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USGS2012

Electrical structure of Newberry Volcano, Oregon

From the interpretation of magnetotelluric, transient electromagnetic, and Schlumberger resistivity soundings, the electrical structure of Newberry Volcano in central Oregon is found to consist of four units. From the surface downward, the geoelectrical units are (1) very resistive, young, unaltered volcanic rock, (2) a conductive layer of older volcanic material composed of altered tuffs, (3) a thick resistive layer thought to be in part intrusive rocks, and (4) a lower-crustal conductor. This model is similar to the regional geoelectrical structure found throughout the Cascade Range. Inside the caldera, the conductive second layer corresponds to the steep temperature gradient and alteration minerals observed in the USGS Newberry 2 test hole. Drill hole information on the south and north flanks of the volcano (test holes GEO N-1 and GEO N-3, respectively) indicates that outside the caldera the conductor is due to alteration minerals (primarily smectite) and not high-temperature pore fluids. On the flanks of Newberry the conductor is generally deeper than inside the caldera, and it deepens with distance from the summit. A notable exception to this pattern is seen just west of the caldera rim, where the conductive zone is shallower than at other flank locations. The volcano sits atop a rise in the resistive layer, interpreted to be due to intrusive rocks. The intrusive material has served as a heat source to produce enhanced hydrothermal alteration and, perhaps in the case of the west-flank anomaly, elevated fluid temperatures. While no public drill hole information is available to confirm this hypothesis, the west-flank anomaly appears to be a good geothermal target. In addition to the possibility that a region on the west side of the volcano could be favorable for prospecting, part of the resistive structure under the center of the volcano could be due to a vapor-dominated environment with temperatures above 300°C. In other parts of the Cascades, pervasive alteration has produced mixed layer clays and zeolites, resulting in low-resistivity anomalies. Low resistivities cannot be assumed to indicate high-temperature pore fluids. The use of electrical methods that measure resistivity as a function of excitation frequency, such as spectral induced polarization, may provide a way of obtaining information about the type and extent of alteration.

Journal of Geophysical Research Solid Earth↗

USGS1982

Ultrasonic borehole televiewer investigation of oceanic crustal layer 2A, Costa Rice Rift

In situ lithostratigraphy and the distribution of fracture and void zones in oceanic layer 2A were examined with an ultrasonic borehole televiewer at IPOD sites 501 and 504B on the south flank of the Costa Rice Rift. These records indicate a decrease in the size of basalt pillows with depth and a corresponding increase in fracture and void density. The observed increase in P-wave velocity with depth in this hole, as layer 2A merges into 2B, is best explained by an increase in the degree to which clays and zeolites fill fractures and voids. This also explains the order-of-magnitude drop in permeability observed in hole 504B. An increase in alteration-filling with age could explain the transition from convective to conductive heat flow and the correlative disappearance of layer 2A on the flanks of oceanic spreading centres. ?? 1982 Nature Publishing Group.

Nature↗

USGS2025

Widespread occurrence of former anhydrite phenocrysts in Laramide-age magmas related to porphyry-skarn Cu mineralization at Santa Rita and Hanover-Fierro, New Mexico, USA

Reports of magmatic anhydrite are relatively rare, with only ~30 occurrences documented worldwide so far. However, magmatic anhydrite saturation is difficult to recognize because anhydrite decomposes rapidly in near-surface environments. In most cases, only anhydrite inclusions shielded within other phenocryst phases were able to survive. Alternatively, since anhydrite phenocrysts preserved in fresh volcanic rocks are characteristically intergrown with apatite phenocrysts, the former presence of anhydrite phenocrysts can be recognized based on the occurrence of lath-shaped cavities that show a strong spatial association with apatite phenocrysts. These cavities can be either empty or filled with low-temperature, secondary minerals such as zeolites, carbonates, or microcrystalline silica. A systematic search for the occurrence of such cavities, combined with optical and Raman-spectroscopic identification of anhydrite inclusions preserved within apatite, hornblende and quartz phenocrysts, demonstrates that most of the Laramide-age magmas associated with the Santa Rita and Hanover-Fierro porphyry-skarn Cu (Zn, Mo, Au, Pb) deposits were saturated in magmatic anhydrite. The anhydrite typically coexisted with monosulfide solid solution (MSS), suggesting oxygen fugacities of ~2.0±0.5 log units above the fayalite-magnetite-quartz buffer. The magmas range from andesitic to rhyodacitic in composition, and from shortly pre-mineralization (~61 Ma) to shortly post-mineralization (~57 Ma) in age. In three samples with particularly well-recognizable former anhydrite phenocrysts, their modal abundance could be quantified based on high-resolution scans of polished hand specimens. The observed modal anhydrite abundances of 0.63–1.8 vol% translate into minimum magma sulfur contents of 0.20–0.56 wt% S. The highest sulfur content of 0.56 wt% S is difficult to reconcile with available anhydrite solubility models, but it could be reproduced in an anhydrite solubility experiment performed at 950 °C and 1.15 GPa on a natural latite containing 13.1 wt% dissolved H2O. The sample with the second-highest sulfur content of 0.26 wt% S requires ~10 wt% H2O in the silicate melt, and, consequently, a minimum pressure of ~0.5 GPa. Taken together, the results suggest that the magmas of the Central Mining District were extremely hydrous and thus originated from great depth. Indeed, their major element compositions and reconstructed H2O and S contents agree well with experimentally observed and numerically predicted compositions of residual silicate melts after 50–70 wt% crystallization of ordinary arc basalts at high pressure and high oxygen fugacities.

New Mexico↗

USGS1948

Petrography of Iwo Jima

The rocks of Iwo Jima are augite and augite-hornblende trachyandesites, distinctly more alkalic than the ordinary andesites of Japanese volcanoes or the average world andesite. They are described, and chemical analyses are listed. Fumarolic gases have altered some rocks to a mixture of opal and clay minerals and have cemented tuff with opal, calcite, zeolite, and iron oxides.

Iwo Jima↗

USGS1966

Rhizoconcretions in vitric ash-fall tuff, Nye County, Nevada

Small, vertically elongate concretionlike structures, here called rhizoconcretions , are found in scattered localities in and near the southern part of the Belted Range of Nye County , Nevada . The rhizoconcretions occur within a stratigraphically restricted zone in bedded vitric ash - fall tuff of late Tertiary age. The rhizoconcretions differ in mineralogy from their host rock in that they are cemented by a binder of undetermined composition, possibly a zeolite, whereas the host tuffs are un-cemented. These structures appear to have formed during diagenesis of the tuff by means of capillary movement of subsurface waters that reacted with volcanic glass within the conduits to form the cementing material.

Nevada↗

USGS2001

Updip limit of the seismogenic zone beneath the accretionary prism of southwest Japan: An effect of diagenetic to low-grade metamorphic processes and increasing effective stress

Off southwest Japan the seaward limit of coseismic displacement (or updip limit of the seismogenic zone) of the 1946 M w 8.3 thrust earthquake reaches to 4 km depth and ∼40 km landward of the trench. This limit coincides with the estimated location of the 150 °C isotherm, and has been linked to changes in physical properties associated with the smectite to illite clay-mineral transition. Here we show that this limit correlates with a suite of diagenetic to low-grade metamorphic processes characterized by (1) declining fluid production and decreasing fluid pressure ratio (λ * ) and (2) active clay, carbonate, and zeolite cementation and the transition to pressure solution and quartz cementation. These diagenetic to low-grade metamorphic changes cause the onset of velocity weakening during thrust faulting, an increase in effective stress, and strengthening of the hanging wall, which together combine to produce recordable earthquakes.

Geology↗

USGS2012

The paleohydrology of unsaturated and saturated zones at Yucca Mountain, Nevada, and vicinity

Surface, unsaturated-zone, and saturated-zone hydrologic conditions at Yucca Mountain responded to past climate variations and are at least partly preserved by sediment, fossil, and mineral records. Characterizing past hydrologic conditions in surface and subsurface environments helps to constrain hydrologic responses expected under future climate conditions and improve predictions of repository performance. Furthermore, these records provide a better understanding of hydrologic processes that operate at time scales not readily measured by other means. Pleistocene climates in southern Nevada were predominantly wetter and colder than the current interglacial period. Cyclic episodes of aggradation and incision in Fortymile Wash, which drains the eastern slope of Yucca Mountain, are closely linked to Pleistocene climate cycles. Formation of pedogenic cement is favored under wetter Pleistocene climates, consistent with increased soil moisture and vegetation, higher chemical solubility, and greater evapotranspiration relative to Holocene soil conditions. The distribution and geochemistry of secondary minerals in subsurface fractures and cavities reflect unsaturated-zone hydrologic conditions and the response of the hydrogeologic system to changes in temperature and percolation flux over the last 12.8 m.y. Physical and fluid-inclusion evidence indicates that secondary calcite and opal formed in air-filled cavities from fluids percolating downward through connected fracture pathways in the unsaturated zone. Oxygen, strontium, and carbon isotope data from calcite are consistent with a descending meteoric water source but also indicate that water compositions and temperatures evolved through time. Geochronological data indicate that secondary mineral growth rates are less than 1–5 mm/m.y., and have remained approximately uniform over the last 10 m.y. or longer. These data are interpreted as evidence for hydrological stability despite large differences in surface moisture caused by climate shifts between the Miocene and Pleistocene and between Pleistocene glacial-interglacial cycles. Secondary mineral distribution and δ18O profiles indicate that evaporation in the shallower welded tuffs reduces infiltration fluxes. Several near-surface and subsurface processes likely are responsible for diverting or dampening infiltration and percolation, resulting in buffering of percolation fluxes to the deeper unsaturated zone. Cooler and wetter Pleistocene climates resulted in increased recharge in upland areas and higher water tables at Yucca Mountain and throughout the region. Discharge deposits in the Amargosa Desert were active during glacial periods, but only in areas where the modern water table is within 7–30 m of the surface. Published groundwater models simulate water-table rises beneath Yucca Mountain of as much as 150 m during glacial climates. However, most evidence from Fortymile Canyon up gradient from Yucca Mountain limits water-table rises to 30 m or less, which is consistent with evidence from discharge sites in the Amargosa Desert. The isotopic compositions of uranium in tuffs spanning the water table in two Yucca Mountain boreholes indicate that Pleistocene water-table rises likely were restricted to 25–50 m above modern positions and are in approximate agreement with water-table rises estimated from zeolitic-to-vitric transitions in the Yucca Mountain tuffs (less than 60 m in the last 11.6 m.y.).

Nevada↗

USGS2011

Significance of zircon U-Pb ages from the Pescadero felsite, west-central California coast ranges

Weathered felsite is associated with the late Campanian–Maastrichtian Pigeon Point Formation near Pescadero, California. Poorly exposed, its age and correlation are uncertain. Is it part of the Pigeon Point section west of the San Gregorio–Hosgri fault? Does it rest on Nacimiento block basement? Is it dextrally offset from the Oligocene Cambria Felsite, ∼185 km to the southeast? Why is a calc-alkaline hypabyssal igneous rock intrusive into the outboard accretionary prism? To address these questions, we analyzed 43 oscillatory-zoned zircon crystals from three incipiently recrystallized pumpellyite ± prehnite ± laumontite-bearing Pescadero felsite samples by sensitive high-resolution ion microprobe–reverse geometry (SHRIMP-RG) and laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) techniques. Thirty-three zircons gave late Mesozoic U-Pb ages, with single-grain values ranging from 81 to 167 Ma; ten have pre-Mesozoic, chiefly Proterozoic ages. A group of the four youngest Pescadero zircons yielded an apparent maximum igneous age of ca. 86–90 Ma. Reflecting broad age scatter and presence of partly digested sandstone inclusions, we interpret the rest of the zircons (perhaps all) as xenocrysts. Twenty-three zircons were separated and analyzed from two samples of the similar Cambria Felsite, yielding a unimodal 27 Ma U-Pb age. Clearly, the origin of the Upper Oligocene Cambria Felsite is different from that of the Upper Cretaceous Pescadero felsite; these rocks are not correlated, and do not constrain displacement along the San Gregorio–Hosgri fault. Peak ages differ slightly, but relative probability curves for Mesozoic and pre-Mesozoic Pescadero zircons compare well, for example, with abundant U-Pb age data for detrital zircons from Franciscan metaclastic strata ∼100 km to the east in the Diablo Range–San Francisco Bay area, San Joaquin Great Valley Group turbidites, Upper Cretaceous Nacimiento block Franciscan strata, and Upper Cretaceous forearc units of the Transverse Ranges. Based on zircon U-Pb ages, geologic and petrographic relations, the Pescadero felsite and a capping, sheared metaconglomerate underlie the Pigeon Point Formation. We infer that the magma formed by anatexis of Franciscan or Great Valley clastic sedimentary rocks originating from a parental Mesozoic Sierran-Mojave-Salinian calc-alkaline arc. The felsite erupted during Late Cretaceous time, was metamorphosed to pumpellyite-prehnite grade within the subduction zone, and then was rapidly exhumed, weakly zeolitized, and exposed before Pigeon Point forearc deposition. Pescadero volcanism apparently reflects a previously unrecognized ca. 86–90 Ma felsic igneous event in the accretionary margin.

California↗

USGS2019

Pleistocene hydrothermal activity on Brokeoff volcano and in the Maidu volcanic center, Lassen Peak area, northeast California: Evolution of magmatic-hydrothermal systems on stratovolcanoes

Partially eroded stratovolcanoes worldwide, notably Mounts Rainier and Adams in the Cascades and several volcanoes in Japan, record episodic periods of eruption and geothermal activity that produce zones of hydrothermal alteration. The partly eroded core of late Pleistocene Brokeoff volcano on the south side of Lassen Peak exposes the upper 1 km of multiple ancient (ca. 410–300 ka) magmatic-hydrothermal alteration zones in a 3.5 by 5 km area that allows characterization of the three-dimensional hydrothermal evolution of the volcano. Both acid- and neutral-pH hydrothermal solutions produced distinctive alteration mineral assemblages in close proximity. Early hydrothermal activity is characterized by alunite-rich alteration that is temporally and spatially related to shallow intrusions in the center of the volcano. Younger acid alteration and a large area of neutral-pH alteration formed along the volcano’s flanks. The neutral-pH alteration is vertically zoned over 1000 m from shallow zeolite ± adularia through intermediate argillic (smectite-pyrite ± illite) to deep propylitic (chlorite-calcite-albite-illite) alteration. Pleistocene alteration is partly overprinted by surficial, steam-heated alteration related to Lassen’s modern hydrothermal activity. A large (∼3.5 km 2 ), shallow (≤300 m), ca. 1.5 Ma alunite-rich magmatic-hydrothermal alteration zone is exposed on the northeast flank of the nearby Maidu volcanic center.

California↗

USGS1941

Igneous rocks of the Highwood Mountains, Montana: Part VI. Mineralogy

The minerals of the igneous rocks of the Highwood Mountains are described. The primary hornblende of the quartz latites is basaltic and it has been partly replaced by a common green hornblende. Hornblende is rare in the alkalic rocks. Augite is an abundant mineral of the alkalic rocks; in the syenites it contains some acmite, especially on the borders of the crystals. Aegirite is a common metamorphic mineral. The olivine of the shonkinites contains from 12 to 22 mol per cent of fayalite, that of the syenite from Square Butte 50 per cent. Monticellite twinned into trillings on (031) is an abundant mineral in one small intrusive body. In the alkalic rocks barium sanidine is abundant and plagioclase rare; leucite, primary analcime, and pseudoleucite are common and sodalite rare. Natrolite, thomsonite, and other zeolites are widespread and very abundant.

Montana↗

USGS1979

Petrology, composition, and age of intrusive rocks associated with the Quartz Hill molybdenite deposit, southeastern Alaska

A large porphyry molybdenum deposit (Quartz Hill deposit) was recently discovered in the heart of the Coast Range batholithic complex about 70 km east of Ketchikan, southeastern Alaska. Intrusive rocks associated with the mineral deposit form two composite epizonal to hypabyssal stocks and many dikes in country rocks. The stocks are characterized by a variety of textural rock types varying from equigranular or weakly seriate biotite granite to porphyries with aphanitic or very fine grained and aplitic groundmasses. These rocks contain about equal amounts of quartz, albitic plagioclase, and microperthitic microcline and less than 2.5% biotite. Unaltered rocks contain between 0.2 and 1% CaO, less than 1.7% combined Fe 2 O 3 , FeO, and MgO, and 74.4 to 77.7% SiO 2 . Total alkalis are between 8 and 9%, and K 2 O/Na 2 O is about 1.1. The range of major-oxide variation is small, but it is systematically related to lithology. Many trace-elements, including B, Pb, Sn, and Li have low concentrations. Intrusive rocks associated with the Quartz Hill deposit are more albitic and possibly trace-element depleted compared to some other rocks associated with porphyry molybdenum deposits.All observed metallization and alteration is within the Quartz Hill stock. Molybdenite forms fracture coatings and occurs in veins with quartz. Alteration is widespread and includes development of secondary quartz, pyrite, K-feldspar, biotite, white mica, chlorite, and zeolite. Field relations indicate that the stocks were emplaced after regional uplift and erosion of the Coast Range batholithic complex, and K–Ar data show that intrusion and alteration took place in late Oligocene time, about 27 to 30 Ma ago. Data from the Ketchikan quadrangle indicate that porphyry molybdenum metallization in the Coast Range batholithic complex is associated with regionally extensive but spotty, middle Tertiary or younger, felsic magmatism.

Alaska↗

USGS1984

Paleogene geology and chronology of southwestern Umnak Island, Aleutian Islands, Alaska

The rocks of southwestern Umnak Island (Nikolski area) consist mainly of a slightly deformed marine sedimentary sequence intruded by hypabyssal quartz diorite sills and small plutons. Laminated and thin-bedded argillaceous and tuffaceous mudstone and siltstone reflect volcanic arc sedimentation from late Eocene to early Oligocene time. Zeolite facies minerals formed during diagenesis include heulandite, analcime, quartz, chlorite, and smectite. Radiometric dates from sill rock and plutons yield apparent ages of about 30 Ma, i.e, middle Oligocene. Chemical data from igneous rocks exhibit calc-alkaline and tholeiitic volcanic arc differentiation trends. Dikes of pyroxene basalt yield K/Ar apparent ages as old at 17 Ma, and a columnar-jointed plug of pyroxene basalt is as young as 1 Ma. The fossil ages and radiometric dates from southwestern Umnak Island are similar to those reported from other central and eastern Aleutian islands, and indicate uniformity in the chronology and tectonic development of the archipelago during the Paleogene. Paleomagnetic data suggest possible northward movement but remain equivocal and more work is indicated.

Alaska↗

USGS2017

Assessment of the potential respiratory hazard of volcanic ash from future Icelandic eruptions: A study of archived basaltic to rhyolitic ash samples

Background The eruptions of Eyjafjallajökull (2010) and Grímsvötn (2011), Iceland, triggered immediate, international consideration of the respiratory health hazard of inhaling volcanic ash, and prompted the need to estimate the potential hazard posed by future eruptions of Iceland’s volcanoes to Icelandic and Northern European populations. Methods A physicochemical characterization and toxicological assessment was conducted on a suite of archived ash samples spanning the spectrum of past eruptions (basaltic to rhyolitic magmatic composition) of Icelandic volcanoes following a protocol specifically designed by the International Volcanic Health Hazard Network. Results Icelandic ash can be of a respirable size (up to 11.3 vol.% < 4 μm), but the samples did not display physicochemical characteristics of pathogenic particulate in terms of composition or morphology. Ash particles were generally angular, being composed of fragmented glass and crystals. Few fiber-like particles were observed, but those present comprised glass or sodium oxides, and are not related to pathogenic natural fibers, like asbestos or fibrous zeolites, thereby limiting concern of associated respiratory diseases. None of the samples contained cristobalite or tridymite, and only one sample contained quartz, minerals of interest due to the potential to cause silicosis. Sample surface areas are low, ranging from 0.4 to 1.6 m2 g−1, which aligns with analyses on ash from other eruptions worldwide. All samples generated a low level of hydroxyl radicals (HO•), a measure of surface reactivity, through the iron-catalyzed Fenton reaction compared to concurrently analyzed comparative samples. However, radical generation increased after ‘refreshing’ sample surfaces, indicating that newly erupted samples may display higher reactivity. A composition-dependent range of available surface iron was measured after a 7-day incubation, from 22.5 to 315.7 μmol m−2, with mafic samples releasing more iron than silicic samples. All samples were non-reactive in a test of red blood cell-membrane damage. Conclusions The primary particle-specific concern is the potential for future eruptions of Iceland’s volcanoes to generate fine, respirable material and, thus, to increase ambient PM concentrations. This particularly applies to highly explosive silicic eruptions, but can also hold true for explosive basaltic eruptions or discrete events associated with basaltic fissure eruptions.

Environmental Health↗

USGS1981

Northwest margin of California continental borderland: Marine geology and tectonic evolution

The northwest margin of the California continental borderland consists mainly of two northwest-trending pre-Neogene lithologic belts blanketed by Miocene and younger strata. These belts, which are lithologically and structurally correlated with the Franciscan Complex and Great Valley sequence of northern California, are interpreted to represent facies corresponding to the subduction complex and forearc-basin deposits of a late Mesozoic and Paleogene continental-margin arc-trench system. The outer belt, which forms the acoustic basement underlying the continental slope and Patton Ridge, is characterized by: (1) moderately high compressional velocities (5.1 to 6.2 km/sec), (2) discordant and discontinuous seismic reflectors, and (3) nonfossiliferous and diverse rock types t at range in metamorphic grade from zeolite-bearing arkosic sandstone to blueschist. Landward, the inner Great Valley sequence belt underlies Santa Rosa-Cortes Ridge and Santa Cruz and San Nicolas Basins; in contrast to the outer belt, this belt is characterized by: (1) intermediate compressional velocities (4.5 to 4.6 km/sec), (2) concordant and relatively continuous reflectors, and (3) a thick turbidite sequence of relatively undeformed and unmetamorphosed fossiliferous Cretaceous and Paleogene strata. The inner belt overlies a 6.6-km/sec layer that is probably oceanic crust, a relation consistent with the hypothesis that trapped old oceanic crust underlies the forearc basins along parts of some convergent margins. Well-developed wrench-fault structures in overlying Miocene strata record a transition from Paleogene subduction to Neogene transform faulting. Related events include widespread andesitic to dacitic volcanism and concurrent uplift and erosion of Miocene and older rocks. The proximity of these Miocene silicic volcanic rocks to the paleotrench (< 20 km) is best explained as resulting from passage of the southward-migrating Rivera ridge-fault-trench triple junction along the margin. On the basis of the age of the adjacent sea floor, passage of this triple junction occurred about 17 m.y. ago, a date that corresponds closely to the inception of volcanism and uplift in the northwest margin of the borderland. A widespread unconformity that separates highly deformed lower and lower middle iocene strata from relatively undeformed younger strata is assumed to mark the subsequent onset of wrench faulting. The timing of volcanism and uplift, and of the slightly younger wrenching event, closely matches the plate-tectonic model of Atwater and Molnar if the northwest margin of the borderland is restored to an earlier tectonic setting off Baja California, approximately 260 km southeast of its present position.

California↗

USGS2024

Evidence for low-grade metamorphism, hydrothermal alteration, and diagenesis on Mars from phyllosilicate mineral assemblages

The enhanced spatial and spectral resolution provided by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) on the Mars Reconnaissance Orbiter (MRO) has led to the discovery of numerous hydrated silicate minerals on Mars, particularly in the ancient, cratered crust comprising the southern highlands. Phases recently identified using visible/near-infrared spectra include: smectite, chlorite, prehnite, high-charge phyllosilicates (illite or muscovite), the zeolite analcime, opaline silica, and serpentine. Some mineral assemblages represent the products of aqueous alteration at elevated temperatures. Geologic occurrences of these mineral assemblages are described using examples from west of the Isidis basin near the Nili Fossae and with reference to differences in implied temperature, fluid composition, and starting materials during alteration. The alteration minerals are not distributed homogeneously. Rather, certain craters host distinctive alteration assemblages: (1) prehnite-chlorite-silica, (2) analcime-silica-Fe,Mg-smectite-chlorite, (3) chlorite-illite (muscovite), and (4) serpentine, which furthermore has been found in bedrock units. These assemblages contrast with the prevalence of solely Fe,Mg-smectites in most phyllosilicate-bearing terrains on Mars, and they represent materials altered at depth then exposed by cratering. Of the minerals found to date, prehnite provides the clearest evidence for subsurface, hydrothermal/metamorphic alteration, as it forms only under highly restricted conditions ( T = 200–400ºC). Multiple mechanisms exist for forming the other individual minerals; however, the most likely formation mechanisms for the characteristic mineralogic assemblages observed are, for (1) and (2), low-grade metamorphism or hydrothermal (<400ºC) circulation of fluids in basalt; for (3), transformation of trioctahedral smectites to chlorite and dioctahedral smectites to illite during diagenesis; and for (4), low-grade metamorphism or hydrothermal (<400ºC) circulation of fluids in ultramafic rocks. Evidence for high-grade metamorphism at elevated pressures or temperatures >400ºC has not been found.

Clays and Clay Minerals↗

USGS1963

Studies of fluid inclusions; [Part] 2, Freezing data and their interpretation

Aqueous and non-aqueous inclusions in 84 samples of various minerals from a wide range of geologic environments were studied with the freezing stage in order to gain an insight into the range of concentrations and compositions of fluid inclusions . Inclusions in most Mississippi Valley-type ore minerals contain highly concentrated saline solutions, showing freezing temperatures between -23.4° and -10.5° C; minerals from ore deposits of more typically hydrothermal affiliations mainly show temperatures of -9.4° to nearly 0° C; and inclusions in quartz crystals from sedimentary, metamorphic, and igneous rock environments show a wide range of freezing temperatures. Inclusions in pegmatite minerals in particular vary over a wide range, from the most concentrated solutions found in any inclusion (> 40% salts) to fairly dilute solutions (< 5% salts). Most inclusions in quartz from Swiss Alpine-type veins, and from Brazilian quartz veins and pegmatites, show freezing temperatures in the range -8.5° to - 2 .5°, and considerable free C02. Other geologic environments sampled include Colombian emerald, pegmatitic topaz and fluorite, the Triassic traprock zeolite assemblage, and sedimentary halite beds. Not all the phenomena exhibited by inclusions at low temperature are completely understood at present but several crystalline hydrate phases, such as NaCl-2H,0 and CO,-5%H,0 (structural formula 8C02-46H20), are shown to be stable in inclusions of appropriate composition even at temperatures above 0° C, and probably exist in the inclusions in natural rocks in polar regions. More significantly, the formation and recognition of such phases aid in establishing the gross composition of individual inclusions far too small for chemical analysis. The data obtained are useful in a variety of ways, such as: discriminating among gas, liquid, and supercritical fluid , and among liquid water, liquid oil, and liquid C02 in inclusions ; improving precision of the pressure corrections applied to inclusion filling temperature determinations; proving the general lack of leakage into or out of inclusions ; estimating the minimum pressure at the time of deposition of certain samples; verifying the lack of extraneous solid crystallization nuclei in the inclusions and hence their formation from exceedingly quiet, clean solutions; determining the total equivalent NaCl concentrations and some information concerning the composition of the fluids from which ores have formed; and determining changes in the composition of the fluids bathing a single crystal during its growth, and at certain times throughout its history.

Economic Geology↗

USGS2025

Gulf Coast Basin CORE-CM initiative final report

The Bureau of Economic Geology at the University of Texas at Austin (UT-BEG) is leading the Gulf Coast Carbon Ore, Rare Earth, and Critical Minerals (CORE-CM) Initiative to assess the potential to produce critical minerals (CMs), including rare earth elements (REEs) from coal, coal ash, and produced water related to oil and gas production, and related materials (alumina processing waste [red mud], heavy mineral sands, graphite, and zeolite) within the Gulf Coast Basin. This project represents the first phase in a long-term program and provides reconnaissance data that will be foundational for future work by assessing resources and suggesting plans to be conducted in future work and expanding stakeholder engagement. The project includes several tasks designed to identify, characterize, and assess several necessary aspects for development of CMs and REEs in the Gulf Coast Basin.

Gulf Coast basin↗

USGS2012

Initiation of long-term coupled microbiological, geochemical, and hydrological experimentation within the seafloor at North Pond, western flank of the Mid-Atlantic Ridge

Integrated Ocean Drilling Program (IODP) Expedition 336 successfully initiated subseafloor observatory science at a young mid-ocean-ridge flank setting. All of the drilled sites are located in the North Pond region of the Atlantic Ocean (22°45′N, 46°05′W) in 4414–4483 m water depth. This area is known from previous ocean drilling and site survey investigations as a site of particularly vigorous circulation of seawater in permeable 8 Ma basaltic basement underlying a <300 m thick sedimentary pile. Understanding how this seawater circulation affects microbial and geochemical processes in the uppermost basement was the primary science objective of Expedition 336. Basement was cored and wireline-logged in Holes U1382A and U1383C. Upper oceanic crust in Hole U1382A, which is only 50 m west of Deep Sea Drilling Project (DSDP) Hole 395A, recovered 32 m of core between 110 and 210 meters below seafloor (mbsf). Core recovery in basement was 32%, yielding a number of volcanic flow units with distinct geochemical and petrographic characteristics. A unit of sedimentary breccia containing clasts of basalt, gabbroic rocks, and mantle peridotite was found intercalated between two volcanic flow units and was interpreted as a rock slide deposit. From Hole U1383C we recovered 50.3 m of core between 69.5 and 331.5 mbsf (19%). The basalts are aphyric to highly plagioclase-olivine-phyric tholeiites that fall on a liquid line of descent controlled by olivine fractionation. They are fresh to moderately altered, with clay minerals (saponite, nontronite, and celadonite), Fe oxyhydroxide, carbonate, and zeolite as secondary phases replacing glass and olivine to variable extents. In addition to traditional downhole logs, we also used a new logging tool for detecting in situ microbial life in ocean floor boreholes—the Deep Exploration Biosphere Investigative tool (DEBI-t). Sediment thickness was ~90 m at Sites U1382 and U1384 and varied between 38 and 53 m at Site U1383. The sediments are predominantly nannofossil ooze with layers of coarse foraminiferal sand and occasional pebble-size clasts of basalt, serpentinite, gabbroic rocks, and bivalve debris. The bottommost meters of sections cored with the advanced piston corer feature brown clay. Extended core barrel coring at the sediment/ basement interface recovered <1 m of brecciated basalt with micritic limestone. Sediments were intensely sampled for geochemical pore water analyses and microbiological work. In addition, high-resolution measurements of dissolved oxygen concentration were performed on the whole-round sediment cores Major strides in ridge-flank studies have been made with subseafloor borehole observatories (CORKs) because they facilitate combined hydrological, geochemical, and microbiological studies and controlled experimentation in the subseafloor. During Expedition 336, two fully functional observatories were installed in two newly drilled holes (U1382A and U1383C) and an instrument and sampling string were placed in an existing hole (395A). Although the CORK wellhead in Hole 395A broke off and Hole U1383B was abandoned after a bit failure, these holes and installations are intended for future observatory science targets. The CORK observatory in Hole U1382A has a packer seal in the bottom of the casing and monitors/samples a single zone in uppermost oceanic crust extending from 90 to 210 mbsf. Hole U1383C was equipped with a three-level CORK observatory that spans a zone of thin basalt flows with intercalated limestone (~70–146 mbsf), a zone of glassy, thin basaltic flows and hyaloclastites (146–200 mbsf), and a lowermost zone (~200–331.5 mbsf) of more massive pillow flows with occasional hyaloclastites in the upper part.

Integrated Ocean Drilling Program: Preliminary Rep↗