bluuebunny/crossref_metadata_2025 huggingface dataset:
Porosity reduction and crustal pore pressure development
Authors: Walder Joseph , Nur Amos
Published: February 2008

The influence of porosity reduction processes on the hydrologic characteristics of the earth's crust is examined. We present a simple mathematical model that shows the effect of porosity reduction on pore pressure development in the crust. Pore pressure in excess of hydrostatic can be generated if porosity reduction rates are sufficiently high. Elevated pore pressure could in turn affect properties such as strength and seismic reflectivity. Indirect evidence from laboratory, theoretical, and field studies indicates that porosity reduction processes are active, or very likely to be so, in large portions of the crust. Porosity, permeability, and pore pressure in the earth's crust may well be time‐dependent, even in regions not subjected to large deviatoric stresses.
Pristine rocks (8th Foray): “Plagiophile” Element ratios, crustal genesis, and the bulk composition of the Moon
Authors: Warren Paul H. , Kallemeyn Gregory W.
Published: February 2008

Compositional diversity among pristine nonmare rocks is probably too great for both the Mg‐rich and ferroan anorthosite groups to be generated by a single magma, based on ratios of “plagiophile” elements (which have plagioclase/liquid distribution coefficients close to unity, but much smaller crystal/liquid distribution coefficients for olivine and low‐Ca pyroxene, e.g., Al, Sr, Eu, Ga, and to lesser degrees K and Ba). Because plagiophile elements are virtually confined to a single mineral, bulk‐rock ratios of plagiophile elements are nearly immune to sampling errors. On a plot of Eu/Al versus mg, ferroan anorthosites are separated from all other pristine nonmare rocks by a considerable gap. Stochastic processes (e.g., differential entrapment of interstitial liquid, or differential crystallization of minor phases) might conceivably explain the general diversity of plagiophile ratios, but a nonrandom process must be invoked to account for a gap in the spectrum of ratios. A single magma probably cannot account for even the Mg‐rich pristine rocks subset, based on diversity of plagiophile ratios among samples with similar mg ratios. Plagiophile ratios also constrain the bulk composition of the moon. Previous work suggests that ferroan anorthosites formed by relatively uncomplicated processes, i.e., flotation of plagioclase cumulates over a primordial “magmasphere.” This model implies that ferroan anorthosite plagiophile ratios are straightforwardly related to those of the bulk silicate fraction of the moon. For refractory elements, plagiophile ratios among ferroan anorthosites exactly match those expected under such a model, assuming the bulk moon ratios are approximately chondritic. Plagioclase ratios indicate that K, which is volatile, and Ga, which is volatile and siderophile, are depleted relative to chondrites. Ratios among nonvolatile elements confirm that the moon formed out of materials akin to chondritic meteorites.
Petrology and geochemistry of lunar dimict breccia 61015
Authors: James Odette B. , Flohr Marta K. , Lindstrom Marilyn M.
Published: February 2008

Dimict breccia 61015 consists dominantly of granulated anorthosite and impact melt rock of VHA (very‐high‐alumina)‐basalt composition. The anorthosite is a typical Apollo 16 ferroan anorthosite, except that it is slightly more augite‐rich than most other ferroan anorthosites. The breccia probably formed in the floor of an expanding crater cavity by the following process: Impact melt generated within the cavity was injected into underlying anorthositic bedrock and solidified rapidly, and the anorthosite and melt rock were then fragmented by deformation in the later stages of the impact. All the Apollo 16 dimict breccias (except perhaps 61016) appear to have formed in the same impact. The source crater cannot yet be identified, but it may be an intermediate‐sized (50–150 km diameter) crater within a few hundred kilometers of the Apollo 16 site, in the direction of the Imbrium basin, of either pre‐Nectarian or post‐Nectarian/pre‐Imbrian age. The breccia has been affected by at least one, and perhaps as many as four, recent small impacts (the most recent was probably the South Ray impact). The recent impact(s) produced internal shock‐induced melting of the breccia and injected and coated the rock with externally derived impact melts. The coating glass is heterogeneous. Its average composition is similar to the compositions of other Apollo 16 coating glasses, except that its TiO2 content is lower. The vein glass is unusually mafic and has very low TiO2 for its Al2O3 content. The TiO2 contents of these glasses establish that they cannot be melted soils; their compositions require a mafic, low‐TiO2 protolith component that has not yet been identified among the large rock samples, perhaps a very mafic member of the ferroan‐anorthosite suite.
Petrologic profile of Apollo 16 regolith at station 4
Authors: Basu Abhijit , McKay David S.
Published: February 2008

The Apollo 16 double drive tube core 64001/2 (total depth∼60 cm) consists of 4 petrologic units based on the modal abundance of lithic, monomineralic, and other particles in the 20–500 μm size range of samples from 12 levels. Variation in the abundance of particles derived directly from bedrocks is low and the core soils probably represent only one set of source rocks. The chemical signature of an excessively high mare basalt component at ∼42 cm correlates with an excess of regolith breccias, which may then be the physical carrier. The core soils as a whole show a systematic size‐composition relationship that may be attributed to a large‐scale macroscopic reworking and the consequent masking of any or all previous surface process related mixing event. However, on a much smaller scale (∼0.5 cm), individual soil layers seem to have evolved more via mixing than via in situ reworking.
Magnetic field amplification and generation in hypervelocity meteoroid impacts with application to lunar paleomagnetism
Authors: Hood L. L. , Vickery A.
Published: February 2008

A continuing difficulty with the core dynamo hypothesis for the origin of lunar paleomagnetic fields is the small maximum size of a present‐day iron core and correspondingly small estimates for maximum lunar surface paleofields based on available models for planetary magnetic field generation. This paper explores the possibility that magnetic fields may be significantly amplified or generated in large‐scale meteoroid impacts and that these fields may have contributed substantially to the observed magnetization of lunar materials. Using a one‐dimensional numerical model for expansion of impact‐produced vapor clouds, scalar electrical conductivities for the expanding partially ionized gas are calculated to be in the range 100–106 S m−1 for meteoroids with representative compositions impacting at velocities greater than the threshold for significant vaporization (∼10 km s−1). Magnetic diffusion will therefore be small in comparison to advection on relevant time scales. The available gas kinetic and internal energy densities are ≫ probable magnetic energy densities for time periods that increase with increasing impactor velocity and size. These periods represent limits on the time available for magnetic field amplification/generation and are in the approximate range 102–104 s for meteoroids with radii between 1 and 200 km impacting at 15 km s−1. Preexisting solar wind magnetic fields will be compressed by the expanding plasma cloud and amplified by induction in the presence of an electrically conducting lunar interior. Within the limitations of the one‐dimensional modeling results, maximum surface field amplitudes expected via these mechanisms are ≲0.01 G so that their importance with respect to lunar paleointensities (0.01–1.0 G) is marginal. However, existence of a core dynamo during the 3–4 b.y. time period with probable surface fields of ≲0.003 G would increase maximum induced field amplitudes to values compatible with the range of lunar paleointensities. Direct generation of transient fields in impact‐produced plasmas via currents forced by thermal pressure gradients will occur, but estimated field amplitudes for large‐scale impacts (∼10−4–10−5 G) are too small to contribute to lunar paleointensities. Dynamo amplification of these fields as a result of expansion velocity gradients and turbulence within the expanding plasma cloud is possible within available time periods, but necessary conditions for achieving lunar paleointensity amplitudes have not been demonstrated.
The case for a melt matrix in plagioclase‐POIK mesosiderites
Authors: Hewins Roger H.
Published: February 2008

The plagioclase‐POIK mesosiderites, Bondoc, Budulan, and Mincy, have poikilitic matrix textures exactly like igneous rocks including the olivine zone of the Palisades sill and Apollo 17 mare basalts. They contain olivine grains with embayed margins characteristic of resorption in liquid rather than coronas formed during solid state reactions. There is a nonrandom (i.e., nonmetamorphic) arrangement of pyroxene phases in the matrix: orthopyroxene is enclosed poikilitically by plagioclase, but inverted pigeonite is interstitial and locally subophitic to plagioclase, as in an igneous crystallization sequence. These mesosiderites are therefore reclassified from subgroup 3B (highly recrystallized matrix) to 4B (igneous matrix). The 4A mesosiderites have fine‐grained intergranular textures because they are richer in plagioclase than subgroup 4B, so that plagioclase crystallized earlier than in 4B, but also because they contained abundant nuclei associated with relict plagioclase clasts. The only 4A melt‐rock mesosiderite, Pinnaroo, with an intergranular‐poikilitic matrix texture transitional between 4A and 4B mesosiderites, contains very few plagioclase clasts. Over half the mesosiderites contain melt matrix or melt‐rock clasts. The heterogeneity within mesosiderites as a whole and within specific mesosiderites (e.g., Estherville) may be due to variations in the proportions of hot melt and cold clasts incorporated.
U‐Pb geochronology of zircons from lunar breccia 73217 using a sensitive high mass‐resolution ion microprobe
Authors: Compston W. , Williams I. S. , Meyer C.
Published: February 2008

U‐Pb ages of four zircons in thin‐sections of lunar breccia 73217 have been determined using the sensitive high‐resolution ion microprobe, SHRIMP. All crystals show remarkably little loss of radiogenic Pb and give U‐Pb ages that are within 10% of concordance at 4356−14+23 m.y. Two of the crystals show evidence of radiogenic initial Pb that evolved in a source with μ ∼2000. One zircon is an inclusion in an ilmenite of the clast assemblage, which shows that the zircons belong to that assemblage and strongly suggests that they date formation of the (igneous) rock from which the clasts were derived. The principal loss of radiogenic Pb from the zircons occurred significantly later than 3900 m.y. but probably earlier than 1000 m.y. Two of the zircons are optically and chemically zoned and only partly metamict. U, Th, Yb, Y, and P are relatively enriched in the metamict region of one.
Partition coefficients of Hf, Zr, and ree between phenocrysts and groundmasses
Authors: Fujimaki Hirokazu , Tatsumoto Mitsunobu , Aoki Ken‐ichiro
Published: February 2008

Partition coefficients of Hf, Zr, and REE between olivine, orthopyroxene, clinopyroxene, plagioclase, garnet, amphibole, ilmenite, phlogopite, and liquid are presented. Samples consist of megacrysts in kimberlite, phenocrysts in alkaline basalts, tholeiitic basalts and andesitic to dacitic rocks, and synthetic garnet and clinopyroxene in Hawaiian tholeiites. The Hf‐Lu and Zr‐Lu elemental fractionations are as large as the Lu‐Sm or Lu‐Nd fractionation. The Hf and Zr partition coefficients between mafic phenocrysts and liquids are smaller than the Lu partition coefficients, but are similar to the Nd or Sm partition coefficients. The Hf and Zr partition coefficients between ilmenite, phlogopite, and liquid are larger than the Lu partition coefficients for these minerals and their corresponding liquids. The Hf‐Zr elemental fractionation does not occur except for extreme fractionation involving Zr‐minerals and extremely low fo2. These data have an important bearing on chronological and petrogenetic tracer studies involving the Lu‐Hf isotopic system.
Lateral heterogeneity in the convecting mantle
Authors: Jarvis G. T. , Peltier W. R.
Published: February 2008

Thermally induced lateral heterogeneity in the earth's mantle has been studied in terms of idealized two‐dimensional, constant viscosity, numerical models of mantle convection. Steady model solutions of the temperature and velocity fields were analyzed with respect to both the spatial and spectral signatures of their inherent lateral heterogeneity. By examining the changes in spectral signature produced by varying the Rayleigh number, the degree of internal heating, and the depth in the convection cell, it has been possible to identify those features of the spectra of lateral heterogeneity which are most indicative of these variations. Particular attention has been paid to the spectral characteristics of thermal boundary layers within the convecting layer. The spectral analysis of known temperature fields represents the forward problem corresponding to the inverse problem of inferring the thermal structure of the mantle from the spectral components of its lateral heterogeneity as determined by seismic tomography. The forward problem presented here illustrates a new approach in convection modeling which may ultimately provide important diagnostic criteria for the interpretation of recent tomographic data.
Dynamic motion of a single degree of freedom system following a rate and state dependent friction law
Authors: Rice James R. , Tse Simon T.
Published: February 2008

Sequences of dynamic instabilities are analyzed for a single degree of freedom elastic system which slides along a surface having frictional resistance depending on slip rate and slip rate history, in the manner of Dieterich, Ruina and others. The system is represented as a rigid block in contact with a fixed surface and having a spring attached to it whose opposite end is forced to move at a uniform slow speed. The resulting “stick‐slip” motions are well understood in the classical case for which there is an abrupt drop from “static” to “sliding” frictional resistance. We analyze them here on the basis of more accurate frictional constitutive models. The problem has two time scales, an inertial scale set by the natural oscillation periodTof the analogous frictionless system asT/2π and a state relaxation scaleL/Voccurring in evolution, over a characteristic slip distanceL, of frictional stress τ towards a “steady state” value τss(V) associated with slip speedV. We show that τ ≃ τss(V) during motions for which acceleration a satisfiesaL/V2≪ 1, and that this condition is met during an inertia controlled instability in typical circumstances for which the unstable slip is much greater thanL. SinceV/ais of orderT/2π during inertia controlled motion, one hasL/V≪T/2π, whereasL/V≫T/2π during much of the essentially quasi‐static “stick” part of the cycle when there is a sufficiently small imposed velocity at the load point. Thus the physically irrelevant time scale (L/Vduring inertial controlled motion,T/2π during quasi‐static motion) is much shorter than the relevant scale, which is troublesome from a numerical point of view as it is the shorter time scale which constrains allowable step size. We propose efficient numerical procedures to deal with such response, in which the full equations with inertia and state relaxation are solved only in a transition regime when both time scales are significant. We show results for several friction laws, all having history dependence based on a single evolving state variable and all having properties that ∂τ/∂V> 0 for instantaneous changes inV, that τ evolves towards τss(V) as exp (−δ/L) with ongoing slip δ whenV= const, and that dτss(V)/dV< 0 except possibly at highV. During the dynamic instabilities we find that motion continues at a nearly steady state condition, τ≃τss(V), until dynamic overshoot becomes so significant that “arrest” begins. In the arrest stage,Vdrops rapidly to very much lower values (never zero in our models) under nearly fixed state conditions, and then the long quasi‐static “stick” phase of the motion begins again.
Paleomagnetism and K‐Ar ages of volcanic rocks from Long Valley Caldera, California
Authors: Mankinen Edward A. , Grommé C. Sherman , Dalrymple G. Brent , Lanphere Marvin A. , Bailey Roy A.
Published: February 2008

Paleomagnetic measurements and K‐Ar age determinations on volcanic rocks from Long Valley caldera, California, have enabled further refinement of eruptive activity within this large silicic volcanic center. K‐Ar age determinations show that postcaldera volcanic eruptions began 0.73 m.y. ago and continued periodically until about 50,000 years ago. The eruptions were not temporally random but tended to occur in distinct episodes separated by periods of quiescence. Volcanism in the western half of the caldera was particularly intense between 0.15 and 0.50 m.y. ago, when many units ranging in composition from basalt to rhyolite were erupted. An average of the virtual geomagnetic poles for 33 units from the caldera yields a paleomagnetic pole at 89.7°N, 138.4°E, (α95 = 5.1°), which is indistinguishable from the earth's rotational axis. The ancient geomagnetic field dispersion about this mean pole is 16.0°, with upper and lower limits of 19.3° and 13.6°, respectively.
Temporal change in coda Q before the Tangshan Earthquake of 1976 and the Haicheng Earthquake of 1975
Authors: Jin Anshu , Aki Keiiti
Published: February 2008

A new earthquake precursor called “coda Q” is demonstrated for the Tangshan earthquake of 1976 and for the Haicheng earthquake of 1975. The precursor is based on the measurement of coda decay rate for small local earthquakes in the general area that is preparing for a major earthquake. During the 3‐year period preceding the Tangshan earthquake the coda Q was about 3 times lower than it was before or after this period. A comparable change also occured at the time of the Haicheng earthquake. The observed change cannot be due to the change in the source parameters, such as the focal depth or the source spectrum. The coda Q precursor appears to detect the same physical process detected by other precursors such as b value, quiescence, radon, water quality, Vp/Vs, and animal behavior but has advantages over them in its clearer physical mechanism, easier sampling, and better defined anomalous region.
Convection beneath young oceanic lithosphere: Implications for thermal structure and gravity
Authors: Buck W. Roger , Parmentier E. M.
Published: February 2008

Small‐scale convection under the oceanic lithosphere which begins in the first 5 m.y. of cooling can produce a gravity signal with the amplitude and wavelength observed for large areas of the central Pacific and southern Indian oceans using Seasat altimeter data. The trend of the observed anomalies is parallel to the direction of plate motion as might be expected if they were produced by small‐scale convection. Models predict that the wavelengths of gravity anomalies increase more rapidly with age than is observed. The persistence of short relatively uniform wavelength anomalies (<200 km) to crustal ages of 50 Ma may indicate that they were produced when the lithosphere was very young and thin and were “frozen in” as cooling thickened the elastic lithosphere. Small‐scale convection which begins under very young lithosphere does not violate other geophysical data such as the rate of seafloor subsidence and variations of geoid height with age. After convection has begun, the subsidence due to thermal contraction within the lithosphere varies linearly with age, in the absence of mantle heat sources, although the rate of change of these quantities is affected by convection. Much of the variation of the geoid height across fracture zones can be fit by a model which includes small‐scale convection.
Pore pressure generation in sedimentary basins: Overloading versus aquathermal
Authors: Shi Yaolin , Wang Chi‐Yuen
Published: February 2008

The principal mechanism for abnormally high pore pressure generation has been an important matter of debate for more than a decade. Among the various mechanisms proposed, mechanical overloading and aquathermal pressuring have probably received the greatest attention. Most previous discussions, however, have been based upon qualitative conceptual models. Here we propose a quantitative analysis by first examining the basic principles of porous flow and the fundamental properties of sediments and, second, by comparing the relative importance of the overloading versus the aquathermal mechanisms. The coupling between pore pressure and compaction as well as the nonlinear and path‐dependent material properties are found to be critical in the present analysis. We find that although changes in stress and temperature both contribute to pressure generation, mechanical overloading is the main mechanism under normal geological conditions and is capable of producing the observed pore pressure profiles.
Morphotectonic fabric of the Orozco Transform Fault: Results From a Sea Beam investigation
Authors: Madsen John A. , Fox Paul J. , Macdonald Ken C.
Published: February 2008

We have used data collected with the Sea Beam system to produce a detailed regional bathymetric map of the medium to fast slipping (90 mm/yr) Orozco Transform Fault. The morphotectonic fabric of the 90 × 50 km transform domain is spatially heterogeneous and is composed of relatively shallow areas (<2800 m) bounded by E‐W and N‐S trending troughs and basins (>3200 m). The plate boundary geometry is clearly identified morphologically and is characterized by a series of linked basins and troughs that approximate a zed pattern in plan view. East‐west trending basins define the northern and southern boundaries of the transform domain, and these troughs are thought to mark the location of strike‐slip tectonism. A central, north‐south trending ridge with an axial valley, called here the Mid‐Orozco spreading center, is thought to represent a short (30 km) ridge segment that links two transform faults. Morphotectonic relationships developed along the southeastern flank of the survey area suggest that this plate boundary geometry has recently become unstable and is presently undergoing a transformation as the tip of the East Pacific Rise axis propagates northward across the transform domain bypassing the Mid‐Orozco spreading center. If the southern ridge axis continues to propagate northward it will link with the aseismic extension of the northern strike‐slip strand and the present‐day plate boundary will evolve into a simple ridge‐transform‐ridge (R‐T‐R) geometry with an offset of approximately 70 km.
Crustal structure of the Mid‐Atlantic Ridge at 23°N from seismic refraction studies
Authors: Purdy G. M. , Detrick Robert S.
Published: February 2008

An explosive seismic refraction experiment using four ocean bottom hydrophone receivers was carried out along a 120‐km‐long section of the Mid‐Atlantic Ridge (MAR) median valley at latitude 23°N immediately south of its intersection with the Kane Fracture Zone. The data are interpreted in terms of laterally homogeneous horizontally layered structures using travel time inversion and synthetic seismogram modeling techniques. These solutions are combined using two‐dimensional ray tracing to produce a single model defining the major along‐axis structural changes occurring beneath this section of the median valley. With the exception of slightly low layer 3 velocities, the crust beneath several tens of kilometers of the MAR median valley has all the characteristics of simple, mature ocean crust including a well‐defined Moho transition zone, ∼8 km/s upper mantle velocities, and a total crustal thickness of 6–7 km. No evidence is found for the presence of a steady state axial magma chamber in the crust or upper mantle, and the excellent propagation characteristics and orderly amplitude versus range relationships imply the existence of a considerable degree of lateral homogeneity beneath the median valley over distances of several tens of kilometers along‐axis. We infer from these observations that the basic seismic characteristics of the ocean crust are “frozen‐in” in a time period less than that separating the major volcanic injection events (10,000–50,000 years) by hydrothermal circulation penetrating to the base of the crust. Along the 100‐km‐long ridge segment studied here, two major along‐axis structural changes occur. The first is a 10‐ to 15‐km‐wide (along‐axis) zone of lowered velocities in the lower crust centered beneath a major along‐axis topographic high. This is interpreted to be the remnants of the most recent phase of injection that has temporarily left behind it a region of elevated temperatures and pervasive cracking and thus reduced velocities. The second occurs abruptly at latitude 23°15′N and is accompanied by both a major change in rift valley and crestal mountain morphology and the apparent noncoincidence of the median valley and central magnetic anomaly. North of this boundary the ∼30‐km‐long ridge segment that abuts the Kane Fracture Zone has a crustal thickness of 4–5 km and no distinctive layering to produce the characteristic amplitude patterns that typify mature oceanic crust. Interpretation of the cause of this structural change is uncertain but may be related to local tectonic events, perhaps a recent 10‐ to 20‐km ridge jump to the east.
A record of high‐temperature embrittlement of peridotite in CO2 permeated xenoliths from basalt
Authors: Rovetta Mark R. , Delaney John R. , Blacic James D.
Published: February 2008

Four ultramafic xenoliths recovered from Hawaiian basalts contain CO2‐fluid inclusion arrays which originated as healed microcracks. These healed microcracks have been used to study the microcracking mechanisms in the xenolith source region. Fluid inclusion arrays in olivine have a preferred crystallographic orientation which is consistent with a hypothesis of microcrack nucleation upon dislocation pileups by the Stroh mechanism on the following olivine slip systems: (0kl)[100], (110)[001], and (010)[001]. Previous investigators have shown that all these slip systems of olivine are active at high stress or low temperature. Olivine neoblasts occur as small grains in local shear zones (0.04 mm diameter and less than 3% of total volume) and larger grains which embay porphyroclasts throughout the specimens (0.22‐mm diameter and approximately 30% percent of total volume). Olivine neoblast diameters provide estimates of the deviatoric stress of 160–200 MPa using the smaller grains and 50–75 MPa using the larger grains. The absence of clinopyroxene mechanical twinning in the xenoliths provides an independent paleopiezometer that limits the maximum deviatoric stress in the peridotite source region to 200 MPa. Fluid inclusion arrays are parallel to each other, are perpendicular to penetrative foliation planes produced by intragranular plasticity, and have an average spacing between arrays of 0.5–2.0 mm. Recrystallized olivine grains (neoblasts) are free of fluid inclusions. Estimates of the contribution of Stroh cracks to the deformation of the xenolith source region indicate a maximum decrease of a few percent in the net energy required to produce an increment of strain, dependent upon confining pressure.
Melt segregation in plagioclase‐poikilitic mesosiderites
Authors: Hewins Roger H. , Harriott Theresa A.
Published: February 2008

The Budulan and Mincy mesosiderites contain a poikilitic‐plagioclase matrix with orthopyroxene chadacrysts and interstitial‐subophitic inverted pigeonite. Orthopyroxene chadacrysts in Mincy and Budulan are uniformly more aluminous (and in Mincy some are more magnesian) than orthopyroxene clasts. They could not have been derived from clasts by metamorphism. Interstitial inverted pigeonite is more ferroan than adjacent orthopyroxene in the matrix, consistent with the crystallization of a melt with the sequence orthopyroxene followed by pigeonite. The magnesian chadacrysts in Mincy could not have formed from a melt in equilibrium with the clasts (anatexis) but could have crystallized from impact melt. The most magnesian chadacrysts are enclosed in large reversely zoned plagioclase crystals. Such zoning can be caused by undercooling, which is expected in melt lacking plagioclase clasts and associated nuclei. Mincy contains plagioclase‐poor regions in which plagioclase is interstitial‐poikilitic and and more plagioclase‐rich regions with coarsely poikilitic texture. This is interpreted as separation of silicate melt into pools. Reckling Peak A80258, a plagioclase‐poikilitic mesosiderite with a very high chadacryst/plagioclase ratio, resembles Mincy material from which melt has been extracted. The suggested origin of the plagioclase‐poikilitic mesosiderites is impact melting of a metal‐silicate mixture.
“On‐line” analyses of simulated solar wind implantations of terrestrial analogs of lunar materials
Authors: Blanford G. E. , Børgesen P. , Maurette M. , Möller W. , Monart B.
Published: February 2008

To help assess possible water reserves on the moon derived from the solar wind that could supply a lunar base, oligoclase, ilmenite, and simulated lunar glass have been irradiated with a beam of 2.5 keV/amu deuterium ions to simulate solar wind proton bombardment of lunar materials. For ilmenite and simulated lunar glass the areal density of deuterium D(ϕ) (D/cm2) increases with the incident ion fluence ϕ until a critical reemission fluence is reached (ϕ = 3.3–3.5×1017 D/cm2). At this point deuterium begins to be lost from both targets until D(ϕ) reaches a saturation value Ds = 2–4×1017 D/cm2 at ϕ8 = 1–2 × 1018 D/cm2. Oligoclase begins losing deuterium when the beam is turned on; D(ϕ) never reaches a saturation value up to the limit of our experiment (≈ 3×103 D/cm2). There is a small yield of ∼10−4 D2O+/Dincident during the implantation. Thermal release patterns for D2 in our targets do not correspond to those observed for H2 in lunar soils, but the differences may be explainable after further experiments. Although ilmenite stores about twice as much deuterium as the other target materials, it is unknown whether this small enrichment factor will be sufficient to make it a potential source of lunar water.
The laser microprobe: A technique for extracting carbon, nitrogen, and oxygen from solid samples for isotopic measurements
Authors: Franchi I. A. , Wright I. P. , Gibson E. K. , Pillinger C. T.
Published: February 2008

The laser microprobe extraction technique has been adapted for the determination of concentrations and stable isotopic compositions of carbon, nitrogen, and oxygen. The power from a focused laser beam is used to selectively pyrolyse, or combust, specific areas of a solid sample of interest, which is located within a vacuum extraction line. Gases released from the sample are collected, purified, and then admitted to an appropriate mass spectrometer as either molecular nitrogen (for δ15N measurements) or carbon dioxide (for δ13C and δ18O measurements). The minimum amounts of gas that can be measured Isotopically are less than a nanomole. A number of samples have been analysed in order to evaluate the efficacy of the laser microprobe including two carbonate minerals (calcite and siderite), graphite, and titanium nitride. Initial studies on the distribution of nitrogen in an iron meteorite (Uwet) and a carbonaceous chondrite (Murchison) have been undertaken. Nitrogen in Uwet was found to be concentrated in the phosphide mineral schreibersite rather than in the kamacite. Laser extractions of the dark matrix of Murchison reveal 620 to 790 pprn of nitrogen with a δ15N = +38.5 to +44.7‰ (in agreement with stepped heating extractions of bulk Murchison) while much lower concentrations, down to 80 ppm (and δ15N = +12 to +60‰), were found in the light‐coloured high‐temperature inclusions. For carbon‐containing phases, there appears to be an isotope fractionation associated with the formation of CO2 and CO, However, preliminary results suggest that the extent of isotope fractionation may be reproducible, allowing suitable corrections to be applied to the raw data.
Geodesy by radio interferometry: Studies of the forced nutations of the Earth: 1. Data analysis
Authors: Herring T. A. , Gwinn C. R. , Shapiro I. I.
Published: February 2008

The analysis of 235 very long baseline interferometry observing sessions carried out between July 1980 and December 1984 has yielded estimates of corrections to the seven largest nutation series coefficients (with periods ≤1 year) in the Wahr nutation series, the most complete such model available at this time. The largest correction is to the amplitude of the retrograde annual nutation: −1.80±0.18 ‐ i(0.42±0.18) milliarc seconds (mas), where the estimated standard deviations contain contributions from the effects of systematic as well as random errors. We interpret this correction as being due to an incompleteness in the model of the core‐mantle coupling used by Wahr. We have also examined our results for evidence of any “free core nutation” and can place an upper bound of 1.0 mas (99.5% confidence interval) on its amplitude.
Sediment subduction and magma genesis in the Lesser Antilles: Isotopic and trace element constraints
Authors: White William M. , Dupré Bernard
Published: February 2008

We present analyses of Pb, Sr, and Nd isotope ratios and of K, Rb, Cs, Sr, Ba, U, Th, Pb, and rare earth elements (REE) of a suite of volcanic rocks from the Lesser Antilles island arc, including representatives from all the major islands. An unusually wide range of compositions is present. 87Sr/86Sr and 143Nd/144Nd overlap extensively with those of oceanic islands and indicate slightly more “enriched” source signatures than most island arcs. Pb isotope ratios range to extremely radiogenic values (206pb/204pb = 19.75, 207pb/204pb = 15.83) and extend significantly beyond the oceanic basalt field. REE patterns range from slightly light‐rare‐earth depleted to strongly light‐rare‐earth enriched; several exhibit negative Ce anomalies. The Pb isotopic compositions are more radiogenic than those of most marine sediments but are similar to isotope compositions of sediments in front of the arc. This, and other evidence, such as high Cs/Rb ratios and a southward increase in Pb isotope ratios in both arc volcanics and sediments in front of the arc, is regarded as strong evidence for a sediment component in arc magmas. Metasedimentary xenoliths in arc magmas and other evidence indicate that sediment can be a high‐level contaminant of ascending magmas. However, the ubiquitous nature of the “sedimentary” isotopic and trace element signature in arc magmas, including primitive and peridotite‐xenolith‐bearing ones, indicates subducted sediment must also be a component of arc magma sources. Mixing calculations show the isotope geochemistry of most arc magmas is readily explained if their sources are mixtures of depleted mantle and 3% or less subducted sediment. On average, subduction of 90 m of sediment would be required to explain the isotopic compositions of arc magmas. Simple binary mixing models also explain the REE patterns of low‐K basalts but fail to explain all the excess enrichment of the alkalis and alkaline earths relative to the REE. The excess abundance of these elements is probably due to their enrichment (relative to the REE) in an aqueous fluid generated by dehydration of the subducting sediment and basalt. This fluid mixes with (metasomatizes) overlying depleted mantle, which melts to produce the arc magmas.
Multiple sources for basaltic arc rocks from the southern volcanic zone of the Andes (34°–41°S): Trace element and isotopic evidence for contributions from subducted oceanic crust, mantle, and continental crust
Authors: Hickey Rosemary L. , Frey Frederick A. , Gerlach David C. , Lopez‐Escobar Leopoldo
Published: February 2008

Potential sources for arc magmas include subducted midocean ridge basalt and sediment, subarc mantle peridotite, and overlying crust. Geochemical data for arc rocks in general, including those for the southern volcanic zone (SVZ) of the Andes (34°–41°S in Chile), cannot be explained by one‐stage partial melting of subducted oceanic crust or overlying mantle peridotite. Sr and Nd isotope systematics in arc rocks are similar to those in oceanic island basalts (OIB's). However, plots of 207Pb/204Pb versus 208Pb/204Pb show clearly that there is an input from subducted sediment in most arcs. In arc rocks, radiogenic isotope ratios are decoupled from abundance anomalies of Th, U, Pb, alkali metals, Sr, and Ba. This decoupling is expected if the trace element abundance anomalies are caused by a process such as partial melting of or volatile release from the subducted oceanic crust. In basalts from different volcanoes in the SVZ, anomalous trace element abundance ratios, such as Ba/Nb, are coupled with incompatible element abundances. This feature is consistent with mixing of a component derived from the subducted oceanic crust (i.e., having high Ba/Nb, Cs/Rb, and 207Pb204Pb) with enriched asthenosphere or lithosphere. Although crustal contamination may have had minor effects in all SVZ volcanoes, basaltic lavas from volcanoes 34°–38°S, which are sited on thicker crust, exhibit more evidence for continental crustal contamination, specifically, lower 143Nd/144Nd and higher Al2O3/CaO, than lavas from volcanoes 38°–41°S, which are sited on thinner crust.
Stress history and tectonic development of the Rio Grande Rift, New Mexico
Authors: Aldrich M. J. , Chapin C. E. , Laughlin A. W.
Published: February 2008

Paleostress determinations from radiometrically dated dikes indicate ENE‐WSW regional extension began by about 32 Ma in New Mexico, and perhaps earlier in the southern part of the state. The extension direction maintained this orientation until sometime after 23 Ma when it rotated clockwise to an E‐W or WNW‐ESE direction, which is the present orientation. The spreading direction in the Rio Grande rift, however, has been E‐W since the beginning of rifting about 31 Ma and reflects control of spreading by preexisting structural grain. NE‐SW extension occurred in the Grants‐Pie Town area of the Colorado Plateau and in the roofs of batholiths in the Socorro and Questa areas concurrently with E‐W spreading in the Rio Grande rift. Nearly orthogonal orientations of late Eocene‐Oligocene (38–32 Ma) paleostress indicators in southern New Mexico indicate a nearly isotropic middle Tertiary stress field. Both the western and eastern boundaries of the zone of E‐W spreading along the Rio Grande rift in northern New Mexico shifted 60–200 km eastward sometime between 23 and 16 Ma, perhaps coeval with rotation of the direction of regional extension. This eastward shift also coincided approximately with the renewal of volcanism following an early Miocene (22–17 Ma) lull. During the late Miocene (9–5 Ma), the eastern boundary of E‐W spreading shifted back to the present eastern margin of the Rio Grande rift. The N‐S to NNE‐SSW orientation of the least principal horizontal stress in the southern Great Plains province has persisted since at least 28 Ma. Stress province boundaries can be remarkably sharp with stress orientations changing by 90° within 30 km or less. The NE trending Jemez lineament transects the Colorado Plateau, Rio Grande rift (Basin and Range), and southern Great Plains stress provinces, but except for a transition zone along the present southeastern boundary of the Colorado Plateau, both paleostress and contemporary stress reflect the respective stress provinces, not the lineament. Chemical analyses of dated dikes reveal highly variable compositions but indicate that true basalts are more common among early rift (>20 Ma) volcanic rocks than is generally recognized. A histogram constructed from nearly 200 radiometric ages of igneous rocks in New Mexico between 104°W and 108°W longitude indicates that volcanism along the Rio Grande rift has been episodic with peaks at 37–34, 31–27, 26–22, 17–13,12–6, and 5–0 Ma.
An investigation of low‐stress ice rheology on the Ward‐Hunt Ice Shelf
Authors: MacAyeal Douglas R. , Holdsworth G.
Published: February 2008

Deformation of the Ward‐Hunt Ice Shelf, Northwest Territories, Canada, can be used to study the rheology of ice in the low‐stress, low‐strain rate regime (10−12s−1). Finite element simulations are separately displayed to demonstrate what the effects of ice salinity, ice temperature, and sea‐ice back pressure have on the spreading rates of this ice shelf. We conclude that in general these effects tend to obscure any differences between the commonly used non‐Newtonian flow law and the Newtonian flow law recently reinterpreted from field measurements. The numerical simulations indicate that there are specific locations where useful field measurements could be carried out on the ice shelf.
Solitary wave propagation in a fluid conduit within a viscous matrix
Authors: Olson Peter , Christensen Ulrich
Published: February 2008

Conduits of low‐viscosity, buoyant fluid imbedded in a highly viscous matrix have been used to model the dynamics of magma transport in vertical dikes and within zones of partial melt. The theory predicts that disturbances can propagate along the conduit in the form of large‐amplitude waves. A set of experiments have been made to investigate the behavior of this system. Uniform, vertical conduits of low‐viscosity liquid (dilute aqueous solutions of ethyl alcohol and sucrose) were established in a column of high‐viscosity, high‐density matrix fluid (concentrated sucrose solution). Perturbations were introduced in the form of pulses of conduit liquid. Two classes of travelling disturbances were observed: slowly propagating, periodic wave trains, and fast propagating solitary waves. The slow, periodic waves form during the development of a conduit, behind an ascending diapir. The fast, solitary waves form in response to disturbances introduced into fully developed conduits. Both of these wave types are correctly predicted by the theory. Measurements were made of solitary wave propagation speed versus wave volume in two different conduit liquids over a wide range of background (undisturbed) conduit flux. Satisfactory agreement was found between the measured and the theoretical propagation speeds except in two limits. For very large wave volumes the observed propagation was always faster than expected. This deviation may be due to finite wave slope effects and to departures from Poiseuille flow in the conduit which are not included in the theory. For very thin conduits the observed propagation was generally slower than expected, an effect that can be attributed to mass diffusion between the conduit fluid and the matrix. Overall, our results indicate that the two‐fluid models of magma transport are adequate for describing the behavior of homogeneous, nondiffusive systems.
Synthetic seismograms from coupled free oscillations: Effects of lateral structure and rotation
Authors: Park Jeffrey
Published: February 2008

Synthetic seismograms from coupled free oscillations are calculated for a regionalized continent‐ocean model of the upper mantle and a recently proposed spherical harmonic lateral structure model. Spectral correlation analysis of these synthetic records, containing Rayleigh waves with periodT≳ 160 s, reveals the size of departures from the assumption that the waveform perturbation can be explained by just the structure under the great circle between source and receiver. Arrival azimuth deflections of up to 10° are observed on horizontal synthetics using a 2.3% variation in shear velocity between continents and oceans. Among the M84A synthetics, the great circle travel timesR3–R1,R4–R2,R5–R3vary on average by 3–5 s for 3 ≲f≲ 6 mHz surface wave motion. Comparison between synthetics, calculated to be sensitive to even‐order structure or to both even‐ and odd‐order structure, gives a 5‐s average time delay attributable to odd‐order structure. Significant Rayleigh wave packet amplitude anomalies are common in the synthetic records and are nearly as common in synthetics calculated for the even‐order structure as in those sensitive to all structure. Examples suggest that model M84A does not consistently reproduce the amplitude anomalies observed in International Deployment of Accelerometers (IDA) and Global Digital Seismographic Network (GDSN) data. Lastly, we demonstrate the effect of Coriolis coupling in the time domain. Visible precursors to Rayleigh wave packets are predicted in cases of favorable source‐receiver geometry. These precursors are caused by the spheroidal component of the hybrid, predominantly toroidal, low‐frequency (f≲ 4.2 mHz) Love wave, whose arrival precedes that of the Rayleigh wave. We present examples of this anomalous waveform in IDA data from a large strike‐slip earthquake in New Guinea, showing a qualitative fit to the coupled‐mode prediction.
Crustal structure of southwestern Saudi Arabia
Authors: Gettings M. E. , Blank H. R. , Mooney W. D. , Healey J. H.
Published: February 2008

A self‐consistent regional‐scale model of the crust and upper mantle for the southern Arabian Shield and Red Sea paar has been constructed from an integrated interpretation of seismic deep refraction, regional gravity, aeromagnetic, heat flow, and surface geologic data. The Shield consists of two 20‐km‐thick layers of crust with an average compressional wave velocity in the upper crust of about 6.3 km/s and in the lower crust of about 7.0 km/s. This crust thins abruptly to less than 20 km near the southwestern end of the transect, where Precambrian outcrops abut the Cenozoic rocks, and to 8 km beneath the Farasan Islands. The data over the Red Sea westward of Precambrian outcrop are fit satisfactorily by an oceanic crustal model. The major velocity discontinuities occur at about the same depth across the entire shield and indicate horizontal metamorphic stratification of the Precambrian crust. Several lateral inhomogeneities in bulk physical properties have been identified in both the upper and lower crust of the shield, indicating bulk compositional variations. The subcrustal portion of the model is composed of a hot, low‐density lithosphere and asthenosphere beneath the Red Sea which is systematically cooler and denser to the northeast. This model provides a mechanism that explains the observed topographic uplift, regional gravity anomaly pattern, heat flow, and mantle seismic velocities. Such a lithosphere could be produced by upwelling of hot asthenosphere beneath the Red Sea which then flows laterally beneath the lithosphere of the Arabian plate.
Electromagnetic study of the Atlantic continental margin using a section of a transatlantic cable
Authors: Lanzerotti L. J. , Thomson D. J. , Meloni A. , Medford L. V. , Maclennan C. G.
Published: February 2008

The induction of currents in an approximately 1200 km length of the TAT‐7 transatlantic telecommunications cable by geomagnetic field fluctuations is studied. Analyses of the induced voltage drop across the essentially geomagnetically east‐west oriented cable and of magnetic field data obtained at field sites at the cable head (Tuckerton, New Jersey) and in New Hampshire show a very high correlation over a wide frequency range between the induced currents in the cable and the east‐west magnetic field fluctuations. The observations do not correspond to the classical ocean‐coast effects discussed in the literature. The results of the analyses suggest that there are telluric currents flowing in an approximately north‐south direction off the east coast of the United States. The electrical current flow could be primarily confined in the ocean water or channeled in a conductivity structure within the earth's crust and upper mantle. If the latter situation exists, as may be likely because of the wide range of frequencies over which the effect is seen, then the region of the induced current flow could be related to the east coast magnetic anomaly (ECMA). In this case, these results would indicate that in addition to being a static magnetic anomaly, the ECMA is a structure with significant electrical characteristics such as higher conductivity.