Magnesium isotopes constraints on the origin of Mg-rich olivines from the Allende chondrite: Nebular versus planetary?

High precision Mg isotope measurements by multi-collector ion microprobe show that refractory olivines from the Allende chondrite, either olivines isolated in the matrix (2 samples studied) or olivines in type I chondrules (6 samples studied), have variable δ26Mg* enrichments and deficits (calculated in permil as the 26Mg deviation from the instrumental mass fractionation line) relative to the Earth. Most average δ26Mg* (noted δ26Mg*av) values (between 10 and 20 analyses per chondrule) are negative but the total range is from −0.029 (± 0.010) ‰ (2 sigma errors) to + 0.011 (± 0.011) ‰ with an exception of one olivine at + 0.043 (± 0.023) ‰. These variations in δ26Mg*av reflect the formation of the olivines from reservoirs enriched in various amounts of 26Mg by the decay of short-lived 26Al (T1/2 = 0.73 Ma). Similarly, 30 analyses of olivines from the Eagle Station pallasite show a δ26Mg*av value of −0.033 ± 0.008‰, as negative as some olivines from Allende chondrules and the Solar system initial δ26Mg* value of −0.038 ± 0.004‰ (defined at the time of formation of type B Ca-Al-rich inclusions - CAIs - when 26Al/27Al = 5.23 × 10−5, Jacobsen et al., 2008).Because olivines are Al-poor and because their Mg isotopic compositions are not reset during the chondrule forming events, their δ26Mg*av can be used to calculate model crystallization ages relative to various theoretical Mg isotope growth curves. The two end-member scenarios considered are (i) a “nebular” growth in which the Al/Mg ratio remains chondritic and (ii) a “planetary” growth in which a significant increase of the Al/Mg ratio can be due to, for instance, olivine magmatic fractionation. The low δ26Mg*av value of olivines from the Eagle Station pallasite demonstrate that metal-silicate differentiation occurred as early as ~ 0. 15- 0. 23+ 0. 29 Ma after CAIs in either of the growth scenarios. Similarly the variable δ26Mg*av values of refractory olivines can be understood if they were formed in planetesimals which started to differentiate as early as the Eagle Station parent body. Accretion of these planetesimals must have been coeval to the formation of CAIs and their disruption could explain why their fragments (Mg-rich olivines) were distributed in the chondrule forming regions of the disk.

Research Highlights► Allende Mg-rich olivines could have crystallized as early as ~ 0.3 Ma after CAIs. ► Eagle Station pallasite parent body differentiated as early as ~ 0.15 Ma after CAIs. ► Formation of some planetesimals predate the formation of most chondrules. ► Formation of some planetesimals may have occurred before the formation of type B CAIs.