Alkali magmatism on a carbonaceous chondrite planetesimal

Karavannoe is a pallasite found in Russia in 2010. The mineralogy, chemistry, and oxygen isotopic composition indicate that Karavannoe is a member of the Eagle Station Pallasite (ESP) group. Karavannoe contains mostly olivine and subdued interstitial Fe,Ni‐metal. Zoned distribution of FeO in small, rounded grains of olivine and FeO and Al2O3 in chromite shows that the cooling rate of the melt was fast during the crystallization of the round olivine grains. Siderophile element distribution and correlations of Au‐As and Os‐Ir concentrations in Karavannoe and the other ESP metal record its magmatic origin. FeO‐rich composition of olivine, low W and Ga, and high Ni abundances in the Karavannoe metal indicate the formation of the metal from an oxidized chondrite precursor. Model calculations demonstrate that the ESPs’ metal compositions correspond to the solids of the fractional crystallization of CV‐ or CO‐chondrite‐derived metallic liquids. The Karavannoe metal composition corresponds to the solid fraction crystallized after ~40% fractional crystallization. The Mg/(Mg+Fe) atom ratio of complementary silicate liquid corresponds to Fo70, possibly indicating that the olivine is not in equilibrium with the metal and could have been a product of the late evolutionary processes in the Karavannoe parent body mantle. In any ESP genesis Karavannoe was not in equilibrium with its metal and is a product of mantle differentiation processes. Olivine of Karavannoe and ESPs is similar in composition, while the metal is different. We propose a model of ESP formation involving an impact‐induced intrusion of liquid core metal into a basal mantle layer, followed by fractional crystallization of the metal. The metal textures and chemical zoning of Karavannoe minerals point to remelting and rapid cooling due to a later impact event.

Section: Discussionsupporting

confidence: 80%

Karavannoe: Mineralogy, trace element geochemistry, and origin of Eagle Station group pallasites

Teplyakova

Lorenz

Ivanova

et al. 2022

“…It might be noted, incidentally, that other metamorphic clasts, and also igneous clasts (e.g., Aléon et al., 2020), have been reported from CR chondrites, and they also testify to early high‐temperature processing and breakup of CR‐related planetesimals before the final accretion of the CR parent body (or parent bodies). However, no other clast appears to have experienced anything close to the extreme pressure and temperature of the one being discussed here.…”

Section: Background To the Journey Down And Back: Clarification Of As...mentioning

confidence: 97%

Discussion of the unique “eclogitic” clast reported from Northwest Africa 801 (CR2) and the possibility of ejection of foundered crust from deep in a molten Moon‐sized projectile

Scott,

Sanders,

Asphaug

et al. 2023

A unique 2 mm‐wide clast of fine‐grained garnet–omphacite peridotite with chondritic chemistry was reported from the CR2 chondrite Northwest Africa 801 by Hiyagon et al. (2016, Geochimica et Cosmochimica Acta, 186, 32–48). Those authors described the clast as eclogitic and inferred from its mineral compositions that the rock formed quickly, during perhaps 100–1000 years of burial, deep within a Moon‐sized body at about 3 GPa (30 kbar) pressure and 1000°C. Such conditions are unprecedented among meteorites and invite scrutiny. Here, we discuss the clast and its origin. The inferred conditions appear justified, but the published idea of burial during a protoplanetary merger and, soon after, exhumation in a violent collision seems improbable and contrary to the clast's low shock levels. We find that exhumation is better explained by pull‐apart of a projectile in a low velocity hit‐and‐run collision. We try inconclusively to explain near‐simultaneous burial and exhumation in a hit‐and‐run return scenario. Taking a different approach, and to conclude, we speculate that an approximately lunar‐mass body was molten beneath a thin dense chondritic crust, of which fragments foundered and sank deep into the magma ocean as an ongoing process. Fragments that were changing to garnet–omphacite peridotite were exhumed when the molten body was pulled apart in a hit‐and‐run collision with a larger body.

“…Achondrites were originally thought to be restricted to basaltic compositions due to the abundance of basalt‐like samples, that is, angrites (Keil, 2012), or the howardite–eucrite–diogenite (HED) clan (Mittlefehldt, 2015). However, the discovery of several andesitic to trachyandesitic achondrites from previously unidentified parent bodies, that is, Graves Nunataks (GRA) 06128 and 06129 (Day et al., 2012; Shearer et al., 2010), Almahatta Sitta MS‐MU‐011 (ALM‐A; Amelin et al., 2015; Bischoff et al., 2014; Goodrich et al., 2022), Northwest Africa 11119 (NWA 11119; Srinivasan et al., 2018), has revealed that planetesimal melting and differentiation processes were more diverse than previously thought (Aléon et al., 2020; Barrat et al., 2021; Bischoff et al., 2014; Srinivasan et al., 2018). These discoveries suggest that planetesimal crusts were not merely restricted to basaltic compositions, but evolved, andesitic crusts were also possible (Barrat et al., 2021; Day et al., 2012; Nicklas et al., 2022; Shearer et al., 2010).…”

Section: Introductionmentioning

confidence: 99%

Igneous intrusion origin for the petrofabric of Erg Chech 002

Beros,

Tait,

Di Cecco

et al. 2024

Achondrites provide an opportunity to examine the igneous processes of differentiated bodies in our solar system. The recent discovery of several silica‐rich achondrites suggests that andesitic crusts were more common among planetesimals than previously thought, though the processes behind their emplacement are not well understood. Here, electron backscatter diffraction (EBSD) is used to investigate the igneous emplacement conditions of Erg Chech 002 (EC 002), a recently discovered ungrouped achondrite representing andesitic magmatism ~2 Myr after the formation of calcium–aluminum‐rich inclusions (CAIs). EBSD analyses of crystallographic preferred orientations (CPOs) for augite and plagioclase feldspar phenocrysts indicate that EC 002 exhibits a weak foliation CPO. Augite misorientation inverse pole figures (mIPF) indicate preferential slip along the (100)[001] system with a distinct shift toward the {0kl}[u0w] system in plastically deformed grains. Our findings support the hypothesis that EC 002 was likely emplaced in the lower regions of a magmatic intrusion. Augite slip signatures suggest that EC 002 crystallization and emplacement were restricted to high temperatures (>800°C) and experienced at least two strain regimes. The distinct shift from a dominant (100)[001] slip system, which corresponds to high temperatures (800–1050°C), to a [0kl][u0w] slip system indicates an increased strain rate due to shock deformation (1–5 GPa) attributed to ejection by hypervelocity impact.