Magma generation on Mars: amounts, rates, and comparisons with Earth, moon, and venus

Total extrusive and intrusive magma generated on Mars over the last approximately 3.8 billion years is estimated at 654 x 10(6) cubic kilometers, or 0.17 cubic kilometers per year (km(3)/yr), substantially less than rates for Earth (26 to 34 km(3)/yr) and Venus (less than 20 km(3)/yr) but much more than for the Moon (0.025 km(3)/yr). When scaled to Earth's mass the martian rate is much smaller than that for Earth or Venus and slightly smaller than for the Moon.     

Highly siderophile element constraints on accretion and differentiation of the Earth-Moon system

A new combined rhenium-osmium- and platinum-group element data set for basalts from the Moon establishes that the basalts have uniformly low abundances of highly siderophile elements. The data set indicates a lunar mantle with long-term, chondritic, highly siderophile element ratios, but with absolute abundances that are over 20 times lower than those in Earth's mantle. The results are consistent with silicate-metal equilibrium during a giant impact and core formation in both bodies, followed by post-core-formation late accretion that replenished their mantles with highly siderophile elements. The lunar mantle experienced late accretion that was similar in composition to that of Earth but volumetrically less than (approximately 0.02% lunar mass) and terminated earlier than for Earth.     

142Nd evidence for early (>4.53 Ga) global differentiation of the silicate Earth

New high-precision samarium-neodymium isotopic data for chondritic meteorites show that their 142Nd/144Nd ratio is 20 parts per million lower than that of most terrestrial rocks. This difference indicates that most (70 to 95%) of Earth's mantle is compositionally similar to the incompatible element-depleted source of mid-ocean ridge basalts, possibly as a result of a global differentiation 4.53 billion years ago (Ga), within 30 million years of Earth's formation. The complementary enriched reservoir has never been sampled and is probably located at the base of the mantle. These data influence models of Earth's compositional structure and require revision of the timing of global differentiation on Earth's Moon and Mars.     

Structure and dynamics of Earth's lower mantle

Processes within the lowest several hundred kilometers of Earth's rocky mantle play a critical role in the evolution of the planet. Understanding Earth's lower mantle requires putting recent seismic and mineral physics discoveries into a self-consistent, geodynamically feasible context. Two nearly antipodal large low-shear-velocity provinces in the deep mantle likely represent chemically distinct and denser material. High-resolution seismological studies have revealed laterally varying seismic velocity discontinuities in the deepest few hundred kilometers, consistent with a phase transition from perovskite to post-perovskite. In the deepest tens of kilometers of the mantle, isolated pockets of ultralow seismic velocities may denote Earth's deepest magma chamber.     

182W evidence for long-term preservation of early mantle differentiation products

Late accretion, early mantle differentiation, and core-mantle interaction are processes that could have created subtle (182)W isotopic heterogeneities within Earth's mantle. Tungsten isotopic data for Kostomuksha komatiites dated at 2.8 billion years ago show a well-resolved (182)W excess relative to modern terrestrial samples, whereas data for Komati komatiites dated at 3.5 billion years ago show no such excess. Combined (182)W, (186,187)Os, and (142,143)Nd isotopic data indicate that the mantle source of the Kostomuksha komatiites included material from a primordial reservoir that represents either a deep mantle region that underwent metal-silicate equilibration or a product of large-scale magmatic differentiation of the mantle. The preservation, until at least 2.8 billion years ago, of this reservoir-which likely formed within the first 30 million years of solar system history-indicates that the mantle may have never been well mixed.     

Probing asthenospheric density, temperature, and elastic moduli below the western United States

Periodic ocean tides continually provide a cyclic load on Earth's surface, the response to which can be exploited to provide new insights into Earth's interior structure. We used geodetic observations of surface displacements induced by ocean tidal loads to constrain a depth-dependent model for the crust and uppermost mantle that provides independent estimates of density and elastic moduli below the western United States and nearby offshore regions. Our observations require strong gradients in both density and elastic shear moduli at the top and bottom of the asthenosphere but no discrete structural discontinuity at a depth of 220 kilometers. The model indicates that the asthenosphere has a low-density anomaly of ~50 kilograms per cubic meter; a temperature anomaly of ~300°C can simultaneously explain this density anomaly and inferred colocated minima in elastic moduli.     

Earth's Core-Mantle Boundary: Results of Experiments at High Pressures and Temperatures

Laboratory experiments document that liquid iron reacts chemically with silicates at high pressures (>/=2.4 x 10(10) Pascals) and temperatures. In particular, (Mg,Fe)SiO(3) perovskite, the most abundant mineral of Earth's lower mantle, is expected to react with liquid iron to produce metallic alloys (FeO and FeSi) and nonmetallic silicates (SiO(2) stishovite and MgSiO(3) perovskite) at the pressures of the core-mantle boundary, 14 x 10(10) Pascals. The experimental observations, in conjunction with seismological data, suggest that the lowermost 200 to 300 kilometers of Earth's mantle, the D" layer, may be an extremely heterogeneous region as a result of chemical reactions between the silicate mantle and the liquid iron alloy of Earth's core. The combined thermal-chemical-electrical boundary layer resulting from such reactions offers a plausible explanation for the complex behavior of seismic waves near the core-mantle boundary and could influence Earth's magnetic field observed at the surface.     

Hf-W chronometry of lunar metals and the age and early differentiation of the Moon

The use of hafnium-tungsten chronometry to date the Moon is hampered by cosmogenic tungsten-182 production mainly by neutron capture of tantalum-181 at the lunar surface. We report tungsten isotope data for lunar metals, which contain no 181Ta-derived cosmogenic 182W. The data reveal differences in indigenous 182W/184W of lunar mantle reservoirs, indicating crystallization of the lunar magma ocean 4.527 +/- 0.010 billion years ago. This age is consistent with the giant impact hypothesis and defines the completion of the major stage of Earth's accretion.     

Melting Temperature and Partial Melt Chemistry of H2O-Saturated Mantle Peridotite to 11 Gigapascals

The H2O-saturated solidus of a model mantle composition (Kilborne Hole peridotite nodule, KLB-1) was determined to be just above 1000°C from 5 to 11 gigapascals. Given reasonable H2O abundances in Earth's mantle, an H2O-rich fluid could exist only in a region defined by the wet solidus and thermal stability limits of hydrous minerals, at depths between 90 and 330 kilometers. The experimental partial melts monotonously became more mafic with increasing pressure from andesitic composition at 1 gigapascal to more mafic than the starting peridotite at 10 gigapascals. Because the chemistry of the experimental partial melts is similar to that of kimberlites, it is suggested that kimberlites may be derived by low-temperature melting of an H2O-rich mantle at depths of 150 to 300 kilometers.     

Martian meteorite launch: high-speed ejecta from small craters

We performed high-resolution computer simulations of impacts into homogeneous and layered martian terrain analogs to try to account for the ages and characteristics of the martian meteorite collection found on Earth. We found that craters as small as approximately 3 kilometers can eject approximately 10(7) decimeter-sized fragments from Mars, which is enough to expect those fragments to appear in the terrestrial collection. This minimum crater diameter is at least four times smaller than previous estimates and depends on the physical composition of the target material. Terrain covered by a weak layer such as an impact-generated regolith requires larger, therefore rarer, impacts to eject meteorites. Because older terrain is more likely to be mantled with such material, we estimate that the martian meteorites will be biased toward younger ages, which is consistent with the meteorite collection.     

Iron partitioning in Earth's mantle: toward a deep lower mantle discontinuity

We measured the spin state of iron in ferropericlase (Mg0.83Fe0.17)O at high pressure and found a high-spin to low-spin transition occurring in the 60- to 70-gigapascal pressure range, corresponding to depths of 2000 kilometers in Earth's lower mantle. This transition implies that the partition coefficient of iron between ferropericlase and magnesium silicate perovskite, the two main constituents of the lower mantle, may increase by several orders of magnitude, depleting the perovskite phase of its iron. The lower mantle may then be composed of two different layers. The upper layer would consist of a phase mixture with about equal partitioning of iron between magnesium silicate perovskite and ferropericlase, whereas the lower layer would consist of almost iron-free perovskite and iron-rich ferropericlase. This stratification is likely to have profound implications for the transport properties of Earth's lowermost mantle.     

Compositional heterogeneity in the bottom 1000 kilometers of Earth's mantle: toward a hybrid convection model

Tomographic imaging indicates that slabs of subducted lithosphere can sink deep into Earth's lower mantle. The view that convective flow is stratified at 660-kilometer depth and preserves a relatively pristine lower mantle is therefore not tenable. However, a range of geophysical evidence indicates that compositionally distinct, hence convectively isolated, mantle domains may exist in the bottom 1000 kilometers of the mantle. Survival of these domains, which are perhaps related to local iron enrichment and silicate-to-oxide transformations, implies that mantle convection is more complex than envisaged by conventional end-member flow models.     

Geodynamic evidence for a chemically depleted continental tectosphere

The tectosphere, namely the portions of Earth's mantle lying below cratons, has a thermochemical structure that differs from average suboceanic mantle. The tectosphere is thought to be depleted in its basaltic components and to have an intrinsic buoyancy that balances the mass increase associated with its colder temperature relative to suboceanic mantle. Inversions of a large set of geodynamic data related to mantle convection, using tomography-based mantle flow models, indicate that the tectosphere is chemically depleted and relatively cold to 250 kilometers depth below Earth's surface. The approximate equilibrium between thermal and chemical buoyancy contributes to cratonic stability over geological time.     

Spin transition zone in Earth's lower mantle

Mineral properties in Earth's lower mantle are affected by iron electronic states, but representative pressures and temperatures have not yet been probed. Spin states of iron in lower-mantle ferropericlase have been measured up to 95 gigapascals and 2000 kelvin with x-ray emission in a laser-heated diamond cell. A gradual spin transition of iron occurs over a pressure-temperature range extending from about 1000 kilometers in depth and 1900 kelvin to 2200 kilometers and 2300 kelvin in the lower mantle. Because low-spin ferropericlase exhibits higher density and faster sound velocities relative to the high-spin ferropericlase, the observed increase in low-spin (Mg,Fe)O at mid-lower mantle conditions would manifest seismically as a lower-mantle spin transition zone characterized by a steeper-than-normal density gradient.     

Cumulate xenolith in oahu, hawaii: implications for deep magma chambers and hawaiian volcanism

The maximum depth at which large (>1000 km(3)) terrestrial mafic magma chambers can form has generally been thought to be the Moho, which occurs at a mean depth of about 35 kilometers beneath the continents and 8 kilometers beneath ocean basins. However, the presence of layers of cumulus magnesium-rich spinel and olivine and intercumulus garnet in an unusual mantle xenolith from Oahu, Hawaii, suggests that this rock is a fragment of a large magma chamber that formed at a depth of about 90 kilometers; Hawaiian shield-building magmas may pond and fractionate in such magma chambers before continuing their ascent. This depth is at or near the base of the 90-million-year-old lithosphere beneath Oahu; thus, rejuvenated stage alkalic magmas containing mantle xenoliths evidently also originate below the lithosphere.     

Asymmetric phase effects and mantle convection patterns

Recent high-pressure experiments and thermodynamic calculations have shown that the Clapeyron slope of the spinel-perovskite phase transition at a depth of 660 kilometers in the Earth's mantle changes from negative to positive at temperatures above 1700 degrees to 2000 degrees C. In numerical experiments that account for this phase behavior, cold downwelling flows were impeded at the phase boundary, but hot plumes ascended to the upper mantle with ease. The resultant mantle convection was partially layered and strongly time-dependent. Mantle layering was weaker when the mantle was hotter and when the Rayleigh number was larger.     

Seismic observations of splitting of the mid-transition zone discontinuity in Earth's mantle

The transition zone of Earth's mantle is delineated by globally observed discontinuities in seismic properties at depths of about 410 and 660 kilometers. Here, we investigate the detailed structure between 410 and 660 kilometers depth, by making use of regional stacks of precursors to the SS phase. The previously observed discontinuity at about 520 kilometers depth is confirmed in many regions, but is found to be absent in others. There are a number of regions in which we find two discontinuities at about 500 and 560 kilometers depth, an effect which can be interpreted as a "splitting" of the 520 kilometer discontinuity. These observations provide seismic constraints on the sharpness and observability of mineralogical phase transitions in the mantle transition zone.     

Electrical conductivity of olivine, wadsleyite, and ringwoodite under upper-mantle conditions

Geophysical models show that electrical conductivity in Earth's mantle rises about two orders of magnitude through the transition zone in the depth range 410 to 660 kilometers. Impedance measurements obtained on Mg1.8Fe0.2SiO4 olivine, wadsleyite, and ringwoodite at up to 20 gigapascals and 1400 degreesC show that the electrical conductivities of wadsleyite and ringwoodite are similar and are almost two orders of magnitude higher than that of olivine. A conductivity-depth profile to 660 kilometers, based on these laboratory data, shows a conductivity increase of almost two orders of magnitude across the 410-kilometer discontinuity; such a profile favors a two-layer model for the upper mantle. Activation enthalpies of 1.2 to 1.7 electron volts permit appreciable lateral variations of conductivity with lateral temperature variations.     

Splitting of the 520-kilometer seismic discontinuity and chemical heterogeneity in the mantle

Seismic studies indicate that beneath some regions the 520-kilometer seismic discontinuity in Earth's mantle splits into two separate discontinuities (at approximately 500 kilometers and approximately 560 kilometers). The discontinuity near 500 kilometers is most likely caused by the (Mg,Fe)2SiO4 beta-to-gamma phase transformation. We show that the formation of CaSiO3 perovskite from garnet can cause the deeper discontinuity, and by determining the temperature dependence for this reaction we demonstrate that regional variations in splitting of the discontinuity arise from variability in the calcium concentration of the mantle rather than from temperature changes. This discontinuity therefore is sensitive to large-scale chemical heterogeneity. Its occurrence and variability yield regional information on the fertility of the mantle or the proportion of recycled oceanic crust.     

Phobos: preliminary results from mariner 7

Analysis of an image of Phobos on Mariner 7 frame 7F91 indicates that the martian satellite is larger and has a darker surface than had previously been thought. The limb profile measures 18 by 22 kilometers and is elongated along the orbital plane. Phobos has an average visual geometric albedo of 0.065 lower than that known for any other body in the solar system. It seems probable that Phobos did not form by accretion around primordial Mars, but was captured at some later time.