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Solar System formation - Meteorites

Solar System formation - Meteorites. Classification and geologic context of meteorites. How are meteorites classified? Hierarchy of classification, first based on process. Next level is based on texture, geochemistry, and isotopic composition.

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Solar System formation - Meteorites

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  1. Solar System formation - Meteorites • Classification and geologic context of meteorites. • How are meteorites classified? • Hierarchy of classification, first based on process. • Next level is based on texture, geochemistry, and isotopic composition. • Names (How does a meteorite get an official name?).

  2. Solar System formation - Meteorites • Iron meteorites • Cores of small planetesimals. • FeNi - rich metal, which is only found essentially in the core of Earth (natural, one major location).

  3. Solar System formation - Meteorites

  4. a

  5. O-isotopes: 16, 17, and 18 with 16 being least abundant. c Isotope: equal protons but different # neutrons, hence different at mass

  6. d

  7. Figure 8

  8. a

  9. b

  10. Why are chondrites important? • Age = ~ 4.566+2Ma/-1Ma billion years (Allégre et al, 1995) or 4.5647±0.0006 billion years (Amelin et al., 2002). • They are accretionary rocks formed within the protoplanetary disk. • They are relatively unprocessed planetary materials that have a solar-like composition. • They contain components that would not be predicted to exist if they did not exist.

  11. Anatomy of a Chondrite • Chondrules • CAI’s or Refractory inclusions • Fragments of the above • Matrix • Opaques: Fe-Ni, FeS • Pre-solar grains

  12. Why are chondrites important? 3. They are relatively unprocessed planetary materials that have a solar-like composition. Rocks = Sun’s photosphere

  13. Why are chondrites important? 4. They contain components that would not be predicted if they did not exist. • Chondrules (igneous rocks) and igneous CAIs

  14. Chondrules and CAIs

  15. Chondrule mineral components? FeMg-rich chondrules: • Olivine (fosterite Mg2SiO4 in solid solution with fayalite, Fe2SiO4) • Othropyroxene (enstatite, Mg2Si2O6 Ca-poor in solid solution with ferrosilite, Fe2Si2O6) • Clinopyroxene (diopside, CaMgSi2O6) • Glass (trash can, varying amounts of Ca, Al, Na, K, etc.) • ± minor abundances of chromite (Fe, Mg)Cr2O4, FeNi metal, FeS, spinel MgAl2O4

  16. CAI mineral components? Refractory inclusions known as CAIs: • Spinel (MgAl2O4) • Melilite (gehlenite, Ca2Al2SiO7 in solid solution with akermanite, Ca2MgSi2O7) • Fassaite (Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6: This is not an official mineral name. It is a complex augite. • Anorthite (Ca2Al2Si2O8) • ± all kinds of other minerals, primary and secondary, in varying abundances.

  17. Fe, Mg-rich Chondrules FeO-poor or type I

  18. Fe, Mg-rich Chondrules FeO-rich or type II

  19. Textural Types • Nonporphyritic textures = nearly complete to complete melting. • Barred, radial, cryptocrystalline and glassy.

  20. Textural Types • Porphyritic textures = partial melting • Porphyritic to micro-porphyitic. • Olivine-rich, olivine + pyroxene, pyroxene-rich ± glassy mesostasis.

  21. Textural Types • Compound chondrules • Information on local chondrule abundances during heating.

  22. Why are igneous CAIs and chondrules important? • They are considered free-floating wanders that are self-contained igneous rocks that reacted with ambient nebular gases. • A high-temperature, transient heating event melted minerals and rocks within the protoplanetary disk. • The mechanism that melted these objects is not intuitive to astrophysics, what was it? • What was the mechanism that melted them?

  23. Why are igneous CAIs and chondrules important? • Why did these object not cool as a black body (or why did they cool slowly)? • What can they tell us about the environment within the disk where they formed? • What is their stable isotope composition telling us about the evolution of planetary materials. • What is the relationship of these objects to terrestrial planet formation?

  24. 1. What was the mechanism(s) that melted these objects? Constraints: Detailed characterization Experimental petrology

  25. 1. What was the mechanism(s) that melted these objects? First, any model that reproduces these rocks MUST do so quantitatively and make testable predictions that match the rock record. • Petrography/Petrology - This is a must! • Geochemistry - Follows with above. • Isotopic Signatures - Critical? • Environment of formation vs. mechanism • Mass-dependant fractionation

  26. 1. What was the mechanism(s) that melted these objects? Petrology/Petrography: • Igneous textures • Fractionated chemistry of crystals • Bulk composition • Redox conditions (e.g., FeO vs. Fe) • They are controlled by kinetic reactions and are not systems in equilibrium. Basically, they are igneous rocks and all characteristics of such rocks must be determined and reproduced.

  27. 1. What was the mechanism(s) that melted these objects? • The first-order constraint that must be quantitatively predicted and hence tested by any model is the reproduction of the rock’s thermal histories. • Zero-order observation = Igneous rock • This means whether we are discussing Fe, Mg-rich chondrules, Al-rich chondrules, type B or C, CAIs, etc. • Critical to understanding the problem.

  28. Thermal Histories of Chondrules • Pre-melting conditions • Peak melting • Cooling rates • Post-melting conditions First order constraints on thermal histories.

  29. 1. Constraints on Pre-melting • Limited to temperatures between 650 - 1000 K for seconds to many minutes (Lauretta et al., 2001). • Abundances of primary S phases and moderately volatile elements such as Na.

  30. 2. Constraints on Peak Melting • Tmax set from Tliq of barred olivine chondrule production - Texture • Tmax = 1700-2100 K • Time = Minutes • Average (porphyritic) = ~1800 K Bulk Composition!

  31. 2. Constraints on Peak Melting • Constraints on CAI formation are essentially restricted to type B1. • Tmax = 1750 K • Melilite appearance • Time = Mins - “Hrs”

  32. 3. Constraints on Cooling Rate • Non-porphyritic chondrules: Textures • BO = 500-3000 K/hr • Radial = 5-3000 K/hr • Abundance = 10-14%

  33. 3. Constraints on Cooling Rate • Porphyritic chondrules: Texture and chemistry • PO = 5-1000 K /hr with 5-100 K/hr best for producing the majority. • Abundance = 85%

  34. 3. Constraints on Cooling Rate • Type B1 CAIs • Cooling rate 0.5-50 K/hr but best results are from 0.5 - 10 K/hr. • Based on melilite texture and composition.

  35. Type B1 CAIs 0.5 -50 K/hr 0.5-10 K/hr best Stolper and Paque, 86 Porphyritic Chondrules 5-100 K/hr 10 K/hr preferred Jones and Lofgren, 93 Type II bulk 3. Constraints on Cooling Rate

  36. 4. Constraints on Post-melting • Recycling of chondrules and CAIs • With some CAIs, this occurred after alteration in the nebula. • That’s about it! Dusty relict olivine grains

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