Autor Thema: 'Meteoriten-Mix' bei der Entstehung der Erde vielseitiger als bisher angenommen  (Gelesen 3015 mal)

Offline karmaka

  • Administrator
  • Foren-Legende
  • *****
  • Beiträge: 6136
    • karmaka
Die Materie, die zur Entstehung der Erde beitrug, ist vermutlich vielseitiger gewesen, als bisher angenommen.
Es waren wohl nicht vor allem Enstatit-Chondriten.

http://www.sciencemag.org/content/early/2012/02/29/science.1219509 (Science ; 1. März 2012)

Zitat
The compositions of Earth materials are strikingly similar to those of enstatite chondrite meteorites in many isotope systems. Although this suggests that Earth largely accreted from enstatite chondrites, definitive proof of this model has been lacking. By comparing the Si isotope signatures of several extraterrestrial materials with terrestrial samples, we show that they cannot be explained by core formation scenarios involving a bulk Earth of enstatite chondrite composition. Si isotope similarities between the bulk silicate Earth and the Moon preclude the existence of a hidden reservoir in the lower mantle, a necessary condition of the enstatite chondrite model, and require an equilibrium process after the Moon-forming impact. A three-endmember chondritic mixing model for the Earth reconciles the Si isotope similarities between enstatite chondrites and Earth.

http://www.space.com/14752-earth-formation-meteorite-history.html?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+spaceheadlines+%28SPACE.com+Headline+Feed%29

Zitat
Scientists had thought that most of the bodies that merged to make Earth formed from a narrow zone in space and were similar to each other, belonging to a subclass of meteorites called enstatite chondrites. This idea was based on measurements of numerous striking similarities between different types of atoms (called isotopes) of elements such as oxygen, nickel and chromium, between the Earth and enstatite chondrites. [...]
Using computer models of Earth's formation, the researchers calculated that a mix of three types of meteorite ingredients could have produced the right blend of oxygen, nickel, and chromium isotopes previously measured in Earth samples, as well as their new findings about silicon isotopes in terrestrial and meteorite samples.

The results suggest that a mixture of chondrites, rather than enstatite chondrites alone, probably combined to create the Earth.

"This is the first time that a different composition in isotopes is observed for a major element between the enstatite chondrites and the Earth," Fitoussi told SPACE.com. "So that's quite different from what has been observed before."

The fact that the silicon isotope compositions measured were similar in both the Earth and moon rock samples suggests that the material that formed the moon must have mixed with the Earth's mantle before the moon formed so that both bodies hold the same signature.

 :hut:

Martin
« Letzte Änderung: März 01, 2012, 23:29:38 Nachmittag von karmaka »

Offline karmaka

  • Administrator
  • Foren-Legende
  • *****
  • Beiträge: 6136
    • karmaka
Re: 'Meteoriten-Mix' bei der Entstehung der Erde vielseitiger als bisher angenommen
« Antwort #1 am: April 14, 2014, 17:00:55 Nachmittag »
Um diesen Thread wieder ein wenig zu beleben...

Eine neue Studie kommt auf Grundlage einer neuen Methode der Analyse von Calcium-Isotopenverhältnissen in der Erdkruste und in Meteoriten zu folgender Schlussfolgerung:

Zitat
The calcium isotope data presented in this study suggest that both ordinary and enstatite chondrites are representative of the bulk of the refractory materials that formed Earth. On the basis of calcium isotopes, carbonaceous chondrites (with the exception of CO) are not representative of the fraction of condensable material that accreted to form the terrestrial planets and can be excluded as unique contenders for the building blocks of Earth; however, on the basis of other isotopic systems, CO chondrites can be excluded as well.

The nature of Earth's building blocks as revealed by calcium isotopes

Maria C. Valdes, Manuel Moreira, Julien Foriel, Frédéric Moynier

Earth and Planetary Science Letters, Volume 394, 15 May 2014, Pages 135–145

LINK

Zitat
ABSTRACT: Calcium is the fifth most abundant element in the Earth and in chondrites and is a pure lithophile element which does not partition into planetary cores. Therefore, the calcium isotopic composition of the mantle represents the bulk Earth and calcium isotopes have the potential to reveal genetic links between Earth and meteorites. However, whether calcium exhibits significant mass-dependent variations among Earth and the various chondrite groups, and the magnitude of these variations, is still contentious. Here we have developed a new method to analyze calcium isotope ratios with high precision using multiple-collector inductively-coupled-plasma mass-spectrometry. The method has been applied to a range of terrestrial and meteoritic samples. We find that the Earth, the Moon, and the aubrite parent body are indistinguishable from enstatite, ordinary, and CO chondritic meteorites. Therefore, enstatite chondrites cannot be excluded as components of Earth's building blocks based on calcium isotopes, as has been proposed previously. In contrast, CI, CV, CM and CR carbonaceous chondrites are largely enriched in lighter calcium isotopes compared to Earth, and, overall, exhibit a wide range in calcium isotopic composition. Calcium is the only major element, along with oxygen, for which isotopic variations are observed among carbonaceous chondrite groups. These calcium isotope variations cannot be attributed to volatility effects, and it is difficult to ascribe them to the abundance of isotopically light refractory inclusions. The calcium isotope data presented in this study suggest that both ordinary and enstatite chondrites are representative of the bulk of the refractory materials that formed Earth. On the basis of calcium isotopes, carbonaceous chondrites (with the exception of CO) are not representative of the fraction of condensable material that accreted to form the terrestrial planets and can be excluded as unique contenders for the building blocks of Earth; however, on the basis of other isotopic systems, CO chondrites can be excluded as well.

Offline karmaka

  • Administrator
  • Foren-Legende
  • *****
  • Beiträge: 6136
    • karmaka
Re: 'Meteoriten-Mix' bei der Entstehung der Erde vielseitiger als bisher angenommen
« Antwort #2 am: September 23, 2015, 21:04:45 Nachmittag »
Ein ganz aktueller Beitrag (open access!) zu einem alten Thema:

Cosmochemical fractionation by collisional erosion during the Earth’s accretion

Boujibar, A. et al.
Nat. Commun. 6:8295 doi: 10.1038/ncomms9295 (2015)
Published 23 September 2015

LINK

 

   Impressum --- Datenschutzerklärung