Formation of non-magmatic iron-meteorite group IIE

Abstract

Instrumental neutron-activation (INAA) data for metal in 22 nonmagmatic IIE meteorites show narrow ranges in Ir and other refractory siderophiles; the Ir range is a factor of 2.6, a factor of ∼2 smaller than in nonmagmatic IAB-MG, and orders of magnitude smaller than in the large magmatic groups. Siderophile data show no evidence of fractional crystallization. IIE irons can be split into two sets, a larger main-set and a set of 6 Cu- (or FeS) rich irons. Elemental concentrations in metal from veins in H5 chondrite Portales Valley fall within the IIE range with the exceptions of Co (high) and Ga (low).H-group-chondrite and Au-normalized IIE abundances for siderophiles show that IIE irons are ∼30% higher than H in refractory siderophiles Re, Ir and W and are about 30% lower than H chondrites in the volatiles Ga and Sb, inconsistent with proposals that IIE irons formed from H chondrites. The IIE fractionations contrast with those in L chondrites which are about 15% lower than H in the three refractory elements and 40% higher than H in volatiles indicating that IIE irons did not form from H chondrites but from a more reduced and siderophile-rich kind of ordinary chondrite (“HH” chondrites). Most O-isotope data support a close relationship between IIE irons and H or HH chondrites; lower Δ17O in primitive (chondritic) silicates support an HH classification. Literature isotopic data for Ru and Mo also show that IIE metal formed from an ordinary chondrite parent; it appears that the silicates and metal were formed by melting of a single asteroid. There is no evidence for radiogenic (26Al) heating; this, the rapid cooling recorded in the sizes of parental gamma crystal in the metal and the absence of fractional crystallization strongly support the hypothesis that IIE melting was the result of impacts.To summarize, the weight of the evidence favors the conclusion that IIE meteorites were formed by one or more impacts on an HH asteroid. The target probably had a composition like the chondritic materials in Netschaevo, but was unequilibrated and had much higher porosity.