A.E. Ringwood

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

Abstract The mineral assemblages of a variety of basaltic compositions have been studied experimentally in the pressure range from 1 bar to 30 kb at temperatures above 1000°C and below the basalt solidus. At low pressures, less than 10 kb at 1100°C, the mineral assemblages match those of gabbros and pyroxene granulites but at pressures above 21 kb at 1100°C the major phases are pyrope-almandine garnet and clinopyroxene and the mineral assemblages match those of eclogites. At intermediate pressures the mineral assemblages are characterized by co-existence of garnet, clinopyroxene, plagioclase and, commonly, quartz. The transition interval between the gabbroic and eclogitic assemblages is a broad one characterized by gradual increase in garnet and in the pyrope content of the garnet, and decrease in plagioclase and in the anorthite content of the plagioclase. The roles of variable SiO 2 -saturation, Na 2 O content, albite: anorthite ratio of plagioclase, oxidation state, and of variations in the Mg/(Mg + Fe 2+ ) (atomic) ratio have been studied to determine the influence of particular chemical parameters on the pressure at which a given phase (e.g. garnet) appears or disappears. Low values of the Mg/(Mg + Fe 2+ ) ratio cause garnet to appear at lower pressures. In undersaturated compositions (olivine-normative), garnet appears at lower pressures than in quartz-normative compositions and in addition there is an intermediate assemblage of aluminous pyroxenes + plagioclase + spinel present, particularly in magnesian basalts, between the low pressure olivine-bearing and higher pressure garnet-bearing assemblages. The pressure required for elimination of plagioclase varies from 15 to 20 kb at 1100°C in the spectrum of basaltic compositions studied. The pressure required for the appearance of garnet and disappearance of plagioclase in a given composition is strongly dependent on temperature. P-T gradients for these boundary reactions in a quartz tholeiite composition have been established in the 1000–1250°C temperature range. When extrapolated to lower temperatures, these gradients suggest that eclogite mineralogy is stable in dry basaltic rocks along normal geothermal gradients throughout the entire crust. The observed mineral assemblages at various pressures and the effects of chemical parameters on mineralogy in a given P-T field are closely matched with natural pyroxene granulite and eclogite occurrences and with experimental work in simple systems. The experimental work provides some quantitative data on dry solid-solid reactions which are strongly pressure and temperature dependent and which, in natural rocks, provide criteria for subdivision of granulite facies metamorphic rocks into high pressure, intermediate pressure and low pressure types. By comparison of the experimental data with estimates of the P H 2 O , T conditions of other metamorphic facies, based in part on dehydration reactions, it is argued that eclogite mineralogy may be stable in dry basaltic rocks within the almandine amphibolite facies, the glaucophane schist facies and part of the greenschist facies of regional metamorphism.

Chemical evolution of the terrestrial planets

The terrestrial planets are believed to have formed by accretion from an initially cold and chemically homogeneous cloud of dust and gas. The iron occurring in the dust particles of the cloud was present in a completely oxidised form. Either before or during accretion of dust into planets, partial reduction of oxidised iron to metal occurred. The role of oxidationreduction equilibria during the formation of terrestrial planets is discussed and it is concluded that the differing zero-pressure densities of the planets are caused dominantly by differing mean states of oxidation which were established during the primary accretion processes. This interpretation avoids the necessity for assuming the occurrence of physical fractionation of metal from silicates in the solar nebula before accretion. A detailed study is made of the evidence shed by chondritic meteorites upon oxidationreduction equilibria occurring early in the history of the solar system. The significance of the widely varying oxidation states of chrondrites is discussed. It is concluded that the different classes of chondrites have formed by an autoreduction process operating upon primitive material similar in composition to the Type I carbonaceous chondrites. Reduction occurred when this material accreted into parent bodies which were heated internally, perhaps by extinct radioactivities. Under these conditions, trapped carbonaceous material reacted with oxidised iron to produce a metallic phase in situ. Such a process explains the primary oxidation-reduction relationships in chondrites established by prior. The chemistry of the reduction process which operated in chondrites is studied. The evidence strongly indicates that the principal reducing agent was carbon and not hydrogen. Furthermore, reduction occurred in a condensed environment and not in the dispersed solar nebula. The origins and chemical evolution of other terrestrial planets are discussed in the light of evidence yielded by the chondrites. The hypothesis is advanced that each of the terrestrial planets formed by a single-stage autoreduction process operating upon primitive material similar to the Type 1 carbonaceous chondrites. In the case of the earth, autoreduction occurred at higher mean temperatures than in chondrites because of the large gravitational energy source which was involved. Accordingly, it is suggested that selective volatility played a more important part than in chondrites, and that many relatively volatile elements were lost from the earth. On the other hand the earth may have retained essentially the primordial abundances of elements which are not readily volatile under high-temperature, reducing conditions. A detailed study of the earths chemical composition supports this hypothesis. It is possible to construct a self consistent model from the primordial abundances of elements which are not readily volatile under high-temperature reducing conditions. The model implies the presence of silicon as an important component of the earths core. Independent evidence supporting this implication is cited. The distribution and fractionation of oxyphile non-volatile elements imply that much or all of the mantle has been subjected to complete or partial melting at some stage in its history. In contrast to the non-volatile elements, it appears that the earth has suffered strong depletion in a large number of volatile elements—Na, K, Rb, Cs, Zn, Cd, Hg, Bi, Tl, Pb, Cl, S and many others. It is suggested that loss of these elements by volatilization occurred during the primary accretion of the earth from primitive oxidised material, and that reduction, complete melting, formation of the core, and fractionation of the mantle occurred during and immediately after the primary accretion process. Studies of the abundance of siderophile elements in the mantle, of the mean oxidation state of the mantle and of the nature of the volatile components which have been degassed from the mantle show that the mantle is not and never has been in equilibrium with the core. This conclusion places an important constraint on the core-formation process. It is shown to be incompatible with the currently accepted theory that the material from which the earth accreted was composed of an intimate mixture of silicate and metal particles similar to ordinary chondrites. The formation of the earth by direct accretion and autoreduction of primitive material resulted in the generation of an enormous primitive atmosphere composed principally of CO and H2, together with the volatile elements mentioned above. It is suggested that this atmosphere subsequently escaped from the earth carrying the volatile elements mentioned above. Possible mechanisms of escape are discussed. In the terminal phase of accretion, the temperature was sufficiently high to reduce and volatilise magnesium and silicon monoxide from the infalling planetesimals and dust. At this stage, the condensed matter accreting on the earth consisted principally of metallic iron and calcium and aluminum silicates. When the primitive atmosphere was disrupted and escaped, the accompanying expansion and cooling caused precipitation of the non-volatile silicate components of the atmosphere in the form of planetesimals and smoke. Precipitation of this material, mainly as iron-poor magnesian silicates occurred in a sediment-ring around the earth. This material became mixed with primitive planetesimals possessing the composition of Type 1 carbonaceous chondrites, which had not accreted upon the earth. The sediment-ring of highly reduced magnesian silicate planetesimals and primitive oxidised planetesimals became unstable and coagulated to form the moon. The properties of the moon are discussed in terms of its formation from such material. Possible explanations of the moons density, luminescent properties, surface heterogeniety, thermal history and stress history emerge. A possibility that stoney meteorites are derived from the moon is also discussed. It is distinctly possible that ordinary chondrites may have formed by autoreduction and fractionation which occurred when primitive Type I carbonaceous chondrite planetesimals collided with the moon during its terminal period of formation. Other theories of lunar origin are briefly reviewed. The origins and internal constitution of the other terrestrial planets are discussed. Mercury is believed to have accreted from the solar nebula at an initially high temperature, maintained by an early stage of high solar luminosity. As a result, Mercury suffered depletion of magnesian silicates which were reduced and volatilised under these conditions. The abundance of iron, which was not volatilised was correspondingly increased, resulting in a high mean density for this planet. The evolution of Venus was very similar to that of the earth. Its mean state of oxidation may be slightly higher. The material from which Venus accreted possessed a higher C/H ratio than the source material of the earth. Differing atmospheric compositions are attributable to this factor. Mars is composed of highly oxidised primordial material, with little or no metal phase. Lack of reduction is attributed to the small content of carbonaceous material in the primordial material from which Mars accreted. Physical properties and the thermal history of Mars are discussed in terms of the proposed chemical constitution and the possibility of a self consistent solution is demonstrated.

Phase transformations and their bearing on the constitution and dynamics of the mantle

Abstract The bulk chemical composition of the Upper Mantle (“pyrolite”) is derived from experimental and petrological studies of the complementary relationships between basaltic magmas and refractory peridotites. The phase transformations which are experienced by pyrolite between depths of 100–800 km are reviewed in some detail, particularly with regard to their capacity to explain the seismic P and S velocity profiles throughout this region. The transition of olivine and pyroxene to β-(Mg,Fe)2SiO4 plus garnet provides a satisfactory explanation of the velocity changes associated with the 400 km discontinuity within the limits of error of the seismic velocity determinations. Seismic velocities between 400 and 650 km are likewise consistent with this region crystallizing as an assemblage of β,γ(Mg,Fe)2SiO4 plus garnet. The depth of the 650 km seismic discontinuity corresponds closely to the pressure at which (Mg,Fe)2SiO4 spinel disproportionates to MgSiO3 perovskite + (Mg,Fe)O magnesiowustite. This transformation is completed over a narrow depth interval (

Potassium, rubidium, and cesium in the Earth and Moon and the evolution of the mantle of the Earth☆

Estimates of the abundances of volatile, alkali elements (K, Rb, and Cs) in the bulk Silicate Earth vary considerably. The K and Rb abundances are constrained by the K/U (~ 1.3 × 104), K/Rb (~380), Rb/Sr (~0.03), and Ba/Rb (~11) ratios of the bulk Earth and by Sr, Nd, and Hf isotope systematics. The Cs abundance of the Silicate Earth is constrained by estimates of the RbCs ratios of the continental crust and mantle. The continental crust has a RbCs ratio of about 25, whereas the depleted MORB source and OIB plume source regions have a RbCs ratio of about 80. There is evidence suggestive of a secular change in the RbCs ratios of the depleted mantle, which may have been caused by continental crust formation and crust-mantle recycling processes. The RbCs ratio in the Silicate Earth is estimated to be about 28, based upon studies of the continental crust, MORB source, and OIB (plume) source. The continental crust contains about 37% of the total K present in the Silicate Earth, 50% of its Rb, and 55% of its Cs, whereas the residual mantle (the MORB and OIB source) contains about 20% of the K, 10% of the Rb, and only 4% of the Cs. Together, these reservoirs account for only about 60% of the total inventory of K, Rb, and Cs in the Earth today, indicating the existence of a less depleted reservoir in the mantle that contains the remainder of these elements. The average RbCs ratio for all lunar samples is about 22 and this is believed to represent the bulk RbCs ratio of the Moon. Within the limits of uncertainty which apply to both estimates, this ratio is similar to that of the Silicate Earth. Hence, we conclude that the existence of significant difference in the RbCs ratios of the Earth and Moon cannot be inferred from the presently available data base. Thus, we disagree with the claim of Kreutzberger et al. (1986) that the Moon has a significantly lower RbCs ratio than the Earth.

Chemical and genetic relationships among meteorites

Abstract Optical and X-ray studies have been made on the olivines, pyroxenes and metal phases from thirty-four chondrites for which chemical analyses exist. These data enable a selection of reliable analyses to be made, and confirm a conclusion first expressed by Prior —namely, that chondritos are samples of a homogeneous parental material which varied widely in its state of oxidation. There is an almost continuous range in oxidation states from carbonaceous chondrites, in which all the iron and nickel is oxidized, through to enstatite chondrites, in which all the iron is reduced, and which contain, in addition, reduced Si, Cr, Ca and P. The mineralogy and chemistry of silicate and metal phases in chondrites are dominated by this oxidation-reduction equilibrium. The original state of the parental chondritic material probably resembled that displayed by carbonaceous chondrites. This material was cold and oxidized, containing up to 5 per cent carbon, and 20 per cent water, together with obscure compounds of carbon, hydrogen, nitrogen, sulphur and other volatiles. Subsequently it was subjected to higher temperatures under such conditions that the carbon (and hydrogen) reacted with oxidized iron and nickel to form a metal phase in situ , whilst volatiles (principally water and CO 2 ) were lost. Mineralogical and textural evidence shows that the silicates in most chondrites crystallized from the liquid state at a temperature below 1000°C. A high pressure of volatiles (H 2 O and CO 2 ) must have been present to produce the observed melting point depression of the silicates. Primary chondritic textures are tuffaceous in nature and therefore of volcanic origin. They have formed by the rapid liberation of H 2 O and CO 2 from the chondritic magma. Sudden loss of volatiles has caused rapid crystallization and chondrule formation. The principal difference between formation of chondrites and terrestrial tuffs is due to the relatively low viscosity of chondritic magma, which has facilitated chondrule formation. Chondrites have subsequently been exposed to varying degrees of metamorphism, which has caused compaction and recrystallization. The range of pressures indicated is such that at least one of the parent bodies of chrondrites was of lunar size. The genesis of other groups of meteorites can be understood in terms of melting and differentiation of a small amount of parental chondritic material. Several lines of evidence indicate that the irons have crystallized under high pressures,—probably exceeding 30,000 atm. It therefore seems probable that melting and differentiation occurred near the centre of a parent meteoritic planet, and that the metal partially segregated to form a small core. Melting and differentiation of the parent meteoritic planet (or planets) occurred about 4.5 × 10 9 years ago. Subsequent cooling below 450°C occurred within approximately 10 8 years. Cooling of the small molten core occurred by adiabatic heat exchange with the outer chondritic mantle, which had been rapidly cooled to about 300°C, during the endothermic volcanic phase of evolution. The meteoritic planet(s) broke up less than 10 9 years after melting and differentiation, and the fragments have since been colliding and becoming further reduced in size, thus forming the asteroids. Some suggestions regarding the cause of the initial break-up are hazarded.

Some aspects of the thermal evolution of the earth

Empirical data relating to the thermal history of the earth are examined. Recent astronomic and geochemical evidence strongly suggests that the earth formed by accretion from an initially low-temperature gas-dust cloud of solar composition. The distribution of U, Pb, Th and K within the earth imply that it passed through a melting or partial melting process about 4.5 × 109 years ago. This conclusion is confirmed if the core is assumed to consist dominantly of iron-nickel. Formation of the core, which likewise occurred about 4.5 × 109 years ago would liberate sufficient gravitational energy to cause melting. Evidence in favour of melting is also provided by analogy with meteorites. An examination is made of possible causes of this early melting stage and it is concluded that gravitational energy is chiefly responsible. Radioactive heating does not appear to be important. A critical factor in the early heating and chemical evolution is the interaction of accreting dust falling with high velocity into the primitive reducing atmosphere surrounding the earth. Because of this interaction, a metallic phase is produced by reduction. The distribution of temperature within the earth 4.5 × 109 years ago will be given by the melting-point gradient. Recent data on the electrical conductivity of the mantle and the melting point of metals under high pressures suggest that the present temperature distribution is much less than the melting point gradient. This implies that the earth has cooled considerably. The inferred cooling is consistent with present data on the abundance of radioactive elements in meteorites and in the earth, and also with possible modes of internal heat transfer—particularly convection and radiation.

Silicon in the metal phase of enstatite chondrites and some geochemical implications

Abstract Some recent hypotheses have proposed that silicon occurs as a major component of the earths core. Since the chemical evolution of the earth may have been analogous in some respects to that which occurred in meteorites, a search for silicon was made in the metal phases of chondrites. Using X-ray and chemical methods, it was established that between 2 and 6 atomic per cent of silicon occurred in solid solution in the metal phases of all eight enstatite chondrites which were examined. No silicon was found in the metal phase of ordinary chondrites. Evidence relating to the possible occurrence of silicon in the earths core is reviewed, and the significance of the new meteoritic data in relation to this hypothesis is discussed in some detail. A study of the conditions which accompany the segregation of a core, both in the earth and in the parent meteoritic body, lead to the conclusion that the core is unlikely to be in chemical equilibrium with the surrounding mantle. One of the principal effects of this disequilibrium is the diffusion of silicon from the core into the adjacent region of the mantle, resulting in the reduction of oxidized iron and precipitation of metallic iron. This effect may be responsible for the anomalous seismic velocity gradients which are observed in the bottom 200 km of the mantle. This region is likely to be mechanically unstable, owing to the tendency of precipitated iron to collect and sink into the core. Owing to inhomogenieties in ionic and electronic transport properties in this region, attainment of chemical equilibrium across the core-mantle boundary is accompanied by generation of electric currents. These may be relevant to the origin of the earths magnetic field, and may also affect the electromagnetic coupling between core and mantle.

Prediction and confirmation of olivine—spinel transition in Ni2SiO4

Abstract It is possible to predict the pressure at which a compound will transform to a denser polymorph by a study of solid solutions of the given compound with compounds possessing closer atomic packing. This enables the free energy of transition and the density of the closer packed polymorph to be found. From these data, the transition pressure may be calculated. The method has been applied to calculate the pressures at which the olivine—spinel transition should occur in Ni2SiO4. The data required for the calculations were provided from an experimental investigation of solid-solution equilibria in the system Ni2SiO4-Ni2GeO4 between 650°C and 1500°C. The transition curve was then calculated from the zero pressure phase diagram. Pressures required to cause Ni2SiO4 (olivine structure) to invert to spinel rose from 16,000 bars at 650°C, to 58,000 bars at 1500°C. A direct search was made for the spinel polymorph of Ni2SiO4 using high-pressure techniques. An olivine-spinel transition in Ni2SiO4 was discovered at 650°C and 18,000 ± 5,000 bars. This result is in excellent agreement with the theoretical prediction, and confirms the validity and usefulness of the prediction method. Ni2SiO4 spinel is 8.7 per cent denser than the olivine, and possesses a lattice parameter of 8.045 A. The same prediction method, when applied to the non-metals Si, Ge, P and B which display solid solubility in α-iron, suggests that they should invert to metals at high pressure.

The Novo Urei meteorite

Abstract The occurrence of diamond in Novo Urei has been confirmed by X-ray diffraction. Chemical and mineralogical studies of this meteorite suggest that it is an intensely metamorphosed chondrite.

Core-mantle equilibrium - Comments on a paper by R. Brett

Abstract Bretts arguments in the preceding paper are critically reviewed. It is concluded that the abundances of Ni, Co, Cu, Au and Pt in the upper mantle, the oxidation state of this region and the nature of the volatiles inferred to have been degassed from the upper mantle are not readily explained if the earth has accreted from a well mixed reservoir of preexisting metal and silicate particles in the solar nebula which were equilibrated within the earth prior to separation of the core. The data can be readily explained if it is assumed that the earth accreted inhomogeneously in a state which was initially out of overall chemical equilibrium and that equilibrium, although approached, was not finally achieved during core-segregation.