In search of a bulk-Earth composition

Abstract

Refined estimates of primitive upper-mantle (PUM) composition are derived here from the major-element and rare-earth-element (REE) contents of mantle lherzolites, using the approaches pioneered by Loubet et al. (1975) and Jagoutz et al. (1979). Because the PUM composition of Jagoutz et al. (1979) is distinctly non-chondritic in certain “refractory” element ratios, such as Mg/Si and Ca/Al, either the upper and lower mantles are chemically different, or the bulk silicate Earth (BSE) is non-chondritic with respect to these elements. We argue that the high Ca/Al ratios observed in lherzolites are not in fact representative of PUM composition, but are caused by excess and non-representative modal abundances of clinopyroxene in analyzed lherzolite samples. Corrections for this excess Cpx lead to REE ratios (to Ca or Al) in the least depleted lherzolites which are essentially chondritic. Furthermore, we argue that this corrected PUM composition is also equivalent to a BSE composition, as it seems unlikely that significant intramantle chemical fractionations can occur without serious distortion of the relative abundances of highly incompatible (e.g., Nd) and moderately compatible (e.g., Ca) elements. If the upper and lower mantles are chemically similar, then the Earth is deficient in Si and Mg relative to C1 chondrites; that is, Si and Mg do not behave as true refractory elements during the Earth's condensation and accretion. Any C1 model which treats Si and Mg as purely refractory elements demands different compositions for upper and lower mantles, and demands that O and/or S cannot be the only light elements in the core; Si (and/or Mg) must be present as well. In addition, heterogeneous accretion or intramantle differentiation processes are then required which preserve C1 ratios of REE/Al in the upper mantle, while simultaneously allowing for non-chondritic relative abundances of Ca, Al, Mg and Si. On the other hand, if Si and Mg are allowed to be slightly deficient in the Earth (19% and 8%, respectively), relative to Ca or Al in C1 chondrites (LOSIMAG C1 model), then the upper and lower mantles need not be chemically different (i.e. PUM = BSE), and an Earth size core is attainable with only ∼ 9% light (O or S) element in it (no Si or Mg is required). Our LOSIMAG C1 model, with PUM = BSE, leads to refractory element abundances in BSE of 2.51 × C1 chondrites (as given by Anders and Ebihara, 1982). Crust-mantle budgets constructed using this PUM-derived concentration for Nd (= 1.17 ppm) show that the present-day lower mantle (> 670 km) is moderately depleted in the light REE's and cannot be wholly characterized by undifferentiated (chondritic) Sm/Nd ratios. This argues for at least minor mass transfer (convection) between the upper and lower mantles in the past. The U content of the BSE (20.8 ppb) leads to a relatively high value (∼ 2.8) for the reduced Urey ratio (mantle heat flow/heat production); this documents poor thermal efficiency of present mantle convection regimes.