Mechanisms of metal–silicate equilibration in the terrestrial magma ocean     

《Earth and Planetary Science Letters》2003,205(3-4):239-255

It has been proposed that the high concentrations of moderately siderophile elements (e.g. Ni and Co) in the Earth’s mantle are the result of metal–silicate equilibration at the base of a deep magma ocean that formed during Earth’s accretion. According to this model, liquid metal ponds at the base of the magma ocean and, after equilibrating chemically with the overlying silicate liquid at high pressure (e.g. 25–30 GPa), descends further as large diapirs to form the core. Here we investigate the kinetics of metal–silicate equilibration in order to test this model and place new constraints on processes of core formation. We investigate two models: (1) Reaction between a layer of segregated liquid metal and overlying silicate liquid at the base of a convecting magma ocean, as described above. (2) Reaction between dispersed metal droplets and silicate liquid in a magma ocean. In the liquid-metal layer model, the convection velocity of the magma ocean controls both the equilibration rate and the rate at which the magma ocean cools. Results indicate that time scales of chemical equilibration are two to three orders of magnitude longer than the time scales of cooling and crystallization of the magma ocean. In the falling metal droplet model, the droplet size and settling velocity are critical parameters that we determine from fluid dynamics. For likely silicate liquid viscosities, the stable droplet diameter is estimated to be ∼1 cm and the settling velocity ∼0.5 m/s. Using such parameters, liquid metal droplets are predicted to equilibrate chemically after falling a distance of <200 m in a magma ocean. The models indicate that the concentrations of moderately siderophile elements in the mantle could be the result of chemical interaction between settling metal droplets and silicate liquid in a magma ocean but not between a segregated layer of liquid metal and overlying silicate liquid at the base of the magma ocean. Finally, due to fractionation effects, the depth of the magma ocean could have been significantly different from the value suggested by the apparent equilibration pressure.  相似文献   

Melting temperatures of the Allende meteorite: implications for a Hadean magma ocean     

Carl B. Agee 《Physics of the Earth and Planetary Interiors》1997,100(1-4):41-47

Melting temperatures of the silicate fraction of the Allende CV3 meteorite, at upper mantle pressures, are several hundred degrees lower than that of fertile peridotite xenoliths or ‘pyrolite’. If the Earth accreted from material similar to chondrites, then deep mantle melting could have occurred with a relatively modest heat budget. It is concluded that initial chemical composition is an important variable in realistic magma ocean models.  相似文献   

Core formation and chemical equilibrium in the Earth—I. Physical considerations     

Shun-ichiro Karato  V. Rama Murthy 《Physics of the Earth and Planetary Interiors》1997,100(1-4):61-79

Current models of planetary formation suggest a hierarchy in the size of planetesimals from which planets were formed, causing formation of a hot magma ocean through which metal-silicate separation (core formation) may have occurred. We analyze chemical equilibrium during metal-silicate separation and show that the size of iron as well as the thermodynamic conditions of equilibrium plays a key role in determining the chemistry of the mantle (silicates) and core (iron) after core formation. A fluid dynamical analysis shows that the hydrodynamically stable size of iron droplets is less than 10⁻² m for which both chemical and thermal equilibrium should have been established during the separation from the surrounding silicate magma. However, iron may have been separated from silicates as larger bodies when accumulation of iron on rheological boundaries and resultant large scale gravitational instability occurred or when the core of colliding planetesimals directly plunged into the pre-existing core. In these cases, iron to form the core will be chemically in dis-equilibrium with surrounding silicates during separation. The relative role of equilibrium and dis-equilibrium separation has been examined taking into account of the effects of rheological structure of a growing earth that contains a completely molten near surface layer followed by a partially molten deep magma ocean and finally a solid innermost proto-nucleus. We show that the separation of iron through a completely molten magma ocean likely occurred with iron droplets assuming a hydrodynamically stable size ( 10⁻² m) at chemical equilibrium, but the sinking iron droplets are likely to have been accumulated on top of the partially molten layer to form a layer (or a lake) of molten iron which sank to deeper portions as a larger droplet. The degree of chemical equilibrium during this process is determined by the size of droplets which is in turn controlled by the size and frequency of accreting planetesimals and the rheological properties of silicate matrix. For a plausible range of parameters, most of the iron that formed the core is likely to have been separated as large droplets or bodies and chemical equilibrium with silicate occurred only at relatively low temperatures and pressures in a shallow magma ocean or in their parental bodies. However, a small portion of iron that separated as small droplets was in chemical equilibrium with silicate at high temperatures and pressures in a deep magma ocean during the later stage of core formation. Therefore the chemistry of the core is mostly controlled by the chemical equilibrium with silicates at relatively low temperatures and pressures, whereas the chemistry of the mantle controlled by the interaction with iron during core formation is likely to have been determined mostly by the chemical equilibrium with a small amount of iron at high temperatures and pressures.  相似文献   

Creep of crystals: J.P. Poirier,Cambridge University Press, 1985, xii + 260 pp., £27.50, US$ 49.50, ISBN 0-521-26177-5     

B.K. Atkinson 《Physics of the Earth and Planetary Interiors》1985,41(1):70-71

Fractional crystallization behaviour of a magma ocean extending to lower mantle depths was deduced from estimations of melting relations for the deep mantle and the density relationships between ultrabasic liquid and mantle minerals. The accretional growth of the Earth necessarily involves a molten zone (magma ocean) in the outer layer of the growing Earth. The fractionation by melting during accretion results in primary stratification composed of a molten ultrabasic upper mantle (magma ocean), a perovskite-rich lower mantle, and an iron core. A certain amount of Al₂O₃ and CaO was removed from the magma ocean and retained in the lower mantle due to eclogite fractionation in the early stage of accretion and the perovskite fractionation in the later stage of accretion. Models of the stratification of the upper mantle arising from fractional crystallization of the magma ocean and subsequent convective disturbance were deduced on the basis of estimations of melting relations for the deep mantle and the density relationships between the ultrabasic liquid and mantle minerals. The stratification of the mantle, which is consistent with geophysical constraints is as follows; the upper mantle is composed of two layers, the upper olivine-rich layer and the lower garnet-rich layer with a thickness around 200 km, and the lower mantle with a perovskite-rich composition. In this model, both the 400 and 650 km discontinuities are the chemical boundaries.  相似文献   

The geochemistry of oceanic basalts in the vicinity of transform faults: Observations and implications     

Charles H. Langmuir  John F. Bender 《Earth and Planetary Science Letters》1984,69(1):107-127

Consideration of geochemical data from basalts formed near major Atlantic and Pacific transform faults reveals two significant sets of observations. First, compared to basalts formed far from the transform, basalts near the ridge/transform intersection have, for the same MgO contents, higher abundances of TiO₂ and other incompatible elements, higher La/Sm and La/Yb ratios, and often higher FeO. These enrichments are distinct from and occur in addition to the more variable and fractionated compositions which have been frequently noted [10–13]. Modeling of this “transform fault effect” using data from the Tamayo/EPR intersection suggests the chemical systematics are caused by decreasing extents of melting as the transform is approached.Second, there are chemical discontinuities in the major element, trace element and isotopic chemistry of basalts across many transforms. These “transform discontinuities” occur in normal ocean crust as well as around hot spots.Consideration of the melting zone in the mantle suggests that the transform fault effect is a natural consequence of the ridge/transform plate boundary. The melting zone beneath a ridge segment must terminate across the transform, leading to lower extents of melting at the transform edge. The surface manifestation of the change in the melting zone may be affected by the age of the transform offset, the spreading rate, the transform spacing and the interaction of mantle flow with the local thermal structure; it may be obscured by episodic magma chamber processes and mantle heterogeneity.The significance of transform discontinuities depends on whether they persist with age. If they do not, then temporally variable crust-forming processes may produce changes along a flow line similar to those at zero age across a transform. If, on the other hand, a discontinuity persists with age, then the transform may be related to a fundamental discontinuity in the underlying mantle. Long-lived transform discontinuities would have profound implications for the nature of plate motions, mantle convection and mantle heterogeneity.  相似文献   

A re-evaluation of metal diapir breakup and equilibration in terrestrial magma oceans     

Henri Samuel 《Earth and Planetary Science Letters》2012

Due to mechanisms such as impact heating, early atmospheric thermal blanketing, and radioactive heating, the presence of at least one global magma ocean stage in the early histories of terrestrial planets seems unavoidable. In such a context, a key question to constrain the early thermo–chemical evolution of the Earth is how much iron diapirs provided by differentiated impactors emulsified during their sinking towards the bottom of an early magma ocean.In the past years, several workers have focused on this question, using however various approaches and making different assumptions. While most studies favor rapid breakup and equilibration of iron bodies during their sinking through the magma ocean, recent work suggests that iron bodies of size comparable or greater than a few tens of kilometers may preserve most of their initial volume as they reach the bottom of a magma ocean, therefore leading to metal–silicate disequilibrium.To clarify the discrepancies and the differences among studies I have conducted a series of numerical simulations and theoretical calculations to derive the conditions and the timing for the breakup of metal diapirs of any size, sinking through a silicate magma ocean, with a large range of plausible viscosity values. The obtained breakup criterion is used to derive stable diapir sizes and their ability to equilibrate with the surrounding silicates. I show that for plausible magma ocean viscosities, diapirs with initial radii smaller than the thickness of a magma ocean rapidly break up into stable diapir sizes smaller than 0.2 m, at which metal–silicate equilibration is rapidly achieved.  相似文献   

The age of ferroan anorthosite 60025: oldest crust on a young Moon?     

Richard W. Carlson  Gunter W. Lugmair 《Earth and Planetary Science Letters》1988,90(2)

SmNd isotopic data for mineral separates from the ferroan anorthosite 60025 define a precise isochron of 4.44 ± 0.02Ga age. This age is roughly 110 m.y. younger than the formation of the first large solid objects in the solar nebula, as recorded by the radiometric ages of the differentiated meteorites. In the magma ocean model for early lunar differentiation, ferroan anorthosites are the first crustal rocks to form on the Moon. If the Moon is as old as the oldest meteorites, the relatively young age determined for 60025 implies either that the magma ocean did not form synchronously with lunar formation, or that the magma ocean required over 100 m.y. before reaching the stage of ferroan anorthosite crystallization. Alternatively, we propose that the accumulated body of radiogenic isotope data for lunar rocks permit the Moon to be as young as 4.44–4.51 Ga. If so, isotopic evidence for chemical differentiation on the Earth at about this same time suggests that the formation of the Moon is reflected in the chemical evolution of the Earth. This, in turn, is consistent with the idea that the materials that now make up the Moon were derived from the Earth, perhaps ejected by collision between the Earth and another very large planetesimal during the final stages of accumulation of the terrestrial planets. Terrestrial origin models for the Moon lessen the requirement that the Earth and Moon each have near chondritic relative abundances of the refractory elements and could require that certain chemical and isotopic characteristics of both bodies be considered in the framework of the chemical mass-balance of the combined Earth-Moon system.  相似文献   

Oceanic fissure eruptions,subvolcanic intrusions and volcanic magma Chambers     

F. Machado 《Bulletin of Volcanology》1969,33(4):1229-1236

Rifting along the mid-Atlantic ridge seems to have been accompanied by fissure eruptions which flooded the ocean bottom. Locally these plateau lavas rose above sea level and erosion revealed plutonic bodies emplaced within them. There is also some evidence of shallow magma chambers feeding surface volcanism. All these facts can be conveniently interpreted by assuming fractional melting of the upper mantle, at depths below about 50 km, and a pulsation of the pressure, produced by a varying gravitation, which seems capable of squeezing the molten fraction and of fracturing the solid crust above. Magma chambers can then be formed, probably by subterranean cauldron subsidence of Scottish type, they can leed surface volcanoes and will eventually solidify as plutonic bodies. Phase changes of eclogite, possibly present in the oceanic upper mantle, could also explain the uplift of island platforms.  相似文献   

A worldwide comparison of alkali olivine basalts and their differentiation trends     

Rudy R. Schwarzer  John J.W. Rogers 《Earth and Planetary Science Letters》1974,23(3):286-296

Worldwide alkali olivine basalts (AOB) and their differentiation series have been subdivided into continental, oceanic, or island-arc assemblages according to the inferred crustal environment at their time and place of eruption. No systematic differences have been found in major element composition of the AOB's from these three different environments. As plotted on (Na₂O + K₂O) vs. SiO₂ and AMF diagrams, AOB differentiation trends also show no differences between environments. Thus, AOB appears to be a primary magma generated at sufficient depth in the mantle that its major element content is unaffected by chemical or thermal differences between mantle regions underlying continents, ocean basins, or island arcs. The major element chemistry of AOB is also apparently unaffected by passage through different types of crust.  相似文献   

Komatiites and the structure of the Archaean mantle     

Euan G. Nisbet  David Walker 《Earth and Planetary Science Letters》1982,60(1):105-113

The existence of Archaean komatiites with eruption temperatures greater than 1650°C requires that the mantle be vertically differentiated by the time of komatiite eruption. If in the unlikely event that undifferentiated mantle had survived primordial planetary differentiation and had been hot enough to deliver 1650°C komatiite, it would have been extensively molten to depths of ～250 km, resulting in rapid, profound, vertical differentiation anyway. During primordial differentiation (or Archaean komatiite petrogenesis) the high density and compressibility of ultrabasic melt allowed storage of a global melt layer beneath a buoyant residue of dunite and/or harzburgite. This refractory cap segregated by extraction of melt both upwards and downwards from the depth at which the density contrast between crystals and liquid vanishes. Eruption of komatiite from the melt layer by corrosion of the cap was the Archaean earth's principal means of dissipating excess heat. This subterranean magma ocean precluded vertical homogenization of the Archaean mantle by convection but effectively absorbed lateral mantle heterogeneities and imposed the relative uniformity of maximum eruption temperature and MgO contents (～32%) seen in primitive Archaean komatiites on all continents.Verification of the postulated density relations of liquids and crystals to 100 kbar becomes a pressing concern in view of the expected consequences these relations may have had.  相似文献   

Magma migration beneath an ocean ridge     

J.L. Ahern  D.L. Turcotte 《Earth and Planetary Science Letters》1979,45(1):115-122

Basaltic volcanism which forms the oceanic crust at mid-ocean ridges is the result of pressure release melting associated with ascending mantle convection. We present a model that gives the distribution of melting beneath the ridge and the subsequent migration of magma through the asthenosphere. In order to produce the degree of partial melting associated with the basaltic rocks making up the ocean crust, melting must extend to a depth of at least 70 km. Small degrees of partial melting are expected to result in an interconnected permeability along grain intersections. Due to the differential buoyancy of the magma relative to the residual solid the magma will be rapidly driven upwards. Solid-state creep allows the solid matrix to collapse as the magma migrates upwards and the lithostatic pressure in the matrix is nearly equal to the fluid pressure in the magma. The percentage partial melt present is only slightly greater than that necessary for the development of interconnected permeability and is much less than the degree of partial melting. The first partial melt fraction produced at the greatest depths migrates upwards and mixes with the later partial melt fractions produced at shallower depths. The uniformity of this mixing will have a profound effect on the chemistry of the basalts of the oceanic crust.  相似文献   

The acid rocks of the ocean basins     

P. E. Baker  I. McReath 《Bulletin of Volcanology》1972,36(2):328-341

The occurrences of acid rocks in the ocean basins are reviewed, and their genesis is discussed. It is concluded that fractional crystallisation of basalt magma at low pressure or at high pressure could adequately explain the dominant chemical features of most occurrences. For those cases where anomalously high initial⁸⁷Sr/⁸⁶Sr ratios compared to the basalts are observed, formation by repeated remobilisation of early formed acid material associated with the volcanic pile is suggested. The Daly gap is likely to be a distortion introduced by the limitations of subaerial sampling, and subsequent bias during analysis and interpretation. Although volatile transfer could be a subsidiary factor in the evolution of the acid rocks, there is insufficient data available in any single case to demonstrate the effectiveness of such a process.  相似文献   

Core formation and chemical equilibrium in the Earth—II: Chemical consequences for the mantle and core     

V. Rama Murthy  Shun-ichiro Karato 《Physics of the Earth and Planetary Interiors》1997,100(1-4):81-95

We present here a new model of core formation which is based on the current understanding of planetary accretion and discuss its implications for the chemistry of the Earth's mantle and core. Formation of the Earth by hierarchical accretion of progressively larger bodies on a time scale much longer than that of solid body differentiation in the nebula indicates that a significant fraction of metal in the core could be inherited from preterrestrially differentiated planetesimals. An analysis of the segregation of this iron to form the core suggests that most of the metal settles to the core without interaction with silicates; only a small fraction of the metal chemically equilibrates at high temperatures and pressures with the silicates. The siderophile element abundances in the mantle are considered to be a consequence of a two-step equilibration with iron, once preterrestrially in the planetesimals at low temperatures and pressures, and later in the Earth at high temperatures and pressures. The highly siderophile elements such as Re, Au and the platinum group elements in the mantle are essentially excluded from silicates from the preterrestrial equilibration. We attribute the abundances of these elements in the mantle to the later equilibration in the Earth at substantially reduced metal-silicate partition coefficients (D^met/sil), for which there is a considerable experimental evidence now. Mass balance considerations constrain the fraction of core metal involved in such an equilibration at approximately 0.3 – 0.5%. The model accounts for the levels and the near-chondritic ratios of the highly siderophile elements in the mantle. The mantle abundances of the less siderophile elements are largely determined by preterrestrial metal-silicate equilibrium and are not significantly affected by the second equilibration. The extreme depletion of sulfur and the lack of silicate melt-sulfide signature in the noble metal abundances in the mantle are natural consequences of this mode of core formation. Sulfur was added to the magma ocean during the high-T, high-P equilibration in the Earth, not extracted from it by sulfide segregation to the core. Except for Ni and Co, the overall siderophile abundances of the mantle can be well matched in this two-step equilibration model.

The mantle characteristics of Ni and Co are unique to the Earth and hence suggest a terrestrial process as the likely cause. One such process is the flotation and addition of olivine to the primitive upper mantle. In our model of core formation, neither the elemental and isotopic data of Re---Os, nor the low sulfur content of the mantle remains as an objection to the existence of a magma ocean and olivine flotation.

The small fraction of core metal that equilibrates with silicates at high T and P suggests that the light elements O, Si or H are unimportant in the core, leaving S (and possibly C) as prime candidates. Sulfur, as FeS associated with incoming iron metal, is directly sequestered to the core along with the bulk of the iron metal. It appears unlikely that other light elements can be added to the core after its formation. U and Th are excluded from the core but the model allows for entry of some K; however, the extent to which K serves as a heat source in the core remains uncertain.

The model is testable in two ways. One is by investigation of the metal-silicate partitioning at high temperatures and pressures under magma ocean conditions to determine if the (D^met/sil) values are lowered to the levels required in the model. The other is by experiments to determine if a solvus closure between metal and silicate liquids occurs at high temperatures relevant to a magma ocean.  相似文献   

Trace elements in ocean ridge basalts     

R.W. Kay  N.J. Hubbard 《Earth and Planetary Science Letters》1978,38(1):95-116

A correlary of sea floor spreading is that the production rate of ocean ridge basalts exceeds that of all other volcanic rocks on the earth combined. Basalts of the ocean ridges bring with them a continuous record in space and time of the chemical characteristics of the underlying mantle. The chemical record is once removed, due to chemical fractionation during partial melting. Chemical fractionations can be evaluated by assuming that peridotite melting has proceeded to an olivine-orthopyroxene stage, in which case the ratios of a number of magmaphile elements in the extracted melt closely match the ratios in the mantle. Comparison of ocean ridge basalts and chondritic meteorites reveals systematic patterns of element fractionation, and what is probably a double depletion in some elements. The first depletion is in volatile elements and is due to high accretion temperatures of a large percentage of the earth from the solar nebula. The second depletion is in the largest, most highly charged lithophile elements (“incompatible elements”), probably because the mantle source of the basalts was melted previously, and the melt, enriched in these elements, was removed. Migration of melt relative to solid under ocean ridges and oceanic plates, element fractionation at subduction zones, and fractional melting of amphibolite in the Precambrian are possible mechanisms for depleting the mantle in incompatible elements. Ratios of transition metals in the mantle source of ocean ridge basalts are close to chondritic, and contrast to the extreme depletion of refractory siderophile elements, the reason for which remains uncertain. Variation of ocean ridge basalt chemistry along the length of the ridge has been correlated with ridge elevation. Thus chemically anomalous ridge segments up to 1000 km long appear to broadly coincide with regions of high magma production (plumes, hot spots). Basalt heterogeneity at a single location indicates mantle heterogeneity on a smaller scale. Variation of ocean ridge basalt chemistry with time has not been established, in fact, criteria for recognizing old oceanic crust in ophiolite terrains are currently under debate. The similarity of rare earth element patterns in basalt from ocean ridges, back-arc basins, some young island arcs, and some continental flood basalts illustrates the dangers of tectonic labeling by rare earth element pattern.  相似文献   

Chemical fractionation in the silicate vapor atmosphere of the Earth     

Kaveh Pahlevan  David J. Stevenson  John M. Eiler 《Earth and Planetary Science Letters》2011,301(3-4):433-443

Despite its importance to questions of lunar origin, the chemical composition of the Moon is not precisely known. In recent years, however, the isotopic composition of lunar samples has been determined to high precision and found to be indistinguishable from the terrestrial mantle despite widespread isotopic heterogeneity in the Solar System. In the context of the giant-impact hypothesis, this level of isotopic homogeneity can evolve if the proto-lunar disk and post-impact Earth undergo turbulent mixing into a single uniform reservoir while the system is extensively molten and partially vaporized. In the absence of liquid–vapor separation, such a model leads to the lunar inheritance of the chemical composition of the terrestrial magma ocean. Hence, the turbulent mixing model raises the question of how chemical differences arose between the silicate Earth and Moon. Here we explore the consequences of liquid–vapor separation in one of the settings relevant to the lunar composition: the silicate vapor atmosphere of the post-giant-impact Earth. We use a model atmosphere to quantify the extent to which rainout can generate chemical differences by enriching the upper atmosphere in the vapor, and show that plausible parameters can generate the postulated enhancement in the FeO/MgO ratio of the silicate Moon relative to the Earth's mantle. Moreover, we show that liquid–vapor separation also generates measurable mass-dependent isotopic offsets between the silicate Earth and Moon and that precise silicon isotope measurements can be used to constrain the degree of chemical fractionation during this earliest period of lunar history. An approach of this kind has the potential to resolve long-standing questions on the lunar chemical composition.  相似文献   

Recent volcanism in Masaya-Granada Area,Nicaragua     

T. Ui 《Bulletin of Volcanology》1972,36(1):174-190

Masaya-Granada area is located in the middle part of the Central American volcanic zone. A basaltic shield volcano with a caldera, an acidic pyroclastic flow plateau with a caldera, cinder cones, maars, a lava dome and a composite andesitic volcano were formed by recent volcanic activities. Magmas of basic and intermediate ejecta are supposed to be formed by partial melting of the upper mantle material. Most of basalts and andesites was derived from common parental magma after crystallization differentiation history, but some basalts, which have extremely high MgO content and low K₂O content might be derived from primary magma of different type. There is no evidence to deny the possibility of differentiation product of acidic rock from basic magma, but compositional gap on variation diagram suggest the possibility of partial melting origin. Strike-slip fault systems might have been formed in association with plate movement, and fluidal basaltic magma was erupted also along these fault zones.  相似文献   

Internal constitution and evolution of the moon     

Sean C. Solomon  M. Nafi Toksöz 《Physics of the Earth and Planetary Interiors》1973,7(1):15-38

The composition, structure and evolution of the moon's interior are narrowly constrained by a large assortment of physical and chemical data. Models of the thermal evolution of the moon that fit the chronology of igneous activity on the lunar surface, the stress history of the lunar lithosphere implied by the presence of mascons, and the surface concentrations of radioactive elements, involve extensive differentiation early in lunar history. This differentiation may be the result of rapid accretion and large-scale melting or of primary chemical layering during accretion; differences in present-day temperatures for these two possibilities are significant only in the inner 1000 km of the moon and may not be resolvable. If the Apollo 15 heat-flow result is representative of the moon, the average uranium concentration in the moon is 0.05–0.08 p.p.m.Density models for the moon, including the effects of temperature and pressure, can be made to satisfy the mass and moment of inertia of the moon and the presence of a low-density crust inferred from seismic refraction studies only if the lunar mantle is chemically or mineralogically inhomogeneous. The upper mantle must exceed the density of the lower mantle at similar conditions by at least 5%. The average mantle density is that of a pyroxenite or olivine pyroxenite, though the density of the upper mantle may exceed 3.5 g/cm³. The density of the lower mantle is less than that of the combined crust and upper mantle at similar temperature and pressure, thus reinforcing arguments for early moon-wide differentiation of both major and minor elements. The suggested density inversion is gravitationally unstable and implies stresses in the mantle 2–5 times those associated with the lunar gravitational field, a difficulty that can be explained or avoided by: (1) adopting lower values for the moment of inertia and/or crustal thickness, or (2) postulating that the strength of the lower mantle increases with depth or with time, either of which is possible for certain combinations of composition and thermal evolution.A small iron-rich core in the moon cannot be excluded by the moon's mass and moment of inertia. If such a core were molten at the time lunar surface rocks acquired remanent magnetization, then thermal-history models with initially cold interiors strongly depleted in radioactive heat sources as a primary accretional feature must be excluded. Further, the presence of ～||pre|40 K in a FeFeS core could significantly alter the thermal evolution and estimated present-day temperatures of the deep lunar interior.  相似文献   

Temporal change in chemistry of magma source under Central Kyushu,Southwest Japan: progressive contamination of mantle wedge     

Setsuya Nakada  Hiroki Kamata 《Bulletin of Volcanology》1991,53(3):182-194

Volcanism related to subduction of the Philippine Sea (PHS) plate began in Central Kyushu at 5 Ma, after a pause of igneous activity lasting about 10 m.y. It formed a large volcano-tectonic depression, the Hohi volcanic zone (HVZ), and has continued to the present at a decreasing eruption rate. The products are largely andesite and dacite, which became enriched in K with time. The proportion of tholeiitic to calc alkalic rocks also increases with time. Calc-alkalic high-Mg basaltic andesites (YbBs) were erupted in the early stage of the HVZ activity (5–3 Ma), and high-alumina basalts (KjBs) were erupted in the later stage (2–0 Ma). In contrast to the basalts in the HVZ, Northwest Kyushu basalts (NWKBs) have been erupted on the backarc side of the HVZ since 11 Ma, and hence are not related to the PHS plate subduction. They are mainly high-alkali tholeiitic to alkali basalt that shows no notable chemical change with time. NWKB, YbB, and KjB have MORB-normalized incompatible-element spectra that differ from each other, as is well expressed in both Nb and Sr anomalies. The patterns of KjB and NWKB are typical of those for island-arc basalt (IAB) and ocean-island basalt (OIB), respectively. YbB shows a pattern intermediate between the two. We suggest that the magma source beneath the HVZ changed in composition from an OIB-type mantle to an IAB-type mantle as the subduction of PHS plate advanced. However, the magma source remained fertile under Northwest Kyushu. In order to explain the temporal change of source mantle beneath the HVZ, we propose a model for progressive contamination of the mantle wedge, in which three processes (contamination by a slab-derived component, subtraction of magma from the mantle, and mixing of the mantle residue and slab-derived component) are repeated as subduction continues. As long as the progressive contamination of mantle wedge proceeds, its trace-element composition converges at a steady-state value for a short period. This value does not depend on the initial composition of the mantle wedge but instead on the composition of the slab-derived component. The trace-element composition of the magma produced in such a mantle wedge approaches that of the slab-derived component with time, but the major-element composition is determined by the phase relations of mantle peridotite. The slab-derived component may be basaltic liquid that is partially melted from rutile-bearing eclogite.  相似文献   

Some remarks on contact anatexis     

V. Gottini 《Bulletin of Volcanology》1970,34(2):406-413

The chemical variability of the products of contact-anatexis, completely different from the normal trend of magmatic differentiation, may be explained by the quantitative variation of gaseous transfer, according to the state of the basaltic magma which may be pyromagma or hypomagma at the contact with the surrounding sialic rocks. Therefore, two types of contact-anatexis must be distinguished: 1st.Anatexis at the contact with pyromagma. If the tectonical conditions are favourable, then the basaltic magma rises so high in the sialic crust that the gas tension overcomes the hydrostatic pressure. A gas phase will separate and cause a considerable gas transfer by which pneumatophilic substances (Na, Fe, Ti etc.) are supplied to the overlying anatectic magma. 2nd.Anatexis at the contact with hypomagma. If the rising basaltic magma cannot reach very high levels in the sialic crust, then the gas tension remains lower than the hydrostatic pressure, and the gases are molecularly dispersed within the melt. The gas transfer will be insignificant, and the anatexis is merely due to the supply of heat without any appreciable change of the chemical composition of the anatectic magma.  相似文献   

Mass balance and phase density constraints on early differentiation of chondritic mantle     

Carl B. Agee  David Walker 《Earth and Planetary Science Letters》1988,90(2)

A surprisingly simple and precise major element mass balance is consistent with derivation of average upper mantle peridotite from a partially molten chondritic Earth by subtraction of perovskite and addition of olivine. Majorite involvement is precluded unless some as yet unidentified components play a role. Perovskite subtraction during a primordial melting event is expected to occur by crystal fractionation at depth, while olivine addition is accomplished by a combination of buoyancy mechanisms: crystal flotation from a deep layer of melt buried by its own compressibility to the base of the solidifying upper mantle and subsequent solid state convection of this buoyant magnesian olivine upward. These processes are consistent with known density relations of crystals and liquid at very high pressure. Mass balance predicts that the residual magma body at depth after supplying olivine by flotation upward can be komatiitic. Distribution of originally C1 chrondritic bulk Earth material a few 100 m.y. after primordial differentiation is solid peridotite upper mantle, perovskite lower mantle, and a komatiitic liquid sandwich horizon.  相似文献