Chronology and petrology of silicates from IIE iron meteorites: evidence of a complex parent body evolution

Abstract

Abstract-IIE iron meteorites contain silicate inclusions the characteristics of which suggest a parent body similar to that of H-chondrites. However, these silicates show a wide range of alteration, ranging from Netschaëvo and Techado, the inclusions of which are little altered, to highly differentiated silicates like those in Kodaikanal, Weekeroo Station, and Colomera, which have lost metal and sulfur and are enriched in feldspar. We find these inclusions to show varying degrees of shock alteration. We made 39 Ar-40 Ar age determinations of Watson, Techado, Miles, Colomera, and Sombrerete. Watson has an Ar-Ar age of 3.677 Ϯ 0.007 Gyr, similar to previously reported ages for Kodaikanal and Netschaëvo. We suggest that the various determined radiometric ages of these three meteorites were probably reset by a common impact event. The space exposure ages for these three meteorites are also similar to each other and are considerably younger than exposure ages of other IIEs.39 Ar ages inferred for the other four meteorites analyzed are considerably older than Watson and are: Techado ϭ 4.49 Ϯ 0.01 Gyr, Miles ϭ 4.405 Ϯ 0.012 Gyr, Colomera ϭ 4.470 Ϯ 0.010 Gyr, and Sombrerete ϭ 4.541 Ϯ 0.0012 Gyr. These ages are in fair agreement with previously reported Rb-Sr isochron ages for Colomera and Weekeroo Station. Although several mechanisms to form IIE meteorites have been suggested, it is not obvious that a single mechanism could produce a suite of meteorites with very different degrees of silicate differentiation and with isotopic ages that differ by Ͼ0.8 Gyr. We suggest that those IIEs with older isotopic ages are a product of partial melting and differentiation within the parent body, followed by mixing of silicate and metal while both were relatively hot. Netschaëvo and Watson may have formed by this same process or by impact mixing ϳ4.5 Gyr ago, but their isotopic ages may have been subsequently reset by shock heating. Kodaikanal apparently is required to have formed more recently, in which case impact melting and differentiation seems the only viable process. We see no compelling reasons to believe that IIE silicate and metal derived from different parent bodies or that the parent body of IIEs was the same as that of H-chondrites.

39Ar–40Ar ages of martian nakhlites

“…Ages were calculated relative to the age of the Nl-25 hornblende (2.65 Ga) using the K decay parameters reported by Steiger and Jäger (1977). Characteristics of the NL-25 hornblende standard and experimental details are given in Bogard et al (1995Bogard et al ( , 2000. Nakhlite ArAr data are reported in Electronic annex EA-2.…”

39Ar‐40Ar chronology of R chondrites

“…The experimental methods and data reduction procedure used in this study are similar to those described by Bogard and Hirsch (1980) and Bogard, Garrison, and McCoy (2000), with the exception of the acid treatments described below. The R chondrite samples were rock fragments obtained from at least 1 cm from the outer fusion crust of the meteorite to avoid sampling the portions of the meteorite that were heated during atmospheric entry.…”

“…The age uncertainties in these plots were derived from all uncertainties in the 40 Ar/ 39 Ar ratio, including uncertainties in ratio measurement and corrections for system blanks and reactorproduced interferences. In contrast, reported Ar-Ar ages for a given sample (Table 3) were obtained by taking a mean age (weighed according to the amount of 39 Ar released) among individual temperature extractions possessing similar ages (Bogard, Garrison, and McCoy 2000). Uncertainties in the reported 39 Ar-40 Ar ages are derived both from the uncertainty in this mean age and the error in the J-value (Bogard, Garrison, and McCoy 2000).…”

Graves Nunataks 95209: A snapshot of metal segregation and core formation

“…The average age for these nine extractions is 4.521 ± 0.006 Ga. The uncertainty in this age is derived from one sigma of the distribution of these nine ages about the mean, combined with the uncertainty in irradiation constant, J (Bogard et al, 2000). It does not include a 60.5% uncertainty in absolute age of the NL-25 hornblende monitor.…”