3D porosity, flow, and transport characteristics of two L chondrites reveal wet accretion-related cosmic web-like porosity

Abstract

Porosity, with its structure-dependent flow properties (permeability and tortuosity) and transport properties (thermal conductivity and thermal diffusivity), is closely related to the accretion, thermal metamorphism, and associated hydrothermal alteration of ordinary chondrite (OC) parent bodies. Using synchrotron radiation microtomography (SRμCT), we reveal the varying porosity structures in two L chondrite falls of low (Mezö-Madaras L3.7) and high (Bath Furnace L6) petrologic types and quantify porosity properties, such as shape and connectivity, and related effective permeability and tortuosity factor. Although the two specimens demonstrate similar effective permeabilities, they exhibit significantly different tortuosity factors and textures of porosity, which include notable differences in void throat diameters, complexity and density of the interconnected void network, heterogeneity in void distribution, and the extent of primary and secondary porosity. The complex relationships among porosity, permeability, tortuosity, and thermal conductivity can be explained by the varying void arrangements related to varying grain sizes among the petrologic types of OCs, which in turn reflect their varying evolutionary paths.

Electron microprobe and attached energy-dispersive X-ray spectrometer reveal signs of hydrothermal alteration in both petrologic types. High-energy SRμCT imaging (0.65 μm voxel size) reveals the presence of a new microporosity substructure resembling a microscopic cosmic web, which may be linked to fluid-assisted metamorphism and hydrothermal alteration during wet accretion of the parent body. Furthermore, the proportion of this continuous porosity may be related to the temperatures associated with different petrologic types, and the wet accretion model may resolve the lack of correlation between petrologic types and porosity of OCs. Finally, the uncovered cosmic web-like microporosity structure may explain the observed concurrent high thermal conductivity, low permeability, and high porosity of the high-petrologic-type OCs.