Oxygen isotope and petrological study of silicate inclusions in IIE iron meteorites and their relationship with H chondrites

The origin of silicate-bearing irons, especially those in groups IAB, IIICD, and IIE, is poorly understood as silicate should

have separated rapidly from molten metal. Here we report the results of high precision oxygen isotope analysis of silicate

inclusions in eleven group IIE meteorites and a petrological study of silicate inclusions in ten IIE irons including those in Garhi

Yasin and Tarahumara, which have not been described in detail before. Oxygen isotopes have also been analysed in 20 H

chondrites to investigate their possible relationship with the IIE irons.

Based on petrographic observations and mineral analysis, the silicate-bearing IIE meteorites have been divided into four

types according to the nature of their silicate inclusions: (1) primitive chondritic, (2) evolved chondritic, (3) differentiated with

>10 vol.% orthopyroxene, and (4) differentiated with <10 vol.% orthopyroxene. Each meteorite contains a single inclusion

type. While inclusions in an individual IIE meteorite tend to show relatively limited D17O variation, a wide range of values

is seen in the dataset as a whole. Group IIE irons with differentiated silicates, with the exception of Colomera, have a range

of mean D17O values that is essentially identical to those of the H4-6 chondrites: 0.60–0.77‰ and 0.61–0.76‰, respectively.

Colomera inclusions, which are differentiated with <10 vol.% orthopyroxene, have an anomalously high D17O value and plot

?2r away from the next nearest IIE iron. However, in view of the textural similarities to other IIE inclusions, a separate

source for Colomera is deemed unlikely. Three IIE irons with primitive chondritic inclusions, Garhi Yasin, Netschae¨vo,

and Techado, have relatively low mean D17O values of 0.56–0.57‰ as well as relatively reduced silicates with Fa15–17 olivine,

which have been called HH chondrites. Given the significant overlap in their oxygen isotope compositions, a genetic relationship

between IIE irons and H chondrites is supported by our new data. However, derivation of both groups from one parent

body seems unlikely. Instead, both groups probably sampled similar precursor materials and accreted at a similar nebular

location.

Our data suggest that the IIE meteorites formed on an internally heated H/HH chondrite-like body that experienced the

initial stages of differentiation in response to radiogenic heating. However, prior to full differentiation the IIE parent body

experienced a major hit-and-run style collision that resulted in silicate-metal mixing. The initial stages of this event involved

a phase of rapid cooling that prevented unmixing of metal and silicates. Reassembly of the IIE parent body produced a large

regolith blanket that facilitated subsequent slow cooling. The IIE parent body has probably experienced numerous subsequent

less catastrophic collisions. The development of alkali glass textures in some differentiated inclusions is probably the result of

one of these later events.