Cosmochemical constraints on asteroid accretion
Cosmochemical constraints on asteroid accretion (in Goldschmidt abstracts 2013, Anonymous)
Mineralogical Magazine (2013) 77 (5): 1026
Multiple lines of chemical, isotopic and textural evidence constrain how kilometer-sized bodies accreted. Km-size chondrite parent bodies were likely precursors to planetesimals and the modern planets. First: Chondritic meteorites ( approximately 90% of known meteorites) contain widely variant modal abundances of their primary components: Chondrules, Ca-Al-rich inclusions (CAIs), and fine-grained matrix containing insoluble organic matter (IOM) and presolar grains (PSG) including graphite and SiC , yet are all "chondritic" in composition [2, 3]. Second: Chondrules, themselves highly variable in Fe/Si, combine to make rocks with bulk chondritic Fe/Si . Third: Chondritic meteorites contain most major elements (Si, Mg, Al, Ca, Ti) and rare earth elements (REEs) in solar proportions, with the most variation in Fe, yet CAIs have highly variable REE inventories . Fourth: Oxygen isotopes, CAI abundances, and clast sizes suggest each chondrite group formed under locally distinct conditions. Variations in Fe do not affect the present argument. That extraterrestrial materials universally tend towards chondritic major and trace element abundances is, alone, a powerful constraint on disk processes, that has been dubbed "complementarity" [2-4]. That is, the primary components (chondrules, CAIs, matrix) complement each other to produce "chondritic" bulk meteorite composition. Why is this so? We will present major and trace element (REE) evidence that the parent bodies of chondritic meteorites accreted from local, gravitationally unstable (overdense) accumulations of material of bulk solar or dust-enriched composition , subjected, prior to accretion, to varying degrees of highly local heating that produced melted objects (e.g., chondrules) with varying degrees of efficiency .