Oxygen isotope heterogeneity of the mantle deduced from global 18O systematics of basalts from different geotectonic settings

Based upon a compilation and analysis of O-isotope data for Neogene volcanic rocks worldwide, the ¹⁸O variation for 743 basalts (historic lavas, submarine glasses, and lavas with <0.75 wt% H₂O) is +2.9 to +11.4. Mid-ocean-ridge basalt (MORB) has a uniform O-isotope composition with ¹⁸0=+5.7±0.2. Basalts erupted in different tectonic settings have mean ¹⁸O/¹⁶O ratios that are both lower and higher than MORB, with continental basalts enriched in ¹⁸O by ca. 1 over oceanic basalts. The ¹⁸O range for the subset of 88 basalts with Mg# [100·Mg(Mg+Fe²⁺)] 75–68, considered to be unmodified primary mantle partial melts, is +3.6 to +8.7. These features are a clear indication that: (1) the Earth's upper mantle is heterogeneous with respect to its O-isotope composition; (2) that both low-¹⁸O and high-¹⁸O reservoirs have contributed to basalt petrogenesis. Large-ion lithophile element-enriched basalts associated with subduction at convergent plate margins are slightly enriched in ¹⁸O, a characteristic that is considered to be an intrinsic feature of the subduction process. Intraplate oceanic and continental basalts have highly variable ¹⁸O/¹⁶O ratios, with individual localities displaying ¹⁸O ranges in excess of 1.5 to 2. Systematic co-variations between O-, Sr-, Nd-, and Pb-isotope ratios reflect the same principal intramantle end-member isotopic components (DMM, HIMU, EM-I, EM-II) deduced from radiogenic isotope considerations and, therefore, imply that a common process is responsible for the origin of upper mantle stable and radiogenic isotope heterogeneity, namely the recycling of lithospheric material into the mantle.