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Published July 2017 | Supplemental Material
Journal Article Open

The roles of pyroxenite and peridotite in the mantle sources of oceanic basalts


Subduction of oceanic crust generates chemical and lithological heterogeneities in the mantle. An outstanding question is the extent to which these heterogeneities contribute to subsequent magmas generated by mantle melting, but the answer differs depending on the geochemical behaviour of the elements under investigation: analyses of incompatible elements (those that preferentially concentrate into silicate melts) suggest that recycled oceanic crust is an important contributor, whereas analyses of compatible elements (those that concentrate in crystalline residues) generally suggest it is not. Recently, however, the concentrations of Mn and Ni—two elements of varying compatibility—in early-crystallizing olivines, have been used to infer that erupted magmas are mixtures of partial melts of olivine-rich mantle rocks (that is, peridotite) and of metasomatic pyroxene-rich mantle rocks (that is, pyroxenite) formed by interaction between partial melts of recycled oceanic crust and peridotite. Here, we test whether melting of peridotite alone can explain the observed trend in olivine compositions by combining new experimental data on the partitioning of Mn between olivine and silicate melt under conditions relevant to basalt petrogenesis with earlier results on Ni partitioning. We show that the observed olivine compositions are consistent with melts of fertile peridotite at various pressures—importantly, melts from metasomatic pyroxenites are not required. Thus, although recycled materials may well be present in the mantle source regions of some basalts, the Mn and Ni data can be explained without such a contribution. Furthermore, the success of modelling the Mn–Ni contents of olivine phenocrysts as low-pressure crystallization products of partial melts of peridotite over a range of pressures implies a simple new approach for constraining depths of mantle melting.

Additional Information

© 2017 Macmillan Publishers Limited. Received 13 October 2016. Accepted 11 May 2017. Published online 12 June 2017. C. Ma and S. Creighton are thanked for their guidance and support using the electron microprobe, and for sharing xenolith data, respectively. Funding was provided by National Science Foundation grant EAR-1019886, National Aeronautics and Space Administration grant NNG04GG14G, and European Research Council grant 267764. Author Contributions: A.K.M. analysed the experiments, constructed the geochemical models and wrote the manuscript. B.J.W., M.B.B. and E.M.S. provided intellectual guidance throughout the project and wrote the manuscript. Code availability. A spreadsheet that contains our forward model of batch peridotite melting is included in the Supplementary Information. Data availability. The authors declare that the data supporting the findings of this study are available within the article and its Supplementary Information files. In addition to the high-precision measurements reported in Table 1, data from other authors used in this study are referenced in the main text, the Methods, and in the Supplementary Information. The authors declare no competing financial interests.

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