Summary Trace-element data for mid-ocean ridge basalts (MORBs) and ocean island basalts (OIB) are used to formulate chemical systematics for oceanic basalts. The data suggest that the order of trace-element incompatibility in oceanic basalts is Cs ≈ Rb ≈ (≈ Tl) ≈ Ba(≈ W) > Th > U ≈ Nb = Ta ≈ K > La > Ce ≈ Pb > Pr (≈ Mo) ≈ Sr > P ≈ Nd (> F) > Zr = Hf ≈ Sm > Eu ≈ Sn (≈ Sb) ≈ Ti > Dy ≈ (Li) > Ho = Y > Yb. This rule works in general and suggests that the overall fractionation processes operating during magma generation and evolution are relatively simple, involving no significant change in the environment of formation for MORBs and OIBs. In detail, minor differences in element ratios correlate with the isotopic characteristics of different types of OIB components (HIMU, EM, MORB). These systematics are interpreted in terms of partial-melting conditions, variations in residual mineralogy, involvement of subducted sediment, recycling of oceanic lithosphere and processes within the low velocity zone. Niobium data indicate that the mantle sources of MORB and OIB are not exact complementary reservoirs to the continental crust. Subduction of oceanic crust or separation of refractory eclogite material from the former oceanic crust into the lower mantle appears to be required. The negative europium anomalies observed in some EM-type OIBs and the systematics of their key element ratios suggest the addition of a small amount (⩽1% or less) of subducted sediment to their mantle sources. However, a general lack of a crustal signature in OIBs indicates that sediment recycling has not been an important process in the convecting mantle, at least not in more recent times (⩽2 Ga). Upward migration of silica-undersaturated melts from the low velocity zone can generate an enriched reservoir in the continental and oceanic lithospheric mantle. We propose that the HIMU type (eg St Helena) OIB component can be generated in this way. This enriched mantle can be re-introduced into the convective mantle by thermal erosion of the continental lithosphere and by the recycling of the enriched oceanic lithosphere back into the mantle.

Chemical and isotopic systematics of oceanic basalt : implications for mantle composition and processes

Chemical and isotopic systematics of oceanic basalts. Implications for Mantle Composition and Processes

The composition of the Earth

This chapter reviews the present-day composition of the continental crust, the methods employed to derive these estimates, and the implications of the continental crust composition for the formation of the continents, Earth differentiation, and its geochemical inventories. We review the composition of the upper, middle, and lower continental crust. We then examine the bulk crust composition and the implications of this composition for crust generation and modification processes. Finally, we compare the Earth's crust with those of the other terrestrial planets in our solar system and speculate about what unique processes on Earth have given rise to this unusual crustal distribution.

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Composition of the Continental Crust

We report here the results of a study to develop natural zircon geochemical standards for calibrating the U-(Th)-Pb geochronometer and Hf isotopic analyses. Additional data were also collected for the major, minor and trace element contents of the three selected sample sets. A total of five large zircon grains (masses between 0.5 and 238 g) were selected for this study, representing three different suites of zircons with ages of 1065 Ma, 2.5 Ma and 0.9 Ma. Geochemical laboratories can obtain these materials by contacting Geostandards Newsletter.

Three natural zircon standards for u‐th‐pb, lu‐hf, trace element and ree analyses

Partial melt distributions from inversion of rare earth element concentrations

https://faculty.washington.edu/stn/ess_501/reading/Goldstein_River_Nd_EPSL_1984.pdf

A Sm-Nd isotopic study of atmospheric dusts and particulates from major river systems

Rare-earth abundances in chondritic meteorites

Variations in143Nd/144Nd and87Sr/86Sr ratios in oceanic basalts

A simple two-reservoir. model with time-dependent transport coefficients between the reservoirs has been used to model the abundances of K, Ar, Rb, Sr, Sm, Nd, U, Th, and Pb and the isotopic compositions of Ar, Sr, Nd, and Pb in the earth's mantle and continental crust. The transport coefficients, like heat production, are considered to decay exponentially with time. Models which involve the whole mantle in generating the continental crust yield 87Sr/144 and 143Nd/144Nd ratios for the residual mantle which are higher and lower than midocean ridge basalts, respectively. A model which involves only half of the mantle in the production of continental crust produces a residual mantle with isotope ratios similar to those of midocean ridge basalt. The 40Ar/36Ar ratios in the atmosphere and suboceanic mantle are reproduced by this model without any inequality in the upward transport coefficients of K and Ar but with a smaller downward transport coefficient for Ar than for K. The results imply that the earth's crust may have developed by extraction of material from only half of the mantle and constrain the possible convective regimes that have existed in the mantle.

R.K. O'Nions

Papers

Partial melt distributions from inversion of rare earth element concentrations

A Sm-Nd isotopic study of atmospheric dusts and particulates from major river systems

Rare-earth abundances in chondritic meteorites

Variations in143Nd/144Nd and87Sr/86Sr ratios in oceanic basalts

Geochemical modeling of mantle differentiation and crustal growth