Ureilite precursor formation from an isotopically and chemically heterogeneous, isolated protoplanetary disk reservoir

• Ureilites record ε 50 Ti, ε 54 Cr and O isotope heterogeneities in their accretion region. • Ureilite precursors formed by mixing of chemically and isotopically constrained end-member components. • Ureilite precursors accreted in an isolated NC sub-reservoir and represent an end-member in the ε 50 Ti-ε 54 Cr space. • Correlations between Δ 17 O with Mg# in olivine cores suggest coupling of photodissociation and self-shielding with photo-evaporation of gas in the protoplanetary disk. Isotopic and chemical compositions of meteorites are fundamental for reconstructing the spatial evolution of the protoplanetary disk and for assessing the physical processes that took place in the disk during its early evolution. Ureilites, a group of achondrites, are of particular interest in this regard because they exhibit chemical and isotopic heterogeneity for some elements inherited from their precursor materials. Oxygen, Ti and Cr isotope compositions of whole-rock fractions of main group ureilites provide constraints on their accretion region. The O-isotope data from main group ureilites correlate in δ 17 O-δ 18 O space and define the Ureilite Line with a slope of 1.07 ± 0.05. In the δ 17 O-δ 18 O diagram, the Ureilite Line is shifted to the right of other mass-independent O-isotope fractionation lines (e.g., Young and Russell Line and Primitive Chondrule Mineral Line). This peculiarity of ureilites is further recorded by their ε 50 Ti (= -1.95 ± 0.30 (2SD)) and ε 54 Cr (= -0.87 ± 0.18 (2SD)) compositions, which define them as an isolated end-member of the non-carbonaceous chondrite reservoir in ε 50 Ti-ε 54 Cr space. These distinct signatures may have been established by the formation of gaps at specific locations in the protoplanetary disk due to dust trapping near pressure maxima or due to intense photo-evaporative mass loss, preventing significant mixing with other Solar System materials. The variation of ε 50 Ti and ε 54 Cr among different ureilites suggests that their accreted material was characteristic of the ureilite-forming reservoir and reflects isotope variability within this sub-reservoir. The material with different Δ 17 O TFL and redox states within this sub-reservoir was mixed by heterogeneous accretion, as indicated by two discrete trends (Δ 17 O TFL vs. Mg# in olivine cores). Ureilite precursors were possibly formed by coupled processes involving photodissociation and self-shielding of CO molecules, and photo-evaporation of gas from the protoplanetary disk surface.