Olivine-dominated asteroids: Mineralogy and origin

Abstract

Olivine-dominated asteroids are a rare type of objects formed either in nebular processes or through magmatic differentiation. The analysis of meteorite samples suggest that at least 100 parent bodies in the main belt experienced partial or complete melting and differentiation before being disrupted. However, only a few olivine-dominated asteroids, representative of the mantle of disrupted differentiated bodies, are known to exist. Due to the paucity of these objects in the main belt their origin and evolution have been a matter of great debate over the years. In this work we present a detailed mineralogical analysis of twelve olivine-dominated asteroids. Within our sample we distinguish two classes, one that we call monomineralic-olivine asteroids and another referred to as olivine-rich asteroids. For the monomineralic-olivine asteroids the olivine chemistry was found to range from ~ Fo49 to Fo70, consistent with the values measured for brachinites and R chondrites. In the case of the olivine-rich asteroids we determined their olivine and low-Ca pyroxene abundance using a new set of spectral calibrations derived from the analysis of R chondrites spectra. We found that the olivine abundance for these asteroids varies from 0.68 to 0.93, while the fraction of low-Ca pyroxene to total pyroxene ranges from 0.6 to 0.9. A search for dynamical connections between the olivine-dominated asteroids and asteroid families found no genetic link (of the type core-mantel-crust) between these objects.Comment: 28 pages, 11 figures, 5 tables. This version includes a few minor changes in the text to match the final version published in Icaru

Basalt or Not? Near-infrared Spectra, Surface Mineralogical Estimates, and Meteorite Analogs for 33 Vp-type Asteroids

“…Individual bodies subject to compositional zoning into canonical crust, mantle, and core components, whether partially or fully differentiated, were later disrupted via collisions and became constituents of the solar system. However, previous researchers have noted the dearth of mantle (i.e., olivine) and basaltic NIR spectral signatures on asteroids in the present era, which could be attributed to mass loss from the solar system (Burbine et al 1996;Sanchez et al 2014;Brasil et al 2017).…”

“…This technique can potentially be applied to all asteroid NIR spectra with measurable spectral absorption features but is most useful for well-calibrated spectra dominated by pyroxene, olivine, or both (Burns 1993;Gaffey et al 1993;Hardersen et al 2014Hardersen et al , 2015Sanchez et al 2014). Calibrations of laboratory pyroxene and basaltic achondrites are the most relevant calibrations for this work because pyroxene is the dominant mafic silicate component in basaltic achondrites and the HED clan (Mittlefehdlt et al 1998).…”

“…Absorption band depth is another commonly measured spectral band parameter, which represents the percentage decrease in reflectance of an absorption relative to the isolated continuum (e.g., band depth (%)=1 -X =Y, where X is the relative reflectance value of the minimum reflectance of the absorption band and Y is the continuum-removed absorption band depth represented as a percentage). Band depths are not diagnostic of mineralogy and can vary based on the asteroid's phase angle during observations, space weathering on an asteroid's surface, or a surface metal component (Britt & Pieters 1988;Blewett et al 2015;Cloutis et al 1990Cloutis et al , 2015Sanchez et al 2014). Our analysis of band parameters utilizes a set of MATLAB routines (Reddy et al 2012a) and the IDL-based Spectral Analysis Routine for Asteroids (SARA) (Lindsay et al 2015).…”

More chips off of Asteroid (4) Vesta: Characterization of eight Vestoids and their HED meteorite analogs

“…All eight asteroid band-band data plot slightly above the HED data, which suggests a possible minor contribution from olivine or a high-Ca pyroxene phase. These minerals (olivine, Type A clinopyroxene) only exhibit Band I absorption features; however, Band I centers for olivine will increase with increasing relative abundances while no band center trend is seen for Type A clinopyroxenes (King and Ridley, 1987;Burns, 1993a;Schade et al, 2004;Sanchez et al, 2014). However, any contribution from these phases will be minor as the Figure 4 data plots only slightly above the HED data and a significant contribution from another mineral phase is not apparent in Figure 3 or Figure 5.…”

“…Other minerals are detectable in this spectral region, such as water ice and magnetite, and, to a lesser extent, plagioclase feldspar, phyllosilicates, and other less diagnostic minerals (Rivkin and Emery, 2010;Yang and Jewitt, 2010;Takir and Emery, 2012), but the olivine and pyroxene mineral groups are the dominant minerals found on asteroid surfaces for many taxonomic types (i.e., V, S, M, R, A: Gaffey et al, 1993;Gaffey, 1997;Abell and Gaffey, 2000;Moskovitz et al, 2010;Hardersen et al, 2011;Sanchez et al, 2014).…”