Shock‐darkening in ordinary chondrites: Determination of the pressure‐temperature conditions by shock physics mesoscale modeling

Abstract

We determined the shock-darkening pressure range in ordinary chondrites using the iSALE shock physics code. We simulated planar shock waves on a mesoscale in a sample layer at different nominal pressures. Iron and troilite grains were resolved in a porous olivine matrix in the sample layer. We used equations of state (Tillotson EoS and ANEOS) and basic strength and thermal properties to describe the material phases. We used Lagrangian tracers to record the peak shock pressures in each material unit. The postshock temperatures (and the fractions of the tracers experiencing temperatures above the melting point) for each material were estimated after the passage of the shock wave and after the reflections of the shock at grain boundaries in the heterogeneous materials. The results showed that shock-darkening, associated with troilite melt and the onset of olivine melt, happened between 40 and 50 GPa with 52 GPa being the pressure at which all tracers in the troilite material reach the melting point. We demonstrate the difficulties of shock heating in iron and also the importance of porosity. Material impedances, grain shapes, and the porosity models available in the iSALE code are discussed. We also discuss possible notshock-related triggers for iron melt.

Partial amorphization of experimentally shocked plagioclase: A spectroscopic study

“…Darkening of silicates, that is, the reduction of the transparency in transmitted light, is commonly associated with shock and is due to the dispersion of tiny Fe‐bearing droplets, activated even at lower shock conditions than those required for melting (e.g., Rubin ; Moreau et al. ). Natural plagioclase generally contains minor amounts of Fe (e.g., Deer et al.…”

Compositional distributions and evolutionary processes for the near-Earth object population: Results from the MIT-Hawaii Near-Earth Object Spectroscopic Survey (MITHNEOS)

“…The second case, which may be described as impact melting, occurs for higher temperatures where the silicates also melt and then recrystallize with both sulfides and iron mixed in. The corresponding peak shock pressure range for these two shock processes was estimated by Moreau et al (2017) with a corresponding pressure range of ∼40-50 GPa for sulfide melting and > 60 GPa for whole-rock melting. Reddy et al (2014) and Kohout et al (2015) show that these two processes yield similar outcomes in terms of their measureable reflectance spectra.…”

“…We depict here "the space weathering vector," which we find parallels the diagonal line in the figure corresponding to the progressive spectral dispersion for the S-complex that was revealed empirically in the asteroid data and denoted as "line α" in the (DeMeo et al, 2009) analysis. We also explored the progressive role of shock darkening, guided by the work of Reddy et al (2014), Kohout et al (2014Kohout et al ( , 2015 and Moreau et al (2017), as described in Section 4.3. Within the inset figure, "x" symbols show the detailed outcomes for progressive alteration that depicts a "shock darkening vector" starting with the unshocked Chelyabinsk meteorite then adding increasing mixtures of shocked material (0, 5, 10, 20, 30, .…”

Experimental constraints on the ordinary chondrite shock darkening caused by asteroid collisions

“…Optical changes in the chondritic materials are associated with a partial or complete melting caused by the impact-related post-shock heating (Heymann 1967;Smith & Goldstein 1977;Scott 1982;Rubin 1985Rubin , 1992. Shock modeling results suggest that at ∼40-50 GPa, troilite starts to extensively melt due to its lower melting point with only a small amount of silicate or metal melt localized in the thin shock veins (Moreau et al 2017(Moreau et al , 2019. The molten troilite typically forms a web of fine veins dispersed in the cracks within solid silicate grains, which is similar to the Chelyabinsk dark-colored lithology (Keil et al 1992;Kohout et al 2014;Righter et al 2015;Kaeter et al 2018).…”