Half-life and initial Solar System abundance of ¹⁴⁶ Sm determined from the oldest andesitic meteorite

Abstract

Significance 146 Sm- 142 Nd radioactive systematics can provide constraints on the timing of early differentiation processes on Earth, Moon, and Mars. The uncertainties related to the initial abundance and half-life of the extinct isotope 146 Sm impede the interpretation of the 146 Sm- 142 Nd systematics of planetary materials. The accurate determinations of Sm, Nd, and Mg isotopic compositions of the oldest “andesitic” achondrite Erg Chech 002 (EC 002) define a crystallization age of 1.8 Myr after the formation of the Solar System and provide the most accurate and reliable initial ratio of 146 Sm/ 144 Sm for the Solar System at 0.00840 ± 0.00032 using a 146 Sm half-life of 103 Ma, making EC 002 an anchor for 146 Sm- 142 Nd systematics for Earth and planetary materials.

U-Pb age of the oldest achondrite points to 26Al heterogeneity in the early Solar System

“…The initial 26 Al/ 27 Al in EC 002 45 is clearly resolved from other studied achondrites (except NWA 2976, which has large uncertainty of initial 26 Al/ 27 Al) but overlaps with the "canonical" value in the CAIs. Well resolved difference in initial 26 Al/ 27 Al between EC 002 and volcanic angrites persists if the value 26 Al/ 27 Al in EC 002 measured by SIMS 44 is accepted instead of the value measured by MC-ICPMS 45 . This is reliable evidence that 26 Al was heterogeneously distributed among the feeding zones of parent asteroids of achondrites.…”

“…Such analytical biases due to imperfect matrix matching are common in SIMS 48 . If the shallower slope of the SIMS isochron was caused by slow cooling 45 , then the SIMS data for plagioclase would depend on the crystal size and SIMS spot location, and would have been widely dispersed below the MC-ICPMS isochron, rather than forming a tight array without excess scatter 44 . The 26 Al/ 27 Al ratio yielded by isochrons for EC 002 is ca.…”

“…Recent discovery of andesitic achondrite Erg Chech 002 (EC 002) and its exceptionally old age according to 26 Al-26 Mg systematics 44,45 provides an opportunity to further explore the initial distribution of 26 Al. Combined regression of 19 plagioclase and 11 pyroxene Al-Mg SIMS (secondary-ion mass spectrometry) analyses 44 yielded an isochron corresponding to 26 Al/ 27 Al = (5.72 ± 0.07)×10 − 6 .…”

“…Combined regression of 19 plagioclase and 11 pyroxene Al-Mg SIMS (secondary-ion mass spectrometry) analyses 44 yielded an isochron corresponding to 26 Al/ 27 Al = (5.72 ± 0.07)×10 − 6 . The MC-ICPMS (multi collector inductively coupled plasma mass spectrometry) analysis of six pyroxene and plagioclase fractions and one whole rock 45 yielded a steeper isochron corresponding to 26 Al/ 27 Al = (8.89 ± 0.09)×10 − 6 . The absence of excess dispersion in both regressions -which include multiple analyses of two minerals with different rates of Mg diffusion 46,47 and cover a wide range of Al/Mg ratios -indicates that the event dated by 26 Al-26 Mg isochrons was very rapid, i.e., probably crystallisation rather than cessation of diffusion upon cooling.…”

“…It also suggests that the minerals remained close to migration of Al and Mg at micron scale 44 ever since. The difference in the slope between the isochrons based on SIMS 44 and MC-ICPMS 45 data is probably caused by uncorrected systematic analytical uncertainty in Al/Mg ratios measured in SIMS due to using Ca-rich plagioclase as a standard in analysis of Na-rich plagioclase. Such analytical biases due to imperfect matrix matching are common in SIMS 48 .…”