Sulfur and REE zoning in apatite: the example of the Colli Albani magmatic system

We investigate the distribution of major and trace elements in apatite crystals hosted in granular alkaline rocks composed mainly of leucite and clinopyroxene, representative of the hypabyssal crystallization of a magma body in the Quaternary ultra-potassic Colli Albani Volcanic District (CAVD), which was emplaced into thick limestone units along the Tyrrhenian margin of Italy. Results show that the analyzed crystals are the SrO-richest (up to 4.6 wt%) fluorapatite (F = 2.6–3.7 wt%) of the Italian alkaline rocks. The strontium enrichment is caused by the lack of other Sr-compatible mineral phases, such as plagioclase, alkali feldspar and melilite, in these leucite- and clinopyroxene-bearing rocks. The studied samples show core–rim zoning with rims enriched in Si, S, and REE whereas the cores are enriched in Ca and P. The LREE-oxides contents of apatite, reaching 4.2 wt%, represent more than 95% of the total REE budget; SiO₂ contents range from 1.3 to 3.6 wt%, and SO₃ concentrations between 0.6 and 1.4 wt%. We support the idea that REE and sulfur enrichment are a response to the crystallization caused by the pressure drop in the magmatic system during the eruption and follow the substitution mechanisms Si⁴⁺ + REE³⁺ = P⁵⁺ + Ca²⁺ and S⁶⁺ + Si⁴⁺ = 2P⁵⁺. The results also suggest the presence of the substitution Th⁴⁺ + 2Si⁴⁺ = Ca²⁺ + 2P⁵⁺.