Microstructures of enstatite in fine-grained CAIs from CV3 chondrites: Implications for mechanisms and conditions of formation

Enstatite is a ubiquitous phase in chondritic meteorites, interplanetary dust particles, and cometary samples. In equilibrium condensation models, enstatite predicted to condense via reaction between pre-condensed forsterite gaseous SiO. However, previous studies have shown that some enstatites chondrite matrices AOAs do not genetic relationship with forsterite, arguing against formation by the forsterite-gas reaction. Here we report occurrence of unique, fine-grained, spinel-rich inclusion (FGI) Ef1014-01 Efremovka CV3 chondrite. this FGI present as an outer layer on spinel-anorthite-diopside nodules separates from amoeboid olivine aggregate (AOA) -like material. shows elevated CaO Al 2 O 3 contents (up few weight percent). Four FIB sections were extracted investigate microstructures its other phases using TEM techniques. The observations show dominantly low-temperature clinoenstatite (LCLEN), which displays abundant twinning, sometimes associated thin orthoenstatite (OREN) lamellae. Clinoenstatite grains commonly crystallographic orientation adjacent diopside, but exhibit any replacement AOA-like material surrounding FGI. Investigations several fine-grained CAIs Leoville chondrites more common these inclusions than previously thought typically forms discontinuous layers or islands diopside layers. Based SEM observations, suggest LCLEN-OREN intergrowths formed transformation protoenstatite (PEN) precursor, may be product direct reheating solar nebula. suggests epitaxial growth occurred, lowering activation energy for nucleation facilitating direction gas phase, rather forsteritic olivine. are indicative extremely rapid cooling rate (∼10 4 K/h) within range chondrule rates. Such imply rates FGIs indeed much higher types refractory inclusions. Alternatively, reflect primary FGIs, result after short-lived secondary event A fractionated lower Mg/Si ratio value required enstatite. could produced isolation repeated evaporation-recondensation processes. presence both enstatite-bearing enstatite-free at least two reservoirs different ratios CAI-forming regions.