The abundances of lithium and boron in olivine, orthopyroxene, clinopyroxene and spinel in 4 anhydrous, spinel bearing peridotite xenoliths (McDonough et al., 1992) have been determined by SIMS analyses (Ottolini et al., 1993; Hawthorne et al., 1995). These data are used to establish subsolidus distribution coefficients for these elements, to constrain the estimated abundances of these elements in the Silicate Earth and to examine their partitioning behavior during mantle melting.
The peridotites are representative of anhydrous peridotites and range in composition from fertile lherzolite to depleted harzburgite (cpx mode content from 21% to 3%). Their minerals have homogeneous major and trace element compositions from cores to rims (McDonough et al., 1992). Two pyroxene equilibration temperatures show a limited range of values at about 1150°C (Sachtleben and Seck, 1981) and several of these and related peridotites are in Nd isotopic equilibrium (Stosch and Lugmair, 1986).
Lithium abundances in individual minerals are inversely correlated to the degree of refractoriness of the whole rock, with the most refractory whole rocks containing minerals with the lowest Li abundances. Li shows the following partitioning: ol > cpx opx >> spinel, with the bulk of the Li inventory being hosted in olivine (60-90%) and opx (20-10%). Distribution coefficients for Li between ol/opx (1.8-2.1) and ol/cpx (1.4-2.1) show less variation than that for ol/sp (3-5). The similar ionic radii of Li and Mg allow for the substitution of Li in mafic minerals, likely through a coupled substitution with P in the tetrahedral site (Brown, 1980). Calculated bulk rock concentrations for Li in these peridotites show a limited range (1.1 to 1.3 ppm). Estimates of Li in the Silicate Earth range from 1.6 to 2.1 ppm (Jagoutz et al., 1979; Ryan and Langmuir, 1987; McDonough and Sun, 1995) and are based on data from bulk rock peridotites
and basalts. These estimates may be too high, given that
bulk rock peridotite data may have been influenced by serpentinization. Based on the chemical systematics for the spectrum of peridotites studied here, we predict 1.3-1.4 ppm Li in the Silicate Earth. Primitive basalts (9-12 wt% MgO) have 3 to 5 ppm Li, indicating that the bulk distribution coefficient for Li during mantle melting is similar to that of V or Sc (Ryan and Langmuir, 1987).
Boron abundances in individuals minerals also inversely correlate to refractoriness of the whole rock, with the most refractory whole rocks containing minerals with the lowest B abundances. B shows the following partitioning: cpx opx ol >> spinel, with B mainly hosted in the silicates: olivine (40-80%), opx (30-20%) and cpx (30-1%). Distribution coefficients for B between ol/opx (0.6-1.3) and ol/cpx
(0.5-0.9) show less variation than that for ol/sp (10-30). The similar ionic radii of B and Si allow for the substitution of B in the tetrahedral site as is the case for Al, another 3+ cation. The low B contents of spinels are consistent with boron's chemical similarity to Si, not Al, and reflects the influence of boron's high ionic potential in subsolidus partitioning. Calculated bulk rock concentrations for B in these peridotites vary considerably (25 to 110 ppb). Boron is a highly incompatible element with a bulk distribution coefficient during mantle melting similar to that of K (Chaussidon and Jambon, 1994). Values of K/B in the bulk continental crust and various oceanic basaltic reservoirs vary by at least a factor of 3 (McDonough and Sun, 1995). Estimates of B in the Silicate Earth range from 0.25 to 0.5 ppm (McDonough and Sun, 1995; Chaussidon and Jambon, 1994; Higgins and Shaw, 1984) and are based on data from bulk rock peridotites, basalts and the continental crust. Estimates based on whole rock analyses of peridotites are likely to be too high, given B addition during serpentinization. Based on the systematics for the spectrum of peridotites studied here we suggest a lower limit of B in the Silicate Earth of about 100 ppb and an upper limit of about 300 ppb (McDonough and Sun, 1995; Chaussidon and Jambon, 1994).
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