How and where did GEMS form?

In the paper "On the Origins of GEMS" Keller and Messenger (2011) present the results of a study of GEMS (glass with embedded metal and sulfides) in chondritic porous interplanetary dust particles (CP IDPs). They find that non-solar oxygen isotopic abundances are detectable in a few typically the largest GEMS, confirming that they are surviving presolar amorphous silicates derived from the interstellar medium (ISM). They argue that those without detectable anomalies, the majority, formed in the solar system even though they are otherwise identical to the anomalous GEMS. Their conclusion is based on element-to-Si ratios, but GEMS-rich CP IDPs are collected, stored in and permanently contaminated with silicone oil. They are also thermally unstable and chemically reactive during atmospheric entry, which further modifies element-to-Si ratios. Both of the approaches employed to argue negligible silicone oil contamination are problematical: First, carbon is used as a tracer for silicone oil contamination, but carbon is both native to the IDPs and introduced throughout the sample collection and preparation processes. Second, the absence of an similar to 8 mu m infrared (IR) feature is cited, based on refitting of previously published spectra using a polynomial spline. (C) 2012 Elsevier Ltd. All rights reserved.