Petrography and geochemistry of the Ribbeck aubrite recovered from asteroid 2024 BX1, the closest analog to Mercury?

Meteorite recovery

Asteroid 2024 BX₁ was discovered by astronomer K. Sárneczky at 21:48 UTC on 20 January 2024. NASA’s Scout and ESA’s Meerkat impact assessment systems soon identified it as a potential impactor and predicted that it would pass over Nennhausen, ~60 km W of Berlin, Germany, between 00:15 and 00:51 UTC on 21 January. At this time, a fireball was observed by eyewitnesses and recorded by allsky cameras of the European Fireball Network, IMO/All-Sky7, and FRIPON. Bolide analysis and strewn field calculations [1] indicated that strong winds blew the surviving meteorites to the SE, predicting a fall just south of Ribbeck. Starting on 22 January, a systematic search was carried out by scientists and students of MfN, DLR, FU Berlin, TU Berlin, the SETI Institute, and the Arbeitskreis Meteore. The first meteorite, totaling 171 g, was found by meteorite hunters on 25 January just west of Ribbeck. Two students from our team found two smaller meteorites (8.1 and 4.7 g) on 26 January, proving that the strewn field model was correct. Searching continued until 20 March and revealed about 200 reported finds with a total mass of >1770 g.

Strewn field

The strewn field extends along a ~1 km wide and ~8 km long, WNW-ESW oriented corridor just south of Ribbeck (where meteorites of between 50 and 230 g were found) and Berge and Lietzow (where meteorites <10 g were found). The average meteorite mass as a function of distance along the strewn field agrees well with predictions by [1], but more small meteorites were recovered at distances >5 km than predicted. The distribution of total mass per kilometer along the strewn field is approximately constant (~230 g/km) until the distribution of small (<4 g) meteorites drops off at distances >6 km due to sampling bias or a lack of small masses during fragmentation.

Petrography of the Ribbeck aubrites

Petrographic observations revealed that Ribbeck is an aubrite, which are rare enstatite achondrites [2–4]. The Ribbeck meteorites are fragmental breccias predominantly composed of up to 1.2-cm-sized, mostly angular, FeO-free, homogeneous enstatite (En_99.2Fs_0.0Wo_0.8), less abundant (~1–11%), up to-1.5-cm sized, homogeneous forsterite (Fo_99.9), and highly variable amounts (~1–17%) of sodic feldspar (An_2.4Ab_95.1Or_2.5) set in a fine-grained, comminuted matrix of related material. Less abundant silicates include K-feldspar (An_0.1Ab_7.0Or_92.2) and diopside (En_53.5Fs_0.1Wo_46.4). Opaque phases include Ti-bearing troilite, exotic sulfides such as alabandite, keilite, djerfisherite, oldhamite, caswellsilverite, schöllhornite, and cronusite, Si-bearing (0.7–1.0 wt% Si) and Si-poor (<0.1 wt% Si) kamacite, rare taenite, iron, and copper as well as very rare schreibersite and perryite. Shock stage is low to moderate.

Geochemistry of the Ribbeck aubrites

The classification of Ribbeck as an aubrite is supported by element abundances as well as O and Ti isotopic compositions. Ribbeck’s O isotopic composition (δ¹⁷O = 2.987‰; δ¹⁸O = 5.682‰; Δ¹⁷O = 0.010‰) was determined by laser-assisted fluorination and found to be consistent with that of other aubrites [4,5], in particular Bishopville [5]. Ribbeck plots very close to the terrestrial fractionation line and onto the aubrite trend [5]. Bulk rare earth element (REE) patterns of Ribbeck have a positive Eu anomaly due to its high abundance of feldspar. REE patterns of Ribbeck are similar to that of Bishopville and distinct from those of other aubrites such as Aubres, Norton County, and Mount Egerton [4,5]. FTICR-MS on solvent extracts furthermore revealed a rich, C-H-N-O-S-Mg-based organic chemistry. Trace-element analysis by LA-ICP-MS furthermore revealed the presence of at least two varieties of FeNi metal that might represent various stages of melting of an enstatite chondrite-like precursor followed by fractional crystallization. The Ti isotopic compositions of Ribbeck (ε⁴⁶Ti = –0.11 ± 0.12; ε⁴⁸Ti = 0.02 ± 0.08; ε⁵⁰Ti = –0.09 ± 0.07) and Bishopville (ε⁴⁶Ti = –0.11 ± 0.04; ε⁴⁸Ti = 0.04 ± 0.03; ε⁵⁰Ti = –0.10 ± 0.05) were determined by MC-ICP-MS and found to be similar. Our data suggest that Ribbeck and Bishopville originate from a similar portion of the aubrite parent body. Preliminary ²¹Ne-derived cosmic ray exposure ages would be consistent with this scenario.

Outlook

2024 BX₁ is only the eighth asteroid for which an impact with Earth was predicted by impact assessment systems and only the fourth from which meteorites have been recovered. It is among the best-documented fall events to date. Ribbeck provides a snapshot of planetary formation and differentiation in the inner solar system and serves as a natural analog material for the upcoming BepiColombo mission to Mercury [6,7]. For the planned Mercury flyby of BepiColombo in December 2024, the fall of the Ribbeck Aubrite is an extremely fortunate coincidence, as fresh analog material was delivered to Earth just in time. In the compation abstract of Van den Neucker et al. [7], we further illustrate how the Ribbeck aubrites may serve as reference materials for the MErcury Radiometer and Thermal Infrared Spectrometer (MERTIS) on board of BepiColombo. The pristine state of Ribbeck combined with the comprehensive orbital data of 2024 BX₁ will also provide opportunities for source region modeling.

References

[1] Spurný P. et al. (2024) Astronomy & Astrophysics in press. [2] Watters and Prinz (1979) Proc. 10th LPSC pp. 1073–1093. [3] Keil K. Chemie der Erde 70:295–317. [4] Wilbur Z. E. et al. (2022) Meteoritics & Planetary Science 57:1387–1420. [5] Barrat J.-A. et al. (2016) Geochimica et Cosmochimica Acta 192:29–48. [6] Cartier C. and Wood B. J. Elements 15:39–45. [7] Van den Neucker A. et al. (2024) this meeting.