Calcium-aluminum-rich inclusions

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Allende meteorite: sanded disc with white, irregularly shaped calcium-aluminum-rich inclusions (CAIs), gray, round chondrules in a black matrix of fayalite- rich olivine and carbon .

Calcium-aluminum-rich inclusions (abbreviated as "CAI" of Engl. C a- A l-rich I nclusions) are micrometer to centimeter-sized light-colored inclusions , which in many chondritic meteorites , especially in carbonaceous chondrites , occur. In addition, CAI-like fragments were found in samples from comet Wild 2 , which the Stardust probe brought to Earth in early 2006. CAIs formed at the beginning of the formation of our solar system and, with an age of ~ 4.6 billion years, are among the oldest known objects.

composition

The composition of these inclusions is determined by high-melting elements such as calcium (Ca), aluminum (Al), magnesium (Mg), titanium (Ti) and, to a lesser extent, scandium (Sc) and rare earth metals such as cerium (Ce), europium ( Eu) and gadolinium (Gd), which already condensed at very high temperatures up to 2000 ° Kelvin (~ 1700 ° C) from the cooling protoplanetary nebula from which the planetary system was formed. The iron contents are consistently negligibly low.

There are 3 types of CAIs based on their mineral inventory.

  • Type A CAIs (Melilith Spinel CAIs) consist of 80–85% melilite , 15–20% spinel and 1–2% perovskite . As an accessory , small amounts of plagioclase , hibonite , wollastonite and grossular can be added. Clinopyroxene , when present, forms narrow margins around inclusions or cavities and is poor in aluminum and titanium. The melilites are rich in gehlenite with 10–30 mol% Åkermanite , the spinels are pure Mg spinels.
  • Type B CAIs (pyroxene-spinel CAIs) consist of 35-60% clinopyroxene, 15-30% spinel, 5-25% plagioclase and 5-20% melilite. The melilites are richer in Åkermanite and vary more in their composition than those of the type A CAIs. In contrast to the pyroxenes the type A CAIs the pyroxenes in Type B CAIs are rich in aluminum ( Kushiroit ) and titanium ( Grossmanit ).
  • Type-C CAIs (anorthite-pyroxene-CAIs) consist of 30–60% anorthite and up to 35% each of Al-Ti-pyroxene, melilite and spinel. Smaller, sodium-rich type C CAIs contain albite-containing anorthite, pyroxene, olivine and spinel. The pyroxenes are diopside - grossmanite - kushiroite mixed crystals, the melilites are gehlenite with 35–55 mol% Åkermanite. They also contain small grains of complex alloys of various platinum metals and foreign matter .

According to the mineral structure, CAIs are divided into 2 groups:

  • Fuzzy ( fluffy ) CAIs are loose agglomerates of small crystals with a snowflake-like structure. They are assumed to be formed by resublimation directly from the gas of the presolar nebula.
  • Compact ( igneus ) CAIs are dense, rounded aggregates with the appearance of solidified melt droplets. Consequently, they are assumed to be formed by crystallization from a melt.

Both structural variants were observed for both Type A and Type B CAIs.

Education and occurrence

Using uranium-lead dating , it was possible to determine an age of 4.5672 ± 0.0006 billion years for CAIs, which can be interpreted as the beginning of our planetary system . The measured lead ratios are within the error tolerances on the so-called concordia , a theoretical curve that is used to determine the reliability of measured lead-lead aging. Old age can be seen as very well secured. Nevertheless it was argued that the error tolerances of the isotope measurements allowed a very slight disturbance of the uranium-lead isotope system in CAIs, the age based on this would only represent a lower limit of the actual age, which is in reality somewhat higher. Other methods based on manganese-chromium and magnesium-aluminum dating actually gave a slightly older age, at 4.571 billion years.

Dates of different CAIs from different meteorites yield identical ages within the scope of their errors. According to this, calcium-aluminum-rich inclusions are among the oldest surviving objects in our solar system . Their formation began with the collapse of the prestellar core and lasted only a short period of ~ 160,000 years ago during the existence of a protostar of Class 0 in the center of our solar system. At the same time, the formation of silicate melt droplets began, which were retained as chondrules and make up a large part of the chondrites . Their formation spanned around 3,000,000 years, during which our sun developed from a protostar of class 0 to protostar of class 3.

Thermodynamic equilibrium calculations allow simplified conclusions to be drawn about the sequence of minerals that are deposited from the hot gas when the solar nebula cools. Accordingly, the deposition of corundum (Al 2 O 3 ) begins at 0.001 bar and ~ 1730 ° K (~ 1460 ° C ). From ~ 1700 ° K this reacts with calcium from the gas phase to form the increasingly calcium-rich compounds hibonite (CaAl 12 O 19 ), grossite (CaAl 4 O 7 , from ~ 1660 ° K) and crotite (CaAl 2 O 4 , from ~ 1600 ° K). From this temperature the gas phase contains almost no aluminum and from ~ 1560 ° K on the one hand, the deposition of tetravalent titanium as perovskite (CaTiO 3 ) and, on the other hand, the formation of melilite (gehlenite-Åkermanite mixed crystals Ca 2 Al 1) -x Mg x Al 1-x Si 1 + x O 7 ) through the reaction of gaseous magnesium and silicon with Ca-Al oxides. These become richer in aluminum again and first grossite (up to ~ 1500 ° K), then hibonite (up to ~ 1470 ° K) again. From ~ 1500 ° K, all of the calcium has also disappeared from the gas phase. From ~ 1470 ° K, hibonite reacts with magnesium from the gas phase to form spinel (MgAl 2 O 4 ) and Melilite reacts from ~ 1430 ° K with the gas phase to clinopyroxene, which is rich in kushiroite (CaAlAlSiO 6 ) (Fassaite). Trivalent titanium is as Grossmanit (CaTi 3+ AlSiO 6 bound). The high ratio of Ti 3+ to Ti 4+ in the pyroxenes proves extremely low oxygen contents, which correspond to the solar composition of the primeval nebula. Finally, from ~ 1375 ° K, spinel and clinopyroxene react to anorthite (CaAl 2 Si 2 O 8 ).

This simple sequence of mineral deposits from and reaction with the cooling gas of the presolar nebula can best be observed in the flaky CAIs of types A and B. This pristine state was often not preserved in the further development of the solar system and the aggregation of meteorites. Many of the CAIs were partially or completely melted, lost more volatile elements such as silicon, magnesium and lighter rare earth elements when heated , and crystallized again. This could be repeated several times. Most compact CAIs were created this way.

1–2 million years after the formation of the CAIs, multiphase, metamorphic overprints often set in. They extended over a period of up to ~ 15 million years and took place at temperatures below 1000 ° K and the presence of an aqueous, fluid phase. Minerals such as grossular , monticellite , wollastonite and forsterite or, at lower temperatures, sodalite , nepheline and phyllosilicates were formed . With the participation of Cl -rich fluids, minerals of the mayenite upper group ( adrianite , wadalite , chloromayenite ) were also formed.

The first description of a calcium-aluminum-rich inclusion comes from Mireille Christophe-Michel-Lévy of the CNRS in Paris. In 1968 she described a "Melilith-Spinel-Chondrule" in the Vigarano meteorite, a carbonaceous chondrite from Italy. Since then, numerous CAIs have been described in most carbonaceous chondrites as well as some enstatite chondrites.

Individual evidence

  1. ↑ KD McKeegan et al .: Isotopic Compositions of Cometary Matter Returned by Stardust. In: Science, . tape 314 , 2006, pp. 1724-1728 , doi : 10.1126 / science.1135992 .
  2. ↑ Thomas Henning: Astromineralogy . 2nd Edition. Springer Verlag, Berlin and Heidelberg 2010, ISBN 978-3-642-13258-2 , p. 219, 224 .
  3. ↑ Refractory elements in: Planetary Science Research Discoveries Glossary CAI in: Planetary Science Research Discoveries Glossary
  4. ^ A b Lawrence Grossman: Petrography and mineral chemistry of Ca-rich inclusions in the Allende meteorite . In: Geochimica et Cosmochimica Acta . tape 39 (4) , 1975, pp. 433-434 , doi : 10.1016 / 0016-7037 (75) 90099-X .
  5. ↑ DA Wark: plagioclase-rich inclusions in carbonaceous chondrite meteorites: Liquid condensates? In: Geochimica et Cosmochimica Acta . tape 51 (2) , 1987, pp. 221-242 , doi : 10.1016 / 0016-7037 (87) 90234-1 .
  6. ^ Lawrence Grossman: Refractory inclusions in the Allende meteorite . In: Annual review of earth and planetary sciences. tape 8 , 1980, p. 559-608 ( harvard.edu [accessed March 23, 2019]).
  7. ^ Jamie Gilmour: The Solar System's First Clocks . In: Science . tape 297 , 2002, pp. 1658–1659 ( uiuc.edu [PDF; 269 kB ; accessed on December 22, 2018]). The Solar System's First Clocks ( Memento of the original from March 3, 2016 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.  @1@ 2Template: Webachiv / IABot / ijolite.geology.uiuc.edu
  8. ^ Yuri Amelin, Alexander N. Krot, Ian D. Hutcheon, Alexander A. Ulyanov: Lead isotopic ages of chondrules and calcium-aluminum-rich inclusions . In: Science . tape 297 , 2002, pp. 1678–1683 ( psu.edu [PDF; 198 kB ; accessed on December 22, 2018]).
  9. ↑ Alexander N. Krot: Dating the Earliest Solids in our Solar System . In: Planetary Science Research Discoveries . 2002, p. 1–5 ( hawaii.edu [PDF; 450 kB ; accessed on December 22, 2018]).
  10. ↑ Shukolyukov A., Lugmair GW (2002) Chronology of Asteroid Accretion and Differentiation 687-695, in Asterois III, Bottke WF, Cellino A., Paolicchi P., Binzel RP, eds., University of Arizona Press (2002), ISBN 0-8165-2281-2
  11. ↑ James N. Connelly, Martin Bizzarro, Alexander N. Krot, Åke Nordlund, Daniel Wielandt, Marina A. Ivanova: The Absolute Chronology and Thermal Processing of Solids in the Solar Protoplanetary Disk . In: Science . tape 338 , 2012, p. 651–655 ( researchgate.net [PDF; 488 kB ; accessed on March 23, 2019]).
  12. ↑ Makoto Kimura, Takashi Mikouchi, Akio Suzuki, Masaaki Miyahara, Eiji Ohtani, Ahmed El Goresy: Kushiroite, CaAlAlSiO6: A new mineral of the pyroxene group from the ALH 85085 CH chondrite, and its genetic significance in refractory inclusions . In: American Mineralogist . tape 94 , 2009, p. 1479–1482 ( rruff.info [PDF; 503 kB ; accessed on December 10, 2018]).
  13. ↑ Chi Ma, Steven B. Simon, George R. Rossman, Lawrence Grossman: Calcium Tschermak's pyroxene, CaAlAlSiO6, from the Allende and Murray meteorites: EBSD and micro-Raman characterizations . In: American Mineralogist . tape 94 , 2009, p. 1483–1486 ( rruff.info [PDF; 741 kB ; accessed on December 10, 2018]).
  14. ^ Chi Ma, John R. Beckett, George R. Rossman: Grossmanite, Davisite, and Kushiroite: Three Newly-approved Diopside-Group Clinopyroxenes in CAIs . In: Lunar and Planetary Science Conference . tape 41 , 2010 ( usra.edu [PDF; 996 kB ; accessed on December 17, 2018]).
  15. ^ A b Lawrence Grossman: Vapor-condensed phase processes in the early solar system . In: Meteoritics & Planetary Science . tape 45 , 2010, p. 7–20 ( wiley.com [PDF; 2.0 MB ; accessed on December 23, 2018]).
  16. ↑ Emma S. Bullock, Kim B. Knight, Frank M. Richter, Noriko T. Kita, Takayuki Ushikubo, Glenn J. MacPherson, Andrew M. Davis, Ruslan A. Mendybaev: Mg and Si isotopic fractionation patterns in types B1 and B2 CAIs: Implications for formation under different nebular conditions . In: Meteoritics & Planetary Science . tape 48 (8) , 2013, pp. 1440-1458 ( wisc.edu [PDF; 1.7 MB ; accessed on March 23, 2019]).
  17. ^ GJ MacPherson: Calcium-Aluminum-rich Inclusions in Chondritic Meteorites . In: Treatise on Geochemistry . tape 1 , 2003, p. 201–246 , bibcode : 2003TrGeo ... 1..201M .
  18. ↑ Alexander N. Krot, Ian D. Hutcheon, Adrian J. Brearley, Olga V. Pravdivtseva, Michael I. Petaev, Charles M. Hohenberg: Timescales and settings for alteration of chondritic meteorites . Ed .: Office of Scientific and Technical Information. November 16, 2005, p. 525–553 (English, usra.edu [PDF; 4.2 MB ; accessed on March 24, 2019]).
  19. ↑ Chi Ma, Alexander N. Krot: Adrianite, Ca 12 (Al 4 Mg 3 Si 7 ) O 32 Cl 6 , a new Cl-rich silicate mineral from the Allende meteorite: An alteration phase in a Ca-Al-rich inclusion. In: American Mineralogist . tape 103 , no. 8 , 2018, p. 1329–1334 , doi : 10.2138 / am-2018-6505 ( minsocam.org [PDF; 1.5 MB ; accessed on July 22, 2018]).
  20. ↑ Hope A. Ishii, Alexander N. Krot, John P. Bradley, Klaus Keil, Kazuhide Nagashima, Nick Teslich, Benjamin Jacobsen, Qing-Zhu Yin: Discovery, Mineral Paragenesis and Origin of Wadalite in Meteorites . In: American Mineralogist . tape 95 , 2010, p. 440–448 ( llnl.gov [PDF; 1.4 MB ; accessed on June 30, 2018]).
  21. ↑ Chi Ma, Harold C. Connolly Jr., John R. Beckett, Oliver Tschauner, George R. Rossman, Anthony R. Kampf, Thomas J. Zega, Stuart A. Sweeney Smith, Devin L. Schrader: Brearleyite, Ca12Al14O32Cl2, a new alteration mineral from the NWA 1934 meteorite. In: The American Mineralogist . tape 96 , 2011, p. 1199–1206 ( rruff.info [PDF; 539 kB ; accessed on August 7, 2018]).
  22. ↑ Mireille Christophe-Michel-Lévy: Un chondre exceptionnel dans la météorite de Vigarano . In: Bulletin de la Société française deMinéralogie et de Cristallographie . tape 91 , 1968, p. 212–214 ( persee.fr [PDF; 658 kB ; accessed on March 23, 2019]).
  23. ↑ Timothy J. Fagan, Alexander N. Krot, Klaus Keil: Calcium-aluminum-rich inclusions in enstatite chondrites (I): Mineralogy and textures . In: Meteoritics & Planetary Science . tape 35 , 2000, pp. 771-781 ( wiley.com [PDF; 5.5 MB ; accessed on December 22, 2018]).