Microstructural analysis of phosphorus (P)-bearing assemblages in type 3 chondrites: Implications for P condensation and processing in the early solar nebula

As the limiting element in the development of living systems, it is crucial to understand the history of phosphorus (P), from its stellar origins to its arrival on planetary surfaces. A key component in this cycle is understanding the forms of P delivered to the presolar nebula and their subsequent evolution on planetary bodies, including meteorites. Here, we report on the P distribution in the Bishunpur (LL3.15), Queen Alexandra Range (QUE) 97008 (L3.05), and Allan Hills (ALHA) 77307 (CO3.0) chondrites to determine its origins and secondary processing in the solar protoplanetary disk and on meteorite parent bodies using a coordinated analytical approach. In support of the microstructural characterization, we used density functional theory (DFT) to calculate the Gibbs free energy of the Fe₃P – Ni₃P binary under non-ideal mixing conditions in its entire range of composition and temperature space and performed equilibrium condensation modeling. We identified 106 P-bearing regions in these petrologic type-3 chondrites and find that the major P-bearing minerals are schreibersite ((Fe, Ni)₃P) and merrillite (Ca₉NaMg(PO₄)₇). Bishunpur predominately contains merrillite, which occurs in rims on chondrules and as hopper crystals. QUE 97008 primarily contains merrillite in association with metal and sulfides. Microstructural evaluation of merrillite in Bishunpur suggests igneous origins within the chondrule-forming region, whereas merrillite in QUE 97008 formed via condensation. In comparison, the dominant P-bearing phase in ALHA 77307 is P-bearing metal, including two Ni-rich schreibersite grains that are composed of 45 and 52.5 at.% Ni, far higher than predicted by equilibrium condensation. The equilibrium thermodynamic model, including our newly described non-ideal schreibersite solid solution, predicts the formation of a miscibility gap where (Fe_0.63, Ni_0.37)₃P and Ni₃P form via nebular condensation. We therefore suggest that Ni-rich schreibersite formed through non-equilibrium condensation.