Study of Crust and Mantle Differentiation Processes from Variations in Nd, Sr, and Pb Isotopes

Citation

Jacobsen, Stein Bjørnar (1980) Study of Crust and Mantle Differentiation Processes from Variations in Nd, Sr, and Pb Isotopes. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/q5z7-d408. https://resolver.caltech.edu/CaltechTHESIS:01242024-233510543

Abstract

This study discusses two simple transport models for the evolution of the mantle and the crust. In Model I, the continents grow by extraction of melts over the history of the earth from undepleted mantle; the residue forms a depleted mantle, which today is the source of mid-ocean ridge basalts. In Model II, new additions to the continents are derived from a mantle reservoir which becomes increasingly depleted through time by repeated extraction of melts. In developing these models, I solved the transport equations for stable, radioactive, and daughter isotopes for arbitrary crustal growth curves. For both models the isotopic composition and concentrations of trace elements reduce to simple mathematical expressions which readily permit calculations of basic evolutionary parameters from new and published data.

New measurements of ¹⁴³Nd/¹⁴⁴Nd in chondrites yield a range in values of 3.4 ϵ-units (0.511767 to 0.511845), which correlates with the variation of 2.6% in ¹⁴⁷Sm/¹⁴⁴Nd (0.1920 to 0.1969). From these data a new set of reference values for CHUR ("chondritic uniform reservoir") have been selected. The evolution of ¹⁴³Nd/¹⁴⁴Nd in CHUR can be described by a self-consistent set of present-day values of (¹⁴³Nd/¹⁴⁴Nd)°_(CHUR) = 0.511836 and (¹⁴⁷Sm/¹⁴⁴Nd)°_(CHUR) = 0.1967. Some terrestrial samples of Archean age show clear deviations from the new CHUR curve. If the CHUR curve is representative of undifferentiated mantle then it demonstrates that some Archean rocks were formed from depleted sources. This may indicate that the depleted source of present-day mid-ocean ridge basalts (MORB) originated very early in the earth's history.

Two Sm-Nd internal isochrons for pyroxene-gabbros of the Bay of Islands Ophiolite Complex give well-defined ages of 508±6 m.y. and 501±13 m.y. with initial ¹⁴³Nd/¹⁴⁴Nd of ϵ_(Nd) = +7.9∓0.1 and ϵ_(Nd) = +7.7∓0.2, respectively. Total rock samples from pillow basalts, sheeted dikes, trondhjemites, hornblende gabbros, pyroxene gabbros, and an orthopyroxenite layer from the harzburgite give initial ϵ_(Nd) in the range from +6.7 to +8.3 with an average value of +7.8. However, the initial ⁸⁷Sr/⁸⁶Sr within the different phases of the complex is found to be highly variable(~52 ϵ-units) and shows the effect of seawater alteration. The magnitude of the initial ϵ_(Nd) values (+7.8) are somewhat smaller than for typical present-day mid-ocean ridge basalts (+10). This is most likely due to differential evolution over the past 0.5 AE of the oceanic mantle relative to the bulk earth.

Sm-Nd and Rb-Sr data for the Oslo Rift demonstrate that this province which is strongly enriched in large ion lithophile (LIL) elements is derived from depleted.to undepleted mantle materials. A source with a long history of light rare earth (LREE) enrichment can clearly be ruled out.

Sm-Nd data for the Archean granulite and amphibolite facies migmatites of Langøy and Hinnøy in Vesterålen indicate that their protoliths formed ~2.6 AE ago. Rb and U loss during a granulite facies metamorphism at ~1.8 AE caused serious disturbance of total rock U-Pb and Rb-Sr systems. Therefore these systems do not provide any precise age information for the granulite facies migmatites. For the amphibolite facies migmatites of Vesterålen both Sm-Nd, Rb-Sr, and Pb-Pb total rock systems give model ages of ~2.6 AE.

Sm-Nd and Rb-Sr isotopic data for continental crust, depleted and undepleted mantle, have been used to evaluate both models and yield young mean ages for the mass of the continental crust of 1.8 AE and 1.5 AE for Model I and Model II, respectively. Both models also suggest that the rate of growth of the continents for the last 0.5 AE is much less than the average growth rate. The young mean age of the continents implies either rapid refluxing of crustal materials to the mantle in the period from 4.5 AE to 3.6 AE or that very little early crust ever formed. Mass balance calculations for both models show that the continents were only formed from ~30% of the total mantle leaving 70% of the mantle as undepleted. The major difference in the two models lies in the difference in the compositions of newly derived crust. For Model I the trace element concentrations in new additions to the crust is constant and the isotopic values are those of the undepleted mantle reservoir in agreement with recent Nd isotopic studies. Model II implies that new additions to the continents have the isotopic characteristics of the depleted mantle and that the concentrations of Rb, U, Ba and other highly incompatible trace elements in newly added material have changed by a factor of ~10 through time. There is no evidence, however, for such a large change in the concentration of these elements with time.

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