The formation of phosphoran olivine and stanfieldite from the pyrometamorphic breakdown of apatite in slags from a prehistoric ritual immolation site (Goldbichl, Igls, Tyrol, Austria)

Abstract

In this study we report P-rich olivine and the tric- calciumphosphate(TCP)stanfielditeinpartiallymoltenquartz- phyllites from the ritual immolation site at the Goldbichl, near Innsbruck in the Tyrol, Austria. During partial melting, foamy patches of dark glassy material formed at the surface of the rocks and also as layers within the rocks. The pyrometamorphic rocks contain mostly the mineral assemblage olivine + ortho- pyroxene + plagioclase + spinel + glass. During the investiga- tion of slag samples from this prehistoric ritual immolation site, extremely P-rich, apatite-bearing micro-domains were found. In these domains phosphoran olivine was found whose P con- tents are approaching the maximum P contents in olivine according to the experimental investigations of Boesenberg and Hewins (Geochim Cosmochim Acta 74:1923-1941, 2010). The textures within these domains indicate strongly disequilibrium conditions. The phosphoran olivines formed due to reactions involving apatite and the mineral assemblage of the quartzphyllites, and coexist with plagioclase and a tri- calcium phosphate phase (TCP) showing stanfieldite Ca4(Mg, Fe 2+ ,M n 2+ )5(PO4)6 composition. In terms of its chemical composition, olivine shows a wide range in composition with P ranging from 0.3 to 0.54 a.p.f.u, which corresponds to max- imal 23 wt.% P2O5. These are the highest P-contents in olivine reported from rocks so far. The incorporation of P correlates with decreasing Si contents according to the charge balancing scheme 2P 5þ þ M1;2 ¼ 2Si 4þ þ Mg;Fe ðÞ 2þ M1;2. Therefore P can only be incorporated in combination with a vacancy on the M1,2 position. Micro-Raman spectroscopy of phosphoran olivines indicates that these olivines can easily be identified with this method due to the strong signals of the SiO4 and PO4 vibrations. The external vibrations of the M1,2 sites at low wave-numbers are more complex than for P-free olivine. This might be due to the effect of P 5+ on the M1 2+ and M2 2+ positions and the formation of vacancies on these sites. Since micro- Raman investigations of the TCP phase yielded no conclusive match with a known Raman spectrum of a phosphate mineral so far, therefore it is most likely that the TCP phase is stanfieldite, whose Raman spectrum has not been obtained yet. Schematical Schreinemakers analysis in the system CaO-Al2O3-FeO-SiO2- P2O5-H2O shows that P-rich olivine (fayalite-sarcopside solid solution) can form from mineral reactions involving chlorite, apatite and quartz and show that the occurrence of P-rich Fe- olivines spans a large T-range but is restricted to domains with high aSiO2. The mineral assemblage in the P-rich micro- domains shows that the formation of phosphoran olivine is not only restricted to the interaction between bone material and rocks in slags from ritual immolation sites as suggested by Tropper et al. (Eur J Mineral 16:631-640, 2004 )f rom the immolation site in Oetz but can form locally due to the pyro- metamorphic breakdown of a P-rich accessory precursor phase such as apatite.