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

Summary A number of possible mechanisms have recently been proposed for driving the motions of the lithospheric plates, such as pushing from mid-ocean ridges, pulling by downgoing slabs, suction toward trenches, and coupling of the plates to flow in the mantle. We advance a new observational method of testing these theories of the driving mechanism. Our basic approach is to solve the inverse problem of determining the relative strength of the plausible driving forces, given the observed motions and geometries of the lithospheric plates. Since the inertia of the plates is negligible, each plate must be in dynamic equilibrium, so that the sum of the torques acting on a plate must be zero. Thus, our problem is to determine the relative sizes of the forces that minimize the components of net torque on each plate. The results indicate that the forces acting on the downgoing slab control the velocity of the oceanic plates and are an order of magnitude stronger than any other force. Namely, all the oceanic plates attached to substantial amounts of downgoing slabs move with a ' terminal velocity ' at which the gravitational body force pulling the slabs downward is nearly balanced with the resistance acting on the slab; regardless of the other features of the trailing horizontal part of the plates. The drag on the bottom of the plates which resist motion is stronger under the continents than under the oceans.

On the Relative Importance of the Driving Forces of Plate Motion

Responding to the latest developments in rock physics research, this popular reference book has been thoroughly updated while retaining its comprehensive coverage of the fundamental theory, concepts, and laboratory results. It brings together the vast literature from the field to address the relationships between geophysical observations and the underlying physical properties of Earth materials - including water, hydrocarbons, gases, minerals, rocks, ice, magma and methane hydrates. This third edition includes expanded coverage of topics such as effective medium models, viscoelasticity, attenuation, anisotropy, electrical-elastic cross relations, and highlights applications in unconventional reservoirs. Appendices have been enhanced with new materials and properties, while worked examples (supplemented by online datasets and MATLAB® codes) enable readers to implement the workflows and models in practice. This significantly revised edition will continue to be the go-to reference for students and researchers interested in rock physics, near-surface geophysics, seismology, and professionals in the oil and gas industries.

The Rock Physics Handbook

However, it can be a residue of komatiite melt extraction at >7 Melting experiments on fertile peridotite KR4003, a ‘pyrolitic’ GPa from a mantle enriched in SiO2 relative to pyrolite. composition, were made from 3 to 7 GPa in piston-cylinder and multi-anvil apparatus. Temperature gradients across the sample were minimized (<25°C), and the compositions of all phases were determined. Modal abundances of coexisting phases were calculated by mass balance, and the results were used to determine phase

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Melting of Garnet Peridotite and the Origin of Komatiite and Depleted Lithosphere

Theory of Earth

The single-crystal elastic moduli of the spinel (gamma) phase of Mg2SiO4 have been experimentally determined from Brillouin spectroscopy at ambient conditions. They are C11 = 327, C44 = 126, and C12 = 112 (GPa) yielding a bulk modulus of 184. and a shear modulus of 119 GPa. This value of bulk modulus is lower than expected from previous measurements. The elastic properties of the spinel and modified spinel (beta) phases of Mg2SiO4 are quite similar with the greatest differences related to the c axis of the beta which is relatively soft under compression. The polycrystal 1ine acoustic velocities differ by only 1% for these two phases. Thus there should be no seismic discontinuity associated with the beta to gamma phase transition in the mantle. The similarity of elastic properties of these two phases suggests that effects of pressure, temperature, and iron content on the elastic properties should be similar for both phases. Estimates of the effect of iron content are made from the extant data on fayalite spinel in conjunction with the data presented here. The derivatives of elastic moduli with respect to iron content are very similar to those observed for the olivine phase. Based on comparisons with aluminate analogue compositions in the spinel structure, there is no justification to predict the pressure or temperature derivatives of the elastic moduli of these high pressure phases at this time.

Single‐crystal elastic properties of the spinel phase of Mg2SiO4

beta -Mg 2 SiO 4 ; single-crystal X-ray diffraction study

THIS is the first announcement of success in the experimental confirmation of the postspinel phase of forsterite at high pressure and high temperature. Forsterite (Mg2SiO4) decomposes to periclase (MgO) and stishovite (SiO2) at 330 kbar and 1,000° C. Forsterite is a major constituent of the Earth's mantle and the phase of forsterite composition at high pressure places a strong constraint on the nature of the transition layer and also on the composition and evolution of the Earth.

Postspinel Phase of Forsterite and Evolution of the Earth's Mantle

Experimental data are reported for the melting of komatiite, peridotite, and chondrite compositions in the pressure range 5–16.5 GPa. All experiments were run using the multiple-anvil apparatus facilities at Nagoya and Stony Brook. Equilibrium between coexisting crystals and liquid is demonstrated to occur in less than 3 min in the 2100°C range. The anhydrous solidus in CaO-MgO-Al2O3-SiO2 has been calibrated and is shown to be about 100° higher than that for naturally occurring peridotite (KLB1). All melting curves have positive dT/dP. The effect of pressure is to expand the crystallization field of garnet at the expense of all other phases, resulting in a change in the liquidus phase from olivine to garnet at high pressures. The melting of rocks which contain the four crystalline phases olivine, orthopyroxene, clinopyroxene, and garnet is restricted to enstatite-rich compositions such as chondrite. For these it is demonstrated that melting is peritectic, rather than eutectic, and takes the form L+Opx = Ol+Cpx+Gt. Partial melting yields liquids with the following properties: 5 GPa for komatiite; and 10–15 GPa for liquid peridotite with about 40% MgO, but one that is unlike mantle peridotite in that it is distinctly enriched in silica. These results provide a test and refutation of the model that upper mantle peridotite originated by direct initial melting of a chondritic mantle (Herzberg and O'Hara, 1985). Unlike chondrite, partial melting of peridotite does not usually involve orthopyroxene. Instead, it occurs by the generation of ultrabasic liquids along a cotectic involving L+Ol+Cpx+Gt. Although the thermal and compositional characteristics of this cotectic have not been fully calibrated, it is very likely that it will degenerate into a thermal minimum (L+Ol+Cpx+Gt), compositionally similar to komatiite at 5 GPa and mantle peridotite at 10–15 GPa. Peridotite liquids that occupy a thermal minimum can be derived from those formed from the melting of chondrite by removal of orthopyroxene, followed by fractional crystallization of olivine, clinopyroxene, and garnet. The possibility exists that the thermal minimum is compositionally identical to mantle peridotite in the 10–15 GPa range. If this can be confirmed by experiment, the upper mantle can be understood as having originated by the fractional crystallization of peridotite liquids in a large-scale differentiation event, consistent with magma ocean models for an early Earth.

Origin of mantle peridotite: Constraints from melting experiments to 16.5 GPa

Calculations of the sthermal evolution of the Earth during accretion suggest that the outer layer may once have been molten, perhaps to a depth of 1,000 km or more1,2. If such global melting did occur, fractionation processes could have produced a chemical stratification in the mantle. Recent technical developments in experimental petrology have made it possible to achieve temperatures in excess of 2,000°C at a pressure of 20 GPa3,4, thereby permitting studies of multi-component systems under conditions that prevail in the Earth's mantle. We have conducted melting experiments using a composition in the five-component system CaO–FeO–MgO–Al2O3–SiO2 (CFMAS) corresponding to a model chondritic mantle, and find that the liquidus phase changes from olivine to majorite at a pressure between 12 and 15 GPa. The liquid coexisting with majorite at 20 GPa has a peridotitic composition. In addition, because the CaO/A12O3 ratio of the liquidus majorite at 20 GPa is lower than that of the sub-solidus majorite, partial melting and majorite fractionation at the base of the upper mantle could produce a peridotitic liquid with a CaO/A12O3 ratio greater than that of the chondritic starting material, consistent with current views on mantle geochemistry.

Hiroshi Sawamoto

Papers

Single‐crystal elastic properties of the spinel phase of Mg2SiO4

beta -Mg 2 SiO 4 ; single-crystal X-ray diffraction study

Postspinel Phase of Forsterite and Evolution of the Earth's Mantle

Origin of mantle peridotite: Constraints from melting experiments to 16.5 GPa

Melting of a model chondritic mantle to 20 GPa