Volcaniclastic Material Research Articles

Podrobnosti o počátcích sedimantace a vulkanické činnosti na začátku permu (asselu); rudnický obzor v sz. části podkrkonošské pánve

A succession of the early Permian (Asselian) Rudník member exposed at the locality Škodějov – Hrádecký potok in the northwestern part of the Krkonoše Piedmont Basin (eastern Bohemia) contains light-colored clastic rocks with macroscopically conspicuous lamination, overlying lacustrine bituminous claystones and siltstones (ʻblack shalesʻ). This work aims to constrain the origin of those laminated strata. They can be macroscopically described as heterolithic sequence formed by fine- to medium-, exceptionally coarse-grained sandstone with intraclasts of mudstone, calcified fine-grained sandstone, mudstone and claystone with interlayers of organic-rich laminae. Volcaniclastic rocks are also present. The heterolithic volcanosedimentary sequence comprises cyclic (up to 4 dm), mostly horizontally laminated rocks, in places with soft- sediment deformation structures. Diagenetic phenomena, related to calcification of volcaniclastic layers and growth of pyrite crystals and framboids, are common. Volcaniclastic rocks with a predominance of albitized volcanic glass fragments are interpreted as vitritic tuffs. Fragments of volcanic glass floating in silty to fine-grained sandy matrix contain vesicules and are locally brown- colored. They are reinterpreted as being derived from contemporary intermediate (andesitic) volcanism, most likely extrabasinal. Extensive alteration of volcanic material makes characterization of the original composition of volcaniclastic deposits difficult. Trace element analysis using a portable XRF spectrometer revelaed negligible concentrations of V, Ni and Ti, which are indicative of rather basic volcanism. Results of outcrop-scale gammaspectrometry are presented here as trends of Th/U ratio of relevant lithotypes from several localities of the Rudník member. Organic-rich laminae present within the heterolithic succession at Hrádecký potok section are commonly rich in pyrite clusters. Those are the most probably related to formation of algal mats stimulated by input of fine-grained volcaniclastic material into the Rudník Lake. However, elevated siliciclastic and volcaniclastic input, particularly during periods of intensified volcanic activity, with related seismic events (that may have triggered gravity flows), may have disturbed or punctuated growth of algae on the lake floor. Our work has shown that the volcanic activity at the onset of deposition of the Rudník member in the northwestern part of Krkonoše Piedmont Basin (Škodějov – Hrádecký potok) was probably of intermediate nature and that it influenced depositional environment. Particularly, the fluctuating input of clastic and volcanogenic material could have caused an alternation of oxic and dysoxic conditions in the lake lacustrine environment over time. The intermediate character of volcanism in this locality contrasts with confirmed acidic volcanism documented in volcaniclastic layers in another lacustrine succession at Vrchlabí – roadcut (Martínek et al. 2006) that probably represent a different eruptive phase recorded within the Rudník member.

Jurassic to Early Paleogene sedimentation in the Amazon region of Ecuador: Implications for the paleogeographic evolution of northwestern South America

The retroarc Amazon region of Ecuador recorded distinct events that are preserved in the Mesozoic-Cenozoic sedimentary record of the Subandean Zone and Oriente Basin. In this contribution, we review the Jurassic to Paleocene geologic and paleogeographic evolution of the Ecuadorian foreland using stratigraphic, sedimentologic, provenance, and geochronological data. Our results provide new insights into the sedimentary systems, stratigraphic framework, depositional ages, and sediment dispersal patterns of retroarc basin fill. During the Early Jurassic, marine deposits of the Santiago Formation formed a carbonate platform in eastern Ecuador. At ~180 Ma, an eastward-prograding deltaic system transported volcaniclastic material from a western volcanic arc. Nonmarine conditions prevailed during most of the Middle to Late Jurassic, characterized by deposition of red beds and intercalated volcanic and volcaniclastic deposits of the Chapiza Formation. Clastic and volcanic deposition dated from ~160 to 130 Ma was coeval with regional extension, as suggested by sedimentologic and seismic data from the Oriente Basin. Provenance results show that a ~ 120 Ma fluvial system at the base of the Hollín Formation was initially sourced from the west and progressively evolved to a mixed source with contributions from the Amazonian Craton and incipient Andean topography. Deposition of the Hollín Formation occurred in a northwest-flowing fluvial system that evolved into an estuary system confined by the Andes to the west and the Amazonian Craton to the east. Coastal to shallow marine deposits of the Napo Formation accumulated during regional sea-level variations throughout the Late Cretaceous. The geometry of coastal deposits and supporting provenance data suggest a drainage system characterized by west-flowing rivers derived chiefly from the Amazonian Craton. The latest Cretaceous (~70 Ma) onset of shortening in the Northern Andes coincided with collision of the Caribbean Plateau. This tectonic episode induced a continental-scale drainage reorganization documented in provenance data showing an Andean origin of the east-directed fluvial deposits of the Maastrichtian–Paleocene Tena Formation.

Dating North Pacific Abyssal Sediments by Geomagnetic Paleointensity: Implications of Magnetization Carriers, Plio-Pleistocene Climate Change, and Benthic Redox Conditions

Non-carbonaceous abyssal fine-grained sediments cover vast parts of the North Pacific’s deep oceanic basins and gain increasing interests as glacial carbon traps. They are, however, difficult to date at an orbital-scale temporal resolution and still rarely used for paleoceanographic reconstructions. Here, we show that sedimentary records of past geomagnetic field intensity have high potential to improve reversal-based magnetostratigraphic age models. Five sediment cores from Central North Pacific mid-latitudes (39–47°N) and abyssal water depths ranging from 3,900 to 6,100 m were cube-sampled at 23 mm resolution and analyzed by automated standard paleo- and rock magnetic methods, XRF scanning, and electron microscopy. Relative Paleointensity (RPI) records were determined by comparing natural vs. anhysteretic remanent magnetization losses during alternating field demagnetization using a slope method within optimized coercivity windows. The paleomagnetic record delivered well interpretable geomagnetic reversal sequences back to 3 Ma. This age span covers the climate-induced transition from a biogenic magnetite prevalence in the Late Pliocene and Early Pleistocene to a dust-dominated detrital magnetic mineral assemblage since the Mid-Pleistocene. Volcaniclastic materials from concurrent eruptions and gravitational or contouritic sediment re-deposition along extinct seamount flanks provide a further important source of fine- to coarse-grained magnetic carriers. Surprisingly, higher proportions of biogenic vs. detrital magnetite in the late Pliocene correlate with systematically lowered RPI values, which seems to be a consequence of magnetofossil oxidation rather than reductive depletion. Our abyssal RPI records match the astronomically tuned stack of the mostly bathyal Pacific RPI records. While a stratigraphic correlation of rock magnetic and element ratio logs with standard oxygen isotope records was sporadically possible, the RPI minima allowed to establish further stratigraphic tie points at ∼50 kyr intervals. Thus, this RPI-enhanced magnetostratigraphy appears to be a major step forward to reliably date unaltered abyssal North Pacific sediments close to orbital-scale resolution.

Modeling of the Dec. 22nd 2018 Anak Krakatau volcano lateral collapse and tsunami based on recent field surveys: Comparison with observed tsunami impact

The Dec. 22, 2018 lateral collapse of the Anak Krakatau (AK) volcano in the Sunda Straits of Indonesia discharged volcaniclastic material into the 250 m deep caldera southwest of the volcano and generated a large tsunami, causing runups of up to 85 m in the near-field, and 13.5 m in the far-field, on the nearby coasts of Sumatra and Java. The tsunami caused 437 fatalities, the greatest number from a volcanically-induced tsunami since the catastrophic explosive caldera-forming eruption of Krakatau in 1883 and the sector collapse of Ritter Island in 1888. For the first time in over 100 years, the 2018 AK event provides an opportunity to study a major volcanically-generated tsunami that caused widespread loss of life and significant damage. Here, we present numerical simulations of the collapse and tsunami generation, propagation, and coastal impact, with state-of the-art numerical models, using both a new parametrization of the collapse and a near-field bathymetric dataset based on our 2019 field surveys and satellite images. These subaerial and submarine data sets are used to constrain the geometry and magnitude of the landslide mechanism, which show that the primary landslide scar bisected the AK edifice, cutting behind the central vent and removing 50% of its subaerial volume. The primary landslide volume is estimated to range from 0.175–0.313 km3, based on uncertainties in the shape of the submerged part of the failure plane. This is supported by an independent estimate of the primary landslide deposit volume of 0.214 ± 0.036 km3. Given uncertainties in the failure volume, we define a range of potential failure surfaces that span these values in 4 collapse scenarios of volume ranging from 0.175 to 0.313 km3. These AK collapses are modeled, assuming either a granular or viscous fluid rheology, together with their corresponding tsunami generation and propagation. Observations of a single tsunami, with no subsequent waves, are consistent with our interpretation of landslide failure in a rapid, single phase of movement rather than a more piecemeal process, generating a tsunami which reached nearby coastlines within ~30 min. For both modeled rheologies, the 0.224 km3 collapse (second and preferred scenario) most successfully reproduces the near- and far-field tsunami flow depth and runup observed in all post-event field survey results, tide gauge records, and eyewitness reports to date, suggesting our estimated landslide volume range is appropriate. This event highlights the significant hazard posed by relatively small-scale lateral volcanic collapses, which can occur en-masse, without any precursory signals, and are an efficient and unpredictable tsunami source. Our successful simulations demonstrate that current numerical models can accurately forecast tsunami hazards from these events. In cases such as Anak Krakatau's, the absence of precursory warning signals, together with the short travel time following tsunami initiation present a major challenge for mitigating tsunami coastal impact, stressing the need to develop and install early warning systems for such events.

At the origins of Pompeii: the plant landscape of the Sarno River floodplain from the first millennium bc to the ad 79 eruption

The ad 79 eruption of the Vesuvius severely affected the floodplain surrounding the ancient city of Pompeii, i.e. the Sarno River floodplain. The landscape was covered with volcaniclastic materials that destroyed the ecosystem but, at the same time, preserved the traces of former environmental conditions. This study provides—for the first time—a pollen sequence reconstructing the environmental evolution and the plant landscape of the Sarno floodplain between 900 and 750 cal bc and ad 79, i.e. before and during the foundation of the city, and during its life phases. Previous geomorphological studies revealed that the portion of the Sarno floodplain under the “Pompeii hill” was a freshwater backswamp with patchy inundated and dry areas. Palynology depicts a thin forest cover since the Early Iron Age, suggesting an open environment with a mosaic of vegetation types. The local presence of Mediterranean coastal shrubland, hygrophilous riverine forest and mesophilous plain forest is combined with the regional contribution of mountain vegetation through the sequence. Oscillations between inundated and wet ground characterized the studied area until the ad 79 eruption. Such a natural environment shows anthropogenic traits since pre-Roman times: pasturelands, cultivated fields and olive groves, which probably occupied drier soils. The most important change in the land use system was the introduction of cabbage cultivation in the fourth century bc and its intensification from the second century bc, when Roman influence grew. The presence of tree crops and of ornamental trees reveals the opulence of the Imperial age until the catastrophic eruption.

Submarine and Subaerial Morphological Changes Associated with the 2014 Eruption at Stromboli Island

Stromboli is an active insular volcano located in the Southern Tyrrhenian Sea and its recent volcanic activity is mostly confined within the Sciara del Fuoco (SdF, hereafter), a 2-km wide subaerial–submarine collapse scar, which morphologically dominates the NW flank of the edifice. In August-November 2014, an effusive eruption occurred along the steep SdF slope, with multiple lava flows reaching the sea. The integration of multisensor remote sensing data, including lidar, photogrammetric, bathymetric surveys coupled with SAR amplitude images collected before and after the 2014 eruption enabled to reconstruct the dynamics of the lava flows through the main morphological changes of the whole SdF slope. Well-defined and steep-sided ridges were created by lava flows during the early stages of the eruption, when effusion rates were high, favoring the penetration into the sea of lava flows as coherent bodies. Differently, fan-shaped features were emplaced during the declining stage of the eruption or in relation to lava overflows and associated gravel flows, suggesting the prevalence of volcaniclastic breccias with respect to coherent lava flows. The estimated volume of eruptive products emplaced on the SdF slope during the 2014 eruption, accounts for about 3.7 × 106 m3, 18% of which is in the submarine setting. This figure is different with respect to the previous 2007 eruption at Stromboli, when a large lava submarine delta formed. This discrepancy can be mainly related to the different elevation of the main vents feeding lava flows during the 2007 eruption (around 400 m) and the 2014 eruption (around 650 m). Besides slope accretion, instability processes were detected both in the subaerial and submarine SdF slope. Submarine slope failure mobilized at least 6 × 105 m3 of volcaniclastic material, representing the largest instability event detected since the 2007 lava delta emplacement.

The Tarim Basin, China, a prospect for plume-related Zr(Hf)-Nb(Ta)-REY-Ga-U mineralization

This study is based on published data from the Tarim and Emeishan Large Igneous Provinces (TLIP and ELIP, respectively). The data are interpreted in a comparative manner, using the well-studied ELIP’s mineralization as a reference. Some additional information on related U and REY-carbonate ores from Far Eastern Russia are also included in this comparison. The results show that the Tarim Basin has both positive and negative indicators for tuffaceous Zr(Hf)–Nb(Ta)–REY–Ga–U ores. Like in ELIP, the source of the mineralization in the Tarim Basin is likely to be the Permian volcaniclastic deposits. The volcaniclastic deposits in the Tarim are not well studied geochemically; however, since the composition of the parent magmatic rocks are similar between TLIP and ELIP, the volcaniclastic material is also thought to be similar. Positive indications are that the TLIP’s deposits are larger (volumetrically), more felsic than the ELIP’s volcaniclastics. In addition, the former are concentrated in an almost closed terrestrial basin (Tarim Basin), whereas the latter are disseminated throughout a wide continental margin. The Paleozoic strata of the Tarim Basin are deeply altered in and around fracture zones, which are widespread. The alteration is caused by acidic volcanic-related fluids that are a key factor in metal mobilization and redistribution. However, in contrast to the ELIP’s area, the Tarim Basin was under influence of semi-arid climate that probably resulted in a weak activity of meteoric waters and poorly developed vegetation during deposition of the Permian volcaniclastic strata, which have a relatively low content of organic matter (<5%, commonly <2%) and contain no coal seam. These may be considered negative indicators for tuffaceous Zr(Hf)–Nb(Ta)–REY–Ga–U ores because organic acids might play a significant role in the leaching, redistribution, and the finishing processes that concentrated Zr, Hf, Nb, Ta, REY, Ga, and U within rocks with clay alteration. Finally, the Permian tuffaceous mudstones located in and around deep faults of the Tarim Basin may be a likely target for geochemical prospecting for Zr(Hf)–Nb(Ta)–REY–Ga–U ores similar to those found in southwestern China. In addition, the fracture zones in the Tarim Basin are likely to contain REY-carbonate and U ores similar to those discovered in Primorye, the Far Eastern Russia.

Ground-Penetrating Radar Modeling Across the Jezero Crater Floor

This article assesses how the ground-penetrating radar RIMFAX will image the crater floor at the Mars 2020 landing site, where lithological compositions and stratigraphic relationships are under discussion prior to mission operation. A putative mafic unit (lava flow, volcanic ash, or volcaniclastic deposit) on the crater floor will be crucial in piecing together the chronology of deposition and for understanding the volcanic history in the region. In order to see how lithological properties and subsurface geometries affect radar sounding, a synthetic radargram is generated through forward modeling with a finite-difference time-domain method. The acquisition is simulated across the mafic unit as a succession of lava flows, exploring detection of internal structures and contacts to adjacent lithologies. To compare modeling results with the alternative formation scenarios, a discussion about sounding over a tephra or volcaniclastic material is presented. Similarities and differences between Martian and terrestrial lithologies can be related to electromagnetic properties relevant for radar sounding. This article, therefore, evaluates potential scientific insights gained from acquisition across the disputed mafic unit, in light of proposed hypotheses of lithological generation.

Shallow-water models for volcanic granular flows: A review of strengths and weaknesses of TITAN2D and FLO2D numerical codes

The behaviour of dry and wet volcanic granular flows is one of the main research topics in present day geophysics and volcanology. It involves various disciplines (e.g. sedimentology, geophysics, fluid dynamics) and investigation techniques (e.g. field studies, laboratory experiments, computational fluid dynamics). The vast interest is justified by the complex nature of these flows and their very dangerous nature that threaten millions of people around the world. In the last decade, computational fluid dynamics has become one of the main instruments used to reproduce past events of volcanic granular flows or to predict their behaviour and potential hazard. In this study, we tested two of the most used codes for simulating volcanic granular flows, TITAN2D and FLO2D, against well studied natural cases (the 1998 wet granular flows in the Sarno area and the 2005 block and ash flows at Colima volcano) and large-scale experiments on granular flows. Comparison between simulated parameters and real ones were carried out in order to evaluate strengths and weaknesses of the two numerical codes. TITAN2D results showed how the basal friction angle is fundamental to control numerical simulations and its dependence on the topographic complexities, DEM resolution and slope-angle ratio. Simulation of large scale experiments offered a good relationship between slope angle ratio at break in slope and basal friction angle, which is useful for application to small drainage basins with not complex channel morphology. FLO2D suffers the lack of rheometric parameters for volcaniclastic material, but is less sensitive of DEM resolution with respect to TITAN2D.

Hydrogeomorphic and sedimentary response to the Late Pleistocene violent Strombolian eruption of the Croscat volcano (Garrotxa Volcanic Field, Spain)

Explosive eruptions can severely alter the boundary conditions of fluvial systems around volcanoes by depositing large volumes of erodible fragmental material, increasing erosion rate and drainage mass flux (water and sediment) in the affected basin. In this paper, the environmental response to the 13 ka Croscat Violent Strombolian eruption in the Garrotxa Volcanic Field (NE Spain) has been investigated using a stratigraphic approach. Volcaniclastic material was mainly delivered at headwater tributaries (Les Tries plain in the Ser river basin) of the Fluvia River, one of the main drainage systems of NE Spain. Deposits in the Les Tries valley was initially reworked by hyperconcentrated-flows to braided stream-flows and by debris-flows involving mixed terrains (volcaniclastic materials and sands–gravels of the Eocene sedimentary basement) as registered in the stratigraphic record. The erosive pattern observed at the Les Tries valley reflects the erosion model of well-sorted and coarse-grained deposits (as the tephra deposited during the Violent Strombolian phase of the Croscat eruption), in which deep gullying and downstream aggradation are typically retarded where coarse tephra deposits remained highly permeable. Moreover, the Croscat eruption occurred soon after the Last Glacial Maximum, implying the shift from dry to wet conditions during the remobilization of the Croscat tephra. In this framework, the semiarid conditions retarded soil stabilization, driving the development of hyperconcentrated-flow-dominated alluvial fans, whereas the more humid environmental conditions favoured the partial stabilization of loose pyroclastic material, until it is periodically remobilization by mass-wasting processes as debris-flow-dominated lahars.

Lake conditions and detrital sources of Paleolake Olduvai, Tanzania, reconstructed using X-ray Diffraction analysis of cores

The Olduvai Basin of northern Tanzania contains well-known Pleistocene archaeological sites in a palaeoenvironmental setting of interfingering paleolake and volcanic fan deposits. This study documents the mineral assemblages, as determined qualitatively through bulk X-ray Diffraction (XRD) of systematically sampled lacustrine sediments from Olduvai Gorge Coring Project (OGCP) Cores 1A, 2A, and 3A, from about 45 m below the surface (mbs) to the base of all three cores. This interval includes fluvio-lacustrine sediments below and between two major progradations of Ngorongoro volcanic deposits recorded in Core 2A, and all of Olduvai Bed I and the lower part of Bed II in all three cores. Changes in mineral assemblage track changes in sediment sources and lake water chemistry. Abundant quartz indicates sediment sources derived from eroded Precambrian basement quartzite and gneiss exposures to the west, southwest, and north. In contrast, the presence of anorthoclase feldspar (sometimes associated with minor augite) indicates southerly and easterly sources, derived from the then active volcanoes (Ngorongoro, Olmoti) of the Ngorongoro Volcanic Highlands. The presence and relative abundance of specific zeolites provides information about Paleolake Olduvai's pH conditions and the relative activities of various dissolved cations (K+, Na+, Ca2+, Si) at the time of alteration, and the presence and abundance of authigenic K-feldspar indicates post-depositional exposure to saline-alkaline pore fluids.The mineral assemblages recorded in the cores reveal both large-scale trends in sediment composition over time, and smaller-scale changes related to fluctuating lake levels. In the lacustrine deposits near the base of Core 2A (interval NSF-1), analcime dominates the zeolite assemblage (with minor clinoptilolite), whereas carbonates and authigenic K-feldspar are rare. This interval is followed by more lacustrine deposits in which phillipsite and chabazite are the dominant zeolites (and analcime is absent). Lacustrine deposition was interrupted by the first of two pulses of coarse Ngorongoro volcaniclastic material (NVF-1). In the lacustrine interval between the two Ngorongoro volcanic pulses (NSF-2, recovered only in Core 2A), chabazite and erionite dominate the zeolite assemblage, carbonate and K-feldspar are rare, and analcime is absent, indicating less extreme saline-alkaline conditions and greater Ca2+ availability for zeolite formation. Volcanic glass is preserved, along with abundant zeolites, including phillipsite, chabazite, and erionite, in altered volcanic materials from the upper Ngorongoro volcanic pulse (NVF-2).In the lacustrine deposits of Lower Bed I (below the Bed I basalts but above NVF-2, only recovered in Core 3A), zeolites and K-feldspar are rare to absent, consistent with wetter conditions and fresher lake waters. Carbonates are ubiquitous throughout the lacustrine intervals of Beds I and II. In lacustrine deposits above the Bed I basalts (in Upper Bed I), phillipsite is the dominant zeolite, and quartz is rare. Alternating intervals of zeolite and K-feldspar-rich and poor sediments suggest variations between wetter and drier conditions throughout Upper Bed I and into Lower Bed II. Aragonite and associated dolomite in Bed I during the deposition of a claystone Mg-anomaly points to high magnesium concentrations and elevated Mg2+/Ca2+ ratios during that time. A major change between lowermost Bed II and overlying Bed II lacustrine sediments includes an abrupt increase in quartz coupled with an increase in the zeolite analcime, consistent with the onset of more saline-alkaline conditions, contemporaneous with overall drier conditions.

Volcaniclastic member of the richly fossiliferous Kaiparowits Formation reveals new insights for regional correlation and tectonics in southern Utah during the latest Campanian

Lithostratigraphic investigation of the richly fossiliferous Kaiparowits Formation in southern Utah reveals the presence of a previously unidentified stratigraphic unit herein named the Upper Valley Member. The 255-m-thick Upper Valley Member is latest Campanian to earliest Maastrichtian in age and records a significant sedimentological change in the Kaiparowits Formation. This change is illustrated in the member by a significant increase in near syn-sedimentary aged zircons, coincident with the introduction of white, volcaniclastic sandstones, as well as a paucity of Jurassic grains, which dominate the provenance of the rest of the formation. The source of the late Campanian volcaniclastic material, including near syn-sedimentary zircons, is most likely from nearby volcanic centers within the Laramide porphyry copper province to the south of the Kaiparowits Plateau in the Mogollon region. Measured sections reported here stratigraphically expand the Kaiparowits Formation to a total of 1005 m and find that the upper boundary of the formation is largely gradational with the overlying Canaan Peak Formation. Lithological changes documented in this study are interpreted to signify a sedimentological response to proximal magmatism and emerging uplifts within the Cordilleran foreland basin during early Laramide orogenesis, which resulted in paleo-drainage rearrangement in southern Laramidia in the latest Campanian. The fossil-bearing Upper Valley Member can be correlated regionally to the Kirtland, Tuscher and Bearpaw formations and other latest Campanian – and possibly early Maastrichtian – units across western North America and represents the capping member one of the most continuous terrestrial records of the Campanian biosphere found anywhere in the world.

Geochemical Evolution of Arc and Slab Following Subduction Initiation: a Record from the Bonin Islands, Japan

AbstractVolcanism following the initiation of subduction is vital to our understanding of this specific magma-generation environment. This setting is represented by the first development of the Izu–Bonin–Mariana arc system as subduction commenced along the Western Pacific margin in the Eocene. A new collection of volcanic rocks recovered from the islands and exposed crustal sections of the Bonin Ridge spans the first 10 Myr of arc evolution. An elemental and radiogenic isotope dataset from this material is presented in conjuction with new 40Ar/39Ar ages and a stratigraphic framework developed by a detailed mapping campaign through the volcanic sections of the Bonin Islands. The dating results reveal that both the locus and type of magmatism systematically changed with time in response to the progressive sinking of the slab until the establishment of steady-state subduction at around 7–8 Ma. Following initial mid-ocean ridge basalt (MORB)-like spreading-related basalt magmatism, volcanic centres migrated away from the trench and changed from high-Si boninite to low-Si boninite or high-Mg andesite, then finally tholeiitic or calcalkaline arc magma. Subducting pelagic sediment combined with Pacific-type igneous ocean crust dominates the slab input to the shallow source of high-Si boninites at 49 Ma, but high-precision Pb isotope data show that this sediment varies in composition along the subducting plate. At around 45 Ma, volcanism switched to low-Si boninite and the pelagic sediment signature was almost entirely replaced by volcanic or volcaniclastic material originating from a HIMU ocean island source. These low-Si boninites are isotopically consistent with a slab component comprising variable proportions of HIMU volcaniclastic rocks and Pacific MORB. In turn, this signature was replaced by a Pacific MORB-dominated flux in the post 45 Ma tholeiite and calcalkaline volcanic rocks. Notably, each change in slab-derived flux coincided with a change in the magma type. Fluctuations in the slab-derived geochemical signature were superimposed on a change in the mantle wedge source from highly depleted harzburgite to a depleted MORB-type mantle-type source. In turn, this may correspond to the increasing depth of the leading edge of the slab through this 5 Myr period.

Identification and Description of a Silicic Volcaniclastic Layer in Gale Crater, Mars, Using Active Neutron Interrogation

AbstractThe Dynamic Albedo of Neutrons instrument aboard the Mars Science Laboratory rover, Curiosity, has been used to map a stratigraphically conformable layer of high‐SiO material in Gale crater. Previous work has shown that this material contains tridymite, a high‐temperature/low‐pressure felsic mineral, interpreted to have a volcanic source rock. We describe several characteristics including orientation, extent, hydration, and geochemistry, consistent with a volcaniclastic material conformably deposited within a lacustrine mudstone succession. Relationships with widely dispersed alteration features and orbital detections of hydrated SiO suggest that this high‐SiO layer extends at least 17 km laterally. Mineralogical abundances previously reported for this high‐SiO material indicated that hydrous species were restricted to the amorphous (non‐crystalline) fraction, which is dominated by SiO . The low mean bulk hydration of this high‐SiO layer (1.85 0.13 wt.% water‐equivalent hydrogen) is consistent with silicic glass in addition to opal‐A and opal‐CT. Persistent volcanic glass and tridymite in addition to opal in an ancient sedimentary unit indicates that the conversion to more ordered forms of crystalline SiO has not proceeded to completion and that this material has had only limited exposure to water since it originally erupted, despite having been transported in a fluviolacustrine system. Our results, including the conformable nature, large areal extent, and presence of volcanic glass, indicate that this high‐SiO material is derived from the product of evolved magma on Mars. This is the first identification of a silicic volcaniclastic layer on another planet and has important implications for magma evolution mechanisms on single‐plate planets.

Rapid Shifts in Chemical and Isotopic Compositions of Sediment Pore Waters in the Amami Sankaku Basin in Response to Initial Arc Rifting in the Mid‐Oligocene

AbstractPore waters were recovered from the sediment cores drilled in the back‐arc Amami Sankaku Basin (ASB) during the International Ocean Discovery Program Expedition 351. The sediment sequence is composed of the recent mud and ooze from late Oligocene/early Miocene accumulated at 7 m/Ma and rapid deposition of thick volcaniclastic sequence at 50 m/Ma. Such variations are reflected in the evolutions of chemical and isotopic compositions of pore waters. Cl and Ca concentrations in pore waters increased rapidly with depth and became enriched in the deep portion. Before arc rifting, extremely high deposition rates of volcaniclastic gravity‐flow sediments from the nearby Kyushu‐Palau arc into the ASB resulted in strong alteration, removing Mg and Si from pore waters. Mg concentrations were initially almost zero but abruptly shifted to substantial increases after ~29.5 Ma, indicating extended rifting of the active volcanic axis from the ASB. The δ18O and δD values of pore waters also evolved from more depleted and wide range to more positive and narrow range as a response to reduced alteration and increasing influence of seawater. Concentrations of Si and Mn in pore waters rapidly increased since ~29.5 Ma but suddenly declined at ~17.5 Ma due to the cessation of input of volcaniclastic materials. Covariations of Ca and Mg in pore waters are very similar to those in the back‐arc West Philippine Basin but significantly different from those in the Izu‐Bonin fore‐arc basin. Variations in chemical and isotopic compositions of pore waters can assist the reconstruction of tectonic activities.

Mineralogical Studies on Some Selected Semi-Precious Stones from Northeast Jordan

A semi-precious stone is known as a gem or gemstone (also a jewel, a gem, a precious stone), which is a portion of mineral in refined and cut form that could be used to create a jewelry or other embellishments. A gem also can be defined as any mineral that is highly prized for its beauty, durability, and rarity, is used for personal adornment and has been enhanced in some manner by altering its shape usually by cutting and polishing. Semi-precious stones occurred as xenolithes are widespread in Jordan within different source of rocks. High grade of copper as malachite and chrysocolla and plancheite is suitable for gemstone manufacture found in Finan area. High purity of garnet can be found in metamorphic rocks in Wadi Araba area, whereas xenoliths of pyroxene, olivine and spinel-perioditite and garnet are wide spread in basalt and pyroclastic materials in northeast Jordan. The studied semi-precious stones are part of the Miocene alkali basaltic rocks that covered the northeastern part of Jordan within Harrat Al-Shaam Basaltic Super Group Plateau concentrated mainly in volcanic centers and volcaniclastic materials of Aritian, Hassan, Ufiyahim and Bishriyya formations. Some of these xenoliths and xenocrysts can be considered as semi-precious stones. This study is focused on characterization of petrography, mineralogy and geochemistry of these semi-precious stones like olivine, pyroxene and garnet xenoliths that have been collected from selected areas in northeast Jordan. The samples were analyzed by polarizing microscope, X-Ray Diffraction (XRD), X-Ray Fluorescence (XRF) and Scanning Electron Microscope (SEM). Cutting and polishing tests have shown that some of the studied xenoliths are suitable for manufacturing attractively colored gemstones and could be considered as semi-precious stones. The geological chances for the discovery of some hundreds of tons of these gemstone-raw materials are considered to be good in volcaniclastic materials that are associated within different volcanic centers at northeast Jordan.

Long-term non-erosive nature of the south Costa Rican margin supported by arc-derived sediments accreted in the Osa Mélange

Understanding the erosive and accretionary nature of convergent margins is significant to understand tectonics and the crustal mass balance at subduction zones. The Costa Rican margin is commonly regarded as an archetypal example of an erosive margin, where subduction of sediments and basal removal of the upper plate in the subduction zone have occurred for most of the Cainozoic. This view is supported by structural constraints from 3D seismic reflection data in the outer forearc, as well as periods of forearc subsidence at ODP/DSDP/IODP drill sites. However, determining the origin of the Upper Eocene-Miocene Osa Mélange that is exposed in south Costa Rica only 10–30 km from today's trench offers another opportunity to constrain the long-term erosive, accretionary, and/or non-erosive evolution of the margin. Existing models for formation of the mélange propose that it resulted from (i) accretion of arc-derived trench-fill sediments, (ii) punctual accretion of the clastic apron of an ocean islands system, (iii) local dismemberment of the margin due to tectonic erosion, or (iv) in-situ deformation of a forearc sedimentary cover. To test the validity of these models and provide new constraints on the accretionary and/or erosive nature of the margin we studied the provenance of volcaniclastic material in the Upper Eocene San Pedrillo Unit of the Osa Mélange using geochemical analysis of detrital pyroxenes, amphiboles and igneous rocks. This innovative approach to determine the origin(s) of dismembered sedimentary sequences reveals that the volcaniclastic fraction of the mélange is, without ambiguity, predominantly composed of forearc material that preserves an assemblage of arc basement, proto-arc and arc sequences of the pre-Oligocene Costa Rican margin. This result and previous geological constraints show that the Osa Mélange formed through accretion of arc-derived trench-fill deposits in the Late Eocene to Miocene, with possibly minor tectonic incorporation of intra-oceanic material (ocean floor and seamount sequences). Therefore, consistently with recent seismic observations in south Costa Rica that document a phase of accretion within the past ∼5 m.yr., preservation of arc-derived sediments in the Osa Mélange shows that the margin was predominantly non-erosive over the past ∼35 m.yr. Cycles of subsidence and uplift in the south Costa Rican forearc during this period could represent a tectonic response to episodic subduction of seamounts formed at the Galapagos Hotspot, without causal link to short-term subduction erosion or accretion. Fundamentally, new results from the Osa Mélange show that large (10 km-thick) accretionary complexes can form durably due to local sedimentary recycling of the forearc in seamount collisional settings. The recycled component of the NW Osa Mélange exemplifies that quantifying the extent of local recycling vs net crustal addition through accretion of intra-oceanic sequences or net crustal loss due to tectonic erosion poses a serious challenge to determine the crustal mass balance at subduction zones.

Armoured sponge spicules from Panarea Island (Italy): Implications for their fossil preservation

A variety of calcareous and siliceous skeletal components are associated with exceptional, still active deposits of modern iron ooids off Panarea, one of the volcanic islands of the Aeolian Arc (Tyrrhenian Sea, Italy). Both ooids and skeletal components occur as loose sediments influenced by submarine hydrothermal fluid vents. Whereas iron ooids are exclusively made of goethite laminae primarily nucleated on volcaniclastic material, sponge spicules – that represent most of the siliceous skeletal component – develop laminated coatings (concretions) of varying sizes. This partial or total coating consists of regularly banded Fe‒rich layers exhibiting the same textural features and mineralogical composition (goethite) of the ooid cortex developed around inorganic cores. Spicules did not reveal any obvious attaching structure or interfingering with the surrounding coating and did not undergo bioerosion or any other evidence of biological intervention during their development. A hydrothermal origin, compatible with the general setting, is therefore proposed for these armoured sponge spicules. We believe that this unique modern case of iron concretions produced on siliceous spicules can contribute to explain other known fossil cases as well as to understand the taphonomy of this type of biogenic silica that seems rarely to have been preserved in such an extreme habitat.

Modelling of the tsunami from the December 22, 2018 lateral collapse of Anak Krakatau volcano in the Sunda Straits, Indonesia

On Dec. 22, 2018, at approximately 20:55–57 local time, Anak Krakatau volcano, located in the Sunda Straits of Indonesia, experienced a major lateral collapse during a period of eruptive activity that began in June. The collapse discharged volcaniclastic material into the 250 m deep caldera southwest of the volcano, which generated a tsunami with runups of up to 13 m on the adjacent coasts of Sumatra and Java. The tsunami caused at least 437 fatalities, the greatest number from a volcanically-induced tsunami since the catastrophic explosive eruption of Krakatau in 1883 and the sector collapse of Ritter Island in 1888. For the first time in over 100 years, the 2018 Anak Krakatau event provides an opportunity to study a major volcanically-generated tsunami that caused widespread loss of life and significant damage. Here, we present numerical simulations of the tsunami, with state-of the-art numerical models, based on a combined landslide-source and bathymetric dataset. We constrain the geometry and magnitude of the landslide source through analyses of pre- and post-event satellite images and aerial photography, which demonstrate that the primary landslide scar bisected the Anak Krakatau volcano, cutting behind the central vent and removing 50% of its subaerial extent. Estimated submarine collapse geometries result in a primary landslide volume range of 0.22–0.30 km3, which is used to initialize a tsunami generation and propagation model with two different landslide rheologies (granular and fluid). Observations of a single tsunami, with no subsequent waves, are consistent with our interpretation of landslide failure in a rapid, single phase of movement rather than a more piecemeal process, generating a tsunami which reached nearby coastlines within ~30 minutes. Both modelled rheologies successfully reproduce observed tsunami characteristics from post-event field survey results, tide gauge records, and eyewitness reports, suggesting our estimated landslide volume range is appropriate. This event highlights the significant hazard posed by relatively small-scale lateral volcanic collapses, which can occur en-masse, without any precursory signals, and are an efficient and unpredictable tsunami source. Our successful simulations demonstrate that current numerical models can accurately forecast tsunami hazards from these events. In cases such as Anak Krakatau’s, the absence of precursory warning signals together with the short travel time following tsunami initiation present a major challenge for mitigating tsunami coastal impact.

Llandovery microfossils and microfacies of the Hýskov section, Prague Basin

Abstract The lower Silurian strata of the Prague Basin, as well as in many other regions, are primarily represented by black shales, with limited to no record of benthic life. In this paper, we describe an exceptional late Aeronian volcanic-carbonate succession from the Hýskov locality near Beroun, which contains a diverse and well-preserved fossil assemblage and specific microfacies features. The studied section is mostly represented by well-washed, poorly sorted skeletal grainstones, rudstones and floatstones along with volcaniclastic material. The environment is interpreted as agitated, above storm wave base, and with rapid sedimentation and burial. The collection of acid-resistant microfossils includes abundant scolecodonts, conodonts and chitinozoans, of which only the chitinozoans have previously been studied. The scolecodonts recovered represent at least 14 polychaete genera and 20 species. The fauna bears a great similarity to the early Silurian assemblages previously reported from Baltica, and some environmentally sensitive species allow constraining the assemblage to shallow-shelf settings. The recovered conodonts similarly show close affinities to Baltic faunas, and suggest a Rhuddanian – Aeronian age for the studied section. The chitinozoan assemblage, containing at least eight species, is dominated by Conochitina, and most likely belongs to the C. alargada Biozone.