Possible non-equilibrium oxygen isotope effects in mantle nodules, an alternative to the Kyser-O'Neil-Carmichael ¹⁸O/¹⁶O geothermometer

Abstract

Kyser, O'Neil, and Carmichael (1981, 1982) measured the δ18O values of coexisting minerals from peridotite nodules in alkali basalts and kimberlites, interpreting the nodules as equilibrium assemblages. Using Ca-Mg-Fe element-partition geothermometric data, they proposed an empirical 18O/16O geothermometer: T(°C)=1,151−173Δ−68Δ 2, whereΔ is the per mil pyroxene-olivine fractionation. However, this geothermometer has an unusual “crossover” at 1,150 °C, and in contrast to what might be expected during closed-system equilibrium exchange, the most abundant mineral in the nodules (olivine) shows a much greater range in δ18O (+4.4 to +7.5) than the much less abundant pyroxene (all 50 pyroxene analyses from spinel peridotites lie within the interval +5.3 to +6.5). On δ18O-olivinevs. δ18O-pyroxene diagrams, the mantle nodules exhibit data arrays that cut across the Δ18O=zero line. These arrays strongly resemble the non-equilibrium quartzfeldspar and feldspar-pyroxene δ18O arrays that we now know are diagnostic of hydrothermally altered plutonic igneous rocks. Thus, we have re-interpreted the Kyser et al. data as non-equilibrium phenomena, casting doubt on their empirical geothermometer. The peridotite nodules appear to have been open systems that underwent metasomatic exchange with an external, oxygen-bearing fluid (CO2, magma, H2O, etc.); during this event, the relatively inert pyroxenes exchanged at a much slower rate than did the coexisting olivines and spinels, in agreement with available exchange-rate and diffusion measurements on these minerals. 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.