OXYGEN-ISOTOPE HETEROGENEITY OF THE MANTLE DEDUCED FROM GLOBAL O-18 SYSTEMATICS OF BASALTS FROM DIFFERENT GEOTECTONIC SETTINGS

Based upon a compilation and analysis of O-isotope data for Neogene volcanic rocks worldwide, the delta(18)O variation for 743 basalts (historic lavas, submarine glasses, and lavas with < 0.75 wt% H2O) is + 2.9 to + 11.4 parts per thousand Mid-ocean-ridge basalt (MORB) has a uniform O-isotope composition with delta(18)O = + 5.7 +/- 0.2 parts per thousand. Basalts erupted in different tectonic settings have mean O-18/O-16 ratios that are both lower and higher than MORB, with continental basalts enriched in O-18 by ca. 1 parts per thousand over oceanic basalts. The delta(18)O 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 parts per thousand. 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-O-18 and high-O-18 reservoirs have contributed to basalt petrogenesis. Large-ion lithophile element-enriched basalts associated with subduction at convergent plate margins are slightly enriched in O-18, a characteristic that is considered to be an intrinsic feature of the subduction process. Intraplate oceanic and continental basalts have highly variable O-18/O-16 ratios, with individual localities displaying delta(18)O ranges in excess of 1.5 to 2 parts per thousand. 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.