Oxygen isotope systematics of ordinary chondrite chondrules: insights into the inner solar system planetary reservoir

Chondrules are fundamental objects from which important insights in the conditions of the solar protoplanetary disk can be inferred. In this regard, oxygen isotope composition of chondrules have played a key role tracking the physicochemical environment under which their formation occurred. It has been proposed that chondrite asteroids accreted specific chondrule groups or ‘chondrule populations’ in terms of oxygen isotopes but this hypothesis has never been statistically assessed. Systematic in situ measurements of oxygen isotope composition of chondrule olivine from ordinary chondrites (H, L, and LL groups) by SHRIMP-SI and a robust statistical evaluation allowed us to conclude for the first time that the different ordinary chondrite parent bodies sampled the same main population of chondrules in their accretion location. This population is characterised by a mean of Δ17O = 0.74 ± 0.10‰ (σ95%) with a variability of 0.53‰ (σ95%). Ordinary chondrite chondrules were then formed in the same gaseous oxygen isotope reservoir or region of the protoplanetary disk. This population is formed by chondrules with different chemical composition (Type I and Type II) indicating that their formation region was chemically differentiated and form a chemical continuum rather than two separated classes. Our results together with literature data lead us to propose an oxygen isotope gradient of the protoplanetary disk ambient gas under which these and other astrophysical objects (e.g. refractory inclusions) were formed, 16O-richer closer to the Sun and 16O-poorer at longer radial distances. Ultimately, we suggest a change in the paradigm related to the oxygen isotope reservoirs of the solar system, particularly the so-called planetary reservoir. It is proposed that the gaseous oxygen isotope planetary reservoir is a discrete reservoir enriched in SiOgas molecules and other rock-forming elements present since the earliest stages of the solar system evolution possibly reflecting the average oxygen isotope composition of the primordial dust of the solar nebula. The mean composition of the planetary reservoir is δ18O = 4.64 ± 0.15‰ (σ95%) and δ17O = 3.03 ± 0.10‰ (σ95%), with variabilities of 0.97‰ and 0.52‰ (σ95%), respectively.