MINERALOGY, PETROGRAPHY, AND OXYGEN ISOTOPIC COMPOSITIONS OF ULTRAREFRACTORY INCLUSIONS FROM CARBONACEOUS CHONDRITES
A.N. Krot, C. Ma, K. Nagashima, A.M. Davis, J.R. Beckett, S.B. Simon, M. Komatsu, T.J. Fagan, F. Brenker, M.A. Ivanova, A.Bischoff
Geochemistry
Available online 15 August 2019
“We report on the mineralogy, petrography, and in situ oxygen isotopic composition of twenty-five ultrarefractory calcium-aluminum-rich inclusions (UR CAIs) in CM2, CR2, CH3.0, CV3.1―3.6, CO3.0―3.6, MAC 88107 (CO3.1-like), and Acfer 094 (C3.0 ungrouped) carbonaceous chondrites. The UR CAIs studied are typically small, < 100 µm in size, and contain, sometimes dominated by, Zr-, Sc-, and Y-rich minerals, including allendeite (Sc4Zr3O12), and an unnamed ((Ti,Mg,Sc,Al)3O5) mineral, davisite (CaScAlSiO6), eringaite (Ca3(Sc,Y,Ti)2Si3O12), kangite ((Sc,Ti,Al,Zr,Mg,Ca,□)2O3), lakargiite (CaZrO3), warkite (Ca2Sc6Al6O20), panguite ((Ti,Al,Sc,Mg,Zr,Ca)1.8O3), Y-rich perovskite ((Ca,Y)TiO3), tazheranite ((Zr,Ti,Ca)O2―x), thortveitite (Sc2Si2O7), zirconolite (orthorhombic CaZrTi2O7), and zirkelite (cubic CaZrTi2O7). These minerals are often associated with 50―200 nm-sized nuggets of platinum group elements. The UR CAIs occur as: (i) individual irregularly-shaped, nodular-like inclusions; (ii) constituents of unmelted refractory inclusions – amoeboid olivine aggregates (AOAs) and Fluffy Type A CAIs; (iii) relict inclusions in coarse-grained igneous CAIs (forsterite-bearing Type Bs and compact Type As); and (iv) relict inclusions in chondrules. Most UR CAIs, except for relict inclusions, are surrounded by single or multilayered Wark-Lovering rims composed of Sc-rich clinopyroxene, ±eringaite, Al-diopside, and ±forsterite. Most of UR CAIs in carbonaceous chondrites of petrologic types 2―3.0 are uniformly 16O-rich (Δ17O ˜ ―23‰), except for one CH UR CAI, which is uniformly 16O-depleted (Δ 17O ˜ ―5‰). Two UR CAIs in Murchison have heterogeneous Δ17O. These include: an intergrowth of corundum (˜ ‒24‰) and (Ti,Mg,Sc,Al)3O5 (˜ 0‰), and a thortveitite-bearing CAI (˜ ‒20 to ˜ ‒5‰); the latter apparently experienced incomplete melting during chondrule formation. In contrast, most UR CAIs in metamorphosed chondrites are isotopically heterogeneous (Δ17O ranges from ˜ ―23‰ to ˜ ―2‰), with Zr- and Sc-rich oxides and silicates, melilite and perovskite being 16O-depleted to various degrees relative to uniformly 16O-rich (Δ17O ˜ ―23‰) hibonite, spinel, Al-diopside, and forsterite. We conclude that UR CAIs formed by evaporation/condensation, aggregation and, in some cases, melting processes in a 16O-rich gas of approximately solar composition in the CAI-forming region(s), most likely near the protoSun, and were subsequently dispersed throughout the protoplanetary disk. One of the CH UR CAIs formed in an 16O-depleted gaseous reservoir providing an evidence for large variations in Δ17O of the nebular gas in the CH CAIs-forming region. Subsequently some UR CAIs experienced oxygen isotopic exchange during melting in 16O-depleted regions of the disk, most likely during the epoch of chondrule formation. In addition, UR CAIs in metamorphosed CO and CV chondrites, and, possibly, the corundum-(Ti,Mg,Sc,Al)3O5 intergrowth in Murchison experienced O-isotope exchange with aqueous fluids on the CO, CV, and CM chondrite parent asteroids. Thus, both nebular and planetary exchange with 16O-depleted reservoirs occurred.”
