Incipient melting in and shock classification of L-group chondrites

Because the meteorites with the oldest (Rumuruti, ~4.47 Ga) and the youngest (Acfer 217, ~4.30 Ga) ages are both breccias, these ages probably do not record slow cooling within an undisrupted asteroidal parent body. Instead, the process of breccia formation may have differentially reset the ages of the constituent material, or the differences in their age spectra may arise from mixtures of material that had different ages. Two end-member type situations may be envisioned to explain the age range observed in the R chondrites. The first is if the impact(s) that reset the ages of Acfer 217 and Rumuruti was very early. In this case, the ~170 Ma maximum age difference between these meteorites may have been produced by much deeper burial of Acfer 217 than Rumuruti within an impact-induced thick regolith layer, or within a rubble pile type parent body following parent body re-assembly. The second, preferred scenario is if the impact that reset the age of Acfer 217 was much later than that which reset Rumuruti, then Acfer 217 may have cooled more rapidly within a much thinner regolith layer. In either scenario, the oldest age obtained here, from Rumuruti, provides evidence for relatively early (~4.47 Ga) impact events and breccia formation on the R chondrite parent body.

Section: Shockmentioning

confidence: 99%

³⁹Ar‐⁴⁰Ar chronology of R chondrites

Dixon

Bogard

Garrison

2003

“…These criteria were developed through empirical relationships between melt veins and apparent shock intensity (e.g., Anders 1964;Heymann 1967;Dodd and Jarosewich 1979). In particular, melt veins are generally interpreted as "shock" veins, and those ordinary chondrites with pervasive dark veins have been referred to as "shockdarkened" (Heymann 1967).…”

Section: Introductionmentioning

confidence: 99%

Impact‐induced frictional melting in ordinary chondrites: A mechanism for deformation, darkening, and vein formation

Bogert

Schultz

Spray

2003

Abstract-High speed friction experiments have been performed on the ordinary chondrites El Hammami (H5, S2) and Sahara 97001 (L6, S3) using an axial friction-welding apparatus. Each sample was subjected to a strain rate of 10 3 to 10 4 s -1 , which generated 250 to 500 mm-deep darkened zones on each sample cube. Thin section analyses reveal that the darkened areas are composed of silicate glass and mineral fragments intermingled with dispersed submicron-size FeNi and FeS blebs.Fracturing of mineral grains and the formation of tiny metallic veins define the extent of deformation beyond the darkened shear zone. These features are not present in the original meteorites. The shear zones and tiny veins are quite similar to certain vein systems seen in naturally deformed ordinary chondrites. The experiments show that shock deformation is not required for the formation of melt veins and darkening in ordinary chondrites. Therefore, the presence of melt veins and darkening does not imply that an ordinary chondrite has undergone severe shock deformation. In fact, high strain rate deformation and frictional melting are especially important for the formation of veins at low shock pressures.

“…Shock-induced localized melting is a common feature in shocked ordinary chondrites (Dodd and Jarosewich, 1979;Rubin, 1985;Stoffler et a/., 1991). In naturally shocked chondrites, opaque melt veins, melt pockets, melt dikes, and metal or sulfide in.jections can be distinguished (Stoffler et a/., 1991).…”

Section: Shock-induced Localized Meltingmentioning

confidence: 99%

“…The shock-metamorphic overprint of ordinary chondrites ranges from very weak to intense, including shock-induced melting (Dodd and Jarosewich, 1979;Stoffler et al, 1991). Although ordinary chondrites are quite porous and heterogeneous materials, terrestrial analogues such as single crystals of olivine, orthopyroxene, and plagioclase, or magmatic rocks were used in past shock experiments for the pressure calibration of shock effects in ordinary chondrites (Carter et al, 1968;Pollack, 1969: James, 1969;Muller and Hornernann.…”

Section: Introductionmentioning

confidence: 99%

Shock experiments with the H6 chondrite Kernouvé: Pressure calibration of microscopic shock effects

Schmitt

2000

104

113

Abstract-Shock-recovery experiments were carried out on samples of the H6 chondrite Kernouve at shock pressures of 10, 15, 20, 25, 30, 35, 45, and 60 GPa and preheating temperatures of 293 K (low-temperature experiments) and 920 K (high-temperature experiments). Using a calculated equation of state of Kernouve, pressure-pulse durations of 0.3 to 1.2 ,us were estimated. The shocked samples were investigated by optical microscopy to calibrate the various shock effects in olivine, orthopyroxene, oligoclase, and troilite.The following pressure calibration is proposed for silicates: (1) undulatory extinction of olivine <15 GPa; (2) weak mosaicism of olivine from 10-15 GPa to 20-25 GPa; (3) onset of strong mosaicism of olivine at 20-25 GPa; (4) transformation of oligoclase to diaplectic glass completed at 25-30 GPa (low-temperature experiments) and at 20-25 GPa (high-temperature experiments); ( 5 ) onset of weak mosaicism in orthopyroxene at 30-35 GPa (low-temperature experiments) and at 25-30 GPa (high-temperature experiments); and (6) recrystallization or melting of olivine starting at 4 5 4 0 GPa (low-temperature experiments) and at 3 5 4 5 GPa (high-temperature experiments), and completed above 45-60 GPa in the high-temperature experiments.Troilite displays distinct differences between the samples shocked at low and high temperatures. In the low-temperature experiments, the following effects can be observed in troilite: (1) undulatory extinction up to 25 GPa, (2) twinning up to 45 GPa, (3) partial recrystallization from 30 to 60 GPa, and (4) complete recrystallization >35 GPa; whereas in the high-temperature experiments, troilite shows (1) complete recrystallization from 10 up to 45 GPa and (2) melting and crystallization above 45 GPa.Localized shock-induced melting is observed in samples shocked to pressures >15 GPa in the hightemperature experiments and >30 GPa for the low-temperature experiments in the form of FeNi metal and troilite melt injections and intergrowths and as pockets and veins of whole-rock melt. Obviously, the onset and abundance of shock-induced localized melting strongly depends on the initial temperature of the sample.