Ar-Ar and I-Xe ages and the thermal history of IAB meteorites

Abstract

Ar-Ar and I-Xe ages and the thermal history of IAB meteoritesWe interpret these older Ar ages to represent cooling after the time of last significant metamorphism on the parent body and the younger ages to represent later 40 Ar diffusion loss. The older Ar-Ar ages for IABs are similar to Sm-Nd and Rb-Sr isochron ages reported in the literature for Caddo County. Considering the possibility that IAB parent body formation was followed by impact disruption, reassembly, and metamorphism (e.g., Benedix et al. 2000), the Ar-Ar ages and IAB cooling rates deduced from Ni concentration profiles in IAB metal (Herpfer et al. 1994) are consistent if the time of the postassembly metamorphism was as late as about 4.53 Gyr ago. However, I-Xe ages reported for some IABs define much older ages of about 4558-4566 Myr, which cannot easily be reconciled with the much younger Ar-Ar and Sm-Nd ages. An explanation for the difference in radiometric ages of IABs may reside in combinations of the following: a) I-Xe ages have very high closure temperatures and were not reset during metamorphism about 4.53 Gyr ago; b) a bias exists in the 40 K decay constants which makes these Ar-Ar ages approximately 30 Myr too young; c) the reported Sm-Nd and Rb-Sr ages for Caddo are in error by amounts equal to or exceeding their reported 2-sigma uncertainties; and d) about 30 Myr after the initial heating that produced differentiation of Caddo silicate and mixing of silicate and metal, a mild metamorphism of the IAB parent body reset the Ar-Ar ages.

Iron and Stony-Iron Meteorites

“…These cooling rates are the slowest for any natural geological material, suggesting that the mesosiderite parent body must have been large. Mesosiderites have young Ar-At ages of -3.95 Ga (Bogard and Garrison, 1998). The young At-At ages have been attributed to extended cooling within a large asteroid (Haack et al, 1996b;Bogard and Garrison, 1998) or impact resetting (Bogard er al., 1990;Rubin and Mittlefehldt, 1992).…”

K‐Ar dating of rocks on Mars: Requirements from Martian meteorite analyses and isochron modeling

“…Faster 39 Ar diffusion in MIL 03346 compared to the other two nakhlites most probably is the result of the K phases in MIL 03346 possessing even smaller grain sizes, and significantly smaller than the grain size range of the mineral separates analyzed. Figure 8 compares Ar diffusion characteristics in plagioclase and pyroxene separates of Martian meteorites with whole rock samples of a eucrite (Yamaguchi et al 2001), a mesosiderite (Bogard and Garrison 1998b), and a H-5 chondrite , all JSC data. Potassium in the three non-Martian meteorites is primarily contained in plagioclase.…”

Interpreting the I‐Xe system in individual silicate grains from Toluca IAB

“…In all IAB meteorites studied so far, I-Xe ages are generally older, from 4.5579 ± 0.0001 to 4.5762 ± 0.0024 Ga (Podosek 1970;Niemeyer 1979a;Bogard et al 2005). The I-Xe ages reported by Niemeyer (1979a) are among the oldest, seeming to predate the Pb-Pb ages of CAIs by about 7 Ma (Chen et al 1981;Göpel et al 1991;Amelin et al 2002).…”

“…Even without this set of seven ages, the remaining five I-Xe absolute ages are consistently older then corresponding Ar-Ar ages. Bogard et al (2005) suggested two possible explanations for this age discrepancy in IABs and winonaites: That the IXe system in IAB meteorites was not reset during early parent body breakup and reassembly, or that there is a bias in the decay parameters for 40 K, with the renormalized Ar-Ar ages for most meteorites about 30 Ma older (Renne 2000;Trieloff et al 2003). While a revised decay constant can address an age bias between I-Xe and Ar-Ar, it cannot address the observed age spread.…”

“…Based on these relatively slow cooling rates, the spread in apparent radiometric ages that is observed and the proposed complex formation scenarios (Benedix et al 2000;Wasson and Kallemeyn 2002;Bogard et al 2005) suggested that different chronometers might record different events in the history of the IAB parent body, and that the I-Xe system was not reset during metamorphism reflected in much younger Ar-Ar and Sm-Nd ages. In this work we present new I-Xe data for the Toluca IAB iron meteorite.…”