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

TL;DR

This study examines sulfur and rare earth element zoning in apatite crystals from the Colli Albani volcanic system, revealing core-rim compositional variations linked to magmatic processes and mineral phase absence.

Contribution

It provides detailed geochemical analysis of apatite zoning in alkaline rocks, highlighting the influence of mineral assemblage on element distribution in magmatic systems.

Findings

Apatite crystals are SrO-rich fluorapatite with up to 4.6 wt% SrO.

Core-rim zoning shows rims enriched in Si, S, and REE, cores in Ca and P.

REE contents in apatite account for over 95% of total REE budget.

Abstract

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%.