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Isotopic modeling of the evolution of the mantle and crust
Author(s) -
Turcotte D. L.,
Kellogg L. H.
Publication year - 1986
Publication title -
reviews of geophysics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 8.087
H-Index - 156
eISSN - 1944-9208
pISSN - 8755-1209
DOI - 10.1029/rg024i002p00311
Subject(s) - geology , continental crust , mantle (geology) , geochemistry , crust , isotopes of strontium , isochron , crustal recycling , strontium , geophysics , physics , nuclear physics
The geochemical reservoirs and cycles of the earth are discussed. Models involving the isotopic evolution of the enriched continental crustal reservoir and the complementary depleted mantle reservoir are considered. Analytic solutions for the fractionation factors f and isotope ratios ε are obtained using consistent approximations. The results are applied to data on the samarium‐neodymium, rubidium‐strontium, and thorium‐uranium‐lead systems. Using a model for instantaneous crustal differentiation, data on young rocks provide constraints on the mean age of the continental crust and on the mass of the depleted mantle reservoir. Although there are a variety of uncertainties, we favor a mean continental age of τ c =2.1±0.7 Ga. A mass balance based on samarium‐neodymium and rubidium‐strontium data favors a mass for the depleted layer near the mass of the upper mantle. Mass balances for argon and helium, isotope correlations between strontium and neodymium and between thorium and uranium, and the substantial quantities of primordial helium and argon found in ocean island basalts (OIB) from Hawaii and Iceland favor the existence of a sizable near‐primitive reservoir. We conclude that the isotope data favor a near‐primitive lower mantle with a barrier to mantle convection at a depth of 650 km. Thus the isotope data favor layered mantle convection rather than whole mantle convection. Isotope data on older rocks provide information on the time evolution of the continents. An important question is whether significant volumes of continental crust are recycled back into the mantle. Isotope data on old continental rocks that can be associated with the depleted mantle appear to favor significant volumes of crustal recycling. We have also studied the implications of preferential recycling of either parent or daughter isotopes. Recycling of either isotope increases the age of the continental crust inferred from isotope data. Studies of lead isotopes in mid‐ocean ridge basalts (MORB) and OIB show that they evolved in uranium rich environments for several billion years. This is the “missing lead” paradox; we associate the missing lead with the lower continental crust. We conclude that the radiogenic lead evolved either in the depleted mantle reservoir or in the radiogenic upper crust. The isotopic heterogeneities of OIB are also considered; these are attributed to (1) entrainment of near‐primitive lower mantle rock and (2) inclusion of recently subducted and delaminated continental crust and mantle.