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SNC Ages, Mantle Sources, and the Differentiation of Mars

SNC Ages, Mantle Sources, and the Differentiation of Mars. Crystallization ages of Martian meteorites Mostly young, but initial differentiation very early Isotopic record of the time of formation of source regions in the Martian mantle At least three mantle reservoirs

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SNC Ages, Mantle Sources, and the Differentiation of Mars

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  1. SNC Ages, Mantle Sources, and the Differentiation of Mars • Crystallization ages of Martian meteorites • Mostly young, but initial differentiation very early • Isotopic record of the time of formation of source regions in the Martian mantle • At least three mantle reservoirs • Preserved for 4.5 billion years • No recycling of crust • Magma ocean modeling Ages, Mantle Sources, Differentiation

  2. Complications in Age Determinations Weathering on Mars and Earth Shock effects during launch from Mars Ages, Mantle Sources, Differentiation

  3. Ages, Mantle Sources, Differentiation

  4. Age of QUE 94201 Borg et al. (1997) Ages, Mantle Sources, Differentiation

  5. Age of QUE 94201 Borg et al. (1997) Ages, Mantle Sources, Differentiation

  6. Age of DaG 476 Sm-Nd age well defined (474 Gy), though affected a bit by terrestrial weathering, as shown by mixing line with clays etc. Rb-Sr system is disturbed and gives no age info. However, can get initial Sr by assuming Sm-Nd age and using low Rb maskelynite Borg et al. (2003) Ages, Mantle Sources, Differentiation

  7. Ages of Dated Martian Events 4091 ±30 Ma Borg and Drake (2005) Ages, Mantle Sources, Differentiation

  8. The Old-Shergottite Interpretation Bouvier et al. (2005) Ages, Mantle Sources, Differentiation

  9. Shergottite Age Conundrum (From Borg et al., 2003, GCA, v. 67(18), p. 3519-3536.) Pb isotopic data indicate old ages for Martian meteorites, including shergottites. But internal isochrons suggest young ages for shergottites. Ages, Mantle Sources, Differentiation

  10. Shergotty 0.725 depleted enriched Ages Zagami ALH84001 Los Angeles 1 165-212 Ma 327-575 Ma 1300 Ma 4500 Ma 0.720 T = 4.4 Ga ISr = 0.6985 ALH77005 0.715 87Sr/86Sr whole rock LEW88516 EET79001A/B 0.710 Y793605 Governador Valadares Lafayette Earth (recycling and smaller range in Sr composition) 0.705 Nakhla QUE94201, DaG476, Dhofar 019 0.700 0.0 0.1 0.2 0.3 0.4 0.5 87Rb/86Sr whole rock Martian Meteorites Imply Early Global Differentiation 87Rb  87Sr No recycling on Mars Borg et al (1997, 2003) Ages, Mantle Sources, Differentiation

  11. depleted enriched Ages Shergotty 0.725 4.49 Ga mixing line Zagami 165-212 Ma 327-575 Ma 1300 Ma 4500 Ma ALH84001 Los Angeles 1 0.720 ALH77005 0.715 87Sr/86Sr whole rock LEW88516 EET79001A/B 0.710 Y793605 Earth (recycling) Governador Valadares Lafayette 0.705 Nakhla QUE94201, DaG476, Dhofar 019 0.700 0.0 0.1 0.2 0.3 0.4 0.5 87Rb/86Sr whole rock Rb-Sr whole rock mixing diagram 87Rb  87Sr Borg et al (1997, 2003) Two reservoirs established at ~4.5 Ga: No recycling on Mars Ages, Mantle Sources, Differentiation

  12. Independent Support for Early Differentiation • 147Sm143Nd (t1/2=106 Gy) • 146Sm 142Nd (t1/2= 103 My) • These can be used together to determine time of differentiation if that happened early enough (before 142Nd decayed away) • Graph at right shows that SNCs lie along a line indicating initial fractionation of Sm from Nd took place 4.5 Gy ago Borg and Drake (2005), based on Borg et al. (2003), and Foley et al (2005) Ages, Mantle Sources, Differentiation

  13. Hf-W and Core Formation • 182Hf decays to 182W with a half life of 9 My • During core formation, W goes into core (Hf/W in core is 0), while remaining Hf stays in mantle (Hf/W is >10. • 142 Nd is also shortlived • Assuming a 2-stage model, the apparent isochron suggests age of 11.6 ± 0.4 My after beginning of solar system. Foley et al. 2005 tAges, Mantle Sources, Differentiation

  14. Dating core formation: • W goes into core • Hf does not • 182W in mantle indicates amount decayed after core formed • Hf-W data from Martian meteorites indicate core formation by 11.6 My after solar system formation • Other work suggests 2-4 My Ages, Mantle Sources, Differentiation

  15. Enriched Depleted Depleted and Enriched Shergottites Ages, Mantle Sources, Differentiation

  16. Shergottite Mixing: Two Ancient Reservoirs 1.6 Shergotty Nakhlites 1.2 Zagami La/Yb 0.8 LEW ALH77 0.4 EETA/B DaG QUE 0.0 enriched 0.705 0.695 0.725 0.715 Initial 87Sr/86Sr Earth depleted mixing Courtesy of Lars Borg • Long-term incompatible-element enriched. • Long-term incompatible-element depleted. Ages, Mantle Sources, Differentiation

  17. enriched depleted Earth mixing Jones (1989) Longhi (1991) Borg et al (1997, 2003) • Long-term incompatible-element enriched. • Long-term incompatible-element depleted. Ages, Mantle Sources, Differentiation

  18. Oxidation State of Reservoirs • We can determine oxygen fugacity (fO2) from that amount of Fe2+ and Fe3+ in ilmenite and coexisting spinel, or among spinel,olivine, and pyroxene • Basic relation: 6Fe2SiO4 + O2 = 3Fe2Si2O6 + 2Fe3O4 oliv pyrox spinel fO2 = Fe3O4(sp)*Fe2+(pyx)/Fe2+(oliv) Has been calibrated experimentally Ilm = ilmenite; TMt = titanomagnetite Ages, Mantle Sources, Differentiation

  19. Uncertainties • Some concern that use of oxides does not reflect temperature of magma because of lower-T requilibration • Use of olivine and pyroxene avoids that problem • Electron microprobe analyses must be “perfect” and so must the minerals because amount of Fe3+ is calculated from stociometry of minerals • Chris Herd, who has done most of this for SNCs, estimates that uncertainty in fO2 is ±0.5 log units Ages, Mantle Sources, Differentiation

  20. Oxidation state of Sources • Meenakshi Wadhwa (ASU) measured Eu and Gd in pyroxenes in Martian basaltic meteorites, using ion microprobe • Found correlation with initial Sr • DEu related to fO2 (experiments) • So initial Sr isotopic composition related to fO2 • Was first to show this Laboratory experiments (in G. McKay’s Lab at JSC) calibrate Eu partitioning as Function of fO2 Ages, Mantle Sources, Differentiation

  21. Olivine-Phyric Oxidation states inherited from source regions Shergottites Enriched (“crust”) Basaltic La/Yb No correlation with type of shergottite Depleted (“mantle”) log fO2 (relative to QFM) Herd et al (2002) Goodrich et al (2003) Ages, Mantle Sources, Differentiation

  22. Martian differentiation history 1. Formation of 2 reservoirs at ~4.5 Ga: “Crust-like”: high La/Yb low Sm/Nd (-eNd) high Rb (high 87Sr/86Sr) oxidized “Mantle-like”: lowLa/Yb high Sm/Nd (+ eNd ) low Rb (low 87Sr/86Sr) reduced 2. No significant recycling 3. Later magmatism samples mixtures of 2 reservoirs Ages, Mantle Sources, Differentiation

  23. Diversity of Mantle Sources • Calculated 147Sm/144Nd in sources for shergottites have a big range • Ditto for 87Rb/86Sr • This means that almost all of the meteorites must come from separate sources • Exceptions: nakhlites and chassigny, and EET 79001 A and B • Implies a complicated mantle Ages, Mantle Sources, Differentiation

  24. Assimilation of Altered Crust Reduced basalt (mantle characteristics) e.g., QUE 94201 Oxidized basalt (crustal characteristics) e.g., Shergotty Alteration Alteration Long-term, incompatible element enrichment Insert diagram of martian mantle/crust with amphibole layer CRUST Long-term, incompatible element depletion MANTLE Upper Mantle Source ~ IW (~QFM - 3.5) Herd et al (2002) Ages, Mantle Sources, Differentiation

  25. Assimilation of Hydrous Minerals Throughout Crust Reduced basalt (mantle characteristics) e.g., QUE 94201 Oxidized basalt (crustal characteristics) e.g., Shergotty Long-term, incompatible element enrichment Hydrous Minerals Insert diagram of martian mantle/crust with amphibole layer CRUST Hydrous Long-term, incompatible element depletion MANTLE Upper Mantle Source ~ IW (~QFM - 3.5) Herd et al (2002) Ages, Mantle Sources, Differentiation

  26. Heterogeneous Mantle Model(Magma Ocean) Reduced basalt (mantle characteristics) e.g., QUE 94201, Dhofar 019 Oxidized basalt (crust-like characteristics) e.g., Shergotty CRUST Long-term, incompatible element enrichment Insert diagram of martian mantle/crust with amphibole layer Long-term, incompatible element depletion ~ IW MANTLE Herd et al (2003) Ages, Mantle Sources, Differentiation

  27. So which one, crustal assimilation or differences in the mantle? • Consensus is that the mantle dominates and that it all involves the magma ocean • No correlation with petrographic type—both basaltic and olivine-phyric shergottites are in enriched and depleted, and in between, categories • No strong correlation between indices of differentiation (SiO2 concentrations, Mg/(Mg+Fe) and trace element or isotopic ratios (see next slide) Ages, Mantle Sources, Differentiation

  28. Mantle Sources No Correlation with SiO2 Calculated mantle sources assuming differentiation at 4.513 Ga Ages, Mantle Sources, Differentiation

  29. Nakhlite and ShergSouces Are Different Foley et al. (2005) Ages, Mantle Sources, Differentiation

  30. Mantle Sources “Depleted” Shergottites “Enriched” Shergottites Taylor et al. (2008) Ages, Mantle Sources, Differentiation

  31. Mantle Sources - Plots at left show that nakhlite source regions are distinct from those of shergottites. - The Ba/La plot has both nakhlites and shergottites above the chondrite line, indicating that there must be an additional (low Ba/La) region in the mantle. Taylor et al. (2008) Ages, Mantle Sources, Differentiation

  32. Magma Ocean Geochemical Modeling • In contrast to modeling the lunar magma ocean, we have to take into account high-pressure phases on Mars • This cross section assumes Dreibus-Wänke bulk composition (high FeO) and is based on experiments by Bertka and Fei. • Majorite has garnet structure but pyroxene composition Ages, Mantle Sources, Differentiation

  33. Evidence for a Martian Magma Ocean • Early differentiation • Rapid accretion Red curve is calculated accretion rate of Mars that is consistent with core formation from Hf-W systematics. Dauphas and Pourmand (2011) Ages, Mantle Sources, Differentiation

  34. Magma Ocean Geochemical Modeling • Borg and Draper (2003) have modeled crystallization of magma ocean from expected crystallization sequence of bulk Mars composition • Results are reasonable and when assorted cumulates are remelted, SNC parent magmas are produced, including high Ca/Al • Elements that do not fit very well: Rb, U, Ta, light REE Borg and Draper (2003) Ages, Mantle Sources, Differentiation

  35. Mantle Turn Over after Magma Ocean Crystallization Ages, Mantle Sources, Differentiation

  36. Making a Complicated Mantle Ages, Mantle Sources, Differentiation

  37. Styles of Differentiation Ages, Mantle Sources, Differentiation

  38. Styles of Differentiation Th on the Moon • Moon has much lower K/Th than Mars, but also much larger range in both elements (cumulates, highly evolved materials, KREEP) • Implies that on Mars: no large areas dominated by cumulates or highly evolved magmatic products • Does this mean Mars had no magma ocean, or different magma ocean processes? Th on Mars Ages, Mantle Sources, Differentiation

  39. Styles of Differentiation • Mars: no magma ocean? Maybe, but: • Meteorite data show that there was an early, extensive differentiation event • Processes in Martian magma ocean might have been different than in lunar magma ocean (hydrous? Choked with crystals?) • If no magma ocean • Accreting planetesimals were not substantially melted • Not enough 26Al to cause substantial melting, implying later accretion than isotopic systems indicate. Ages, Mantle Sources, Differentiation

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