Calcium phosphates associated with chondrules in the CR chondrite Queen Alexandra Range (QUE) 99177: Evidence for solar nebular and parent body processes

Abstract

Phosphorus-bearing minerals in carbonaceous chondrites record early aqueous alteration effects in the parent asteroid and potentially provide clues on early solar nebular processes. Despite their importance, only a few studies exist dedicated to investigating P-bearing minerals in primitive carbonaceous chondrites and thus, their origins are not well constrained. Work on Ca phosphates around the edges of type IIA chondrules in primitive CR and CM chondrites has shown that Ca phosphates are generally associated with aqueous alteration in the parent body. The present study examines two different Ca phosphate occurrences in one of the least altered CR chondrites known, QUE 99177, by SEM, EPMA, and FIB-TEM techniques to better constrain their origins. The first type consists of elongate, submicron-sized rods of merrillite that occur in regions of mesostasis at the edge of type IIA chondrules adjacent to the surrounding matrix. The second type occurs as nanometer-sized grains around some type IA chondrules that are surrounded by smooth rims. These smooth rims are a type of rim that consists of an amorphous, Fe-rich, hydrous silicate phase that results from low-temperature aqueous alteration of silica in Silica-rich Igneous Rims (SIRs) at the earliest stages of parent body alteration. The Ca phosphates are located within discrete regions at the interface between smooth rims and adjacent matrix, ranging from whitlockite to apatite compositions. We argue that the first type of Ca phosphate has a solar nebular origin, formed by quenching of Ca- and P-bearing melts in chondrules at the final stages of crystallization, whereas the second type has a parent body origin, formed by oxidation of Fe,Ni metal grains in SIRs surrounding chondrules. Therefore, our new data and a reappraisal of previous data demonstrate, for the first time, that Ca phosphates formed by both primary (solar nebular) and secondary (parent body) processes. These results also provide additional insights into the formation conditions of type IIA chondrules in the protoplanetary disk and constrain the earliest stages of aqueous alteration in the CR chondrite parent body.