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Hydroxylapatite (HA) nanoparticles were functionalized by depositing rare-earth-doped Y2O3Y2O3 nanoparticles on the surface, and the structural evolutions of both HA and Y2O3Y2O3 phases at different annealing temperatures were investigated by x-ray diffraction and transmission electron microscopy. Laser spectroscopy indicated that the surface functionalized HA nanoparticles exhibited strong visible emissions. No visible emissions were observed from rare-earth-doped Y2O3Y2O3 without any substrate, suggesting a doping-induced environmental change of optically active rare-earth elements in the functionalized HA nanoparticles. The luminescent hydroxylapatite nanoparticles may find important applications as a biodegradable substrate for biomarking and drug delivery.

We report a quantitative analysis of regional differences in the the oxygen isotope composition of river water and precipitation across the USA because data are now available to undertake a more geographically and temporally extensive analysis than was formerly possible. Maps of modern, mean annual?? 18 O values for both precipitation (?? 18 O PPT) and river water (?? 18 O RIV) across the 48 contiguous states of the USA have been generated using latitude and elevation as the primary predictors of stable isotope composition while also incorporating regional and local deviations based on available isotopic data. The difference between these two maps was calculated to determine regions where?? 18 O RIV is significantly offset from local?? 18 O PPT . Additional maps depicting seasonal and extreme values for?? 18 O RIV and?? 18 O PPT were also constructed. This exercise confirms the presence of regions characterized by differences in?? 18 O RIV and?? 18 O PPT and specifically identifies the magnitude and regional extent of these offsets. In particular, the Great Plains has?? 18 O RIV values that are more positive than precipitation, while much of the western USA is characterized by significantly lower?? 18 O RIV values in comparison with local?? 18 O PPT . The most salient feature that emerged from this comparison is the???catchment effect??? for the rivers. Because river water is largely derived from precipitation that fell upstream of the sample locality (i.e. at higher elevations)?? 18 O RIV values are often lower than local?? 18 O PPT values, particularly in catchments with high-elevation gradients. Seasonal patterns in the isotopic data substantiate the generally accepted notion that amplitudes of?? 18 O variation are greatly dampened in river water relative to those of local precipitation. Copyright?? 2005 John Wiley & Sons, Ltd.

Electron microbeam techniques have been used to examine submicroscopically intergrown paragonite, phengite and chlorite from the South Fork Mountain Schist of the Franciscan Terrane of northern California, which was subjected to blueschist facies metamorphism. The sample also contains quartz, albite, lawsonite, and rutile. The subassemblage albite-lawsonite-rutile requires metamorphic conditions on the low-temperature side of the equilibrium albite+lawsonite+rutile=paragonite+sphene+quartz+H 2 O (T<200?? C and P<7.4 kbars based on thermodynamic data of Holland and Powell 1990). The white micas appear to be optically homogeneous, but back-scattered electron images can distinguish two different micas by their slight difference in contrast. Electron microprobe analyses (EMPA) of micas show Na/(Na+K) ranging from 0.2 to 0.8. The two micas are resolved by transmission electron microscopy (TEM) as packets of phengite and paragonite that range from 20 to several hundred nm in thickness. The compositions, determined by analytical electron microscopy (AEM), constrain the limbs of the phengite-paragonite solvus to values of Na/(Na+K)=<0.02 and 0.97, representing less mutual solid solution than ever reported by EMPA. The textural relations imply that the sheet silicates were derived from reactions between fluids and detrital clays and that they are in an intermediate stage of textural development. We caution that microprobe analyses of apparently homogeneous sheet silicates may yield erroneous data and lead to faulty conclusions using phengite barometry and paragonite-muscovite thermometry, especially in fine-grained rocks that formed at relatively low temperatures.

The Sr, Nd, and O isotopic compositions of garnet and clinopyroxene mineral separates from nine eclogite xenoliths from the Bellsbank kimberlite (erupted at 120 Ma) define three groups. Group A eclogites, considered to be mantle cumulates, are characterized by [delta]18O, and 87Sr/86Sr values typical of mantle-derived materials (+5.1 to +5.6[per mille sign] and 0.7042-0.7046, respectively), and very low Sm/Nd ratios, (apparent) Rb/Sr ratios and [epsilon]Nd[120] values (0.057-0.078, 0.00005-0.00136 and -14 to -16 respectively). The REE and isotopic data for these eclogites can be modelled in terms of crystallization from a Group II kimberlite magma at ~ 1-1.5 Ga. Group B and C eclogites, believed to be the metamorphosed products of ancient subducted oceanic crust, are characterized by low [delta]18O (+2.9 to +4.7), extremely high [epsilon]Nd[120] (~ +40 to +219), and radiogenic 87Sr/86Sr ratios (0.708-0.710). The Sm/Nd ratios of the Group B eclogites are very high (up to 1.6). The data for Group B and C eclogites define a linear correlation on Sm/Nd and 1/Nd vs. [epsilon]Nd[120] diagrams. These relationships are consistent with mixing of the Bellsbank kimberlite ([epsilon]Nd[120] = -10; Sm/Nd = 0.10) with a depleted eclogite end-member ([epsilon]Nd[120] +219; Sm/Nd = 1.6) during a cryptic metasomatic event. The Sr isotopic variations in Group B and C eclogites cannot be generated by simple two-component mixing. The Sr, Nd, and O isotope data for Group B and C eclogites probably reflect a complex sequence of depletion and enrichment events, in both crust and mantle settings. Enrichments which possibly affected the Group B and C eclogites include seawater-alteration of a MORB-like protolith, which lowered the [delta]18O and raised the 87Sr/86Sr ratio, but left the Nd isotopic compositions unchanged, and cryptic metasomatism by the magmatism that produced the Bellsbank kimberlite. The high Sm/Nd ratio of the depleted eclogite end-member cannot be generated by extraction

The thin film of carbonfluorine was deposited on the surfaces of aligned carbon nanotubes using a plasma polymerization treatment. High-resolution transmission electron microscopy images revealed that a thin film of the polymer layer (20???nm)(20nm) was uniformly deposited on the surfaces of the aligned carbon nanotubes. Time-of-flight secondary ion mass spectroscopy and Fourier transform infrared experiments identified the carbonfluorine thin films on the carbon nanotubes. The plasma deposition mechanism is discussed.

In this paper, we report a new method to prepare the polymer/inorganic nanoparticle composites using electron irradiation-induced polymerization. The mixture of nanoparticles and MMA solution were co-irradiated by 1.6??MeV electron beam to a dose of 10, 20 and 30??kGy at a dose-rate of 60??kGy/h in air at room temperature. The products after irradiation were extracted using a soxhlet extractor with boiling xylene and investigated by X-ray diffraction (XRD), Fourier transmission infrared (FTIR), X-ray photoelectron spectroscopy (XPS), optical absorption spectra (OAP) and photoluminescence (PL). The FTIR and XPS results show that there exist some unextractable PMMA in the nanocomposites after extraction, indicating a strong interaction between the PMMA and nanoparticles. PL results show that new luminescence peaks appear at 415 and 420??nm for the nanocomposites of anatase and??-Al 2 O 3.

???Hydrogen and oxygen isotope analyses have been made of hydrous minerals in gabbros and basaltic xenoliths from the Eocene Kap Edvard Holm intrusive complex of East Greenland. The analyzed samples are of three types: (1) primary igneous hornblendes and phlogopites that crystallized from partial melts of hydrothermally altered basaltic xenoliths, (2) primary igneous hornblendes that formed during late???magmatic recrystallization of layered gabbroic cumulates, and (3) secondary actinolite, epidote and chlorite that formed during subsolidus alteration of both xenoliths and gabbros. Secondary actinolite has a?? 18 O value of???5.8??? and a??D value of???158???. These low values reflect subsolidus alteration by low????? 18 O, low?????D hydrothermal fluids of meteoric origin. The??D value is lower than the???146 to???112??? values previously reported for amphiboles from other early Tertiary meteoric???hydrothermal systems in East Greenland and Scotland, indicating that the meteoric waters at Kap Edvard Holm were isotopically lighter than typical early Tertiary meteoric waters in the North Atlantic region. This probably reflects local climatic variations caused by formation of a major topographic dome at about the time of plutonism and hydrothermal activity. The calculated isotopic composition of the meteoric water is??D=???110?????10???,?? 18 O??????15???. Igneous hornblendes and phlogopites from pegmatitic pods in hornfelsed basaltic xenoliths have?? 18 O values between???6.0 and???3.8??? and??D values between???155 and???140???. These are both much lower than typical values of fresh basalts. The oxygen isotope fractionations between pegmatitic hornblendes and surrounding hornfelsic minerals are close to equilibrium fractionations for magmatic temperatures, indicating that the pegmatites crystallized from low????? 18 O partial melts of xenoliths that had been hydrothermally altered and depleted in 18 O prior to stoping. The pegmatitic minerals may hav

The thermal regime in the moist surficial peat layer of a palsa, measured during a clear and calm July day in arctic Alaska, indicates that coupled heat and water flow retard ablation of the ice core. A simulation model of coupled-flow effects was constructed using rough estimates of meteorological boundary conditions and material properties. The model indicates that ablation is inhibited by advection of cold water above the ice/peat interface, and by internal evaporation near the surface. Evaporation also limits thermal maxima near the surface; because the ratio of the heat of evaporation to the heat of fusion is approximately 7.5, the coupled-flow regime effectively retards ablation of the palsa's ice core. Das Temperaturregime in der feuchten, oberfl??chennahen Torfschicht eines Pals wurde w??hrend eines klaren und windstillen Julitages in der Arktis von Alaska vermessen. Es wird gezeigt, da?? der gekoppelte W??rme- und Wasserstrom die Ablation des Eiskernes verz??gert. Ein Simulationsmodell der gekoppelten Stromeffekte wurde konstruiert, wobei grobe Absch??tzungen der meteorologischen Grenzbedingungen und der Bodenbeschaffenheit angebracht wurden. Das Modell deutet an, da?? die Ablation durch Advektion von kaltem Wasser oberhalb der Eis-Torf-Grenzschicht und durch interne Verdunstung nahe der Oberfl??che behindert wird. Verdunstung beschr??nkt au??erdem die Temperaturmaxima nahe der Oberfl??che. Da das Verh??ltnis der latenten W??rmen der Verdunstung und der Sublimation ungef??hr 7,5 betr??gt, verz??gert das gekoppelte Stromregime effektiv die Ablation des Eiskernes der Pals.

Das Tr??gergestein der Chapada Kupferlagerst??tte in Goi??s ist in metamorpher Hoch-Amphibolitfazies ausgebildet. Gesamtgesteins- und individuelle Mineral Rb-Sr-Isochronen mit einem 87 Sr/ 86 Sr-Initialwert (I) von 0,70414??0.00005 ergeben f??r die Kristallisation der Vulkanite und Magmatite, die die Metamorphose vordatieren, ein Alter von 561??9 Mio Jahre. Die Datierung der Metamorphose w??hrend der panafrikanisch/brasilianischen Orogenese (700 bis 450 Mio Jahre) ergibt 532??1 Mio Jahre. Das Mineral-Isochronenalter eines Dioritk??rpers porphyrischen Gef??ges, der in die Schiefer intrudierte, betr??gt 534??16 Mio Jahre (I=0,70415 ??0,00004). Das Ergebnis niedriger Initial-Werte dieser Isochronen ergibt f??r die Schiefer bei Chapada ein Ausgangs-Material im Bereich des oberen Mantels oder der tieferen Kruste; diese Ausgangswerte sind charakteristisch f??r magmatische T??tigkeit in Verbindung mit Inselb??gen. Das unterkambrische Alter des Tr??gergesteins bei Chapada steht im Widerspruch zu dem bisher angenommenen mittelproterozoischen Alter und ist ein Hinweis auf eine m??gliche Phase intensiverer pan-afrikanisch/brasilianischer magmatischer Aktivit??t in Zentral-Brasilien als urspr??nglich angenommen wurde. Whole rock and mineral Rb-Sr isochrons from the upper amphibolite grade metamorphic host rocks of the Chapada copper deposit in Goi??s, Brazil, yield an age of 561??9 Ma, with a 87 Sr/ 86 Sr initial (I) of 0.70414??0.00005, for the crystallization of pre-metamorphic volcanic and igneous rocks and 532 ??1 Ma for their metamorphism during the Pan-African/Brasiliano orogeny (700-450 Ma). A porphyritic diorite stock which intruded the host schists yields a mineral isochron age of 534??16 Ma (I=0.70415??0.00004). The low I values determined from these isochrons suggest that the host schists at Chapada were derived from upper mantle or lower crust material and are characteristic of magmatic products associated with island arcs. The Early Cambrian age of the host rocks a

Subduction-related volcanism in the Nevados de Payachata region of the Central Andes at 18??S comprises two temporally and geochemically distinct phases. An older period of magmatism is represented by glaciated stratocones and ignimbrite sheets of late Miocene age. The Pleistocene to Recent phase (???0.3 Ma) includes the twin stratovolcanoes Volcan Pomerape and Volcan Parinacota (the Nevados de Payachata volcanic group) and two small centers to the west (i. e., Caquena and Vilacollo). Both stratovolcanoes consist of an older dome-and-flow series capped by an andesitic cone. The younger cone, i. e., V. Parinacota, suffered a postglacial cone collapse producing a widespread debris-avalanche deposit. Subsequently, the cone reformed during a brief, second volcanic episode. A number of small, relatively mafic, satellitic cinder cones and associated flows were produced during the most recent activity at V. Parinacota. At the older cone, i. e., V. Pomerape, an early dome sequence with an overlying isolated mafic spatter cone and the cone-forming andesitic-dacitic phase (mostly flows) have been recognized. The two Nevados de Payachata stratovolcanoes display continuous major- and trace-element trends from high-K 2 O basaltic andesites through rhyolites (53%???76% SiO 2) that are well defined and distinct from those of the older volcanic centers. Petrography, chemical composition, and eruptive styles at V. Parinacota differ between pre- and post-debris-avalanche lavas. Precollapse flows have abundant amphibole (at SiO 2> 59 wt%) and lower Mg numbers than postcollapse lavas, which are generally less silicic and more restricted in composition. Compositional variations indicate that the magmas of the Nevados de Payachata volcanic group evolved through a combination of fractional crystallization, crustal assimilation, and intratrend magma mixing. Isotope compositions exhibit only minor variations. Pb-isotope ratios are relatively low (206 Pb/ 204 Pb = 17.95???18.20 and 208 Pb

Located at the volcanic front in the western Mexican arc, in the Colima Rift, is the active Volc??n Colima, which lies on the southern end of the massive (???450??km 3) Colima-Nevado volcanic complex. Along the margins of this andesitic volcanic complex, is a group of 11 scoria cones and associated lavas, which have been dated by the 40 Ar/ 39 Ar method. Nine scoria cones erupted???1.3??km 3 of alkaline magma (basanite, leucite-basanite, minette) between 450 and 60??ka, with>99% between 240 and 60??ka. Two additional cones (both the oldest and calc-alkaline) erupted <0.003??km 3 of basalt (0.5??Ma) and <0.003??km 3 of basaltic andesite (1.2??Ma), respectively. Cone and lava volumes were estimated with the aid of digital elevation models (DEMs). The eruption rate for these scoria cones and their associated lavas over the last 1.2??Myr is???1.2??km 3 /Myr, which is more than 400 times smaller than that from the andesitic Colima-Nevado edifice. In addition to these alkaline Colima cones, two other potassic basalts erupted at the volcanic front, but???200??km to the ESE (near the historically active Volc??n Jorullo), and were dated at 1.06 and 0.10??Ma. These potassic suites reflect the tendency in the west-central Mexican arc for magmas close to the volcanic front to be enriched in K 2 O relative to those farther from the trench.

The oxygen fugacities (fO 2 's) of magnetically-concentrated fractions (MCF) of three rock samples from the Skaergaard Layered Intrusion were measured between 800???1150?? C using oxygen-specific, solid zirconia electrolytes at atmospheric pressure. Two of the bulk rock samples (an oxide cumulate and an oxide-bearing gabbro) are from the Middle Zone (MZ) and the other (an olivine plagioclase orthocumulate) is from the Lower Zone (LZ). All MCF define fO 2 versus T arrays that lie 1.5???0.5 log units above the fayalite-magnetite-quartz (FMQ) buffer. Experiments with different cell-imposed initial redox states (one from a reduced direction and one from an oxidized direction) were run on each sample in an attempt to achieve experimental reversibility. This was accomplished by imposing a known redox memory on the galvanic cell prior to loading each sample. Reversibility for each sample agreed to better than 0.2 of a log unit. Irreversible autoreduction of 0.2 of a log unit was observed on the two MZ samples at temperatures exceeding 1065?? C. Scanning electron microscope and electron microprobe study of pre- and post-run products shows that reaction and textural re-equilibration occurred among the oxide phase assemblages under the experimental conditions employed. Careful characterization of pre- and post-run assemblages is clearly necessary before adequate interpretation of the experimental results can be made in these types of electrochemical studies. Different approaches to investigations of the fO 2 of the Skaergaard Intrusion, be it thermodynamic calculations or experimental methods, should yield concordant results or at least understandable discrepancies. Calculated fO 2 's using thermobarometry applied to the ilmenite-magnetite pairs in the post-experimental assemblages agree with the experimentally determined fO 2 's to within one log unit at a given temperature. These results are also consistent with previously calculated fO 2 values (Buddington and Lindsley 196

A wide variety of rock types are present in the O'Leary Peak and Strawberry Crater volcanics of the Pliocene to Recent San Francisco Volcanic Field (SFVF), AZ. The O'Leary Peak flows range from andesite to rhyolite (56???72 wt % SiO 2) and the Strawberry Crater flows range from basalt to dacite (49???64 wt % SiO 2). Our interpretation of the chemical data is that both magma mixing and crustal melting are important in the genesis of the intermediate composition lavas of both suites. Observed chemical variations in major and trace elements can be modeled as binary mixtures between a crustal melt similar to the O'Leary dome rhyolite and two different mafic end-members. The mafic end-member of the Strawberry suite may be a primary mantle-derived melt. Similar basalts have also been erupted from many other vents in the SFVF. In the O'Leary Peak suite, the mafic end-member is an evolved (low Mg/(Mg+ Fe)) basalt that is chemically distinct from the Strawberry Crater and other vent basalts as it is richer in total Fe, TiO 2, Al 2 O 3, MnO, Na 2 O, K 2 O, and Zr and poorer in MgO, CaO, P 2 O 5, Ni, Sc, Cr, and V. The derivative basalt probably results from fractional crystallization of the more primitive, vent basalt type of magma. This evolved basalt occurs as xenolithic (but originally magmatic) inclusions in the O'Leary domes and andesite porphyry flow. The most mafic xenolith may represent melt that mixed with the O'Leary dome rhyolite resulting in andesite preserved as other xenoliths, a pyroclastic unit (Qoap), porphyry flow (Qoaf) and dacite (Darton Dome) magmas. Thermal constraints on the capacity of a melt to assimilate (and melt) a volume of solid material require that melt mixing and not assimilation has produced the observed intermediate lavas at both Strawberry Crater and O'Leary Peak. Textures, petrography, and mineral chemistry support the magma mixing model. Some of the inclusions have quenched rims where in contact with the host. The intermediate rocks

The Cabo de Gata volcanic field of southeastern Spain contains several recently-recognized calderas. Some of the calderas are mineralized with epithermal gold, alunite, and base metal deposits, and others are barren, and yet they formed under generally similar conditions. Comparison of the magmatic, geochemical, and physical evolution of the Los Frailes, Rodalquilar, and Lomilla calderas provides insight into the processes of caldera evolution that led to precious-metal mineralization. The Los Frailes caldera formed at 14.4 Ma and is the oldest caldera. It formed in response to multiple eruptions of hornblende dacite magma. Following each eruption, the area collapsed and the caldera was invaded by the sea. Dacite domes fill the lower part of the caldera. Pyroxene andesites were erupted through the solidified core of the caldera and were probably initially responsible for magma generation. The Los Frailes caldera did not evolve to rhyolites nor was it subjected to the amount of structural development that the younger, mineralized Rodalquilar and Lomilla calderas were.

While most aspects of subduction have been extensively studied, the process of subduction initiation lacks an observational foundation. The Macquarie Ridge complex (MRC) forms the Pacific-Australia plate boundary between New Zealand to the north and the Pacific-Australia-Antarctica triple junction to the south. The MRC consists of alternating troughs and rises and is characterized by a transitional tectonic environment in which subduction initiation presently occurs. There is a high seismicity level with 15 large earthquakes (M>7) in this century. Our seismological investigation is centered on the largest event since 1943: the 25 MAY 1981 earthquake. Love, Rayleigh, and P waves are inverted to find: a faulting geometry of right-lateral strike-slip along the local trend of the Macquarie Ridge (N30??E); a seismic moment of 5??10 27 dyn cm (M w =7.7) a double event rupture process with a fault length of less than 100km to the southwest of the epicenter and a fault depth of less than 20km. Three smaller thrust earthquakes occurred previous to the 1981 event along the 1981 rupture zone; their shallow-dipping thrust planes are virtually adjacent to the 1981 vertical fault plane. Oblique convergence in this region is thus accommodated by a dual rupture mode of several small thrust events and a large strike-slip event. Our study of other large MRC earthquakes, plus those of other investigators, produces focal mechanisms for 15 earthquakes distributed along the entire MRC; thrust and right-lateral strike-slip events are scattered throughout the MRC. Thus, all of the MRC is characterized by oblique convergence and the dual rupture mode. The???true??? best-fit rotation pole for the Pacific-Australia motion is close to the Minster & Jordan RM2 pole for the Pacific-India motion. Southward migration of the rotation pole has caused the recent transition to oblique convergence in the northern MRC. We propose a subduction initiation process that is akin to crack propagation; the

The early P wave coda (5???15 sec after the first arrival) of underground explosions at the Nevada Test Site is studied in the time domain using 2082 teleseismic short-period recordings, with the intent of identifying near-source contributions to the signals in the frequency range 0.2???2.0 Hz. Smaller magnitude events tend to have relatively high coda levels in the 0.4???0.8 Hz frequency band for both Yucca Flat and Pahute Mesa explosions. Coda complexity in this low-frequency passband is negatively correlated with burial depth for Pahute Mesa events but is only weakly correlated with depth for Yucca Flat events. Enhanced excitation of relatively long-period scattered waves for smaller, less deeply buried events is required to explain this behavior. Coda complexity in the 0.8???1.1 Hz band is positively correlated with magnitude and depth for Pahute Mesa events, but has no such dependence for Yucca Flat events. This may result from systematic variations between the spectra of direct signals and coda arrivals caused by pP interference for the largest events, all of which were detonated at Pahute Mesa. Another possible explanation is a frequency-dependent propagation effect on the direct signals of the larger events, most of which were located in the center of the mesa overlying strong lateral velocity gradients in the crust and upper mantle. Event average complexity varies spatially for both test sites, particularly in the 0.8???1.1 Hz band, providing evidence for frequency-dependent focussing or scattering by near-source structure. Both the direct arrivals and the early coda have strong azimuthal amplitude patterns that are produced by defocussing by mantle heterogeneity. The direct arrivals have stronger coherent azimuthal patterns than the early coda for Pahute Mesa events, indicating more pronounced deep crustal and shallow mantle defocussing for the direct signals. However, for Yucca Flat events the direct arrivals have less coherent azimuthal patterns than the

The 25 April 1992 Cape Mendocino earthquake generated a tsunami characterized by both coastal trapped edge wave and non-trapped tsunami modes that propagated north and south along the U.S. West Coast. Both observed and synthetic time series at Crescent City and North Spit are consistent with the zero-order edge wave mode solution for a semi-infinite sloping beach depth profile. Wave amplitudes at Crescent City were about twice that observed at North Spit, in spite of the fact that the source region was three times farther from Crescent City than North Spit. The largest observed amplitude was due to an edge wave which arrived almost three hours after the initial onset of the tsunami; since such waves are highly localized nearshore, this suggests that the enhanced responsiveness at Crescent City is at least partly due to local dynamic processes. Furthermore, the substantially delayed arrival of this wave, which was generated at the southern end of the Cascadia Subduction Zone, has significant implications for hazard mitigation efforts along the entire U.S. West Coast. Specifically, this study demonstrates that slow-moving but very energetic edge wave modes could be generated by future large tsunamigenic earthquakes in the CSZ, and that these might arrive unexpectedly at coastal communities several hours after the initial tsunami waves have subsided.

Boundaries between the various topographic, geologic, and socio-political entities that span the Earth???s continental surfaces are determined by a myriad of complexly interrelated natural and/or cultural factors. Areas of large river basins, of lithologic units on geologic maps, and of nations exhibit size frequency distributions that are closely approximated by density functions in which diameters of individual areas are distributed exponentially. As such, size distributions of each type of surface unit are closely modeled as randomly delimited areas. In other words, if one were to walk in a straight line across any particular continent, each step along that transect would embody some random continuous probability of passing out of a major drainage basin, or out of an area of more or less homogeneous rock type, or from one country to another. Moreover, this simple truism gives rise to area frequency distributions for large river basin or outcrop or nation area that are primarily dependent on the number of basins or outcrops or countries that exist across that particular landmass. As a consequence, the size frequencies of these areal units can be closely predicted knowing only the total area under consideration and the number of drainage basins, outcrop areas, or nations that exist within the area. The similarity between area frequencies of large river basins and rock types and nations suggests a nontrivial component of geologic influence on the partitioning of continental surfaces into major political divisions, and implies that cultural and economic factors, which serve to divide and unite political entities, do so within a geological framework.

Powdered and pelletized wollastonite (CaSiO 3) was applied to an 11.8 ha forested watershed at the Hubbard Brook Experimental Forest (HBEF) in northern New Hampshire, U.S.A. during October of 1999. The dissolution of wollastonite was studied using watershed solute mass balances, and a 87 Sr/ 86 Sr isotopic tracer. The wollastonite (87 Sr/ 86 Sr = 0.70554) that was deposited directly into the stream channel began to dissolve immediately, resulting in marked increases in stream water Ca concentrations and decreases in the 87 Sr/ 86 Sr ratios from pre-application values of 0.872 mg/L and 0.72032 to values of???2.6 mg/L and 0.71818 respectively. After one calendar year, 401 kg of the initial 631 kg of wollastonite applied to the stream channel was exported as stream dissolved load, and 230 kg remained within the stream channel as residual CaSiO 3 and/or adsorbed on streambed exchange sites. Using previously established values for streambed Ca exchange capacity at the HBEF, the dissolution rate for wollastonite was found to be consistent with dissolution rates measured in laboratory experiments. Initially, Ca was released from the mineral lattice faster than Si, resulting in the development of a Ca-depleted leached layer on mineral grains. The degree of preferential Ca release decreased with time and reached stoichiometric proportions after???6 months. Using Sr as a proxy for Ca, the Ca from wollastonite dissolution can be accurately tracked as it is transported through the aquatic and terrestrial ecosystems of this watershed.

Magnetic anisotropy analysis presents an alternative and fast method for obtaining and quantifying crystallographic preferred orientations in rocks, using relatively simple equipment. Two natural examples and numerical modeling demonstrate that magnetic anisotropy increases with increasing degree of crystallographic preferred orientation. The normalized magnetic parameters Mi = ln(ki/(kmax * kint * kmin)1/3) (kmax>= kint>= kmin are the principal magnetic susceptibilities) correlate directly with March `strains' obtained from X-ray texture goniometry. The additional advantage of our method is that the preferred fabrics are determined from large sample volumes (typically about 11 cm3) rather than the essentially two-dimensional slice used in optical and X-ray methods. Thus, magnetic anisotropy provides a reliable measure of bulk crystallographic preferred orientation in rocks.

We have analyzed a suite of seventy-four formation-water samples from Mississippian and Pennsylvanian carbonate and siliciclastic strata in the Illinois basin for major, minor, and trace element concentrations and for strontium isotopic composition. A subset of these samples was also analyzed for boron isotopic composition. Data are used to interpret origin of salinity and chemical and Sr isotopic evolution of the brines and in comparison with a similar data set from an earlier study of basin formation waters from Silurian-Devonian reservoirs. Systematics of Cl-Br-Na show that present Mississippian-Pennsylvanian brine salinity can be explained by a combination of subaerial seawater evaporation short of halite saturation and subsurface dissolution of halite from an evaporite zone in the middle Mississippian St. Louis Limestone, along with extensive dilution by mixing with meteoric waters. Additional diagenetic modifications in the subsurface interpreted from cation/Br ratios include K depletion through interaction with clay minerals, Ca enrichment, and Mg depletion by dolomitization, and Sr enrichment through CaCO3 recrystallization and dolomitization. Ste. Genevieve Limestone (middle Mississippian) formation waters show 87Sr/86Sr ratios in the range 0.70782-0.70900, whereas waters from the siliciclastic reservoirs are in the range 0.70900-0.71052. Inverse correlations between 87Sr/86Sr and B, Li, and Mg concentrations suggest that the brines acquired radiogenic 87Sr through interaction with siliciclastic minerals. Completely unsystematic relations between 87Sr/86Sr and 1/Sr are observed; Sr concentrations in Ste. Genevieve and Aux Vases (middle Mississippian) waters appear to be buffered by equilibrium with respect to SrSO4. Although there are many similarities in their origin and evolution, these formation waters are distinguished from Silurian-Devonian brines in the basin by elevated Cl/Br and Na/Br ratios and by unsystematic Sr isotope relationships. Thus waters

A new method based on Zn reagent for the quantitative conversion H2O to H2 for the hydrogen isotope analysis of hydrous minerals and whole rocks has been developed. Analyses by this method are of comparable accuracy and precision (~ 1[per mille sign]; 2O) to those of conventional extraction methods based on uranium reagent and is significantly simpler, faster and less expensive.

The metalliferous ore deposits of southwest England are associated with biotite-muscovite granites that intruded upper Paleozoic sediments and volcanic rocks at the end of the Hercynian Orogeny. The hydrothermal mineralization can be subdivided into four stages: 1. (1) exoskarns2. (2) high-temperature tin and tungsten oxide-bearing sheeted greisen bordered veins and Sn-bearing tourmaline veins and breccias3. (3) polymetallic quartz-tourmaline-chlorite-sulfide-fluorite-bearing fissure veins, which represent the main episode of economic mineralization4. (4) late-stage, low-temperature polymetallic fluorite veins. U-Pb dating of monazite and xenotime and 40Ar/39Ar dating of muscovite were used to determine emplacement ages and cooling times for individual plutons within the Cornubian batholith, as well as separate intrusive phases within the plutons. In addition, 40Ar/39Ar ages from hornblende and secondary muscovite and Sm-Nd isochron ages from fluorite were employed to determine the relationship between pluton emplacement and different stages of mineralization. The U-Pb ages indicate that granite magmatism was protracted from ~300 Ma down to ~275 Ma with no evidence of a major hiatus. There is no systematic relation between the age of a pluton and its location within the batholith. The U-Pb ages for separate granite phases within a single pluton are resolvable and indicate that magma emplacement within individual plutons occurred over periods of as much as 4.5 myrs. Felsic porphyry dike emplacement was coeval with plutonism, but continued to ~270 Ma. The geochronologic data suggest that the Cornubian batholith originated from repeated melting events over 30 myrs and was formed by a series of small coalescing granitic bodies. Cooling rates of the main plutons are unrelated to emplacement age, but decrease from the southwest to the northeast from ~210[deg]C myr-1 to ~60[deg]C myr-1 with a mean of 100[deg]C myr-1. These slow cooling rates appear to reflect the addition

We present a new gravity model for the lithosphere beneath the Kenya Rift Valley, the Mozambique Belt, and the Tanzania Craton in western Kenya and northeastern Tanzania. The Kenya Rift lies within the eastern branch of the extensive Cenozoic East African Rift System and has developed almost entirely in the Pan-African Mozambique Belt about 50 to 150 km east of the exposed margin of the Archean Tanzania Craton. The gravity field over western Kenya and northeastern Tanzania is characterized by a long-wavelength Bouguer anomaly. We propose that this anomaly has two components: 1. (1) a "rift" signature, deriving from a shallow rift basin, a lower crustal intrusion and a low-density zone in the mantle lithosphere localized beneath the rift axis2. (2) a "suture" signature, arising from a crustal root along the boundary between the Mozambique Belt and Tanzania Craton and higher density crust in the mobile belt above part of the crustal root. Two lines of reasoning support our interpretation: 1. (1) Recent geological studies of the Mozambique Belt in Kenya and Tanzania suggest that it is a continent-continent collision zone, and continent-continent collision zones worldwide commonly exhibit a characteristic gravity anomaly.2. (2) The long-wavelength Bouguer anomaly has at least two minima, one over the craton-mobile belt boundary, and one or more over the rift valley. Corroborative evidence for our interpretation of the gravity field is provided by recent seismic investigations.

We report Rb---Sr, Sm---Nd and mineralogical data for leached whole rock, 87Sr/86Sr from 0.708 to 0.711 in exchangeable sites. Leached authigenic illite from the 1200 m, independent of the duration of burial. The exchangeable Sr in the clay has remained an open system and has undergone isotopic exchange since that time. Minimum fluid/rock ratios of 0.2 to 2 are modeled from Sr isotopic data, consistent with rock-dominated, open system diagenesis. Leachates and residues of the 147Sm/144Nd (147Sm/144Nd as high as 0.188. This Sm/Nd fractionation, in conjunction with the preserved isotopic equilibrium may be useful for dating diagenesis and associated fluid-rock interaction in older rocks.

Anisotropy of magnetic susceptibility (AMS) has in the past been used as an indicator of finite strain; mathematical relationships between the measured AMS ellipsoid and the finite strain ellipsoid in slates are numerous. The Ordovician Martinsburg Formation at Lehigh Gap, Pennsylvania, provides a unique opportunity to test this correlation, as a complete shale-to-slate transition is preserved. Magnitudes of the principal susceptibility axes appear to reflect a strain gradient at this location. Apparent strain values derived from AMS, using equations from Rathore and Hrouda, have X : Y : Z ratios from 2.79 : 1.87 : 0.19 (shale) to 2.92 : 0.82 : 0.42 (slate). The orientations of the susceptibility axes characterize two fabrics: (i) kint/kmax planes at 20[deg] to bedding and (ii) kint/kmax planes parallel to cleavage. Anisotropy of magnetic susceptibility fails to show any intermediate orientations of the principal susceptibility axes in the zone in which cleavage is developed. Comparison with phyllosilicate crystallographic orientations indicates that the AMS intermediate and maximum susceptibility axes are parallel to chlorite basal planes. Because chlorite reoriented via dissolution and new growth as cleavage developed in this location, and because models correlating AMS and strain instead assume that strain is reflected by the rotation of the magnetic susceptibility carriers, the apparent strain values calculated from AMS reflect only the degree of dissolution and neocrystallization in these rocks and not the finite strain.

The intrinsic oxygen fugacity (IOF) of olivine separates from the Salta, Springwater, and Eagle Station pallasites was measured between 850 and 1150[deg]C using oxygen-specific solid zirconia electrolytes at 105 Pa. The experimental apparatus consisted of a double-opposed electrolyte configuration with a CO-CO2 gas-mix bridging atmosphere. Four initial IOF measurements on Salta olivines revealed the effects of grain size and cell memory on experimental results; experiments with an IW cell memory and fine grain size (log fO2), contains Fa12.5 and Fe90.5Ni9.0; Springwater, of intermediate redox state (~IW), contains Fa18.0 and Fe87.5Ni12.1; Eagle Station, the most oxidized sample (IW + 0.5 logfO2), contains Fa20.5 and Fe85.0Ni14.6. Electron microbeam characterization of the starting materials and run products from these experiments have shown that olivine is the only phase present. Thermodynamic calculations of redox equilibria involving equilibrium pallasite assemblages are in good agreement with our experimental results and provide a lower limit to pallasite redox stability; others involving disequilibrium assemblages, suggest that pallasites experienced localized, late-stage oxidation and reduction effects. Consideration of the redox buffer metal-olivine-orthopyroxene utilizing calculated Eucrite Parent Body (EPB) mantle phase compositions indicates that small redox gradients may have existed in the EPB. Such gradients may have produced strong compositional variation within the EPB. In addition, there is apparently significant redox heterogeneity in the source area of Eagle Station Trio (EST) pallasites and Bocaiuva iron meteorites.

The Hot Spring limestone is a shallow-water algal carbonate within a late Tertiary transgressive lacustrine sequence exposed in the southwestern Snake River Plain. This 5 m thick lensoid sequence crops out over an 80 km2 area that closely approximates original areal extent of nearshore carbonate accumulation. Reefal bodies consist of closely packed algal cylinders, several decimeters in height, each of which includes a dense laminated carbonate wall surrounding porous digitate carbonate that radiates outward and upward from one or more hollow tubes. These coalesce upsection into separate vertical columns several meters in diameter. Moderately well-sorted terrigenous and molluscan debris deposited between columns during growth indicates these structures were resistant to wave erosion and, therefore, were true reefs. Thick rings of littoral carbonate surrounding the upper walls of each column record the final stages of reef development. Structural attributes exhibited by these Miocene carbonate bodies are also common to a number of Tertiary and Quaternary algal buildups reported from other lacustrine settings. Although features within the Hot Spring limestone are complex in gross morphology and structural detail, both columnar reefs and algal cylinders display little variation in size, shape, or internal structure between areas of varying water depth and wave energy, thus reflecting the importance of biological processes as well as physical processes during reef development.

Experimental weathering of sediment-hosted micron gold (SHMG) ore from the Alligator Ridge deposit indicates that ore and adjacent Pilot Shale wallrock release gas with larger concentrations of COS, CS2 and CH4, and lower CH4/(CH4+C2H4) and CH4/(CH4+C2H6) ratios than does barren Pilot Shale. The COS and CS2 are derived from decomposition of disseminated pyrite, which is probably more abundant in mineralized zones, and variations in the CH-gas ratios are thought to reflect increased thermal maturity of organic material in the mineralized zones. Surveys designed to determine whether these gases are present in detectable haloes around SHMG mineralization were carried out over the Yankee prospect, a relatively similar deposit several km south of Alligator Ridge. The surveys included analysis of interstitial soil gas, gases desorbed from soils, and gases desorbed from two sets of Carbotrap collectors, one left on the bedrock interface (at depths of about 30-70 cm below the surface) and another placed in holes drilled about 3 m into bedrock. All of the gases observed in the experiments were detected, as well as organo-sulfur and higher alkane and alkene gases. The surveys indicated systematic variations in gas dispersal related to faults and/or mineralization, with the most consistent patterns being elevated COS values and depressed CH4/(CH4+C2H4) ratios over mineralization. Similar highs in COS, as well as other gases including SO2 and ethyl mercaptan, were associated with faults, and it was not possible to distinguish these two sources of anomalous gas patterns in all cases. In general, best results were obtained with gases desorbed from bulk soil, and no significant advantages was observed with the use of collectors placed in deep holes. Although all of the survey methods yielded anomalous patterns, none of these patterns are sufficiently strong to merit the use of gas geochemistry as a primary guide in exploration for SHMG deposits.

Statistical analyses of geochemical data from ferromanganese nodule samples representing a broad area of the southeast Pacific Ocean indicate nodule compositions are controlled by four factors: (1) suboxic diagenesis; (2) oxic diagenesis; (3) hydrogenous precipitation; and (4) hydrothermal precipitation. The first three are identical to those identified in studies of nodules from the North Pacific; the fourth factor represents the first quantitative determination of a significant hydrothermal end-member in ferromanganese nodules. Nodules apparently acquire a hydrothermal component through the accretion of colloidal-sized hydrothermal precipitates which are widely dispersed from their origin at vents along the East Pacific Rise (EPR). Nodules recovered from beneath the "shadow" of the present-day hydrothermal plumes emanating from the EPR contain significantly higher amounts of hydrothermal material relative to nodules from other areas.

The recent determination of high-quality, short-period P-wave amplitude profiles near the core shadow zone for three source-receiver combinations allows an exploration of lateral variations in P velocity structure at the base of the mantle. Various radially symmetric models are tested by comparison of the data with amplitudes measured from generalized ray theory synthetics. The assumption necessary to justify one-dimensional modeling--that each of the profiles is most sensitive to scales of heterogeneity larger than the discrete regions sampled--is well supported by the coherence of the individual profiles. The observed amplitude versus distance profiles exhibit significant regional variations of the apparent shadow zone boundary, with as much as a 5[deg] shift in onset distance, but it is possible to model the overall behavior using simple, regionally varying, positive P velocity gradients in the lowermost mantle. Velocity models with pervasive negative velocity gradients in the D" layer are not consistent with the data. The modeling indicates that D" velocities beneath the North Pole are ~ 2% slower than those beneath the central Pacific, while velocities beneath the North Pacific are ~ 1% faster. Although the simplest class of successful models begins to deviate from the PREM reference Earth model as much as 690 km above the core-mantle boundary, these models do not violate global mantle velocity constraints, and the theoretical slowness values calculated for the three models are consistent with slowness measurements for the same general regions. More complex, multiple-gradient lower mantle velocity structures with a thinner zone of lateral heterogeneity may be compatible with the P wave data, but such detailed structures cannot be resolved by our modeling (though bounds can be placed on viable structures). The strong lateral variations required by the data support the presence of compositional heterogeneity in D", and the data require that at least in several lo

Oxygen-concentration cells with zirconia solid electrolytes have been used to make equilibrium measurements of the standard molar Gibbs free energy of formation for copper(I) oxide, [Delta]fGmo(Cu2O), over the temperature range from 900 to 1300 K. Compared with previous measurements, systematic errors due to thermal gradients across the zirconia solid electrolyte have been greatly reduced. Measurements with three different types of zirconia solid electrolytes have yielded results that differ by only +/-40 J[middle dot]mol-1 (+/-0.2 mV). This is the best agreement yet achieved between solid electrolytes of different composition. Our recommended value for the standard molar enthalpy of formation, [Delta]fHmo(Cu2O, 298.15 K), is -(170.59+/-0.08) kJ[middle dot]mol-1 (po = 1 x 105 Pa).

The focal mechanisms of intraplate earthquakes within subducting lithosphere are frequently used to infer large-scale stress regimes induced by slab-pull, bending or unbending, and lateral segmentation and undulations of the slab. Numerous studies have further postulated that the intraplate activity is influenced by transitory regional stress regimes such as those associated with interplate thrust events. Temporal variations of the latter type may potentially play an important role in assessing regions of uncertain seismic potential, and possibly even in earthquake forecasting. A systematic analysis of 1130 focal mechanisms for intraplate earthquakes with mb>= 5.0 in the depth range 0-300 km is conducted for nine circum-Pacific subduction zones, all of which are known to have large interplate thrust events. The spatial and temporal relationships of the earthquakes within the subducting slab to the large thrust events in each region are appraised. The earthquake catalog assembled contains all published focal mechanisms, and is probably complete for mb>= 6.5 for the years 1963-1986. For many of the localized regions considered in detail the catalog is complete to lower thresholds of mb>= 6.0 or mb>= 5.5. This analysis provides compelling evidence for a temporal link between large interplate thrust activity and intraplate seismicity. For the seismically coupled regions considered here, outer rise compressional events have occurred prior to several large thrust events or are associated with seismic gaps, while outer rise tensional events generally only follow interplate ruptures. In the intermediate depth range, large down-dip tensional events generally precede interplate thrusts, and are often concentrated at the down-dip edge of the coupled zone. A transition to down-dip compressional stress or diminished tensional activity at intermediate depth is observed after several large thrust events (e.g., 1960 Chile, 1974 Peru, 1957 Aleutian, 1971 New Britain)

The Sitka, Alaska, earthquake of July 30, 1972 (Ms = 7.6) is one of the largest strike-slip events since 1963. This earthquake is thought to have ruptured the Pacific-North America plate boundary segment between the large strike-slip events in 1949 and 1958, and the Sitka earthquake was heralded as one of the first successful `forecasts' of an earthquake based on the seismic gap hypothesis. Another motivation to study this event is the availability of a strong-ground motion recording ~ 50 km from the epicenter, which is near the middle of the 180 km long plate boundary segment. The teleseismically located aftershocks occur only in the southern half of this segment. We have analyzed Rayleigh waves and P waves recorded by WWSSN long-period vertical instruments. A total of 25 very-long-period Rayleigh phases (R2-R5) define a symmetric four-lobed radiation pattern from which we can infer a nearly pure right-lateral strike-slip fault, striking 340[deg] (N20[deg]W), and a bilateral rupture. Comparison of synthetic and observed Rayleigh waves yields a seismic moment of 3 x 1027 dyn cm (MW = 7.6). The P waves are characterized by a dominant complex pulse in the first 40 s, and a succession of smaller pulses after that. The source time functions are deconvolved from 32 P waves with good azimuthal distribution to obtain the moment, depth extent, and rupture history. The mean moment of the source time functions is 5.0 +/- 2.4 x 1027 dyn cm, and the depth extent is 0-10 km. Tomographic inversion of the source time functions produces an image of the earthquake rupture process. This procedure maps the moment release of the complex pulse into two high-displacement regions (i.e., asperities): (1) the epicentral area to 40 km north, and (2) between 60 and 90 km south of the epicenter. The main asperity just north of the epicenter coincides with a quiescent zone of microaftershocks. While the overall average seismic displacement is 6 m, the displacement at the main asperity is 7 m, t

Estimates of subsurface temperatures in the Archean craton of southern Africa during the Archean derived from diamond thermobarometry studies are remarkably similar to temperatures estimated for the same depths today, even though heat production in the earth and the mean global heat flow were probably substantially higher in the Archean. We present multi-dimensional numerical models for the thermal environment of the Archean craton in southern Africa during the Archean in which deep mantle heat is diverted away from the craton toward the surrounding oceanic lithosphere by a lithospheric root beneath the craton. Extrapolation of present-day models to thermal conditions appropriate for the Archean is inadequate to explain the similarity of present-day and Archean temperatures in the cratonic root. Reconciliation of the modern and ancient temperature estimates requires either relaxation of the constraints that the cratonic crustal heat production and/or the earth's mean mantle temperature were higher in the Archean than they are today, or that substantial "erosion" of the lithosphere comprising the cratonic root has occurred since the Archean. The latter possibility could perhaps result from revolatilization of the cratonic root in association with thermal perturbations in the mantle, for which there is evidence in southern Africa in the form of post-Archean tectonic and igneous activity.

The concentrations and mass accumulation rates of rare earth elements (REE) in hydrothermal sediments from two Pacific sites support earlier predictions that hydrothermal REE inputs are significant to the overall marine REE mass balance. Although REE are enriched in hydrothermal vent fluids, they are rapidly and extensively scavenged from the water column by Fe oxyhydroxides and undergo some degree of fractionation prior to removal. Evidence for significant light REE (LREE) hydrothermal inputs tends to be masked by the fact that the LREE patterns and inter-element ratios of both proximal and distal hydrothermal precipitates are similar to those of seawater. However, the heavy REE (HREE) patterns clearly reflect a basaltic source and the HREE ratios from both sites are similar to each other but distinct from their seawater analogs. This suggests HREE ratios potentialy could be used as proxy indicators in our attempts to reconstruct the history of seafloor hydrothermal activity.

A paleomagnetic investigation of the Lower Devonian Llandstadwell Formation in southwest Wales has yielded a characteristic direction (declination = 196[deg], INCLINATION = -5[deg]) that passes a fold test. Comparison of the corresponding virtual geomagnetic pole (334[deg]E, 39[deg]S) to previously published poles for Great Britain indicates that the magnetization is secondary and likely to be of Carboniferous age. No evidence for post-Hercynian rotation of Pembrokeshire is indicated by our data.

Transmission and analytical electron microscopy have been used to characterize the numerous lenticular fissures which were observed in electron micrographs of paragonite. Observations with a very weak electron beam revealed that there were no fissures in the original paragonite and that the degree of damage is a function of exposure to the beam. AEM analysis revealed that there is significant loss of Na in the beam-damaged paragonite. The fissures may be caused by collapse of the paragonite structure to a pyrophyllite-like structure, which has a smaller basal spacing due to loss of interlayer Na. Data from a survey of TEM observations of other phyllosilicates which show similar features imply that the fissures are directly associated with the interlayer cation Na and that beam damage effects are related to the interlayer cations in micas.

The seismicity in the southern Kurile Islands arc is characterized by a complex overlapping of aftershock zones of a few great and many large earthquakes (Ms [greater, double equals] 7.0). The spatial relationships of these aftershock zones can be explained in terms of a simple pattern of fault plane heterogeneity. The four largest earthquakes in this region are, from north to south, the 1963 Kurile Islands (Mw = 8.5), the 1958 Etorofu (Mw = 8.3), the 1969 Kurile Islands (Mw = 8.2) and the 1973 Nemuro-Oki (Mw = 7.8) events. Detailed body wave studies indicate that the moment release associated with each event is concentrated in a small portion of the aftershock area, with scale lengths Ms [greater, double equals] 7.0) earthquakes both before and after the great events. The second mode involves either rupture of relatively weak regions which results in low-stress-drop events, or rupture of smaller asperities. The earthquakes produced by failure of small asperities tend to repeatedly rupture the same regions, and they have impulsive source time functions indicating rupture of highly stressed regions of the fault surface. By analysis of all Ms [greater, double equals] 7.0 events since 1946 in this region we find a segmentation of the southern Kurile Islands thrust zone into strongly coupled regions with two scale lengths (dominant and secondary asperities) and weakly coupled regions (the surrounding areas generating the low-stress-drop events). The largest earthquakes and the smaller asperity earthquakes occur at the down-dip portion of the lithospheric interface, while the low-stress-drop events occur much closer to the trench axis. This implies a difference in the degree of mechanical coupling between the subducting and overriding plates, with the deeper portions having greater strength than the shallower parts.

The surface heat flow in the interior of Archean cratons is typically about 40 mW m-2 while that in Proterozoic and younger terrains surrounding them is generally considerably higher. The eighty-four heat flow observations from southern Africa provide an excellent example of this contrast in surface heat flow, showing a difference of some 25 mW m-2 between the Archean craton and younger peripheral units. We investigate two possible contributions to this contrast: (1) a shallow mechanism, essentially geochemical, comprising a difference in crustal heat production between the two terrains, and (2) a deeper mechanism, essentially geodynamical, arising from the existence of a lithospheric root beneath the Archean craton which diverts heat away from the craton into the thinner surrounding lithosphere. A finite element numerical model which explores the interplay between these two mechanisms suggests that a range of combinations of differences in crustal heat production and lithospheric thickness can lead to the contrast in surface heat flow observed in southern Africa. Additional constraints derived from seismological observations of cratonic roots, the correlation of surface heat flow and surface heat production, petrological estimates of the mean heat production in continental crust and constraints on upper mantle temperatures help narrow the range of acceptable models. Successful models suggest that a cratonic root beneath southern Africa extends to depths of 200-400 km. A root in this thickness range can divert enough heat to account for 50-100% of the observed contrast in surface heat flow, the remainder being due to a difference in crustal heat production between the craton and the surrounding mobile belts in the range of zero to 0.35 [mu]W m-3.

Gases are known to form haloes around presently active geothermal systems and might be useful in exploration for older, extinct hydrothermal systems. Gases from such extinct hydrothermal systems can be liberated for analysis from fluid inclusions in hydrothermal minerals by thermal decrepitation or they can be desorbed from alteration mineral surfaces by heating. The most abundant of these gases, besides H2O, are usually CO2, CH4, CO and N2. We have used a gas chromatograph to analyze these gases in fluid inclusions in jasperoid around the Pueblo Viejo gold-silver deposit, in vein minerals from the Creede silver-lead-zinc deposit, and from clays in the alteration cap overlying veins at Creede to test for gas haloes useful in exploration. At Pueblo Viejo CO2 abundances in the jasperoid range from less than 1 mole percent (with respect to the system CH4-CO2-CO-N2-H2O) in the ore zone to as much as 6 mole percent in surrounding, barren jasperoid. Fluid inclusion analyses at Creede suggest that a drop in the CO2 content of the fluid may relate to ore deposition and clay directly above veins has large amounts of adsorbed CO2. These results suggest that primary gas abundances exhibit patterns in and around hydrothermal ore deposits that can be used in mineral exploration.

Intense intraplate seismic activity has recently been identified in the west-central Pacific, along a line extending from Samoa through the Gilbert Islands and the Ralik Fracture Zone to the Caroline Islands. Over the past 50 years, this seismic line has contributed an estimated 15-30% of the entire intraplate seismicity for the Pacific plate (Hawaii excepted). Focal mechanisms available for ten of the largest events during the 1981-1983 Gilbert Islands swarm, with horizontal compressional stress oriented at right angles to the direction of motion of the plate, cannot be reconciled with simple models of intraplate stresses. We speculate that they express large scale deformation of that part of the Pacific plate, related to incipient detachment of a microplate and relocation of the Pacific-Australian subduction zone some 1000 km to the north of its present position.

From 27 (out of a total of 29) sites a characteristic pre-folding magnetization has been obtained with D = 150[deg], I = +20[deg], [alpha]95 = 3.5[deg], and paleopole at 38[deg]N, 132[deg]E. However, we conclude from a bedding-error test and a conglomerate test, as well as from descriptions of the hematite as a replacement mineral, that the magnetization is a (late) post-depositional chemical remanent magnetization. The age constraints on the magnetization, between Middle Silurian and Early Permian, can be refined by a comparison with the apparent polar wander path for cratonic North America; this comparison suggests a Late Carboniferous age for the magnetization and the hematite, which constitutes the principal component of the iron ores. Similar Late Paleozoic remagnetizations have been noted in other Appalachian and mid-continent formations and suggest a widespread, but as yet ill-defined mechanism for the remagnetization. It is tempting to correlate this event with the early phases of the Alleghenian orogeny in Carboniferous times and with possible fluid migrations resulting from the tectonism.

The Middle to Late Cambrian loop in the North American apparent polar wander path (APWP) has been variously attributed to tectonic rotations, remagnetizations and primary magnetizations. Although no primary thermal remanent magnetizations or primary detrital remanent magnetizations have as yet been demonstrated, the temporally self-consistent nature of the loop has been used as an argument for primary magnetizations. We have studied 535 +/- 5 Ma nepheline syenites and syenites of the McClure Mountain alkalic complex, as well as 495 +/- 10 Ma red trachyte dikes which intruded the complex, in an effort to find a primary TRM. Because Zijderveld analysis yielded consistent results for only one trachyte dike, remagnetization great-circle analysis was employed, giving a pole for the trachyte dikes at the tip of the loop (43[deg]N, 114[deg]E), while the syenites and nepheline synenites gave a pole at the base of the loop (18[deg]N, 142[deg]E). The magnetic carrier in the trachytes is hematite which apparently formed during a pervasive hydrothermal alteration. K---Ar whole rock dating of the trachytes suggests a Pennsylvanian age for the alteration, and thus a late Paleozoic remagnetization of the trachytes. Thus, the low-latitude Cambrian pole is confirmed, but we find no evidence in this study to support the primary nature of the Cambrian APWP.

A naturally-occurring mercuroammonium compound from Pitkin County, Colorado, is shown to be the natural analog of synthetic HgNH2NO3. The crystals are isometric, P4132 or P4332, with a = 10.254(1)A and twelve formula weights per cell. Using 437 symmetry-independent reflections, the crystal structure was partially determined and refined to a residual of 0.090. The positions of the Hg atoms and the N and O atoms of the nitrate group were determined, but the amide ion could not be located, probably due to positional disorder. The structure contains mercury atoms arranged in equilateral triangles 3.421(1) A on a side. These triangles are linked through shared vertices into helical chains wound around the fourfold screw axes. Similar triangular units occur in other inorganic Hg(II) compounds. The distortion of the nitrate ion from trigonal planar symmetry is also discussed.

A magnetization which passes the fold test has been observed in 73 limestone samples (10 sites) from the Middle Jurassic Twin Creek Formation. The pole calculated from the site mean poles is located at 68.4[deg]N, 145.0[deg]E (K = 31.8, A95 = 8.7[deg]). This pole lies in a segment of the North American apparent polar wander (APW) path for which there are only a few reliable poles in the literature. The results corroborate earlier studies which conclude that the Jurassic segment of the APW path does not include the present north pole. However, the position of the Twin Creek pole suggests that significantly more APW took place prior to the late Jurassic than previous studies indicated.

We have evaluated the potential application of sulfur gas analysis to exploration for buried sulfide mineral deposits by: (1) calculating by use of equilibrium thermodynamics, the relative abundances of gases that should be given off by decomposing sulfide minerals; and (2) determining experimentally the abundances of gases that are actually given off. The calculations indicate that the gases that should be given off by decomposing sulfide minerals are (in order of decreasing abundance) H2S, COS, CS2, CH3SH, (CH3)2S2 or SO2 or S2 (depending on Eh and pH). In contrast, our experiments show that decomposing sulfide minerals evolve only CS2 and COS, in order of decreasing abundance. Pyrite produces the largest amounts of sulfur gas. Moist (rather than saturated) and non-sterile (rather than sterile) conditions enhance gas generation from pyrite, although no large difference appeared between sterile and non-sterile experiments for other sulfide minerals. These experiments indicate that the sulfur gases CS2 and COS could be useful indicators of buried metal sulfide deposits.