CHROMITE CHONDRULES AND INCLUSIONS IN ORD INARY CHONDRITES

A petrological study of 270 ordinary chondrites (115 H, 116 L and 39 L (LL) + (LL) revealed chromite chondzules and inclusions occuring mainl s among H (73%) chondrites; they are less common in L (21%) and L(LL) (6%) chondrites. There are no obvious correlations between the occurre nce of Chr-objects and petrological types or shock stages of the meteo rite hosts. These inclusions consist mostly of chromite + plagioclase (or plagioclase-like glass) and accessory ilmenite, high-Ca pyroxene, merrillite and (or) chlorapatite. Based on mineralogy and chemistry th e inclusions can be divided into two types: (1) Those consisting of lo w-Al chromite (Al2O3 < 8.5%), uniform in composition and feldspar of v ariable Na-K-composition. (2) Consisting of hign-Al chromite (Al2O3 > 9%), often variable in composition and feldspar of variable Na-Ca comp osition. Chr-objects are a subst of Al-rich objects that contain more than 10 wt% bulk Al2O3 but they have higher contents of Cr2O3 and FeO and lower contents of SiO2 and MgO than the other Al-rich objects. Mod els for the formation of Chr-objects include the following: (1) meltin g and incomplete evaporation of presolar lumps, this resulted in enric hment of the residue in Cr and Al; (2) formation of Chr-rich objects f rom oxidized precursor materials that originally consisted of Cr-P-Si- rich metal; (3) alteration of Sp-inclusions during parent-body metamor phism.