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At least 433 records · Page 24Linked to original sources

USGS1990

Relationships among macerals, minerals, miospores and paleoecology in a column of Redstone coal (Upper Pennsylvanian) from north-central West Virginia (U.S.A.)

Two distinct paleoenvironments are represented in vertical succession in a column of Redstone coal in north-central West Virginia as indicated by a study of 37 consecutive 3-cm (0.1 ft) increments analyzed for ash yield, petrographic composition, low-temperature ash mineralogy and palynomorph abundances. Abundance profiles were constructed for ash, 12 petrographic components, 3 minerals and 5 miospore assemblages. The profiles and calculated correlation coefficients show close relationships between several constituents. Components that increased in abundance upward in the coal bed were a collinite type > 50 microns in thickness, cutinite, and miospores affiliated with calamites, herbaceous lycopods, cordaites and herbaceous ferns. Components that decreased in abundance upward were a collinite type < 50 microns in thickness, inertodetrinite, tree fern miospores, quartz, illite and ash yield. Components were correlated with ash yield to infer the swamp geochemical conditions that contributed to the low-ash (7%) upper one-third of the coal bed and the higher-ash (16%) lower two-thirds of the coal bed. Components that correlate positively with increased ash were the collinite type < 50 μ m thickness, inertodetrinite, tree fern miospores, illite and quartz. Components that correlate negatively with increased ash were the collinite type > 50 μ m in thickness, cutinite, calamite and cordaite miospores and kaolinite. Significant correlations occurred between ash yield and the collinite types > 50 and < 50 μ m in thickness but no significant correlation was found between ash yield and total vitrinite-group content. This is interpreted to show that division of vitrinite macerals by size is important in petrographic paleoenvironmental studies. Paleoecologic interpretations based upon these correlations suggest that two distinct, planar, probably topogenous paleoecologic environments are represented in this column of the Redstone coal. The lower two-thirds of the coal bed was interpreted to have accumulated in a planar swamp in which significant introduction of detrital or dissolved mineral matter, and significant anaerobic and moderate oxidative degradation of the peat occurred. The flora of this paleoenvironment was dominated by tree ferns. The paleoenvironment during accumulation of the upper one-third of the coal bed was also interpreted to have been a planar swamp, but one in which moderate to low introduction of detrital or dissolved mineral matter, and minor anaerobic and oxidative degradation of the peat occurred. The dominant flora of this paleoenvironment consisted mainly of calamites with fewer cordaites and herbaceous ferns. This study shows that valuable paleoecologic information may be obtained by sampling closely spaced vertical increments. No mixing of detrital sediments with the peat was observed in coal layers immediately adjacent to the parting or the overlying sandstone unit.

International Journal of Coal Geology↗

USGS2022

Maturation study of vitrinite in carbonaceous shales and coals: Insights from hydrous pyrolysis

The presence of vitrinite in sedimentary rocks of post-Silurian age allows its reflectance to be used to estimate the thermal maturation of organic matter in petroleum systems. Increasing reflectance of vitrinite, which is primarily driven by aromaticity, depends primarily on the time and temperature attributes of its evolutionary pathway. This study evaluated carbonaceous shales proximal to coal measures and coal samples via isothermal hydrous pyrolysis (HP) to compare differences in the maturation pathways of vitrinite. Sample residues were analysed via vitrinite reflectance (VRo), geochemical screening tests (organic carbon and programmed temperature pyrolysis), and infrared spectroscopy. The study included samples from Indian and North American basins, to observe differences in vitrinite evolution with respect to enclosing mineral matrix, starting degree of aromaticity, organic matter types, stratigraphic age, and depositional environment. The organic content of HP residues shows an intuitive response to the thermal stress of HP, e.g., a general depletion of total organic carbon (TOC) content, pyrolyzate (S2), and hydrogen index with increasing HP temperature. Infrared proxies including C-factor and CH2/CH3 generally decrease with increasing thermal maturity indicating loss of O via CO2 generation and the thermal cracking of aliphatic organic matter. Tmax, production index (PI), and VRo show intuitive increasing values with respect to HP temperature. The least mature sample (0.48 ± 0.05% VRo) generally experienced the maximum change in these parameters during maturation, whereas the most mature sample (0.99 ± 0.06% VRo) generally showed the least change. This observation is consistent with higher kinetic barriers to reaction in more aromatic vitrinite which contains higher bond dissociation energies. Devolatilization of vitrinite during HP causes formation of gas evacuation vacuoles and contraction cracks in the vitrinite grains of both coal and carbonaceous shale. Similarities in vitrinite response to HP between coal and carbonaceous shale suggest that thermal evolution of the vitrinite maceral is principally controlled by inherent rate-limiting kinetic parameters related to its molecular structure. Whereas, the stratigraphic age, sedimentary environment, surrounding organic matter, lithology, and catalysis by mineral composition have less effect. To further improve our understanding of vitrinite aromatization and kinetic parameters, future studies of vitrinite reflectance thermal evolution with temperature should include coal and carbonaceous shale from the same stratigraphic section and extant woody tissue from modern vascular plants.

International Journal of Coal Geology↗

USGS2018

Geochemical characterization and modeling of regional groundwater contributing to the Verde River, Arizona between Mormon Pocket and the USGS Clarkdale gage

We use synoptic surveys of stream discharge, stable isotopes, and dissolved noble gases to identify the source of groundwater discharge to the Verde River in central Arizona. The Verde River more than doubles in discharge in Mormon Pocket over a 1.4 km distance that includes three discrete locations of visible spring input to the river and other diffuse groundwater inputs. A detailed study of the Verde River between Mormon Pocket and the USGS Clarkdale Gage was conducted to better constrain the location of groundwater inputs, the geochemical signature and constrain the source of groundwater input. Discharge, water quality parameters (temperature, pH, specific conductance, and dissolved oxygen), stable isotopes (δ18O and δ2H), noble gases (He, Ne, Ar, Kr and Xe), and radon (222Rn) from river water were collected. Groundwater samples from springs and wells in the area were collected and analyzed for tracers measured in the stream along with some additional analytes (major ions, strontium isotopes (87Sr/86Sr), carbon-14, δ13C, and tritium). Groundwater isotopic signature is consistent with a regional groundwater source. Groundwater springs discharging to the river have a depleted stable isotopic signature indicating recharge source up to 1000 m higher than the discharge location in the Verde River and are significantly fresher than stream water. Spring water has a radiocarbon age of several thousand years and some areas have tritium less than the laboratory reporting level or low concentrations of tritium (1.5 TU). The strontium isotopes indicate groundwater interaction with tertiary volcanic rock and Paleozoic sedimentary rocks. Along the study reach with distance downstream, Verde stream water chemistry shows increased 222Rn, freshening, increased 4He, and isotopic depletion with distance downstream. We estimated total groundwater discharge by inverting a stream transport model against 222Rn and discharge measured in the stream. The salinity, 4He, and stable isotope composition of discharging groundwater was then estimated by fitting modeled values to observed in-stream values. Estimated groundwater inflow to the stream was well within the ranges observed in springs, indicating that the main source of streamflow is deep, regional groundwater. These results show that synoptic surveys of environmental tracers in streams can be used to estimate the isotopic composition and constrain the source of groundwater discharging to streams. Our data provide direct field evidence that deep, regional groundwater discharge can be a significant source of streamflow generation in arid, topographically complex watersheds.

Arizona↗

USGS1982

Petrology and trace element geochemistry of the Honolulu volcanics, Oahu: Implications for the oceanic mantle below Hawaii

The Honolulu Volcanics comprises small volume, late-stage (post-erosional) vents along rifts cutting the older massive Koolau tholeütic shield on Oahu, Hawaii. Most of these lavas and tuff of the Honolulu Volcanics have geochemical features expected of near-primary magmas derived from a peridotite source containing Fo 87–89 olivine; e. g. 100 Mg/(Mg + Fe 2+ ) >65, >250 p. p. m. Ni, and presence of ultramafic mantle xenoliths at 18 of the 37 vents. Consequently, the geochemistry of the alkali olivine basalt, basanite, nephelinite and nepheline melilitite lavas and tuff of the Honolulu Volcanics have been used to deduce the composition of their mantle source and the conditions under which they were generated by partial melting in the mantle. Compositional trends in 30 samples establish that the magmas were derived by partial melting of a garnet (<10 per cent) Iherzolite source, which we infer to have been carbon-bearing, from analogy with experimental results. This source was isotopically homogeneous (Sr, Lanphere & Dalrymple, 1980; Pb, Sun, 1980; Nd, Roden et al ., 1981), and we infer that the source was compositionally uniform in all major-element oxides except TiO 2 , in compatible trace elements (Sc, V, Cr, Mn, Co and Ni), and in highly incompatible trace elements (P, Th, La, Ce). However, the source appears to have been heterogeneous in TiO 2 , Zr, Hf, Nb, and Ta, elements that were not strongly incompatible during partial melting. Some nepheline melilitite samples may be derived from a source with distinct Sc and heavy-rare-earth-elements (REE) abundances, or which had a phase or phases controlling the distribution of these elements. The relatively limited abundance range for several elements, such as Ti, Zr, Nb, is partly a consequence of the low degrees of melting inferred for the series (2 per cent for nepheline melilitite, 11 per cent for alkali olivine basalt), which failed to exhaust the source in minor residual phases. We infer that these residual phases probably included phlogopite, amphibole, and another Ti-rich phase (an oxide?), but not apatite.

Journal of Petrology↗

USGS2009

Magnetostratigraphic correlations of Permian-Triassic marine-to-terrestrial sections from China

We have studied three Permian–Triassic (PT) localities from China as part of a combined magnetostratigraphic, 40 Ar/ 39 Ar and U–Pb radioisotopic, and biostratigraphic study aimed at resolving the temporal relations between terrestrial and marine records across the Permo-Triassic boundary, as well as the rate of the biotic recovery in the Early Triassic. The studied sections from Shangsi (Sichuan Province), Langdai (Guihzou Province), and the Junggar basin (Xinjiang Province), span marine, paralic, and terrestrial PT environments, respectively. Each of these sections was logged in detail in order to place geochronologic, paleomagnetic, geochemical, conodont and palynologic samples within a common stratigraphic context. Here we present rock-magnetic, paleomagnetic and magnetostratigraphic results from the three localities. At Shangsi, northern Sichuan Province, we sampled three sections spanning Permo-Triassic marine carbonates. Magnetostratigraphic results from the three sections indicate that the composite section contains at least eight polarity chrons and that the PT boundary occurs within a normal polarity chron a short distance above the mass extinction level and a reversed-to-normal (R-N) polarity reversal. Furthermore, the onset of the Illawarra mixed interval lies below the sampled section indicating that the uppermost Permian Changhsingian and at least part of the Wuchiapingian stages postdate the end of the Kiaman Permo-Carboniferous Reversed Superchron. At Langdai, Guizhou Province, we studied magnetostratigraphy of PT paralic mudstone and carbonate sediments in two sections. The composite section spans an R-N polarity sequence. Section-mean directions pass a fold test at the 95% confidence level, and the section-mean poles are close to the mean PT pole for the South China block. Based on biostratigraphic constraints, the R-N transition recorded at Langdai is consistent with that at Shangsi and demonstrates that the PT boundary occurred within a normal polarity chron a short distance above the mass extinction level. In the southern Junggar basin, Xinjiang Province, in northwest China, we determined the magnetostratigraphy of three sections of a terrestrial sequence. Normal and reversed polarity directions are roughly antipodal, and magnetostratigraphies from the three sections are highly consistent. Combined bio- and magneto-stratigraphy used to correlate this sequence to other PT sequences suggests that the previously-proposed biostratigraphic PT boundary in the Junggar sections was most likely misplaced by earlier workers suggesting that further work is necessary to confidently place the PT boundary there.

Journal of Asian Earth Sciences↗

USGS2022

Perspectives on premetamorphic stratabound tourmalinites

Stratabound tourmalinites are metallogenically important rocks that locally show a close spatial association with diverse types of mineralization, especially volcanogenic massive sulfides (VMS) and clastic-dominated (CD) Zn-Pb deposits. These tourmalinite occurrences pan the geologic record from Eoarchean to Jurassic. Host lithologies are dominated by clastic metasedimentary rocks but in some areas include metavolcanic rocks, marble, or metaevaporites. Stratabound and stratiform (conformable) tourmalinites commonly display sedimentary structures such as graded beds, cross-beds, and rip-up clasts. In most cases, field and microtextural relationships are consistent with a synsedimentary to the early diagenetic introduction of boron as a precursor to tourmaline formation. Whole-rock geochemical data for major, trace, and rare earth elements (REE) provide valuable insights into tourmalinite origins. Al-normalized values relative to those for least-altered host metasedimentary rocks suggest that tourmalinites in proximal settings at or near hydrothermal vent sites characterized by high fluid/rock regimes (e.g., Sullivan Pb-Zn-Ag deposit, Canada) have very different signatures than those in low fluid/rock, distal settings (e.g., Broken Hill Pb-Zn-Ag deposit, Australia). The high fluid/rock regimes at Sullivan show large mass changes of +60 % for Mg and +180 % for Mn, as well as large variations in abundances of light and middle REE. In contrast, tourmalinite formation in low fluid/rock regimes yields minimal Al-normalized changes in major elements, trace elements, and REE. Boron isotope values of tourmalinite-hosted tourmaline vary widely from -26.1 to +27.5 ‰, and are attributed mainly to boron sources (e.g., sediments, evaporites) with generally minor influence from processes such as formational temperature, fluid/rock ratio, and secular variation in seawater δ 11 B values. Laterally extensive stratiform tourmalinites formed mainly by syngenetic or early diagenetic processes on or beneath the seafloor. The syngenetic process is attributed to the interaction of vented B-rich brines with aluminous minerals in sediments, whereas the diagenetic process involves the selective replacement of aluminous sediments by B-rich fluids. Modern examples of tourmalinites, as yet undiscovered, may exist in metalliferous sediments of the Red Sea and the eastern Pacific Ocean, in altered volcaniclastic sediments within active seafloor-hydrothermal systems of the South Pacific, and in hydrothermal mounds and vents associated with mafic sill complexes in extensional basins as in the North Sea and South China Sea. Stratabound tourmalinites that contain base-metal sulfides, high Mn concentrations (>1 wt. % MnO), or positive Eu anomalies can be valuable exploration guides for base-metal sulfide deposits in sedimentary and volcanic terranes.

Journal of Geosciences↗

USGS2007

Spatial and temporal geochemical trends in the hydrothermal system of Yellowstone National Park: Inferences from river solute fluxes

We present and analyze a chemical dataset that includes the concentrations and fluxes of HCO 3 − , SO 4 2− , Cl − , and F − in the major rivers draining Yellowstone National Park (YNP) for the 2002–2004 water years (1 October 2001 – 30 September 2004). The total (molar) flux in all rivers decreases in the following order, HCO 3 − > Cl − > SO 4 2− > F − , but each river is characterized by a distinct chemical composition, implying large-scale spatial heterogeneity in the inputs of the various solutes. The data also display non-uniform temporal trends; whereas solute concentrations and fluxes are nearly constant during base-flow conditions, concentrations decrease, solute fluxes increase, and HCO 3 − /Cl − , and SO 4 2− /Cl − increase during the late-spring high-flow period. HCO 3 − /SO 4 2− decreases with increasing discharge in the Madison and Falls Rivers, but increases with discharge in the Yellowstone and Snake Rivers. The non-linear relations between solute concentrations and river discharge and the change in anion ratios associated with spring runoff are explained by mixing between two components: (1) a component that is discharged during base-flow conditions and (2) a component associated with snow–melt runoff characterized by higher HCO 3 − /Cl − and SO 4 2− /Cl − . The fraction of the second component is greater in the Yellowstone and Snake Rivers, which host lakes in their drainage basins and where a large fraction of the solute flux follows thaw of ice cover in the spring months. Although the total river HCO 3 − flux is larger than the flux of other solutes (HCO 3 − /Cl − ≈ 3), the CO 2 equivalent flux is only ∼ 1% of the estimated emission of magmatic CO 2 soil emissions from Yellowstone. No anomalous solute flux in response to perturbations in the hydrothermal system was observed, possibly because gage locations are too distant from areas of disturbance, or because of the relatively low sampling frequency. In order to detect changes in river hydrothermal solute fluxes, sampling at higher frequencies with better spatial coverage would be required. Our analysis also suggests that it might be more feasible to detect large-scale heating or cooling of the hydrothermal system by tracking changes in gas and steam flux than by tracking changes in river solute flux.

Journal of Volcanology and Geothermal Research↗

USGS2013

Short-term variability of 7 Be atmospheric deposition and watershed response in a Pacific coastal stream, Monterey Bay, California, USA

Beryllium-7 is a powerful and commonly used tracer for environmental processes such as watershed sediment provenance, soil erosion, fluvial and nearshore sediment cycling, and atmospheric fallout. However, few studies have quantified temporal or spatial variability of 7 Be accumulation from atmospheric fallout, and parameters that would better define the uses and limitations of this geochemical tracer. We investigated the abundance and variability of 7 Be in atmospheric deposition in both rain events and dry periods, and in stream surface-water samples collected over a ten-month interval at sites near northern Monterey Bay (37°N, 122°W) on the central California coast, a region characterized by a rainy winters, dry summers, and small mountainous streams with flashy hydrology. The range of 7 Be activity in rainwater samples from the main sampling site was 1.3–4.4 Bq L −1 , with a mean (±standard deviation) of 2.2 ± 0.9 Bq L −1 , and a volume-weighted average of 2.0 Bq L −1 . The range of wet atmospheric deposition was 18–188 Bq m −2 per rain event, with a mean of 72 ± 53 Bq m −2 . Dry deposition fluxes of 7 Be ranged from less than 0.01 up to 0.45 Bq m −2 d −1 , with an estimated dry season deposition of 7 Bq m −2 month −1 . Annualized 7 Be atmospheric deposition was approximately 1900 Bq m −2 yr −1 , with most deposition via rainwater (>95%) and little via dry deposition. Overall, these activities and deposition fluxes are similar to values found in other coastal locations with comparable latitude and Mediterranean-type climate. Particulate 7 Be values in the surface water of the San Lorenzo River in Santa Cruz, California, ranged from <0.01 Bq g −1 to 0.6 Bq g −1 , with a median activity of 0.26 Bq g −1 . A large storm event in January 2010 characterized by prolonged flooding resulted in the entrainment of 7 Be-depleted sediment, presumably from substantial erosion in the watershed. There were too few particulate 7 Be data over the storm to accurately model a 7 Be load, but the results suggest enhanced watershed export of 7 Be from small, mountainous river systems compared to other watershed types.

California↗

USGS2026

Magmatic source of the opening phase of Kīlauea’s 2018 Lower East Rift Zone eruption

The 2018 eruption of Kīlauea volcano in its Lower East Rift Zone began with the discharge of evolved high-Ti basalt as weak lava fountains and short, slow-moving lava flows. The lavas were quickly geochemically recognized as being derived from magmas stored within the rift zone and remobilized by a new intrusion, a sequence that is common at Kīlauea. This initial phase of the 2018 eruption, referred to as phase 1a, lasted for 6 days and was followed by extrusion of mixed magma after a 3-day pause. Even though remobilization of older rift zone magmas is common within Kīlauea’s rift zones, it is difficult to determine which past intrusion(s) may have initially emplaced those stored magmas. This difficulty stems from the tendency for Kīlauea magmas to follow very similar differentiation paths without significant variations in major, minor, or even trace element chemistry. We investigate possible magma sources for the lavas erupted during phase 1a of the 2018 eruption using whole-rock, mineral, and glass major and trace element compositions from historical East Rift Zone eruptions with adjacent fissures. We consider two primary hypotheses for the phase 1a source: magmas associated with the 1955 Lower East Rift Zone eruption or the nine eruptions in the Middle and Upper East Rift Zone during the 1960s. Our results suggest that magma associated with the earliest phases of Kīlauea’s 1955 eruption was the most likely source of the 2018 phase 1a remobilized magma. We determine volatile saturation pressures from melt inclusion chemistry and find similar storage depths for the 2018 phase 1a and early 1955 magmas. The phase 1a and early 1955 lavas are nearly indistinguishable in all of the compositional criteria considered, implying that the leftover 1955 magma body barely cooled and differentiated in the 63 years between eruptions (cooling rates of ~0.1 °C/year). This study sheds light on the potential for protracted storage of eruptible magmas in rift zones at Kīlauea, and highlights some of the challenges and solutions to identifying genetic relationships between magmas at Kīlauea.

Hawaii↗

USGS1999

Trace-element geochemistry of metabasaltic rocks from the Yukon-Tanana Upland and implications for the origin of tectonic assemblages in east-central Alaska

We present major- and trace- element geochemical data for 27 amphibolites and six greenstones from three structural packages in the Yukon-Tanana Upland of east-central Alaska: the Lake George assemblage (LG) of Devono-Mississippian augen gneiss, quartz-mica schist, quartzite, and amphibolite; the Taylor Mountain assemblage (TM) of mafic schist and gneiss, marble, quartzite, and metachert; and the Seventymile terrane of greenstone, serpentinized peridotite, and Mississippian to Late Triassic metasedimentary rocks. Most LG amphibolites have relatively high Nb, TiO2, Zr, and light rare earth element contents, indicative of an alkalic to tholeiitic, within-plate basalt origin. The within-plate affinities of the LG amphibolites suggest that their basaltic parent magmas developed in an extensional setting and support a correlation of these metamorphosed continental-margin rocks with less metamorphosed counterparts across the Tintina fault in the Selwyn Basin of the Canadian Cordillera. TM amphibolites have a tholeiitic or calc-alkalic composition, low normalized abundances of Nb and Ta relative to Th and La, and Ti/V values of <20, all indicative of a volcanic-arc origin. Limited results from Seventymile greenstones indicate a tholeiitic or calc-alkalic composition and intermediate to high Ti/V values (27-48), consistent with either a within-plate or an ocean-floor basalt origin. Y-La-Nb proportions in both TM and Seventymile metabasalts indicate the proximity of the arc and marginal basin to continental crust. The arc geochemistry of TM amphibolites is consistent with a model in which the TM assemblage includes arc rocks generated above a west-dipping subduction zone outboard of the North American continental margin in mid-Paleozoic through Triassic time. The ocean-floor or within-plate basalt geochemistry of the Seventymile greenstones supports the correlation of the Seventymile terrane with the Slide Mountain terrane in Canada and the hypothesis that these oceanic rocks originated in a basin between the continental margin and an arc to the west.

Alaska↗

USGS2008

Application of multiple isotopic and geochemical tracers for investigation of recharge, salinization, and residence time of water in the Souss-Massa aquifer, southwest of Morocco

Groundwater and surface water in Souss-Massa basin in the west-southern part of Morocco is characterized by a large variation in salinity, up to levels of 37 g L-1. The high salinity coupled with groundwater level decline pose serious problems for current irrigation and domestic water supplies as well as future exploitation. A combined hydrogeologic and isotopic investigation using several chemical and isotopic tracers such as Br/Cl, ??18O, ??2H, 3H, 87Sr/86Sr, ??11B, and 14C was carried out in order to determine the sources of water recharge to the aquifer, the origin of salinity, and the residence time of water. Stable isotope, 3H and 14C data indicate that the high Atlas mountains in the northern margin of the Souss-Massa basin with high rainfall and low ??18O and ??2H values (-6 to -8??? and -36 to -50???) is currently constitute the major source of recharge to the Souss-Massa shallow aquifer, particularly along the eastern part of the basin. Localized stable isotope enrichments offset meteoric isotopic signature and are associated with high nitrate concentrations, which infer water recycling via water agricultural return flows. The 3H and 14C data suggest that the residence time of water in the western part of the basin is in the order of several thousands of years; hence old water is mined, particularly in the coastal areas. The multiple isotope analyses and chemical tracing of groundwater from the basin reveal that seawater intrusion is just one of multiple salinity sources that affect the quality of groundwater in the Souss-Massa aquifer. We differentiate between modern seawater intrusion, salinization by remnants of seawater entrapped in the middle Souss plains, recharge of nitrate-rich agricultural return flow, and dissolution of evaporate rocks (gypsum and halite minerals) along the outcrops of the high Atlas mountains. The data generated in this study provide the framework for a comprehensive management plan in which water exploitation should shift toward the eastern part of the basin where current recharge occurs with young and high quality groundwater. In contrast, we argued that the heavily exploited aquifer along the coastal areas is more vulnerable given the relatively longer residence time of the water and salinization processes in this part of the aquifer. ?? 2008 Elsevier B.V. All rights reserved.

Journal of Hydrology↗

USGS1999

The Cenomanian/Turonian carbon burial event, Bass River, NJ, USA: Geochemical, paleoecological, and sea-level changes

The Bass River borehole (ODP Leg 174AX) recovered approximately 200 ft (61 m) of upper Cenomanian to lower Turonian strata from the New Jersey Coastal Plain, USA, including the expression of a global carbon burial event. The Cenomanian/Turonian (C/T) boundary is recognized at Bass River at ???1935.5 ft (589.9 m) based on the contact of nannofossil Microstaurus chiastius and Eiffellithus eximius Subzones of the Parhabdolithus asper Zone. Carbon isotopic records of both Gavelinella and Epistomina show a large (>2???) increase immediately below the C/T boundary, with maximum values of 6??? in Epistomina and 4.3??? in Gavelinella. The ??13C offset between these taxa is constant and we conclude that Epistomina, like Gavelinella, faithfully records seawater ??13C changes. Above the sharp ??13C increase, elevated ??13C and sedimentary organic carbon (>0.9%) values continue into the lower Turonian, culminating in a sharp ??13C decrease. High ??13C values in the uppermost Cenomanian-lower Turonian at Bass River correlate with a global carbon burial event recorded in Europe and the U.S. Western Interior; we estimate the duration of this event at Bass River as 400-500 k.y. Although the carbon burial event occurred during a long-term eustatic rise (10 m.y. scale), it occurs within a 1-2 m.y. long sequence at Bass River that indicates no relationship with sea-level lowering on the m.y. scale. The carbon burial event does not appear to be associated with maximum flooding either, indicating little correlation with sea-level rise on a m.y. scale. Within the sequence spanning the carbon event, there are at least 4 shallowing-upward parasequences (durations ???350-460 k.y.) indicated by changes in abundance and type of Epistomina species, ??18O variations, and minor lithologic variations. The highest occurrences of 6 Epistomina species and the origination of Epistomina sliteri Olsson n. sp. are associated with the parasequences and possibly with higher ??18O values. There is no clear association of parasequence boundaries inferred at Bass River with the carbon burial event; thus, there does not appear to be a relationship of the event with sea-level change on the 100 k.y. scale. We conclude that while the organic carbon burial event was associated with a general long-term (10 m.y. scale) eustatic rise, the initiation and termination of the peak organic burial event itself were unrelated to sea-level change.

Journal of Foraminiferal Research↗

USGS2023

Thermal evolution of graptolite and solid bitumen properties at high maturity under natural and artificial conditions

To refine the use of graptolite and solid bitumen as thermal proxies at overmature conditions, we evaluated their evolution via Raman and infrared (IR) spectroscopies, reflectance, and geochemical screening using high-temperature pyrolysis experiments in comparison to naturally matured samples. Naturally matured samples included marine shales from the overmature Upper Ordovician Wufeng-Lower Silurian Longmaxi Formations (herein referred to as Wufeng-Longmaxi) of the Sichuan Basin, China. Immature samples for pyrolysis experiments included Mesoproterozoic (Ectasian) Xiamaling marine shale from Hebei, China (graptolite absent) and graptolite-bearing Ordovician (Tremadocian) Alum marine shale from Grönhögen, Sweden. Pyrolysis experiments at 360 °C for 10 days and 27 days (hydrous conditions), and 450 °C 3 days, 550 °C 3 days, and 550 °C 10 days (all anhydrous) created Xiamaling and Alum residues with solid bitumen and graptolite, respectively, of similar and higher maturity compared to the naturally matured Wufeng-Longmaxi samples. Compositional proxies from IR spectroscopy were inconclusive. Raman spectral properties including Raman band separation (RBS) and the full-width at half-maximum of the G-band (G-FWHM) exhibited robust positive and inverse relationships with increasing reflectance in pyrolysis residues, respectively. RBS values were systematically lower in artificially matured samples versus naturally matured samples with equivalent reflectance values, whereas G-FWHM values were systematically higher. This observation indicates that the limited duration or low internal reactor pressure of pyrolysis experiments is insufficient to allow alignment or ordering of aromatic carbon arrays in both organic matter types, relative to natural maturation occurring at geologic time scales. Reflectance of graptolite was typically higher than co-occurring solid bitumen in Wufeng-Longmaxi samples and the untreated Alum sample, suggesting inherently higher aromaticity. However, aromaticity in pyrolysis residues was systematically higher in solid bitumen compared to graptolite with equivalent reflectance values, indicating a higher kinetic barrier to molecular rearrangement in graptolite versus solid bitumen. These data further clarify differences in the molecular evolution of sedimentary organic matter in natural versus analogue laboratory environments and distinguish the properties of individual organic matter types in response to thermal stress.

International Journal of Coal Geology↗

USGS1998

Episodic fluid flow in the Nankai accretionary complex: Timescale, geochemistry, flow rates, and fluid budget

Down-hole geochemical anomalies encountered in active accretionary systems can be used to constrain the timing, rates, and localization of fluid flow. Here we combine a coupled flow and solute transport model with a kinetic model for smectite dehydration to better understand and quantify fluid flow in the Nankai accretionary complex offshore of Japan. Compaction of sediments and clay dehydration provide fluid sources which drive the model flow system. We explicitly include the consolidation rate of underthrust sediments in our calculations to evaluate the impact that variations in this unknown quantity have on pressure and chloride distribution. Sensitivity analysis of steady state pressure solutions constrains bulk and flow conduit permeabilities. Steady state simulations with 30% smectite in the incoming sedimentary sequence result in minimum chloride concentrations at site 808 of 550 m M , but measured chlorinity is as low as 447 m M . We simulate the transient effects of hydrofracture or a strain event by assuming an instantaneous permeability increase of 3–4 orders of magnitude along a flow conduit (in this case the décollement), using steady state results as initial conditions. Transient results with an increase in décollement permeability from 10 −16 m 2 to 10 −13 m 2 and 20% smectite reproduce the observed chloride profile at site 808 after 80–160 kyr. Modeled chloride concentrations are highly sensitive to the consolidation rate of underthrust sediments, such that rapid compaction of underthrust material leads to increased freshening. Pressures within the décollement during transient simulations rise rapidly to a significant fraction of lithostatic and remain high for at least 160 kyr, providing a mechanism for maintaining high permeability. Flow rates at the deformation front for transient simulations are in good agreement with direct measurements, but steady state flow rates are 2–3 orders of magnitude smaller than observed. Fluid budget calculations indicate that nearly 71% of the incoming water in the sediments leaves the accretionary wedge via diffuse flow out the seafloor, 0–5% escapes by focused flow along the décollement, and roughly 1% is subducted.

Journal of Geophysical Research B: Solid Earth↗

USGS2003

Inferences on the hydrothermal system beneath the resurgent dome in Long Valley Caldera, east-central California, USA, from recent pumping tests and geochemical sampling

Quaternary volcanic unrest has provided heat for episodic hydrothermal circulation in the Long Valley caldera, including the present-day hydrothermal system, which has been active over the past 40 kyr. The most recent period of crustal unrest in this region of east-central California began around 1980 and has included periods of intense seismicity and ground deformation. Uplift totaling more than 0.7 m has been centered on the caldera's resurgent dome, and is best modeled by a near-vertical ellipsoidal source centered at depths of 6-7 km. Modeling of both deformation and microgravity data now suggests that (1) there are two inflation sources beneath the caldera, a shallower source 7-10 km beneath the resurgent dome and a deeper source ???15 km beneath the caldera's south moat and (2) the shallower source may contain components of magmatic brine and gas. The Long Valley Exploration Well (LVEW), completed in 1998 on the resurgent dome, penetrates to a depth of 3 km directly above this shallower source, but bottoms in a zone of 100??C fluid with zero vertical thermal gradient. Although these results preclude extrapolations of temperatures at depths below 3 km, other information obtained from flow tests and fluid sampling at this well indicates the presence of magmatic volatiles and fault-related permeability within the metamorphic basement rocks underlying the volcanic fill. In this paper, we present recently acquired data from LVEW and compare them with information from other drill holes and thermal springs in Long Valley to delineate the likely flow paths and fluid system properties under the resurgent dome. Additional information from mineralogical assemblages in core obtained from fracture zones in LVEW documents a previous period of more vigorous and energetic fluid circulation beneath the resurgent dome. Although this system apparently died off as a result of mineral deposition and cooling (and/or deepening) of magmatic heat sources, flow testing and tidal analyses of LVEW water level data show that relatively high permeability and strain sensitivity still exist in the steeply dipping principal fracture zone penetrated at a depth of 2.6 km. The hydraulic properties of this zone would allow a pressure change induced at distances of several kilometers below the well to be observable within a matter of days. This indicates that continuous fluid pressure monitoring in the well could provide direct evidence of future intrusions of magma or high-temperature fluids at depths of 5-7 km. ?? 2003 Elsevier B.V. All rights reserved.

California↗

USGS2013

Hydrogen isotope investigation of amphibole and glass in dacite magmas erupted in 1980-1986 and 2005 at Mount St. Helens, Washington

In active, shallow, sub-volcanic magma conduits the extent of the dehydrogenation–oxidation reaction in amphibole phenocrysts is controlled by energetic processes that cause crystal lattice damage or conditions that increase hydrogen diffusivity in magmatic phases. Amphibole phenocrysts separated from dacitic volcanic rocks erupted from 1980 to 1986 and in 2005 at Mount St. Helens (MSH) were analyzed for δD, water content and Fe 3 +/Fe 2 +, and fragments of glassy groundmass were analyzed for δD and water content. Changes in amphibole δD values through time are evaluated within the context of carefully observed volcanic eruption behavior and published petrological and geochemical investigations. Driving forces for amphibole dehydrogenation include increase in magma oxygen fugacity, decrease in amphibole hydrogen fugacity, or both. The phenocryst amphibole (δD value c. –57‰ and 2 wt % H 2 O) in the white fallout pumice of the May 18, 1980 plinian eruptive phase is probably little modified during rapid magma ascent up an ∼7 km conduit. Younger volcanic rocks incorporate some shallowly degassed dacitic magma from earlier pulses, based on amphibole phenocryst populations that exhibit varying degrees of dehydrogenation. Pyroclastic rocks from explosive eruptions in June–October 1980 have elevated abundances of mottled amphibole phenocrysts (peaking in some pyroclastic rocks erupted on July 22, 1980), and extensive amphibole dehydrogenation is linked to crystal damage from vesiculation and pyroclastic fountain collapse that increased effective hydrogen diffusion in amphibole. Multiple amphibole δD populations in many 1980 pyroclastic rocks combined with their groundmass characteristics (e.g. mixed pumice textures) support models of shallow mixing prior to, or during, eruption as new, volatile-rich magma pulses blended with more oxidized, degassed magma. Amphibole dehydrogenation is quenched at the top surface of MSH dacite lava lobes, but the diversity in the δD amph populations in original fresh lava flow surfaces may occur from blending magma domains with different ascent histories in the sub-volcanic environment immediately before eruption. Multi-stage open-system magma degassing operated in each parcel of magma rising toward the surface, whereas the magma below ∼7 km was a relatively closed system, at least to the October 1986 eruption based on the large population of minimally dehydrogenated, rim-free amphibole in the lavas. Magma degassing and possibly H isotope exchange with low-δD fluids around the roof zone may have accompanied the ∼1·5 km upward migration of the 1980 magma body. The low-δDamph (c. –188 to –122‰) oxy-amphibole phenocrysts in lava spines extruded in May 2005 reflect dehydrogenation as ascending viscous magma degassed and crystallized, and fractures that admitted oxygen into the hot solidified lava spine interior facilitated additional iron oxidation.

Washington↗

USGS2016

Organic petrology and geochemistry of mudrocks from the lacustrine Lucaogou Formation, Santanghu Basin, northwest China: Application to lake basin evolution

Exploration for tight oil in the frontier Santanghu Basin of northwest China has resulted in recent commercial discoveries sourced from the lacustrine Upper Permian Lucaogou Formation, already considered a “world class source rock” in the Junggar Basin to the west. Here we apply an integrated analytical program to carbonate-dominated mudrocks from the Lucaogou Formation in Santanghu Basin to document the nature of organic matter (OM) in the context of an evolving lake system. The organic-rich samples (TOC 2.8–11.4 wt%; n = 10) were widely spaced from an ~ 200 m cored section, interpreted from textural and mineralogical evidence to document transition from a lower under-filled to an overlying balanced-filled lake. Organic matter is dominated by moderate to strongly fluorescent amorphous material with Type I geochemical signature (HI values 510–755; n = 10) occurring in a continuum from lamellar stringers, 10–20 μm thick, some ≥ 1 mm in length (possible microbial mat; preserved only in lower under-filled section) to finely-disseminated amorphous groundmass intimately intermixed with mineral matrix. Biomarkers for methanotrophs and photosynthetic cyanobacteria indicate a complex microbial consortium. A unicellular prasinophyte green alga(?), similar to Tasmanites in marine rocks, is present as discrete flattened discs 50–100 μm in diameter. Type III OM including vitrinite (some fluorescent) and inertinite also is abundant. Solid bitumen, indicating local kerogen conversion, fills voids and occurs throughout the cored section. Vitrinite reflectance values are 0.47–0.58%, consistent with strong OM fluorescence but may be “suppressed”. Other proxies, e.g., biomarker parameters, indicate the Lucaogou Formation is in the early oil window at this location. On average, slightly more amorphous OM and telalginite are present in the lower section, consistent with a shallow, stratified, saline environment with low sediment dilution. More inertinite is present in the upper section, indicating greater terrestrial influx and consistent with higher quartz and plagioclase content (dominantly authigenic chalcedony and albite). Laminated mudstones in the upper section indicate anoxia prevented bioturbation from benthic grazing, also indicating stratified water column conditions. A decrease upsection in authigenic dolomite with reciprocal increase of ankerite/siderite is consistent with decreasing salinity, as is an overall decrease in gammacerane index values. These observations suggest evolution from a shallow, stratified evaporative (saline) setting to a deeper, stratified freshwater basin with higher water input during Lucaogou deposition. The evolution from an under-filled to balance-filled lake in Santanghu Basin is similar to Lucaogou deposition in Junggar Basin, suggesting similar tectonic and climatic controls. Paleoclimate interpretations from other researchers in this area suggested an evolution from semi-arid to humid conditions during the Roadian; we interpret that the evolution from an under-filled to balanced-filled lake seen in our data is in response to climate change, and may represent increased groundwater delivery to the Santanghu Basin.

International Journal of Coal Geology↗

USGS2009

Petrogenesis of basaltic volcanic rocks from the Pribilof Islands, Alaska, by melting of metasomatically enriched depleted lithosphere, crystallization differentiation, and magma mixing

The Pribilof Islands, Alaska, are located in the Bering Sea in a continental intraplate setting. In this study we examine the petrology and geochemistry of volcanic rocks from St. Paul (0??54-0??003 Ma) and St. George (2??8-1??4 Ma) Islands, the two largest Pribilof Islands. Rocks from St. George can be divided into three groups: group 1 is a high-MgO, low-SiO. 2 suite composed primarily of basanites; group 2 is a high-MgO, high-SiO 2 suite consisting predominantly of alkali basalts; group 3 is an intermediate- to low-MgO suite that includes plagioclase-phyric subalkali basalts and hawaiites. Major and trace element geochemistry suggests that groups 1 and 2 formed by small-degree partial melting of amphibole-bearing to amphibole-free garnet peridotite. Group 1 rocks were the earliest melts produced from the most hydrous parts of the mantle, as they show the strongest geochemical signature of amphibole in their source. The suite of rocks from St. Paul ranges from 14??4 to 4??2 wt % MgO at relatively constant SiO 2 contents (43??1-47??3 wt %). The most primitive St. Paul rocks are modeled as mixtures between magmas with compositions similar to groups 1 and 2 from St. George Island, which subsequently fractionated olivine, clinopyroxene, and spinel to form more evolved rocks. Plagioclase-phyric group 3 rocks from St. George are modeled as mixtures between an evolved melt similar to the evolved magmas on St. Paul and a fractionated group 2 end-member from St. George. Mantle potential temperatures estimated for primitive basanites and alkali basalts are ???1400??C and are similar to those of mid-ocean ridge basalts (MORB). Similarly, 87 Sr/. 86 Sr and 143 Nd/. 144 Nd values for all rocks are MORB-like, in the range of 0??702704-0??703035 and 0??513026-0??513109, respectively. 208 Pb/. 204 Pb vs 206 Pb/. 204 Pb values lie near the MORB end-member but show a linear trend towards HIMU (high time-integrated 238 U/. 204 Pb). Despite isotopic similarities to MORB, many of the major and trace element characteristics are similar to those of ocean island basalts (OIB), including enrichment in alkalis and incompatible trace elements. These characteristics are interpreted to indicate that their mantle source experienced an ancient melt-removal event that is reflected in depleted radiogenic isotopic compositions and was then re-enriched by metasomatism that elevated incompatible trace element contents, but was too young to produce a time-integrated change in radiogenic isotopic ratios. Evidence suggests that the Pribilof Island basalts did not form in either a plume or a back-arc basin tectonic setting. Rather, they were produced by melting of metasomatically hydrated upper mantle peridotite at relatively low temperatures and were able to erupt at the surface through extensional or transtensional faults that served as conduits for the magmas. ?? The Author 2009. Published by Oxford University Press.

Journal of Petrology↗