An early I‐Xe age for CB chondrite chondrule formation, and a re‐evaluation of the closure age of Shallowater enstatite

Abstract

Abstract-The iodine-xenon system has been analyzed in samples of 7 chondrules from the CB chondrites Gujba and Hammadah al Hamra (HaH) 237. One sample from Gujba defined a high temperature iodine-xenon isochron corresponding to closure 1.87 ± 0.4 Ma before closure of Shallowater enstatite. Motivated by this result, we employ outlier rejection to re-evaluate the Shallowater age, leading to a modified value of 4562.3 ± 0.4 Ma (1σ). In this process, the datum obtained by combining our I-Xe age for Gujba with the literature Pb-Pb age is rejected as an outlier, indicating that in this sample the I-Xe system closed earlier than the accepted Pb-Pb age of chondrules from CB chondrites. The need for a formation environment distinct from that of chondrules from other meteorites is thus reduced.

Collisional modification of the acapulcoite/lodranite parent body revealed by the iodine‐xenon system in lodranites

“…A negative age implies closure after Shallowater, the absolute age of the sample being given by the sum of its relative age and the absolute age of Shallowater. Therefore these relative ages correspond to absolute ages of 4558.2 ± 0.8 Ma and 4558.3 ± 1.0 Ma, respectively (Table 1) if an absolute age of 4562.3 ± 0.4 Ma is adopted for Shallowater (Gilmour et al 2009). …”

“…Our data allow us to address two (linked) issues of importance: the history of the acapulcoite/lodranite parent body and the departure from concordance of the datum derived from Acapulco phosphate in the proposed calibration between the Pb-Pb and I-Xe systems of Gilmour et al (2006Gilmour et al ( , 2009. Fig.…”

“…If this I-Xe interval is associated with the Pb-Pb age of phosphates on the assumption that this too was set during rapid cooling, a procedure similar to that adopted by Gilmour et al (2009) leads to identification of the same three outliers and an absolute age for Shallowater of 4562.25 ± 0.5 Ma. Relaxing the constraint somewhat and requiring only that the Pb-Pb age of phosphates be bracketed by the I-Xe ages of phosphate and our GRA 95209 separates, the new "best fit" age of Shallowater enstatite is 4562.4 ± 0.4 Ma rather than 4562.3 ± 0.4 Ma (Gilmour et al 2009)-once again a change that is (Göpel et al 1992(Göpel et al , 1994 are taken as fixed points in time and temperature. The I-Xe ages (Brazzle et al 1999) are related to the Pb-Pb time scale using the absolute age of Shallowater.…”

“…In examining the absolute calibration of the I-Xe and Pb-Pb chronometers, Gilmour et al (2009) considered a range of samples in which the Pb-Pb and I-Xe chronometers could be plausibly compared. They employed a method of outlier rejection in which the datum involving an association between the I-Xe and Pb-Pb systems in Acapulco phosphate was rejected.…”

“…As noted, in the case of I-Xe, formation intervals can be converted to absolute ages with reference to the absolute age of the Shallowater standard. Such absolute ages are open to continuous revision, but at present this is taken to correspond to 4562.3 ± 0.4 Ma, based on an observed correlation between absolute Pb-Pb ages and I-Xe formation intervals across a range of samples (Gilmour et al 2006(Gilmour et al , 2009. The additional required step of identifying closure of the short lived chronometer with an absolute time necessarily introduces an element of uncertainty beyond that quantified by the experimental error (Gilmour et al 2009).…”