Mineralogy, Petrography, and Oxygen Isotopic Compositions of Ultrarefractory Inclusions from Carbonaceous Chondrites

Abstract

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 (Sc_4Zr_3O_(12)), and an unnamed ((Ti,Mg,Sc,Al)_3O_5) mineral, davisite (CaScAlSiO_6), eringaite (Ca_3(Sc,Y,Ti)_2Si_3O_(12)), kangite ((Sc,Ti,Al,Zr,Mg,Ca,□)_2O_3), lakargiite (CaZrO_3), warkite (Ca_2Sc_6Al_6O_(20)), panguite ((Ti,Al,Sc,Mg,Zr,Ca)_(1.8)O_3), Y-rich perovskite ((Ca,Y)TiO_3), tazheranite ((Zr,Ti,Ca)O_(2―x)), thortveitite (Sc_2Si_2O_7), zirconolite (orthorhombic CaZrTi_2O_7), and zirkelite (cubic CaZrTi_2O_7). 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 ^(16)O-rich (Δ^(17)O ˜ ―23‰), except for one CH UR CAI, which is uniformly ^(16)O-depleted (Δ^(17)O ˜ ―5‰). Two UR CAIs in Murchison have heterogeneous Δ17O. These include: an intergrowth of corundum (˜ ‒24‰) and (Ti,Mg,Sc,Al)_3O_5 (˜ 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 (Δ^(17)O ranges from ˜ ―23‰ to ˜ ―2‰), with Zr- and Sc-rich oxides and silicates, melilite and perovskite being ^(16)O-depleted to various degrees relative to uniformly ^(16)O-rich (Δ^(17)O ˜ ―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 ^(16)O-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 ^(16)O-depleted gaseous reservoir providing an evidence for large variations in Δ^(17)O of the nebular gas in the CH CAIs-forming region. Subsequently some UR CAIs experienced oxygen isotopic exchange during melting in ^(16)O-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)_3O_5 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 ^(16)O-depleted reservoirs occurred.

Additional details