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

Abstract

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.

EBSD Analysis of Iron‐Nickel Metal in L Type Ordinary Chondrites: 2. Formation of Net, Acicular, Duplex, and Pearlitic Plessite

“…However, natural plessite in iron and stony iron meteorites can exhibit mixed morphological characteristics. For example, the plessite in Mangui (Figure 1) has round kamacite grains in duplex plessite, and the plessite in NWA 6369 (IAB) was defined as the spheroidized plessite (Teplyakova et al., 2011), but it is actually duplex plessite. Moreover, it should be noted that the same morphological type of plessite can be formed through various mechanisms and require distinct formation conditions and processes; an example being net plessite observed in L group chondrites and irons (see discussion on net plessite below).…”

EBSD Analysis of Iron‐Nickel Metal in L Type Ordinary Chondrites: 1. The Microstructural Shock Signatures

“…It can be inferred that Hoba might have experienced a moderate shock event. Teplyakova et al (2011) reported the spheroidized plessite in NWA 6369 (IAB). Although some taenite within the plessite exhibits nearly circular shapes, it strictly represents duplex plessite.…”