Pyroxene thermobarometry in LL‐group chondrites and implications for parent body metamorphism

Abstract

Abstract-Geothermometry based on the compositions of clinopyroxenes in type 6 and 7 LL chondrites gives coherent results, but the estimated temperatures from coexisting orthopyroxenes are consistently lower than for clinopyroxenes. Orthopyroxene thermometry is suspect because of compositional effects of polymorphic inversions and/or unknown kinetic factors. Lack of clinopyroxene equilibration precludes accurate estimation of peak metamorphic temperatures for type 4 and 5 chondrites. There is no apparent correlation between Al content (a pressure-dependent variable) and equilibration temperature in chondritic pyroxenes. This finding, which is at variance with a previously published conclusion that temperature and pressure are correlated in metamorphosed chondrites, may have important implications for asteroid thermal models.

Sodic plagioclase thermometry of type 6 ordinary chondrites: Implications for the thermal histories of parent bodies

“…The maximum metamorphic temperatures estimated are 725-742 "C for the H6 chondrites, 808-820 "C for the L6 chondrites, and 800 "C for the LL6 chondrite. These temperatures are lower than those based on Ca contents of clinopyroxenes (Dodd, 1981;McSween et al, 1988) but are consistent with those based on Ca contents of orthopyroxenes (McSween and Patchen, 1989;Langenhorst et al, 1995;Jones, 1997). The K content of sodic plagioclase correlates with the temperature obtained from the structural state.…”

Mineralogy of four Itokawa particles collected from the first touchdown site

“…The Enstatite (En) and Wo contents (mol%) in the highCa pyroxene in RB-QD04-0008 are 46.1 and 41.9, respectively, which are outside the range of the En and Wo contents in high-Ca pyroxene in the highly equilibrated grains investigated in the initial analysis (Figure 5). The mol% of Wo is similar to those of high-Ca pyroxene in LL6 chondrites (McSween and Patchen 1989). By considering the above discussion, RB-QD04-0008 may have been formed by recrystallization and it contains mineral fragments that experienced thermal metamorphism as high as most LL6 chondrites experienced.…”

Mineralogy of some evolved LL chondrites with reference to asteroid materials and solar system evolution

“…Geothermometry based on the compositions of pyroxenes of the LL-group chondrites has been discussed by McSween and Patchen (1989). The sampled materials of Itokawa were thermally metamorphosed, as is the case for most ordinary chondrites.…”

“…It is important to recognize that high-temperature events took place in a region of the LL chondrite parent body, from where Itokawa and parent asteroids of the Antarctic meteorites were ejected. It is to be remembered that chondritic materials keep records of primitive solar system (McSween and Patchen 1989), but some parts are partly melted and heavily metamorphosed.…”