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

Gregory, Robert T. and Taylor, Hugh P., Jr. and Kyser, T. Kurtis and O'Neil, James R. and Carmichael, Ian S. E. (1986) Possible non-equilibrium oxygen isotope effects in mantle nodules, an alternative to the Kyser-O'Neil-Carmichael ¹⁸O/¹⁶O geothermometer. Contributions to Mineralogy and Petrology, 93 (1). pp. 114-119. ISSN 0010-7999. doi:10.1007/bf00963589. https://resolver.caltech.edu/CaltechAUTHORS:20221026-664855800.2

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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.


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https://doi.org/10.1007/bf00963589DOIArticle
Alternate Title:Possible non-equilibrium oxygen isotope effects in mantle nodules, an alternative to the Kyser-O'Neil-Carmichael 18O/16O geothermometer
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Caltech Division of Geological and Planetary Sciences3978
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DOI:10.1007/bf00963589
Record Number:CaltechAUTHORS:20221026-664855800.2
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20221026-664855800.2
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ID Code:117577
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:29 Oct 2022 22:05
Last Modified:29 Oct 2022 22:05

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