Period Of Volcanism Research Articles

Substantial gas enrichment in shales influenced by volcanism during the Ordovician–Silurian transition

The substantial gas enrichment in shales of the Ordovician–Silurian transition is associated with the development of the organic matter (OM)-rich source rock. While organic matter enrichment has been linked to intensive volcanism, it remains a challenge to precisely evaluate the impact of the volcanism on substantial gas enrichment containing the largest gas storage capacity. This study focused on consecutive borehole shale samples from the Wufeng–Longmaxi formations during the Ordovician–Silurian transition in southern China. We conducted a comprehensive analysis, integrating the major geological volcanism with high-resolution analysis, including QEMSCAN, argon-ion SEM, thin-section examination, XRD mineralogy, TOC, Hg concentration, petrophysical properties and nanopore structure analysis (low-pressure CO2/N2 gas adsorption, helium porosity and permeability). The results link the significant shale gas enrichment in Wufeng–Longmaxi formations to intensive volcanism across the Ordovician–Silurian transition. We identified the most favorable shale intervals in the lower Longmaxi Formation, aligning with the peak period of volcanism. This period showed synchronous spikes in Hg, Hg/TOC, and TOC contents. Shale deposited during this favorable paleoenvironment exhibited the highest values of TOC, porosity, permeability, specific surface area, pore volume, and maximum gas adsorption capacity, leading to the largest amount of gas content and substantial gas enrichment. Our work, therefore, provides new insights into identifying the most favorable shale gas resources. This knowledge assists in accurate predictions of the stratigraphic ‘sweet spot’ intervals for large shale gas storage capacity, providing crucial information for engineering explorations and developments in shale formations.

Nature and Origin of a Massive Sulfide Occurrence in the Karrat Group: Evidence for Paleoproterozoic VMS Mineralization in Central West Greenland

Abstract Mafic volcanic rocks of the Kangilleq Formation of the Paleoproterozoic Karrat Group host volcanogenic massive sulfide (VMS) mineralization in the area of central Kangiusap Kuua, central West Greenland. The mafic volcanic rocks display evidence of subaqueous, effusive eruption and redeposition by mass debris flows generated along fault scarps on the sea floor. A zone of semiconformable quartz alteration and disconformable chlorite alteration within hydrothermal breccias and mafic tuff breccias near the top of the volcanic sequence is interpreted to reflect a synvolcanic hydrothermal system. Conformable, massive to semimassive, and discordant, stringer-style sulfide mineralization is hosted within the quartz- and chlorite-altered volcanic rocks. The massive to semimassive sulfide mineralization is ~10 m thick and crops out along strike for ~2,000 m. The stringer zone is ≤10 m thick with individual sulfide stringers ranging in width from 5 to 90 cm. All sulfide zones are dominated by coarse pyrrhotite and pyrite, with trace amounts of sphalerite and chalcopyrite. The pillow lavas are subalkaline with geochemical characteristics typical of modern transitional to tholeiitic mid-ocean ridge or back-arc basin basalt. Trace element and Nd isotope data suggest that these lavas erupted in an epicratonic, back-arc basin. Characteristics of the host rocks indicate a period of localized rifting, volcanism, and VMS formation during genesis of the Karrat Group, which is dominated by siliciclastic rocks.

GRAIL gravity gradients evidence for a potential lava tube at Marius Hills on the moon

Lunar lava tubes are underground channels formed by volcanic eruptions and lava flow during the Moon's volcanic period. Intact lunar lava tubes have the potential to serve as secure shelters for humans and facilities, as they provide a safe and pristine environment with a constant temperature. In this study, the lunar gravity field model derived from NASA's GRAIL mission data is analyzed for detecting of an empty lava tube in the Marius Hills region. Compared to the classical Hessian matrix, the gravitational gradient method used in this study is capable of detecting anomaly signals both in the north-south and east-west directions. Using this method, we detect a north-south anomaly signal in the Marius Hills region, located at 57.5°W and 14.3°N, indicating a negative density anomaly near the known lunar sinuous rilles and the Marius Hills Hole skylight. This suggests the presence of a potential lava tube, which is confirmed by the results of the forward method and lunar radar echo data. The extent of the lava tube is estimated with the forward method to be ∼60 km in length, ∼9 km in width, ∼605 m in depth, and ∼55 m in height. Furthermore, the gradient features indicate that the skylight located on the eastern side of the lava tube could serve as an entry to the main part of the lava tube.

Flare Up of Hot-Dry-Reduced Ignimbrites Related to Extension in the Cascades Arc: The Deschutes Formation, Central Oregon

Abstract Ignimbrite flare-ups are rare periods of intense silicic volcanism during which the pyroclastic volume and eruptive frequency is more than an order of magnitude higher than background activity. Investigating the compositional differences between flare-up and steady-state magmas provides critical constraints on the petrogenetic causes for the event and can offer unique opportunities to investigate the role of large-scale tectonic or geodynamic processes in arc magmatism. In this study, we focus on the bimodal Deschutes Formation ignimbrite flare-up of Central Oregon, which erupted unusually high volumes of pyroclastic material 6.25–5.45 Ma from a new axis of volcanism in the Cascades arc. This episode is marked by increased eruption rates and eruption of more silicic compositions relative to the Quaternary Cascade arc, which rarely erupts rhyolites. Ignimbrites are crystal-poor (<10%) dacite to rhyolites (mostly 65–77 wt.% SiO2) with anhydrous mineral assemblages and higher FeO/MgO, Y, Eu/Eu*, MREE and Zr/Sr, indicating drier magmatic evolution compared to the Quaternary arc, and are more similar to those from the rear-arc High Lava Plains (HLP) province that lies to the east. Magnetite-ilmenite oxybarometry indicates that Deschutes Formation felsic magmas tend to be hotter and more reduced (NNO-1 to NNO) than the Quaternary arc (NNO to NNO + 1.5). Rhyolite-MELTS geobarometry suggests complex storage of diverse Deschutes Formation magmas within the shallow crust (50–250 MPa), and the common co-eruption of multiple plagioclase populations, pumice compositions, and compositionally banded pumice suggest variable degrees of mixing and mingling of distinct magmas. Deschutes magmas also have low δ18Oplagioclase values that indicate partial melting and assimilation of hydrothermally altered shallow crust. Trace element systematics and rhyolite-MELTS modeling suggests that felsic pumice cannot be produced by simple fractionation of co-erupted mafic pumice or basaltic lavas, and requires a crustal melting origin, and trace elements and Pb isotopes suggest that young mafic crust may have been the primary protolith. We suggest that partial melting produced low-Si rhyolite melt (~72 wt.%) that acted as both a parent for the most evolved rhyolites, and as a mixing endmember to create the dacite to rhyodacite magmas with heterogenous plagioclase populations. Unlike the predominantly calc-alkaline basalts erupted in the Quaternary Cascade arc, Deschutes Formation primary basalts are mostly low-K tholeiites, indicative of decompression melting. These are similar to the compositions erupted during a contemporaneous pulse of low-K tholeiite volcanism across the whole HLP that reached into the Cascades rear-arc. We suggest that intra-arc extension focused decompression melts from the back-arc into the arc and that tensional stresses allowed this high flux of hot-dry-reduced basalt throughout the crustal column, causing partial melting of mafic protoliths and the production of hot-dry-reduced rhyolite melts. Depletion of incompatible elements in successive rhyolites implies progressive depletion in fertility of the protolith. Extension also allowed for the establishment of a robust hydrothermal system, and assimilation of hydrothermally-altered rocks by magmas residing in a shallow, complex storage network lead to low δ18O melts. Our findings suggest the integral role that extensional tectonics played in producing an unusual ignimbrite flare-up of hot-dry-reduced rhyolite magmas that are atypical of the Cascades arc and may be an important contributor to flare-ups at arcs worldwide.

Magmatic addition rates differentiate periods of steady-state versus flare-up magmatism in the Central Andean arc

Cordilleran arcs are built by long periods of steady-state magmatism punctuated by transient high-flux magmatic episodes or flare-ups. Such flare-ups, manifested as periods of prodigious silicic volcanism and magmatism, result from geodynamic perturbations that cause elevated rates of magma addition to the crust. Questions remain, however, about how magmatic addition rates quantitatively compare between steady-state and flare-up modes of arc magmatism, and how long after the major geodynamic perturbation the flare-up begins. Here, we compute new estimates of erupted volumes over the last 35 Myr for the 22.5–29°S segment of the Central Andes based on a new volcanic geospatial database. These yield magmatic addition rates at least an order of magnitude higher during flare-up compared to steady-state conditions. A lag time of ~8–12 Myr between ocean ridge subduction (the major geodynamic perturbation in the Central Andean arc) and the onset of flare-up conditions is estimated.

Fresh volcanic aerosols injected in the atmosphere during the volcano eruptive activity at the Cumbre Vieja area (La Palma, Canary Islands): Temporal evolution and vertical impact

For the first time in fifty years, the Cumbre Vieja volcanic area (La Palma, Canary Islands, Spain) erupted on September 19, 2021, giving birth to a new volcano. Fresh volcanic aerosols were continuously injected into the troposphere at different height levels, decreasing with time until the end of December 2021 (15 weeks duration). A wide set of different instrumentation was deployed all over the Island in order to evaluate the effects of the volcanic plumes on the atmosphere and the air quality. For the first time, a long-term study of the relative mass contribution and vertical impact of the volcanic components, ash and non-ash particles separately, during the eruptive activity was carried out in this work. In particular, a polarized Micro-Pulse Lidar (P-MPL) was deployed at Tazacorte (at around 8 km west from the volcano) in 24/7 operation from October 17, 2021 until the end of the volcano activity (11 weeks) for vertical monitoring of the volcanic particles. First, a statistical study of the mass conversion factors for mass concentration estimation of the volcanic (ash and non-ash) particles was performed by using the AERONET sun/sky-photometer dataset at Fuencaliente (at around 18 km south from the volcano). A representative mass conversion factor was obtained for ash and non-ash particles: 1.89 ± 0.53 and 0.31 ± 0.06 g m−2, respectively, with no dependence on time and optical depth. Second, these factors were used to calculate the ash and non-ash mass concentrations from P-MPL observations. Ash particles dominated 11% of the time and mostly until week 3 (i.e. week 7 from the volcanic eruption). Their mass concentration decreased by one order of magnitude: the relative ash mass contribution was 73 ± 18% with a total mass loading of 566 ± 281 mg m−2 at week 1, reducing gradually down to 38 ± 32% and 120 ± 49 mg m−2, respectively, at week 11. Layer-to-layer, it decreased with increasing layer-height; no ash was detected above 4 km at the end of the volcanic period. Third, in order to analyse the potential AERONET underestimation of the coarse mass conversion factor due to the 15 μm cutoff effect in the AERONET retrieval, two worst-case-scenarios (WCS) were examined, representing aged-like ash particles (WCS1, 4-μm radius) and fresh-like (WCS2, 10-μm radius). For both scenarios, the mass concentration of the volcanic plumes exceeded the first contamination level (>200 μg m−3, as defined by the UK Meteorological Office) up to 5–6 km height mostly during week 1 and up to 1–2 km until week 9. The extreme contamination level (>2000 μg m−3, aircraft flight limitations) was only exceeded from week 1 to week 6 under WCS2 conditions. This work infers a new long-term insight on the volcanic matter injected in the atmosphere with relevance for Air Quality issues and air traffic safety policies.

Geologic setting of the Pueblo Viejo Au–Ag–Cu-(Zn) mining district, Dominican Republic - Links to volcanic domes and volcanogenic massive sulfide mineralization

Geologic mapping at the Pueblo Viejo Au–Ag–Cu-(Zn) mine, Dominican Republic, and across the surrounding Pueblo Viejo district, reveals the geologic setting at the time of mineralization. Ore deposits consisting of disseminated sulfide, bedded massive sulfide, and high-grade massive sulfide veins formed (∼112 Ma) in a volcanic dome field during a period of extension across an emergent, intraoceanic island arc. Geologic maps reveal the links between volcanic domes and structure, hydrothermal alteration and mineralization, and ore deposits and gossans. Whole-rock geochemical data establish a tholeiitic signature for Early Cretaceous magmatism including the volcanic dome field. Northerly-striking, high-angle faults controlled the emplacement of volcanic domes and served as feeders for hydrothermal solutions. The largest ore deposits formed in a subsiding, marginal marine, volcano-sedimentary basin. Stacked intervals of bedded massive sulfide mineralization indicate that the basin was subsiding while mineralization was underway. Calc-alkaline dikes and plutons in the Pueblo Viejo district intruded at ∼88 Ma, well after mineralization had ended. Low-angle reverse faulting, greenschist facies metamorphism, and milky quartz veining and replacement were also Late Cretaceous (post-mineral) events. Ore deposits in the Pueblo Viejo district are best described as hybrid epithermal – VMS deposits, formed during a period of Early Cretaceous extension, tholeiitic volcanism, and volcanic dome emplacement in a shallow, marginal marine environment. Exploration for Au–Ag–Cu-(Zn) deposits like Pueblo Viejo should focus on intraoceanic island arcs, tholeiitic volcanic dome fields, extensional volcano-sedimentary basins, and VMS mineral occurrences.

Organic Carbon Burial With Reactive Iron Across Global Environments

AbstractPreservation of organic carbon (OC) in marine and terrestrial deposits is enhanced by bonding with reactive iron (FeR). Association of OC with FeR (OC‐FeR) provides physical protection and hinders microbiological degradation. Roughly 20% of all OC stored in unconsolidated marine sediments and 40% of all OC present in Quaternary terrestrial deposits is preserved as OC‐FeR, but this value varies from 10% to 80% across global depositional environments. Here, we provide a new assessment of global OC‐FeR burial rates in both marine and terrestrial environments, using published estimates of OC associated with FeR, carbon burial, and probabilistic modeling. We estimate the marine OC‐FeR sink between 31 and 70 Mt C yr−1 (average 52 Mt C yr−1), and the terrestrial OC‐FeR sink at between 146 and 917 Mt C yr−1 (average 446 Mt C yr−1). In marine environments, continental shelves (average 17 Mt C yr−1) and deltaic/estuarine environments (average 11 Mg C yr−1) are the primary settings of OC‐FeR burial. On land, croplands (279 Mt C yr−1) and grasslands (121 Mt C yr−1) dominate the OC‐FeR burial budget. Changes in the Earth system through geological time impact the OC‐FeR pools, particularly in marine settings. For example, periods of intense explosive volcanism may lead to increased net OC‐FeR burial in marine sediments. Our work highlights the importance of OC‐FeR in marine carbon burial and demonstrates how OC‐FeR burial rates may be an order of magnitude greater in terrestrial environments, but here OC‐FeR stocks are most sensitive to the anthropogenic impacts of climatic change.

Emeishan felsic volcanism lasted until the Changhsingian? New evidence from volcanic ash in the northern South China block

The main episode of volcanism in the Emeishan large igneous province (ELIP) occurred ca. 260 Ma. However, the ending period of volcanism has not been well established. We present the SHRIMP and LA-ICP-MS zircon U–Pb geochronology, zircon Hf isotopes, and geochemistry of ashes in the Shuanghe and Xintang sections, northern South China block (SCB). The results show that the Al/Ti ratio and high field strength elements content of Changsingian ashes (253.5–253.0 Ma) are consistent with those of alkaline ashes in the Wuchiapingian coal measure of the western SCB. These volcanic rocks contain zircon grains with high εHf(t) values (mean + 2.53) and low Th/Nb ratios (mostly < 10), consistent with the origin from mantle-derived magmas within the plate. These magmas are interpreted to be related to the Emeishan felsic volcanism. Furthermore, the zircon grains separated from ashes at the Permian-Triassic boundary (PTB) (ca. 252 Ma) and the upper Longtan Formation (256.8 Ma), have low εHf(t) values (mean − 7.04) and high Th/Nb ratios (mostly greater than 40). These features are consistent with a magmatic arc source, and the arc was likely related to the subduction of Paleo-Tethys in the middle Wuchiapingian. Therefore, we propose that the waning stage of the Emeishan felsic volcanism continued until the middle Changhsingian (253 Ma), which is not a previously suggested older age of 257 Ma. However, the ashes near the PTB on the SCB should have come from the magmatic arc. The Emeishan volcanism may not be the main cause of the PTB extinction.

Isotopic evidence for changes in the mercury and zinc cycles during Oceanic Anoxic Event 2 in the northwestern Tethys, Austria

The Cenomanian-Turonian Oceanic Anoxic Event 2 (OAE 2, ca. 94 Ma) was one of the most extreme carbon cycle and climatic perturbations of the Phanerozoic Eon. Widespread deposition of organic-rich shales during OAE 2 has been attributed to a rapid rise in atmospheric CO2, global heating, and marine anoxia triggered by intense large igneous province (LIP) volcanism. Here, we present new Hg and Zn elemental and isotopic analyses from samples spanning OAE 2 in a hemipelagic section from Rehkogelgraben, Austria, which was part of the north-western Tethys. We compare our data to existing records from a range of sites to constrain the relative timing, magnitude and geographic extent of the perturbation. We find a prominent Hg concentration peak and an overall positive Δ199Hg excursion, with no correlation between Hg content and organic matter (OM), Mn-Fe-oxyhydroxides, and/or clay minerals. We interpret this to indicate a terrestrial volcanic origin of Hg. The Hg excursion is coincident with an osmium (Os) isotope excursion, and together, this supports a global period of intense LIP volcanism. The δ66Zn record from the Rehkogelgraben section decreases abruptly by ~0.5‰ prior to the onset of OAE 2, a change recorded consistently among all reference sections. Combined with the Hg data, we interpret this to result from isotopically light Zn sourced from LIP activity. However, the second negative excursion in δ66Zn during the Plenus Cold Event (PCE), which is recorded in the proto-North Atlantic and adjacent areas and has been attributed to Zn released from OM during re‑oxygenation, is not recorded in this section. We suggest that the cool, oxygenated deep water mass did not invade the Penninic Ocean in the northwestern Tethys. Alternatively, this excursion could be missing in our section due to the presence of carbonate-free sediments during the PCE. After the PCE, the positive excursion in δ66Zn recorded in all sections reveals a recovery of the atmosphere-ocean system. Our findings highlight the significance of spatial and temporal variations in Hg and Zn isotopes during OAE 2.

Constraining marine anoxia under the extremely oxygenated Permian atmosphere using uranium isotopes in calcitic brachiopods and marine carbonates

The redox chemistry change in ancient oceans has profoundly shaped the evolutionary trajectories of animals. Uranium isotopes (δ238U) in marine carbonate sediments have widely been used to place quantitative constraints on the oxygenation state of the oceans through geological history. However, syndepositional and post-depositional diagenesis impose a positive and variable δ238U offset in the carbonate sediments relative to contemporaneous seawater, leaving uncertainties on quantification of anoxic seafloor areas in the past. Studies from modern settings suggest that Low-Magnesium Calcite (LMC) in articulate brachiopod shells are diagenetic resistant materials that may faithfully record the δ238U value of ancient seawater. However, this notation has not yet been validated in geological records. Here, we test this hypothesis by analyzing paired brachiopod shells and the host carbonate rocks from several Permian stratigraphic successions in South China. Forty-nine articulate brachiopod fossil shells and twenty-six host carbonate rocks from the Early Permian to the Late Permian were investigated. We performed a rigorous screening to monitor the diagenetic process that might have influenced the δ238U of the shells using a scanning electronic microscope, cathodoluminescent microscopy, and major and trace elements. We estimate that 53% of the shells preserve the primary seawater δ238U signals. Using the screened δ238U values of shells and bulk carbonates, we suggest there were four episodes of expanding marine anoxia during the Permian that are temporally coincident with periods of volcanism. The first two oceanic anoxic events occurred at a time when atmospheric pO2 levels were predicted to be higher than today. The last two events are temporally coincident or immediately preceding the end-Guadalupian and the end-Permian mass extinctions, respectively, confirming previous suggestions that marine anoxia was a critical factor in driving marine animal extinctions.

Climate/ocean dynamics and possible atmospheric mercury depletion events during the Late Sturtian deglaciation

Late Sturtian deglaciation, resulting in development of oxic conditions and deposition of banded iron formations (BIFs), was a pivotal period in evolution of global ocean chemistry. The driving forces behind these Neoproterozoic BIFs and climatic/oceanic changes remain controversial, however. Here we use mercury (Hg) to elucidate the cause(s) of these dramatic changes to earth biogeochemical cycles. In the Guandong section, South China, we observe anomalously low Hg concentrations and Hg to total organic carbon (Hg/TOC) ratios in the Late Sturtian BIFs, suggesting a period of limited continental volcanism during the Late Sturtian deglaciation. We hypothesize that instead of a major LIP event, long-lasting and subdued submarine volcanism slowly increased atmospheric CO2 to trigger the greenhouse climate that caused deglaciation. Negative shifts of Δ199Hg in the studied section suggest that deglaciation was accompanied by atmospheric mercury depletion events (AMDEs). Extremely low TOC levels in the studied section may suggest low oceanic productivity, and this may support a non-biological cause of oceanic oxygenation during the Late Sturtian glaciation, through the release of photochemically produced H2O2 in melting glaciers.

The Surprising Greenhouse Gas That Caused Volcanic Summer

Extended periods of volcanism known as flood basalt eruptions lead to volcanic winters, which are often followed by an extended period of warming. But it was more than just carbon dioxide that warmed the globe.

Implications for carbonate mass-wasting complexes induced by volcanism from Upper Cretaceous Austin Chalk strata in the Maverick Basin and San Marcos Arch areas of south-central Texas, USA

The upper part of the Austin Chalk (AC-B1 subunit), as seen in a series of cores in areas of the Maverick Basin and San Marcos Arch in south-central Texas, contains a stratigraphic section of mass-wasting complexes resulting from penecontemporaneous volcanic-induced gravity-flow deposition. The types of gravity flows are variable in composition depending on sources of volcanic or carbonate sediments available and position of initiation of the flow on the volcanic mounds.The gravity flows occurred in a deeper-water drowned-shelf setting where argillaceous chalk-rich sediments were being deposited. The mafic volcanoes are part of the Balcones Igneous Province that was active during very Late Santonian to Early Campanian time. Some of the volcanoes formed mounds that created elevated areas in the photic zone or above sea level. In the shallow-water photic zones, carbonate factories formed as atolls and shoals. These carbonate factories were a source for shallow-water allochems, and along with the volcanic debris, constituted gravity flows that were likely triggered by volcanic-associated earthquakes. As gravity flows moved downslope, they eroded deeper-water, bottom sediments, which were incorporated into the flows. The two major transport mechanisms were debris flows and hyperconcentrated-density flows.The debrites appear as two types: those dominated by mud clasts and those dominated by cm-scale skeletal fragments such as inoceramid or disarticulated oyster shells. The clasts in the mud-clast-dominant debrites range in rigidity from very soft (i.e., easily deformed) to firm (i.e., resist deformation and preserved burrows). Since the mud clasts are composed of planktic foraminifers and calcispheres in a coccolith-hash matrix, they are assumed to have originated from the deeper flanks of the volcanic mounds. The rigidity of the mud clasts are likely related to initial (i.e., early) depth of burial of the unlithified mud.The skeletal-dominated debrites with disarticulated oyster shells appear to have been sourced from the upper slopes of the volcanic mounds and underwent little abrasion during debris-flow transportation. The inoceramid-fragment-dominated debrites may have been initiated lower on the volcanic mounds where these low-oxygen-tolerating bivalves would dominate the macrofauna.The hyperconcentrated-density-flow deposits are grain-rich in skeletal fragments with a mixture of shallow- and deep-water biota, suggesting some direct sourcing from shallow-water shoals and later incorporation of deeper-water sediments from the flanks of the volcanic mounds. These hyperconcentrated-density-flow deposits are positioned close to the base of the mounds.This distinct stratigraphic interval varies greatly from the rest of the Austin Chalk section in the Maverick Basin and San Marcos Arch areas. Deeper-water chalk deposition was the general background environmental setting, but it was disrupted by this period of volcanism that initiated numerous mass-wasting complexes punctuating the deeper-water chalk deposits. Within the gravity-flow deposits, volcanic components such as lapilli, ash, and glauconite-altered volcanic grains occurred along with both shallow- and deep-water carbonate allochems. Investigation of this stratigraphic unit within the Austin Chalk provides an analog for mass-wasting complexes associated with active volcanism and explains the variations in composition of the gravity-flow deposits.

Petrogenesis and tectonic implication of the Late Triassic A1-type alkaline volcanics from the Xiangride area, eastern segment of the East Kunlun Orogen (China)

A series of alkaline volcanic rocks with pronounced Nb contents, were identified in the Xiangride area of the eastern segment of the East Kunlun Orogen (EKO). An integrated investigation, involving mineral composition, whole-rock elemental and Sr − Nd isotopic geochemistry and zircon UPb dating and LuHf isotopes was conducted on the alkaline volcanics as well as a spatially associated andesite. The Xiangride alkaline volcanics are mainly composed of trachyandesite, quartz trachyte, rhyolitic ignimbrite, and rhyolite. Zircon UPb dating reveals that the Xiangride alkaline volcanics were formed at 212–209 Ma, apparently postdating the associated andesite (231 Ma) and the volcanic rocks of the Elashan Formation (232–218 Ma) as well as the widespread I-type granitoids in the eastern EKO (265–217 Ma). The Xiangride alkaline volcanics display variably high total alkali, (Zr + Nb + Ce + Y) content, Ga/Al ratio, and zircon saturation temperature, typical of A1-type granite. They are also characterized by high HFSEs and LREEs contents, along with strong depletion of Ba, Sr, P, Ti and Eu, similar to the OIB-type alkaline trachytic to rhyolitic rocks and obviously different from the coeval arc-related magmatic rocks in the eastern EKO. In contrast, the associated Xiangride andesite shows similar chemical signatures to the Elashan andesites, resembling the IAB-series rocks. Notably, the Xiangride alkaline volcanics all contain high Nb contents (up to 120 ppm) as compared to other Triassic granitoids and volcanic equivalents of the EKO (Nb = 5–60 ppm). The dominate Nb-hosts include primary titanite and rutile and secondary ilmenorutile and euxenite, indicating both contributions of magmatic and hydrothermal processes. The Xiangride alkaline volcanics yield slightly higher ɛNd(t) values (−3.7 to −2.4) and zircon ɛHf(t) values (−0.6 to 2.6) than the associated Xiangride andesites (−4.2 to −4.1 and − 4.7 to −3.5, respectively) and the widespread Elashan andesites (−6.4 to −3.4 and − 7.4 to −2.0, respectively).Based on our results, we suggest that the Xiangride A1-type alkaline volcanics were likely generated via partial melting of a metasomatized lithospheric mantle followed by prolonged fractional crystallization and minor crustal contamination, whereas the associated Xiangride andesite might have originated from partial melting of the juvenile mafic lower crust, similar to the Elashan andesites. We propose that the Xiangride A1-type alkaline volcanics may represent a within-plate anorogenic extensional magmatic pulse, whereas the associated andesite and the Elashan andesites recorded an earlier period of post-collisional volcanism. Therefore, the Triassic continental collision orogeny of the EKO is likely to have ended before ca. 212 Ma, followed by a transition from post-collisional to intracontinental extension.

Geology, zircon U–Pb dating and εHf data for the Julie greenstone belt and associated rocks in NW Ghana: Implications for Birimian-to-Tarkwaian correlation and crustal evolution

The Julie greenstone belt of NW Ghana is characterized by a predominately volcanic succession of basalt with minor rhyolite and a mainly sedimentary succession of siltstone, shale and greywacke. The first unit is correlated with the Sefwi Group, and the second with the Kumasi Group of the Birimian Supergroup. The volcano-sedimentary succession is separated by a sheared unconformable contact from clastic sedimentary rocks, including sandstones and conglomerates, correlated with the Tarkwaian Group of Ghana. The supracrustal rocks are intruded by various granitoids. Two major deformational events have been identified. A regional N–S shortening event (D1) affected rocks of the Birimian Supergroup and associated granitoids. A later NW-SE shortening event (D2) affected both the Birimian and Tarkwaian units. Rocks have undergone regional greenschist to amphibolite facies metamorphism. Peak metamorphism took place during D2.Several samples of igneous and sedimentary rocks were subjected to U–Pb zircon dating and Lu–Hf isotope analyses. All zircons provided Paleoproterozoic ages in the range of 2112 ± 5 to 2337 ± 11 Ma, which is consistent with existing zircon ages from volcanic and plutonic rocks in various parts of Ghana, and other Birimian and Tarkwaian terrains within the West African Craton. Rhyolite of the Sefwi Group has an age of 2182 ± 3 Ma and marks a period of explosive felsic volcanism. Detrital zircon ages from greywacke of the Kumasi Group yield a maximum depositional age of 2158 ± 4 Ma. This age compares well to a maximum depositional age of 2154 ± 2 Ma defined in southern Ghana. Zircon ages from Tarkwaian sandstones indicate a maximum depositional age of 2135 ± 5 Ma, which is in accord with similar depositional ages for the Tarkwaian in SW Ghana and Houndé in Burkina-Faso. Thus the Tarkwaian rocks from the Julie greenstone belt area are correlative to those in SW Ghana and elsewhere. Detrital zircon ages point to a derivation from erosion of felsic volcanic and plutonic rocks within the Paleoproterozoic terrain. Detrital zircon ages of Birimian and Tarkwaian rocks show a ∼70% overlap. This suggest that the Tarkwaian detritus may have been derived from reworking of older Birimian rocks. Alternatively, both groups sampled a similar source but were deposited at different periods. Dated granitoids include a 2209 ± 5 Ma phase and a post-Tarkwaian intrusion of 2112 ± 5 Ma in age. The older granitoid is in sheared contact with basalt, and both share similar D1 structures.The ϵHf values for zircon grains from analyzed rocks are in the range of −1.78 to +6.37, in agreement with most Hf isotopic signatures currently available from the Paleoproterozoic Baoulé-Mossi Domain, and indicate crust components with apparent mantle derivation ages averaging c. 2300 Ma and ranging from c. 2100 to c. 2500 Ma.

Dike Propagation During Global Contraction: Making Sense of Conflicting Stress Histories on Mercury

Thrust fault-related landforms, smooth plains units, and impact craters and basins have all been observed on the surface of Mercury. While tectonic landforms point to a long-lived history of global cooling and contraction, smooth plains units have been inferred to represent more punctuated periods of effusive volcanism. The timings of these processes are inferred through impact cratering records to have overlapped, yet the stress regimes implied by the processes are contradictory. Effusive volcanism on Mercury is believed to have produced flood basalts through dikes, the propagation of which is dependent on being able to open and fill vertical tensile cracks when horizontal stresses are small. On the contrary, thrust faults propagate when at least one horizontal stress is very large relative to the vertical compressive stress. We made sense of conflicting stress regimes through modeling with frictional faulting theory and Earth analogue work. Frictional faulting theory equations predict that the minimum and maximum principal stresses have a predictable relationship when thrust faulting is observed. The Griffith Criterion and Kirsch equations similarly predict a relationship between these stresses when tensile fractures are observed. Together, both sets of equations limit the range of stresses possible when dikes and thrusts are observed and permitted us to calculate deviatoric stresses for regions of Earth and Mercury. Deviatoric stress was applied to test a physical model for dike propagation distance in the horizontally compressive stress regime of the Columbia River Flood Basalt Province, an Earth analogue for Borealis Planitia, the northern smooth plains, of Mercury. By confirming that dike propagation distances from sources observed in the province can be generated with the physical model, we confidently apply the model to confirm that dikes on Mercury can propagate in a horizontally compressive stress regime and calculate the depth to the source for the plains materials. Results imply that dikes could travel from ∼89 km depth to bring material from deep within the lithosphere to the surface, and that Mercury’s lithosphere is mechanically layered, with only the uppermost layer being weak.

Records of organic carbon isotopic composition (δ13Corg) and volcanism linked to changes in atmospheric pCO2 and climate during the Late Paleozoic Icehouse

The Late Paleozoic Icehouse (LPI) provides a deep-time perspective for climate-glaciation-environment coevolution and offers potential insights into future climatic and environmental predictions. Most previous studies attributed climatic and environmental changes during the LPI to perturbations of atmospheric p CO 2 , yet the driving mechanism for p CO 2 changes remains controversial. Although the environmental and climatic effects of volcanism have been recognized, its high-resolution geological records and links with changes in atmospheric p CO 2 and climate are rarely reported. We address this by investigating volcanic records (indicating by Hg/TOC ratio) and organic carbon isotope composition (δ 13 C org ) of paralic strata from the latest Gzhelian to Roadian stages in the North China Platform (NCP), China. Four δ 13 C org negative excursions (latest Gzhelian, middle Artinskian, middle Kungurian and latest Kungurian stages) and two positive isotope plateaus (Asselian and Roadian stages) are identified. Each of the four negative isotope excursions coincides with peaks in Hg/TOC ratio and rising periods of atmospheric p CO 2 . The co-occurrence of the negative isotope excursions and Hg/TOC peaks suggests volcanic origins for both the Hg and perturbations in the carbon cycle. The two positive isotope plateaus correspond to the glacial P1 and the climatic transition period to glacial P3 in Australia and to weak periods of volcanism. Our results provide clear insights into volcanism driving perturbations in environment, climate and the carbon cycle in deep time, and support the assertion that volcanism was sufficient to shift global climates from glacial to interglacial through greenhouse gas emissions during the LPI. • Early Permian continental δ 13 C org record presented from North China Platform. • Negative δ 13 C org excursions and Hg/TOC ratios identify episodes of volcanism. • Volcanism is important driver for changes in atmospheric p CO 2 and climate. • Two positive δ 13 C org plateaus correspond to P1 and P3 glaciations in Australia.

Effusive and explosive eruptive history of the Ilopango caldera complex, El Salvador

The Ilopango caldera in central El Salvador is the source of at least 13 voluminous rhyolitic eruptions over the last two million years, including the 431 CE Tierra Blanca Joven (TBJ) which blanketed areas inhabited by the Mayan civilization, and the 1.707 ± 0.022 Ma Olocuilta ignimbrite, one of the largest Quaternary eruptions in Central America. These rhyolitic deposits have been the subject of numerous detailed studies. In contrast, the abundant effusive and monogenetic eruptions at Ilopango have not been studied. These small-volume eruptions range from basalt to rhyolite (49.7–72.8 wt% SiO2) and occur both within the caldera margin and on the south flank of the Antiguo Ilopango stratovolcano. Here we present 28 new 40Ar/39Ar ages and geochemical data from domes, lavas, dikes, and ignimbrites from the Ilopango caldera and the Antiguo Ilopango stratovolcano as well as from adjacent volcanic systems. Collectively, these new data are used to construct a revised eruptive sequence for the Ilopango caldera that spans almost three million years. The new chronostratigraphy for Ilopango is highlighted by two volcanic cycles. Each cycle starts with rhyolite dome effusions, and then a prolonged period of explosive rhyolitic volcanism followed by a ~120–175 kyr period of less evolved eruptions. The spatio- and chemo-temporal patterns in the Ilopango eruptive sequence are similar in many ways to those observed in other well-studied Quaternary silicic centers.

Intermittent volcanic activity detected in the Von Kármán crater on the farside of the Moon

Sensitive to permittivity differences, lunar penetrating radar (LPR) can detect stratification due to material property differences in a medium. We examined the subsurface structures of Von Kármán crater within the South Pole-Aitken (SPA) basin on the farside of the Moon using LPR data obtained by the Chang'E-4 mission. Various distinct strata with depth are clearly recognized, consistent with multiple periods of intermittent lava flows, paleo-regolith, and crater ejecta. Numerous concave and arc shape structures observed at various depths are consistent with buried craters and broken rocks, indicating that the ancient lunar surfaces which are currently buried by up to 30 m deep in Von Kármán, as well as the current surface, suffered numerous impacting events. An imaged subsurface thick paleo-regolith infers a quiescent period with low volcanic activity within the Von Kármán crater. Following the quiescent period (shallower in the profile), a relatively young period of volcanism in the Von Kármán crater is inferred. The mapped stratigraphy and chronology of these events present a self-consistent history of the Von Kármán crater within the SPA, which suggests connections to events on the lunar nearside.