A new method for determining the petrologic type of unequilibrated ordinary chondrites that can be applied to asteroids

1,2,5Derek Sears,1,2,5Daniel Ostrowski,3Heather Smith,1,6Adonay Sissay,4Mihir Trivedi
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2021.114442]
1Arkansas Center for Space and Planetary Sciences, University of Arkansas, Fayetteville, AR 72701, USA
2BAER Institute/NASA Ames Research Center, Moffett Field, CA 94035, USA
3USRA/NASA Ames Research Center, Moffett Field, CA 94035, USA
4NASA Ames Research Center, Moffett Field, CA 94035, USA
5Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville, AR 72701, USA
Copyright Elsevier

In order to find an additional quantitative way to estimate the petrographic type of unequilibrated ordinary chondrites (UOC), and one that can be used remotely in the study of asteroids, we have analyzed the near-infrared spectra of a suite of UOC observed falls. We obtained spectra from the RELAB database at Brown University and applied several methods for determining the amount of clinopyroxene (CPX) as a percentage of the total pyroxene in the meteorites. The presence of low-Ca CPX has long been known to be characteristic of little-metamorphosed ordinary chondrites. The methods we used were (1) naked-eye determination of the wavelength of the absorption features at ~1 μm and ~2 μm, (2) determination of the wavelengths of these features by fitting polynomial equations, and (3) determining the relative intensities of the CPX and OPX features after isolation by a curve fitting procedure. The measurements were then “calibrated” using data from the literature to obtain values for the amount of CPX in the total pyroxene. We find that there is an empirical relationship between the amount of CPX detected by these methods of spectrum analysis and the petrologic type.

Petrologic type = +4.402–0.019 × CPX%

We explain this empirical relationship (1) as evidence that in pyroxene bearing rocks the spectrum of pyroxene dominates (this has been known in the 1970s), (2) that low-Ca CPX is so abundant in these meteorites (up to 40 vol%) that it is easily detected by reflectance spectroscopy, and (3) compositional effects caused by Ca and Fe in the pyroxenes partially cancel out or are small. We thus have a new method of quantitatively measuring the level of metamorphic alteration experienced by these important meteorites and of assigning them a petrologic type of 3.0 to 3.9. More importantly, unlike existing methods, this can be applied remotely so that chondritic asteroid surfaces (i.e. those of Q and S asteroids) can also be characterized in terms of their metamorphic history. As an example, (433) Eros and (25143) Itokawa were found to be types ~3.5 and ~3.4, respectively. We briefly discuss the implications of this for understanding the history of meteorites and asteroids.

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