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
© 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
Stable (C and O) isotope data from carbonates are one of the most important methods used to infer genetic processes in most 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 48 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‰, 52 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
We are grateful for the editorial handling of Michael Böttcher and the comments of Alan Cooper, Roberto Ventura Santos and an anonymous reviewer on an earlier manuscript version. This work was funded by a NERC BGS studentship to SBF (NEE/J50318/1; S208), through a NIGL Steering Committee grant (IP-1387-1113) and by the NERC SoS RARE consortium (NE/M011429/1). Hilary Sloane carried out carbonate analyses. Mkango Resources Ltd. provided access and field assistance at the Songwe carbonatite. K. Moore and P. Scott provided material from Jacupiranga and Fen. AGG publishes with permission from the Executive Director of the BGS.
This is the final version of the article. Available from the publisher via the DOI in this record.
Vol 440, pp. 275–287