Tracing the fluid source of heavy REE mineralisation in carbonatites using a novel method of oxygen-isotope analysis in apatite: The example of Songwe Hill, Malawi

Sam Broom-Fendley,Timothy Heaton,Frances Wall,Gus Gunn

doi:10.1016/j.chemgeo.2016.07.023

Sam Broom-Fendley, Timothy Heaton + Show 2 more

Open Access

https://doi.org/10.1016/j.chemgeo.2016.07.023

Copy DOI

Abstract

Stable (C and O) isotope data from carbonates are one of the most important methods used to infer genetic processes in carbonatites. However despite their ubiquitous use in geological studies, it is suspected that carbonates are susceptible to dissolution-reprecipitation and isotopic resetting, especially in shallow intrusions, and may not be the best records of either igneous or hydrothermal processes. Apatite, however, should be much less susceptible to these resetting problems but has not been used for O isotope analysis. In this contribution, a novel bulk-carbonatite method for the analysis of O isotopes in the apatite PO4 site demonstrates a more robust record of stable isotope values. Analyses of apatite from five carbonatites with magmatic textures establishes a preliminary Primary Igneous Apatite (PIA) field of δ18O=+2.5 to +6.0‰ (VSMOW), comparable to Primary Igneous Carbonatite (PIC) compositions from carbonates.Carbonate and apatite stable isotope data are compared in 10 carbonatite samples from Songwe Hill, Malawi. Apatite is heavy rare earth element (HREE) enriched at Songwe and, therefore, oxygen isotope analyses of this mineral are ideal for understanding HREE-related mineralisation in carbonatites. Carbonate C and O isotope ratios show a general trend, from early to late in the evolution, towards higher δ18O values (+7.8 to +26.7‰, VSMOW), with a slight increase in δ13C (−4.6 to −0.1‰, VPDB). Oxygen isotope ratios from apatite show a contrary trend, decreasing from a PIA field towards more negative values (+2.5 to −0.7‰, VSMOW). The contrasting results are interpreted as the product of the different minerals recording fluid interaction at different temperatures and compositions. Modelling indicates the possibility of both a CO2 rich fluid and mixing between meteoric and deuteric waters. A model is proposed where brecciation leads to depressurisation and rapid apatite precipitation. Subsequently, a convection cell develops from a carbonatite, interacting with surrounding meteoric water. REE are likely to be transported in this convection cell and precipitate owing to decreasing salinity and/or temperature.

Highlights

Stable C and O isotope ratios are powerful tools for investigating the evolution of carbonatites, allowing interpretation of either crystallisation temperature (e.g. Haynes et al, 2003; Demény et al, 2004a) or fluidAbbreviations: PIC, Primary Igneous Carbonatite; PIA, Primary Igneous Apatite; δ18O in the apatite PO4 site (δ18OPO4), δ18O value of oxygen from the apatite PO4 site; TREO, total rare earth oxides; CL, cathodoluminescence; HREE, heavy rare earth element; ML-ON, silver phosphate preparation method after McLaughlin et al (2004) and O'Neil et al (1994)
To understand more about the, potentially unique, formation of HREE-enriched apatite at Songwe, samples were selected to test: 1. Are there variations in isotope ratios at Songwe, and can these be related to Rayleigh fractionation, fluid alteration or sediment assimilation?
For three rock samples there was no significant difference between the δ18O values of silver phosphate prepared from apatite separates compared with whole rock powder; and only a 0.6‰ difference for a fourth sample

Summary

Introduction

Stable C and O isotope ratios are powerful tools for investigating the evolution of carbonatites, allowing interpretation of either crystallisation temperature (e.g. Haynes et al, 2003; Demény et al, 2004a) or fluid. The processes affecting the isotope composition include: 1) Rayleigh fractionation, where calcite crystallises in equilibrium with H2O and CO2 from a fluid/magma, leading to increasing δ13C and δ18O in subsequently crystallised carbonates (Deines, 1970, 1989; Ray and Ramesh, 2000). 3) Degassing, attributed to carbonatites which have lower δ13C than PIC through preferential partitioning of heavy carbon into the gas phase (Suwa et al, 1975; Demény et al, 1994). Modelling indicates that this can be accompanied by a corresponding decrease in δ18O values which becomes more extreme as the temperature of degassing decreases from 500 to 100 °C (Santos and Clayton, 1995)

Methods

Results

Discussion

Conclusion