I-Xe and Ar-Ar chronology of enstatite chondrite and enstatite achondrite parent bodies in the Early Solar System
Final Report Abstract
A chronologic study was conducted on a suite of enstatite chondrites and achondrites utilizing the 40Ar/39Ar dating system (based on the decay of 40K with half-life 1.25 Ga) and the 129 129 I- Xe dating system (half-life 15.7 Ma). With the Ar-Ar method it is possible to derive absolute ages whereas the I-Xe system represents a method for determination of relative ages between meteorites, which commonly is anchored to the absolute time-scale by comeasurement of the Shallowater meteorite (4562.3±0.4 Ma). The aubrites showed low concentrations of xenon and no significant anomalies in 129Xe/132Xe related to 129I decay, indicating a late resetting or formation of the aubrites included in this study. Only one aubrite (Peña Blanca Spring) displayed an isochron age indistinguishable to a previously reported age of 4-6 Ma more recent than Shallowater. The I-Xe ages of EH4, 4/5 and 5 chondrites are old (+0.6 to -1 Ma, negative/positive sign = more recent/ancient than Shallowater), again in agreement with previous reports. The EH3 chondrite Sahara 97096 showed a significantly younger age of -7.87±0.43 Ma, likely reflecting resetting of the I-Xe system during an impact-induced shock event early in the history of the EH chondrite parent body. The minimum I-Xe age of EH-impact melt LAP 02225 is -5 Ma. The two EL6 chondrites included in this I-Xe study show the same age offset towards younger ages relative to the EH4 to 5 chondrites as reported previously. Their ages are -3.8 to -5.2 Ma. Whether this dichotomy is a consequence of the different metamorphic history (documented in the distinct petrologic types) or mirrors a basic age difference in formation of both parent bodies remains open. An important result of our Ar-Ar dating study was the detection of a trapped 40Ar/36Ar component with non-solar composition in EL chondrites, likely reflecting incomplete degassing and subsequent re-mobilization of Ar within the respective meteorites. Therefore, we found two isochron relations (at lower and higher argon release temperatures) with essentially the same slope (i.e. apparent age) but different intercepts (40Ar/36Ar = 180-270 in lower-T extractions, 40Ar/36Ar ~ zero in higher-T extractions). Most EL chondrites showed isochron ages in the range of 4.45 to 4.51 Ga. The only two EL 4 chondrites included in this study point to disturbance of the K-Ar system at ca. 4 Ga. Most EH chondrites also show a likely shock-induced reset of the K-Ar system mostly precluding determination of a welldefined age. Only for Indarch (EH4) and St. Marks (EH5) a moderately well-defined age plateau of 4.42 to 4.44 Ga might pinpoint one single impact event in the history of the EH chondrite parent body whereas much younger apparent ages of Sahara 97096 (EH3) indicate a later collisional history at ca. 2 Ga. The oldest Ar-Ar age of this study was observed for EH-impact melt LAP 02225 with an age of 4.53±0.01 Ga - note the likely necessary upward correction of 20-30 Ma accounting for the age bias between the U-Pb and K-Ar systems. No correlation of petrologic type with Ar-Ar age (so far) has been observed for either subgroup of enstatite chondrites as might be expected for a simple metamorphic cooling history. Instead, both enstatite parent bodies appear strongly affected by impactinduced thermal events.
Publications
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(2011). Comment on „Joint determination of 40K decay constants and 40Ar*/40K for the Fish Canyon sanidine standard, and improved accuracy for 40Ar/39Ar geochronology“ by Paul R. Renne, Mundil R., Balco G., Min K. and Ludwig K. (2010). Geochimica et Cosmochimica Acta 75, 5094-5096
Schwarz W.H., Kossert K., Trieloff M., Hopp J.
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(2011). I-Xe and Ar-Ar dating of enstatite meteorites. 74th Annual Meeting of the Meteoritical Society, London, Meteoritics and Planetary Science 46, Suppl., A5241
Hopp J., Trieloff M., Ott U., Pravdivtseva O.V.
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(2013). I-Xe chronology of enstatite chondrites. 76th Annual meeting of the Meteoritical Society, Edmonton, Meteoritics and Planetary Science 48, Suppl. A5098
Hopp J., Trieloff M., Ott U.
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(2013). New Ar-Ar ages of EL chondrites. 44th Lunar and Planetary Science Conference, Houston, A1865
Hopp J., Trieloff M., Ott U., Korochantseva E.K., Buykin A.I.
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(2014). 39Ar-40Ar chronology of the enstatite chondrite parent bodies. Meteoritics and Planetary Science 49, 358-372
Hopp J., Trieloff M., Ott U., Korochantseva E.K., Buykin A.I.
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(2016). I-Xe ages of enstatite chondrites. Geochimica et Cosmochimica Acta 174, 196-210
Hopp J., Trieloff M., Ott U.
