Testing an integrated chronology: I-Xe analysis of enstatite meteorites and a eucrite

Abstract

We have determined initial 129I/127I ratios for mineral concentrates of four enstatite meteorites and a eucrite. In the case of the enstatite meteorites the inferred ages are associated with the pyroxene-rich separates giving pyroxene closure ages relative to the Shallowater standard of Indarch (EH4, -0.04 ± 0.67 Ma), Khairpur (EL6, ?4.22 0.67 Ma), Khor Temiki (aubrite, -0.06 Ma), and Itqiy (enstatite achondrite, -2.6 ± 2.6 Ma), negative ages indicate closure after Shallowater. No separate from the cumulate eucrite Asuka (A-) 881394 yielded a consistent ratio, though excess 129Xe was observed in a feldspar separate, suggesting disturbance by thermal metamorphism within 25 Ma of closure in Shallowater. Iodine-129 ages are mapped to the absolute Pb-Pb time scale using the calibration proposed by Gilmour et al. (2006) who place the closure age of Shallowater at 4563.3 ± 0.4 Ma. Comparison of the combined 129I-Pb data with associated 53Mn ages, for objects that have been dated by both systems, indicates that all three chronometers evolved concordantly in the early solar system. The enstatite chondrites are offset from the linear array described by asteroid-belt objects when 53Mn ages are plotted against combined 129I-Pb data, supporting the suggestion that 53Mn was radially heterogeneous in the early solar system.