Intra- and Intercrystalline Oxygen Isotope Variations in Minerals from Basalts and Peridotites

Abstract

Igneous phenocrysts commonly exhibit zoning in major and trace element composition, reflecting (and potentially constraining) the differentiation and/or mixing histories of their parent melts. To date, little work has been done characterizing zonation of oxygen isotopes in minerals from mafic and ultramafic rocks. We present 259 ion probe (CAMECA ims-1280) measurements of δ^(18)O in 34 natural magmatic and mantle olivines and pyroxenes from five hand samples from diverse igneous environments. We compare δ^(18)O variations with zonation in other elements [especially P; analyzed by electron microprobe analysis (EMPA) and nano-secondary ionization mass spectrometry (nanoSIMS)]. There is generally a close (average within ~0·1–0·2 ‰) agreement between average δ^(18)O values of olivines measured by SIMS (standardized against San Carlos olivine) and independently known values for bulk separates from the same samples measured by laser fluorination. These data demonstrate that current ion microprobe techniques are not only precise but also accurate enough for study of sub-per-mil oxygen isotope variations in silicates (within ~0·2 ‰), provided samples are prepared and analyzed following strict guidelines. All but one of the 34 studied grains are homogeneous in δ^(18)O within a small multiple of analytical precision [estimated ±0·2‰, 1σ for most data; poorer for a subset of measurements made on small (~5 µm) spots]. This population of isotopically homogeneous grains includes some with oscillatory micrometer-scale P banding. The lack of δ^(18)O variations suggests that whatever factors lead to this common mode of trace element zonation have no detectable effect on melt–crystal partitioning of oxygen isotopes. Large (2‰) oxygen isotope variations are observed in one olivine glomerocryst from Mauna Kea, Hawaii. This glomerocryst contains P-rich domains that are either equant or skeletal or feathery in outline, and these P-rich domains are systematically low in δ^(18)O compared with adjacent, later-grown, P-poor olivine. This unusual oxygen isotope zonation pattern might reflect a kinetic fractionation during nucleation and growth of the cores of some olivine phenocrysts. We tested this hypothesis through measurements of δ^(18)O distributions in synthetic olivines grown at a range of rates and exhibiting diverse patterns of P zoning. These synthetic olivines are homogeneous in δ^(18)O, within the limits of our analyses (± 0·3–0·4‰ in this case) and show no connection between P zonation and oxygen isotope heterogeneity. We therefore think it more plausible that unusual O isotope zonation in the Mauna Kea glomerocryst reflects addition of a low-δ^(18)O component to some Hawaiian magmas just before nucleation of olivine. More generally, this study demonstrates the utility of modern SIMS techniques for in situ study of the subtle (~1‰ range) oxygen isotope variations characteristic of common mafic and ultramafic rocks.

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