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\begin{document}

\title{Size and composition of the residual and depleted mantle reservoir}



\author[1]{Albrecht W. Hofmann}%
\author[2]{Cornelia Class}%
\author[3]{Steven L. Goldstein}%
\affil[1]{MPI}%
\affil[2]{Lamont-Doherty Earth  Observatory}%
\affil[3]{Columbia University}%


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  \date{May 26, 2024}


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\begin{abstract}
Most previous efforts to characterize the size and composition of the
upper mantle, the source of mid-ocean ridge basalts (MORBs), have
assumed that this MORB source is the residue of continental crust
extraction. The use of Nd isotopes to model this process led to the
near-consensus that the ``depleted MORB reservoir'' is more-or-less
confined to the upper mantle (above 670 km, \textasciitilde{}30\% of the
mantle), with a severe degree of depletion of incompatible elements,
leaving the lower mantle in a more primitive state. Here, we reassess
the mass and composition of the mantle reservoir depleted by continental
crust extraction. We initially apply simple mass balance considerations,
using alternatively \selectlanguage{greek}ε\selectlanguage{english}(Nd) and ``canonical'' (Nb,Ta)/U tracers, to a
conventional three-reservoir silicate Earth consisting of primitive
mantle, continental crust, and depleted mantle. The (Nb,Ta)/U tracer
yields a `depleted reservoir' exceeding 60\% by mass of the total mantle
(X(DM) \textgreater{} 0.6) with average \selectlanguage{greek}ε\selectlanguage{english}(Nd) [?] 3, whereas the
\selectlanguage{greek}ε\selectlanguage{english}(Nd)-based mass balance, using \selectlanguage{greek}ε\selectlanguage{english}(Nd) = 8.5, yields a ``depleted
reservoir'' of X(DM) [?] 0.3. This discrepancy requires additional
processes/reservoirs that impact the fractionation of Sm/Nd in the
depleted mantle. Simple segregation of enriched OIB sources is shown to
be insufficient. Permanent sequestration of a fourth, early-enriched,
mafic reservoir (EER), leaving behind an early-depleted reservoir (EDR)
can resolve the dilemma. Segregation of the present-day continental
crust from EDR generates a moderately depleted, ``residual-mantle''
reservoir (RM), which occupies 80-98\% of the total mantle (X(RM) =
0.8-0.98). This leads to concordant results for the two crust-mantle
mass balances.%
\end{abstract}%



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