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Meteorites Francis - 2012

Suggested Readings: Weisberg, M.K. et al.; 2006: Systematics and Evaluation of Meteorite Classification, in Meteorites and the Early Solar System II, 19-52, Arizona University Press.

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Meteorites Francis - 2012

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  1. Suggested Readings: Weisberg, M.K. et al.; 2006: Systematics and Evaluation of Meteorite Classification, in Meteorites and the Early Solar System II, 19-52, Arizona University Press. Brown, et al.; 2000: The fall, recovery, orbit, and composition of the Tagish Lake meteorite: a new type of carbonaceous chondrite. Science 290, 320-285. Righter,K, Drake, M.J., and Scott, E.; 2006: Compositional relationships between Meteorites and Terrestrial Planets in Meteorites and the Early Solar System II, 803-826, Arizona University Press. MeteoritesFrancis - 2012

  2. Source of Meteorites There are now ~70 Apollo asteroids with radii > 1 km which are now known to have orbits that cross that of the Earth and the total number of Apollo asteroids may exceed 3000. Apollo asteroids are typically discovered just after they have almost hit the Earth. These asteroids are thought to have dynamic lifetimes on the order of only 10 million years, but are thought to be continuously re-supplied from the asteroid belt because of orbital perturbations resulting from interactions with Jupiter. There is probably not, however, a single source of meteorites; for example the annual meteor showers that we experience are caused by the Earth passing through the orbits of known comets as it revolves around the Sun. These meteorite showers are thought to consist of cometary debris strung out along the orbit of their parent comet. Furthermore, meteorites have now been returned from Antarctica that appear to have been derived from the Moon, and isotopic similarities in noble gases between SNC meteorites and the atmosphere of Mars suggest that the former are derived from Mars. Recent studies of meteorite impact dynamics confirm that large impacts are capable of ejecting the surface material of planets into orbit.

  3. Classification of Meteorites: • The classification of meteorites is complex and there are a number of disagreements, especially about fine scale subdivisions. In addition, the classification scheme predates 1960, and is presently undergoing 'contortions' to accommodate the exponential increase in the number meteorite specimens because of the new Antarctic and Northwest African finds. • Most classifications include the same first two subdivisions: • On the basis of total metal alloy content into: stones, stony-irons, and irons. • The stones are subdivided according to the presence or absence of 'chondrules' into: chondrites and achondrites. • The chondrite meteorites are thought by many to approximate the primordial condensed material of the solar • nebula, while the achondrite meteorites appear to have undergone 'igneous' processes which have fractionated • their compositions. 2500 Antarctic meteorites Previous Finds Stones

  4. Stones Feo < 30% Forest City Beardsley

  5. < 30% metal

  6. Chondrite Components Chondrules Calcium-Aluminum Inclusions (CAI’s) Amoeboid olivine aggregates AOA’s Fe-Ni alloy Fine-grained matrix

  7. The chondrite meteorites are characterized by the presence of 0.01-10 mm spheres called chondrules, which are not observed in terrestrial rocks. Chondrules are comprised largely of olivine and/or low-Ca pyroxene crystals, and/or glass. Similar spheres have been found in Lunar soils, where they are thought to have formed by the crystallization of droplets of silicate melt during free fall in a vacuum, following formation by meteorite impact. Implications of chondrules: standard view is that condensation of the solar nebula occurred at pressures that were too low (P < 10-3 atm) to involve a liquid phase, but chondrules would appear to be evidence of widespread conditions under which liquid silicate was a stable phase in space?

  8. LL3 Allende CV

  9. Cooling Rate Versus Grain Size and Shape ~ 1500 oC/hr Rapid cooling rates leads to large undercooling, high nucleation and growth rates and therefore diffusion controlled growth 0.5 oC/hr ~ 80 oC/hr olivine ~ 20 oC/hr

  10. 2 “types” of chondrules Type 2 Chondrules Type 1 Chondrules Olivine Fo > 92 Relatively reduced, most Fe is metal High temperature ? Olivine Fo < 92 Relatively oxidized, most Fe as Fe2+ in silicates Lower temperature ? MgSiO3 + Feometal + H2O (Mg,Fe)2SiO4 + H2 For olivine: log XFeO = -5.85 + 2775 / T

  11. Allende CV3 CAI’s Calcium Aluminum-Rich Refractory Inclusions Particularly abundant in CV Chondrites

  12. Calcium Aluminum-Rich Refractory Inclusions • Composed of high temperature condensates: • CaAl12O19 hibonite • MgAl2O4 spinel • CaTiO3 perovskite • Ca2Al2SiO7 – Ca2MgSi2O7 melilite • CaAl2Si2O8 anorthite 182Hf 182W + e- +  +  t1/2 = 9 106 years CAI’s are the oldest objects in the solar system (~ 4.569 Ga).

  13. Calcium Aluminum-Rich Refractory Inclusions The presence of anomalous 129Xe in chondrites and 26Mg anomalies in refractory calcium-aluminum rich inclusions (CAI) in CV carbonaceous chondrites, daughters of short-lived 129I and 26Al radioactive isotopes, requires that the chondritic meteorites formed shortly after the supernova that created their unstable parental isotopes. 129I 129Xe + e- + + t1/2 = 16 106 yrs = 4.3  10-8/ yr 26Al 26Mg + e+ + + t1/2 = 0.72 106 years = 9.8  10-7/ yr

  14. Calcium Aluminum-Rich Refractory Inclusions 26Al 26Mg + e+ + + t1/2 = 0.72 106 years = 9.8  10-7/ yr Anomalous 26Mg/24Mg ratios that correlate with 27Al/24Mg ratios are taken as evidence for the presence of 26Al in the early solar nebula. The short half-life of 26Al suggests that something like a supernova occurred immediately prior to the collapse of the solar nebula – possibly the cause of the cloud collapse itself.

  15. AOA - Amoeboid Olivine Aggregates Porous fine-grained (5-20microns) aggregates of forsteritic (Fo:1-3) olivine, with lesser Fe-Metal, low-Ca Pyrox, and refractory Ca-Al-Ti oxides. Thought to represent an early high temperature condensate of the Solar Nebula

  16. CAI Matrix Allende CV3

  17. Fine Grained Matrix: 10nm to 5 microns Murchison CM3 In the Carbonaceous Chondrites (especially CI and CM), the matrix is composed of hydrated silicates (phyllosilicates), with minor aromatic hydrocarbons compounds, magnetite, sulfides, sulfates, and carbonates, and traces of CAI and pre-solar grains (10-50 ppm), including: SiC, graphite, diamonds, and refractory oxides. The fine-grained matrix commonly appears to coat the chrondrules and other fragments. Is it the primordial solar dust? NWA5239 CV3

  18. DAG 442 H3.6 Fine Grained Matrix: 10nm to 5 microns In ordinary Chondrites, matrix is less abundant and typically composed of crystalline Mg-rich olivine and orthopyroxene, Fe-metal, Fe-rich amorphous silicates, sulfides, and refractory oxides. Very similar mineralogy to IDP’s (Interplanetary Dust Particles) NWA5479 L3.0

  19. Pre-Solar Grains SiC 1micron graphite These compounds have very high condensation/evaporation temperatures. SiC, for example, has a condensation temperature (~2700oC), and does not melt.

  20. Carbon and nitrogen isotopic compositions of presolar: SiC, graphite, and Si3N4. Davis A M PNAS 2011;108:19142-19146

  21. Most SiC grains (90+%) are thought to have been produced in Carbon-rich AGB red-Giant stars. So called SiC X-grains and Si3N4 grains are thought to be produced in supernovae

  22. Chondrites: All chondrite meteorites have similar chemical compositions, despite the fact that many are complex breccias consisting of un-equilibrated fragments and mineral grains. To a first approximation, the differences between the different subdivisions of chondrites largely reflect the oxidation state of Fe and the contents of volatile elements such as Cu, Zn, Pb, C, etc. The Enstatite Chondrites represent one extreme end-member of the Chondrite meteorites in which all the iron occurs as a metallic alloy, because reactions such as the following have gone all the way to the right: H2 +FeOFeo + H2OaFeO =  aFe XH2O / iron oxide => iron metal + water H2 + (Mg,Fe)2SiO4 MgSiO3 + Feo + H2O olivine => pyroxene + iron metal + water There is a general increase in the oxidation state of Fe, with a decrease in the proportion of metallic alloy, from the Enstatite Chondrites through the Ordinary Chondrites to the Carbonaceous Chondrites, as Fe enters the Y site of the silicate minerals (Prior's Second Rule) forming olivine at the expense of pyroxene, with a concomitant increase in the levels of the relatively volatile elements.

  23. MgSiO3 + Feo + H2O H2 + (Mg,Fe)2SiO4 3H2O+ 2 (Mg,Fe)2SiO4 (Mg,Fe)3Si2O5(OH)4 + Fe(OH)2

  24. In detail, however, there are compositional differences between the chondrite meteorite types. In particular, the differences between the high, low, and very low Fe Ordinary chondrites reflect variations in the total Fe (and siderophile element abundances) that are not simply a function of oxidation state. In addition, there are systematic increases in the ratios of refractory lithophile elements to Si (eg. Al/Si, Ca/Si, Sc/Si, Sm/Si) from Enstatite through Ordinary to Carbonaceous chondrites, with the highest ratios observed in the CV carbonaceous chondrites, which are rich in refractory Ca-Al inclusions.

  25. Relative depletion in volatile elements?

  26. Oxygen Isotopes gas dust CCAM Sun? CCAM = Carbonaceous Chondrite Anhydrous Minerals

  27. Origin of Chondrules and CAI’s The solar nebula originally consisted of 16O-rich dusts and 16O-poor gas. CAIs having a 16O-rich signature formed at the inside of the inner edge of the gas disk. CAIs with 16O-poor signature and chondrules formed in the gas disk. Expected radial excursions of the X-point are shown as red and blue colours. CAIs having both 16O-rich and 16O-poor signatures formed at the fluctuation zone of the inner edge.

  28. Textural Classification The chondrite meteorites are further subdivided into petrologic types according to a numeric scale (1 through 6) that reflects the extent of post-formation 'metamorphism' or 'alteration' processes they have experienced: Type 3 meteorites are characterized by the best preserved chondrules and refractory inclusions, the presence of clear igneous glass, opaque matrices, and highly variable, un-equilibrated silicate mineral compositions. These are thought to represent the most pristine samples of the solar nebula condensate. Types 2 through 1 are characterized by increasing degree of replacement of anhydrous silicate phases by hydrated phyllosilicates, a greater abundance of volatile elements, an absence of metallic iron, and the destruction of chondrules. These meteorites are thought to have suffered low temperature alteration or weathering in the presence of water. Types 4 through 6 correspond to an increasing degree re-equilibration reflecting post formation heating to temperatures on the order of 1300oC. They are characterized by relatively equilibrated mineral compositions, lower concentrations of volatile elements, crystalline matrices containing feldspar, and poor preservation of chondrules. This metamorphism is thought to have occurred in small planetesimals such as asteroids. Higher temperatures would lead to partial melting.

  29. Frequency of Meteorite Textural Types “Ordinary” chondrites are by far the most abundant.

  30. Ages Chondrite meteorites range in age from ~4.4 to 4.5+ Ga, and are the oldest 'rocks' that have ever been dated. The ages of the chondritic meteorites are thus taken as the age of the solar system and the upper limit for the accretion of the Earth. CAI inclusions (4.567 Ga) appear to be slightly older (~2 ma) than the chondrules found in the same chondrite meteorite.

  31. Age of the Earth

  32. Summary of the time scale of formation of various solar system objects from small scale CAIs and chondrules to differentiated planetesimals and embryos (such as angrites and HED parent body), to Mars, Earth, and Moon. Righter K , O’Brien D P PNAS 2011;108:19165-19170

  33. W isotope evolution of Fe-Metal in chondrites • 182Hf decays to 182W with a half life of only ~ 10 mys. • 182Hf 182W + e- + +  • t1/2 = 9  106 years • = 0.077 /my (Kleine et al. 2004, GCA 68)

  34. 182Hf - 182W chronometry Hf is a strongly lithophile element, entering silicate phases, whereas W is strongly siderophile, preferentially entering metallic Fe. There is essentially no Hf in the Fe metal, so that there is no in growth of 182W with time. Because of the short half life of 182Hf, the 182W deficiencies in the Fe metal in the ordinary chondrites and in Fe meteorites indicate that metal segregation occurred very early in the history of the solar nebula, within ~ 7 ma of initial condensation. In a complimentary manner, the positive εWvalues of the achondritic meteorites indicates a very early separation of their metal cores (< 30 ma). Eucrites SNC meteorites Lunar samples Terrestrial samples Iron meteorites

  35. CI Carbonaceous Chondrites • The CI carbonaceous chondrites represent an extreme end member of the spectrum of chondrite meteorites, they contain more than 3 % carbon as an amorphous black 'gunk', and virtually all of the silicate minerals have been replaced by their hydrated equivalents, ie. the olivine and pyroxene have been converted to serpentine and clay minerals. Although they have similar total Fe contents as the other chondrites, there is no metallic Fe present, all the Fe is oxidized and sits in the Y sites of the hydrated silicates, or as Fe3+ oxides. • Despite their relatively low abundance, CI carbonaceous chondrites are commonly taken as a basic reference for all chemical studies because: • The compositions of these meteorites appears to be almost identical to that of the Sun, except for the most volatile of elements. • The dominant asteroid type in the outer asteroid belt have dark spectral characteristics and appear to correspond to carbonaceous chondrite compositions. These appear to mark the transition from the fractionated inner rocky planets to the outer gaseous planets.

  36. Tagish Meteorite CI2 The newly arrived Tagish Lake meteorite is the first CI2, containing some fresh olivine, and recognizable chondrules and CAI’s. Because it is considered “fresher” than the CI carbonaceous chondrites, it is likely to become the new standard for estimated composition of the solar nebula.

  37. Significance of Chondritic Meteorites • The composition of a CI Carbonaceous Chondrite is typically used as a standard reference because its chemical composition can be determined more precisely in the lab than the composition of the Sun can be determined at a distance. There are, however, differing opinions about the significance of Carbonaceous Chondrites and chondrites in general: • The similarity of the composition of the CI Carbonaceous Chondrites to that of the Sun, combined with their great age, have lead to a common assumption that they represent the composition of the primordial matter of the solar nebula from which the planets were initially formed. In such models, the other chondrite meteorites represent primordial material that has been heated since its initial condensation and, as a result, lost some proportion of their volatile elements. • Others distrust these clastic, unequilibrated, 'gunky' rocks consisting of low temperature oxidized and hydrated minerals. They believe that CI Carbonaceous chondrites have suffered low temperature alteration, and prefer CV or CO Carbonaceous chondrites. Others prefer ordinary chondrites, or even enstatite chondrites, as models of the primordial material for the terrestrial planets, because, unlike the carbonaceous chondrites, their O isotopes are similar to those of the Earth – Moon system. • Still others attach little significance to the bulk chemical composition of chondrite meteorites, viewing them as random mixtures of different temperature components that could have come from completely different parts of the solar system. Their importance lies in the components of which they are comprised, which are thought to represent the fractionating phases of the early solar system. For example, knowing that the major mineral phases are Fe metal, olivine and pyroxene highlights the importance of distinguishing between siderophile and lithophile trace elements.

  38. Whatever one’s view about the bulk composition of chondritic meteorites, the components of the chondrite meteorites approximate that expected of the early solar condensate: Ca–Al Inclusions (CAI’s):melilite, hibonite, spinel, anorthite, corundum, perovskite T = 1400K Chondrules: olivine, low-Ca pyroxene, Feo metal T = 500K Matrix:hydrous phyllosilicates, carbonaceous matter

  39. Implications for the Earth 15 – 20 % melting MgO = 37.8 Al2O3 = 4.4 SiO2 = 45.0 CaO = 3.5 FeO = 8.1 Total = 98.8

  40. Implications for the Earth

  41. Implications for the Earth Y2TO4 = Mg2SiO4 - Fe2SiO4 dashed lines:Kd = (Fe/Mg)olivine / (Fe/Mg)liquid = 0.30.03 olivine line: Fe + Mg = 66.67 thin tie-lines: join coexisting olivine and liquid curved arrow: liquid line of descent dotted lines: isotherms, slope = -Kd

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