Extent of alteration, paleomagnetic history, and infrared spectral properties of the Tarda ungrouped carbonaceous chondrite

Abstract

Tarda is an ungrouped, hydrated carbonaceous chondrite (C2-ung) that was seen to fall in Morocco in 2020. Early studies showed that Tarda chemically resembles another ungrouped chondrite, Tagish Lake (C2-ung), which has previously been linked to the dark D-type asteroids. Samples of D-type asteroids provide an important opportunity to investigate primitive conditions in the outer solar system. We show that Tarda contains few intact chondrules and refractory inclusions and that its composition is dominated by secondary Mg-rich phyllosilicates (>70 vol%), carbonates, oxides, and Fe-sulfides that formed during extensive water–rock reactions. Quantitative assessment of first-order reversal curve (FORC) diagrams shows that Tarda's magnetic mineralogy (i.e., framboidal magnetite) is comparable to that of the CI chondrites and differs notably from that of most CM chondrites. These traits support a common formation process for magnetite in Tarda and the CI chondrites. Furthermore, Tarda's pre-terrestrial paleomagnetic remanence is similar to that of Tagish Lake and samples returned from asteroid Ryugu, with a very weak paleointensity (<0.6 μT) suggesting that Tarda's parent body accreted more distally than that of the CM chondrites, possibly at a distance of >5.4–8.3 AU. An origin in the cold, outer regions of the solar system is further supported by the presence of distinct, porous clasts enriched in aliphatic-rich organics that potentially retain a pristine interstellar composition. Together, our observations support a genetic relationship between Tarda and Tagish Lake.