Refractory inclusions and aluminum-rich chondrules in Sayh al Uhaymir 290 CH chondrite: Petrography and mineralogy

Abstract

Here we report the petrography, mineralogy, and bulk compositions of Ca,Al-rich inclusions(CAIs), amoeboid olivine aggregate (AOA), and Al-rich chondrules (ARCs) in Sayh al Uhaymir (SaU) 290 CH chondrite. Eighty-two CAIs (0.1% of the section surface area) were found. They are hibonite-rich (9%), grossite-rich (18%), melilite ±  spinel-rich (48%), fassaite ± spinel-rich (15%), and fassaite-anorthite-rich (10%) refractory inclusions. Most CAIs are rounded in shape and small in size (average = 40 μm). They are more refractory than those of other groups of chondrites. CAIs in SaU 290 might have experienced higher peak heating temperatures, which could be due to the formation region closer to the center of protoplanetary disk or have formed earlier than those of other groups of chondrites. In SaU 290, refractory inclusions with a layered texture could have formed by gas-solid condensation from the solar nebula and those with an igneous texture could have crystallized from melt droplets or experienced subsequent melting of pre-existing condensates from the solar nebula. One refractory inclusion represents an evaporation product of pre-existing refractory solid on the basis of its layered texture and melting temperature of constituting minerals. Only one AOA is observed (75 μm across). It consists of olivine, Al-diopside, anorthite, and minor spinel with a layered texture. CAIs and AOA show no significant low-temperature aqueous alteration. ARCs in SaU 290 consist of diopside, forsterite, anorthite, Al-enstatite, spinel, and mesostasis or glass. They can be divided into diopsiderich, Al-enstatite-rich, glass-rich, and anorthite-rich chondrules. Bulk compositions of most ARCs are consistent with a mixture origin of CAIs and ferromagnesian chondrules. Anorthite and Al-enstatite do not coexist in a given ARC, implying a kinetic effect on their formation.