The Correlation Between Mn and Ni in Primitive Olivine Phenocrysts from MORBs and OIBs—is Recycled Crust Necessary

The creation and recycling of oceanic and continental crust are the principal mechanisms by which the Earth's mantle chemically evolves. Assuming constant subduction rates, the mantle contains 5% recycled oceanic crust—a potentially significant geochemical reservoir [e.g., 1]. Trace elements and radiogenic isotopes of basalts, and the Ni and Mn contents of magnesian olivine (ol) phenocrysts [2] have been used to infer the presence of recycled oceanic crust as a source component in magmas. We report high-precision measurements of the Mn contents of olivine and silicate liquid (liq) from experimental run products. Olivine-liquid partition coefficients (DMn, by wt.) are independent of temperature but depend on liquid composition: decreasing from 0.90 to 0.65 as MgOliqincreases from 12 to 21 wt. %. We model the MnO and NiO concentrations in crystals and liquids during isobaric peridotite melting using the results of 1-4.5 GPa peridotite partial melting experiments [3, 4]; ol-liq and crystal-ol (e.g., opx-ol) partitioning expressions fit to data from the literature [5, 6] and this work; and bulk mantle MnO and NiO contents of 0.131 wt. % and 0.25 wt. %, respectively [7]. With increasing pressure of melting, there is a decrease in the MnO content of residual olivine, at fixed Mg#. Two factors contribute to this pressure effect: (1) Partial melts of peridotite become richer in MgO as pressure increases, decreasing DMn ; and (2) garnet appears at the expense of spinel plus pyroxene at 3 GPa, and Mn is appreciably more compatible in garnet than in pyroxene. Finally, we show that the inverse correlation between Mn and Ni contents of magnesian olivine phenocrysts previously ascribed to pyroxenite melting [2] can be generated by partial melting of peridotite under lithosphere of varying thickness. Thus, although recycled materials are likely present in the mantle source regions of many basaltic magmas, no contribution from recycled oceanic crust is required to explain the Ni and Mn contents of the olivine phenocrysts in these magmas. [1] Stracke et al (2003) G34, 8003. [2] Sobolev et al (2007) Nature 316, 412. [3] Baker et al (1995) Nature 375, 308. [4] Walter (1998) JPet 39, 29. [5] Hirschmann et al (2008) G3Q03011. [6] Matzen et al (2013) JPet 54, 2521. [7] McDonough & Sun (1995) Chem Geol 120, 223.