Time and duration of chondrule formation: Constraints from Al-26-(26) Mg ages of individual chondrules

摘要

Chondrules from unequilibrated ordinary and carbonaceous chondrites belong to the oldest and most primitive materials from the early solar system and record chemical and isotopic signatures relating to their formation and evolution. These signatures allow tracing protoplanetary disk processes that eventually led to the formation of planetary building blocks and rocky planets. Al-26-Mg-26 ages based on mineral-mesostasis isochrons of 31 porphyritic ferromagnesian chondrules, that belong mainly to type-II, constrain the time of chondrule melting prior to incorporation into the respective chondrite parent bodies. For this study chondrules from the unequilibrated L, L(LL) and LL ordinary chondrites (UOCs) NWA 5206, NWA 8276, MET 96503, MET 00452, MET 00526, NWA 7936 and QUE 97008 were selected, which are of petrologic types 3.003.15 and were thus least metamorphosed after formation. Magnesium and Al isotopes were measured in-situ by Secondary Ion Mass Spectrometry (SIMS) using a CAMECA 1280 ims. Mg-26 excess from in-situ decay of Al-26 correlating with Al-27/Mg-24 has been detected in the mesostasis of all but one chondrule. The initial Al isotopic compositions (Al-26/Al-27)(0) and Mg-26/Mg-24 ratios (delta Mg-26*(0)) deduced from internal mineral isochron regressions range from (9.5 +/- 2.8) x 10(-6) to (3.1 +/- 1.2) x 10(-6) and -0.020 +/- 0.028 parts per thousand to 0.011 +/- 0.039 parts per thousand, respectively. The corresponding chondrule ages (Delta t(CAI)), calculated relative to calcium-aluminum-rich inclusions (CAIs) using the canonical Al-26/Al-27 = (5.23 +/- 0.13) x 10(-5), are between 1.76(-0.27)(+0.36) and 2.92(-0.34)(+0.51) Ma and date the melt formation and thus primary chondrule formation from dust-like precursors or reprocessing of older chondrules. The age range agrees with those acquired with different short-lived chronometers and with published Al-26-Mg-26 ages, the majority of which were obtained for chondrules from the Bishunpur and Semarkona meteorites, although no chondrule with (Al-26/Al-27)(0) > 10(-5) was found.