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.

SourceResearch connectionFrom yearThrough year

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

At least 1,459 records · Page 81Linked to original sources

USGS1995

40Ar/39Ar and 18O/16O studies of the Chegem ash-flow caldera and the Eldjurta Granite: Cooling of two late Pliocene igneous bodies in the Greater Caucasus Mountains, Russia

Volcanic and intrusive rocks of the Chegem caldera and the nearby Eldjurta (Eldzhurtinskiy) Granite record a late Pliocene episode of silicic magmatism in the north-central Caucasus Mountains. Surface exposures, created by the recent rapid uplift and erosion of the Caucasus Mountains, span a 2 km vertical section of Chegem caldera fill and 1150 m of the Eldjurta Granite; cored mineral-exploration drillholes in the Eldjurta Granite extend the sampling to a depth of 4 km. The unique sampling range available in these two young igneous bodies affords an excellent opportunity to study their denudation and cooling histories, which we examine by means of 40 Ar/ 39 Ar and 18 O/ 16 O measurements on an extensive sample suite. Total-fusion biotite and sanidine ages from eight Chegem Tuff samples, both intracaldera and outflow, are analytically indistinguishable with a weighted mean of 2.82 ± 0.02 Ma. A cross-cutting granodiorite porphyry intrusion has a sanidine total fusion age of 2.84 ± 0.03 Ma, and whole-rock incremental heating of a post-caldera andesite flow, which caps the caldera fill, yields an age of 2.82 ± 0.02 Ma. Thus, caldera formation and post-caldera resurgence and volcanism all occurred within a very short time (< 50,000 yr). Biotite total-fusion ages of ten Eldjurta Granite samples, including seven from ∼ 500 m intervals in the 4 km deep drillhole, show a systematic linear decrease in age with depth from 1.90 Ma near the roof contact of the granite to 1.56 Ma at a depth of 3700 m. Assuming these ages were set at the same temperature, this age/depth gradient implies an isotherm migration rate of 13 mm/yr between 1.90 and 1.56 Ma. This migration rate is due to a combination of rapid denudation and downward relaxation of isotherms, with cooling rates between 200 and 500°C/Ma during this period. Oxygen isotopic compositions of quartz, K-feldspar, plagioclase and biotite from the drillhole samples below the 800 m depth are fairly uniform and record primary igneous δ 18 O values with little evidence for subsolidus hydrothermal activity. However, in surface outcrop samples and in the shallowest drillhole sample, mineral δ 18 O values have been lowered by up to 3‰ by interaction with an external (meteoric-hydrothermal?) fluid. The primary mineral δ 18 O values of the Eldjurta Granite are distinctly higher than the corresponding phenocryst δ 18 O values in the Chegem volcanic rocks, indicating that the two bodies evolved as separxate magma batches.

Caucasus Mountains, Chegem Caldera↗

USGS1995

A mass proportion method for calculating melting reactions and application to melting of model upper mantle lherzolite

We present a method for calculating quantitative melting reactions in systems with multiple solid solutions that accounts for changes in the mass proportions of phases between two points at different temperatures along a melting curve. This method can be applied to any data set that defines the phase proportions along a melting curve. The method yields the net change in mass proportion of all phases for the chosen melting interval, and gives an average reaction for the melting path. Instantaneous melting reactions can be approximated closely by choosing sufficiently small melting intervals. As an application of the method, reactions for melting of model upper mantle peridotite are calculated using data from the system CaO-MgO-Al 2 O 3 -SiO 2 -Na 2 O (CMASN) over the pressure interval 0.7 – 3.5 GPa. Throughout almost this entire pressure range, melting of model lherzolite involves the crystallization of one or more solid phases, and is analogous to melting at a peritectic invariant point. In addition, we show that melting reactions for small melting intervals (< 5%) along the solidus of mantle peridotite are significantly different from those calculated for large melting intervals. For large melting intervals (> 10%), reaction stoichiometries calculated in CMASN are usually in good agreement with those available for melting of natural peridotite. The coefficients of melting reactions calculated from this method can be used in equations that describe the behavior of trace elements during melting. We compare results from near-fractional melting models using (1) melting reactions and rock modes from CMASN, and (2) constant reactions representative of those used in the literature. In modeling trace element abundances in melt, significant differences arise for some elements at low degrees of melting (< 10%). In modeling element abundances in the residue, differences increase with increase in degree of melting. Reactions calculated along the model lherzolite solidus in CMASN are the only ones available at present for small degrees of melting so we recommend them for accurate trace element modeling of natural lherzolite.

Earth and Planetary Science Letters↗

USGS1996

A brief Oligocene period of flood volcanism in Yemen: Implications for the duration and rate of continental flood volcanism at the Afro-Arabian triple junction

40 Ar 29 Ar dating of mineral separates and whole-rock (WR) samples has established that basaltic continental flood volcanism (CFV) began between 30.9 and 29.2 Ma in northwestern and southwestern Yemen, respectively. Rhyolitic volcanism commenced at 29.0–29.3 Ma throughout Yemen. Lower basaltic lavas were erupted every 10–100 kyr, whereas upper bimodal volcanic units were erupted every 100–500 kyr, which reflects generation of rhyolitic magmas from basalts that resided for longer periods in lithospheric magma chambers than during the early phase of exclusively mafic magmatism. The youngest dated flood volcanic units were erupted between 26.9 and 26.5 Ma throughout Yemen. The duration of preserved CFV defined by 40 Ar 29 Ar dating (4.4 myr) contrasts with the wide range of WR K Ar dates previously obtained in Yemen (> 50 myr). 40 Ar 39 Ar "> 40Ar39Ar step-heating studies of WR samples has shown that this discrepancy is due to the disturbed Ar systematics of volcanic samples. Most samples have experienced post-crystallization loss of radiogenic Ar and/or contain excess Ar, with only ca. 25% of the WR K Ar dates within 1–2 myr of true crystallization ages. WR K Ar data can be screened for reliability using the radiogenic Ar yield and 40 K 36 Ar "> 40K36Ar ratio, which reflect the Ar retentivity of the sample, the likelihood that alteration has disturbed a sample's Ar systematics, and the susceptibility of the sample to a finite amount of Ar loss or the presence of a finite amount of excess Ar. Examination of existing WR KAr data in the Ethiopian part of this flood volcanic province, using these parameters, suggests that much of these data are also misleading. Two phases of flood volcanism are inferred in Ethiopia and Eritrea at 38-30 Ma and ca. 20 Ma. The older phase is equivalent to that in Yemen, and is consistent with the progression in basal volcanic ages obtained in Yemen moving from north to south. The younger phase is related to the onset of upper crustal extension and incipient Red Sea-Gulf of Aden rifting. The sequence of events — surface uplift (?), flood magmatism and subsequent upper crustal extension — in Yemen is consistent with the involvement of a mantle plume at the Afro-Arabian tripe junction. However, the overall eruption rate for this flood volcanic province is only 0.03 km 3 /yr, much slower than that postulated for other plume-related provinces such as the Deccan or Siberian Traps, but perhaps comparable to the Paraná-Etendeka province, which also contains significant amounts of rhyolitic volcanic products like those of Yemen-Ethiopia. The highly variable eruption rates in individual provinces must reflect the very different character of individual plumes, or the control of lithospheric structure and plate tectonic stresses on the surface manifestations of plumes. The long duration of CFV and large amounts of rhyolitic volcanism at the Afro-Arabian triple junction may be attributed to the relatively slow separation of the African and Arabian plates compared with, for example, the rifting of India and the Deccan Traps.

Earth and Planetary Science Letters↗

USGS1996

Velocity structure of a bottom simulating reflector offshore Peru: Results from full waveform inversion

Much of our knowledge of the worldwide distribution of submarine gas hydrates comes from seismic observations of Bottom Simulating Reflectors (BSRs). Full waveform inversion has proven to be a reliable technique for studying the fine structure of BSRs using the compressional wave velocity. We applied a non-linear full waveform inversion technique to a BSR at a location offshore Peru. We first determined the large-scale features of seismic velocity variations using a statistical inversion technique to maximise coherent energy along travel-time curves. These velocities were used for a starting velocity model for the full waveform inversion, which yielded a detailed velocity/depth model in the vicinity of the BSR. We found that the data are best fit by a model in which the BSR consists of a thin, low-velocity layer. The compressional wave velocity drops from 2.15 km/s down to an average of 1.70 km/s in an 18m thick interval, with a minimum velocity of 1.62 km/s in a 6 m interval. The resulting compressional wave velocity was used to estimate gas content in the sediments. Our results suggest that the low velocity layer is a 6-18 m thick zone containing a few percent of free gas in the pore space. The presence of the BSR coincides with a region of vertical uplift. Therefore, we suggest that gas at this BSR is formed by a dissociation of hydrates at the base of the hydrate stability zone due to uplift and subsequently a decrease in pressure.

Earth and Planetary Science Letters↗

USGS1996

The nature, distribution, and origin of gas hydrate in the Chile Triple Junction region

A bottom simulating reflector (BSR) is regionally distributed throughout much of the Chile Triple Junction (CTJ) region. Downhole temperature and logging data collected during Ocean Drilling Program (ODP) Leg 141 suggest that the seismic BSR is generated by low seismic velocities associated with the presence of a few percent free gas in a ??? 10 m thick zone just beneath the hydrate-bearing zone. The data also indicate that the temperature and pressure at the BSR best corresponds to the seawater/methane hydrate stability field. The origin of the large amounts of methane required to generate the hydrates is, however, problematic. Low total organic carbon contents and low alkalinities argue against significant in situ biogenic methanogenesis, but additional input from thermogenic sources also appears to be precluded. Increasing thermal gradients, associated with the approach of the spreading ridge system, may have caused the base of the hydrate stability field to migrate 300 m upwards in the sediments. We propose that the upward migration of the base of the stability field has concentrated originally widely dispersed hydrate patches into the more continuous hydrate body we see today. The methane can be concentrated if the gas hydrates can form from dissolved methane, transported into the hydrate zone via diffusion or fluid advection. A strong gradient may exist in dissolved methane concentration across the BSR leading to the steady reabsorbtion of the free gas zone during the upward migration of the BSR even in the absence of fluid advection.

Earth and Planetary Science Letters↗

USGS1996

Recent volcanism in the Siqueiros transform fault: Picritic basalts and implications for MORB magma genesis

Small constructional volcanic landforms and very fresh-looking lava flows are present along one of the inferred active strike-slip faults that connect two small spreading centers (A and B) in the western portion of the Siqueiros transform domain. The most primitive lavas (picritic and olivine-phyric basalts), exclusively recovered from the young-looking flows within the A-B strike-slip fault, contain millimeter-sized olivine phenocrysts (up to 20 modal%) that have a limited compositional range (Fo91.5-Fo89.5) and complexly zoned Cr-Al spinels. High-MgO (9.5-10.6 wt%) glasses sampled from the young lava flows contain 1-7% olivine phenocrysts (Fo90.5-Fo89) that could have formed by equilibrium crystallization from basaltic melts with Mg# values between 71 and 74. These high MgO (and high Al2O3) glasses may be near-primary melts from incompatible-element depleted oceanic mantle and little modified by crustal mixing and/or fractionation processes. Phase chemistry and major element systematics indicate that the picritic basalts are not primary liquids and formed by the accumulation of olivine and minor spinel from high-MgO melts (10% < MgO < 14%). Compared to typical N-MORB from the East Pacific Rise, the Siqueiros lavas are more primitive and depleted in incompatible elements. Phase equilibria calculations and comparisons with experimental data and trace element modeling support this hypothesis. They indicate such primary mid-ocean ridge basalt magmas formed by 10-18% accumulative decompression melting in the spinel peridotite field (but small amounts of melting in the garnet peridotite field are not precluded). The compositional variations of the primitive magmas may result from the accumulation of different small batch melt fractions from a polybaric melting column.

Earth and Planetary Science Letters↗

USGS1996

Global distribution of beryllium isotopes in deep ocean water as derived from Fe-Mn crusts

The direct measurement of the ratio of cosmogenic 10 Be ( T 1 2 = 1.5 Ma "> T12= 1.5 Ma ) to stable terrigenously sourced 9 Be in deep seawater or marine deposits can be used to trace water mass movements and to quantify the incorporation of trace metals into the deep sea. In this study a SIMS-based technique has been used to determine the 10 Be 9 Be "> 10Be9Be ratios of the outermost millimetre of hydrogenetic ferromanganese crusts from the worlds oceans. 10 Be 9 Be "> 10Be9Be ratios, time-corrected for radioactive decay of cosmogenic 10 Be using 234 U 238 U "> 234U238U , are in good agreement with AMS measurements of modern deep seawater. Ratios are relatively low in the North and equatorial Atlantic samples (0.4–0.5 × 10 −7 ). In the Southwest Atlantic ratios increase up to 1 × 10 −7 , they vary between 0.7 and 1.0 × 10 −7 in Indian Ocean samples, and have a near constant value of 1.1 ± 0.2 × 10 −7 for all Pacific samples. If the residence time of 10 Be ( τ 10 Be ) in deep water is constant globally, then the observed variations in 10 Be 9 Be "> 10Be9Be ratios could be caused by accumulation of 10 Be in deep water as it flows and ages along the conveyor, following a transient depletion upon its formation in the Northern Atlantic. In this view both 10 Be and 9 Be reach local steady-state concentration in Pacific deep water and the global τ10 Be ≌ 600 a ">≌ τ10Be≌ 600a . An alternative possibility is that the Be isotope abundances are controlled by local scavenging. For this scenario τ 10 Be would vary according to local particle concentration and would ≌ 600 a in the central Pacific, but τ10 Be ≌ 230 a ">≌ τ10Be≌ 230a in the Atlantic. Mass balance considerations indicate that hydrothermal additions of 9 Be to the oceans are negligible and that the dissolved riverine source is also small. Furthermore, aeolian dust input of 9 Be appears insufficient to provide the dissolved 9 Be inventory. The dissolution of only a small proportion (2%) of river-derived particulates could in principle supply the observed seawater 9 Be content. If true, ocean margins would be the sites for 9 Be addition. Due to the particle-reactive nature of Be, these would also be the primary sites of Be removal. A possible net result of horizontal water masses passing through these marginal areas might be a decrease in seawater 10 Be 9 Be "> 10Be9Be , and establishment of a relatively constant 9 Be concentration. As τ 10 Be (∼ 600 a) is less than the apparent age of deep water in the Pacific (∼ 1500 a), the Pacific record of 10 Be 9 Be "> 10Be9Be is not expected to show secular variations due to changes in deep-water flow, despite the large variations in 10 Be 9 Be "> 10Be9Be between different water masses. Because of this insensitivity to deep-water flow, however, it is suggested that the 10 Be 9 Be "> 10Be9Be ratio, determined in the authigenic phase of marine sediments or hydrogenetic precipitates, should be a suitable tool for monitoring changes in continental input or cosmic ray intensity on longer time scales.

Earth and Planetary Science Letters↗

USGS1996

Distinguishing shocked from tectonically deformed quartz by the use of the SEM and chemical etching

Multiple sets of crystallographically-oriented planar deformation features (PDFs) are generated by high-strain-rate shock waves at pressures of > 12 GPa in naturally shocked quartz samples. On surfaces, PDFs appear as narrow (50-500 nm) lamellae filled with amorphosed quartz (diaplectic glass) which can be etched with hydrofluoric acid or with hydrothermal alkaline solutions. In contrast, slow-strain-rate tectonic deformation pressure produces wider, semi-linear and widely spaced arrays of dislocation loops that are not glass filled. Etching samples with HF before examination in a scanning electron microscope (SEM) allows for unambiguous visual distinction between glass-filled PDFs and glass-free tectonic deformation arrays in quartz. This etching also reveals the internal 'pillaring' often characteristic of shock-induced PDFs. This technique is useful for easily distinguishing between shock and tectonic deformation in quartz, but does not replace optical techniques for characterizing the shock features.

Earth and Planetary Science Letters↗

USGS1996

Stability of hydrous phases in subducting oceanic crust

Experiments in the basalt-H 2 O system at 600–950°C and 0.8–3.0 GPa, demonstrate that breakdown of amphibole represents the final dehydration of subducting oceanic tholeiite at T ≥650°C; the dehydration H 2 O occurs as a free fluid or in silicate melt co-existing with an anhydrous eclogite assemblage. In contrast, about 0.5 wt% of H 2 O is stored in lawsonite at 600°C, 3.0 GPa. Our results suggest that slab melting occurs at depths shallower than 60 km for subducting young oceanic crust; along a subduction zone with an average thermal gradient higher than 7°C/km, H 2 O stored in hydrated low-potassium, metabasaltic layers cannot be subducted to depths greater than 100 km, then released to generate arc magma.

Earth and Planetary Science Letters↗

USGS2004

Tracing the history of submarine hydrothermal inputs and the significance of hydrothermal hafnium for the seawater budget - A combined Pb-Hf-Nd isotope approach

Secular variations in the Pb isotopic composition of a mixed hydrogenous-hydrothermal ferromanganese crust from the Bauer Basin in the eastern Equatorial Pacific provide clear evidence for changes in hydrothermal contributions during the past 7 Myr. The nearby Galapagos Rise spreading center provided a strong hydrothermal flux prior to 6.5 Ma. After 6.5 Ma, the Pb became stepwise more radiogenic and more similar to Equatorial Pacific seawater, reflecting the westward shift of spreading to the presently active East Pacific Rise (EPR). A second, previously unrecognized enhanced hydrothermal period occurred between 4.4 and 2.9 Ma, which reflects either off-axis hydrothermal activity in the Bauer Basin or a late-stage pulse of hydrothermal Pb from the then active, but waning Galapagos Rise spreading center. Hafnium isotope time-series of the same mixed hydrogenous-hydrothermal crust show invariant values over the past 7 Myr. Hafnium isotope ratios, as well as Nd isotope ratios obtained for this crust, are identical to that of hydrogenous Equatorial Pacific deep water crusts and clearly indicate that hydrothermal Hf, similar to Nd, does not travel far from submarine vents. Therefore, we suggest that hydrothermal Hf fluxes do not contribute significantly to the global marine Hf budget. ?? 2004 Elsevier B.V. All rights reserved.

Earth and Planetary Science Letters↗

USGS2004

Three-dimensional distribution of gas hydrate beneath southern Hydrate Ridge: Constraints from ODP Leg 204

Large uncertainties about the energy resource potential and role in global climate change of gas hydrates result from uncertainty about how much hydrate is contained in marine sediments. During Leg 204 of the Ocean Drilling Program (ODP) to the accretionary complex of the Cascadia subduction zone, we sampled the gas hydrate stability zone (GHSZ) from the seafloor to its base in contrasting geological settings defined by a 3D seismic survey. By integrating results from different methods, including several new techniques developed for Leg 204, we overcome the problem of spatial under-sampling inherent in robust methods traditionally used for estimating the hydrate content of cores and obtain a high-resolution, quantitative estimate of the total amount and spatial variability of gas hydrate in this structural system. We conclude that high gas hydrate content (30-40% of pore space or 20-26% of total volume) is restricted to the upper tens of meters below the seafloor near the summit of the structure, where vigorous fluid venting occurs. Elsewhere, the average gas hydrate content of the sediments in the gas hydrate stability zone is generally <2% of the pore space, although this estimate may increase by a factor of 2 when patchy zones of locally higher gas hydrate content are included in the calculation. These patchy zones are structurally and stratigraphically controlled, contain up to 20% hydrate in the pore space when averaged over zones ???10 m thick, and may occur in up to ???20% of the region imaged by 3D seismic data. This heterogeneous gas hydrate distribution is an important constraint on models of gas hydrate formation in marine sediments and the response of the sediments to tectonic and environmental change. ?? 2004 Published by Elsevier B.V.

Earth and Planetary Science Letters↗

USGS2004

Isotopic evidence bearing on Late Triassic extinction events, Queen Charlotte Islands, British Columbia, and implications for the duration and cause of the Triassic/Jurassic mass extinction

Stable isotope analyses of Late Triassic to earliest Jurassic strata from Kennecott Point in the Queen Charlotte Islands, British Columbia, Canada shows the presence of two distinct and different organic carbon isotope anomalies at the Norian/Rhaetian and Rhaetian/Hettangian (=Triassic/Jurassic) stage boundaries. At the older of these boundaries, which is marked by the disappearance of the bivalve Monotis, the isotope record shows a series of short-lived positive excursions toward heavier values. Strata approaching this boundary show evidence of increasing anoxia. At the higher boundary, marked by the disappearance of the last remaining Triassic ammonites and over 50 species of radiolarians, the isotopic pattern consists of a series of short duration negative anomalies. The two events, separated by the duration of the Rhaetian age, comprise the end-Triassic mass extinction. While there is no definitive evidence as to cause, the isotopic record does not appear similar to that of the impact-caused Cretaceous/Tertiary boundary extinction. ?? 2004 Published by Elsevier B.V.

Earth and Planetary Science Letters↗

USGS2004

The global variation in the iron isotope composition of marine hydrogenetic ferromanganese deposits: Implications for seawater chemistry?

The iron (Fe) isotope compositions of 37 hydrogenetic ferromanganese deposits from various oceans have been analysed by MC-ICPMS; they permit the construction of a global map of Fe isotopic values. The isotopic compositions range between -1.2 and -0.1??? in ??57FeIRMM14. Averages for the Atlantic and the Pacific are -0.41 and -0.88???, but their standard deviations are identical (0.27, 1 ??) and the data very largely overlap. No correlation is found with Pb or Nd isotope compositions and there is no evidence that the observed oceanic Fe isotopic heterogeneity is directly controlled by variations in continental sources. The small quantities of Fe that can be introduced from hydrothermal sources render as unlikely the possibility that the isotopic variations reflect variable proportions of continental and hydrothermal Fe, as recently proposed. The more likely explanation is that the variations are induced locally within the ocean. The exact sources of fractionation remain unclear. Likely possibilities are the dissolution and reprecipitation processes that liberate Fe from sediments during anoxic events, dissolution in surface waters or processes occurring during growth of the crusts. ?? 2004 Elsevier B.V. All rights reserved.

Earth and Planetary Science Letters↗

USGS2004

Zircon geochronology and ca. 400 Ma exhumation of Norwegian ultrahigh-pressure rocks: An ion microprobe and chemical abrasion study

Understanding the formation and exhumation of the remarkable ultrahigh-pressure (UHP) rocks of the Western Gneiss Region, Norway, hinges on precise determination of the time of eclogite recrystallization. We conducted detailed thermal ionization mass spectrometry, chemical abrasion analysis and sensitive high-resolution ion-microprobe analysis of zircons from four ultrahigh- and high-pressure (HP) rocks. Ion-microprobe analyses from the Flatraket eclogite yielded a broad range of apparently concordant Caledonian ages, suggesting long-term growth. In contrast, higher precision thermal ionization mass spectrometry analysis of zircon subject to combined thermal annealing and multi-step chemical abrasion yielded moderate Pb loss from the first (lowest temperature) abrasion step, possible minor Pb loss or minor growth at 400 Ma from the second step and a 407-404 Ma cluster of slightly discordant 206Pb/238U ages, most likely free from Pb loss, from the remaining abrasion steps. We interpret the latter to reflect zircon crystallization at ???405-400 Ma with minor discordance from inherited cores. Zircon crystallization occurred at eclogite-facies, possibly post-peak conditions, based on compositions of garnet inclusions in zircon as well as nearly flat HREE profiles and lack of Eu anomalies in zircon fractions subjected to chemical abrasion. These ages are significantly younger than the 425 Ma age often cited for western Norway eclogite recrystallization, implying faster rates of exhumation (>2.5-8.5 km/Myr), and coeval formation of eclogites across the UHP portion of the Western Gneiss Region. ?? 2004 Published by Elsevier B.V.

Earth and Planetary Science Letters↗

USGS2005

Late Pleistocene granodiorite source for recycled zircon and phenocrysts in rhyodacite lava at Crater Lake, Oregon

Rhyodacite tephra and three lavas erupted ∼27 ka, interpreted to be early leaks from the climactic magma chamber of Mount Mazama, contain ubiquitous resorbed crystals (antecrysts) that were recycled from young granodiorite and related plutonic rocks of the same magmatic system. The shallow composite pluton is represented by blocks ejected in the 7.7-ka climactic eruption that formed Crater Lake caldera. Plagioclase crystals in both rhyodacite and granodiorites commonly have cores with crystallographically oriented Fe-oxide needles exsolved at subsolidus conditions. At least 80% of plagioclase crystals in the rhyodacite are antecrysts derived from plutonic rocks. Other crystals in the rhyodacite, notably zircon, also were recycled. SIMS 238 U– 230 Th dating indicates that zircons in 4 granodiorite blocks crystallized at various times between ∼20 ka and ≥ 300 ka with concentrations of analyses near 50–70, ∼110, and ∼200 ka that correspond to periods of dacitic volcanism dated by K–Ar. U–Th ages of zircon from a rhyodacite sample yield similar results. No analyzed zircons from the granodiorite or rhyodacite are pre-Quaternary. Zircon minimum ages in blocks from different locations around the caldera reflect ages of nearby volcanic vents and may map the distribution of intrusions within a composite pluton. Survival of zircon in zircon-undersaturated hydrous magma and of Fe-oxide needles in plagioclase suggests that little time elapsed from entrainment of antecrysts to the ∼27-ka eruption of the rhyodacite. The ∼27-ka rhyodacite is an example of young silicic magma that preserved unstable antecrysts from a known source early during growth of a large high-level magma chamber. In contrast, the voluminous 7.7-ka climactic rhyodacite pumice is virtually lacking in zircon, indicating dissolution of any granodioritic debris in the intervening period. Mineralogical evidence of assimilation may be destroyed in hot, vigorously growing silicic magma bodies such as ultimately produced the climactic eruption of Mount Mazama.

Oregon↗

USGS2005

Antipodal hotspots and bipolar catastrophes: Were oceanic large-body impacts the cause?

One aspect of the hotspot distribution that has received little attention is its antipodal character. Of 45 ‘primary’ hotspots found in most hotspot compilations 22 (49%) form antipodal pairs within observed hotspot drift limits (≤ 20 mm/yr). In addition, the available ages, or possible age ranges, for both hotspots of an antipodal pair tend to be similar (≤ 10 Myr difference) or overlap. Monte Carlo simulations indicate that the antipodal primary hotspots' locations and ages are not due to chance at the > 99% confidence level ( p < 0.01). All hotspot pairs include at least one oceanic hotspot, and these are consistently opposite those hotspots related to large igneous provinces (LIPs) and continental volcanism. A mechanism of formation is considered in which minor hotspot volcanism is induced at, and flood basalt volcanism is triggered by seismic energy focused antipodal to, oceanic large-body impact sites. Because continental impacts are expected to have lower seismic efficiencies, continents possibly acted as shields to the formation of antipodal hotspot pairs. Published numerical models indicate that large oceanic impacts (10-km-diameter bolide) generate megatsunami capable of altering coastal depositional environments on a global scale. Past impact-generated megatsunami, consequently, could have left widespread stratigraphic records, possibly misinterpreted as indicating large rapid changes in eustatic sea level, and widely disrupted continental and marine sediment reservoirs responsible for abrupt changes in the isotopic composition of seawater. Phanerozoic mass extinction events, therefore, might have resulted primarily from catastrophic megatsunami in a dominantly oceanic hemisphere and the near contemporaneous effusion of vast quantities of noxious gases from flood basalt eruptions in a dominantly continental one.

Earth and Planetary Science Letters↗

USGS2005

Systematics of halogen elements and their radioisotopes in thermal springs of the Cascade Range, Central Oregon, Western USA

This study quantifies the cycling of halogen elements through the Cascadia subduction zone based on the chemistry of thermal springs in the Central Oregon Cascade Range and of a mineral spring in the forearc (Willamette Valley). Considerations based on mass balances, element ratios, and 36 Cl/Cl and 129 I/I ratios suggest that halogens discharged through the thermal springs in the Cascade Range are probably derived from magma degassing. Our results indicate that < 35% of the subducted Cl and < 20% of the subducted Br and I could be transported through arc volcanism and the thermal springs, a considerably lower percentage than estimated for other volcanic arcs along the Pacific Rim. A likely explanation for this difference is that a large fraction of the halogens is released from the slab at shallow depths into the serpentinized sub-forearc mantle because of the relatively high temperatures in the subducting Juan de Fuca plate. The small fraction of halogens subducted to depth probably also indicates a low rate of water transport, which is consistent with the observation that the Cascade Range sub-arc mantle is relatively dry and has a low degree of volcanic vigor, compared with other arcs.

Oregon↗

USGS2005

The South China sea margins: Implications for rifting contrasts

Implications regarding spatially complex continental rifting, crustal extension, and the subsequent evolution to seafloor spreading are re-examined for the northern and southern-rifted margins of the South China Sea. Previous seismic studies have shown dramatic differences in the present-day crustal thicknesses as the manifestations of the strain experienced during the rifting of the margin of south China. Although the total crustal extension is presumed to be the same along the margin and adjacent ocean basin, the amount of continental crustal extension that occurred is much less along the east and central segments of the margin than along the western segment. This difference was accommodated by the early formation of oceanic crust (creating the present-day South China Sea basin) adjacent to the eastern margin segment while continued extension of continental crust was sustained to the west. Using the observed cross-sectional areas of extended continental crust derived from deep penetration seismics, two end-member models of varying rift zone widths and varying initial crustal thicknesses are qualitatively examined for three transects. Each model implies a time difference in the initiation of seafloor spreading inferred for different segments along the margin. The two models examined predict that the oceanic crust of the South China Sea basin toward the west did not begin forming until sometime between 6-12 my after its initial formation (???32 Ma) toward the east. These results are compatible with crustal age interpretations of marine magnetic anomalies. Assuming rifting symmetry with conjugate margin segments now residing along the southern portions of the South China Sea basin implies that the total width of the zone of rifting in the west was greater than in the east by about a factor of two. We suggest the most likely causes of the rifting differences were east-west variations in the rheology of the pre-rift crust and associated east-west variations in the thermal structure of the pre-rift lithosphere. The calculated widths of rifted continental crust for the northern and southern margins, when combined with the differential widths of seafloor generated during the seafloor spreading phase, indicate the total crustal extension that occurred is about 1100 km and is remarkably consistent to within ???10% for all three (eastern, central, western) segments examined. ?? 2005 Elsevier B.V. All rights reserved.

Earth and Planetary Science Letters↗