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

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, (δ¹⁸O_VSMOW = 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 δ⁵⁷Fe_IRMM-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 δ⁵⁷Fe_IRMM-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.