Assessment of O and Fe isotope heterogeneity in garnet from Kakanui (New Zealand) and Erongo (Namibia)

Abstract

Oxygen and iron isotope variations have been investigated in three compositionally distinct garnet samples to assess natural variations and search for suitable reference material. We report in situ major, trace element and O isotope analyses for mantle-derived garnet xenocrysts from Kakanui, New Zealand, as well as magmatic and hydrothermal garnets (skarn) from two different localities in Erongo, Namibia. The in situ analyses are complemented by bulk mineral separate Fe isotope analyses for all samples and CO2 laser fluorination oxygen isotope analysis for Kakanui garnet. Mantle-derived pyrope-rich garnet megacrysts from Kakanui are chemically homogeneous in major and trace elements, and in O isotopes, (δ18OVSMOW = 5.67 ± 0.02‰). Magmatic garnet from Erongo, Namibia, is rich in Mn and Fe2+ and very poor in Ca showing minor variations along the almandine–spessartine join [(Fe,Mn)3Al2Si3O12]. Although rare earth elements vary over one order of magnitude, no resolvable O isotope zoning is observed (δ18O = 9.3 ± 0.3‰, 1σ). Hydrothermal garnet from Namibia is rich in Ca and Fe3+ and shows strong zonation along the andradite–grossular join [Ca3(Fe3+,Al)2Si3O12] with a considerable spread in trace-element contents, accompanied by a limited, but resolvable, spread in O isotopes values between cores (8.3 ± 0.3‰, 1σ) and rims (7.4 ± 0.3‰, 1σ). Iron isotopes (expressed as δ57FeIRMM-014) within bulk garnet separates are heterogeneous in both crustal garnet from Erongo with a large spread ranging from −0.15 to +0.30‰ in igneous garnet and from +0.4 to +1.1‰ in hydrothermal garnet. Igneous garnet from Kakanui are homogeneous with an average δ57FeIRMM-014 of +0.09 ± 0.01, 1σ. The Fe3+-dominated andradite shows very heavy Fe isotopes, suggesting a link between preferential ferric iron incorporation into garnet and Fe isotope signatures. Combined O and Fe isotope analyses in garnet can provide potentially important insights into the nature of parental medium from which the garnet forms (based on O isotopes) and associated petrogenetic processes (e.g., redox conditions based on Fe isotopes), though more systematic studies are required to further assess these proxies in natural systems. Finally, we propose that Kakanui garnet might represent a suitable reference material for both, O and Fe isotope analyses.