Evidence for the insignificance of ordinary chondritic material in the asteroid belt

Abstract

We review the meteoritical and astronomical literature to answer the question: What is the evidence for the importance of ordinary chondritic material to the composition of the asteroid belt? From the meteoritical literature, we find that currently (1) our meteorite collections sample at least 135 different asteroids; (2) out of 25+ chondritic meteorite parent bodies, 3 are (by definition) ordinary chondritic; (3) out of 14 chondritic grouplets and unique chondrites, 1 1 are affiliated with a carbonaceous groupklan of chondrites; (4) out of 24 differentiated groups of meteorites, only the IIE iron meteorites clearly formed from ordinary chondritic precursor material; (5) out of 12 differentiated grouplets and unique differentiated meteorites, 8 seem to have had carbonaceous chondritic precursors; (6) a high frequency of carbonaceous clasts in ordinary chondritic breccias suggests that ordinary chondrites have been embedded in a swarm of carbonaceous material. The rare occurrence (only one example) of ordinary chondritic clasts in carbonaceous chondritic breccias indicates that ordinary chondritic material has not been widespread in the asteroid belt; (7) cosmic spherules, micrometeorites, and stratospheric interplanetary dust particles-believed to represent a less biased sampling of asteroidal material-show that only a very small fraction (less than-1%) of asteroidal dust has an ordinary chondritic composition. From the astronomical literature, we find that currently (8) spectroscopic surveys of the main asteroid belt are finding more and more nonordinary chondritic primitive material in the inner main belt; (9) the increase in spectroscopic data has increased the inferred mineralogical diversity of main belt asteroids; and (10) no ordinary chondritic asteroids have been directly observed in the main belt. These lines of evidence strongly suggest a scenario in which ordinary chondritic asteroids were never abundant in the main belt. The S-type asteroids may currently be primarily differentiated, but the precursor material is more likely to have been carbonaceous chondritic, not ordinary chondritic. Historically, carbonaceous material could have dominated the entire main belt. This could explain the presence in the inner main belt of asteroids linked to the primitive carbonaceous chondrites, and the absence of asteroids linked to the ordinary chondrites. The implications of this scenario for the asteroid heating mechanism(s) are briefly discussed.

Magnetic evidence for a partially differentiated carbonaceous chondrite parent body

“…Our results suggest that asteroids with differentiated interiors could be present today but masked under chondritic surfaces, which would explain the great discrepancy between the >80% of meteorite parent bodies that melted versus the paucity of asteroids with basaltic surfaces (37). In fact, CV chondrites have spectral signatures similar to many members of the Eos dynamical asteroid family; the spectral diversity of this family has already led to suggestion that the parent asteroid was partially differentiated (38).…”

Graves Nunataks 95209: A snapshot of metal segregation and core formation

“…The diversity and numbers of igneous meteorites suggest that the majority of sampled asteroids ($105 of 135 or $80%; Meibom and Clark, 1999) experienced some degree of melting and differentiation. The vast majority of these asteroids are represented by iron meteorites from both the major groups and the $70 parent bodies represented by the $120 ungrouped iron meteorites (Wasson, 1995).…”

A database of chondrite analyses including platinum group elements, Ni, Co, Au, and Cr: Implications for the identification of chondritic projectiles

“…So far, 14 chondrite groups (EH, EL, H, L, LL, R, CV, CK, CO, CM, CR, CH, CB, CI) and one grouplet (K) have been classified based on oxygen isotopic compositions, characteristic element abundances, petrographic features, mineralogy, and oxidation state (e.g., Krot et al 2003). There are also a number of ungrouped chondritic meteorites (e.g., Meibom and Clark 1999;Krot et al 2003), most of which are related to carbonaceous chondrites. The bulk compositional variations of chondrites probably reflect the conditions at the time and place of the formation of their components (refractory inclusions, metal/sulfides, chondrules, matrix) as well as the abundance of each of these components.…”