Andesitic Magmatism on a Differentiated Asteroid

The Earth differs from other terrestrial planets in having a substantial silica-rich continental crust with a bulk andesitic composition 1 . The compositional dichotomy between oceanic and continental crust is likely related to water-rich subduction processes 2 . Over the past decade, the discovery of meteorites with andesitic bulk compositions have demonstrated that continental-crust like compositions can be attained through partial melting of chondritic protoliths 3,4,5 . Here we show that a newly identied achondrite meteorite, Erg Chech (EC) 002, is a high-Mg andesite but that, unlike previous andesitic achondrites has strongly fractionated and low abundances of the highly siderophile elements (HSE), reminiscent of Earth’s upper continental crust 6 . The major and HSE composition of EC 002 can be explained if its asteroid parent body underwent metal-silicate equilibrium prior to silicate partial melting without losing signicant volatile components. The chemistry of pyroxene grains in EC 002 suggests it approximates a parental melt composition, which cannot be produced by partial melting of pre-existing basaltic lithologies, but more likely requires a metal-free chondritic source. Erg Chech 002 likely formed by ~ 15% melting of the mantle of an alkali-undepleted differentiated asteroid. The discovery of EC 002 shows that extensive silicate differentiation after metal-silicate equilibration was already occurring in the rst two million years of solar system history 7 , and that andesitic crustal compositions do not always require water-rich subduction processes to be produced. which was incorporated into the melt as indicating rapid melt in order to entrain solid The segregated melt cools and forms augite crystals, some of which xenocrysts, and then crystallizes plagioclase An 15-25 and nally a residual 5% of Si-rich melt is quenched as thin veins. The modal quartz abundance and plagioclase An content are consistent with those predicted for the experimental melts 5 . Preliminary Mg isotopic data 7 show that EC 002 was formed with the rst 2 million years of solar system history, when 26 Al was still present. This short-time scale indicates that the EC 002 parent body experienced two separate heating events to segregate a core and then melt to form EC 002 shortly after accretion. Short-lived nuclides like 26 Al are signicant sources of heat and could have provided sucient heat for differentiation of the EC 002 parent body. The new data show that differentiated asteroids were present within the rst 2 Ma of solar system formation, indicating that some early-formed parent bodies may be underrepresented in the meteorite record. These results emphasize that the continental crust composition of Earth may not be unique to a plate tectonic process, that planetary scale differentiation to metal cores, and andesitic crusts could have taken place on relatively small planetesimals to large planets without complete melting and that discovery of exoplanets with andesitic crusts may not mean plate tectonics acts upon them.


Introduction
Numerous properties of the Earth set it apart from the other rocky planets in the solar system, one of the most striking being the presence of Si-rich continental crust (CC). In contrast to Earth, the vast majority of achondritic meteorites range between chondritic and basaltic in bulk rock composition 8 . However, a small but growing number of achondrites with SiO 2 content up to ~61 wt.% have been identi ed over the past decade 3,4,[9][10][11] . Most high-Si meteorites have been concluded to be the result of low-degree melting of alkali-undepleted chondritic lithologies 5  The rst high-Si achondrites identi ed, Graves Nunatak (GRA) 06128 and 06129 show high concentrations of the highly siderophile elements (HSE; Os, Ir, Ru, Pt, Pd, Re) relative to terrestrial rocks 3 .
The HSE partition extremely strongly into Fe-metal over silicate phases and are thus highly depleted in the silicate portions of differentiated planets, including Earth. The high concentrations of the HSE in GRA 06128/9 indicates that its parent body had not segregated a metallic core and was thus likely chondritic.
The GRA 06128/9 meteorites have been linked to the brachinite parent body 3,12 , and many primitive achondrites such as winonites, brachinites, ureilites and acapulcoites/lodranites, probably result from the extraction and removal of a high SiO 2 melt from a chondritic lithology 5,13,14 . Like GRA 06128/9, most other evolved achondrites formed by low-degree (~13%) melting of chondrites 3,4,9,11,12 . The only known evolved lithology in asteroidal meteorites that formed by melting of a ma c rock is a single dacitic clast in the howardite Dominion Range (DOM) 2010 that likely results from 10-20% melting of a rock similar in composition to the eucrite Juvinas 10 . This dacite lithology is probably minor in the Vestan crust, as the bulk of Vestan meteorites are ma c 10 . A new meteorite, Erg Chech 002 (~31.78 kg) was found in the Algerian Sahara in May 2020 and recognized as a new evolved achondrite 15 . Here we present new geochemical data for this meteorite to constrain its petrogenesis and to compare its composition to terrestrial continental crust.

Results
Erg Chech 002 has a plumose variolitic texture of 44.7 modal % 0.5-3 mm long, high aspect ratio (~2:1 long to short axis) pyroxene grains enclosed by a groundmass of 50 modal % plagioclase feldspar with 4.4 modal % Si-rich phase ( Figure 1). Modal abundances were determined quantitatively using the ImageJ software. Accessory minerals include a total of ~0.7 modal% chromite, ilmenite, sul de, and iron metal that occur interstitial to the silicate grains. The plagioclase grains in-ll the space between pyroxene crystals and long, thin silica patches are present in the center of the feldspar regions. Two textural types of pyroxene are evident, including pyroxenes with distinctly colored cores and rims that are both larger and typically rounder than the more common pyroxenes that have more elongate shapes Highly siderophile element abundances as well as 187 Re-187 Os isotopic systematics of EC 002 show that it has a fractionated HSE pattern and much lower HSE compared to the achondrite andesites GRA 068128/9, with highly fractionated (Re/Os) N  share a parent body. The large augite grains in EC 002 are close to equilibrium with the bulk composition with respect to Na and Ti, whose partitioning into pyroxene is relatively unaffected by temperature and pressure 22 . The Mg-rich grain is likely a xenocryst due to its strongly differing composition and the fact that it is rimmed by the predominant augite composition. Therefore, despite the presence of xenocrysts of enstatite, bulk EC 002 is likely close to its parental melt composition. This is consistent with the textures showing large pyroxenes with interstitial oligoclase containing pockets of evolved melts (Figure 1a), and no clear cumulate texture.

Two-Stage Petrogenesis of EC 002
Erg Chech 002 is distinct from the andesitic achondrite GRA 06128/9 in having low and fractionated HSE concentrations (Figure 4). The HSE pattern shows chondrite-normalized (Os/Ir) N ~1 and elevated (Re/Ir) N , (Pd/Ir) N , (Pt/Ir) N and (Ru/Ir) N , similar to the upper terrestrial CC. These fractionations are likely the result of silicate melting during which Re, Pd, Pt and Ru are more incompatible than Os and Ir. Low HSE concentrations in rocks re ect metal removal from source regions, where metal-silicate equilibrium leads to near-quantitative removal of highly siderophile elements. This scenario is supported by the low Ni concentration of bulk EC 002 (13.1 ppm) as well as the lack of low Ni (0.02 wt.%) in the metal grains, which are almost pure Fe. Graves Nunataks 06128/9 formed by melting of a chondritic parent body that never segregated a core 3 . The HED parent body (likely 4-Vesta) and the angrite parent bodies both segregated cores 17,18 , and EC 002 shows an HSE pattern overlapping both of these groups (Figure 4). Therefore, EC 002 is the rst identi ed evolved achondrite from a differentiated asteroidal parent body.
Four possible scenarios for the petrogenesis of EC 002 are considered: 1) formation in a subduction zone like terrestrial CC, 2) impact melting or metamorphic heating and anatexis of a ma c achondrite, 3) melting of a chondritic body leading to simultaneous metal and silicate segregation, and 4) silicate melting on a differentiated asteroid. The rst scenario can be readily discounted due to the early formation of EC 002 7 , and the lack of any evidence for subduction zone-like processes on non-Earth bodies It is possible that EC 002 is the result of melting of a ma c precursor through impact or metamorphic heating. Melting experiments of basaltic eucrites 23,24 show that eucrite melts differ strongly in major-element composition from EC 002 ( Figure 2). Additionally, eucrites and angrites show higher concentrations of the incompatible REE than EC 002 (Figure 2), making them unlikely protoliths. The DOM 2010 dacite clast formed by impact-related eucrite partial melting and differs strongly from EC 002 with very low Na content 10 . Erg Chech 002 therefore is unlikely to represent a partial melt of a basaltic precursor. Petrogenetic modeling of the evolution of eucrite melts also does not yield compositions consistent with EC 002 10 . Finally, it is unlikely that impact melts would have such low HSE contents as contamination with chondritic material would be predicted to add large quantities of the HSE, as observed in lunar impact rocks 25 .
It is possible that EC 002 was the product of melting on a chondritic body in which silicate and metallic melts equilibrated with each other and segregated in a single event. The high concentration of moderately volatile elements, such as Na, in EC 002 indicates that its protolith did not reach high enough temperatures to volatilize these elements, which is unlikely if a single high temperature event led to its formation but we cannot rule out this scenario with the available evidence. The nal petrogenetic mechanism involves melting of the silicate portion of a differentiated parent body that had already formed a core, which can readily explain the low fractionated HSE signature of EC 002. Melting experiments of chondrites yield a number of liquids with composition close to bulk EC 002 with both H and LL chondrites melted at 1202°C and oxygen fugacity (fO 2 ) of Iron-Wüstite -1.38, producing plausible parental magmas 5 . The low fO 2 of these experiments is consistent with the presence of metal in EC 002.
Disequilibrium melting of R chondrites can also yield high Na and Si liquids similar to EC 002 26 .