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

Abstract

AbstractIIE 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).