Report on recent 40 Ar- 39 Ar results obtained from lunar samples

Transcription

1 NWA 4734 Report on recent 40 Ar- 39 Ar results obtained from lunar samples Vera Assis Fernandes Thanks to: Ray Burgess, Grenville Turner, David Blagburn, Bev Clementson, David Plant, Steve Caldwell, Kent Ross, Tim Teague, Tim Becker, Peter Czaja and Kirstin Born

2 Lunar History based on Apollo and Luna samples Lunar Origin - Big Impact Theory (accretionary process) assembled from debris after impact of Mars-sized object with the early Earth Formation of Lunar Magma Ocean, LMO ( km depth) Artist s impression: Don Davis

3 Lunar History LMO differentiation cooling outer portions of the Moon were entirely molten differentiation of the earliest crust and mantle and produced the rocks observed today Formation KREEP-layer Courtesy: Steph Flude, Univ. Manchester Courtesy: Steph Flude, Univ. Manchester Model of the internal structure of the Moon (C. Hamilton)

4 Lunar History ÛEarth-Moon system bombardment (~ ~3.9 Ga) 9 frozen single plate template of bombardment history affecting the inner Solar System Copernicus Crater, 93 km Ø, one of the youngest and freshest impact craters on lunar nearside of Moon, a well-developed complex crater Moltke Crater, 7 km Ø, a simple crater

5 Lunar History Earth-Moon system bombardment (~ ~3.9 Ga) Catastrophic vs. Continuous bombardment Oldest Earth s rocks Dashed curve: exponential decay of impact rate Solid curve: cool early Earth late heavy bombardment (Hartmann et al., 2000)

6 Lunar History Volcanism: Two hemispheres with different lava flow distribution Nearside Farside

7 Volcanic Features: sinuous rilles, lobate fronts and domes Lobate mare-basalt flows in Mare Imbrium. Aristarchus Plateau. H, Herodotus crater (35 km). Small meandering rille, Vallis Schroteri, probably formed in massive lava flow(s). Mare Domes in northern mare Tranquilitatis Images from LPI website

8 Volcanic Features: Cones, Craters and Dark mantling Image C. Wietz Cones along linear rile and amidst darkmantling material or mare lava (Apollo 16) Craters aligned along a graben on south margin of Mare Serenitatis Scrcnitatis (Apollo 17) Meyer et al.1975 Apollo 17 Orange glass seen under the microscope (image from G. Ryder).

9 Landing sites.and the maria Apollo 15 Apollo 17 Apollo 12 Apollo 11 Luna 24 Luna 20 Apollo 14 Apollo 16 Luna 16

10 Lunar basalts: mineral composition Apollo and Luna missions samples: Typical lunar basalts composed of plagioclase (anorthite), Pyroxene, olivine and ilmenite. based on the vol%: olivine basalts, pigeonite basalts, ilmenite basalts, qtznormative basalt. minor phases: spinels, ülvospinels, merrilite, troilite, kamacite, K-fldsp, armalcolite, hapkeite. Luna 24 basalt Apollo 12 basalt Luna 16 basalt Photomicrographs of 12035,23 Scale is 2.6 mm NASA # S

11 Lunar basalts: chemical composition The primary classification of mare basalts is based on their TiO 2 content: [Ti] - very-low Ti or VLT, <1 wt% low-ti, 1 4 wt% - e.g. A12 pigeonite basalt intermediate, 4 8 wt% - e.g. A11 high-ti >8 wt%. - e.g. A17 low-k Ti content is also used to characterize the different groups of picritic volcanic glasses: wt% Lunar rock sample TiO 2 distribution is bimodal VLT Low-Ti interm. High-Ti [Al] - >11 wt% Al 2 O 3 aluminous basalts as clasts in Apollo 14 breccias. [K] most K <0.1 %; - high-k basalts ~0.3 wt% K 2 O (Apollo 11), - very-high K basalt wt% K 2 O (Apollo 14)

12 Recent views based on remote sensing orbiter data: chemical composition Global Remote Sensing TiO 2 distribution is continuous Giguere et al. (2000) using Galileo and Clementine data Remote sensing data of Orbiter I-V, Clementine and Lunar Prospector missions. Ti content of the lunar basalts shows a continuous range in abundance, Eruptions of low and high Ti content occurred contemporaneously.

13 Ages of Apollo and Luna basalts.. Cryptomare >4.2 Ga, Pre-Nectarian A 4.14 ± 0.05 Ga aluminous mare basalt clast in breccia (Rb-Sr, Taylor, 1983) Samples suggest that most volcanism occurred during the Imbrium ( Ga) Lunar Sites Age (Ga) Apollo Apollo Apollo Apollo Luna Luna ( ) (Part of table after compilation by Stöffler and Ryder, 2001; and Fernandes and Burgess 2005)

14 Age distribution of different lavas within the lunar maria: Age range of Apollo & Luna samples Hiesinger et al. (2003) using Orbiter I-IV and Clementine data Hiesinger et al. (2003) Oceanus Procellarum, Mare Imbrium, Mare Sereninitatis, Mare Tranquilitatis,Mare Insularum, Mare Humorum, Mare Cognitum & Mare Humboldtianum. Lunar volcanism may have occurred to as recently as ~1.2 Ga

15 Lunar Prospector global Th map: Procellarum-KREEP Terrain (PKT) Feldspathic-Highland Terrain (FHT) Maria

16 Lunar mare meteorites Found in cold and hot deserts on Earth 10 lunar basalts (out of a total of 55 lunar meteorites) Random sampling of the lunar surface: near- and farside Chemical composition varies from low to very low-ti NWA 032 LAP02205 NWA773 BSE image NEA 003 Asuka ~6 mm

17 Lunar mare basalt meteorites chemistry Basaltic meteorites vs. lunar basalts Th (ppm NWA032 NWA773 Dhofar287 A Asuka Yamato EET96008 LAP02205 Apollo 11 Hi Ti and K Luna 24 Kalahari009 NEA003-A TiO2 (wt%) Korotev and Zeigler (2007)

18 Ages and Dating Techniques K-Ar and Ar-Ar K relatively abundant in most rock types Applied to whole rocks (step-heating) Applied to individual minerals (spot-laser) Rb-Sr and Sm-Nd Low abundance relatively large samples. Mineral separates and whole rocks needed. U-Pb Resistance to thermal disturbance U-rich minerals (zircon, phosphate) are rare.

19 Ar-Ar (K-Ar) systematics K-isotopes - 41 K, 40 K, 39 K at a constant proportion in nature 40 K 40 Ar 40 1 Ar t = ln λ K Natural Ar-isotopes 40 Ar, 38 Ar and 36 Ar In the late 1960 s Surface-correlated the technique Ar was taken to a new level where only Ar isotopes needed Solar wind to be measured: Ar-Ar Method * Apparent Age (Ga) ideal Nuclear reactor induced Ar-isotopes 39 K 39 Ar & 42 Ca 37 Ar Step-T 40 Ar*/ 39 Ar Ar-loss an age is determined * 1 Ar t = ln J λ Ar Fraction of 39 Ar T K λ J = φ( ε ) σ ( ε ) dε 40 ' K λ + λ Parentless Ar 0 e e 0 * 39 Ar/ 40 Ar Radiogenic Ar 39 λt Ar m J = e 40 * Assumptions: Rock or mineral cannot have gained or lost any parent or daughter nuclide. λ must be well known (no change!!). D 0 must be determined correctly, unless D 0 = 0 Propotions of the different K-isotopes are well constrained 36 Ar/ 40 Ar Trapped Ar Ar Solar wind/radiogenic mixture monitor

20 Mare basalt lunar meteorites NWA032 NWA 032/479 is a unbrecciated, low-ti mare meteorite Ar/Ar age=2.779±0.056 Ga (Fernandes et al. [2003]) Recent Rb/Sr (2.947±0.016 Ga) and Sm/Nd (2.931±0.092 Ga) (Borg et al. [2009]) Apparent Age (Ga) K/Cax1000 (molar) NWA032/479 Furnace Step-Heating G3 150 G4 100 E (c) 1.0 G4 39 Fraction Ar released G3 E2 (photo courtesy of Bruno Fectay & Carine Bidaut)

21 Ar/Ar compared with other systematics in lunar basaltic meteorites Age (Ga) Age (Ga) Age (Ga) (a) A (d) LAP (b) Y Plag Pyx Bulk Bulk Plag Fernandes et al (2009) Sm-Nd Rb-Sr Ar-Ar U-Pb Th-Pb Pb-Pb Pu-Xe K-Ar Fernandes et al (2009) Data sources:1. Misawa et al. (1993); 2. Thalmann et al. (1996); 3. Torigoye-Kita et al. (1995); 5. Rankenburg et al (2007); 6. Nyquist et al (2005); 7. Anand et al. (2005); Ar-Ar ages are not affected by shock events, but to a prolonged heating regime that may occur as a result...e.g. burial under hot ejecta.

22 Mare basalt lunar meteorites NWA 4734 & NWA 4898 NWA 4734 coarse-grained basalt compositionally and texturally indistinguishable from the LaPaz Icefield basalts of Antarctica and is a potential launch pair (Fernandes et al [2009]) 9 LAP (6 stones) 8 Sm (ppm) 7 6 NWA 032/479 NW A NW A Sc (ppm) NWA 4898 first feldspathic basalt among the lunar meteorites; major-element composition is similar Apollo 12, but REE concentrations are lower and LREE are depleted. Preliminary Rb-Sr whole rock analyses suggest age of 3.600±0.059 Ga; impact shock features: plag totally converted to mask, and olv and px show irregular fractures, with olivine shows strong mosaicism 2b stage shock, ~28-34 GPa (Gershake et al. [2008]).

23 Apollo and Luna basalts vs. basaltic Meteorites. TiO 2 (wt%) (after Warren, 2005) Brecciated basalts Unbrecciated mare basalts Mingled breccias High Ti Medium Ti A11 A17 Luna 16 NWA Low Ti 0 VLT A12 NWA032/479 Dho 287 A15 NWA4734 LAP02205 A15 A Y NEA003 Luna 24 EET96008 NWA773/2977 MIL05035 KAL Age (Ga) Gaffney et al. (2008) Apollo and Luna missions samples suggest that most volcanism occurred during the Imbrium ( Ga) Lunar mare meteorites are showing a longer period of volcanic: 4.3 ~2.65 Ga (1.5 Ga period)

24 Mare basalt lunar meteorites Kalahari 009 Mingled fragmental breccia of basaltic lithologies embedded in fine-grained matrix Very-low Ti Shocked to GPa Ar/Ar age = 1.787±0.32 Ga Lu/Hf age = 4.286±0.095 Ga (Sokol et al. 2008) U/Pb age = 4.349±0.150 Ga (Terada et al. 2007) The oldest lunar basalt ever analysed Opaques WR and Plag fractions px-rich 176 Hf/ 177 Hf Clasts Photo by Frank Bartschart Age = 4286 ± 95 Ma (2σ) Initial 176 Hf/ 177 Hf = ± MSWD = Sokol et al. (soon to be submitted) 176 Lu/ 177 Hf K-rich melt veins Similar data uncertainties seen in 87 Sr/ 86 Sr and no age was possible from Sm-Nd

25 Summary.. Lunar meteorites are refining our understanding of lunar evolution. Chemistry of these basalts extends that of the Apollo and Luna samples. The lower limit for the duration of mare volcanism has now been extended to ~2.65 Ga. The upper limit for the beginning of volcanism was at 4.3 Ga Lunar meteorites are the only new samples we will have from the Moon for the near future.

26 Current age determination: Nectaris= Ga Crisium= Ga Serenitatis= Ga Imbrium= Ga the youngest basin, Orientale= Ga. (Stöffler et al., 2006 and references therein)

27 Landing sites Apollo 16 Landing site Cayley Plains o Descartes Formation Nectaris ejecta ( Ga) o Cayley Formation - Imbrium ejecta ( Ga) o Station 13 Apollo 17 Landing site North Ray Crater ejecta o Valley of Taurus Littrow, SE Mare Serenitatis (~50 Ma; 1km Ø x 230m) o Station 8 North Ray Crater Station 13 Station 8 LM LM 1 km 1 km

28 Apollo 16: Ar-Ar release 63503,9 Impact melt 63503,13 Impact melt 4.210±0.176 Ga 4.296±0.176 Ga Partial re-setting 3.921±0.184 Ga Partial re-setting 3.866±0.098 Ga 20 um Ages reported at 2σ 20 um

29 Apollo 16: Ar-Ar release 63503,14 Anorthosite showing shockrelated fractures in plagioclase 63503,11 Anorthosite showing shock-related fractures in plagioclase and few olivine Implanted Ar: cosmic and atmospheric 4.453±0.050 Ga 4.237±0.078 Ga 4.191±0.034 Ga Partial re-setting 3.306±0.194 Ga 100 um Ages reported at 2σ 100 um

30 Summary of Apollo 16 and 17 Ar-Ar ages 12 Soil samples from Station LM sample Crustal rocks -8 2 show max. apparent ages at 4.42 and 4.45 Ga 5 showing Early event at ~ Ga at high-t 2 show later event at 3.9 and 4.0 Ga partial re-setting at low-t 2 show later partial re-setting at low-t event at ~3.3 Ga Fragmental Breccias -3 1 Relic old age ~4.55 Ga Ga Ga 1 event at 3.9 Ga Ga 1 event at 4.2 Ga Ga Impact melt -2 2 show event at ~4.2 Ga 1 event at 3.9 Ga Similar ~4.2 Ga impact age for samples of Apollo 16 Soil Fragments were previously obtained by Kirsten SHRIMP et al. (1973), U-Pb Schaffer measurements & Husain (1975), on zircons Mauerfound et al. in Apollo (1978), 14 Aeschilman and 17 breccias et al. (1982), by Pidgeon Smith et et al, al (2007) and Nemchin et al. (2008) suggest re-setting at Apollo ~4.2 Ga 17 Station 8 Breccia & Crustal rock 2 showing an Early event at ~ Ga Apollo 16 raking at Station 13 Breccia 73235, Pidgeon et al. (2007)

31 ...a view of the impact flux onto the Earth-Moon System Relative Frequency (arbitrary unit) Ga Meteorites+Apollo+Luna (no Culler) Apollo+Luna (No-Culler) Meteorites 40 Ar/ 39 Ar 0.75Ga 1.7 Ga 3.95 Ga 4.25 Ga Apollo 14 Zircon and apatite data Apollo 17 Zircon data 207 Pb/ 206 Pb 4.20 Ga 4.05 Ga 4.34 Ga Age (Ga) Data: Cohen et al. (2005), Culler et al. (2000), Fernandes et al. (2000, 2004, 2005, 2006, 2007, 2008 and unpublished data), Levine et al. (2005), Zellner et al. (2002), Hudgins et.al (2007), Norman et. Al (2006 and 2007) Oldest Earth s rocks Age (Ga) Data: Pidgeon et al. (2007), Nemchin, et al. (2008 and 2009) and Grange et al. (2009).

32 Conclusions Lunar basaltic meteorites are enabling to further know the chemical composition and period of volcanic activity onto the lunar surface o The Lunar mantle is rather heterogeneous, and not well known o Long living heat source mantle bulk composition Further studying of the Apollo fines permit higher resolution on the impact flux of the Earth-Moon System o Several peaks of impact events are observed better determination of basin formation events o Apollo samples are surficial, or the top geological layer, and thus an incomplete record of the impact flux present a bias o Likely this top layer represents material from the last big event which covered previous BIG events. o Gardening of the lunar surface reduced depth over time Lunar basaltic meteorites are enabling to further know the chemical composition and Increase in equipment sensitivity and UHV permit from a small sample to get different data sets: SEM, EMPA, Ar-Ar, Raman but there is the need to have new sample from other areas on the Moon o Wider coverage of the planet o Deep drill cores to get more of lunar history o Farside basalts

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