Oxygen isotope and petrological study of silicate inclusions in IIE iron meteorites and their relationship with H chondrites
Kathryn H. McDermott, Richard C. Greenwood, Edward R.D. Scott, Ian A. Franchi, Mahesh Anand
Geochimica et Cosmochimica Acta
In Press, Accepted Manuscript, Available online 28 October 2015
“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 Δ17O 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 Δ17O 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 Δ17O value and plot ∼2σ 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, Netschaëvo, and Techado, have relatively low mean Δ17O 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."