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 nuclides.

Thermal History 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%.

Alpha  Recoil Cartoon

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 sample heterogeneity. 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.

Related publications:

Min K, Reiners PW, 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.

Min K, Reiners PW (2007) High-Temperature Mars-to-Earth Transfer 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.