NEAR-IR REFLECTANCE SPECTROSCOPY OF NEAR-EARTH ASTEROID (23183) 2000 OY21: EVIDENCE FOR CA-RICH CLINOPYROXENE IN THE SURFACE ASSEMBLAGE

Near-infrared reflectance spectra of Near-Earth Asteroid 2000 OY21 were obtained on Feb. 1, 2006, using the SpeX instrument on NASA IRTF, Mauna Kea, Hawai'i. High- quality spectrum of 2000 OY21 displays a deep asymmetric feature at ~1-microns (Band I) and a broad feature at ~2-microns (Band II) suggesting the presence of pyroxene in the surface assemblage. The calculated Band I center is 1.027±0.01 microns with a Band II center located between 2.18±0.05 and 2.30±0.05 microns and a lower limit Band Area Ratio (BAR) of 0.55±0.01. The uncertainty in the Band II center and BAR arises from the weakly constrained continuum. Using methods developed by Adams (1974), and Cloutis & Gaffey (1991), 2000 OY21 plots on the clinopyroxene (CPX) side of the ortho-clinopyroxene trend line on the band-band plot suggesting the presence of Type B CPX along with a possible second phase.

The presence of a second phase is suggested by a weak inflection at ~1.3-microns. While pure CPX are known to show a small inflection at that wavelength due to Fe²⁺ in M1 crystallographic site, the feature has also been observed in the spectrum of olivine, plagioclase feldspar and high-Fe orthopyroxene. Using calibration work described in Gaffey et al. (2002); we estimate the CPX chemistry to range from Wo_51±3 Fs_21±4 to Wo_51±3 Fs_7±4, given the uncertainties in Band II center. Spectra of Fe-poor (< Fs₂₉) high-Ca CPX (diopside) show 2-µm absorption feature despite the high wollastonite content unless they have high Fe³⁺/Fe²⁺ ratio (Schade et al., 2004). The calculated chemistries plot in the high-Ca CPX region on the pyroxene quadrilateral, away from chondritic pyroxenes which have much lower Fe and Ca contents. We suggest that this assemblage formed from an extracted early partial melt of a chondritic parent body. An alternate interpretation of the spectrum would be a mixture of olivine with spinel which we think is unlikely based on the band parameters. This research was supported by NASA Near-Earth Objects Observations Program grant NNG04GI17G.