The 2.5–5.1 μm reflectance spectra of HED meteorites and their constituent minerals: Implications for Dawn

Abstract

Reflectance spectra of multiple grain sizes and slabs of a suite of 13 howardite–eucrite–diogenite (HED) meteorites have been characterized from 1.7 to 25.4μm. The 4.5–5.1μm region, which is accessible by the Dawn VIR spectrometer, shows multiple absorption bands that vary among the HED groups and can be related to underlying mineralogy. These bands are overtones of asymmetric Si–(Al–)O fundamental stretches in the 9–11μm region. They appear in HED reflectance spectra ranging from fine-grained powders to slabs. The strongest absorption bands in eucrite spectra are found in the 4.74–4.78, 4.86–4.91, and 4.99–5.05μm regions, and are attributable to the various pyroxenes in eucrites. Less frequent bands are found near 4.35, 4.47, and 4.64–4.67μm in larger grain size eucrite spectra, and are likely attributable to high-An plagioclase feldspar. A shocked eucrite (JaH626) shows some differences from presumed unshocked samples, with bands near 4.68, 4.83, and 5.02μm, the first two of which fall outside the range of the presumed unshocked eucrites. The strongest absorption bands in diogenite spectra are found near 4.70 and 4.83–485μm, and are attributable to the abundant low-Ca pyroxene. At larger grain sizes and in slabs, an additional band can appear near 4.79μm. When olivine is dominant, an absorption band is found near 4.93μm, which coincides with the strongest olivine absorption band in this region. The addition of olivine can lead to a progressive shift of the 4.70μm diogenite band toward 4.65μm (where olivine has an additional weaker absorption band). Howardite spectra, as expected, are more diverse. The most persistent absorption features are in the 4.73–4.77, 4.84–4.85, and 4.94–5.00μm regions, and can be attributed to contributions by both diogenitic and eucritic components.For all of the HEDs, with the exception of PRA 04401, a CM xenolith-bearing howardite, the <45μm fraction has the highest overall reflectance. The <45μm HED spectra generally have the shallowest absorption bands compared to the larger grain sizes, with the exception of the diogenites. Relative band depths change with grain size, likely due to different bands saturating at different grain sizes, leading to merging of adjacent absorption bands (causing movement of apparent band minima) and changes from resolvable absorption bands to inflections. The number of bands that appear in the HED reflectance spectra varies with both composition and grain size. In general, weak absorption bands become more resolvable in slab and coarse grain spectra, but multiple diagnostic absorption bands are present for all classes of HED meteorites regardless of grain size. Because these 4–5μm region absorption bands are due to different mechanisms than those responsible for shorter wavelength absorption bands (e.g., Fe2+ crystal field transitions), they provide complementary information on the composition of HEDs and the surface of Vesta. Importantly, these longer wavelength bands can provide direct evidence for the presence of plagioclase feldspar, which is difficult to derive from shorter wavelength observations. Finally, we have found that HEDs and their constituent minerals exhibit even stronger absorption bands just beyond the range of the Dawn VIR spectrometer, from 5.15 to 5.60μm, that are also attributable to Si–(Al–)O overtones.