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

The composition of the continental crust

Composition and Petrology of the Earth's Mantle

This paper reports the results of a detailed experimental investigation of fractionation of natural basaltic compositions under conditions of high pressure and high temperature. A single stage, piston-cylinder apparatus has been used in the pressure range up to 27 kb and at temperatures up to 1500° C to study the melting behaviour of several basaltic compositions. The compositions chosen are olivine-rich (20% or more normative olivine) and include olivine tholeiite (12% normative hypersthene), olivine basalt (1% normative hypersthene) alkali olivine basalt (2% normative nepheline) and picrite (3% normative hypersthene). The liquidus phases of the olivine tholeiite and olivine basalt are olivine at 1 Atmosphere, 4.5 kb and 9 kb, orthopyroxene at 13.5 and 18 kb, clinopyroxene at 22.5 kb and garnet at 27 kb. In the alkali olivine basalt composition, the liquidus phases are olivine at 1 Atmosphere and 9 kb, orthopyroxene with clinopyroxene at 13.5 kb, clinopyroxene at 18 kb and garnet at 27 kb. The sequence of appearance of phases below the liquidus has also been studied in detail. The electron probe micro-analyser has been used to make partial quantitative analyses of olivines, orthopyroxenes, clinopyroxenes and garnets which have crystallized at high pressure.

The genesis of basaltic magmas

I Composition and Constitution of the Earth.- 1 The Mantle-Crust System.- 1.1 Principal Subdivisions of the Earth's Interior.- 1.2 The Crust.- 1.3 The Upper Mantle.- 1.4 The Transition Zone.- 1.5 The Lower Mantle.- 2 Geochemistry of the Mantle.- 2.1 Chemical Homogeneity versus Heterogeneity.- 2.2 Abundance Patterns of Compatible Elements and Their Significance.- 2.3 Major Elements and Involatile Lithophile Elements in Pyrolite as Compared to Their Primordial Abundances.- 2.4 Abunance Patterns of Volatile and Siderophile Elements.- 3 Composition and Formation of the Core.- 3.1 The Role of Metallic Iron.- 3.2 Light Elements in the Outer Core.- 3.3 Iron Oxide as a Major Component of the Outer Core.- 3.4 Formation of the Core.- 4 Boundary Conditions for the Origin of the Earth.- II Origin of the Earth.- 5 Protostars, Disks, and Planets.- 5.1 Cosmogonic Background.- 5.2 Development of Protostars.- 5.3 The Formation of Disks.- 5.4 The Outer Planets and Their Regular Satellite Systems.- 5.5 Angular Momentum Relationships within the Solar System.- 5.6 Possible Relevance for Lunar Origin.- 6 Aspects of Planet Formation in the Primordial Solar Nebula.- 6.1 Mass of the Nebula.- 6.2 Accretion of the Planets.- 6.3 Temperature Regime and the Compositions of Planetesimals.- 6.4 Gas-Solid Equilibria in the Nebula.- 6.5 Chemical Fractionations among the Terrestrial Planets.- 7 Early Theories of Accretion of the Earth.- 7.1 Homogeneous Accretion.- 7.2 Single-Stage Hypothesis.- 7.3 The Heterogeneous Accretion Hypothesis.- 8 Homogeneous Accretion Revisited.- 8.1 Some General Considerations.- 8.2 Accretion of the Earth.- 8.3 Formation of the Core.- 8.4 Siderophile Elements and Oxidation State within the Mantle.- 8.5 Atmosphere and Hydrosphere.- 8.6 Conclusion.- III The Moon and Planets.- 9 The Terrestrial Planets.- 9.1 Introduction.- 9.2 Mars.- 9.3 Venus.- 9.4 Mercury.- 9.5 Asteroids and Meteorites.- 9.6 Cosmogonic Models Based on Chondrites.- 10 Constitution and Composition of the Moon.- 10.1 Internal Structure.- 10.2 The Lunar Crust-Upper Mantle System.- 10.3 Mare Basalts and the Lower Mantle.- 10.4 Further Limits on the Composition of the Lunar Interior.- 10.5 Lunar Thermal Regime.- 11 Geochemistry of the Moon.- 11.1 Introduction.- 11.2 Siderophile Elements in Lunar-Mare and Terrestrial Basalts.- 11.3 Tungsten and Phosphorus.- 11.4 Eucritic Parent Body.- 11.5 Indigenous Siderophile Component of the Lunar Highlands.- 11.6 Geochemical Constraints on the Existence of a Lunar Core.- 11.7 Volatile Elements in Mare Basalts and Their Source Regions.- 11.8 Volatile Elements in the Lunar Highlands.- 12 Towards a Theory of Lunar Origin.- 12.1 Introduction.- 12.2 The Pre-Apollo Scene.- 12.3 New Variations on Old Themes.- 12.4 The Post-Apollo Scene : Geochemical Testimony.- 12.5 Conclusion.- References.

Origin of the Earth and Moon

An experimental investigation of the gabbro to eclogite transformation and its petrological applications

Major regions of inhomogeneity are present in the mantle at depths less than 1000 km The thermal gradient also greatly exceeds its adiabatic value at relatively shallow depths Hence the Williamson-Adams equation cannot be used in this part of the earth to derive the density variation from seismic data In this paper the density in the upper mantle is obtained by explicitly introducing the constitution of the material there In the lower mantle the extended Williamson-Adams equation is used, and the constitution of this region is deduced from the density curve

Recent seismic results for the upper mantle, particularly those relating to low-velocity zones, are examined Significant regional differences are present Beneath the oceans there is a definite LV zone for S, and possibly one for P as well Beneath Precambrian shields the LV zone for S is less pronounced, and the LV zone for P seems to be absent Other continental regions are intermediate between these zones The LV zone is considered to be due to high thermal gradients with mineralogical and chemical heterogeneity superimposed Differences between the behavior of P and S are largely due to different temperature coefficients of the two velocities Regional contrasts arise from regional differences in thermal gradients and mineralogy

Consideration of temperature-depth relations leads to the conclusion that the mantle is hottest beneath the oceans because of the absence of a thick, radioactive crust, and coolest beneath Precambrian shields because of low heat flow A self-consistent model of the mantle requires that the thermal flux at a depth of 400 km be about 05 μcal/cm² sec, because geochemical evidence indicates that the K/U ratio in the upper mantle is much smaller than in chondrites The self-consistent model requires very high thermal conductivity at high temperatures, such as would be provided by radiative transfer or possibly by movement of material

Petrological models of the upper mantle are constructed on the assumptions of an over-all pyrolite (ultrabasic) composition and an eclogitic composition Densities in the mantle are then calculated from known densities, thermal expansions, and compressibilities of minerals inferred to be present Corrections for the effect of pressure on thermal expansion and compressibility are made from results of the theory of finite strain

The transition zone, at depths between 400 and 1000 km, is the site of a series of major phase transformations leading to close-packed structures with silicon in sixfold coordination The density curve in this region is approximated by a linear increase in density with depth The lower mantle, between 1000 and 2900 km, is considered to be homogeneous, and the density is computed from the Williamson-Adams equation, modified in some cases to take account of a superadiabatic thermal gradient The magnitude of the density increase in the transition zone is adjusted to satisfy the restrictions imposed by the total mass and moment of inertia of the earth A complete density curve for the earth is given for each petrological model of the upper mantle In constructing them it is assumed that the outer core is homogeneous and adiabatic, and that the inner core is of uniform density

A density curve derived from the third-order theory of finite strain is fitted to the densities calculated in the lower mantle by least squares The density of the lower mantle at low temperature and pressure can then be calculated The results at 20°C and atmospheric pressure are 42–43 g/cm³ for the adiabatic pyrolite model, and 40–41 g/cm³ for the adiabatic eclogite model These values are in good agreement with estimates for the density of the lower mantle based upon recent investigations of phase transformation in olivines and pyroxenes at very high pressures A superadiabatic gradient of 1°C/km in the lower mantle produces results inconsistent with a plausible constitution; therefore we conclude that the thermal gradient in this part of the earth is smaller than 1°C/km

Our models imply that important differences in density persist to depths of 400 km, and it is inferred that isostatic compensation is not complete before that depth This conclusion is consistent with gravity data, and it leads to crustal densities more plausible, in terms of observed seismic velocities, than those obtained by assuming compensation at the continental M discontinuity Because of these deep-seated density contrasts, the position of the present pole of rotation may well be in equilibrium with the present distribution of continents and oceans

The relatively low temperature beneath the shields could explain their behaving like rigid blocks in Paleozoic and later orogenesis

A.E. Ringwood

Papers

Composition and Petrology of the Earth's Mantle

The genesis of basaltic magmas

Origin of the Earth and Moon

An experimental investigation of the gabbro to eclogite transformation and its petrological applications

Density distribution and constitution of the mantle