
  4. Department of Earth and Planetary Sciences, Rutgers University, Piscataway, New Jersey 08854, USA

    • B. Zanda &
    • R. H. Hewins
  5. Department of Applied Physics, Curtin University, Perth, Western Australia 6845, Australia

    • A. Kennedy
  6. Laboratoire de Planétologie et Géodynamique de Nantes, CNRS UMR 6112, Université de Nantes, 2 Rue de la Houssinière, BP 92208, 44322 Nantes Cedex 3, France

    • J.-P. Lorand
  7. Institut de Physique du Globe, Sorbonne Paris Cité, University Paris Diderot, CNRS UMR 7154, F-75005 Paris, France

    • C. Göpel
  8. Ecole Normale Supérieure, UMR 8538, 75231 Paris Cedex 5, France

    • D. Deldicque
  9. Present address: Laboratory for Isotope Geology, Swedish Museum of Natural History, SE-104 05 Stockholm, Sweden.

    • A. Nemchin

Contributions

M.H., A.N., B.Z. and R.H.H. had the idea behind and directed the research, and wrote the manuscript. M.H. and B.Z. performed laser ablation ICP–MS analyses at Florida State University; A.N., M.G. and A.K. performed the SHRIMP ion probe U–Pb analyses at Curtin University and interpreted the chronology; B.Z. and C.F. prepared polished samples; R.H.H. and B.Z. performed petrological studies; J.-P.L. and S.P. investigated the mineralogy of the sulphide phases and searched for the carriers of platinum-group elements; C.G. provided separated CLIMR clasts; S.P., D.D., J.-P.L. and B.Z. located and imaged zircon and baddeleyite by scanning electron microscopy.

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The authors declare no competing financial interests.

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    This file contains Supplementary Text, Supplementary References, Supplementary Figures 1-8 and Supplementary Tables 1-2.

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