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73. 60Fe-60Ni-Chronology of Core Formation in Mars

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73. 60Fe-60Ni-Chronology of Core Formation in Mars Earth and Planetary Science Letters 390 (2014) 264–274 Contents lists available at ScienceDirect Earth and Planetary Science Letters www.elsevier.com/locate/epsl 60Fe–60Ni chronology of core formation in Mars ∗ Haolan Tang ,1, Nicolas Dauphas Origins Lab, Department of the Geophysical Sciences and Enrico Fermi Institute, The University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60637, United States article info abstract Article history: The timescales of accretion, core formation, and magmatic differentiation in planetary bodies can Received 20 August 2013 be constrained using extinct radionuclide systems. Experiments have shown that Ni becomes more Received in revised form 26 December 2013 siderophile with decreasing pressure, which is reflected in the progressively higher Fe/Ni ratios in the Accepted 7 January 2014 mantles of Earth, Mars and Vesta. Mars formed rapidly and its mantle has a high Fe/Ni ratio, so the Available online xxxx 60Fe–60Nidecaysystem(t = 2.62 Myr) is well suited to establish the timescale of core formation in Editor: B. Marty 1/2 this object. We report new measurements of 60Ni/58Ni ratios in bulk SNC/martian (Shergotty–Nakhla– 60 Keywords: Chassigny) meteorites and chondrites. The difference in ε Ni values between SNC meteorites and the iron-60 building blocks of Mars assumed to be chondritic (55% ordinary chondrites + 45% enstatite chondrites) accretion is +0.028 ± 0.023 (95% confidence interval). Using a model of growth of planetary embryo, this translates + core formation ∼ 1.7 into a time for Mars to have reached 44% of its present size of 1.9−0.8 Myr with a strict lower limit Mars of 1.2 Myr after solar system formation, which agrees with a previous estimate based on 182Hf–182W age systematics. The presence of Mars when planetesimals were still being formed may have influenced the formation of chondrules through bow shocks or by inducing collisions between dynamically excited planetesimals. Published by Elsevier B.V. 1. Introduction (Shergotty–Nakhla–Chassigny) meteorites constrain the time after formation of the solar system for the core of Mars to have reached +0.9 The timescales of accretion, core formation, and mantle differ- approximately half of its present size to 1.8−1.0 Myr (Dauphas entiation in planetary bodies are key observables that can help and Pourmand, 2011; Kobayashi and Dauphas, 2013). However, us test theories regarding the origin and early evolution of as- this estimate is really an upper-limit (Mars could have accreted teroids and planets (see recent reviews by Wadhwa et al., 2006; more rapidly than that), as SNC meteorites contain variable excess 182 Dauphas and Chaussidon, 2011). The extinct 60Fe–60Ni system W produced by early silicate differentiation and the accretion timescale was calculated using the least radiogenic 182Wvalues (t1/2 = 2.62 Myr; Rugel et al., 2009) is a potentially powerful tool (Kleine et al. 2004a, 2009; Foley et al., 2005). The 60Fe–60Ni system to constrain the timescale of core formation in these bodies be- has seldom been used to constrain early solar system chronology cause (1) Ni is more siderophile than Fe, therefore the Fe/Ni ratio and most studies have focused instead on establishing the initial left in the mantle after core segregation can be strongly fraction- abundance of 60Fe in meteorites, with important implications for ated relative to chondrites and (2) core formation in planetesi- the astrophysical context of solar system formation. Indeed, high mals and embryos is thought to have occurred within the first 60Fe/56Fe ratios obtained by measuring nickel isotopic composi- several million years of the formation of the solar system, when tions as well as Fe/Ni ratios by secondary ion mass spectrometers 60Fe was still extant. Much work has been done previously to es- (SIMS) in chondrites were initially taken as fingerprints of the in- timate the timescales of core formation in various bodies using jection of fresh nucleosynthetic products into the solar system by 182Hf–182Wsystematics(t = 9Myr) but ambiguities remain on 1/2 the explosion of a nearby supernova (Tachibana and Huss, 2003; several aspects of early solar system chronology (see Kleine et al., Tachibana et al., 2006; Mostefaoui et al. 2004, 2005; Guan et al., 2009 forareview).Forexample,WisotopemeasurementsofSNC 2007; Marhas and Mishra, 2012; Mishra et al., 2010; Mishra and Chaussidon, 2012; Mishra and Goswami, in press). Other studies of achondrites and chondritic constituents found lower 60Fe/56Fe * Corresponding author. ratios that were interpreted to reflect late disturbance (Chen et E-mail address: [email protected] (H. Tang). al., 2013) or heterogeneous distribution of 60Fe in the early so- 1 Present address: Ion Probe Group, Department of Earth and Space Sciences, Uni- versity of California, Los Angeles, 595 Charles E. Young Drive East, Los Angeles, CA lar system (Sugiura et al., 2006; Quitté et al. 2010, 2011). Recently, 90095, United States. Tang and Dauphas (2012) determined by MC-ICPMS the Ni isotopic 0012-821X/$ – see front matter Published by Elsevier B.V. http://dx.doi.org/10.1016/j.epsl.2014.01.005 H. Tang, N. Dauphas / Earth and Planetary Science Letters 390 (2014) 264–274 265 compositions of bulk angrites, bulk HEDs, bulk chondrites, mineral compositions of 5 martian (SNC) meteorites and 11 chondrites that separates of quenched angrite D’Orbigny, Gujba chondrules, and are thought to represent the building blocks of Mars. The Ni iso- +1.7 chondrules and mineral separates from unequilibrated ordinary topic results give a timescale of 1.9−0.8 Myr for core formation and chondrites (Semarkona and NWA 5717). In all objects analyzed accretiononMars,witharobustlowerlimitof1.2Myr. (chondrites and achondrites), they found uniformly low 60Fe/56Fe ratios corresponding to an initial ratio at the time of condensation 2. Methodology of the first solids in the solar nebula (calcium–aluminum-rich in- − clusions – CAIs) of (1.15 ± 0.26) × 10 8. Strengthening the case for 2.1. Sample preparation, digestion, and chemical separation auniform60Fe/56Fe ratio, they also reported high-precision 58Fe isotope measurements and detected no anomaly in this isotope. All the chemistry was performed under clean laboratory con- The rationale for measuring this isotope of iron is that 58Fe and ditions at the Origins Lab of the University of Chicago. Optima 60Fe are produced together by neutron captures in supernovae, so grade HF, reagent grade acetone, and double distilled HCl and 60 any heterogeneity in Fe should be accompanied by heterogeneity HNO3 were used for digestion and column chromatography. Mil- in 58Fe (Dauphas et al., 2008). The contrapositive of this statement lipore Milli-Q water was used for acid dilution. Bulk chondrites is that 58Fe homogeneity implies 60Fe homogeneity at a level that were selected to help estimate the Ni isotopic composition of bulk rules out the highest 60Fe/56Fe ratios measured by SIMS in chon- Mars and comprise 5 carbonaceous chondrites (Allende, Murchi- drules. Spivak-Birndorf et al. (2012) also concluded that 60Fe was son, Mighei, Vigarano, Orgueil, Leoville) and 6 H-chondrites (Bath, present in low abundance in the early solar system. Bielokrynitschie, Kesen, Kernouvé, Ochansk, Ste. Marguerite). These The study of Tang and Dauphas (2012) puts 60Fe–60Ni on solid samples were provided by the Field Museum. The martian mete- footings to investigate early solar system chronology. Tang and orites (Shergotty, Chassigny, Nakhla, Zagami, Lafayette) were pro- Dauphas (2012) were thus able to constrain the time of core for- vided by the Smithsonian Institution. All samples were crushed +2.5 mation on Vesta to 3.7−1.7 Myr after solar system formation, using into powder in an agate mortar before digestion. To assess data Ni isotope measurements of HED (Howardite–Eucrite–Diogenite) quality and make sure that no analytical artifacts were present, meteorites. Here, we report Ni isotope measurements of SNC terrestrial standards were processed and analyzed together with meteorites and chondrites that provide new constraints on the the meteorite samples. timescale of core formation in Mars. Sample digestion and chemical separation are described by The dynamical context of Mars’ accretion is the subject of Tang and Dauphas (2012). Chondrites and martian meteorites much work and speculation. Terrestrial planets are thought to have weighing 8 to 160 mg were digested in 5–30 mL HF–HNO3 (in formed through collisions between Moon to Mars-size planetary a 2:1 volume ratio) in Teflon beakers placed on a hot plate at ◦ embryos over a duration of several tens of million years (Chambers ∼90 C for 5–10 days. The solutions were subsequently evaporated and Wetherill, 1998). Modeling of terrestrial planet formation to dryness and re-dissolved in a 5–30 mL mixture of concentrated can reproduce the size and accretion timescale of Earth but the HCl–HNO3 with a volume ratio of 2:1. For the samples that were same simulations fail to explain the small size of Mars (Wetherill, not digested completely, the residues were separated by centrifu- 1991; Raymond et al., 2009). One possibility is that Mars grew gation and digested in HF–HNO3 (2:1 volume ratio) and HNO3 ◦ rapidly into a planetary embryo by accretion of planetesimals and using Parr bombs at 90 C for 5–10 days until complete digestion. evaded collisions with other embryos during the subsequent stage The solutions were dried down and the residues taken back in so- of chaotic growth, as was experienced by Earth (Chambers and lution with a minimum amount of concentrated HCl (∼11 M) for Wetherill, 1998; Chambers, 2004; Kobayashi and Dauphas, 2013). loading on the first column. In order to obtain sufficiently clean The short accretion timescale of Mars is indeed consistent with Ni cuts for isotopic measurements, chemical separation of Ni from this view (Dauphas and Pourmand, 2011). The rapid growth and matrix elements and isobars was done using three steps of liquid small mass of Mars can be achieved in the framework of the chromatography.
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