Stranger (meteorite)

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As a stranger opaque inclusions are calcium-aluminum-rich inclusions (CAI) of chondritic meteorites designated. They are some of the most exotic and least understood objects in meteorites. The rounded inclusions in clinopyroxene or melilite crystals , which are rarely more than 0.1 mm in size, differ fundamentally from their surroundings. In contrast to the white, more transparent calcium - aluminum - silicates , they are black, opaque and predominantly contain nickel-iron alloys , sulfides or oxides of iron (Fe), nickel (Ni), tungsten (W), molybdenum (Mo) , Vanadium (V) and metallic granules made of platinum metals . All of these elements are absent from the surrounding minerals of the CAIs and strangers are the main source of platinum metals in meteorites. While the CAI is assumed to be formed at very high temperatures under extremely reducing conditions, the foreign bodies show compositions that indicate significantly lower temperatures and oxidizing conditions.

composition

Foreigners can be of varying complexity and inhomogeneity in their structure and composition.

Type 1 foreign bodies consist of 50–70% by volume of metallic nickel-iron, sulfides (e.g. pentlandite , molybdenite MoS 2 , tungstenite WS 2 ) and grains of alloys of platinum metals. 30-50% by volume make calcium phosphates (eg. B. apatite ), oxides (eg. B. magnetite ) and silicates (eg. B. Burnettit -rich Klinopyroxen, melilite, anorthite , wollastonite , nepheline , sodalite ) is made. The crystals of nickel-iron, pentlandite, calcium phosphate and platinum metal alloys are often idiomorphic and show no reaction structure. The platinum metal grains are very small (<1 µm) and distributed fairly evenly over all of the foreigner's minerals.

Type 2 foreigners consist of 70–90% by volume of metallic nickel-iron and only a few sulfides (molybdenite, tungstenite). Silicates and phosphates are found, if at all, as an aggregate on one side of the inclusion and tiny grains of platinum metal alloys are found finely distributed in the nickel-iron as well as in the silicates. Their composition varies greatly and ranges from almost pure ruthenium (Ru) or platinum (Pt) to rhenium- ruthenium alloys to complex alloys made of Os , Ir , Pt, Ru , Rh , Re , Mo, W.

Type 3 aliens are similar in their composition to type 1 aliens, but show a different grain structure and mostly a concentric shell structure. They consist of 0–70% by volume of metallic nickel-iron, which together with sulphides (molybdenite, tungstenite) in an often spongy, loose structure makes up the core of the foreigners. These cores are surrounded by a crust of oxides, phosphates and silicates, which contain both high-melting and volatile elements. Documented were z. B. pyroxene, spinel , baddeleyite , anorthite, nepheline and sodalite. The smallest grains of platinum metal alloys can be found distributed in all minerals of the aliens, in the silicate crust sometimes also in lumpy aggregates.

Education and occurrence

Strangers are found in calcium-aluminum-rich inclusions (CAI) of carbonaceous chondrites . Here they occur both in type A CAIs and in type B CAIs, accumulated as inclusions in spinel and façade. In the literature on meteorites, the term Fassaite has been used for Kushiroite - Grossmanite - Davisite - Burnettite - Diopside - mixed crystals .

Spinel framboids, which are raspberry-shaped, spherical aggregates of small spinel crystals, are among the first condensates from the cooling solar nebula . They are found as inclusions in CAIs and contain foreign substances that were probably inserted into the spinel aggregates after the condensation of the spinels and before the formation of the silicates (melilite, pyroxene, anorthite).

A ~ 12 µm large alien was also found in cosmic dust particles that were recovered from deep sea sediments of the Indian Ocean. It is free from tungsten and molybdenum and shows a simple zoning with a core made of platinum metals surrounded by metallic Ni-Fe and an outer edge made of sulfides.

Strangers are generally no larger than a few µm and the few large and well-examined specimens have their own names.

Willi is a huge, complex type 3 alien with a diameter of ~ 0.15 mm, which was discovered in a melilite crystal of type B CAI 5241 of the Allende meteorite . Its core consists of several groups of idiomorphic V-magnetite crystals, which are surrounded by loosely packed skeletal crystals of a Ni-Fe alloy. The Ni-Fe crystals consist of ~ 60% Ni with 36% Fe and small amounts of cobalt (Co), Pt, V, Cr and phosphorus (P). In the Ni-Fe crystals there are tiny, idiomorphic inclusions made of V-magnetite. The largest group of V-magnetites in the core is surrounded by a ring of troilite . Also in Willi's core, the mineral scheelite was found in a meteorite for the first time . It was probably formed here by the reaction of ferberite with a calcium-containing mineral, e.g. B. apatite. This complex core is encased in a dense layer of V magnetite. At the contact with the surrounding melilite, the alien is surrounded by vanadium-rich pyroxene (burnettite) and spinel. Distributed throughout the alien are micron-sized crystals of a complex, low-platinum osmium-ruthenium alloy.

The structural features suggest that Willi and the numerous small strangers of this CAI existed before the silicates and, possibly together with the spinels, were absorbed by a CAI melt droplet and then cooled down quickly.

Zelda is a gigantic Type 3 alien with a diameter of ~ 1 mm. This sulphide-rich granule was recovered from a type B CAI Egg6 facade crystal of the Allende meteorite. It consists of ~ 1/3 each of pentlandite and troilite with ~ 20% vanadium magnetite, 5% Ni-Fe alloy, 3% merrillite , 2% molybdenite and small amounts of Os-Ru and Pt-Ir alloys. The core consists of troilite and is surrounded by a rim made of pentlandite. In these sulfides there are small, isolated crystals of Ni-Fe and irregularly shaped V-magnetite crystals. Idiomorphic crystals of merrilite and molybdenite are found evenly distributed in pentlandite and troilite. Pt-Ir grains occur exclusively in the vicinity of NiFe metal in pentlandite and are a residue of the Pt-Ir-containing NiFe alloy that was converted to pentlandite. Unlike the Pt-Ir grains, the Os-Ru grains appear evenly throughout the foreign matter and are formed before the sulfides. An edge area made up of V-spinel and pyroxene was not observed, but may have remained in the host crystal when the foreign material was released from it.

Zelda was formed by transforming an alien, which, like Willi, was rich in Ni-Fe metal and magnetite at temperatures around 800-900 ° C.

Zorba is another, huge, sulphide-rich alien from the CAI Egg6 of the Allende meteorite, with higher contents of the Ni-Fe alloy than Zelda. Its composition is estimated to be 41% Ni-Fe, 45% pentlandite and troilite, 4.9% V-magnetite and 7.5% molybdenite. Unlike Willi, Zorba only contains small amounts of tungsten.

There are different models for the formation of foreigners and none can explain all the characteristics of foreigners.

The working group around El Gorsey at the Max Planck Institute for Nuclear Physics in Heidelberg assumes that the aliens already existed before the CAIs in which they are trapped and were absorbed by them as solid foreign bodies. They explain the high variability, especially in the composition of the platinum metal alloys, with the many temporally and spatially independent condensation processes and presolar material that existed before the formation of the solar nebula and was later baked into the complex alien structures. Another possible explanation for the variable compositions of the platinum metal alloys would be that not one alloy condensed from differently composed areas of the presolar nebula, but several, structurally different and chemically immiscible alloys from one, chemically homogeneous area.

The group around H. Palme regards foreigners as the products of a multi-stage process that begins with a condensation of high-melting, siderophilic elements , followed by their inclusion in CAIs and subsequent oxidation (formation of magnetite) and sulphidation (troilite, pentlandite).

Current electron microscopic studies on these granules of refractory metals point in the same direction. Each examined grain was found to be unique in structure and composition, even if they were included in the same crystal immediately adjacent. Every metal grain has its own history of origin in the early phase of the solar system and was enclosed by the minerals of the aliens at a later point in time.

These models hardly explain how the oxides, phosphates and silicates, which are stable at low temperatures, survived the high temperatures and reducing conditions that prevailed when the CAIs were formed. The researchers working with Blum and Wasserburg from the California Institute of Technology in Pasadena propose an alternative model for the development of aliens . They suspect that the aliens did not emerge before, but rather during the formation of CAIs. Accordingly, droplets of metallic melts of siderophilic elements separated from the calcium aluminosilicate melts. These alloys reacted at low temperatures (~ 500 ° C) with penetrating oxygen and sulfur to form magnetite and sulfides. This model is supported by current experiments and investigations of platinum metal grains from various meteorites. Most of the properties of these alloys are consistent with segregation from a silicate melt.

credentials

  1. ↑ a b c d e A. El Goresy, K. Nagel & P. ​​Ramdohr: Fremdlinge and their noble relatives . In: Lunar and Planetary Science Conference, 9th . tape 1 , 1978, p. 1279-1303 , bibcode : 1978LPSC .... 9.1279E .
  2. ↑ A. El Goresy, K. Nagel & P. Ramdohr: Spinel framboids and strangers in Allende inclusions - Possible sequential markers in the early history of the solar system . In: Lunar and Planetary Science Conference, 10th . tape 1 , 1979, p. 833-850 ( harvard.edu [accessed March 5, 2019]).
  3. ↑ NG Rudraswami and Prasad M. Shyam: STRANGERS TYPE OBJECT IN A COSMIC SPHERULE FROM THE INDIAN OCEAN. In: LunarandPlanetaryScienceConference . tape 44 , 2013 ( nio.org [PDF; 146 kB ; accessed on March 5, 2019]).
  4. ^ JT Armstrong, A. El Goresy, GP Meeker & GJ Wasserburg: Willy: a Prize Noble Fremdling . In: LUNAR AND PLANETARY SCIENCE . tape 15 , 1984, pp. 13-14 ( harvard.edu [accessed March 5, 2019]).
  5. ↑ A. El Goresy, JT Armstrong & GJ Wasserburg: Allende 5241: Anatomy of a stranger-Rich CAI . In: LUNAR AND PLANETARY SCIENCE . tape 15 , 1984, pp. 242–243 ( harvard.edu [accessed March 5, 2019]).
  6. ↑ a b c d JT Armstrong, ID Hutcheon, and GJ Wasserburg: Zelda Revealed . In: LUNAR AND PLANETARY SCIENCE . tape 16 , 1985, pp. 15-16 ( harvard.edu [accessed March 5, 2019]).
  7. ^ A b John T. Armstrong, Ian D. Hutcheon, GJ Wasserburg: Zelda and Company: Petrogenesis of sulfide-rich Fremdlinge and constraints on solar nebula processes . In: Geochimica et Cosmochimica Acta . tape 51 (12) , 1987, pp. 3155-3173 , doi : 10.1016 / 0016-7037 (87) 90125-6 .
  8. ^ L. Grossman, AM Davis, V. Ekambaram, JT Armstrong, ID Hutcheon & GJ Wasserburg: Bulk Chemical Composition of a Fremdling from an Allende Type B Inclusion . In: LUNAR AND PLANETARY SCIENCE . tape 17 , 1986, pp. 295–296 ( harvard.edu [accessed March 5, 2019]).
  9. ^ A b Lawrence Grossman: Refractory inclusions in the Allende meteorite . In: Annual review of earth and planetary sciences. tape 8 , 1980, p. 559-608 ( harvard.edu ).
  10. ↑ P. Sylvester, B. Ward, & L. Grossman: Chemical compositions of Fremdling from an Allende inclusion . In: Meteoritics . tape 24 , 1989, pp. 330 , bibcode : 1989Metic..24S.330S .
  11. ↑ H. Palme, ID Hutcheon, B. Spettel: Composition and origin of refractory-metal-rich assemblages in a Ca, Al-rich Allende inclusion . In: Geochimica et Cosmochimica Acta . tape 58 (1) , 1994, pp. 495-513 , doi : 10.1016 / 0016-7037 (94) 90479-0 .
  12. ↑ Luke Daly, Phil A. Bland, Kathryn A. Dyl, Lucy V. Forman, David W. Saxey, Steven M. Reddy, Denis Fougerouse, William DA Rickard, Patrick W. Trimby, Steve Moody, Limei Yang, Hongwei Liu, Simon P. Ringer, Martin Saunders, Sandra Piazolo: Crystallography of refractory metal nuggets in carbonaceous chondrites: A transmission Kikuchi diffraction approach . In: Geochimica et Cosmochimica Acta . tape 216 , 2017, p. 42–60 , doi : 10.1016 / j.gca.2017.03.037 .
  13. ↑ JD Blum, GJ Wasserburg, ID Hutcheon, JR Beckett, & EM Stolper: Opaque Assemblages in the Allende Meteorite: Evidence for Equilibrium at Low Temperatures (<770K) and High fO2-fS2 . In: Abstracts of the Lunar and Planetary Science Conference . tape 19 , 1988, pp. 106-107 ( harvard, edu ).
  14. ↑ D. Schwander, L. Kööp, T. Berg, G. Schönhense, PR Heck, AM Davis, U. Ott: Formation of refractory metal nuggets and their link to the history of CAIs . In: Geochimica et Cosmochimica Acta . tape 168 , 2015, p. 70–87 , doi : 10.1016 / j.gca.2015.07.014 .