¹⁸O/¹⁶O Evidence for Fluid-Rock Interaction in the Upper Mantle: Data from Ultramafic Nodules and K-Rich Volcanic Rocks in Italy

Abstract

Based mainly on the systematics revealed in δ¹⁸O-olivine vs. δ¹⁸O-pyroxene diagrams, ¹⁸O/¹⁶O data on coexisting minerals from peridotite nodules in alkali basalts and kimberlites are interpreted as non-equilibrium phenomena. The mantle nodules exhibit data arrays that cut steeply across the Δ¹⁸O = zero line on such δ-δ diagrams. These arrays resemble the non-equilibrium quartz-feldspar and feldspar-pyroxene δ¹⁸O arrays that we now know are diagnostic of hydrothermally altered plutonic igneous rocks. Thus, the peridotites appear to have been open systems that underwent metasomatic exchange with an external, oxygenbearing fluid (CO₂, magma, H₂O, etc.); during this event, the relatively inert pyroxenes exchanged at a slower rate than did the coexisting olivines and spinels. This accounts for the correlation between Δ¹⁸O pyroxene-olivine and the whole-rock δ¹⁸O of the peridotites, which is a major difficulty with the equilibrium interpretation. The metasomatic ¹⁸O-enrichments of the peridotites can be related to metasomatic enrichments in LIL elements and the development of amphibole and phlogopite. In recent studies of leucite-bearing lavas with ⁸⁷Sr/⁸⁶Sr = 0.7102 to 0.7106 from the Alban Hills and M. Vulsini in Italy, we have identified a primary magmatic range of δ¹⁸O = +5.5 to +8.0, similar to the range of δ¹⁸O in olivine (+4.4 to +7.5) and in phlogopite (+5.0 to +8.0) that is observed in the peridotite nodules. This suggests that the abundant K-rich magmas erupted in Central Italy during the past 500,000 years were produced from source regions that were metasomatized by ¹⁸O-rich and ⁸⁷Sr-rich fluids. This type of precursor metasomatic activity can in general also explain the development of alkali basalt magmas (which tend to be slightly ¹⁸O-rich relative to MORB, with δ¹⁸O = +6 to +7). Fluids with appropriate δ¹⁸O values to explain the opensystem metasomatic effects can be produced by exchange with ancient subducted oceanic crust (eclogite). Fluid/rock ratios of about 0.5 to 2.5 are required to explain the nodule data, indicating that the metasomatism cannot be a mantle-wide phenomenon. At characteristic mantle temperatures, the isotopic disequilibrium effects would likely disappear in a few tens of millions of years, or less, also implying that these ultramafic nodules are not typical samples of the upper mantle. The non-equilibrium effects are thus apparently transient phenomena, probably associated with the eruptive events that brought the nodules to the surface. Massif-type ultramafic bodies like Lanzo, Ronda, and Beni Bouchera may therefore constitute better samples of the average continental upper mantle.