IIE irons: Origin, relationship to ordinary chondrites, and evidence for siderophile‐element fractionations caused by chondrule formation

Abstract

IIE irons were derived from chondritic precursors that were the most reduced ordinary chondrites. The bulk chemical (e.g., Ir/Ni, Ir/Au, Au/Ni, Co/Ni) and bulk isotopic (i.e., D 17 O and d 74/70 Ge) compositions of IIE irons lie along extensions of LL-L-H trends. Chondrule-bearing silicate clasts in IIE irons have mineralogical and petrological characteristics that extend LL-L-H trends; these clasts have higher modal metallic Fe-Ni and lower values for olivine Fa, low-Ca-pyroxene Fs, kamacite Co, and mean chondrule diameter. IIE irons are modeled as agglomerating before H-L-LL chondrites; they acquired more 26 Al and reached the Fe,Ni-FeS eutectic temperature (~940 °C). An FeS-rich metallic melt separated from unmelted silicate and drained to the core, eventually generating a dynamo. Most IIE metal remained within the crust/mantle region alongside recrystallized chondritic clasts. Alkali-rich IIE silicate inclusions formed from late-stage impacts via preferential melting of plagioclase. Some separation of K from Na occurred during vapor transport. Because most type I chondrules formed before most type II chondrules, the (type I)/(type II) modal ratio decreased from IIE to H to L to LL during agglomeration. Earlier-formed chondrules acquired higher abundances of refractory metal nuggets within CAI-fragment precursors, accounting for systematic changes in bulk OC of refractory/common siderophile and refractory/volatile siderophile ratios (IIE>H>L>LL). Because more Au and Co than Ni were retained in silicates, loss of metal globules from spinning partly molten type I chondrules caused systematic whole-rock decreases in Au/Ni and Co/Ni from IIE through LL. Expelled globules had different nebular aerodynamic properties than chondrules and drifted away (accounting, in part, for the metal/silicate fractionation).

Geochemical evidence for the origin of the IIE parent body from H chondrite‐like material

“…4d). Rubin (2022) also showed that most IIE metal remained within the crust/mantle region alongside recrystallized chondritic clasts, that alkali‐rich IIE silicate inclusions formed from late‐stage impacts via preferential melting of plagioclase, and that some separation of K from Na occurred during vapor transport.…”

“…Thus, the IIE metal could be formed from H chondrite material by its complete melting and reduction of W, Ga and other siderophile elements, segregation of the metallic liquid, and its fractional crystallization. Rubin (2022) suggested that IIE irons were derived from chondritic precursors that were the most reduced ordinary chondrites, probably from a composition close to "HH chondrites." However, our data showed that the IIE metal could be formed from H chondrite-like material by complete melting and reduction of W, Ga and other siderophile elements, segregation of the metallic liquid, and its fractional crystallization.…”

“…(2011), who proposed a common origin for the low‐FeO chondrites Burnwell, EET 96010, and LAP 04757, and the H chondrites based on oxygen isotopes and trace element compositions. A recent investigation (Rubin, 2022) showed that meteorites of the IIE‐H‐L‐LL trend have mineralogical, chemical, isotopic, and physical similarities.…”

“…IIE meteorites contain silicate inclusions of chondritic composition in Netschaevo and Watson, and highly fractionated silica‐, alkali‐rich silicate inclusions in Colomera, Miles, and Elga (Olsen et al., 1994; Ruzicka et al., 1999; Ruzicka & Hutson, 2010; Teplyakova et al., 2018; Wasserburg et al., 1968). The formation and coexistence of silicate inclusions with the metal in IIE irons are difficult to explain, considering the endogenous conditions of asteroids, and likely may be connected with late reprocessing of the IIEs' parent bodies (Kruijer & Kleine, 2019; Rubin, 2022; Ruzicka, 2014; Teplyakova et al., 2018). However, despite the progress described above, modeling of the IIE metal from H chondrite‐like material has not yet been performed.…”

“…However, the concept of an LL-L-H-HH metal-silicate differentiation trend in ordinary chondrites (Russell et al, 1998) was not supported by Troiano et al (2011), who proposed a common origin for the low-FeO chondrites Burnwell, EET 96010, and LAP 04757, and the H chondrites based on oxygen isotopes and trace element compositions. A recent investigation (Rubin, 2022) showed that meteorites of the IIE-H-L-LL trend have mineralogical, chemical, isotopic, and physical similarities.…”