USGS Science⌕ Search

SEARCH · USGS Science

Results for “Earth Science”

Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,441 records · Page 80Linked to original sources
USGS1991

Numerical simulations of hydrothermal circulation resulting from basalt intrusions in a buried spreading center

A two-dimensional, one by two-kilometer section through the seafloor was simulated with a numerical model to investigate coupled fluid and heat flow resulting from basalt intrusions in a buried spreading center. Boundary and initial conditions and physical properties of both sediments and basalt were constrained by field surveys and drilling in the Guaymas Basin, central Gulf of California. Parametric variations in these studies included sediment and basalt permeability, anisotropy in sediment permeability, and the size of heat sources. Faults were introduced through new intrusions both before and after cooling. Background heat input caused fluid convection at velocities ≤ 3 cm a −1 through shallow sediments. Eighty to ninety percent of the heat introduced at the base of the simulations exited through the upper, horizontal surface, even when the vertical boundaries were made permeable to fluid flow. The simulated injection of a 25–50 m thick basalt intrusion at a depth of 250 m resulted in about 10 yr of pore-fluid expulsion through the sea-floor in all cases, leaving the sediments above the intrusions strongly underpressured. A longer period of fluid recharge followed, sometimes accompanied by reductions in total seafloor heat output of 10% in comparison to pre-intrusion values. Additional discharge-recharge events were dispersed chaotically through the duration of the cooling period. These cycles in heat and fluid flow resulted from the response of the simulated system to a thermodynamic shock, the sudden emplacement of a large heat source, and not from mechanical displacement of sediments and pore fluids, which was not simulated. Water/rock mass ratios calculated from numerical simulations are in good agreement with geochemical estimates from materials recovered from the Guaymas Basin, assuming a bulk basalt permeability value of at least 10 −17 m 2 /(10 −2 mD). The addition of faults through intrusions and sediments in these simulations did not facilitate continuous, rapid venting. Increased heat input at the base of the faults resulted in temporarily greater fluid discharge, but the flow could not be sustained because the modeled system could not recharge cold fluid quickly enough to remove sufficient heat through the vents.

Earth and Planetary Science Letters↗
USGS1991

ReOs isotope systematics of NiCu sulfide ores, Sudbury Igneous Complex, Ontario: Evidence for a major crustal component

Sudbury Igneous Complex sublayer ores from the Levack West, Falconbridge and Strathcona mines were analyzed for their Re and Os concentrations and Os isotopic compositions. The Re-Os isotope systematics of three ores from the different mines give isochron ages of1 840 ± 60 Ma, 1770 ± 60 Ma and 1780 ± 110 Ma, suggesting that the Re-Os system became closed at the time of, or soon after the 1850 ± 1 Ma crystallization age of the complex. The Os isotopic compositions of different portions of the complex at the time of crystallization varied considerably, with initial 187 Os/ 186 Os 1850 ranging from 4.64 at Levack West to 7.55 at Strathcona. These heterogeneities require that the Os, and probably also the other platinum-group elements contained in the ores, were derived from at least two sources. In addition, the high initial 187 Os/ 186 Os ratios indicate that the Os was derived predominantly from ancient crust. Previous studies have suggested that the complex either crystallized from a mixture of mantle-derived basaltic melt and ancient continental crust, or was derived exclusively from the fusion of ancient continental crust resulting from a meteorite impact. Results of modelling suggest that if a contemporaneous mantle-derived basaltic melt was involved in the origin of the SIC, it likely contributed < 50% of the Os to all three ores. The large percentage of ancient crust involved in the production of the ores is most consistent with an interpretation of substantial crustal fusion resulting from meteorite impact.

Earth and Planetary Science Letters↗
USGS1991

Fractal patterns of fractures in granites

Fractal measurements using the Cantor's dust method in a linear one-dimensional analysis mode were made on the fracture patterns revealed on two-dimensional, planar surfaces in four granites. This method allows one to conclude that: (1)|The fracture systems seen on two-dimensional surfaces in granites are consistent with the part of fractal theory that predicts a repetition of patterns on different scales of observation, self similarity. Fractal analysis gives essentially the same values of D on the scale of kilometres, metres and centimetres (five orders of magnitude) using mapped, surface fracture patterns in a Sierra Nevada granite batholith (Mt. Abbot quadrangle, Calif.). (2)|Fractures show the same fractal values at different depths in a given batholith. Mapped fractures (main stage ore veins) at three mining levels (over a 700 m depth interval) of the Boulder batholith, Butte, Mont. show the same fractal values although the fracture disposition appears to be different at different levels. (3)|Different sets of fracture planes in a granite batholith, Central France, and in experimental deformation can have different fractal values. In these examples shear and tension modes have the same fractal values while compressional fractures follow a different fractal mode of failure. The composite fracture patterns are also fractal but with a different, median, fractal value compared to the individual values for the fracture plane sets. These observations indicate that the fractal method can possibly be used to distinguish fractures of different origins in a complex system. It is concluded that granites fracture in a fractal manner which can be followed at many scales. It appears that fracture planes of different origins can be characterized using linear fractal analysis.

Earth and Planetary Science Letters↗
USGS1992

Brine history indicated by argon, krypton, chlorine, bromine, and iodine analyses of fluid inclusions from the Mississippi Valley type lead-fluorite-barite deposits at Hansonburg, New Mexico

Argon, krypton, chlorine, bromine, and iodine were measured in a homogeneous population of high-salinity hydrothermal fluid inclusions from the Tertiary-age Mississippi Valley-type (MVT) lead-fluorite-barite deposits at Hansonburg, New Mexico to establish new types of evidence for the history of both the fluid and the major dissolved salts. Noble gases and halogens in fluid inclusions containing 10 −10 –10 −9 L of brine ( Cl= 3 molal) were analyzed by laser microprobe noble-gas mass spectrometry ( lmngms ) on neutron-irradiated samples. The concentrations of 36 Ar ( 4.7 × 10 −8 molal) and 84 Kr 1.8 × 10 −9 molal) in the fluid inclusions are equal to those of fresh surface waters in equilibrium with air at approximately 20 ± 5 °. The mole ratios of Br/Cl ( 1.2 × 10 −4 ) and I/Cl ( 1–2 × 10 −6 ) are among the lowest measured in any natural waters, similar to those of modern brines formed by dissolution of Permian NaCl-bearing evaporites in southeast New Mexico. 40 Ar/ 36 Ar ratios (600) are twice that of air, and indicate that the fluid inclusions had excess radiogenic 40 Ar ( 1.4 × 10 −5 molal) when trapped. The amount of excess 40 Ar appears to be too large to have been acquired with Cl by congruent dissolution of halite-bearing evaporites, and possibly too small to have been acquired with Pb by congruent dissolution of granitic basement rocks with Proterozoic K Ar ages. From the lmngms data, combined with published Pb and S isotope data, we infer the following sequence of events in the history of the Hansonburg MVT hydrothermal brine: (1) the brine originated as relatively dilute meteoric water, and it did not gain or lose atmospheric Ar or Kr after recharge; (2) the originally dilute fluid acquired the bulk of its Cl and sulfate in the subsurface after recharge by dissolving halite-bearing Permian? marine evaporites; (3) the high salinity brine then acquired most of its Pb and excess radiogenic 40 Ar from interactions with aquifer rocks other than evaporites, possibly clastic sedimentary rocks or basement rocks with Phanerozoic K Ar “ages”; and (4) the brine deposited fluorite without having boiled or degassed.

Earth and Planetary Science Letters↗
USGS1992

Erratum: Geochronology and petrogenesis of MORB from the Juan de Fuca and Gorda ridges by 238U-230Th disequilibrium

A highly precise mass spectrometric method of analysis was used to determine 238 U 234 U 230 Th 232 Th in axial and off-axis basalt glasses from Juan de Fuca (JDF) and Gorda ridges. Initial 230 Th activity excesses in the axial samples range from 3 to 38%, but generally lie within a narrow range of 12 to 15%. Secondary alteration effects were evaluated using δ 234 U and appear to be negligible; hence the 230 Th excesses are magmatic in origin. Direct dating of MORB was accomplished by measuring the decrease in excess 230 Th in off-axis samples. 238 U 230 Th ages progressively increase with distance from axis. Uncertainties in age range from 10 to 25 ka for U Th ages of 50 to 200 ka. The full spreading rate based on U Th ages for Endeavour segment of JDF is 5.9 ± 1.2 cm/yr, with asymmetry in spreading between the Pacific ( 4.0 ± 0.6 cm/yr) and JDF ( 1.9 ± 0.6 cm/yr) plates. For northern Gorda ridge, the half spreading rate for the JDF plate is found to be 3.0 ± 0.4 cm/yr. These rates are in agreement with paleomagnetic spreading rates and topographic constraints. This suggests that assumptions used to determine ages, including constancy of initial 230 Th/ 232 Th ratio over time, are generally valid for the areas studied. Samples located near the axis of spreading are typically younger than predicted by these spreading rates, which most likely reflects recent volcanism within a 1–3 km wide zone of crustal accretion. Initial 230 /Th/ 232 Th ratios and 230 Th activity excesses were also used to examine the recent Th/U evolution and extent of melting of mantle sources beneath these ridges. A negative anomaly in 230 Th/ 232 Th for Axial seamount lavas provides the first geochemical evidence of a mantle plume source for Axial seamount and the Cobb-Eickelberg seamount chain and indicates recent depletion of other JDF segment sources. Large 230 Th activity excesses for lavas from northern Gorda ridge and Endeavour segment indicate formation from a lower degree of partial melting than other segments. An inverse correlation between 230 Th excess and 230 Th/ 232 Th for each ridge indicates that these lower degree melts formed from slightly less depleted sources than higher degree melts. Uniformity in 230 Th excess for other segments suggests similarity in processes of melt formation and mixing beneath most of the JDF-Gorda ridge area. The average initial 230 Th/ 232 Th activity ratio of 1.31 for the JDF-Gorda ridge area is in agreement with the predicted value of 1.32 from the Th Sr isotope mantle array.

Earth and Planetary Science Letters↗
USGS1992

Sr, Nd, and Pb isotopes of ultramafic xenoliths in volcanic rocks of Eastern China: Enriched components EMI and EMII in subcontinental lithosphere

The U-Th-Pb, Sm-Nd, and Rb-Sr isotopic systematics of mafic and ultramafic xenolithic rocks and associated megacrystic inclusions of aluminous augite and garnet, that occur in three alkalic volcanic suites: Kuandian in eastern Liaoning Province, Hanluoba in Hebei Province, and Minxi in western Fujian Province, China are described. In various isotopic data plots, the inclusion data invariably fall outside the isotopic ranges displayed by the host volcanic rocks, testifying to the true xenolithic nature of the inclusions. The major element partitioning data on Ca, Mg, Fe, and Al among the coexisting silicate minerals of the xenoliths establish their growth at ambient mantle temperatures of 1000–1100°C and possible depths of 70–80 km in the subcontinental lithosphere. Although the partitioning of these elements reflects equilibrium between coexisting minerals, equilibria of the Pb, Nd, and Sr isotopic systems among the minerals were not preserved. The disequilibria are most notable with respect to the 206 Pb/ 204 Pb ratios of the minerals. On a Nd-Sr isotopic diagram, the inclusion data plot in a wider area than that for oceanic basalts from a distinctly more depleted component than MORB with higher 143 Nd/ 144 Nd and a much broader range of 87 Sr/ 86 Sr values, paralleling the theoretical trajectory of a sea-water altered lithosphere in Nd-Sr space. The garnets consistently show lowerμ andκ values than the pyroxenes and pyroxenites, whereas a phlogopite shows the highestμ andκ values among all the minerals and rocks studied.

Earth and Planetary Science Letters↗
USGS1993

Volcanic eruption of the mid-ocean ridge along the East Pacific Rise crest at 9°45-52'N: direct submersible observations of seafloor phenomena associated with an eruption event in April, 1991

In April, 1991, we witnessed from the submersible Alvin a suite of previously undocumented seafloor phenomena accompanying an in-progress eruption of the mid-ocean ridge on the East Pacific Rise crest at 9&deg;45&prime;N&ndash;52&prime;N. The volume of the eruption could not be precisely determined, although comparison of pre- and post-eruption SeaBeam bathymetry indicate that any changes in ridge crest morphology resulting from the eruption were < 10 m high. Effects of the eruption included: (1) increased abundance and redistribution of hydrothermal vents, disappearance of numerous vent communities, and changes in characteristics of vent fauna and mineral deposits within the eruption area since December, 1989; (2) murkiness of bottom waters up to tens of meters above the seafloor due to high densities of suspended mineral and biogenic particulates; (3) destruction of a vent community by lava flows, mass wasting, and possible hydrovolcanic explosion at a site known as &lsquo;Tubeworm Barbecue&rsquo; in the axial summit caldera (ASC) at 9&deg;50.6&prime;N; (4) near-critical temperatures of hydrothermal vent fluids, ranging up to 403&deg;C; (5) temporal variations over a 2 week interval in both temperatures and chemical/isotopic compositions of hydrothermal fluids; (6) unusual compositions of end-member vent fluids, with pH values ranging to a record low of 2.5, salinities ranging as low as 0.3 wt% NaCl (one-twelfth that of seawater), and dissolved gases reaching high concentrations (> 65 mmol/l for both CO 2 and H 2 S); (7) venting at temperatures above 380&deg;C of visually detectable white vapor that transformed to plumes of gray smoke a few centimeters above vent orifices; (8) disorganized venting of both high-temperature fluids (black and gray smoke) and large volumes of cooler, diffuse hydrothermal fluids directly from the basaltic seafloor, rather than from hydrothermal mineral constructions; (9) rapid and extensive growth of flocculent white bacterial mats (species unknown) on and under the seafloor in areas experiencing widespread venting of diffuse hydrothermal fluid; and (10) subseafloor downslope migration of magma normal to the ridge axis in a network of small-scale (1&ndash;5 m diameter) lava tubes and channels to distances at least 100&ndash;200 m outside the ASC. We suggest that, in April, 1991, intrusion of dikes in the eruption area to < 200 m beneath the ASC floor resulted in phase separation of fluids near the tops of the dikes and a large flux of vapor-rich hydrothermal fluids through the overlying rubbly, cavernous lavas. Low salinities and gas-rich compositions of hydrothermal fluids sampled in the eruption area are appropriate for a vapor phase in a seawater system undergoing subcritical liquid-vapor phase separation (boiling) and phase segregation. Hydrothermal fluids streamed directly from fissures and pits that may have been loci of lava drainback and/or hydrovolcanic explosions. These fissures and pits were lined with white mats of a unique fast-growing bacteria that was the only life associated with the brand-new vents. The prolific bacteria, which covered thousands of square meters on the ridge crest and were also abundant in subseafloor voids, may thrive on high levels of gases in the vapor-rich hydrothermal fluids initially escaping the hydrothermal system. White bacterial particulates swept from the seafloor by hydrothermal vents swirled in an unprecedented biogenic &lsquo;blizzard&rsquo; up to 50 m above the bottom. The bacterial proliferation of April, 1991 is likely to be a transient bloom that will be checked quickly either by decline of dissolved gas concentrations in the fluids as rapid heat loss brings about cessation of boiling, and/or by grazing as other organisms are re-established in the biologically devastated area.

Earth and Planetary Science Letters↗
USGS1993

Cycling of sulfur in subduction zones: The geochemistry of sulfur in the Mariana Island Arc and back-arc trough

The sulfur contents and sulfur isotopic compositions of 24 glassy submarine volcanics from the Mariana Island Arc and back-arc Mariana Trough were determined in order to investigate the hypothesis that subducted seawater sulfur (δ 34 S= 21‰) is recycled through arc volcanism. Our results for sulfur are similar to those for subaerial arc volcanics: Mariana Arc glasses are enriched in 34 S (δ 34 S=up to10.3‰,mean= 3.8‰) and depleted in S (20–290 ppm, mean = 100 ppm) relative to MORB (850 ppm S,δ 34 S= 0.1 ± 0.5‰). The back-arc trough basalts contain 200–930 ppm S and haveδ 34 S values of1.1 ± 0.5‰, which overlap those for the arc and MORB. The low sulfur contents of the arc and some of the trough glasses are attributed to (1) early loss of small amounts of sulfur through separation of immiscible sulfide and (2) later vapor-melt equilibrium control of sulfur contents and loss of sulfur in a vapor phase from sulfide-undersaturated melts near the minimum in sulfur solubility atƒO 2 ≈NNO (nickel-nickel oxide). Although these processes removed sulfur from the melts their effects on the sulfur isotopic compositions of the melts were minimal. Positive trends ofδ 34 S with 87 Sr/ 86 Sr, LILE and LREE contents of the arc volcanics are consistent with a metasomatic seawater sulfur component in the depleted sub-arc mantle source. The lack of a 34 S-rich slab signature in the trough lavas may be attributed to equilibration of metasomatic fluid with mantle material along the longer pathway from the slab to the source of the trough volcanics. Sulfur is likely to have been transported into the mantle wedge by metasomatic fluid derived from subducted sediments and pore fluids.

Earth and Planetary Science Letters↗
USGS1993

Elemental abundance anomalies in the late Cenomanian extinction interval: A search for the source(s)

Elemental abundances have been measured by neutron activation methods across the Cenomanian-Turonian (late Cretaceous) extinction interval in samples collected from sixteen sites in the Western Interior Basin of North America and from twelve widely separated locations around the globe, including six ODP/DSDP sites. In most Western Interior Basin sites, in Colombia, and in western Europe (weaker), two closely spaced elemental abundance peaks occur in the upper Cenomanian (∼ 92 m.y.), spanning the ammonite zones of Sciponoceras gracile through Neocardioceras juddii . Elements with anomalously high concentrations include Sc, Ti, V, Cr, Mn, Co, Ni, Ir, Pt and Au. The lower peak coincides with the disappearance (extinction) of the foraminifer Rotalipora cushmani . In North American sections R. greenhornensis also disappears at or just below this horizon, but in Europe it disappears considerably earlier than R. cushmani . A series of molluscan extinction and speciation or migration events also begins near the stratigraphic level of the lower elemental abundance peak.

Earth and Planetary Science Letters↗
USGS1993

Impact-shocked zircons: Discovery of shock-induced textures reflecting increasing degrees of shock metamorphism

Textural effects specifically characteristic of shock metamorphism in zircons from impact environments have not been reported previously. However, planar deformation features (PDF) due to shock metamorphism are well documented in quartz and other mineral grains from these same environments. An etching technique was developed that allows SEM visualization of PDF and other probable shock-induced textural features, such as granular (polycrystalline) texture, in zircons from a variety of impact shock environments. These textural features in shocked zircons from K/T boundary distal ejecta form a series related to increasing degrees of shock that should correlate with proportionate resetting of the UPb isotopic system.

Earth and Planetary Science Letters↗
USGS1993

Fingerprinting the K/T impact site and determining the time of impact by UPb dating of single shocked zircons from distal ejecta

U Pb isotopic dating of single 1–3 μg zircons from K/T distal ejecta from a site in the Raton Basin, Colorado provides a powerful new tool with which to determine both the time of the impact event and the age of the basement at the impact site. Data for the least shocked zircons are slightly displaced from the 544 ± 5 Ma primary age for a component of the target site, white those for highly shocked and granular grains are strongly displaced towards the time of impact at 65.5 ± 3.0 Ma. Such shocked and granular zircons have never been reported from any source, including explosive volcanic rocks. Zircon is refractory and has one of the highest thermal blocking temperatures; hence, it can record both shock features and primary and secondary ages without modification by post-crystallization processes. Unlike shocked quartz, which can come from almost anywhere on the Earth's crust, shocked zircons can be shown to come from a specific site because basement ages vary on the scale of meters to kilometers. With U Pb zircon dating, it is now possible to correlate ejecta layers derived from the same target site, test the single versus multiple impact hypothesis, and identify the target source of impact ejecta. The ages obtained in this study indicate that the Manson impact site, Iowa, which has basement rocks that are mid-Proterozoic in age, cannot be the source of K/T distal ejecta. The K/T distal ejecta probably originated from a single impact site because most grains have the same primary age.

Earth and Planetary Science Letters↗
USGS1995

Denudation rates determined from the accumulation of in situ-produced 10Be in the luquillo experimental forest, Puerto Rico

We present a simple method for estimation of long-term mean denudation rates using in situ-produced cosmogenic 10Be in fluvial sediments. Procedures are discussed to account for the effects of soil bioturbation, mass wasting and attenuation of cosmic rays by biomass and by local topography. Our analyses of 10Be in quartz from bedrock outcrops, soils, mass-wasting sites and riverine sediment from the Icacos River basin in the Luquillo Experimental Forest, Puerto Rico, are used to characterize denudation for major landform elements in that basin. The 10Be concentration of a discharge-weighted average of size classes of river sediment corresponds to a long-term average denudation of &asymp; 43 m Ma &minus;1, consistent with mass balance results.

Earth and Planetary Science Letters↗
USGS1994

A rock-magnetic record from Lake Baikal, Siberia: Evidence for Late Quaternary climate change

Rock-magnetic measurements of sediment cores from the Academician Ridge region of Lake Baikal, Siberia show variations related to Late Quaternary climate change. Based upon the well-dated last glacial-interglacial transition, variations in magnetic concentration and mineralogy are related to glacial-interglacial cycles using a conceptual model. Interglacial intervals are characterized by low magnetic concentrations and a composition that is dominated by low coercivity minerals. Glacial intervals are characterized by high magnetic concentrations and increased amounts of high coercivity minerals. The variation in magnetic concentration is consistent with dilution by diatom opal during the more productive interglacial periods. We also infer an increased contribution of eolian sediment during the colder, windier, and more arid glacial conditions when extensive loess deposits were formed throughout Europe and Asia. Eolian transport is inferred to deliver increased amounts of high coercivity minerals as staining on eolian grains during the glacial intervals. Variations in magnetic concentration and mineralogy of Lake Baikal sediment correlate to the SPECMAP marine oxygen-isotope record. The high degree of correlation between Baikal magnetic concentration/mineralogy and the SPECMAP oxygen-isotope record indicates that Lake Baikal sediment preserves a history of climate change in central Asia for the last 250 ka. This correlation provides a method of estimating the age of sediment beyond the range of the radiocarbon method. Future work must include providing better age control and additional climate proxy data, thereby strengthening the correlation of continental and marine climate records. ?? 1994.

Lake Baikal↗
USGS1994

Block versus continuum deformation in the Western United States

The relative role of block versus continuum deformation of continental lithosphere is a current subject of debate. Continuous deformation is suggested by distributed seismicity at continental plate margins and by cumulative seismic moment sums which yield slip estimates that are less than estimates from plate motion studies. In contrast, block models are favored by geologic studies of displacement in places like Asia. A problem in this debate is a lack of data from which unequivocal conclusions may be reached. In this paper we apply the techniques of study used in regions such as the Alpine-Himalayan belt to an area with a wealth of instrumental data-the Western United States. By comparing plate rates to seismic moment release rates and assuming a typical seismogenic layer thickness of 15 km it appears that since 1850 about 60% of the Pacific-North America motion across the plate boundary in California and Nevada has occurred seismically and 40% aseismically. The San Francisco Bay area shows similar partitioning between seismic and aseismic deformation, and it can be shown that within the seismogenic depth range aseismic deformation is concentrated near the surface and at depth. In some cases this deformation can be located on creeping surface faults, but elsewhere it is spread over a several kilometer wide zone adjacent to the fault. These superficial creeping deformation zones may be responsible for the palaeomagnetic rotations that have been ascribed elsewhere to the surface expression of continuum deformation in the lithosphere. Our results support the dominant role of non-continuum deformation processes with the implication that deformation localization by strain softening must occur in the lower crust and probably the upper mantle. Our conclusions apply only to the regions where the data are good, and even within the Western United States (i.e., the Basin and Range) deformation styles remain poorly resolved. Nonetheless, we maintain that block motion is the deformation style of choice for those continental regions where the data are best.

Earth and Planetary Science Letters↗
USGS1994

Magnetostratigraphy of the Willwood Formation, Bighorn Basin, Wyoming: New constraints on the location of Paleocene/Eocene boundary

The lower Eocene Willwood Formation in the Bighorn Basin of Wyoming preserves a rich and diverse mammalian and floral record. The paleomagnetic behavior of the sequence of floodplain paleosols of varying degrees of maturation ranges from excellent to poor. We present a magnetostratigraphic section for a composite section near Worland, Wyoming, by using a set of strict criteria for interpreting the step-wise alternating field and thermal demagnetization data of 266 samples from 90 sites throughout the composite section. Correlation to the geomagnetic reversal time scale was achieved by combining magnetostratigraphic and biostratigraphic data from this section, from a section in the Clark's Fork Basin in northern Wyoming, and from DSDP Site 550, with the isotopic data determined on a tuff near the top of our section. Our correlation suggests that the Bighorn Basin composite section in the Worland area spans from within Chron C24r to near the top of Chron C24n, or from approximately 55 to 52 Ma. This correlation places the Paleocene/Eocene boundary within the vicinity of the base of the section. Cryptochron C24r.6 of Cande and Kent is tentatively identified some 100 m above the base of the section. The temporal framework provided here enables correlation of the mammalian biostratigraphy of the Bighorn Basin to other continental sequences as well as to marine records. It also provides independent chronological information for the calculation of sediment accumulation rates to constrain soil maturation rates. We exclude an age as young as 53 Ma for the Paleocene/Eocene boundary and support older ages, as recommended in recent time scales. The location of a tuff dated at 52.8 ± 0.3 Ma at the older boundary C24n.1 is consistent with the age of 52.5 Ma estimated by Cande and Kent and inconsistent with that of 53.7 Ma, from Harland et al.

Wyoming↗
USGS1994

East Mariana Basin tholeiites: Cretaceous intraplate basalts or rift basalts related to the Ontong Java plume?

Studies of seafloor magnetic anomaly patterns suggest the presence of Jurassic oceanic crust in a large area in the western Pacific that includes the East Mariana, Nauru and Pigafetta Basins. Sampling of the igneous crust in this area by the Deep Sea Drilling Program (DSDP) and the Ocean Drilling Program (ODP) allows direct evaluation of the age and petrogenesis of this crust. ODP Leg 129 drilled a 51 m sequence of basalt pillows and massive flows in the central East Mariana Basin. 40 Ar 39 Ar "> 40Ar39Ar ages determined in this study for two Leg 129 basalts average 114.6 ± 3.2 Ma. This age is in agreement with the Albian-late Aptian paleontologic age of the overlying sediments, but is distinctively younger than the Jurassic age predicted by magnetic anomaly patterns in the basin. Compositionally, the East Mariana Basin basalts are uniformly low-K tholeiites that are depleted in highly incompatible elements compared to moderately incompatible ones, which is typical of mid-ocean ridge basalts (MORB) erupted near hotspots. The Sr, Nd and Pb isotopic compositions of the tholeiites ( 87 Sr 86 Sr init = 0.70360&#x2013;0.70374 "> 87Sr86Srinit= 0.70360–0.70374 ; 143 Nd 144 Nd init = 0.512769&#x2013;0.512790 "> 143Nd144Ndinit= 0.512769–0.512790 ; 206 Pb 204 Pb meas = 18.355&#x2013;18.386 "> 206Pb204Pbmeas= 18.355–18.386 ) also overlap with some Indian Ocean Ridge MORB, although they are distinct from the isotopic compositions of Jurassic basalts drilled in the Pigafetta Basin, the oldest Pacific MORB. The isotopic compositions of the East Mariana Basin tholeiites are also similar to those of intraplate basalts, and in particular, to the isotopic signature of basalts from the nearby Ontong Java and Manihiki Plateaus. The East Mariana Basin tholeiites also share many petrologic and isotopic characteristics with the oceanic basement drilled in the Nauru Basin at DSDP Site 462. In addition, the new 110.8 ± 1.0 Ma 40 Ar 39 Ar "> 40Ar39Ar age for two flows from the bottom of Site 462 in the Nauru Basin is indistinguishable from the age of the East Mariana Basin flows. Thus, while magnetic anomaly patterns predict that the igneous basement in the Nauru and East Mariana Basins is Jurassic in age, the geochemical and chronological results discussed here suggest that the basement formed during a Cretaceous rifting event within the Jurassic crust. This magmatic and tectonic event was created by the widespread volcanism responsible for the genesis of the large oceanic plateaus of the western Pacific.

Earth and Planetary Science Letters↗
USGS1995

U-Th-Pb zircon dating of the 13.8-Ma dacite volcanic dome at Cerro Rico de Potosi, Bolivia

The temporal relationship between the extrusion of the Miocene dacite volcanic dome at Cerro Rico de Potasi, Bolivia, and the associated Ag-Sn mineralization has an important bearing on the heat and metal sources for this world class mineral deposit. The present study uses U-Th-Pb dating of sparse zircon contained in the dacite to demonstrate that, at most, only several hundred thousand years separate dome emplacement from main stage mineralization. -from Authors

Earth and Planetary Science Letters↗