A 4,565-My-old andesite from an extinct chondritic protoplanet

The initial solar system abundance of 60Fe and early core formation of the first asteroids

High-precision Ni isotope analyses of the differentiated andesitic meteorite Erg Chech 002 (EC 002), the oldest known crustal fragment of a planetesimal, show that short-lived 60Fe was present in the early solar system with an initial 60Fe/56Fe ratio of (7.71 ± 0.47) × 10-9, which is five times more precise than previous estimates and is proposed to be the reference value for further studies. Using this ratio, the Ni isotopic composition of EC 002 implies that metal segregation in the source of the EC 002 parental melts took place [Formula: see text] million years (Myr) after solar system formation, and similar very early metal-silicate differentiation ages are obtained for 4-Vesta ([Formula: see text] Myr) and the angrite parent body ([Formula: see text] Myr). Such an early age dictates a specific accretion and differentiation history for the EC 002 parent body, with metal segregation occurring at relatively low temperatures (1000° to 1200°C), followed by a high-temperature silicate melting event.

A 4,565-My-old record of the solar nebula field

Magnetic fields in protoplanetary disks are thought to play a prominent role in the formation of planetary bodies. Acting upon turbulence and angular momentum transport, they may influence the motion of solids and accretion onto the central star. By searching for the record of the solar nebula field preserved in meteorites, we aim to characterize the strength of a disk field with a spatial and temporal resolution far superior to observations of extrasolar disks. Here, we present a rock magnetic and paleomagnetic study of the andesite meteorite Erg Chech 002 (EC002). This meteorite contains submicron iron grains, expected to be very reliable magnetic recorders, and carries a stable, high-coercivity magnetization. After ruling out potential sources of magnetic contamination, we show that EC002 most likely carries an ancient thermoremanent magnetization acquired upon cooling on its parent body. Using the U-corrected Pb-Pb age of the meteorite's pyroxene as a proxy for the timing of magnetization acquisition, we estimate that EC002 recorded a field of 60 ± 18 µT at a distance of ~2 to 3 astronomical units, 2.0 ± 0.3 My after the formation of calcium-aluminum-rich inclusions. This record can only be explained if EC002 was magnetized by the field prevalent in the solar nebula. This makes EC002's record, particularly well resolved in time and space, one of the two earliest records of the solar nebula field. Such a field intensity is consistent with stellar accretion rates observed in extrasolar protoplanetary disks.

Igneous meteorites suggest Aluminium-26 heterogeneity in the early Solar Nebula

The short-lived radionuclide aluminium-26 (26Al) isotope is a major heat source for early planetary melting. The aluminium-26 - magnesium-26 (26Al-26Mg) decay system also serves as a high-resolution relative chronometer. In both cases, however, it is critical to establish whether 26Al was homogeneously or heterogeneously distributed throughout the solar nebula. Here we report a precise lead-207 - lead-206 (207Pb-206Pb) isotopic age of 4565.56 ± 0.12 million years (Ma) for the andesitic achondrite Erg Chech 002. Our analysis, in conjunction with published 26Al-26Mg data, reveals that the initial 26Al/27Al in the source material of this achondrite was notably higher than in various other well-preserved and precisely dated achondrites. Here we demonstrate that the current data clearly indicate spatial heterogeneity of 26Al by a factor of 3-4 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.

A dry ancient plume mantle from noble gas isotopes

Primordial volatiles were delivered to terrestrial reservoirs during Earth's accretion, and the mantle plume source is thought to have retained a greater proportion of primordial volatiles compared with the upper mantle. This study shows that mantle He, Ne, and Xe isotopes require that the plume mantle had low concentrations of volatiles like Xe and H2O at the end of accretion compared with the upper mantle. A lower extent of mantle processing alone is not sufficient to explain plume noble gas signatures. Ratios of primordial isotopes are used to determine proportions of solar, chondritic, and regassed atmospheric volatiles in the plume mantle and upper mantle. The regassed Ne flux exceeds the regassed Xe flux but has a small impact on the mantle Ne budget. Pairing primordial isotopes with radiogenic systems gives an absolute concentration of 130Xe in the plume source of ∼1.5 × 107 atoms 130Xe/g at the end of accretion, ∼4 times less than that determined for the ancient upper mantle. A record of limited accretion of volatile-rich solids thus survives in the He-Ne-Xe signatures of mantle rocks today. A primordial viscosity contrast originating from a factor of ∼4 to ∼250 times lower H2O concentration in the plume mantle compared with the upper mantle may explain (a) why giant impacts that triggered whole mantle magma oceans did not homogenize the growing planet, (b) why the plume mantle has experienced less processing by partial melting over Earth's history, and (c) how early-formed isotopic heterogeneities may have survived ∼4.5 Gy of solid-state mantle convection.

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

¹⁴⁶Sm-¹⁴²Nd 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 ¹⁴⁶Sm impede the interpretation of the ¹⁴⁶Sm-¹⁴²Nd 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 ¹⁴⁶Sm/¹⁴⁴Sm for the Solar System at 0.00840 ± 0.00032 using a ¹⁴⁶Sm half-life of 103 Ma, making EC 002 an anchor for ¹⁴⁶Sm-¹⁴²Nd systematics for Earth and planetary materials.

The formation and differentiation of planetary bodies are dated using radioactive decay systems, including the short-lived ¹⁴⁶Sm-¹⁴²Nd (T_½ = 103 or 68 Ma) and long-lived ¹⁴⁷Sm-¹⁴³Nd (T_½ = 106 Ga) radiogenic pairs that provide relative and absolute ages, respectively. However, the initial abundance and half-life of the extinct radioactive isotope ¹⁴⁶Sm are still debated, weakening the interpretation of ¹⁴⁶Sm-¹⁴²Nd systematics obtained for early planetary processes. Here, we apply the short-lived ²⁶Al-²⁶Mg, ¹⁴⁶Sm-¹⁴²Nd, and long-lived ¹⁴⁷Sm-¹⁴³Sm chronometers to the oldest known andesitic meteorite, Erg Chech 002 (EC 002), to constrain the Solar System initial abundance of ¹⁴⁶Sm. The ²⁶Al-²⁶Mg mineral isochron of EC 002 provides a tightly constrained initial δ²⁶Mg* of −0.009 ± 0.005 ‰ and (²⁶Al/²⁷Al)₀ of (8.89 ± 0.09) × 10⁻⁶. This initial abundance of ²⁶Al is the highest measured so far in an achondrite and corresponds to a crystallization age of 1.80 ± 0.01 Myr after Solar System formation. The ¹⁴⁶Sm-¹⁴²Nd mineral isochron returns an initial ¹⁴⁶Sm/¹⁴⁴Sm ratio of 0.00830 ± 0.00032. By combining the Al-Mg crystallization age and initial ¹⁴⁶Sm/¹⁴⁴Sm ratio of EC 002 with values for refractory inclusions, achondrites, and lunar samples, the best-fit half-life for ¹⁴⁶Sm is 102 ± 9 Ma, corresponding to the physically measured value of 103 ± 5 Myr, rather than the latest and lower revised value of 68 ± 7 Ma. Using a half-life of 103 Ma for ¹⁴⁶Sm, the ¹⁴⁶Sm/¹⁴⁴Sm abundance of EC 002 translates into an initial Solar System ¹⁴⁶Sm/¹⁴⁴Sm ratio of 0.00840 ± 0.00032, which represents the most reliable and precise estimate to date and makes EC 002 an ideal anchor for the ¹⁴⁶Sm-¹⁴²Nd clock.