The identification of early condensates from the solar nebula

Abstract

Calcium-aluminum-rich chondrules which are highly deficient in alkalis were extracted from the carbonaceous chondrite Allende and yield a range of compositions with the lowest measured isotopic composition of (⁸⁷Sr/⁸⁶Sr)_(ALL) = 0.69877 ± 0.00002 and identify this material as the earliest known condensate from the solar nebula. Other chondrules suggest the possible presence of even more primitive Sr in this meteorite. This result also shows that some chondritic material formed very near the earliest part of the condensation sequence. Using alkali-deficient planetary objects (Moon, basaltic achondrites, Angra dos Reis, Allende), the Sr data indicate a time interval for condensation of 10 m.y. (from ALL to BABI) if condensation occurred in a solar Rb/Sr environment. A variety of alkali-rich olivine chondrules and Ca-Al-rich aggregates from Allende fail to determine an isochron and indicate that the element distribution in this meteorite was disturbed later than 3.6ae, possibly recently, in a cometary nucleus. This disturbance requires that the determination of initial ⁸⁷Sr/⁸⁶Sr be done on essentially Rb-free phases. Strontium data from equilibrated chondrites and from an iron meteorite establish an interval for metamorphism or differentiation in protoplanetary objects which followed the condensation process by ≈80 mm.y. The chronology for condensation and early planetary evolution obtained for Sr is in disagreement with the ¹²⁹I chronology but can be brought into agreement, if it is assumed that the high temperature iodine containing phases have not been affected by the metamorphic events determined by Sr.