Crystallization histories of the group IIF iron meteorites and Eagle Station pallasites

Abstract

The group IIF iron meteorites and Eagle Station pallasites (PES) have highly siderophile element abundances (HSE; Re, Os, Ir, Ru, Pt, and Pd) of metal that are consistent with formation in planetesimal cores by fractional crystallization with minor to major solid metal-liquid metal mixing. Modeling of HSE abundances of the IIF irons indicates a complex formation history that included the mixing of primitive and evolved solid and liquid metals. By contrast, modeling of HSE abundances of PES metal suggests these meteorites formed mainly as equilibrium solids from a common liquid. Abundances of some of the siderophile elements in the IIF irons and PES are permissive of a common core origin; however, the abundances of W and Ni indicate the PES ultimately formed on a more oxidized body. The PES most likely formed by the injection of olivine present at the core-mantle boundary into a metallic core liquid as a result of impact. The core then crystallized inward, trapping the olivine.

Contemporary formation of early Solar System planetesimals at two distinct radial locations

“…This property results from re-condensation of the gas that sublimated off refractory grains at high temperature [21,22,33] . The Ni/Ir inferred for most bulk NC cores is indeed larger than solar [14] (Ni condenses together with silicates, whereas Ir is refractory), but this property is not unique to NC irons [34,35] . So, these data do not provide clear evidence for the formation of NC parent bodies at the silicatesublimation line.…”

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

“…Chromium content obtained by LA-ICP-MS is below the detection level (1.2 ppm) in contrast with data obtained by INAA (1078 ppm), which may indicate the presence of chromite inclusions in the sample analyzed by INAA. A high Ge content in INAA analyses (130 ppm) obtained with 95% error limits for Ge of about AE12% (Wasson, 2017) correlates with high Ga content (7 ppm) compared to LA-ICP-MS data (81 and 3.45 ppm, respectively); however, Ge/Ga ratios (23.5 and 18.6) are similarly high relative to the value (11) obtained by Hilton et al (2020). The Ge/Ga ratios in Eagle Station (13.9) analyzed simultaneously with Karavannoe (Humayun et al, 2014) and Ge/Ga in Eagle Station (15) by Hilton et al (2020) are similar.…”

“…The metal of the five ES pallasites is rich in Ni, Co, and Ge, and depleted in W and Ga (Fig. 8) relative to the metal of the MG (Hilton et al, 2020; McCoy et al, 2019; Scott, 1977a; Wasson & Choi, 2003). However, variation in highly siderophile element (HSE) in members of ESP could be an indicator of fractional crystallization of the metal.…”

“…The Fe/Mn ratio of the Karavannoe olivine (99 ± 4 atom%) is in the average range for ESPs (104; Mittlefehldt et al, 1998), but higher than the Fe/Mn ratio of the MG olivine (36–54; Mittlefehldt et al, 1998) and of the pyroxene pallasites’ olivine (27; Mittlefehldt et al, 1998). Distribution of trace elements shows that olivine in Karavannoe as well as in other pallasites of the ESP and MG groups has the lowest REE abundances among meteoritic and terrestrial olivines (Hsu, 2003). However, the observed prevalence of LREE over HREE in some Karavannoe olivines, together with the high variability of REE concentrations between the different grains, may indicate the presence of captured melt inclusions or microinclusions of phosphates.…”

“…Other authors (Carporzen et al, 2011; Hsu, 2003; Elkins‐Tanton et al, 2011; Tarduno et al, 2014) consider that pallasites have not necessarily formed at the mantle–core boundary of their parent bodies and based on the pallasite thermal histories, they may have formed at a shallow depth and were subsequently buried deep under a regolith blanket.…”