Fig. 4.
μ26Mg* evolution diagrams showing the radiogenic ingrowth of 26Mg* from 26Al decay. (a) Schematics of the predicted μ26Mg* evolution of source reservoirs (melt [red], residue [green] and undifferentiated source rock [black]) generated by magmatic differentiation. Retarded ingrowth of 26Mg* in the residual rock (Al/Mgresidue ≪ Al/Mgsource rock) results from extraction of an Al-rich partial melt. (b) μ26Mg* ingrowth in an ordinary chondrite type of protolith (27Al/24Mg = 0.084) using either the canonical (26Al/27Al)0 of 5.25 × 10−5 and initial μ26Mg0 = −38 ppm (black curve) or a reduced initial (26Al/27Al)0 of 1−2 × 10−5 and μ26Mg0 of −15.9 ppm (Larsen et al., 2011) (gray shades). The μ26Mg* deficits recorded by Main Group, pyroxene, and the ungrouped Zinder pallasites, as well as primitive achondrites is expected for retarded μ26Mg* ingrowth in a residue. The lowest MGP μ26Mg* value constrains the onset of silicate differentiation to ~ 1.4 Myr after CAI-formation using the canonical (26Al/27Al)0 or to ~0.25 Myr using a reduced (26Al/27Al)0. (c) μ26Mg* ingrowth in a CV carbonaceous chondrite protolith (27Al/24Mg = 0.129) using either the canonical (26Al/27Al)0 or a reduced (26Al/27Al)0 of 2.7−3.2 × 10−5 (upper bound constrained by the CV bulk μ26Mg* value (incl. error); Table 2). The latest time for onset of ESP olivine formation is within ~1.75 Myr post-CAIs assuming canonical (26Al/27Al)0 or ~1 Myr for a reduced (26Al/27Al)0. Formation of ESP through crystallization will change the Al/Mg ratio of the source melt only slightly and thus lower the formation timescales.