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At least 433 records · Page 24Linked to original sources

USGS2019

Experimental study on the impact of thermal maturity on shale microstructures using hydrous pyrolysis

Hydrous pyrolysis was applied to four low-maturity aliquots from the Utica, Excello, Monterey, and Niobrara Shale Formations in North America to create artificial maturation sequences, which could be used to study the impact of maturation on geochemical and microstructural properties. Modified Rock-Eval pyrolysis, reflectance, organic petrology, and Fourier transform infrared spectroscopy (FTIR) were employed to analyze their geochemical properties, while gas adsorption (CO 2 and N 2 ) was used to characterize their pore structures (pores < 200 nm). Organic petrography using white and blue light (fluorescence) before and after hydrous pyrolysis showed that amorphous organic matter cracked into solid bitumen, oil, and gas during hydrous pyrolysis. A reduction of the CH 2 /CH 3 ratio in hydrous pyrolysis residues was observed from FTIR analysis. Rock-Eval pyrolysis showed that kerogens in the four samples were dissimilar, and hydrous pyrolysis residues showed smaller hydrogen index and Sh2 values than starting materials. Results from CO 2 and N 2 gas adsorption analysis showed that pore structures (micropore volume, micropore surface area, meso-macropore volume, and meso-macropore surface area) changed significantly during hydrous pyrolysis. However, changes in pore structure were dissimilar among the four samples, which was attributed to different activation energies of organic matter. A thermodynamic fractal model showed a decrease in fractal dimensions of Utica, Monterey, and Excello after hydrous pyrolysis, indicating a decrease in surface roughness. The pore size heterogeneity in the Utica sample increased as hydrous pyrolysis temperature increased, whereas the pore size heterogeneity distributions in the Monterey and Excello decreased based on the N 2 adsorption data.

Energy & Fuels↗

USGS2011

Bacon receives 1999 Bowen Award

Charles R. Bacon received the 1999 Bowen Award, presented by the Volcanology Geochemistry, and Petrology Section during the 1999 AGU Fall Meeting.

Eos, Transactions, American Geophysical Union↗

USGS2001

Geological characterization of remote field sites using visible and infrared spectroscopy: Results from the 1999 Marsokhod field test

Upcoming Mars Surveyor lander missions will include extensive spectroscopic capabilities designed to improve interpretations of the mineralogy and geology of landing sites on Mars. The 1999 Marsokhod Field Experiment (MFE) was a Mars rover simulation designed in part to investigate the utility of visible/near-infrared and thermal infrared field spectrometers to contribute to the remote geological exploration of a Mars analog field site in the California Mojave Desert. The experiment simultaneously investigated the abilities of an off-site science team to effectively analyze and acquire useful imaging and spectroscopic data and to communicate efficiently with rover engineers and an on-site field team to provide meaningful input to rover operations and traverse planning. Experiences gained during the MFE regarding effective communication between different mission operation teams will be useful to upcoming Mars mission teams. Field spectra acquired during the MFE mission exhibited features interpreted at the time as indicative of carbonates (both dolomitic and calcitic), mafic rocks and associated weathering products, and silicic rocks with desert varnish-like coatings. The visible/near-infrared spectra also suggested the presence of organic compounds, including chlorophyll in one rock. Postmission laboratory petrologic and spectral analyses of returned samples confirmed that all rocks identified as carbonates using field measurements alone were calc-silicates and that chlorophyll associated with endolithic organisms was present in the one rock for which it was predicted. Rocks classified from field spectra as silicics and weathered mafics were recognized in the laboratory as metamorphosed monzonites and diorite schists. This discrepancy was likely due to rock coatings sampled by the field spectrometers compared to fresh rock interiors analyzed petrographically, in addition to somewhat different surfaces analyzed by laboratory thermal spectroscopy compared to field spectra.

California↗

USGS2001

Thermal thickness and evolution of Precambrian lithosphere: A global study

The thermal thickness of Precambrian lithosphere is modeled and compared with estimates from seismic tomography and xenolith data. We use the steady state thermal conductivity equation with the same geothermal constraints for all of the Precambrian cratons (except Antarctica) to calculate the temperature distribution in the stable continental lithosphere. The modeling is based on the global compilation of heat flow data by Pollack et al. [1993] and more recent data. The depth distribution of heat‐producing elements is estimated using regional models for ∼300 blocks with sizes varying from 1°×1° to about 5°×5° in latitude and longitude and is constrained by laboratory, seismic and petrologic data and, where applicable, empirical heat flow/heat production relationships. Maps of the lateral temperature distribution at depths 50, 100, and 150 km are presented for all continents except Antarctica. The thermal thickness of the lithosphere is calculated assuming a conductive layer overlying the mantle with an adiabat of 1300°C. The Archean and early Proterozoic lithosphere is found to have two typical thicknesses, 200–220 km and 300–350 km. In general, thin (∼220 km) roots are found for Archean and early Proterozoic cratons in the Southern Hemisphere (South Africa, Western Australia, South America, and India) and thicker (>300 km) roots are found in the Northern Hemisphere (Baltic Shield, Siberian Platform, West Africa, and possibly the Canadian Shield). We find that the thickness of continental lithosphere generally decreases with age from >200 km beneath Archean cratons to intermediate values of 200±50 km in early Proterozoic lithosphere, to about 140±50 km in middle and late Proterozoic cratons. Using known crustal thickness, our calculated geotherms, and assuming that isostatic balance is achieved at the base of the lithosphere, we find that Archean and early Proterozoic mantle lithosphere is 1.5% less dense (chemically depleted) than the underlying asthenosphere, while middle and late Proterozoic subcrustal lithosphere should be depleted by ∼0.6–0.7%. Our results suggest three contrasting stages of lithosphere formation at the following ages: >2.5 Ga, 2.5–1.8 Ga, and <1.8 Ga. Ages of komatiites, greenstone belts, and giant dike swarms broadly define similar stages and apparently reflect secular changes in mantle temperature and, possibly, convection patterns.

Journal of Geophysical Research B: Solid Earth↗

USGS2002

Thermal infrared spectroscopy of experimentally shocked anorthosite and pyroxenite: Implications for remote sensing of Mars

The feldspar and pyroxene mineralogies on Mars revealed by the Thermal Emission Spectrometer (TES) on Mars Global Surveyor likely record a variety of shock effects, as suggested by petrologic analyses of the Martian meteorites and the abundance of impact craters on the planet's surface. To study the effects of shock pressures on thermal infrared spectra of these minerals, we performed shock recovery experiments on orthopyroxenite and anorthosite samples from the Stillwater Complex (Montana) over peak pressures from 17 to 63 GPa. We acquired emissivity and hemispherical reflectance spectra (350–1400 cm −1 ; ∼7–29 μm) of both coherent chips and fine-grained powders of shocked and unshocked samples. These spectra are more directly comparable to remotely sensed data of Mars (e.g., TES) than previously acquired absorption or transmission spectra of shocked minerals. The spectra of experimentally shocked feldspar show systematic changes with increasing pressure due to depolymerization of the silica tetrahedra. For the spectra of chips, this includes the disappearance of small bands in the 500–650 cm −1 region and a strong band at 1115 cm −1 , and changes in positions of a strong band near 940 cm −1 and the Christiansen feature near 1250 cm −1 . Spectra of the shocked powders show the gradual disappearance of a transparency feature near 830 cm −1 . Fewer changes are observed in the pyroxene spectra at pressures as high as 63 GPa. Spectra of experimentally shocked minerals will help identify more precisely the mineralogy of rocks and soils not only from TES but also from Mars instruments such as miniTES and THEMIS.

Journal of Geophysical Research E: Planets↗

USGS2011

Anderson receives 2001 Bowen Award

Alfred T. Anderson, Jr. received the Bowen Award, presented by the Volcanology, Geochemistry, and Petrology Section at the 2001 Fall Meeting in San Francisco, California, last December.

Eos, Transactions, American Geophysical Union↗

USGS2003

Athena Microscopic Imager investigation

The Athena science payload on the Mars Exploration Rovers (MER) includes the Microscopic Imager (MI). The MI is a fixed‐focus camera mounted on the end of an extendable instrument arm, the Instrument Deployment Device (IDD). The MI was designed to acquire images at a spatial resolution of 30 microns/pixel over a broad spectral range (400–700 nm). The MI uses the same electronics design as the other MER cameras but has optics that yield a field of view of 31 × 31 mm across a 1024 × 1024 pixel CCD image. The MI acquires images using only solar or skylight illumination of the target surface. A contact sensor is used to place the MI slightly closer to the target surface than its best focus distance (about 66 mm), allowing concave surfaces to be imaged in good focus. Coarse focusing (∼2 mm precision) is achieved by moving the IDD away from a rock target after the contact sensor has been activated. The MI optics are protected from the Martian environment by a retractable dust cover. The dust cover includes a Kapton window that is tinted orange to restrict the spectral bandpass to 500–700 nm, allowing color information to be obtained by taking images with the dust cover open and closed. MI data will be used to place other MER instrument data in context and to aid in petrologic and geologic interpretations of rocks and soils on Mars.

Journal of Geophysical Research E: Planets↗

USGS2006

Analysis of gravity and topography in the GLIMPSE study region: Isostatic compensation and uplift of the Sojourn and Hotu Matua Ridge systems

The Gravity Lieations Intraplate Melting Petrologic and Seismic Expedition (GLIMPSE) Experiment investigated the formation of a series of non-hot spot, intraplate volcanic ridges in the South Pacific and their relationship to cross-grain gravity lineaments detected by satellite altimetry. Using shipboard gravity measurements and a simple model of surface loading of a thin elastic plate, we estimate effective elastic thicknesses ranging from ???2 km beneath the Sojourn Ridge to a maximum of 10 km beneath the Southern Cross Seamount. These elastic thicknesses are lower than predicted for the 3-9 Ma seafloor on which the volcanoes lie, perhaps due to reheating and thinning of the plate during emplacement. Anomalously low apparent densities estimated for the Matua and Southern Cross seamounts 2050 and 2250 kg m-3, respectively, probably are artifacts caused by the assumption of only surface loading, ignoring the presence of subsurface loading in the form of underplated crust and/or low-density mantle. Using satellite free-air gravity and shipboard bathymetry, we calculate the age-detrended, residual mantle Bouguer anomaly (rMBA). The rMBA corrects the free-air anomaly for the direct effects of topography, including the thickening of the crust beneath the seamounts and volcanic ridges due to surface loading of the volcanic edifices. There are broad, negative rMBA anomalies along the Sojourn and Brown ridges and the Hotu Matua seamount chain that extend nearly to the East Pacific Rise. These negative rMBA anomalies connect to negative free-air anomalies in the western part of the study area that have been recognized previously as the beginnings of the cross-grain gravity lineaments. Subtracting the topographic effects of surface loading by the ridges and seamounts from the observed topography reveals that the ridges are built on broad bands of anomalously elevated seafloor. This swell topography and the negative rMBA anomalies contradict the predictions of lithospheric cracking models for the origin of gravity lineaments and associated volcanic ridges, favoring models with a dynamic mantle component such as small-scale convection or channelized asthenospheric return flow. Copyright 2006 by the American Geophysical Union.

Journal of Geophysical Research B: Solid Earth↗

USGS2007

Mid-Holocene change in types of degassing volcanoes, using indium in Antarctic ice as a tracer of volcanic source type

Proportions of trace metals in Antarctic ice samples indicate that the type of volcanoes that dominated atmospheric emissions changed at about the middle of the Holocene from relatively mafic, deep source volcanoes to more silicic, shallower-source volcanoes. We base this inference on the strong contrast in the abundances of the trace metal indium (In), relative to other trace metals present in ice, deposited at different times in the past, and on contrasting In abundances in modern emissions of volcanoes of different types. Indium is more abundant in the emissions of deep-source mafic volcanoes than in more felsic, shallower-source volcanoes. Earlier workers have shown, on the basis of petrologic and some meteoritic evidence, that In may be partitioned to the interiors (stony mantles) of differentiated planets, or enriched in the liquids of partly crystallized mafic melts.

Geophysical Research Letters↗

USGS2008

Protracted construction of gabbroic crust at a slow spreading ridge: Constraints from 206 Pb/ 238 U zircon ages from Atlantis Massif and IODP Hole U1309D (30°N, MAR)

Sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon ages of 24 samples from oceanic crust recovered in Integrated Ocean Drilling Program (IODP) Hole U1309D and from the surface of Atlantis Massif, Mid-Atlantic Ridge (MAR) (30°N) document a protracted history of accretion in the footwall to an oceanic detachment fault. Ages for 18 samples of evolved Fe-Ti oxide gabbro and felsic dikes collected 40–1415 m below seafloor in U1309D yield a weighted mean of 1.20 ± 0.03 Ma (mean square of weighted deviates = 7.1). However, the ages range from 1.08 ± 0.07 Ma and 1.28 ± 0.05 Ma indicating crustal construction occurred over a minimum of 100–200 ka. The zircon ages, along with petrologic observations, indicate at least 2 major periods of intrusive activity with age peaks separated by 70 ka. The oldest ages are observed below 600 mbsf, an observation inconsistent with models requiring constant depth melt intrusion beneath a detachment fault. The data are most consistent with a “multiple sill” model whereby sills intrude at random depths below the ridge axis over a length scale greater than 1.4 km. Zircon ages from broadly spaced samples collected along the southern ridge of Atlantis Massif yield a detachment fault slip rate of 28.7 ± 6.7 mm/a and imply significant asymmetric plate spreading (up to 100% on the North American plate) for at least 200 ka during core complex formation.

Geochemistry, Geophysics, Geosystems↗

USGS2009

Thorium abundances on the Aristarchus plateau: Insights into the composition of the Aristarchus pyroclastic glass deposits

Thorium (Th) data from the Lunar Prospector gamma ray spectrometer (LP‐GRS) are used to constrain the composition of lunar pyroclastic glass deposits on top of the Aristarchus plateau. Our goal is to use forward modeling of LP‐GRS Th data to measure the Th abundances on the plateau and then to determine if the elevated Th abundances on the plateau are associated with the pyroclastic deposits or with thorium‐rich ejecta from Aristarchus crater. We use a variety of remote sensing data to show that there is a large, homogenous portion of the pyroclastics on the plateau that has seen little or no contamination from the Th‐rich ejecta of Aristarchus crater. Our results show that the uncontaminated pyroclastic glasses on Aristarchus plateau have an average Th content of 6.7 ppm and ∼7 wt % TiO 2 . These Th and Ti values are consistent with Th‐rich, intermediate‐Ti yellow glasses from the lunar sample suite. On the basis of this information, we use petrologic equations and interelement correlations for the Moon to estimate the composition of the source region from which the Aristarchus glasses were derived. We find that the source region for the Aristarchus glasses contained high abundances of heat‐producing elements, which most likely served as a thermal driver for the prolonged volcanic activity in this region of the Moon.

Journal of Geophysical Research E: Planets↗

USGS2012

Deep magmatic degassing versus scrubbing: Elevated CO2 emissions and C/S in the lead-up to the 2009 eruption of Redoubt Volcano, Alaska

We report CO 2, SO 2 , and H 2 S emission rates and C/S ratios during the five months leading up to the 2009 eruption of Redoubt Volcano, Alaska. CO 2 emission rates up to 9018 t/d and C/S ratios ≥30 measured in the months prior to the eruption were critical for fully informed forecasting efforts. Observations of ice-melt rates, meltwater discharge, and water chemistry suggest that surface waters represented drainage from surficial, perched reservoirs of condensed magmatic steam and glacial meltwater. These fluids scrubbed only a few hundred tonnes/day of SO 2 , not the >2100 t/d SO 2 expected from degassing of magma in the mid- to upper crust (3–6.5 km), where petrologic analysis shows the final magmatic equilibration occurred. All data are consistent with upflow of a CO 2 -rich magmatic gas for at least 5 months prior to eruption, and minimal scrubbing of SO 2 by near-surface groundwater. The high C/S ratios observed could reflect bulk degassing of mid-crustal magma followed by nearly complete loss of SO 2 in a deep magmatic-hydrothermal system. Alternatively, high C/S ratios could be attributed to decompressional degassing of low silica andesitic magma that intruded into the mid-crust in the 5 months prior to eruption, thereby mobilizing the pre-existing high silica andesite magma or mush in this region. The latter scenario is supported by several lines of evidence, including deep long-period earthquakes (−28 to −32 km) prior to and during the eruption, and far-field deformation following the onset of eruptive activity.

Alaska↗

USGS2017

Probing magma reservoirs to improve volcano forecasts

When it comes to forecasting eruptions, volcano observatories rely mostly on real-time signals from earthquakes, ground deformation, and gas discharge, combined with probabilistic assessments based on past behavior [Sparks and Cashman, 2017] . There is comparatively less reliance on geophysical and petrological understanding of subsurface magma reservoirs.

Eos, Earth and Space Science News↗

USGS2018

Evolution of the 2015 Cotopaxi eruption revealed by combined geochemical & seismic observations

Through integration of multiple data streams to monitor volcanic unrest scientists are able to make more robust eruption forecast and to obtain a more holistic interpretation of volcanic systems. We examined gas emission and gas geochemistry, seismic and petrologic data recorded during the 2015 unrest of Cotopaxi (Ecuador) in order to decipher the origin and temporal evolution of this eruption. Identification of families of similar seismic events and the use of seismic amplitude ratios reveals temporal changes in volcanic processes. SO2 (300 to 24000 t/d), BrO/SO2 (5-10 x10-5), SO2/HCl (5.8 ± 4.8 and 6.6 ± 3.0) and CO2/SO2 (0.6 to 2.1) measured throughout the eruption indicate a shallow magmatic source. Bulk ash and glass chemistry indicate a homogenous andesitic (SiO2 wt%=56.94 ± 0.25) magma having undergone extensive S-exsolution and degassing during ascent. These data lead us to interpret this eruption as a magma intrusion and ascend to shallow levels. The intrusion progressively interacted with the hydrothermal system, boiled off water, and produced hydromagmatic explosions. A small volume of this intrusion continued to fragment and produced episodic ash emissions until it was sufficiently degassed and rheologically stiff. Based on the 470 kt of measured SO2 we estimate that ~ 65.3 x106 m3 of magma were required to supply the emitted gases. This volume exceeds the volume of erupted juvenile material by a factor of 50. This result emphasizes the importance of careful monitoring of Cotopaxi to identify the intrusion of a new batch of magma, which could rejuvenate the non-erupted material.

Geochemistry, Geophysics, Geosystems↗

USGS2021

Multidisciplinary constraints on magma compressibility, the pre-eruptive exsolved volatile fraction, and the H2O/CO2 molar ratio for the 2006 Augustine eruption, Alaska

Geodetically modeled reservoir volume changes during volcanic eruptions are commonly much smaller than the observed eruptive volumes. This discrepancy is thought to be partially due to the compressibility of magma, which is largely controlled by the presence of exsolved volatiles. The 2006 eruption of Augustine Volcano, Alaska, produced an eruptive volume that was ∼3 times larger than the geodetically estimated syn-eruptive subsurface volume change. In this study, we use a multistep methodology that combines constraints from geodetic, volcanic gas, geologic, and petrologic data together with equations relating physical processes to observable parameters. We apply a Monte Carlo approach to quantify uncertainties. Ultimately, we solve for the exsolved volatile volume fraction and the magma compressibility. We estimate Augustine's 2006 pre-eruptive exsolved volatile phase to be ∼5.5 vol% of the magma at storage depths, yielding a bulk magma compressibility of ∼3.8 × 10 −10 Pa −1 . We develop a novel approach to estimate the H 2 O/CO 2 ratio of the syn-eruptive gas emissions in the absence of direct H 2 O emission measurements which are hard to obtain due to the high background levels in ambient air. We find a best-fit H 2 O/CO 2 molar ratio of 29. We also investigate the effects of applying different equations of state to our model. We find that the Ideal Gas Law might be used as a first approximation due to its simplicity; however, it overestimates volatile density and compressibility significantly at storage depths. This project capitalizes on the insights that can be gained by integrating multidisciplinary data with models of physical processes.

Alaska↗

USGS2022

Mega-depressions on the Cocos Ridge: Links between volcanism, faults, hydrothermal circulation, and dissolution

High-resolution bathymetry and 3D seismic data along the Cocos Ridge reveal a 245 km 2 field of ∼1 to 4 km in diameter seafloor depressions. The seafloor depressions are part of a two-tiered honeycomb pattern. The lower-tier depressions have steep faults that truncate strata with chaotic internal reflections consistent with sediment collapse into the depression. These extend into a lens shaped interval just above igneous basement. Overlying these depressions is a second broader set with rough seafloor morphology with gently dipping boundaries defined by pinch-out stratigraphic patterns. Drilling results indicate that the lens-shaped zones that host the deeper depressions represent anomalous regions of high porosity, low velocity, and low density within calcareous rich sediment. Analysis of nannofossils from IODP Site U1414 suggests the collapse structures formed during the late Miocene, whereas the younger shallower depressions likely formed between the early Pliocene and the Pliocene-Pleistocene boundary. Geochemical and petrological analysis at Site U1414 suggests that hydrothermal circulation during the late Miocene led to carbonate dissolution and collapse. Following collapse, focused fluid-flow and bottom current scouring resulted in formation of the overlying set of depressions and a honeycomb seafloor morphology. Similar sets of depressions along the Carnegie Ridge to the south support the hypothesis that two-tiered depressions formed in response to processes that occurred broadly across the Panama Basin between the late Miocene and the Pliocene-Pleistocene transition. Geochemical results at Site U1414, combined with geophysical data, suggest this two-tiered system of depressions currently guides ongoing fluid outflow.

Geochemistry, Geophysics, Geosystems↗

USGS2026

Constraining sources of mid-Pleistocene to present explosive volcanism in the Gulf of Alaska using machine learning and compositional data analysis

The long-term eruptive record of a region helps better elucidate magmatic processes at depth as well as volcanic hazards at the surface. Typically, reconstructing such records is done using proximal tephrostratigraphy and linking individual tephras to source volcanoes. These records, however, can only be accurately constructed if they are both well preserved and correctly linked to source volcanoes—not a trivial task, especially in areas such as Alaska that have experienced numerous glaciation events since the Pleistocene. This ultimately necessitates another way of assessing the long-term volcanic record such that these histories may be better discerned. Here we present data from 70 marine core tephras from the Gulf of Alaska, which have a virtually uninterrupted depositional record going back through the mid-Pleistocene. We utilize compositional data analysis techniques to quantify 37 eruptions over the span of eight cores, machine learning classification and conformal prediction algorithms to assign the most probable volcanic source(s) to each eruption, and multivariate distance-based metrics when machine learning classification algorithms are inappropriate. We find that the Mount Katmai magmatic system is the most probable volcanic source for analyzed tephras and that large volcanic centers such as Mount Katmai, Fisher Caldera, and Emmons Lake volcanic center have produced nearly invariant incompatible trace element ratio magmas, allowing for them to be confidently identified in long-term tephra records. This highlights the utility of trace elements for tephra studies, especially when paired with compositional data analysis, multivariate statistical tests, and petrologically informed discriminants.

Alaska↗

USGS2026

Corundum discovered by SuperCam and the Perseverance rover at Jezero crater, Mars

Mars is primarily composed of mafic mineral assemblages and their alteration products, but small, scattered rocks strewn across the landscape offer clues to greater petrological diversity. While traversing the Jezero crater rim, the Perseverance rover encountered several plagioclase-rich light-toned float rocks. SuperCam identified the distinctive signature of corundum (α-Al 2 O 3 ) in these rocks using time-resolved luminescence spectroscopy. Two strong peaks (692.7 and 694.1 nm) with millisecond lifetimes, and additional supporting lines, are consistent with Cr 3+ substitution for Al 3+ in corundum. Corundum forms in Al-rich, Si-depleted environments through magmatic or metamorphic processes. Given the rocks' small size, association with plagioclase, and location on the crater rim, we interpret the most plausible formation scenario to be impact induced metamorphism at the interface of a felsic and a mafic/ultramafic member with the likely action of fluids at some stage, although other possibilities are not excluded.

Geophysical Research Letters↗