Neues zur Datierung von CAIs und Chondren

[karmaka]

Solar system's birth record revised

http://news.ku.dk/all_news/2012/2012.11/solar_system_birth_record_revised/

Showing that chondrules are as old as CAIs addresses a long-standing question of why chondrule formation should be delayed by up to 2 million years after CAIs. The answer – it is not.

The Absolute Chronology and Thermal Processing of Solids in the Solar Protoplanetary Disk

abstract: http://www.sciencemag.org/content/338/6107/651

Transient heating events that formed calcium-aluminum–rich inclusions (CAIs) and chondrules are fundamental processes in the evolution of the solar protoplanetary disk, but their chronology is not understood. Using U-corrected Pb-Pb dating, we determined absolute ages of individual CAIs and chondrules from primitive meteorites. CAIs define a brief formation interval corresponding to an age of 4567.30 ± 0.16 million years (My), whereas chondrule ages range from 4567.32 ± 0.42 to 4564.71 ± 0.30 My. These data refute the long-held view of an age gap between CAIs and chondrules and, instead, indicate that chondrule formation started contemporaneously with CAIs and lasted ~3 My. This time scale is similar to disk lifetimes inferred from astronomical observations, suggesting that the formation of CAIs and chondrules reflects a process intrinsically linked to the secular evolution of accretionary disks.

[karmaka]

Im Artikel steht, dass CAIs und Chondren folgender Meteorite untersucht wurden:

3 CAIs von Efremovka und 3 Chondren von Allende.

Weiterhin wurde das 'whole rock' 238U/235U Verhältnis einiger Chondrite (Ivuna, Gujba, NWA 97159   und NWA 5697) und Achondrite (NWA 2976, SAH 99555 und Juvinas) bestimmt.

Das Alter einer 29,7 mg Allende Chondre (C30) wurde mit 4567.32 +- 0.42 Myr bestimmt (207Pb/206Pb)
und liegt damit, wie im Artikel behauptet, im kurzen 160.000 Jahre CAI-Bildungs-Zeitraum, der der durchschnittlichen
Lebenszeit (~0.1 - 0,2 Myr) eines class 0 Protosterns entspricht. Alle anderen analysierten Chondren sind jünger.

The oldest chondrule age overlaps with our estimate
of CAI formation and thus requires that
aggregation of the chondrule precursor material
and its thermal processing occured within the uncertainty of its
Pb-Pb age. Moreover the age of the oldest chondrule indicates that it was
not heated to temperatures above the Pb closure temperature
after 4567.32 +- 0.42 Myr and therefore has a formation and thermal history
indistinguishable from that of CAIs. These data demonstrate
that chondrule formation started contemporaneously with CAIs
(within the uncertainy of our measurement) and continued
for at least ~3Myr.

Letztlich ist natürlich die Frage, welcher der verfügbaren 'Chronometer' der richtige ist und wann er eigentlich gestellt wurde.

 

Martin

[Thin Section]

Ivuna, Gujba, NWA 97159  und NWA 5697

Hallo Martin und Forum,

Wird wohl ein "Verschreiber" sein und es handelt sich wohl um einen dieser beiden Meteorite:

=> QUE 97159 (LL5)

=> Sahara 97159 (EH3)


Bernd