Thermochronology of Meteorites
Thermal histories of
planets and asteroids, which can be inferred from meteorites, provide
key information in understanding accretion processes, dimension, heat
budget and internal structure of the early solar system. Also, timing
and dynamics of ancient impacts in other planets can be deduced from
thermal histories of meteorites. Impact collision is the major
surface-reshaping processes of the most solid bodies in the solar
system, and the main mass exchange processes among the planets. Another
usage of the estimated thermal histories of meteorites is calibrating
decay constants of radio-nuclides, as will be described in (2). Because
meteorites are normally old, they have enough daughter elements to be
measured with high precision and accuracy, thus providing excellent
chances to cross-calibrate decay constants of different radioactive
of LL6 St. Severin
Low-temperature (< ~300 ”ĘC) thermal histories of meteorites were poorly understood probably because of lack of appropriate thermochronometers. The whole rock (U-Th)/He method was applied various meteorites to constrain low temperature histories, but many of the results are with large uncertainties mainly due to cosmogenic 4He (> ~20% of the total 4He). The contribution of the cosmogenic 4He can be significantly reduced by analyzing single grains of U- and Th- concentrated phases, such as apatite and merrillite. The recently improved (U-Th)/He techniques allow for single grain analysis, yielding much more reliable (U-Th)/He ages with cosmogenic 4He correction being less than 5%.
The revised (U-Th)/He
techniques were rarely applied to meteorites. From single phosphate
grains in Acapulco meteorite, we have obtained (U-Th)/He ages of ~4.5
Ga, essentially same as the formation age of the solar system. The
study showed that the (U-Th)/He method can yield reliable results even
for such old samples. Another example of single grain study was on Los
Angeles (LA) Martian meteorite. The (U-Th)/He ages from the LA
merrillites are consistent with the cosmic ray exposure ages (3.1 Ma)
suggesting the impact metamorphism and ejection of the LA from Mars
happened in a very short time interval. The main differences of these
two studies from the previous ones are (1) the amount of samples used
was significantly reduced, and (2) the U, Th and 4He compositions were
measured from the same grains, thus excluding possible age bias due to
One of the hot issues in the planetary science society is if viable
life can be transferred from a planet to others by meteorites. There is
a growing body of evidence that the ejection of meteorite from large
planets does not require high pressure-temperature metamorphism on the
ejected body (theoretical calculation by Univ. of Arizona research
group; natural remanence magnetism and 40Ar/39Ar data interpretation by
CalTech group). Although all the Martian meteorites have signature of
high P-T shock metamorphism, it is not clear if such metamorphism is
directly related to "ejection" of the meteorites from Mars. If the peak
shock metamorphism is not related to ejection, and if spores existed in
Mars after the shock metamorphism, it is possible that the biologic
entity can be transferred in meteorites. However, if the peak shock
metamorphism is related with ejection, it would be less likely for
spores to survive the extreme conditions of shock metamorphism.
Therefore, it is important to properly link the observed metamorphic
conditions with timing of such events in order to assess the
possibility of the panspermia hypothesis. The (U-Th)/He is the unique
isotopic system which is sensitive enough to low-temperature or short
duration shock episodes. We are currently working on numereous Martian
meteorites for single grain (U-Th)/He dating.
Shuster D (2011) Low-T thermochronology of St. Severin LL6
chondrite revealed from single grain (U-Th)/He ages. 42nd
Lunar and Planetary Science Conference, Abstract 2683.
Farah AE, Min K (2011) Phosphate (U-Th)/He
thermochronology of Zagami and ALH77005 Martian meteorites. 42nd
Lunar and Planetary Science Conference, Abstract 2726.
Min K, Lee SR (2010) (U-Th)/He dating on
Martian meteorites: A review and perspective. Journal of
Petrological Society of Korea,
v. 19, p. 255-267.
Yu Y, Doh S-J, Kim W, Min K (2009) Ancient stable
magnetism of the Richardton H5 chondrite. Physics of the Earth
and Planetary Interiors, v. 177, no. 1-2, p. 12-18.
K, Reiners PW (2007)
of Meteorite ALH84001. Earth and Planetary
Science Letters, v. 260, p. 72-85.
Min K (2005) Low-Temperature Thermochronometry of Meteorites. In: Reiners PW and Ehlers T (ed) Thermochronology, Reviews in Mineralogy and Geochemistry, v. 59.
Min K, Reiners PW (2005) Low-temperature thermal history of Martian meteorite ALH84001 from (U-Th)/He thermochronometry. 36th Lunar and Planetary Science, Abstract 2214.
Min K, Reiners PW, Nicolescu S and Greenwood JP (2004) Age and temperature of shock metamorphism of Martian meteorite Los Angeles from (U-Th)/He thermochronology. Geology, v.32, p. 677-680.
Min K, Farley KA, Renne PR and Marti K (2003) Single grain (U-Th)/He ages from phosphates in Acapulco meteorite and implications for thermal history. Earth and Planetary Science Letters, v. 209, p. 323-337.