摘要:

On the Mars rover sample return mission the rover vehicle will collect and select samples from different locations on the Martian surface to be brought back to Earth for laboratory studies. It is anticipated that an in situ energy‐dispersive X ray fluorescence (XRF) spectrometer will be on board the rover. On such a mission, sample selection is of higher priority than in situ quantitative chemical analysis. With this in mind we propose pattern recognition as a simple, direct, and speedy alternative to detailed chemical analysis of the XRF spectra. The validity and efficacy of the pattern recognition technique are demonstrated by the analyses of laboratory XRF spectra obtained from a series of geological samples, in the form both of standardized pressed pellets and as unprepared rocks. It is found that pattern recognition techniques applied to the raw XRF spectra can provide for the same discrimination among samples as knowledge of their actual chemical compositio

ISSN:0148-0227    

DOI:10.1029/JB094iB10p13611

年代:1989

数据来源: WILEY

摘要:

Hapke's model for bidirectional reflectance is used to calculate the mass fractional abundance of components in intimately mixed, particulate surfaces from laboratory reflectance spectra. Application of this model, simplified by the assumptions that all surfaces scatter light with the same constant phase function and the opposition surge is negligible, to binary mineral mixtures are summarized and compared with new results for ternary mixtures of olivine, enstatite, and anorthite. These experimental tests indicate that the simple model is accurate to within 7% for mixtures not containing low albedo components (<10% reflectance). However, several observed systematic deviations of the calculated mass fractions from the actual mass fractions suggest that the simple model may be improved by allowing for variable scattering between the minerals used. An empirical scattering function, based on the physically plausible scattering behavior that low albedo materials are backward scattering and high albedo, transparent materials are forward scattering, reduces the magnitude of the systematic deviations in mass fractions observed with the constant scattering model. To determine if the empirical scattering function is representative of the actual scattering behavior of particulate mineral surfaces, specific photometric parameters (single‐scattering albedo, single‐particle phase function) of five different mineral components (olivine, enstatite, anorthite, magnetite, hematite) and two mineral mixtures (90% olivine 10% magnetite, 25% olivine 75% magnetite) are derived from bidirectional reflectance spectra measured at 17 different viewing geometries between 0.5 and 1.6 μm. Mineral components were crushed and wet sieved with ethanol to a 45 to 75 μm particle size. All reflectance data are corrected for the non‐Lambertian scattering properties of halon. Analysis of the parameters of the single‐particle phase function indicate that each mineral defines a relatively unique suite of scattering parameters as a function of albedo. In particular, the silicates are forward scattering where the degree of forward scattering increases with albedo, magnetite is forward scattering but the degree of forward scattering decreases with albedo, and hematite is entirely backward scattering where the degree of backward scattering increases with albedo. It is apparent that the physical properties of the particles (roughness, transparency, shape) rather than albedo or chemical composition determine the gross scattering properties of the particulate surfaces studied here. The scattering parameters of the mixtures are similar to the parameters of the more abundant mineral in the mixture. The experimentally determined single‐scattering albedos are used to calculate the mass fractions of olivine and magnetite in these mixtures. These new fractions are much improved over the previous analyses and indicate that more detailed information regarding the geometric dependence of reflectance overcomes the systematic deficiencies of the simplified methods. In general, the simplified method is appropriate for mixtures not containing low albedo material using bidirectional reflectance measured at intermediate phase angles (20°–40°) and small emergence angles (near 0°). Abundance estimates using this approach are accurate to within 5–10% when information regarding the particle size of the surface components is available, and in a relative sense for surfaces of unknown particle size distributions. More precise abundance estimates can be derived if the scattering properties of the surface are well characterized and incorporated

ISSN:0148-0227    

DOI:10.1029/JB094iB10p13619

年代:1989

数据来源: WILEY

摘要:

The U.S. Geological Survey has been using the Global Positioning System (GPS) to perform repeated surveys of networks of different sizes since January 1986. The intersite distances from these various networks are 223 km for a single line connecting the Palos Verdes and Vandenberg sites in southern California; 30–50 km for a network of three sites south of the San Francisco Bay near Loma Prieta; 9–30 km for a network located near Hebgen Lake, Montana; 7–12 km for a network of sites in the vicinity of Parkfield, California; and 240 m for a line located at the Goldstone antenna complex in the Mojave desert. The lengths quoted do not include the intersite distances for the “fiducial” sites included in the solutions for orbit determination, which can be up to 8000 km. We have attempted to assess the “long‐term” precision and accuracy of determinations of relative site position for a range of line lengths. This assessment has consisted of several different studies. In assessing precision, we have generally investigated repeatability, usually about a best fit linear temporal trend. The smallest fractional repeatability was yielded by the measurements on the Palos Verdes to Vandenberg line. The root‐mean‐square (rms) residual about the best fit line for these determinations was 6 mm (0.03 parts per million, or ppm) for the north component, 11 mm (0.05 ppm) for the east component, and 40 mm (0.18 ppm) for the vertical component. For these determinations, GPS data from 3–4 consecutive days were combined to produce a single estimate of the vector separation. We also used the day‐to‐day scatter from these measurements to perform an analysis of variance and found that these multiday determinations can be significantly more precise than estimates determined from a single observing session. The worst results were obtained for the Loma Prieta network, where we obtained rms residuals of 11–12 mm (0.3–0.4 ppm) for north, 14–18 mm (0.3–0.6 ppm) for east, and 18–21 mm (0.4–0.7 ppm) for vertical. We cannot definitely explain these poor results, but we have evidence to indicate that this poor scatter is due to the short observing session (∼4.5 hours, compared to ∼6.5 hours for the Palos Verdes to Vandenberg results). In assessing accuracy, we compared the GPS results to those from independent techniques. At Parkfield we compared the GPS estimates of fault‐parallel motion to those from creep meters and alignment arrays and found excellent agreement (within 1–2 mm yr−1) not only for the slip but also for the variation of slip along the San Andreas fault. We compared the Loma Prieta and Hebgen Lake line length estimates to those obtained from a Geodolite, a high‐precision laser distance‐measurement device that the U.S. Geological Survey has used for 20 years for studies of crustal deformation. If we model the Geodolite minus GPS line length differences ΔLas ΔL= α + βL, whereLis the line length, then we find α = 0.6 ± 0.5 mm and β = 0.1 ± 0.1 mm km−1. The uncertainties quoted are one standard deviation. From this comparison, we conclude that at this level of significance there is no detectable difference between lengths determined from Geodolite and GPS data. Finally, we compared the GPS estimates of the Palos Verdes to Vandenberg line length to those obtained from very long baseline interferometry (VLBI). The mean difference between the six GPS estimates of line length and a best fit line through the VLBI estimates was −1 mm, and the rms difference was 12 mm (0.05 ppm), close to the standard deviatio

ISSN:0148-0227    

DOI:10.1029/JB094iB10p13635

年代:1989

数据来源: WILEY

摘要:

The present‐day global geoid and free air gravity anomaly signals induced by Pleistocene deglaciation have been computed, along with their secular variations. A revised Green function has been derived for the free air gravity anomaly, and we find that an Earth model with a lower mantle viscosity moderately larger than the upper mantle value satisfies observations of the anomaly over both Hudson's Bay and Fennoscandia. Using this “preferred” Earth model, the predicted global geoid anomaly map is characterized by peak negative values of −36.7, −9.4, and −21.9 m over Hudson's Bay, northern Europe, and Antarctica, respectively, and by a smooth, small amplitude (≲2.5 m) upwarping over the major ocean basins. Although the field has an order of magnitude less power than the observed, there is a significant spatial correlation between the two. Inferences of mantle viscosity based upon comparisons between the observed geoid and the computed deglaciation‐induced anomaly are hampered by the large contribution from the convective circulation in the Earth's mantle. We have subtracted a predicted convection signal (based on seismic tomography) from the observed field; however, the “residual” geoid thus obtained is still too inaccurate to allow a useful comparison. In contrast, a filtered version of the observed geoid over Canada, which retains harmonic coefficients in the degree range 10–22, provides a relatively uncontaminated (by convection) datum that is indeed satisfied by the preferred Earth model. Secular variations in the Earth's gravitational field are naturally dominated by the signal from Pleistocene deglaciation. In this respect, we compute zonal harmonics in the expansion of the present day rate of change of the geopotential, and in particular, we predict amplitudes comparable to that for the previously analysedJ2forJ4,J5,J7, andJ9, all of which reflect, in large part, the response due to the melting of the massive Laurentide ice sheet. This analysis suggests that measurement of these higher‐order secular variations of the zonal harmonics based upon analysis of long time series of LAGEOS ranging data could provide very useful additional constraints on the radial variation of mantle viscosity. These constraints, along with all others described in this paper, are shown to be relatively insensitive to assumptions concerning the deflection, during the glacial isostatic adjustment process, of the 400‐ and 670‐km density discon

ISSN:0148-0227    

DOI:10.1029/JB094iB10p13651

年代:1989

数据来源: WILEY

摘要:

Maximum entropy spectral analysis has been applied to the annual mean magnetic data around London for the years 1839–1974. These data were compiled by Malin and Bullard. The existence of a ∼60‐year spectral line is clearly found. In addition, cross power spectral density analyses using a two‐channel maximum entropy method were applied to both the variations of the annual mean declination around London from 1900 to 1974 and the fluctuations of the Earth's rotation rate from 1876 to 1974. The coherence is high (0.98), and the phase angle indicates that the changes of the magnetic declinations around London lag behind the fluctuations of the Earth's rotation rate by 20

ISSN:0148-0227    

DOI:10.1029/JB094iB10p13673

年代:1989

数据来源: WILEY

摘要:

The Jurassic Preuss Sandstone, exposed in five thrust plates of the Wyoming‐Idaho‐Utah thrust belt, carries directions of remanent magnetization that group most tightly after only partial unfolding. Field, petrographic, and rock magnetic evidence indicates that the carrier of this magnetization is detrital, low‐Ti titanomagnetite. The detrital titanomagnetite was remagnetized at low temperatures (75°–150°C) probably completely during folding. Anisotropy of magnetic susceptibility and petrographic observations indicate that the detrital titanomagnetite has been affected by tectonic strain. We suggest that low‐temperature remagnetization of the detrital titanomagnetite was either a viscous partial thermoremanent magnetization, the acquisition of which was enhanced by stress, or a piezoremanent magnetization that involved stress‐induced movement of domain walls during intracrystalline strain, or was a combination of the two mechanisms. Stress may promote remagnetization at temperatures much lower than predicted by current theoretical models. Other mechanisms, such as acquisition of chemical remanent magnetization during folding, deflection of a prefolding magnetization by internal strain, or combination of components of magnetization with different direction cannot account for the geometry of magnetization in the Preuss. The locus of acquisition of synfolding magnetization in the Preuss migrated in conjunction with deformation in the thrust belt. A model is presented in which synfolding magnetization was acquired during cooling and folding as strata moved up thrust ramps. A lack of reverse‐polarity directions remains a puzzling feature of the remanence. The remanent direction is tentatively interpreted to reflect the predominant polarity state during its acquisition over an extended rather than a discrete time period during folding in Late Cretaceous and early Tertiary (?) periods of predominantly

ISSN:0148-0227    

DOI:10.1029/JB094iB10p13681

年代:1989

数据来源: WILEY

摘要:

An important issue is whether thermal convection in the Earth's mantle is chaotic and the mechanisms by which this chaos may be generated. In this paper, we conclude that the high Prandtl number/high Raleigh number thermal convection associated with mantle convection is chaotic, and we document the route to chaos. Deterministic chaos was first exhibited by the Lorenz equations, a three‐mode spatial Fourier expansion of the nonlinear thermal convection equations. This transition to chaos is characterized by the two nontrivial fixed points undergoing a reverse Hopf bifurcation into two unstable limit cycles. However, when the limit of infinite Prandtl number is taken for these equations, the fixed points are stable for all Rayleigh numbers. Is the stability of these fixed points a characteristic of infinite Prandtl number convection or is this an artifact of the truncation? To answer this question, we extend the expansion to 12 modes {n= 0, 1, 2;m= 1, 2, 3, 4} wherenis the horizontal wave number andmis the vertical wave number to generate a set of 12 ordinary differential equations. We calculate the loci of roots of this system as a function of Rayleigh number, and we calculate several trajectories. At Rayleigh numbers betweenRa= 0 andRa= 4.140×104we find pitchfork bifurcations to 14 separate nontrivial branches of solution, and we determine their stability by calculating the eigenvalues of the Jacobian matrix along each branch; we thus find 40 Hopf bifurcations. These fixed points and their bifurcations give excellent quantitative agreement with the trajectories, converging to stable fixed points at and belowRa= 4×104, and oscillating aperiodically at and aboveRa= 4.5×104, where many of the Hopf bifurcations occur. Our conclusion is that thermal convection at infinite Prandtl number becomes chaotic by means of symmetry‐breaking pitchfork bifurcations and the appearance of the Hopf bifurcations which produce unstable periodic

ISSN:0148-0227    

DOI:10.1029/JB094iB10p13707

年代:1989

数据来源: WILEY

摘要:

A model for the degassing of Xe, Ar, and He from the Earth has been constructed. The Earth is divided into three idealized reservoirs in terms of their noble gas inventories: undegassed mantle, degassed mantle or mid‐ocean ridge basalt (MORB) mantle, and atmosphere plus continental crust. Degassing is assumed to occur via the partitioning of gases between a vapor phase and basaltic melt, in accord with He‐Ar isotope systematics which require that3He be degassed at a slower rate than36Ar on average. A total inversion technique is used to handle the large uncertainties associated with the input parameters. The model successfully reconciles presently available isotope ratios of129Xe/130Xe,40Ar/36Ar, and4He/3He in the different Earth reservoirs, and accurately predicts the present degassing rates of He and Ar. This is the first time that noble gas isotope data have been shown to be consistent with a single degassing model. The success of this model demonstrates the feasibility of a solubility‐controlled degassing mechanism, and shows that there is a relationship between noble gas isotope ratios and the melt‐vapor partition coefficients for those gases, which has been obscured by the complicated radiogenic growth and degassing equations. The high129Xe/130Xe ratios in MORB are shown to be consistent with the low solubility of Xe in silicate melts, which resulted in a faster degassing rate for Xe than for other gases in the context of this model. Therefore, the mean degassing time derived from Xe is not directly applicable to other gases. The degassed mantle, our end‐member degassed mantle reservoir, is found to be almost completely degassed for the noble gases, while the exact degree of degassing depends on the species under consideration, as well as the choice of isotope ratios to characterize this mantle segment. If extreme isotope ratios are used, the degree of degassing for the degassed mantle is: 99.92% for130Xe, 99.7% for36Ar, 97.5% for3He, 82% for40Ar and 88% for4He. The mean time of degassing, measured from the formation of Earth, is 21±7 Ma for130Xe, 56±19 Ma for36Ar, 310±120 Ma for3He, ∼1.5 Ga for40Ar, and ∼0.8 Ga for4He. Because the degassing is likely to be solubility controlled at some level, study of noble gases alone does not fully constrain the evolution of the atmosphere. However, the model can be extended to major gaseous species in the Earth's primary atmosphere if the relevant solubilities are known. Similarities between the solubilities of CO2, CO, and He, and between N2and Ar, suggest a mean age for the total atmosphere on the order of 4.4 to 4.3 Ga. The formation of the oceans was a much later event because of the much higher solubility of H2O in silicate melt. If H2O has been the primary outgassing species for ocean water, the oceans have a mean age on the order of 2.7 Ga, suggesting that the volume ratio of continental crust to the oceans might have remained relatively constant duri

ISSN:0148-0227    

DOI:10.1029/JB094iB10p13719

年代:1989

数据来源: WILEY

摘要:

Seismic tomography in both the upper and the lower mantle, as well as subducting oceanic slabs defined by seismicity, has been translated into density heterogeneities to generate models of mantle circulation. These models can predict both the surface velocities and the geoid, which can be compared with plate tectonics and gravity data. A given model is specified by 6 parameters related to the viscosities of 3 mantle layers and the absolute amplitudes of density variations in the upper and lower mantle as well as in the slabs. The values of these parameters are chosen at random within an acceptable range. Each model is submitted to an appropriate test comparing observations with predictions. The results of the most successful models selected by this Monte Carlo inversion are displayed. They yield preferred mantle viscosity structures exhibiting large variations at depth. With a physical interface between upper and lower mantle, i.e., with the possibility for the circulation to penetrate the 650 km discontinuity, two classes of viscosity profiles stand out. The first one implies a regular increase of the viscosity in the sublithospheric mantle, with reasonable values for the density parameters. The second one is unexpected in the sense that it predicts a very stiff bottom for the upper mantle. It also requires vanishingly small amplitudes for the upper mantle density heterogeneities defined by tomography, which would thus have to be of lithological rather than thermal origin. With a chemical interface at 650 km the outcome is very similar: the same two classes of viscosity structures do yield a satisfactory geoid prediction. However only the class of models with a stiff layer at midmantle depths predicts acceptable surface velocities. Altogether the best models out of some 60,000 which have been tested only explain one third of the geoid and two thirds of the surface divergence for spherical harmonic degrees 1 to 6. Nevertheless the main features of these two observed patterns are present in the computed maps, and 4 out of 6 correlation coefficients lie close to the 90% confidence level. This is true for the geoid as well as for the surface divergence of the displacement velocity. However, as the internal viscosity structure has been assumed to have spherical symmetry, the rotational component of the surface velocities cannot be predicted.

ISSN:0148-0227    

DOI:10.1029/JB094iB10p13739

年代:1989

数据来源: WILEY

摘要:

We propose a new model for the tectonic evolution of the eastern Indian Ocean from the Late Cretaceous to the present. Two types of data are used to improve previously published reconstructions. First, recent reinterpretations of seafloor magnetic anomalies, between Australia and Antarctica and in the Wharton Basin, provide new constraints on spreading rates and the timing of major reorganizations. Second, vertical deflection profiles (i.e., horizontal gravity anomaly), derived from 22 repeat cycles of Geosat altimeter data, reveal the tectonic fabric associated with fracture zones. These new Geosat data provide tight constraints on paleospreading directions. For example, three prominent fracture zones can be traced from south of Tasmania to the George V Basin, Antarctica, providing an important constraint on the relative motions of Australia and Antarctica through the Late Eocene. In addition, the Geosat profiles are used to locate the conjugate continental margins and continent‐ocean boundaries of Australia and Antarctica, as well as the conjugate rifted margins of Kerguelen Plateau and Broken Ridge. Based on a compilation of magnetic anomaly data from the Crozet Basin, the Central Indian Basin, the Wharton Basin and the Australian‐Antarctic Basin, ten plate tectonic reconstructions are proposed. Reconstructions at chrons 5 (11 Ma), 6 (21 Ma), 13 (36 Ma) and 18 (43 Ma) confirm that the Southeast Indian Ridge behaved as a single plate boundary since chron 18. The constraints from the Geosat data provide an improvement in the fit of the Kerguelen Plateau and Broken Ridge at chron 20 (46 Ma). To avoid overlaps between Broken Ridge and the Kerguelen Plateau prior to their breakup, our model includes relative motions between the northern and southern provinces of the Kerguelen Plateau. Finally, we examine the implications of our model for the relative motions of India, Australia and Antarctica on the tectonic evolution of the Kerguelen Plateau and Broken Ridge, and the adjacent Labuan Basin and Diamantina Zone, as well as the emplacement of the Ninetyeast Ridge and the Kerguelen Plateau over a fixed hot s

ISSN:0148-0227    

DOI:10.1029/JB094iB10p13755

年代:1989

数据来源: WILEY