Particle size distributions in chondritic meteorites: Evidence for pre-planetesimal historiesOPEN ACCESS 

J.I. Simon, J.N. Cuzzi, K.A. McCain, M.J. Cato, P.A. Christoffersen, K.R. Fisher, P. Srinivasan, A.W. Tait, D.M. Olson, J.D. Scargle

Earth and Planetary Science Letters
Volume 494, 15 July 2018, Pages 69–82

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“Highlights

• All measured particle size distributions are significantly broader than previously reported.
• In Allende ∼85% of particles are unrimmed and in direct contact with meteorite host matrix.
• When present, rim thicknesses appear correlated to underlying particle size.
• All particle size distributions measured are very similar in shape after scaling to a common mean size.
• Size-selective aerodynamics allow particle sticking to proceed to larger aggregation sizes.”

“Magnesium-rich silicate chondrules and calcium-, aluminum-rich refractory inclusions (CAIs) are fundamental components of primitive chondritic meteorites. It has been suggested that concentration of these early-formed particles by nebular sorting processes may lead to accretion of planetesimals, the planetary bodies that represent the building blocks of the terrestrial planets. In this case, the size distributions of the particles may constrain the accretion process. Here we present new particle size distribution data for Northwest Africa 5717, a primitive ordinary chondrite (ungrouped 3.05) and the well-known carbonaceous chondrite Allende (CV3). Instead of the relatively narrow size distributions obtained in previous studies (Ebel et al., 2016; Friedrich et al., 2015 ; Paque and Cuzzi, 1997, and references therein), we observed broad size distributions for all particle types in both meteorites. Detailed microscopic image analysis of Allende shows differences in the size distributions of chondrule subtypes, but collectively these subpopulations comprise a composite “chondrule” size distribution that is similar to the broad size distribution found for CAIs. Also, we find accretionary ‘dust’ rims on only a subset (∼15–20%) of the chondrules contained in Allende, which indicates that subpopulations of chondrules experienced distinct histories prior to planetary accretion. For the rimmed subset, we find positive correlation between rim thickness and chondrule size. The remarkable similarity between the size distributions of various subgroups of particles, both with and without fine grained rims, implies a common size sorting process. Chondrite classification schemes, astrophysical disk models that predict a narrow chondrule size population and/or a common localized formation event, and conventional particle analysis methods must all be critically reevaluated. We support the idea that distinct “lithologies” in NWA 5717 are nebular aggregates of chondrules. If ≥cm-sized aggregates of chondrules can form it will have implications for planet formation and suggests the sticking stage is where the preferential size physics is operating.”