Pyrometamorphic formation of phosphorus-rich olivines in partially molten metapelitic gneisses from a prehistoric sacrificial burning site (Ötz Valley, Tyrol, Austria)

Phosphorus-rich olivine with up to 8.8 wt.% P₂O₅ was observed in partially molten biotite-plagioclase gneisses recovered from a presumably La-Tène (450 - 15 B.C.) age sacrificial place in the Ötz Valley, Northen Tyrol, Austria, where immolation of ritual offerings took place. The formation of this unusually P-rich olivine is thought to be due to incongruent melting of biotite through a reaction such as biotite + quartz = olivine + Ti-magnetite + K-rich melt. During partial melting, foamy patches of dark glassy material formed at the surface of the rocks and also as layers within the rocks. Phosphorus was supplied as apatite from animal bones which also led to the formation of whitlockite in the foamy, glassy surface layer. In this layer, microdomains with the assemblages olivine + plagioclase + whitlockite + glass and clinopyroxene + plagioclase + whitlockite + glass were found. Away from the contact area in the residual rock, the assemblage olivine + Ti-bearing magnetite + clinopyroxene + plagioclase + glass occurs. Phosphorus-rich olivine associated with whitlockite shows a wide range in composition with 0 - 8.8 wt.% P₂O₅ and X_Mg ranging from 0.48 to 0.74. The heterogeneity in olivine compositions is thought to be due to disequilibrium and/or local variations in phosphorus concentration. The systematics of Mg, Fe, Si, and P concentrations in olivine indicates that phosphorus is incorporated into olivine via the coupled substitution 2P + (□)M_1,2 ↔ 2Si + (Mg, Fe)M_1,2 and that phosphorus contamination through whitlockite or glass inclusions can be excluded. A slight negative correlation between the Fe contents and the P contents was also observed. The high chemical variability of the newly formed phases (melt and crystals), dendritic textures and the coexistence of forsterite-rich olivine with a SiO₂-rich melt (>62 wt.% SiO₂) strongly indicate rapid, non-equilibrium growth of the phosphorus-rich olivines in agreement with previous natural observations from meteorites and terrestrial occurrences. These data show that mineralogical constraints such as phosphorus-rich olivine and whitlockite can provide evidence for the burning of bone material and thus be very helpful in the identification of prehistoric burning sites in cases where clear archeological evidence is lacking.