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Thermal history of chondrites: Hot accretion vs. metamorphic reheating
Author(s) -
Haack Henning,
Taylor G. J.,
Scott E. R. D.,
Keil Klaus
Publication year - 1992
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/92gl02465
Subject(s) - chondrule , chondrite , accretion (finance) , formation and evolution of the solar system , geology , nebula , meteorite , geochemistry , astrobiology , astrophysics , physics , stars
The different degrees of equilibration observed between different petrologic types of ordinary chondrites have been attributed to either hot accretion and a resulting slower cooling of the higher petrologic types, or to cold accretion followed by a separate metamorphic reheating event. We find that constraints on the thermal evolution of chondrules from primary heating in the nebula to thermal processing in chondrite parent bodies are inconsistent with accretion of hot (>800°C) chondrules: a) Cooling rate experiments suggest that chondrules cooled to below 800°C in less than a day. We find that the well‐mixed state of chondrules and matrix material in chondrites could not have been obtained in such a short period of time, b) Chondrule cooling rates indicate that they cooled in a hot, relatively slowly cooling environment, whereas the volatile contents of matrix material in chondrites indicate formation in a cold environment. Relatively slowly cooling chondrules could therefore not have coexisted with the matrix material in the nebula, as required by the “hot accretion” scenario, c) Our calculations show that retention of heat in the hot accretion scenario requires accretion rates of the order of 100 km/y, i.e., four orders of magnitude above the accretion rates inferred from theoretical studies for accretion of planetary objects. We therefore find it impossible to create a physical scenario, within the existing constraints, that would allow for hot accretion of chondrules and conclude that the petrologic types of ordinary chondrites were established by metamorphic reheating after cold accretion.