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Reply to: GEMS and the devil in their details

The Original Article was published on 05 July 2019

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replying to J. P. Bradley Nature Astronomy https://doi.org/10.1038/s41550-019-0833-9 (2019)

In his critique, Bradley1 has chosen to focus not on the central argument of our Article but on a narrow question of whether or not we have provided sufficient experimental data to justify the use of the term GEMS, that is, glass with embedded metal and sulfides. Our Article2 provided several independent lines of evidence that the C-rich clast we found in LaPaz Icefield 02342 is xenolithic in nature and likely related to putative cometary materials like anhydrous chondritic porous interplanetary dust particles (CP-IDPs) and ultracarbonaceous Antarctic micrometeorites (UCAMMs). Thus, the most important conclusion of our paper that the properties of the C-rich clast demonstrate mixing of materials accreted to asteroids and comets in the outer solar nebula stands without question. Nevertheless, we find it important to address Bradley’s specific concerns regarding the identification of GEMS in LaPaz Icefield 02342.

The critique puts forward a requirement for the positive identification of GEMS based on electron diffraction that is not supported by the available peer-reviewed literature. Electron diffraction confirmation of the metal and sulfide crystal phases associated with the GEMS has in fact been reported for only a very small number of GEMS inclusions from very few IDPs3,4 and in some cases, the studies did not demonstrate that the sulfides were pyrrhotite. The cited reference for GEMS identification in UCAMMs5 does not provide diffraction data for inclusions inGEMS, but relies on EDS, and thus actually supports our GEMS identification and the comparison of the LaPaz Icefield 02342 xenolithic clast to UCAMMs. Even in CP-IDPs, where GEMS were first identified and named by Bradley, the objects show considerable diversity of chemical compositions, sizes, and subgrain distributions and compositions. Given the extremely sparse diffraction data for GEMS subgrains in the literature, there is at present no basis to support diffraction as an exclusive criterion. In short, the properties, and origin(s) of GEMS are not universally agreed on and are in fact vigorously debated in the literature6,7,8. We encourage Bradley and colleagues to describe and justify their criteria for GEMS classification in a full peer-reviewed manuscript, rather than as a short critique.

References

  1. Bradley, J. P. GEMS and the devil in their details. Nat. Astron. https://doi.org/10.1038/s41550-019-0833-9 (2019).

  2. Nittler, L. R. et al. A cometary building block in a primitive asteroidal meteorite. Nat. Astron. https://doi.org/10.1038/s41550-019-0737-8 (2019).

  3. Bradley, J. P. Chemically anomalous preaccretionally irradiated grains in interplanetary dust from comets. Science 265, 925–929 (1994).

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  8. Keller, L. P. & Messenger, S. On the origins of GEMS grains: a reply. Geochim. Cosmochim. Acta 107, 341–344 (2013).

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The manuscript was written by L.R.N. and R.M.S. with input from all of the other authors.

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Correspondence to Larry R. Nittler.

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Nittler, L.R., Stroud, R.M., Trigo-Rodríguez, J.M. et al. Reply to: GEMS and the devil in their details. Nat Astron 3, 606 (2019). https://doi.org/10.1038/s41550-019-0834-8

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