The Density of Hydrous Magmatic Liquids

Density measurements on several hydrous (相似文献   

Diffusion of Cesium Ions in H2O-Saturated Granitic Melt

The intrinsic self-diffusion coefficient (D) for cesium-134 in a granitic melt containing 6.1 to 6.3 percent (by weight) of dissolved H(2)O is three to four orders of magnitude higher than the values reported for chemically similar but dry granitic glass. For the temperature interval 700 degrees to 800 degrees C and a confining pressure of 2 kilobars, cesium diffusivity is given by D = 7.19 x 10(-5) exp (- 19.52 x 10(3)/RT), where R is the gas constant and T is the absolute temperature; the activation energy of about 20 kilocalories per mole is less than half the value for H(2)O-free glass. The observed increase in ionic mobility that accompanies solution of H(2)O implies sharply reduced equilibration times for chemical processes that occur in H(2)2O-bearing silicate melts.     

Activation volume for creep in the upper mantle

The activation volume for creep, V*, of olivine-rich rocks has been determined in pressure-differential creep experiments on dunite at temperatures from 1100 degrees to 1350 degrees C and confining pressures from 5 to 15 kilobars. Values of V* range from 10.6 to 15.4 cubic centimeters per mole with a mean value of 13.4 cubic centimeters per mole, near that expected for oxygen ion self-diffusion. The quantity V* is incorporated into existing flow equations; in combination with observations on naturally deformed mantle xenoliths, estimates are given of the variation with depth of stress, strain rate, and viscosity.     

Weakening of dunite by serpentine dehydration

A shear press has been used to determine the mechanical behavior of serpentinized dunite and forsterite at normal pressures to 50 kilobars, temperatures to 900 degrees C; and strain rates from 10(-1) to 10(-4) per second. The shear strength of dunite, containing less than 5 percent by volume of serpentine, is reduced by at least 30 percent as the temperature is raised from 300 degrees to 520 degrees C. Abundant kink bands develop at normal pressures above 35 kilobars at 27 degrees C and at lower pressures as the temperature is increased.     

Melting of diamond

Radiation from a Q-switched YAG laser, focused on the (100) face of a single crystal diamond anvil in a high-pressure diamond cell, caused a portion of the diamond anvil face to melt. Potassium bromide mixed with graphite was under pressure between the anvils when melting occurred. The diamond surface melted at pressures greater than approximately 120 kilobars and graphitized at lower pressures. Evidence for the melting and graphitization of the diamond was obtained by optical and scanning electron microscopy.     

Melting relations of the aliende meteorite

The proportions of major oxides in the Allende carbonaceous chondrite after partial reduction are remarkably similar to those in possible mantle material of the earth. When heated, the Allende meteorite generates a sulfide melt (47 percent iron, 25 percent nickel, and 24 percent sulfur by weight), a ferrobasaltic melt, and olivine with or without pyroxene, over a wide pressure range (5 to 25 kilobars). The silicate melt contains more sodium and less titanium than lunar ferrobasalts. An aggregate of the Allende chondrite rich in calcium and aluminum produces silica-undersaturated, calcium-rich melt and spinel over a wide pressure and temperature range. From these studies, it is suggested that the earth's core contains significant amounts of both nickel and sulfur and that a 3 : 2 mixture of Allende bulk sample and calcium- and aluminum-rich aggregates is closer in major element abundances than either of these components to the average composition of the moon.     

Pressure-induced coordination changes in alkali-germanate melts: an in situ spectroscopic investigation

The structure of liquid Na(2)Ge(2)O(5).H(2)O, a silicate melt analog, has been studied with Raman spectroscopy to pressures of 2.2 gigapascals. Upon compression, a peak near approximately 240 wavenumbers associated with octahedral GeO(6) groups grows relative to a peak near approximately 500 wavenumbers associated with tetrahedral GeO(4) groups. This change corresponds to an increase in octahedral germanium in the liquid from near 0% at ambient pressures to >50% at a pressure of 2.2 gigapascals. Silicate liquids plausibly undergo similar coordination changes at depth in the Earth. Such structural changes may generate decreases in the fusion slopes of silicates at high pressures as well as neutrally buoyant magmas within the transition zone of the Earth's mantle.     

Effect of water on the composition of partial melts of greenstone and amphibolite

Closed-system partial melts of hydrated, metamorphosed arc basalts and andesites (greenstones and amphibolites), where only water structurally bound in metamorphic minerals is available for melting (dehydration melting), are generally water-undersaturated, coexist with plagioclase-rich, anhydrous restites, and have compositions like island arc tonalites. In contrast, water-saturated melting at water pressures of 3 kilobars yields strongly peraluminous, low iron melts that coexist with an amphibolebearing, plagioclase-poor restite. These melt compositions are unlike those of most natural silicic rocks. Thus, dehydration melting over a range of pressures in the crust of island arcs is a plausible mechanism for the petrogenesis of islands arc tonalite, whereas water-saturated melting at pressure of 3 kilobars and above is not.     

Impact metamorphism of lunar surface materials

Silicate grains from Tranquillity Base have shock-induced features ranging from internal fragmentation through complete disruption of the lattice to thermal melting. Half the crystalline grains with diameters less than 125 micrometers have features of shock equivalent to those produced in the laboratory at pressures greater than about 40 kilobars. One quarter have features indicative of pressures greater than 90 kilobars. These properties together with great quantities of melt glass and aluminum-26 in the fine-grained material are indicative of repeated shock-loading by meteoritic bombardment over long periods of time.     

Continuous mantle melt supply beneath an overlapping spreading center on the East Pacific Rise

Tomographic images of upper mantle velocity structure beneath an overlapping spreading center (OSC) on the East Pacific Rise indicate that this ridge axis discontinuity is underlain by a continuous region of low P-wave velocities. The anomalous structure can be explained by an approximately 16-kilometer-wide region of high temperatures and melt fractions of a few percent by volume. Our results show that OSCs are not necessarily associated with a discontinuity in melt supply and that both OSC limbs are supplied with melt from a mantle source located beneath the OSC. We conclude that tectonic segmentation of the ridge by OSCs is not the direct result of magmatic segmentation at mantle depths.     

Pseudotachylytes generated during seismic faulting and eclogitization of the deep crust

Pseudotachylytes are typically interpreted to have formed by frictional melting during coseismic faulting within the upper to middle crust. Pseudotachylytes in the Bergen arcs of western Norway contain microlites including omphacite, garnet, plagioclase, and quartz. This eclogite facies assemblage is stable at temperatures of about 800 degrees C and pressures of 18 to 19 kilobars, corresponding to depths of 60 kilometers or more. The pseudotachylytes are exposed in Grenvillian granulites that locally underwent fluid-induced eclogitization and corresponding volume reduction of approximately 10 percent during the Caledonian continental collision. The pseudotachylytes may have formed as a result of the rapid relaxation of stresses caused by the eclogitization process.     

Compressibilities of lunar crystalline rock, microbreccia, and fines to 40 kilobars

The compressibilities of three lunar samples were studied at room temperature from 0 to 40 kilobars. The samples were a fine-grained vesicular crystalline rock (type A), a microbreccia (type C), and fines (type D). All samples were porous. The microbreccia and fines were quite compressible at all pressures; the compressibility of the crystalline rock was somewhat less, being 8.4 megabar(-1) at 1 atmosphere and 1.5 megabar(-1) at 35 kilobars. Some porosity appeared to remain in the samples at all pressures. Thus the pressure-volume data derived from these samples may be representative of porous surface and near-surface material in the vicinity of the Apollo 11 landing site but may not be representative of lunar material at depth.     

Thorium-uranium fractionation by garnet: evidence for a deep source and rapid rise of oceanic basalts

Mid-ocean ridge basalts (MORBs) and ocean island basalts (QIBs) are derived by partial melting of the upper mantle and are marked by systematic excesses of thorium-230 activity relative to the activity of its parent, uranium-238. Experimental measurements of the distribution of thorium and uranium between the melt and solid residue show that, of the major phases in the upper mantle, only garnet will retain uranium over thorium. This sense of fractionation, which is opposite to that caused by clinopyroxene-melt partitioning, is consistent with the thorium-230 excesses observed in young oceanic basalts. Thus, both MORBs and QIBs must begin partial melting in the garnet stability field or below about 70 kilometers. A calculation shows that the thorium-230-uranium-238 disequilibrium in MORBs can be attributed to dynamic partial melting beginning at 80 kilometers with a melt porosity of 0.2 percent or more. This result requires that melting beneath ridges occurs in a wide region and that the magma rises to the surface at a velocity of at least 0.9 meter per year.     

Pressure-induced phases of sulfur

At least three phases of sulfur may be induced at pressures between 16 and 65 kilobars. A fibrous form obtained at pressures of 27 kilobars and higher appears to be closely related to the form obtained when plastic sulfur is chilled and stretched. Crystals of the fibrous form are orthorhombic with a = 13.8, b = 32.4, and c = 9.25 A. Thus far the results are in accord with deductions made by Prins and co-workers that the sulfur atoms are arranged in helices with ten atoms and three turns per 13.8-A period, and that these helices are essentially close-packed. The unit cell contains 16 ten-atom chain lengths. The probable space groups to which the crystal may belong are C c m m, C c 2 m, or C c m 2(1). These imply that the structure must contain both right- and left-handed helices and that at least half the helices have some disorder about their axes. The other two phases appear to have structures related to that of the fibrous form, but analyses of them has not progressed as far. One of these phases appears to be uomegasulfur.     

Compressibility of solid c60

Room-temperature powder x-ray diffraction profiles have been obtained at hydrostatic pressures P = 0 and 1.2 gigapascals on the solid phase of cubic C(60) ("fullerite"). Within experimental error, the linear compressibility d(ln a)/dP is the same as the interlayer compressibility d(ln c)/dP of hexagonal graphite, consistent with van der Waals intermolecular bonding. The volume compressibility -d(ln V)/dP is 7.0 +/- 1 x 10(-12) square centimeter per dyne, 3 and 40 times the values for graphite and diamond, respectively.     

The process of formation of ocean crust

Ocean crust is the outermost layer of earth under the oceans. It is separated from the underlying mantle by a seismic transition zone called the Moho. A widely held view is that the Moho represents a petrologic change from basaltic-type rocks to a mantle composed mostly of olivine and pyroxene. According to this view, crust is formed by a steady segregation of basaltic melt, derived from partial melting of the mantle, into a crustal magma chamber wherein cooling and crystallization bring about steady-state accretion to the continuously spreading plates. There is sufficient disagreement between the predictions of this hypothesis and marine geophysical data to cause one to doubt the validity of this formation process. At least two other processes are more compatible with the geophysical data. In one, the crust is formed from the episodic injection of basaltic dikes from a mantle reservoir and the Moho is a primary petrologic boundary. In the other, the crust is treated as a mechanical boundary layer in which thermal contraction results in cracking; by comparison, in the mantle thermal contraction is accommodated by flow. The upper part of the crust is formed from episodic extrusion and intrusion of basaltic melt. The lower crust is formed by rapid hydrothermal alteration of mantle that may be continuously or episodically injected by viscous flow at temperatures below the melting temperature.     

Stress gradient in solid methane at high pressures

The magnitude of the axial pressure gradient at 77 degrees K (11 percent per centimeter) in solid methane compressed along one axis for applied pressures up to 10 kilobars was determined by comparing the electrical resistance of a pair of doped tellurium pressure gages with that of a set of single-crystal gages made of high-purity bismuth. The existence of the pressure gradient revealed the causes of deformation in metal tensile specimens embedded in solid methane and cycled to high pressure.     

A High-Pressure Polymorph of Troilite, FeS

A new polymorph of FeS was produced in a diamond-anvil cell and observed at high pressure both optically and by x-ray diffraction. Fourteen x-ray reflections of the high-pressure FeS were recorded; however, the crystal structure is unknown. This form of FeS is stable at 25 degrees C only at pressures above approximately 55 kilobars. The transition to the lower pressure polymorph, troilite, is rapid and reversible.     

Dynamic compression of Earth materials

Shock wave techniques have been used to investigate the pressuredensity relations of metals, silicates, and oxides over the entire range of pressures present in the earth (3.7 x 10(6) bars at the center). In many materials of geophysical interest, such as iron, wüstite, calcium oxide, and forsterite, major shock-induced phase changes dominate the compression behavior below pressures of 10(6) bars. The shock wave data for the high-pressure phases of these minerals lead to important inferences about the composition of the lower mantle and outer, liquid core of the earth. The lower mantle of the earth appears to have a slightly higher density than is inferred to correspond to the behavior of an olivine-rich assembiage of the same composition as the upper mantle. The core has a density some 10 percent less than that of pure iron and may have 9 to 12 percent sulfur or about 8 percent oxygen by weight.     

Noble gas partitioning between metal and silicate under high pressures

Measurements of noble gas (helium, neon, argon, krypton, and xenon) partitioning between silicate melt and iron melt under pressures up to 100 kilobars indicate that the partition coefficients are much less than unity and that they decrease systematically with increasing pressure. The results suggest that the Earth's core contains only negligible amounts of noble gases if core separation took place under equilibrium conditions.