Association of silica phases as geothermobarometer for eucrites: Implication for two‐stage thermal metamorphism in the eucritic crust

Abstract

Silica mineral is present in different stable polymorphs depending on the temperature and pressure conditions of crystallization. We suggest using silica mineral phases to constrain the thermal history of eucrites. We focused on silica minerals in basaltic clasts of nine non-cumulate eucrites to compare with previously studied cumulate eucrites. Our observations indicate an apparent relationship between thermal metamorphic degrees and silica phase texture in basaltic clasts of non-cumulate eucrites. To reveal complex transformation relations between silica polymorphs in eucrites, we performed cooling experiments (cooling rate: 1 and 0.1°C h À1 ) and heating experiments (heating 500°C for 168 h and 800°C for 96 h) using eucrites. The cooling experiments show that cristobalite is an initial silica phase crystallized from eucritic magma and transforms to quartz at the cooling rate between 0.1 and 1°C h À1 . Based on the cooling experiments and observations of eucrites, we suggest that a combination of silica minerals varies depending mainly on cooling rates. According to the heating experiments, monoclinic tridymite hardly transforms to other phases at low temperature by short reheating events such as brecciation. Monoclinic tridymite can partially transform to quartz with a "hackle" fracture. We conclude that a reheating event partially transformed monoclinic tridymite to quartz to form aggregates of monoclinic tridymite and quartz with the hackle fracture in eucrites. We suggested that some basaltic clasts in non-cumulate eucrites experienced two-stage thermal metamorphism in the eucritic crust. The first metamorphic event has resulted from burial under lava produced by successive eruptions. Igneous intrusions into the preformed crust may have caused the second metamorphic event. The intrusions heated the deep eucritic crust and induced the transformation from monoclinic tridymite to quartz.

Petrographic constraints on the formation of silica‐rich igneous rims around chondrules in CR chondrites

“…In particular, we have shown that cristobalite can crystallize at temperatures well below 1470°C in a system with SIR bulk chemistry. Ono et al (2021) also observed primary cristobalite in experiments on a eucrite composition, during cooling at rates of 0.1-1°C h À1 from peak temperatures 1250-1300°C. In addition, the origin of tridymite in the rim on EET 92042 Ch11 is not clear.…”

“…Ono et al. (2021) also observed primary cristobalite in experiments on a eucrite composition, during cooling at rates of 0.1–1°C h −1 from peak temperatures 1250–1300°C. In addition, the origin of tridymite in the rim on EET 92042 Ch11 is not clear.…”

“…Ono et al. (2021) suggest that formation of tridymite could occur via transformation from cristobalite at very slow cooling rates (<0.1°C h −1 ), but for reasons discussed below, we rule out such slow cooling for SIRs.…”

“…Clearly, this is inconsistent with crystallization of tridymite from a melt, based on the silica phase diagram. Ono et al (2021) suggest that formation of tridymite could occur via transformation from cristobalite at very slow cooling rates (<0.1°C h À1 ), but for reasons discussed below, we rule out such slow cooling for SIRs.…”

“…However, the sharp boundaries retained at the interfaces between host chondrules/IRs/SIRs suggest a limited duration at high temperatures, and relatively rapid cooling, likely hundreds of °C h À1 , at least initially from peak temperatures. In experiments conducted by Ono et al (2021) on a eucrite composition, cristobalite was partially transformed to quartz at a cooling rate of 0.1°C h À1 . Since we did not observe quartz, we can infer that cooling rates in SIRs were not this slow.…”