All ICDP Publications with Abstracts
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2344.
In Situ Stress Orientation From 3 km Borehole Image Logs in the Koyna Seismogenic Zone, Western India: Implications for Transitional Faulting Environment
Tectonics,
39
(1)
2020
ISSN: 02787407
Publisher: Blackwell Publishing Ltd
Keywords:▾
Boreholes; Buildings; Earthquakes; Fracture; Horizontal wells; Infill drilling; Oil field equipment; Seismographs; Stresses; Strike-slip faults; Well logging, Borehole breakouts; Drilling-induced tensile fractures; Principal stress; Seismogenic zones; Stress orientations; Stress rotation, Fault slips, borehole breakout; borehole logging; Deccan Traps; drilling; faulting; focal mechanism; fracture; in situ stress; induced response; seismicity, India
Abstract: ▾ Knowledge of the in situ stress regime is critical to investigate the genesis of recurrent triggered seismicity over the past five decades in the Koyna seismogenic zone. Orientations of in situ horizontal stresses are determined for the first time from analyses of image logs in a 3 km deep scientific borehole KFD1 in the area. KFD1 passed through 1,247 m thick Deccan Traps and continued 1,767 m in the granitic basement. Stress-induced wellbore failures, breakouts and drilling-induced tensile fractures, are identified in the acoustic and microresistivity images of the granitic basement. Additionally, tightly constrained focal mechanisms of 50 earthquakes of M ≥ 3.6, reported in literature, are inverted to constrain the stress regime. Salient results are as follows: (i) wellbore breakouts and drilling-induced tensile fractures constrain N9°W ± 17° orientation for maximum horizontal principal stress (SHmax); (ii) consistency with the regional NNW-SSE orientation of SHmax from inversion of earthquake focal mechanisms shows that the borehole data are representative for the Koyna region; (iii) breakout rotations at multiple depths below 2,100 m indicate that the borehole may have passed through localized fault damage zones; (iv) consistent strike azimuths of steeply dipping fractures with SHmax orientation indicate favorable conditions for strike-slip to normal transitional faulting environment; and (v) stress inversion of 50 well-determined earthquake focal mechanisms supports transitional faulting environment in the Koyna seismogenic zone. Thus, stress orientation and fracture information from borehole data, together with stress regime constrained from inversion of earthquake focal mechanisms, shed new light on the faulting environment in the region. ©2020. American Geophysical Union. All Rights Reserved.
2343.
Insights into the evolution of the young Lake Ohrid ecosystem and vegetation succession from a southern European refugium during the Early Pleistocene
Quaternary Science Reviews,
227
2020
2342.
Koyna earthquakes: a review of the mechanisms of reservoir-triggered seismicity and slip tendency analysis of subsurface faults
Acta Geophysica,
68
(4)
1097-1112
2020
ISSN: 18956572
Publisher: Springer
Keywords:▾
Earthquakes; Infill drilling; Lithology; Reservoirs (water); Solvents, Continental scientific drillings; Fault geometry; Fluid pressures; Generic modeling; Subsurface lithology; Tendency analysis, Fault slips, decadal variation; earthquake event; earthquake magnitude; fault geometry; fault slip; literature review; rock mechanics; seismicity, India; Koyna; Maharashtra
Abstract: ▾ Koyna region in western India experienced more than 1,00,000 earthquakes of different magnitudes (M ~ 1.0–6.3) in the past five decades. Earthquakes in this region are believed to be triggered by a change in fluid pressure due to the percolation of the reservoir (Koyna and Warna reservoir) water into the subsurface. A drilling program was set up by the Ministry of Earth Sciences, India and International Continental Scientific Drilling Program (ICDP) to study the deep subsurface lithology, structure, thermal attributes, etc. as the area is covered by ~ 950 m of thick Deccan basalts. This paper reviews all the hypotheses proposed by earlier workers to explain the mechanism of reservoir trigger causing earthquakes and summarizes such theories to a simple generic model. Slip tendency analysis was further carried out based on the proposed model to explain the dependence of fault slip on fault geometry, rock mechanical properties, stress and fluid gradients. Finally, faults at various depths were characterized (favourably oriented, unfavourably oriented and severely misoriented) based on their potential to go into failure. © 2020, Institute of Geophysics, Polish Academy of Sciences & Polish Academy of Sciences.
2341.
LA-ICPMS U-Pb geochronology of detrital zircon grains from the Coconino, Moenkopi, and Chinle formations in the Petrified Forest National Park (Arizona)
Geochronology,
2
(2)
257-282
2020
ISSN: 26283735
Publisher: Copernicus GmbH
Abstract: ▾ Uranium-lead (U-Pb) geochronology was conducted by laser ablation - inductively coupled plasma mass spectrometry (LA-ICPMS) on 7175 detrital zircon grains from 29 samples from the Coconino Sandstone, Moenkopi Formation, and Chinle Formation. These samples were recovered from ∼520 m of drill core that was acquired during the Colorado Plateau Coring Project (CPCP), located in Petrified Forest National Park (Arizona). A sample from the lower Permian Coconino Sandstone yields a broad distribution of Proterozoic and Paleozoic ages that are consistent with derivation from the Appalachian and Ouachita orogens, with little input from local basement or Ancestral Rocky Mountain sources. Four samples from the Holbrook Member of the Moenkopi Formation yield a different set of Precambrian and Paleozoic age groups, indicating derivation from the Ouachita orogen, the East Mexico arc, and the Permo-Triassic arc built along the Cordilleran margin. A total of 23 samples from the Chinle Formation contain variable proportions of Proterozoic and Paleozoic zircon grains but are dominated by Late Triassic grains. LA-ICPMS ages of these grains belong to five main groups that correspond to the Mesa Redondo Member, Blue Mesa Member and lower part of the Sonsela Member, upper part of the Sonsela Member, middle part of the Petrified Forest Member, and upper part of the Petrified Forest Member. The ages of pre-Triassic grains also correspond to these chronostratigraphic units and are interpreted to reflect varying contributions from the Appalachian orogen to the east, Ouachita orogen to the southeast, Precambrian basement exposed in the ancestral Mogollon Highlands to the south, East Mexico arc, and Permian-Triassic arc built along the southern Cordilleran margin. Triassic grains in each chronostratigraphic unit also have distinct U and thorium (Th) concentrations, which are interpreted to reflect temporal changes in the chemistry of arc magmatism. Comparison of our LA-ICPMS ages with available chemical abrasion thermal ionization mass spectrometry (CA-TIMS) ages and new magnetostratigraphic data provides new insights into the depositional history of the Chinle Formation, as well as methods utilized to determine depositional ages of fluvial strata. For parts of the Chinle Formation that are dominated by fine-grained clastic strata (e.g., mudstone and siltstone), such as the Blue Mesa Member and Petrified Forest Member, all three chronometers agree (to within ∼ 1 Myr), and robust depositional chronologies have been determined. In contrast, for stratigraphic intervals dominated by coarse-grained clastic strata (e.g., sandstone), such as most of the Sonsela Member, the three chronologic records disagree due to recycling of older zircon grains and variable dilution of syn-depositional-age grains. This results in LA-ICPMS ages that significantly predate deposition and CA-TIMS ages that range between the other two chronometers. These complications challenge attempts to establish a well-defined chronostratigraphic age model for the Chinle Formation. © 2020 George Gehrels et al.
2340.
Lacustrine high resolution sequence records from Upper Cretaceous Nenjiang Formation in Songliao Basin
Global Geology,
23
(1)
38-70
2020
2339.
Lithofacies from the 1963-1967 Surtsey eruption in SUSTAIN drill cores SE-2a, SE-2b and SE-03
2338.
Electric discharge evidence found in a new class of material in the Chicxulub ejecta
Scientific Reports,
10
(1)
2020
Abstract: ▾ Chicxulub impact (66 Ma) event resulted in deposition of spheroids and melt glass, followed by deposition of diamectite and carbonate ejecta represented by large polished striated rounded pebbles and cobbles, henceforth, called Albion Formation1 Pook’s Pebbles, name given from the first site identified in central Belize, Cayo District. Here we report that magnetic analysis of the Pook’s Pebbles samples revealed unique electric discharge signatures. Sectioning of Pook’s Pebbles from the Chicxulub ejecta from the Albion Formation at Belize showed that different parts of Pook’s Pebbles had not only contrasting magnetization directions, but also sharply different level of magnetizations. Such behavior is indicative of electric discharge taking place sometimes during the formation of the Chicxulub ejecta blanket. In addition, some of the Pook’s Pebbles’ surface had recrystallized down to 0.2 mm depth. This is evidence of localized extreme pressures and temperatures during the fluidized ejecta formation which was imprinted in the outer layer of Pook’s Pebbles. Recrystallization caused formation of nanophase iron along the surface, which was revealed by mapping of both natural remanent magnetization and of saturation remanence magnetization signatures. While the spheroids’ magnetization orientation is consistent with reversed magnetic field at the time of impact, the study of the Pook’s Pebbles provided, in addition, new evidence of electric charging during the vapor plume cloud processes. © 2020, The Author(s).
2337.
High resolution X--ray computed tomography and scanning electron microscopy studies of multiphase solid inclusions in Oman podiform chromitite: implications for post--entrapment modification
Journal of Mineralogical and Petrological Sciences,
115
(3)
247--260
2020
2336.
Magmatic Processes at the Oman Ophiolite Paleoridge – Perspectives on the Role of Water
Acta Geologica Sinica (English Edition),
9421-22
2020
ISSN: 10009515
Publisher: John Wiley and Sons Inc
Keywords:▾
accretion; clinopyroxene; gabbro; geoaccumulation; harzburgite; magmatism; mid-ocean ridge basalt; olivine; ophiolite; partial melting; plagioclase; subduction zone; water content, Arabian Sea; East Pacific Rise; Germany; Gulf of Oman; Hannover; Indian Ocean; Lower Saxony; Pacific Ocean
Abstract: ▾ The Oman ophiolite is regarded as best proxy for accreted oceanic crust from typical fast-spreading ridge systems on land. However, the Oman ophiolite is influenced by initial subduction zone initiation, and the nature of the details of the subduction zone setting is still under controversial debate. While a first magmatic phase shows features of magmatic accretion very similar to those known from the East Pacific Rise, except that the primary melts were slightly water-enriched, a second type of magmatism is characterized by an apparent subduction-zone related imprint, producing rocks like FAB basalts and boninites in the upper crust, as well as cross-cutting gabbronorites and wehrlites in the deeper crust. In this paper, we apply diverse experimental studies in wet tholeiitic and peridotitic systems performed at lower pressures (100 to 500 MPa) in the experimental lab of the University Hannover, in order to constrain the details of the magmatic processes proceeded at the Oman ophiolite paleoridge during the Cretaceous, with special focus on the influence of water on the phase stabilities and phase relations. The experiments were performed in vertically oriented internally heated pressure vessels (IHPV) (see Berndt et al., 2002; Fig. 1). This facility uses as pressure medium mixtures of Ar and H2 in order to adjust the required fH2 in the vessel, enabling us to control the redox conditions. The fH2 prevailing in the IHPV at high P and T was measured with a Shaw-membrane made of platinum. The overall variation in fO2 in all experimental series was in the range between ∼FMQ-1 and ∼FMQ+3.2, thus covering the range of oxygen fugacities prevailing in natural MORB magmas (Bezos and Humler, 2005). For understanding the magmatic processes during the Oman ophiolite paleoridge accretion, transects through the lower (GT1) and middle (GT2) crust have been drilled in the frame of ICDP (International Continental Scientific Drilling Program). Drill sites have been selected in the Wadi Tayin massif, which is known that the influence of magmatic phase 2 characterized by subduction-related primary melts is minimal. Details and progress obtained in the Oman Drilling Project (OmanDP) can be found here: (https://www.omandrilling.ac.uk/). Regarding the first magmatic phase of the processes at the Oman ophiolite paleoridge, a characteristic observation made during the description of the drilled cores GT1 (lower crust) and GT2 (mid-curst) was that quite often layers in the layered gabbro series occur which show the presence of clinopyroxene joining olivine instead of plagioclase (under near liquidus conditions). In terms of lithologies this could be interpreted as presence of wehrlitic crystal mushes as early cumulates instead of troctolitic, which are the typical ones for primary magmatism at typical fast-spreading ridges. This situation could be experimentally simulated by adding a moderate to high water activity to primitive MORB at pressures ≥ 200 MPa, resulting in a shift of the clinopyroxene-in curve to higher temperatures above the plagioclase-in curve (Feig et al. 2006; see Fig. 2). Regarding the second, late-stage magmatic phase, the formation of typical Oman high-Ca-boninites could be experimentally simulated by water-saturated partial melting of Oman harzburgite at 200 MPa and relatively low temperatures between 1100 and 1200°C. Depleted gabbronorites crosscutting layered gabbros of phase 1 magmatism can be regarded as cumulates formed in these boninitic melts. Late wehrlites crosscutting layered gabbro could be produced by accumulation of olivine and clinopyroxene at temperatures between 1040 and 1080°C in a hydrous gabbroic system at pressures > 100 MPa with bulk water content of 2–3 wt%. © 2020 Geological Society of China
2335.
Glacial-interglacial cycles largely controlled mass movements during the late Quaternary in Lake El'gygytgyn, Siberia
Palaeogeography, Palaeoclimatology, Palaeoecology,
539
2020
ISSN: 00310182
Publisher: Elsevier B.V.
Keywords:▾
glacial-interglacial cycle; lacustrine deposit; lake level; mass movement; paleoclimate; paleoenvironment; Quaternary; reconstruction; slope failure; turbidity current, Chukchi; Elgygytgyn Lake; Russian Federation; Siberia
Abstract: ▾ Density-driven mass movement deposits (MMDs) were mapped throughout the Quaternary sedimentary record of Lake El'gygytgyn (NE Siberia) using high-resolution acoustic data. Three different acoustic facies types were identified in the lake: (1) pelagic sediment, (2) plastic flow deposits and (3) turbidites. Deposits from plastic flows are dominantly present proximal to the lakes' slopes, whereas deposits from turbidity currents occur more spatially distributed. During glacial times, the distribution of MMDs was more uniform, while during interglacial periods, MMDs were deposited predominantly close to the slope of the northwestern area. Furthermore, the overall number of MMDs and accumulated sediment volume significantly varies between glacial/interglacial periods. About 1.6 times more MMDs were mapped during interglacials, contributing to a 3.5 times higher sediment volume. The main reason for this large difference is that a significant increase in plastic flows were formed during interglacials, which account for a much larger volume of sediments when compared with the glacial intervals characterized by increased amount of turbidites. It appears that the most important source areas for MMDs are located at the northern and western shores. Cycles of lake level changes caused by variations in climate conditions between glacials and interglacials are likely the main trigger mechanism for the generation of these MMDs. The climate-dependent genesis and partly erosive potential influencing the sedimentary record contain implications to consider for future paleo-environmental reconstructions in lacustrine settings. © 2019 Elsevier B.V.
2334.
2333.
Electrical conductivity of Icelandic deep geothermal reservoirs up to supercritical conditions: Insight from laboratory experiments
Journal of Volcanology and Geothermal Research,
391
2020
2332.
Emission spectra of a simulated Chicxulub impact-vapor plume at the Cretaceous–Paleogene boundary
Icarus,
346
2020
Abstract: ▾ A plume of vaporized sediments and basement rocks was ejected to the top of atmosphere when a 10–15 km asteroid impacted on Yucatan in the Southern Gulf of Mexico about 66 million years ago. The Chicxulub impact-vapor plume emitted a flash of light that had clues on the chemistry and degree of vaporization of the target surface material. Here we simulate the asteroid impact by vaporizing marine carbonate sediments cored in the Yaxcopoil-1 borehole in the Chicxulub crater using an intense infrared laser pulse. We investigate two sedimentary layers that represent the most dominant mineral phases of the target sequence: carbonates and sulfates. Their main constituents are 86% calcite and 74% anhydrite, respectively. The laser-induced vapor plumes were produced from each layer in a background simulated late Cretaceus atmosphere (0.16% CO2, 30% O2, and 69.84% N2). Time-resolved spectroscopic analyses from the laser-induced plumes were carried out using experimental and synthetic spectra. The vapor plumes had similar temperatures (≥7800 K) at 1 μs and their spectra showed similar emissions. The spectra contained the following lines in nm: Ca+ (mostly at 393.4 and 396.9 with less prominence at 370.6 and 373.7), Ca (422.7, 430.3, 443.6, 445.5, 527.0, 560.3, 616.4, and 657.3), N (746.8 and 821.6), O (777.7), and C (794.5). Molecular bands were not conspicuous which indicated complete vaporization of the target material by the laser pulse. The contribution of the granitic basement was examined using synthetic spectra. The expected emissions according to their intensities are: Na (589.6), Ca+ (393.4), Al (396.2, 309.3), Ca+ (396.9), Ca (422.7), Na (819.5) and K (766.5, 769.9). The results suggest that the emission corresponded to Ca+ and Ca originated mostly from the volatilization of the marine sediments, and Na, Al, and K from the basement rocks. The physico-chemical evolution of the Chicxulub impact-vapor plume could be deduced by deciphering the temperature and electron density from the emission lines of Ca and Ca+. These physical parameters can be used in gas dynamic models to predict the fluxes and nature of gases, vapors and mineral phases that were introduced into the atmosphere and better assess their impact to the environment and the biosphere. © 2020 Elsevier Inc.
2331.
Engineered carbon mineralization in ultramafic rocks for CO2 removal from air: Review and new insights
Chemical Geology,
550
2020
2330.
Environmental changes in Arctic Chukotka during Marine Isotope Stages 38-31: implications for the Early to Middle Pleistocene transition
Boreas,
49
(1)
168 – 180
2020
2329.
Geothermal energy and ore-forming potential of 600 °C mid-ocean-ridge hydrothermal fluids
Geology,
48
(12)
1221 – 1225
2020
DOI:10.1130/G47791.1
2328.
ExoMars raman laser spectrometer: A tool for the potential recognition of wet-target craters on mars
Astrobiology,
20
(3)
349-363
2020
Abstract: ▾ In the present work, near-infrared, laser-induced breakdown spectroscopy, Raman, and X-ray diffractometer techniques have been complementarily used to carry out a comprehensive characterization of a terrestrial analogue selected from the Chesapeake Bay impact structure (CBIS). The obtained data clearly highlight the key role of Raman spectroscopy in the detection of minor and trace compounds, through which inferences about geological processes occurred in the CBIS can be extrapolated. Beside the use of commercial systems, further Raman analyses were performed by the Raman laser spectrometer (RLS) ExoMars Simulator. This instrument represents the most reliable tool to effectively predict the scientific capabilities of the ExoMars/Raman system that will be deployed on Mars in 2021. By emulating the analytical procedures and operational restrictions established by the ExoMars mission rover design, it was proved that the RLS ExoMars Simulator can detect the amorphization of quartz, which constitutes an analytical clue of the impact origin of craters. Beside amorphized minerals, the detection of barite and siderite, compounds crystallizing under hydrothermal conditions, helps indirectly to confirm the presence of water in impact targets. Furthermore, the RLS ExoMars Simulator capability of performing smart molecular mappings was successfully evaluated. © Copyright 2020, Mary Ann Liebert, Inc., publishers 2020.
2327.
Environmental changes in the eastern Arctic 1.6975-1.965 Ma, palynological investigations from El'gygytgyn Lake, northeastern Russia
Review of Palaeobotany and Palynology,
279
2020
2326.
Explosive interaction of impact melt and seawater following the Chicxulub impact event
Geology,
48
(2)
108-112
2020
DOI:10.1130/G46783.1
Abstract: ▾ The impact of asteroids and comets with planetary surfaces is one of the most catastrophic, yet ubiquitous, geological processes in the solar system. The Chicxulub impact event, which has been linked to the Cretaceous-Paleogene (K-Pg) mass extinction marking the beginning of the Cenozoic Era, is arguably the most significant singular geological event in the past 100 million years of Earth's history. The Chicxulub impact occurred in a marine setting. How quickly the seawater re-entered the newly formed basin after the impact, and its effects of it on the cratering process, remain debated. Here, we show that the explosive interaction of seawater with impact melt led to molten fuel-coolant interaction (MFCI), analogous to what occurs during phreatomagmatic volcanic eruptions. This process fractured and dispersed the melt, which was subsequently deposited subaqueously to form a series of well-sorted deposits. These deposits bear little resemblance to the products of impacts in a continental setting and are not accounted for in current classification schemes for impactites. The similarities between these Chicxulub deposits and the Onaping Formation at the Sudbury impact structure, Canada, are striking, and suggest that MFCI and the production of volcaniclastic-like deposits is to be expected for large impacts in shallow marine settings. © 2019 Geological Society of America.
2325.
Filling a gap in the evolution of charophytes during the Turonian to Santonian: Implications for modern physiognomy
Review of Palaeobotany and Palynology,
274
2020
ISSN: 00346667
Publisher: Elsevier B.V.
Keywords:▾
biodiversity; biogeography; charophyte; dominance; flora; fossil record; lacustrine environment; paleoecology; Santonian; sea level; sedimentology; species diversity; species occurrence; taphonomy; Turonian, China; Europe; Songliao Basin, Characeae; Charales; Clavatoraceae; Lamprothamnium; Lychnothamnus
Abstract: ▾ The fossil record of Turonian to Santonian charophytes is very poor worldwide, probably due to the limited non-marine records from this age, when there was a global high stand in the relative sea level and, thus, a reduction in non-marine habitats. After this gap, the charophyte floras changed to their modern physiognomy, with dominance of the Characeae. Charophytes from the Turonian to Santonian in the Songliao Basin help to fill this significant gap in the evolution of charophytes. The recorded flora is formed by one member of the Clavatoraceae, Atopochara trivolvis var. restricta, and a greater diversity of Characeae, including Nodosochara (Turbochara) heilongjiangensis comb. nov., Lamprothamnium ellipticum, Lychnothamnus barbosai and Lychnothamnus quantouensis comb. nov. These findings showed that the Characeae began to dominate charophyte floras during the global sea level high stand, up to 8 Ma earlier than previously reported. The occurrence of the genus Lychnothamnus in the Turonian in the Songliao Basin represents the oldest record in Eurasia of this still-living genus. The occurrence of A. trivolvis var. restricta in the Turonian to Santonian in China in comparison with its previous records in the Albian?–Cenomanian in Europe indicates that this variety survived as a relict in North China, while the more-derived variety Atopochara trivolvis var. multivolvis had evolved elsewhere. The charophyte flora thrived in large, shallow, permanent freshwater lakes and sometimes in palustrine settings of the Quantou, Qingshankou and Yaojia formations, which explains its higher biodiversity in contrast with the other records that belong mainly to brackish settings. © 2019 Elsevier B.V.
2324.
Flood frequency estimation and uncertainty in arid/semi-arid regions
Journal of Hydrology,
590
2020
2323.
Fluid Infiltration Through Oceanic Lower Crust in Response to Reaction-Induced Fracturing: Insights From Serpentinized Troctolite and Numerical Models
Journal of Geophysical Research: Solid Earth,
125
(11)
e2020JB020268
2020
2322.
Generation of sintered fault rock and its implications for earthquake energetics and fault healing
Communications Earth and Environment,
1
(1)
2020
2321.
Geochemistry, geochronology and petrogenesis of Maya Block granitoids and dykes from the Chicxulub Impact Crater, Gulf of México: Implications for the assembly of Pangea
Gondwana Research,
82128-150
2020
Abstract: ▾ The Late Paleozoic tectono–magmatic history and basement of the Maya block are poorly understood due to the lack of exposures of coeval magmatic rocks in the region. Recently, IODP–ICDP Expedition 364 recovered drill core samples at borehole M0077A from the peak ring of the Chicxulub impact crater, offshore of the Yucatán peninsula in the Gulf of México, have been studied comprehensively. In the lowermost ~600 m of the drill core, impact–deformed granitoids, and minor felsite and dolerite dykes are intercalated with impact melts and breccias. Zircon U-Pb dating of granitoids yielded ages of around 326 ± 5 Ma, representing the first recovery of Late Paleozoic magmatic rocks from the Maya block, which could be genetically related to the convergence of Laurentia and Gondwana. The granitoids show the features of high K2O/Na2O, LaN/YbN and Sr/Y ratios, but very low Yb and Y contents, indicating an adakitic affinity. They are also characterized by slightly positive ԑNd(326Ma) of 0.17–0.68, intermediate initial 87Sr/86Sr(326Ma) of 0.7036–0.7047 and two–stage Nd model age (TDM2) of 1027–1069 Ma, which may indicate a less evolved crustal source. Thus, the adakitic granitoids were probably generated by partial melting of thickened crust, with source components similar to Neoproterozoic metagabbro in the Carolina block (Pan–African Orogeny materials) along Peri–Gondwana. Felsite dykes are shoshonitic with typical continental arc features that are sourced from a metasomatic mantle wedge by slab–fluids. Dolerite dykes display OIB–type features such as positive Nb and Ta anomalies and low ThNpm/NbNpm. In our interpretation, the Chicxulub adakitic granitoids of this study are formed by crustal anatexis due to asthenospheric upwelling resulting from slab breakoff. Through comparing sources and processes of Late Paleozoic magmatism along the Peri–Gondwanan realm, a tearing slab breakoff model may explain the discontinuous magmatism that appears to have occurred during the convergence of Laurentia and Gondwana. © 2020 International Association for Gondwana Research
2320.
Geochronology and Stable Isotope Analysis of Fracture Fill and Karst Mineralization Reveal Sub-Surface Paleo-Fluid Flow and Microbial Activity of the COSC-1 Borehole, Scandinavian Caledonides
Geosciences,
10
(2)
56
2020
Keywords:▾
stable isotopes, Caledonides, deep drilling (COSC-1), in situ U-Pb geochronology, secondary minerals
Abstract: ▾ The deep biosphere hosted in fractured rocks within the upper continental crust is one of the least understood and studied ecological realms on Earth. Scarce knowledge of ancient life and paleo-fluid flow within this realm is owing to the lack of deep drilling into the crust. Here we apply microscale high spatial-resolution analytical techniques to fine-grained secondary minerals in a deep borehole (COSC-1) drilled into the Silurian-Devonian Scandinavian Caledonide mountain range in central Sweden. The aim is to detect and date signs of ancient microbial activity and low-temperature fluid circulation in micro-karsts (foliation-parallel dissolution cavities in the rock) and fractures at depth in the nappe system. Vein carbonates sampled at 684 to 2210 m show a decreased C isotope variability at depths below 1050 m; likely due to decreased influence of organic-C at great depth. Micro-karsts at 122\–178 m depth feature at least two generations of secondary calcite and pyrite growth in the voids as shown by secondary ion mass spectrometry analytical transects within individual grains. The younger of these two precipitation phases shows 34S-depleted δ34Spyrite values (−19.8 ± 1.6‰ vs. Vienna-Canyon Diablo Troilite (V-CDT)) suggesting microbial sulfate reduction in situ. The calcite of this late phase can be distinguished from the older calcite by higher δ18Ocalcite values that correspond to precipitation from ambient meteoric water. The late stage calcite gave two separate laser ablation inductively coupled mass spectrometry-derived U-Pb ages (9.6 ± 1.3 Ma and 2.5 ± 0.2 Ma), marking a minimum age for widespread micro-karst formation within the nappe. Several stages of fluid flow and mineral precipitation followed karst formation; with related bacterial activity as late as the Neogene-Quaternary; in structures presently water conducting. The results show that our combined high spatial-resolution stable isotope and geochronology approach is suitable for characterizing paleo-fluid flow in micro-karst; in this case, of the crystalline crust comprising orogenic nappe units.