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The homogeneity of distribution of radioisotope 26Al in the early solar nebula, a major heat source for early planetary differentiation and foundational assumption to high resolution cosmochronology, remains debatable. Here we report a precise Pb-isotopic age of 4565.56±0.12 million years (Ma) for the recently discovered andesitic achondrite Erg Chech 002. Combining this age with published high-precision 26Al-26Mg data, we demonstrate that the initial 26Al/27Al in the source material of this achondrite was distinctly higher than in several well preserved and precisely dated achondrites. We argue that the current data clearly indicate spatial heterogeneity of 26Al in the precursor molecular cloud or the protoplanetary disk of the Solar System, likely associated with the late infall of stellar materials with freshly synthesized radionuclides.

The multidynamic acquisition mode in TIMS isotopic analysis can effectively minimize the bias in isotopic ratios due to Faraday cup deterioration [1, 2]. However, a potential problem of this mode is that the normalising and normalised ratios are not measured simultaneously. If the fractionation is fast, this can lead to a bias in the fractionation-corrected isotopic ratios, as demonstrated in the recent studies on highprecision multidynamic Nd isotope analysis [2, 3]. Here we show that this effect can be pronounced in the high-precision TIMS analysis of 84 Sr. We measured 84 Sr/ 86 Sr ratios of the standard SRM987 and a series of terrestrial samples (BCR-2, BHVO-2, BIR-1, and seawater) using a 3line multidynamic method with the center cup sequencially collecting masses 85, 86, and 87. The µ 84 Sr values (defined as 10 6 [(84 Sr/ 86 Srsampl e)/(84 Sr/ 86 SrSRM987)-1]) of the terrestrial samples are positively correlated with fractionation rates, consistent with the expectation from our cup configuration (measuring 84 Sr/ 86 Sr prior to 88 Sr/ 86 Sr). After correcting the fractionation drifting between magnetic settings using a linear interpolation method, this correlation is eliminated, and a better measurement precision is achieved. The drift-corrected multidynamic measurements yield a weighted average µ 84 Sr of-31 ± 8 ppm (2 S.E., N = 33) for the terrestrial samples, which is in good agreement with our static measurement results (-31 ± 7 ppm) and those reported by [4-7]. The uncorrected multidynamic measurents yield a less negative µ 84 Sr value (-17 ± 9 ppm), similar to the results of [7-10]. We suggest that the higher fractionation rates of terrestrial samples relative to SRM987 may have biased the multidynamic µ 84 Sr in some of the previous reports, and that the Earth has a 84 Sr deficit of ~30 ppm relative to SRM987. [1] Thirlwall (1991) CG, 94, 85-104. [2] Garçon et al.