Common Source Region Research Articles

Transient deformation in southern Mexico in 2006 and 2007: Evidence for distinct deep‐slip patches beneath Guerrero and Oaxaca

We model three slow slip events in 2006 and 2007 recorded by continuous GPS stations in central and southern Mexico to test for overlap between their source regions along the Mexican subduction interface and whether they intrude upward into the rupture zones of previous large earthquakes. Inverse modeling yields source regions beneath central Oaxaca for two of the three slow slip events (SSE), where a previously described SSE occurred in 2004, and beneath Guerrero for the third, where slip events previously occurred in 2001–2002 and possibly 1998. Along with previously published results, our work suggests there are persistent differences between the depths and magnitudes of transient slip beneath Oaxaca and Guerrero. Transient slip beneath Oaxaca in 2004, 2006, and 2007 had a common source region downdip from the seismogenic zone and released elastic strain energy equivalent to Mw∼7.0 earthquakes, equaling most or all energy that accumulated below the seismogenic zone. Transient slip beneath Guerrero in 2006 had a larger moment magnitude (Mw∼7.3) and extended somewhat farther updip, possibly to seismogenic depths. Transient slip thus appears to relieve some elastic strain that accumulates at shallow levels in the Guerrero seismic gap. We find no evidence for spatial or temporal correlations of slow slip along these two widely separated source regions, although better data are needed to test more definitively for any interaction between them.

Review of post-collisional volcanism in the Central Anatolian Volcanic Province (Turkey), with special reference to the Tepekoy Volcanic Complex

Neogene-Quaternary post-collisional volcanism in Central Anatolian Volcanic Province (CAVP) is mainly characterized by calc-alkaline andesites-dacites, with subordinate tholeiitic-transitional-mildly alkaline basaltic volcanism of the monogenetic cones. Tepekoy Volcanic Complex (TVC) in Nigde area consists of base surge deposits, and medium to high-K andesitic-dacitic lava flows and basaltic andesitic flows associated with monogenetic cones. Tepekoy lava flows petrographically exhibit disequilibrium textures indicative of magma mixing/mingling and a geochemisty characterized by high LILE and low HFSE abundances, negative Nb–Ta, Ba, P and Ti anomalies in mantle-normalized patterns. In this respect, they are similar to the other calc-alkaline volcanics of the CAVP. However, TVC lava flows have higher and variable Ba/Ta, Ba/Nb, Nb/Zr, Ba/TiO2 ratios, indicating a heterogeneous, variably fluid-rich source. All the geochemical features of the TVC are comparable to orogenic andesites elsewhere and point to a sub-continental lithospheric mantle source enriched in incompatible elements due to previous subduction processes. Basaltic monogenetic volcanoes of CAVP display similar patterns, and HFS anomalies on mantle-normalized diagrams, and have incompatible element ratios intermediate between orogenic andesites and within-plate basalts (e.g. OIB). Accordingly, the calc-alkaline and transitional-mildly alkaline basaltic magmas may have a common source region. Variable degrees of partial melting of a heterogeneous source, enriched in incompatible elements due to previous subduction processes followed by fractionation, crustal contamination, and magma mixing in shallow magma chambers produced the calc-alkaline volcanism in the CAVP. Magma generation in the TVC, and CAVP in general is via decompression melting facilitated by a transtensional tectonic regime. Acceleration of the extensional regime, and transcurrent fault systems extending deep into the lithosphere favoured asthenospheric upwelling at the base of the lithosphere, and as a consequence, an increase in temperature. This created fluid-present melting of a fluid-enriched upper lithospheric mantle or lower crustal source, but also mixing with asthenosphere-derived melts. These magmas with hybrid source characteristics produced the tholeiitic-transitional-mildly alkaline basalts depending on the residence times within the crust. Hybrid magmas transported to the surface rapidly, favored by extensional post-collision regime, and produced mildly alkaline monogenetic volcanoes. Hybrid magmas interacted with the calc-alkaline magma chambers during the ascent to the surface suffered slight fractionation and crustal contamination due to relatively longer residence time compared to rapidly rising magmas. In this way they produced the mildly alkaline, transitional, and tholeiitic basaltic magmas. This model can explain the coexistence of a complete spectrum of q-normative, ol-hy-normative, and ne-normative monogenetic basalts with both subduction and within-plate signatures in the CAVP.

The age of the martian meteorite Northwest Africa 1195 and the differentiation history of the shergottites

Samarium-neodymium isotopic analyses of unleached and acid-leached mineral fractions from the recently identified olivine-bearing shergottite Northwest Africa 1195 yield a crystallization age of 347 ± 13 Ma and an ε Nd 143 value of +40.1 ± 0.9. Maskelynite fractions do not lie on the Sm–Nd isochron and appear to contain a martian surface component with low 147Sm/ 144Nd and 143Nd/ 144Nd ratios that was added during shock. The Rb–Sr system is disturbed and does not yield an isochron. Terrestrial Sr appears to have affected all of the mineral fractions, although a maximum initial 87Sr/ 86Sr ratio of 0.7016 is estimated by passing a 347 Ma reference line through the maskelynite fraction that is least affected by contamination. The high initial ε Nd 143 value and the low initial 87Sr/ 86Sr ratio, combined with the geologically young crystallization age, indicate that Northwest Africa 1195 is derived from a source region characterized by a long-term incompatible-element depletion. The age and initial Sr and Nd isotopic compositions of Northwest Africa 1195 are very similar to those of Queen Alexandra Range 94201, indicating these samples were derived from source regions with similar Sr–Nd isotopic systematics. These similarities suggest that these two meteorites share a close petrogenetic relationship and might have been erupted from a common volcano. The meteorites Yamato 980459, Dar al Gani 476, Sayh al Uhaymir 005/008, and Dhofar 019 also have relatively old ages between 474 and 575 Ma and trace element and/or isotopic systematics that are indicative of derivation from incompatible-element-depleted sources. This suggests that the oldest group of meteorites is more closely related to one another than they are to the younger meteorites that are derived from less incompatible-element-depleted sources. Closed-system fractional crystallization of this suite of meteorites is modeled with the MELTS algorithm using the bulk composition of Yamato 980459 as a parent. These models reproduce many of the major element and mineralogical variations observed in the suite. In addition, the rare earth element systematics of these meteorites are reproduced by fractional crystallization using the proportions of phases and extents of crystallization that are calculated by MELTS. Other shergottites that demonstrate enrichments in incompatible-elements and have evolved Sr and Nd isotopic systematics have some geochemical systematics that are similar to those observed in the depleted group. Most notably, although they exhibit a very limited range of incompatible trace element and isotopic compositions, they have highly variable major element compositions. This is also consistent with evolution from a common mantle source region by variable amounts of fractional crystallization. If this scenario is correct, it suggests that the combined effects of source composition and fractional crystallization are likely to account for the major element, trace element, and isotopic diversity of all shergottites.

Source apportionment of fine particles utilizing partially speciated carbonaceous aerosol data at two rural locations in New York State

A source apportionment study was conducted at two rural locations, Potsdam and Stockton, to assess the in-state/out-of-state sources of PM 2.5 and Hg in New York State. At both locations, samples were collected between November 2002 and August 2005 and analyzed for fine PM mass and its chemical constituents. The measured chemical constituents included elements, cations, anions, organic and elemental carbon (OC and EC), black carbon (BC), and water-soluble short-chain (WSSC) organic acids. Positive matrix factorization (PMF) was applied to the measured concentrations and eight and seven factors were resolved at Potsdam and Stockton, respectively. Four factors were resolved in common between the two locations including secondary sulfate, secondary nitrate, secondary OC, and a crustal factor. The factor profiles of mixed industrial and motor vehicle factors resolved at Potsdam were different compared with the corresponding profiles for these factors at Stockton. A resuspended road salt factor was identified at Potsdam, while an aged sea salt factor was identified at Stockton. At Potsdam, a wood smoke factor was also resolved. Among the resolved factors, secondary sulfate was the highest contributor to the measured mass at both sites. Potential source contribution function (PSCF) analysis indicated the Ohio River Valley region as a common potential source region for this factor at both locations. For the secondary nitrate factor, at Potsdam PSCF analysis indicated the Midwestern US (NO x emissions), and the US farm belt (ammonia emissions) as potential source regions, while at Stockton, the Midwestern US (power plant NO x emissions) was indicated as a major potential source region.

Minette mafic microgranular enclaves and their relationship to host syenites in systems formed at mantle pressures: major and trace element evidence from the Piquiri Syenite Massif, southernmost Brazil

Mafic microgranular enclaves in the ultrapotassic Piquiri Syenitic Massif (611 ± 1 Ma) in southern Brazil represent a minette magma mingled with a syenitic one, both produced from similar mantle sources and emplaced in a post-collisional setting of Neoproterozoic age. The minette magma is compositionally close to typical minettes and high-SiO2 lamproites, with relatively high contents of LREE, Cs and Rb. It is slightly silica-undersaturated, ultrapotassic and metaluminous, with K2O/Na2O ratios around 2–3, and about 4–7 wt% K2O. The Piquiri minettes contain K-clinopyroxene and pyrope, which are interpreted to have crystallized under pressures about 5 GPa. Whole-rock and mineral chemistry indicate that the most suitable source for the minette magma is clinopyroxene-phlogopite-apatite-amphibole-sulphide ± garnet mantle veins, under pressures of about 5 GPa and melting temperatures over 1,000 °C. Fractional melting is admitted in order to explain the extremely high Rb, Cs and LREE contents of the minette melt, and is consistent with its estimated rheological behavior. The syenitic host-rock parental magma was produced from a similar source, probably at lower pressures, and the co-mingling probably occurred still at large depth, under pressures around 3 GPa. Rheological and geochemical considerations support a model based on fractional melting of a veined mantle which had been metasomatized during previous (760–700 Ma) ocean-plate consumption. The subduction-related metasomatism in the source is indicated by low LREE/(Nb or Ta) ratios, high Nb/Ta and U/Th ratios, and low Ti contents. The compositional similarity and close spatial and temporal association of minette and syenitic magmas can be explained by their common source region. Compared to typical lamproitic magmatism, the major difference is that the Piquiri minette magmas are derived from a subduction-modified source.

Multispacecraft observations of chorus dispersion and source location

We report Cluster Wideband Data (WBD) receiver observations of correlated chorus elements with different frequency/time characteristics as seen on multiple spatially separated Cluster spacecraft. Because chorus packets disperse as they propagate, careful comparison of the signals from multiple spacecraft can provide new information about the origin of these waves. A cross‐correlation analysis is used to quantify the dispersive time delay between each frequency of a chorus element as it arrives at Cluster spacecraft pairs. This data cross‐correlation is then compared with a ray‐tracing technique in order to identify source locations that are consistent with the observed delays. We also consider a time‐variable source that emits frequencies that increase as a function of time. This frequency drift rate is adjusted to force the frequency/time variation of the simulated chorus element on a single spacecraft to match that observed. This process yields possible source locations for each spacecraft, whose location and extent are a function of the amount of source frequency emission drift. By requiring the individual spacecraft source regions to intersect with the multispacecraft source regions, a common static source region at L ∼ 3.9 (McIlwain parameter) and MLAT ∼ −5.9° (magnetic latitude) is identified for a single event (event 1) and upper and lower bounds are placed on the amount of source emission frequency drift within the source. No common source region is found for a second event (event 2).

Petrology and geochemistry of monchiquites from Tchircotch� (Garoua rift, north Cameroon, Central Africa)

Summary Dykes of Cenozoic age (37.5 � 2.3 Ma) crop out in the Tchircotcharea (Garoua rift, north Cameroon). They consist of a lamprophyric (monchiquite) series with diopside, subsilicic kaersutite and apatite phenocrysts, Ba-Ti-rich biotite microphenocrysts and Cr-diopside xenocrysts scattered in a matrix of analcitic composition containing oligoclase, albite and sanidine microlites and carbonate ocelli. Major and trace element distributions are interpreted in terms of crystal fractionation of olivine, clinopyroxene, amphibole and Fe-Ti oxides. The Tchircotchmonchiquites show a relatively restricted range of initial 87 Sr= 86 Sr ratio (0.70366-0.70387), of "Nd values (þ2.5 to þ2.7), and rare earth element patterns similar to those of the least differentiated basalts of the Cameroon Line. This supports a common magma source region. This mantle source is infra-lithospheric and is strongly enriched in incompatible elements (light REE, Zr, Sr, Ba) probably transported by volatile- and halogen-rich fluids. The monchiquites appear to be derived by low degrees of partial melting as attested by steep REE patterns and high contents of other incompatible elements, suggesting the presence of residual garnet in the source. Several lines of evidence support the occurrence of phlogopite in the source region of Tchircotchamprophyre magma.

Hf isotope evidence for a hidden mantle reservoir

High-precision Hf isotopic analyses and U-Pb ages of carbonatites and kimberlites from Greenland and eastern North America, including Earth9s oldest known carbonatite (3 Ga), indicate derivation from an enriched mantle source. This previously unidentified mantle reservoir—marked by an unradiogenic Hf isotopic composition and preserved in the deep mantle for at least 3 b.y.—may account for the mass imbalance in Earth9s Hf-Nd budget. The Hf isotopic data presented here support a common mantle source region and genetic link between carbonatite and some oceanic- island basalt volcanoes.

Fluid composition and thermal regime during Zn-Pb mineralization in the lower Windsor Group, Nova Scotia, Canada

The Zn-Pb-Ba-Cu-Ag-F deposits hosted by Lower Windsor Group (early Carboniferous) carbonates in Nova Scotia are located along the southern margin of the Maritimes basin. Various geochemical studies indicate that the deposits were formed by fluids expelled from the basin, but it remains unclear whether the ore-forming fluids for different deposits were derived from a common source region in the central part of the basin, as proposed previously, or from separate source regions. It is also unknown how the ore fluids were conveyed from the source regions to the sites of mineralization and whether or not the mineralizing systems of different deposits were hydrologically related. This paper addresses these problems through a compilation of regional fluid composition and temperature data and thermal comparisons between the ore-forming fluids and their host rocks. The compositions and temperatures of the fluids were evaluated from fluid inclusion data, whereas host-rock temperatures were estimated from vitrinite reflectance measurements.The ore-forming fluids are characterized by: (1) variable, but generally high salinities (mainly between 15 and 30 wt % NaCl equiv), (2) variable, but generally detectable amounts of bivalent cations (mainly Ca (super 2+) ), (3) significant concentrations of hydrocarbon species, and (4) high temperatures (mainly between 150 degrees and 250 degrees C). NaCl/(NaCl + CaCl 2 ) wt ratios in fluid inclusions show a broad district-scale variation: mainly 0.4 for the Gays River and Walton deposits in mainland Nova Scotia. CH 4 /higher hydrocarbons ratios of the gas components are greater for the Gays River deposit (3.8-9.7) than the Jubilee deposit (0.7-1.3). Fluid inclusion homogenization temperatures do not show a negative correlation with the distance between the deposits and the central part of the Maritimes basin, as might be expected if the fluids for different deposits were derived from the same source region.The background temperatures prior to mineralization were estimated from mean vitrinite reflectance (R o ) in host rocks which were relatively far from the deposit, and were probably least affected by the mineralizing events, and from homogenization temperatures of fluid inclusions in preore minerals. These temperatures are in the range of 70 degrees to 155 degrees C, which are significantly lower than those of the ore-forming fluids. The paleogeothermal gradients estimated from the background temperatures and burial depths are ca. 65 degrees C/km. Vitrinite reflectance values in host rocks adjacent to the deposits indicate that these rocks were heated by the mineralizing fluids to variable degrees, but thermal equilibrium between the fluids and rocks was not attained in most eases. This thermal relationship suggests focused and short-lived, rather than pervasive and long-lived, fluid flow.The regional variation of fluid composition and thermal patterns supports a model in which the ore-forming fluids of individual deposits were derived from separate sources; fluid flow was focused along confined conduits and individual mineralizing systems were unconnected hydrologically. Under paleogeothermal gradients of ca. 65 degrees C/km, fluids with temperatures of ca. 250 degrees C could have been generated in sub-basins proximal to the deposits. Sudden release of overpressured fluids from the basal part of the proximal sub-basins, characterized by high flow rate and short duration, is proposed as the mechanism of ore-forming fluid flow.

An investigation into the source of the springtime tropospheric ozone maximum at Mauna Loa Observatory

Measurements of CH4, CO, O3, and H2O vapor from Mauna Loa Observatory (MLO) are examined in conjunction with isentropic trajectories to investigate the cause of a maximum in tropospheric O3 consistently observed during spring. CO and O3 have been found to be positively correlated in pollution plumes containing O3 precursors downwind of industrialized regions. However, we report that during continental transport from Asia, O3 is not correlated with either CO or CH4, although CO and CH4 are strongly correlated. The relationship between CO and CH4 suggests common source regions. The lack of correlation between these species and O3 probably indicates an O3 source distinct from that of CO and CH4. While Asian pollution does not appear to be a strong candidate for causing the spring increase in O3, transport characteristics and H2O vapor measurements are consistent with both an upper‐tropospheric/stratospheric contribution and an enhancement from transport across O3 gradients.

Long‐duration whistler waves in the magnetosheath: Wave characteristics and the possible source region

A type of whistler waves termed “long‐duration whistler waves” (LDWW) in ELF range (1 ∼ 64 Hz) in the magnetosheath is studied according to the wave characteristics and generation based on data observed by the search coil magnetometer onboard the Geotail satellite. LDWW are band‐limited emissions near the lower hybrid frequency typically lasting several tens of minutes. Orientations of wave normal are determined from waveform data of vector magnetic field with an assumption of plane waves and are observed to be fairly well organized in a certain duration of LDWW. Ambiguity in propagation directions is removed by comparison of the phase relation between magnetic components and one component of electric field. The propagation vectors of LDWW are statistically aligned along the “Parker spiral” but are primarily reversed in the dusksides and in the dawnsides of the magnetosheath; that is, one is sunward, and the other is antisunward. This outstanding asymmetry strongly suggests that the bow shock region is the common source region with waves propagating away from the bow shock along the draped magnetic field in the magnetosheath. Electrons composing a type of distribution function in flat‐topped shape are concurrently observed during a series of LDWW events and are likely to yield a favorite condition for LDWW propagating a long path free from attenuation caused by wave particle interactions.

TEM Characterization of Arsenic and Phosphorus Implanted Silicon Devices

Abstract Ion implantation of arsenic and phosphorus is a common practice in silicon devices for the formation of transistor source/drain regions. We used a TEM equipped with EDX capabilities to investigate effects of ion implantation in actual devices before and after annealing. A 200 kev field emission gun TEM was used in this study. Two implant cases were studied here. Both samples are p-type, (100) Si wafers. Figure 1 shows the microstructure in a common source region of a silicon device after being implanted by phosphorus (4x1014 cm−2 at 30 kv, 0°), while Figure 2 shows a similar region for arsenic implantation (5x1015 cm−2 at 45 kv, 0°). No screen layer was used during implantation. The phosphorus implant results in a ˜0.05 μm amorphous layer sandwiched between heavily damaged crystalline silicon. High resolution images reveal a rough amorphous/damaged crystalline boundary and high density defects due to silicon lattice displacements.

Geology and geochemistry of Archaean granites in the Kalgoorlie region of the Eastern Goldfields, Western Australia: a syn-collisional tectonic setting?

Granites in the Kalgoorlie region of the Archaean Yilgarn Craton are predominantly biotite granodiorite and monzogranite with some hornblende-bearing varieties. Granites are divided, on the basis of field relationships, petrography and chemical composition, into five supersuites. An older supersuite containing four suites pre-dates regional folding and four supersuites post-date regional folding. The potassic composition of the mainly silica-rich granites (> 68% SiO 2) suggest that the most appropriate source for the majority of supersuites is continental crust with a composition in the range tonalite to granodiorite. Most supersuites evolved by fractional crystallization, mainly of biotite and plagioclase. Some suites contain synplutonic dykes and microgranite xenoliths that crystallized from enriched, mantle-derived magmas. There was only limited and local chemical interaction between granite and mantle-derived magmas. Geometric and temporal relationships between regional structures and granites indicate that the granites were emplaced during a major orogenic event. However, S-type granites which are widespread in some modern syn-collision settings, are absent in the Kalgoorlie region. The predominance of high fO 2, I-type granites in the study area probably reflects the absence of large, graphite-bearing metasedimentary accumulations in the lower crustal source regions. The distribution of suites and supersuites implies a common source region beneath the Kalgoorlie and Gindalbie granite-greenstone terranes suggesting that they shared a common geological history dating back to late greenstone deposition.

Lead and Helium Isotope Evidence from Oceanic Basalts for a Common Deep Source of Mantle Plumes

Linear arrays in lead isotope space for mid-ocean ridge basalts (MORBs) converge on a single end-member component that has intermediate lead, strontium, and neodymium isotope ratios compared with the total database for oceanic island basalts (OIBs) and MORBs. The MORB data are consistent with the presence of a common mantle source region for OIBs that is sampled by mantle plumes. 3He/4He ratios for MORBs show both positive and negative correlation with the 206Pb/204Pb ratios, depending on the MORB suite. These data suggest that the common mantle source is located in the transition zone region. This region contains recycled, oceanic crustal protoliths that incorporated some continental lead before their subduction during the past 300 to 2000 million years.

An H chondrite stream: Identification and confirmation

Fall data indicate that a significant, elongate cluster of co‐orbital H chondrite falls in May between 1855 and 1895 (H Cluster 1) records encounters with two or three closely spaced and probably related meteoroid stream components, each of which was met near its perihelion. Although meteorites included in the Cluster vary widely in petrographic type (3–6), shock facies (a‐d), and 21Ne exposure age (<5 to 50 Ma), they have a distinct labile trace element signature that confirms a common thermal history and, thus, a common source region within an H chondrite parent body. Hence, meteorites selected by one criterion (fall parameters) as distinguishable from all other H chondrites, are distinguished from them by another completely different criterion (contents of labile trace elements).

Phase relations in peridotite+CO2 Systems to 12 GPa: Implications for the origin of kimberlite and carbonate stability in the Earth's upper mantle

The phase relations of synthetic peridotites in the systems CaO‐MgO‐SiO2‐CO2 (CMS‐CO2), CaO‐MgO‐Al2O3‐SiO2‐CO2 (CMAS‐CO2) and CaO‐MgO‐SiO2‐CO2‐H2O (CMS‐CO2‐H2O) were studied from 4 to 12 GPa using a multi‐anvil apparatus. Peridotite solidi in both the CMS‐CO2 and CMAS‐CO2 systems have similar topologies. Carbonates are the first phases consumed at, or above, the solidus in peridotite+CO2 at pressures above 4 GPa. Magnesite solid‐solutions are stable subsolidus phases in peridotite+CO2 to pressures of at least 12 GPa. It is demonstrated that if uncontaminated, diamond‐bearing kimberlites emplaced in the Earth's crust are primary magmas, then they could be derived by partial melting of carbonated peridotite at 5 to 7 GPa in the upper mantle beneath the cratonic areas of continents. “Proto‐kimberlites” may be generated at pressures above 7 GPa in the upper mantle, but no such magmas are represented on the Earth's surface. Mineral chemical data from experimental run products indicate that: 1) the discrete sub‐calcic clinopyroxene nodules found in some kimberlites represent phenocrysts which crystallized at high pressure in the kimberlite magma and, 2) the spatial association of kimberlite and melilitite observed in the field is not related to a common source region for both these magmas in the mantle. If the oxygen fugacity recorded in samples from the upper 150 km of the mantle is maintained at greater depths, then carbonates are potential hosts for carbon in the mantle to depths of at least 350 km.

Primitive calc‐alkaline and alkaline rock types from the Western Mexican Volcanic Belt

Since the early Pliocene, small volumes of alkaline magmas have erupted in the western Mexican Volcanic Belt (MVB) in close association with the volumetrically dominant calc‐alkaline magmas. Both suites include relatively rare “primitive” types, characterized by high Mg#, Cr, and Ni contents. Primitive hypersthene‐normative basalts, parental to the calc‐alkaline suite, are found along the volcanic front but are absent at greater distances from the trench. The volcanic‐front calc‐alkaline suites are typically hornblende bearing, indicating relatively high water contents. Associated primitive alkaline magmas at volcanic‐front locations also contain hydrous minerals. These nepheline‐normative suites include basanites, phlogopite‐bearing minettes, and other hornblende‐bearing lamprophyres. These are probably the youngest and freshest lamprophyres yet discovered on Earth, and ideal samples for addressing the origin of this exotic class of rocks. On extended MORB‐normalized elemental plots, all lamprophyres show patterns similar to the calc‐alkaline basalts, but have overall enrichments of 2 to 25X in Ti, K, P, Ba, Sr, light rare earth elements, and other incompatible elements. The lamprophyres show the same strong relative enrichments of Ba, K, and Sr, and depletions in Ti and Nb that characterize the calc‐alkaline rocks of western Mexico and all subduction zones. These similarities in relative element abundances support a common source region for all primitive magmas of the western MVB. The primitive calc‐alkaline and alkaline rocks of western Mexico show narrow, overlapping ranges in Sr, Nd, and Pb isotopic ratios, also consistent with derivation of all magmas from a common source. The presence of the relatively cold subducted Cocos and Rivera plates below the volcanic front demands that all primitive magmas originated in the depth range 30 to 75 km. These magmas are enriched in Ba, K, Sr, and other elements, probably transported by hydrous fluids rising from the subducted slab. Several lines of evidence support the presence of phlogopite in the source region of western MVB magmas, probably in the form of phlogopite‐rich veins cutting asthenospheric peridotite. The first liquids formed upon partial melting of this veined source region will concentrate the vein component and generate the lamprophyres. These lamprophyric melts rarely erupt in arc settings, more commonly forming dikes in the arc crust. Eruption of lamprophyric magmas in the western MVB is favored by through‐crustal extensional fracture systems related to active rifting of the Jalisco block from the N. American plate. Larger volumes of calc‐alkaline basalt are generated by melting of the same source which dilutes the vein component with larger proportions of peridotitic wall rock.

Carbon dioxide and methane in the Arctic atmosphere

Fifty flask air samples were taken during April 1986 from a NOAA WP-3D Orion aircraft which flew missions across a broad region of the Arctic as part of the second Arctic Gas and Aerosol Sampling Program (AGASP II). The samples were subsequently analyzed for both carbon dioxide (CO2) and methane (CH4). The samples were taken in well-defined layers of Arctic haze, in the background troposphere where no haze was detected, and from near the surface to the lower stratosphere. Vertical profiles were specifically measured in the vicinity of Barrow, Alaska to enable comparisons with routine surface measurements made at the NOAA/GMCC observatory. Elevated levels of both methane and carbon dioxide were found in haze layers. For samples taken in the background troposphere we found negative vertical gradients (lower concentrations aloft) for both gases. For the entire data set (including samples collected in the haze layers) we found a strong positive correlation between the methane and carbon dioxide concentrations, with a linear regression slope of 17.5 ppb CH4/ppm CO2, a standard error of 0.6, and a correlation coefficient (r2) of 0.95. This correlation between the two gases seen in the aircraft samples was corroborated by in situ surface measurements of these gases made at the Barrow observatory during March and April 1986. We also find a similar relationship between methane and carbon dioxide measured concurrenty for a short period in the moderately polluted urban atmosphere of Boulder, Colorado. We suggest that the strong correlation between methane and carbon dioxide concentrations reflects a common source region for both, with subsequent long-range transport of the polluted air to the Arctic.

Microtremor Studies in Roosevelt and Beowawe Geothermal Areas: ABSTRACT

The microtremors generated by pressure variations and microcracking of the hydrothermal system have been considered to be direct indicators of geothermal reservoirs. Despite the number of studies on this subject, it is still controversial. It is clear that a prime hypothesis has yet to be tested: that geothermal reservoirs generate a detectable seismic signal. Two passive seismic studies have been conducted in the Basin and Range province of the western United States, using state-of-the-art seismic data acquisition and processing techniques. The purpose of these studies was to investigate the correlation between microtremor activities and existing geothermal reservoirs. A 256-element modified eight-arm geophone array was deployed to monitor microtremors at six sites in the vicinity of Roosevelt Hot Springs, Utah. Some 2 Hz coherent microtremors were detected, intermittently, in four recording sites; however, the directions of propagation did not indicate that the existing reservoir system was the common source region. The microtremor data in Beowawe geothermal area, Nevada, were acquired by a 60-element cross array near the geyser. Low frequency (2 to 3 Hz) microtremors clearly appear on every record. Those microtremor activities are associated with the geothermal system at depth, as indicated by seismic body wave components propagating with extremely high apparent velocities. End_of_Article - Last_Page 975------------

Division of the waters: changing concepts of the continental divide, 1804–1844

One of the major features of the continental geography of North America that was still misunderstood at the beginning of the nineteenth century was the drainage system between the Mississippi River and the Pacific. At the beginning of the century, Americans thought of the drainage system as symmetrical, with all major streams heading in a common source region and flowing in several directions to the Mississippi, the Arctic, the Pacific and the Gulf of California. The desired water route across the continent was based upon this view. During the first half of the nineteenth century the concept of the common source region and the water route underwent revision. Geographical information from early explorers like Lewis and Clark and Pike reinforced the older theories but gradually gave way before more accurate data acquired by the men of the Rocky Mountain fur trade. The fur trade lore was tested by John Charles Frémont in the 1840's and a new image was developed, one of a continental divide rather than a common source region. Although the idea of a commercial route across the continent still persisted after Frémont, it was viewed as a land route, crossing the Continental Divide at South Pass, rather than one by water.