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

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Alan E. Rubin

Meteoritics & Planetary Science
Version of Record online: 19 July 2021

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“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., Δ17O and δ74/70Ge) 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 26Al 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).”

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