Basalt Flows Research Articles

Estudo da mineralização de cobre em basaltos do Grupo Serra Geral, utilizando difratometria de raios X, microscopia eletrônica de varreduraespectroscopia por energia dispersiva e tomografia com nêutrons

Mineralizações de cobre do tipo Michigan ocorrem nos derrames basálticos do Grupo Serra Geral na região de Nova Prata do Iguaçu, sudoeste do estado do Paraná. Nessas mineralizações, o cobre nativo é o principal mineral de minério e, com a alteração supérgena, transforma-se em óxidos de cobre, malaquita e crisocola, sendo esta encontrada na forma de um precipitado gelatinoso que preenche amígdalas e impregna fraturas no basalto, além de recobrir minerais secundários e outros produtos de alteração. Neste estudo, foram utilizadas as técnicas de difratometria de raios X e microscopia eletrônica de varredura-espectroscopia por energia dispersiva (MEV-EDS) para a caracterização do grau de cristalinidade e composição química da crisocola e a identificação de outros materiais hidrogenados, como subsídio para o imageamento 3D por tomografia com nêutrons. A crisocola destacou-se como um eficiente atenuador de nêutrons, obtendo-se coeficiente de atenuação linear da ordem de 0,8–1,3 cm-1, ao passo que, para o corpo de minério (basalto), obteve-se 0,5–0,6 cm-1. O contraste entre os coeficientes permitiu a obtenção de imagens tomográficas e filmes de boa resolução, em que se pôde visualizar a distribuição da crisocola na amostra de basalto e estimar como fração volumétrica de crisocola no corpo de minério valores entre 0,9 e 1,4%, obtendo-se o valor de 7,8% em volume de crisocola para uma das amostras. A utilização integrada das três técnicas analíticas demonstrou que esta pode ser uma ferramenta para avaliação do potencial mineral nas fases iniciais da prospecção mineral, especialmente quando as substâncias de interesse são ricas em hidrogênio.

14C and 40Ar/39Ar radiometric dating and geologic setting of young lavas of Rancho Seco and Mazcuta volcanoes hosting archaeological sites at the margins of the Pátzcuaro and Zacapu lake basins (central Michoacán, Mexico)

Geologic mapping of two neighbouring areas (totalling ∼770km2) centred around the Rancho Seco and Mazcuta monogenetic scoria cones and situated between the Pátzcuaro and Zacapu lake basins in the Michoacán-Guanajuato Volcanic Field (central portion of the Trans-Mexican Volcanic Belt), allowed the recognition of 62 Quaternary monogenetic volcanic structures, which include cinder cones with associated lava flows, domes, and small-to-medium sized shields. 40Ar/39Ar and 14C radiometric dating, petrography, and whole-rock chemical analyses of volcanic products provide a stratigraphic framework and insight into different eruptive styles, and erupted volumes (∼45km3 of calc-alkaline magma). Pliocene to Holocene monogenetic activity has been frequent in this region, forming structures, which are generally aligned along an ENE-WSW direction associated to the Morelia-Cuitzeo-Acambay normal fault system.The young Rancho Seco and Mazcuta volcanoes are of particular interest because their distal lava flows host the pre-Hispanic archaeological urban centres of Angamuco and Cortijo Viejo, respectively. The Rancho Seco scoria cone was radiocarbon-dated at 27,845+445/−425yr BP and emitted at least six andesitic lava flows which were emplaced toward the SW (Lake Pátzcuaro) covering an area of 21.3km2 with a volume of ∼0.64km3. The Mazcuta scoria cone dated at 7970±135yr BP produced five basaltic andesite-andesite lava flows toward the NW (Lake Zacapu), where they cover an area of 28.85km2 with a volume of ∼0.57km3. The recently studied archaeological site of Angamuco became an important urban centre during the rise of the Tarascan Empire in the Post-Classic period (AD 900–1521), while the Cortijo Viejo site remains largely unexplored, but its occupation might fall within the same period. Although the young lava flows on which these sites are built seem inhospitable and barren with a rugged topography and rocky substrate unfit for agriculture, early inhabitants adapted and modified its surface according to their needs. Hence, these archaeological sites exemplify how a seemingly hostile landscape was ingeniously modified to establish population centres with patterns similar to those observed on lava flows elsewhere.

The SPICE project: Production rates of cosmogenic 21Ne, 10Be, and 14C in quartz from the 72 ka SP basalt flow, Arizona, USAu

The SP lava flow is a quartz-, olivine- and pyroxene-bearing basalt with an 40 Ar/ 39 Ar age of 72 ± 4 ka (2σ). The flow is preserved in the desert climate of northern Arizona, USA. Its unweathered appearance and the lack of soil development indicate it has undergone negligible erosion and/or burial, making it an ideal site for direct calibration of cosmogenic nuclide production rates. Cross-calibrated production rates and production rate ratios for cosmogenic 21 Ne, 10 Be, and 14 C have been determined from SP flow quartz. Production rate ratios for 21 Ne/ 10 Be, 21 Ne/ 14 C, and 14 C/ 10 Be are based on the total, local production rates of each cosmogenic nuclide, independent of scaling models, and have error-weighted means (±2σ uncertainty) of 4.44 ± 0.32, 1.43 ± 0.10, and 2.85 ± 0.21, respectively. Error-weighted mean, sea-level, high latitude (SLHL) total reference production rates of 21 Ne, 10 Be, and 14 C are 17.0 ± 1.1, 3.84 ± 0.27, and 11.2 ± 0.6 at/g/yr (2σ), respectively, using time-independent Lal (1991)/Stone (2000) ( St ) scaling factors. St scaled spallogenic 10 Be and 14 C rates are 3.73 ± 0.26 and 9.2 ± 0.6 at/g/yr, respectively. 21 Ne and 10 Be production rates are integrated over the past 72 ka, whereas 14 C production rates are integrated over 25 ka, the time at which SP flow quartz has reached saturation with respect to 14 C. These rates overlap within 2σ uncertainty with other St -scaled production rates in the literature, including the total reference SLHL 21 Ne production rate of Niedermann (2000), which is revised in this paper to 16.8 ± 3.3 at/g/yr (2σ; St scaling) to reflect a recent change in age control at the Sierra Nevada sites. All SLHL production rates are lower if time- dependent Sf , Sa , and Lm scaling factors are used. For example, error-weighted mean, sea-level, high latitude (SLHL) total reference production rates for 10 Be as calculated in the CREp online calculator range from 3.49 ± 0.23 to 3.74 ± 0.25 at/g/yr (2σ), using time-dependent Lm scaling factors. Commonly used SLHL 10 Be and 14 C production rates in the literature were calibrated on surfaces that have been exposed to cosmic rays for less than 20 ka. Between 20 and 50 ka, the geomagnetic field is proposed to have been weaker than it is today. Production rates of cosmogenic nuclides increase during periods of weaker geomagnetic field strength. However, our study finds no measureable difference between St -scaled production rates of cosmogenic 21 Ne and 10 Be over the past 20 ka and St -scaled 21 Ne and 10 Be production rates over the past 72 ka. As such, the study suggests that 21 Ne and 10 Be production rates in quartz were not significantly greater during the proposed period of decreased magnetic strength from 20 to 50 ka. • Total reference production rates at SLHL are calculated in 72 ka quartz. • Cosmogenic 21 Ne, 10 Be, and 14 C rates ( St ): 17.0 ± 1.1, 3.84 ± 0.27, and 11.2 ± 0.6 at/g/yr. • These rates agree with St scaled production rates over past 20 ka in literature. • There is a proposed period of decreased geomagnetic field strength from 20 to 50 ka. • SPICE rates do not record increased cosmogenic nuclide production over past 72 ka.

Emplacement of lobate sheet flows with hyaloclastites onto soft sediment: The Erquy Neoproterozoic lava pile, Armorican Massif (France)

The Neoproterozoic volcano-sedimentary succession of the Heussaye Headland (Erquy) belongs to the Cadomian block located in the north-east of the Armorican Massif (France). The volcanism, dated at approximately 608 ± 7 Ma (zircon U/Pb data), occurred in a back-arc basin. This site exhibits a noteworthy verticalized pile of moderately metamorphosed (greenschist facies) basaltic andesite flows of arc/back-arc tholeiite affinity, alternating with metasedimentary layers (sandstones and siltstones). A large range of volcanic facies, characteristic of an underwater soft-substrate environment, can be continuously studied for over than 200 m parallel to the flow edges. They include pillow lavas, lava lobes, fluidal peperites and sediment-matrix hyaloclastites. The presence of parallel lava lobes in a sector where one lava flow is capped by a variably thick hyaloclastite carapace, together with the occurrence of asymmetric turbiditic folds in interbedded metasedimentary layers, suggests the existence of an easterly-facing gentle paleoslope. The sudden thinning of this hyaloclastite carapace in two spots located approximately 60 m from each other, is attributed to the synvolcanic activity of two conjugate normal paleofaults. This hypothesis is supported by the observation of a hyaloclastite accumulation in one of the two spots close to a strike-slip fault, which may have been extensional at the time of the volcanism. Water entered the lava flow along the faults, resulting in an interstitial network of hyaloclastites by quench fragmentation in the upper part of the hanging wall block. All these observations are consistent with a model of lava emplacement along a subaqueous slope resulting from synmagmatic extensional faulting in a marginal basin context.

Paleomagnetism of the native copper mineralization, Keweenaw Peninsula, Michigan

The age and genesis of Michigan’s world-class native copper deposits are poorly constrained. The copper is hosted by basaltic flow tops and conglomeratic interbeds of the 1095 ± 2 Ma Keweenawan Portage Lake Formation. Progressive thermal demagnetization isolates stable hematite remanent magnetization components at 28 paleomagnetic sites. Paleomagnetic tilt tests show that magnetite in massive flow interiors is primary (1095 ± 2 Ma) and that hematite throughout the formation is syntectonic. The altered cupriferous deposits contain primary ∼1095 Ma and secondary ∼1053 Ma hematite in various proportions. The Caledonia Mine’s basaltic mineralization carries the ∼1053 Ma hematite dominantly whereas the Delaware Mine’s conglomeratic interbed mineralization carries the ∼1095 Ma hematite dominantly. The ∼1095 Ma hematite is attributed mostly to magnetite exsolution during flow extrusion and to weathering oxidation between extrusion events. An infusion of epigenetic hydrothermal fluids emplaced the native copper with additional hematite and polarity self-reversing titanohematite at 1053 ± 7 Ma. Importantly, paleomagnetic evidence supports a 1053 ± 7 Ma age also for the White Pine stratiform sedimentary copper mineralization, for the oxidation of the Oronto Group clastic rocks to red beds, and for the time limits of major tectonic uplift and deformation on the Keweenaw Peninsula.

Serra do Tapa Ophiolite Suite - Araguaia Belt: Geological characterization and Neoproterozoic evolution (central-northern Brazil)

The Araguaia Belt is a Neoproterozoic collisional orogen located in the northern Tocantins tectonic province, North-central Brazil, being one of the most expressive orogens of West Gondwana. The Serra do Tapa Ophiolite Suite is the most important representative of the ophiolitic bodies of the Araguaia Belt, exposed along the low-grade metamorphic domain of the Belt and extending over 80 km in the N-S direction. It constitutes a set of N-S aligned bodies that represent a disrupted ophiolite special sequence comprising serpentinized peridotites and dunites, massive and pillow basaltic lavas, volcanosedimentary rocks, cherts, jaspilites, and their metamorphic products. The serpentinized rocks have protoliths of mantelic harzburgites and dunites, with relict original protogranular texture partially replaced by pseudomorphic texture after olivine and orthopyroxene, and non-pseudomorphic (replacements). Submarine basaltic flows represent the volcanic succession, overlying serpentinized peridotites. The volcanic succession occurs as homogeneous massive basalts or pillow basalts, and locally volcanoclastic breccias (hyaloclastite). The pillow structure shows zoning, where the core consists of homogeneous massive metabasalt with preserved igneous features whereas pillow outer zone consists of hyalobasalt with quenching textures whose pillow surface is transformed into chlorite-rich rocks that appear fine-grained and related to the metasomatic transformations due to interactions with seawater. Geochemically, the massive metabasalts display a subalkaline-tholeiitic affinity, compatible with Middle Oceanic Ridge Basalts types (MORB). The suite evolution is a testimony of an oceanic stage during the Neoproterozoic of the Araguaia Basin that generated an oceanic lithosphere followed by volcanic activity, the interaction between lava flows and pelitic sediments, and deposition of chemical sedimentary rocks (jaspilites and cherts). Subsequently, the detachment of the oceanic lithosphere and the compressional tectonic evolution of the Araguaia Orogen inverted the sequence leading to the obduction of ophiolite bodies along overthrust and transcurrent faults projecting/thrusting a mass toward the Amazonian Craton. The tectonics was accompanied by regional metamorphism from anchymetamorphism to low greenschist facies conditions. All these processes are related to the evolution of the Brasiliano/Pan-African Cycle of the Araguaia Belt. The Serra do Tapa Ophiolite constitutes a mega-slice of an oceanic crust and marks a phase of lithosphere construction and the early opening of the Araguaia Basin related to the crustal evolution of the Araguaia Belt in the Neoproterozoic (transition Tonian-Ediacaran).

Hot spring hydrochemistry of the Rio Grande rift in northern New Mexico reveals a distal geochemical connection between Valles Caldera and Ojo Caliente

Warm and hot springs occur within and on both sides of the Rio Grande rift (RGR) of northern New Mexico. Springs on the western side of the RGR are along the Ojo Caliente fault zone, which intersects the Embudo fault transfer zone that extends northeast of the Valles Caldera. Springs on the eastern side occur along north-south rift-bounding faults near Taos, NM. We compared spring water geochemical data from the western and eastern sides of the RGR to discover spring water origins. Included in our discussion was spring chemistry of the Valles Caldera located to the southwest of our study area. The Valles Caldera is a significant feature because it had major rhyolite eruptions 1.65 and 1.26 Ma, continued rhyolite eruptions until ~30 ka, and an active magmatically-driven geothermal system. The latest volcanic events in the Taos Plateau were basalt flows occurring about ~ 2 Ma. Our spring chemistry findings showed elevated levels of geothermal input in the western spring group, with the greatest input in Ojo Caliente spring waters. The elevated geothermal tracers include CO2, Li, B, Na, Cl, HCO3, Cext, He, Sr, mantle-like δ13C-CO2 values (−3.5 to −6.8‰), radiogenic basement-like 87Sr/86Sr ratios (0.747), and mantle-derived 3He (up to 4.8%) in the presence of highly elevated crustal 4He. Eastern springs had fewer magmatic inputs including tracers listed above and notably lower 87Sr/86Sr ratios ranging from 0.708 to 0.713. Nevertheless, eastern springs contain endogenic carbon and helium isotope values showing up to 4% mantle-derived helium. We interpret the elevated tracer results discovered in Ojo Caliente spring waters as distal influences of the Valles geothermal system, which is about 60 km away. The most conclusive evidence for these results stem from significant contributions of magmatically-derived volatiles including CO2, He, and their isotopes. With evidence of far-traveled water and volatiles, we concluded that the RGR transfer zone fault systems provide conduits for long-distance migration of deep fluids and gases that mix with meteoric water in the shallow subsurface.

High 3He/4He ratios in lower East Rift Zone steaming vents precede a new phase of Kilauea 2018 eruption by 8 months

On May 1, 2018, a magnitude 5.0 earthquake heralded the collapse of the Pu’u O’o Vent on the middle East Rift Zone (ERZ) of Kilauea Volcano, active since 1983. Increased seismicity was recorded on the middle to lower ERZ from April 30 until May 2, 2018. The active lava lakes within both Pu’u O’o Vent and Halema’uma’u Crater began to drain and the summit caldera began to deflate, with the summit collapse ending on August 2, 2018 and lower ERZ eruptive lava activity ending by 4 September 2018. Herein we report on elevated 3He/4He ratios in steaming vents in the lower ERZ from samples collected in early September 2017. Gas isotopic measurements were made with a new, field-portable He isotope detector capable of sub-daily monitoring of the 3He/4He ratio. When corrected for air contamination, these values exceed those previously reported for Kilauea by nearly twofold, resembling a purer hotspot plume signature, such as those measured directly over the mantle plume at Loihi Seamount to the SE of Hawaii Island, and in older basalt flows when Kilauea and its sister Hawaiian shield volcanoes were located more directly over the plume. The discovery, which presages the eruption there by more than eight months, suggests that we either sampled a 3He/4He rich magma already in place in the lower ERZ or a shallow groundwater reservoir in the lower ERZ (Puna district) with anomalously low values of 4He relative to their 3He/4He ratio, similar to previous findings there and suggestive of a previously unknown He isotopic fractionation.

Fluoride behaviour analysis in groundwater with reference to hydrogeochemical parameters in basaltic aquifers using remote sensing and GIS technique in parts of Burhner watershed, MP, India

High fluoride concentration in groundwater leads to health threat to millions of people around the world; therefore, a systematic study is required to understand the behaviour of fluoride in water in terms of the local hydrogeological setting and other hydrogeochemical parameters. The present study is an attempt to assess the hydrogeology of groundwater in the study area to understand the fluoride behaviour in groundwater and to deduce the chemical parameters responsible for the dissolution activity of fluoride. F− value varies from 0.04 to 14.6 mg/l (mean: 3.15 mg/l) in the stuldy area. It is geologically occupied by basalt rocks where groundwater occurs in the weathered and fractured portions of the rocks and under semi-confined to confined conditions in fractured rocks. High F− concentration is observed in deeper aquifers compared to shallow aquifers. Physicochemical conditions like decomposition, dissociation and subsequent dissolution along with long residence time are responsible for leaching of F− into the groundwater. F− has +ve or −ve correlation with other parameters of water samples as per their nature. Simple to compound pahoehoe basaltic lava flows are responsible for the fluoride contamination in the confined aquifers (bore wells) of the study area. Overall confined aquifer water quality on the basis of fluoride concentration was found to be unsatisfactory for drinking purposes. About 57.13% of confined aquifers showed higher fluoride than the permissible limit but 100% unconfined aquifers (dug wells) have a low level of fluoride concentration, i.e. below the permissible limit.

Ovda Fluctus, the Festoon Lava Flow on Ovda Regio, Venus: Not Silica‐Rich

AbstractOvda Fluctus (6.1°S, 95.5°E) is a lava flow complex on the equatorial tessera highlands of Ovda Regio, Venus. We examined its morphology and geologic setting to determine if its apparent emplacement rheology is consistent with silica‐rich (e.g., rhyolitic) lava; this has significant implications for the compositions of Venus's tesserae. Using Magellan radar data, we find that the radar properties (emissivity, roughness, and backscatter) of Ovda Fluctus are similar to those of the surrounding tessera, although the flow is smoother at multiple scales. Ovda Fluctus is a “festoon flow complex,” whose surface is rumpled into arcuate folds. The flow includes multiple flow lobes and at least two distinct flow units. The flow complex includes a portion at high altitude (~5 km above mean planetary radius) that has low radar backscatter and high radar emissivity compared with lower‐altitude portions of the complex. The high‐altitude region of Ovda Fluctus is continuous with the lower‐elevation portions: the change in radar properties does not represent different flows. Outlines of Ovda Fluctus flow lobes have fractal dimensions consistent with basaltic pahoehoe lavas. The margin of Ovda Fluctus is at significantly higher elevation than its center, a characteristic seen in basalt flows on Earth, but not on more silica‐rich flows. Thus, the available evidence suggests that Ovda Fluctus had an emplacement rheology consistent with a basaltic composition; this result provides no support for hypotheses that Ovda Regio (a highlands tessera terrain) is composed of granitic rock.

The timing of basaltic volcanism at the Apollo landing sites

Precise crystallisation ages have been determined for a range of Apollo basalts from Pb-Pb isochrons generated using Secondary Ion Mass Spectrometry (SIMS) analyses of multiple accessory phases including K-feldspar, K-rich glass and phosphates. The samples analysed in this study include five Apollo 11 high-Ti basalts, one Apollo 14 high-Al basalt, seven Apollo 15 low-Ti basalts, and five Apollo 17 high-Ti basalts. Together with the samples analysed in two previous similar studies, Pb-Pb isochron ages have been determined for all of the major basaltic suites sampled during the Apollo missions. The accuracy of these ages has been assessed as part of a thorough review of existing age determinations for Apollo basalts, which reveals a good agreement with previous studies of the same samples, as well as with average ages that have been calculated for the emplacement of the different basaltic suites at the Apollo landing sites. Furthermore, the precision of the new age determinations helps to resolve distinctions between the ages of different basaltic suites in more detail than was previously possible. The proposed ages for the basaltic surface flows at the Apollo landing sites have been reviewed in light of these new sample ages. Finally, the data presented here have also been used to constrain the initial Pb isotopic compositions of the mare basalts, which indicate a significant degree of heterogeneity in the lunar mantle source regions, even among the basalts collected at individual landing sites.

DRAINAGE EVOLUTION AND FRESHWATER FISH ZOOGEOGRAPHY IN COASTAL OREGON AND WASHINGTON

In Oregon and Washington coastal streams, there are relatively diverse faunas on the Olympic Peninsula of Washington (Chehalis fauna), in the lower Columbia River, and on the south-central Oregon coast (Tyee fauna). Adjacent to the Tyee fauna are groups of streams with depauperate faunas (0–2 primary freshwater fishes). To the north, from the Nehalem River to the Alsea River, streams have 0–1 species (Oregon Coast Range Isolates), and to the south, from the Rogue River to the Klamath River, streams have 0–2 species (Klamath Isolates). Throughout the entire area there was no relationship between drainage area and freshwater fish diversity. Multiple processes are responsible for these patterns. Two endemic genera, Oregonichthys and Novumbra, originated by the Miocene (23–5.3 Mya) and current distributions are largely bordered by the continental flood-basalt flows of the Columbia River Basalt Group (CRBG). It is hypothesized that both genera were isolated by early Miocene Grande Ronde basalt flows, Oregonichthys to the south of the flows in an ancestral Umpqua River, and Novumbra to the north of the flows in an ancestral Chehalis River. The depauperate streams of the Oregon Coast Range Isolates are hypothesized to be the result of bisection of ancient coastal streams by uplift of the Oregon Coast Range and subsequent extinction of the ancestral fauna on west-slope streams either from changing habitats or stochastic events in smaller streams. The Klamath Isolate streams drain the southern Oregon Coast Range and Klamath Mountains, an ancient region otherwise known for its faunal and floral diversity. It is hypothesized that these streams, some of which are relatively large and would be expected to have relatively diverse faunas, have been isolated from diverse primary freshwater fish faunas to the north (Columbia) and south (Sacramento), and most taxa have simply never had access to these streams.

New insights of the tectonic and sedimentary evolution of the Rio Grande Rise, South Atlantic Ocean

Oceanic plateaus, such as the Rio Grande Rise (RGR), comprise crucial pieces of information about the opening of the South Atlantic Ocean and the evolution of associated lithospheric plate margins. A detailed seismostratigraphic assessment of the volcano-sedimentary package of the RGR was conducted in its western portion based on correlations between seismic data and DSDP Site 516F. The approximately 330-km long seismic transect is characterized by a complex array of normal faults related to extensional tectonics from Late Cretaceous to the Miocene, and probably into the present-day, along the Cruzeiro do Sul Rift. The bathymetric and seismic data reveal: a) the presence of deep faults that rise from the basement to the sea floor; b) the occurrence of several seamounts with tops at a water depth below 1000 m; and c) prominent graben structures that represent unidentified volcanic intrusions, indicative of the reactivation of Lower Miocene faults by younger extensional events. Based on integration of older data with the results presented here, we propose a five-stage tectono-sedimentary model for the western portion of the RGR: 1) the first basaltic flow began to form the Rio Grande Rise in the Coniacian/Santonian; 2) extensional movements caused rifting during the uplift of the large volcanic structures; 3) volcanic islands emerged above sea level increasing the deposition of volcanic breccia and ash layers; 4) after the volcanism ceased, thermal subsidence took place over the entire rise with intense erosion and sedimentation; and 5) the uppermost sedimentary layers were deposited in pelagic conditions and offset by sub-vertical normal faults.

Vapor transport of silver and gold in basaltic lava flows

Abstract We documented occurrences of native copper (Cu), silver (Ag), and gold (Au) in a pāhoehoe flow from Kīlauea volcano (Hawaii, USA), an a‘ā flow from Mauna Loa volcano (Hawaii), and a mid-oceanic-ridge basalt (MORB) from the Chile Ridge (southeastern Pacific Ocean). Native Ag in Kīlauea and MORB samples consistently contained minor Cl (<1 wt%). Native Ag in Hawaiian basalts can occur at the center of nearly circular patches of relatively evolved minerals, which presumably formed after late-stage silicate liquid infilled pipe vesicles. Sulfur loss and oxidation of a Cu-sulfide phase can explain the native Cu, but not Au and Ag deposition. The rare occurrence of native Cu-Au-Ag alloys and the large native Au and Ag grain size suggest separate metal precipitation mechanisms. A fractional crystallization and degassing model envisions initial Au and Ag enrichment in crystallizing interstitial liquid and further enrichment in a separating vapor phase. From the flow interior, the metals ascend through ephemeral pipe vesicles as bisulfide (Au) or chloride (Ag) vapor complexes and precipitate in the transition zone below the upper vesicular zone, owing to temperature and oxidation state changes. Our results support igneous vapor transport of ore elements in mafic plutonic systems and imply preconcentration of gold during lava solidification before later hydrothermal remobilization.

Characterisation and genesis of the chalcedony occurring within the Deccan lava flows of the LIT hill, Nagpur, India

The Deccan trap basaltic lava flows host a plethora of secondary minerals, notably zeolites, quartz and calcite. These minerals occupy the vesicles and cavities that are formed during the solidification of the lava flows. Much attention has been given to beautifully developed zeolites of the Deccan traps and well-crystallised quartz varieties such as rosy quartz and amethyst. However, a group of minerals consisting of cryptocrystalline and amorphous silica did not receive much attention despite their consistent occurrence in all parts of the Deccan large igneous province. The freshly exposed samples of the chalcedonic silica occurring in the vesicular basalt at the Laxminarayan Institute of Technology (LIT) hill, Nagpur University Campus, were studied, one of which was studied in detail using X-ray diffraction analysis, differential thermal analysis, thermogravimetric analysis, Fourier transform infrared and scanning electron microscope–energy dispersive spectroscopy. The present study has demonstrated that the secondary silica occupying vesicles in the basaltic lava flows was length-fast chalcedony showing fibrous morphology overprinted by grainy morphology. It was concluded that the mother fluid was a hot (~100–300°C) aqueous solution, which was acidic in nature and high in sulphur, probably derived from the host Deccan trap basaltic magma itself.

Accommodation, slip inversion, and fault segmentation in a province-scale shear zone from high-resolution, densely spaced wide-aperture seismic profiling, Centennial Valley, MT, USA

We acquired a ~9-km long, high-resolution reflection seismic profile in the Centennial Valley, Montana, to better understand the kinematics of basin bounding faults and their role in accommodating proposed right-lateral shear in the Northern Basin and Range adjacent to the Yellowstone hotspot. In pursuing these goals, our findings have also shed light on the development of hanging wall stratigraphy and seismic hazards for this part of the SW Montana seismic belt. Here we present the profile and a working interpretation that identifies fault inversion, and an oblique, anticlinal accommodation zone linking the Centennial and Lima Reservoir faults in the Centennial Valley. These interpretations are consistent with seismicity and GPS-geodetically observed right-lateral shear aligned with the Centennial Valley north of the Yellowstone hotspot. Data were acquired using dense, wide-aperture arrays and illuminate the subsurface stratigraphy and faults down to ~1200 m, showing that the basin is a half-graben with a southern depocenter driven by the listric geometry of the north-dipping Centennial fault. Reflectors onlap basement highs with growth geometry against these faults. Our interpretation of a bright basal reflection as the Timber Hill Basalt (~6 Ma) or related flow, is consistent with a late Miocene – Pliocene inception of the basin proposed by other research. We also note a small inversion structure that we interpret as local evidence of transpression in the shear zone. This transpression is part of the accommodation zone and seismogenic faults including the Lima Reservoir fault that has well-expressed Holocene surface ruptures a few kilometres west of the seismic line along the northern edge of the Centennial basin.

Dextral, normal, and sinistral faulting across the eastern California shear zone–Mina deflection transition, California-Nevada, USA

AbstractStrike-slip faults commonly include extensional and contractional bends and stepovers, whereas rotational stepovers are less common. The Volcanic Tableland, Black Mountain, and River Spring areas (California and Nevada, USA) (hereafter referred to as the VBR region) straddle the transition from the dominantly NW-striking dextral faults that define the northwestern part of the eastern California shear zone into a rotational stepover characterized by dominantly NE-striking sinistral faults that define the southwestern Mina deflection. New detailed geologic mapping, structural studies, and 40Ar/39Ar geochronology across the VBR region allow us to calculate Pliocene to Pleistocene fault slip rates and test predictions for the kinematics of fault slip transfer into this rotational stepover. In the VBR, Mesozoic basement is nonconformably overlain by a Miocene sequence of rhyolite, dacite, and andesite volcanic rocks that yield 40Ar/39Ar ages between 22.878 ± 0.051 Ma and 11.399 ± 0.041 Ma. Miocene rocks are unconformably overlain by an extensive sequence of Pliocene basalt and andesite lava flows and cinder cones that yield 40Ar/39Ar ages between 3.606 ± 0.060 Ma and 2.996 ± 0.027 Ma. The Pliocene sequence is, in turn, unconformably overlain by Quaternary tuffs and sedimentary rocks. This sequence of rocks is cut by NS- to NW-striking normal faults across the Volcanic Tableland that transition northward into NS-striking normal faults across the Black Mountain area and that, in turn, transition northward into NW-striking dextral and NE-striking sinistral faults in the River Spring area. A range of geologic markers were used to measure offset across the faults in the VBR, and combined with the age of the markers, yield minimum ∼EW-extension rates of ∼0.5 mm/yr across the Volcanic Tableland and Black Mountain regions, and minimum NW-dextral slip and NE-sinistral slip rates of ∼0.7 and ∼0.3 mm/yr, respectively, across the River Spring region. In the River Spring area, our preferred minimum dextral slip and sinistral slip rates are 0.8–0.9 mm/yr and 0.7–0.9 mm/yr, respectively. We propose three kinematic fault slip models, two irrotational and one rotational, whereby the VBR region transfers a portion of dextral Owens Valley fault slip northwestward into the Mina deflection. In irrotational model 1, Owens Valley fault slip is partitioned into two components, one northeastward onto the White Mountain fault zone and one northwestward into the Volcanic Tableland. Slip from the two zones is then transferred northward into the southwestern Mina deflection. In irrotational model 2, Owens Valley fault slip is partitioned into three components, with the third component partitioned west-northwest onto the Sierra Nevada frontal fault zone. In the rotational model, predicted sinistral slip rates across the southwestern Mina deflection are at least 115% greater than our observed minimum slip rates, implying our minimum observed rates underestimate true sinistral slip rates. A comparison of summed geologic fault slip rates, parallel to motion of the Sierra Nevada block relative to the central Great Basin, from the Sierra Nevada northeastward across the VBR region and into western Nevada are the same as geodetic rates, if our assumptions about the geologic slip rate across the dextral White Mountain fault zone is correct.

Quartz OSL dating of palaeosols intercalated with basaltic lava flows and scoria deposits from monogenetic volcanoes in northeastern Jeju Island, Korea

Jeju Island, which lies on the continental shelf in the southern Korean Peninsula, is the emergent portion of a basaltic volcanic field that has erupted since the Early Pleistocene (ca. 1.8 Ma). Volcanic activity that continued into historic times (ca. 1 ka) formed an elongated shield with a central edifice (Mt. Halla) and more than 300 monogenetic cones and rings. The establishment of a temporal framework for Jeju volcanism, particularly during the Late Pleistocene and Holocene activities, requires geochronological tools other than radiometric dating techniques, which are based on parent-daughter isotope pairs with geologically long half-lives, such as 40Ar/39Ar dating. In this study, we conducted quartz optically stimulated luminescence (OSL) dating of palaeosols intercalated with basaltic lava flows and scoria deposits, presumably ejected from three monogenetic volcanoes (Cheoreum, Darangsh and Dunjibong volcanoes) in the northeastern part of the island. Quartz extracts from the palaeosols had moderate to dim sensitivity to optical stimulation, but several prerequisite tests of the measurement protocols for equivalent dose estimation were successful. The coarse (63–250 μm) and fine (4–11 μm) quartz fractions yielded continuous wave (CW)-OSL ages of 19.9–7.0 and 18.6–6.7 ka, respectively, both of which were broadly consistent with the radiocarbon dates (10245–7440 Cal yr BP). These ages indicate that lava flows and scoria deposits covering the palaeosols are younger than ~7 ka. The pulsed OSL signals in coarse quartz extracts from the palaeosols suffered from poor counting statistics, thus it is recommended that they not be applied solely (i.e., without any other absolute ages to compare) to dating dim samples. From one of the sites investigated here, the averaged sedimentation rate of the palaeosol is estimated to be ~0.05 mm/yr, based on stratigraphically consistent CW-OSL ages. The OSL ages presented in this paper, together with previous OSL and radiocarbon ages, confirm that Jeju volcanic island was regionally active during the Holocene.

Mineralogy of a New Occurrence of Lamprophyre Dyke from the Saurashtra Peninsula of Gujarat, Northwest Deccan Trap, India

ABSTRACT A new occurrence of lamprophyre dyke is being reported from Saurashtra peninsula of Gujarat, north-western Deccan trap. The paper presents and discusses the detail mineralogy, field setting and whole rock major and trace element geochemistry. The newly discovered lamprophyre dyke intruded into the porphyritic olivine basalt flow. It is E-W trending dyke paralleling with Narmada rift trend, 200 m long and ~1 m in width. Petrographic observations suggest the lamprophyre dyke is fine grained, porphyritic and shows typical panidiomoprphic texture. It contains phenocrysts of euhedral to subhedral amphibole (kaersutite), subhedral clinopyroxenes (diposide augite) and olivine pseudomorph enveloped in the groundmass composed of plagioclase, clinopyroxene and amphibole with carbonate, Ti-magnetite, zeolite and apatite occurring as accessory phases. Few clinopyroxenes show reaction and embayed margins thus indicating xenocrystic orgin. The normative chemistry of the lamprophyre dyke suggests alkaline nature with significant amount of olivine and nepheline. Whole rock geochemical data shows comparatively higher SiO2 than the other lamprophyres of the Deccan and similar to the MgO depleted lamprophyres of the Chhota Udepur alkaline province. The primitive mantle normalized multi-elements pattern shows absence of any high field strength element (HFSE) anomaly. Studied dyke displays more similarity with the earlier reported pre- Deccan alkaline lamprophyres and more particular with the Chhota Udepur lamprophyre. Occurrence of this new lamprophyre dyke from the Saurashtra region extends and confirms the presence of lamprophyre magmatism in the far north-western part of the Deccan.

Volcanology and inflation structures of an extensive basaltic lava flow in the Payenia Volcanic Province, extra-Andean back arc of Argentina

The El Puesto lava flow is located in the Payenia Volcanic Province (central-western Argentina), has a length of 70 km and is Middle Pleistocene in age (0.200±0.027 Ma). The flow shows a P-type pahoehoe structure and exhibits several inflation structures, mainly tumuli and also inflation ridges and lava rises. Lava rise pits and radial or annular clefts are common features associated with inflation structures. The gentle slope on which the flow moved (≈0.5°) allowed the lateral coalescence of lobes at the flow front and the development of an external rigid crust that insulated the liquid core. Lava tunnels are frequent and the lava tunnel named “Cueva de Halada” which is located at its medium portion is the best example of a drainage master tube which formed from the cooling of the crust around a stable inflated flow. Tumuli alignments and long inflation ridges reveal the existence of larger tunnels within the flow. Inflation structures may occur in high concentration belts that converge on a single main belt which is assigned to an anastomosed network of internal flow pathways within the main lava body. The development of inflation structures and lava tunnels require low to moderate effusion rates. An average lava supply rate of 1.8x10-4 m3/s and an inflation time of about 15 days were estimated for an average tumulus of this flow. A high and sustained supply of low viscosity lava (η’=1550 - 483 Pa s) was inferred that initially generated a sheet flow of great areal extension. The reduction in effusion rates could then allowed the development of tunnels that carried lava to the distal fronts, generating localized inflation phenomena throughout the lava flow.