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

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 delta O-18 in 34 natural magmatic and mantle olivines and pyroxenes from five hand samples from diverse igneous environments. We compare delta O-18 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 similar to 0 center dot 1-0 center dot 2 parts per thousand) agreement between average delta O-18 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 similar to 0 center dot 2 parts per thousand), provided samples are prepared and analyzed following strict guidelines. All but one of the 34 studied grains are homogeneous in delta O-18 within a small multiple of analytical precision [estimated +/- 0 center dot 2 parts per thousand, 1 Sigma for most data; poorer for a subset of measurements made on small (similar to 5 mu m) spots]. This population of isotopically homogeneous grains includes some with oscillatory micrometer-scale P banding. The lack of delta O-18 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 parts per thousand) 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 delta O-18 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 delta O-18 distributions in synthetic olivines grown at a range of rates and exhibiting diverse patterns of P zoning. These synthetic olivines are homogeneous in delta O-18, within the limits of our analyses (+/- 0 center dot 3-0 center dot 4 parts per thousand 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-delta O-18 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 (similar to 1 parts per thousand range) oxygen isotope variations characteristic of common mafic and ultramafic rocks.