A complex magmatic system beneath the middle and northern Okinawa Trough: evidence from pyroxene characteristics

The Okinawa Trough is an initial marginal back‐arc basin. In this paper, we present a comparative study of pyroxenes in white and black pumice from the Okinawa Trough to explore the magmatic evolution of the region and the characteristics of the magmatic system. The results show two species of pyroxenes in the white pumice (clinopyroxene and orthopyroxene) but only one species in the black pumice (orthopyroxene). Many isomorphic replacements among the cations occurred during clinopyroxene crystallization. Clinopyroxene‐melt thermobarometry yielded a temperature of 847–988°C and a pressure of 2.73–7.42 kbar, which vary considerably and correspond to a maximum depth of approximately 24 km. All of the orthopyroxenes in both the white and black pumice crystallized at similar pressures ranging from 0.67 to 4.59 kbar, which correspond to a maximum depth of approximately 15 km. The clinopyroxenes crystallized mainly in the lower crust, whereas the orthopyroxenes formed at the boundary depth between the upper crust and the lower crust. The similarities in the pyroxene chemical compositions and the whole‐rock characteristics between the two types of pumice indicate that both types of pumice are linked to the same magmatic system at depth. The pyroxenes crystallized from genetically related melts within a magmatic system that had undergone various degrees of differentiation. The physicochemical characteristics of the pyroxenes show that the magma chamber module changed during the crystallization process from clinopyroxenes to orthopyroxenes. This change most likely resulted from local characteristics of the near‐surface structure. These observations support the model of a ‘two‐layered magma chamber’ in the northern Okinawa Trough. Copyright © 2016 John Wiley & Sons, Ltd.

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Morphometric analysis of Early Permian fusulinids from east‐central Iran: a new approach to defining fusulinid species reliably

Fusulinid fauna from the late Sakmarian–early Artinskian Khan Formation in the Kalmard area, east‐central Iran, was studied using morphometric techniques to test the reliability of previous taxonomic descriptions. Fusulinids in the Kalmard area have been previously grouped into three genera (Eoparafusulina, Perigondwania, and Neodutkevitchia) and ‘20 species’. Results of ordination analyses (PCA and RDA) reveal that the genera are readily distinguished, whereas only a few species previously defined within the genera Eoparafusulina and Perigondwania are recognizable. This suggests that the taxonomic descriptions and measurements for different structural elements for some of these fusulinid species are not sufficient to justify erecting separate taxonomic names. Since fusulinids are really important fauna for biostratigraphic and palaeobiogeographic interpretation in the late Palaeozic, defining species based on quantitative analysis of their morphological structures should be more accurate and improve the quality of regional correlations. Copyright © 2016 John Wiley & Sons, Ltd.

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Tectonic settings of continental crust formation: Insights from Pb isotopes in feldspar inclusions in zircon

Most crustal rocks derive from preexisting crust, and so the composition of newly generated (juvenile) continental crust, and hence the tectonic settings of its formation, have remained difficult to determine, especially for the first billion years of Earth’s evolution. Modern primitive mantle–derived magmas have distinct U/Pb ratios, depending on whether they are generated in intraplate (mean U/Pb = 0.37) or in subduction settings (mean U/Pb = 0.10). The U/Pb ratio can therefore be used as a proxy for the tectonic settings in which juvenile continental crust is generated. This paper presents a new way to see back to the U/Pb ratios of juvenile continental crust that formed hundreds to thousands of millions of years ago, based on ion probe analysis of Pb isotopes in alkali feldspar and plagioclase inclusions within well-dated zircons. Pb isotope data are used to calculate the time-integrated U/Pb ratios (i.e., 238U/204Pb = µ) for the period between the Hf model age and the U-Pb crystallization age of the zircons. These time-integrated ratios reflect the composition of the juvenile continental crust at the time it was extracted from the mantle, and so they can be used as a proxy for the tectonic setting of formation of that crust. Two test samples with Proterozoic Hf model ages and Paleozoic crystallization ages have feldspar inclusions with measured Pb isotope ratios that overlap within analytical error for each sample. Sample Z7.3.1 from Antarctica has Pb isotope ratios (mean 206Pb/204Pb = 16.88 ± 0.08, 1) that indicate it was derived from source rocks with low U/Pb ratios (~0.11), similar to those found in subduction-related settings. Sample Temora 2 from Australia has more radiogenic Pb isotope ratios (mean 206Pb/204Pb = 19.11 ± 0.23, 1) indicative of a source with higher U/Pb ratios (~0.36), similar to magmas generated in intraplate settings. Analysis of detrital populations with a range of Hf model ages (e.g., Hadean to Phanerozoic), and for which zircons and their inclusions represent the only archive of their parent magmas, should ultimately open new avenues to our understanding of the formation and the evolution of the continental crust through time.

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First terrestrial occurrence of tistarite (Ti2O3): Ultra-low oxygen fugacity in the upper mantle beneath Mount Carmel, Israel

The minimum oxygen fugacity (fO2) of Earth’s upper mantle probably is controlled by metal saturation, as defined by the iron-wüstite (IW) buffer reaction (FeO -> Fe + O). However, the widespread occurrence of moissanite (SiC) in kimberlites, and a suite of super-reduced minerals (SiC, alloys, native elements) in peridotites in Tibet and the Polar Urals (Russia), suggest that more reducing conditions (fO2 = 6–8 log units below IW) must occur locally in the mantle. We describe pockets of melt trapped in aggregates of corundum crystals ejected from Cretaceous volcanoes in northern Israel which contain high-temperature mineral assemblages requiring extremely low fO2 (IW < –10). One abundant phase is tistarite (Ti2O3), previously known as a single grain in the Allende carbonaceous chondrite (Mexico) and believed to have formed during the early evolution of the solar nebula. It is associated with other reduced phases usually found in meteorites. The development of super-reducing conditions in Earth’s upper mantle may reflect the introduction of CH4 + H2 fluids from the deep mantle, specifically related to deep-seated volcanic plumbing systems at plate boundaries.

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Recycling in debris-filled volcanic vents

Eruptions through debris-filled vents produce deposits containing magmatic juvenile lithic and recycled clasts. Recycled clasts are exposed to multiple transportation and fragmentation events. We used experiments with multiple subsurface explosions to track clasts and highlight dominant recycling processes in eruptions through analog debris-filled vents. Recycled clasts include those that fall back into and reside in the vent for extended time periods and those that return to the vent through crater growth or collapse. Clasts are recycled by any combination of lofting and fallback of material in the crater by explosion jets, mixing and churning of material at depth in the debris fill, and redistribution of extra-crater deposits by explosion-induced excavation or slumping. We compare experimental processes with natural deposits that preserve recycling signatures from discrete explosions through debris-filled vents such as maar-diatremes, Strombolian vents, and hydrothermal craters. Clasts may not preserve textures diagnostic of their complete recycling histories, but can be used to infer if that history occurred in part in the vent debris or in the eruptive jet. Experiment results and natural deposits suggest that for volcanic craters that undergo multiple explosions, clasts likely undergo some form of recycling before final deposition outside the craters. The underestimation of recycled clast contributions to deposits can lead to inaccurate estimates of thermal budgets and eruption processes.

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Control of lithospheric inheritance on neotectonic activity in northwestern Canada?

Lithospheric inheritance is thought to affect the location and reactivation of tectonic structures through successive cycles of supercontinent formation and dispersal; however, its relation to neotectonic activity remains unclear. In northwestern Canada, abundant seismicity throughout the northern Canadian Cordillera (NCC) is geographically confined by several crustal-scale boundaries, yet its southern extent terminates abruptly along the inferred westward extension of a Late Cretaceous rifted margin boundary called the Liard transfer zone (LTZ). We use seismic data to show that the uppermost mantle beneath the Cordillera exhibits a sharp north-south contrast in fabric across the LTZ. South of the LTZ, fast axes of seismic wave propagation align closely with the lithospheric mantle fabric orientation of the adjacent Canadian shield. North of the LTZ, fast axes are reoriented subparallel to the motion of the Pacific plate and follow the strike of the large dextral strike-slip Tintina and Denali faults. We attribute changes in anisotropic delay times across the Tintina and Denali faults to localized shear within the lithosphere; this implies that the crust and lithospheric mantle remained mechanically coupled during shearing. We propose that the contrast in uppermost mantle structure across the LTZ reflects a change in the nature and origin of the lithospheric mantle from inherited rifted margin structures, which affects the stability of the lithosphere and limits the extent of seismic activity within the NCC. These results indicate that neotectonic activity in modern Cordilleras is controlled in part by inherited upper mantle structures.

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Nanoscale deformation twinning in xenotime, a new shocked mineral, from the Santa Fe impact structure (New Mexico, USA)

Shock microstructures in refractory accessory minerals such as zircon and monazite provide crucial evidence for deciphering impact-related deformation in a wide variety of planetary materials. Here we describe the first occurrence of shock deformation in xenotime, YPO4, from a shocked quartz–bearing shatter cone in granite at the Santa Fe impact structure (New Mexico, USA). Backscattered electron imaging shows that shocked xenotime grains near the surface of a shatter cone contain multiple orientations of closely spaced planar fractures. High-resolution electron backscatter diffraction mapping reveals that some of the planar microstructures in {112} contain deformation twin lamellae that range from 50 nm to 200 nm in width on the polished surface and occur in up to three crystallographic orientations. Other features attributed to impact, such as planar low-angle boundaries and planar deformation bands, record crystal-plastic deformation. Shatter cone formation and co-existing shocked quartz constrain minimum shock pressure experienced by the xenotime grains to 5–10 GPa. An upper limit of 20 GPa is tentatively assigned based on the absence of YPO4 polymorphs and shock twins in co-existing zircon. We propose that {112} deformation twins in xenotime constitute a diagnostic record of shock metamorphism, similar to {112} twins in zircon; they have not previously been reported in nature and occur in a rock with conspicuous evidence of shock deformation. Documentation of deformation twins in xenotime, a widely applied U-Pb geochronometer, can be used to identify hypervelocity deformation in shocked rocks, detrital grains, and other materials, and may be particularly ideal for recording low-pressure (<20 GPa) impact conditions that do not produce diagnostic shock microstructures in zircon.

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Kimberlites and the start of plate tectonics

We want to know when plate tectonics began and will consider any important Earth feature that shows significant temporal evolution. Kimberlites, the primary source of diamonds, are rare igneous features. We analyze their distribution throughout Earth history; most are young (~95% are younger than 0.75 Ga), but rare examples are found as far back as the Archean (older than 2.5 Ga). Although there are differing explanations for this age asymmetry (lack of preservation, lack of exposure, fewer mantle plumes, or lack of old thick lithosphere in the Archean and Proterozoic), we suggest that kimberlite eruptions are a consequence of modern-style plate tectonics, in particular subduction of hydrated oceanic crust and sediments deep into the mantle. This recycling since the onset of modern-style plate tectonics ca. 1 Ga has massively increased mantle CO2 and H2O contents, leading to the rapid and explosive ascent of diamond-bearing kimberlite magmas. The age distribution of kimberlites, combined with other large-scale tectonic indicators that are prevalent only in the past ~1 Ga (blueschists, glaucophane-bearing eclogites; coesite- or diamond-bearing ultrahigh-pressure metamorphic rocks; lawsonite-bearing metamorphic rocks; and jadeitites), indicates that plate tectonics, as observed today, has only operated for <25% of Earth history.

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Zipper junctions: A new approach to the intersections of conjugate strike-slip faults

Intersecting pairs of simultaneously active faults with opposing slip sense present geometrical and kinematic problems. Such faults rarely offset each other but usually merge into a single fault, even when they have displacements of many kilometers. The space problems involved are solved by lengthening the merged fault (zippering up the conjugate faults) or splitting it (unzippering). This process can operate in thrust, normal, and strike-slip fault settings. Examples of conjugate pairs of large-scale strike-slip faults that may have zippered up include the Garlock and San Andreas faults in California (USA), the North and East Anatolian faults (Turkey), the Karakoram and Altyn Tagh faults (Tibet), and the Tonale and Giudicarie faults (southern Alps). Intersecting conjugate ductile shear zones behave in the same way on outcrop and micro-scales. Zippering may produce complex and significant patterns of strain and rotation in the surrounding rocks, depending on the angle between the faults and the relative strength of the blocks they bound. A zippered fault will have a slip rate equal to the vector sum of the slip rates on the merging faults, unless that displacement is transferred into or out of the system by distributed strain in the surrounding rocks.

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Widespread dispersal and aging of organic carbon in shallow marginal seas

The occurrence of pre-aged organic carbon (OC) in continental margin surface sediments is a commonly observed phenomenon, yet the nature, sources, and causes of this aged OC remain largely undetermined for many continental shelf settings. Here we present the results of an extensive survey of the abundance and radiocarbon content of OC in surface sediments from the northern Chinese marginal seas. Pre-aged OC is associated with both coarser (>63 µm) and finer (<63 µm) sedimentary components; measurements on specific grain-size fractions reveal that it is especially prevalent within the 20–63 µm fraction of inner shelf sediments. We suggest that organic matter associated with this sortable silt fraction is subject to protracted entrainment in resuspension-deposition loops during which it ages, is modified, and is laterally dispersed, most likely via entrainment within benthic nepheloid layers. This finding highlights the complex dynamics and predepositional history of organic matter accumulating in continental shelf sediments, with implications for our understanding of carbon cycling on continental shelves, development of regional carbon budgets, and interpretation of sedimentary records.

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Relationship link between landward vergence in accretionary prisms and tsunami generation

The very large slip up to the subduction front of the 2011 Tohoku-Oki (Japan) earthquake has challenged our classic view of the megathrust undergoing only aseismic slip at shallow depth. Furthermore, the enhancement of tsunamis during frontal rupturing has increased concern about tsunami risks. Recent seismic reflection images from the Sumatra subduction zone show frontal landward-vergent thrusts in the accretionary prism in the area of supposed shallow ruptures and enhanced tsunamis. Using mechanical analysis, we here show that sudden and successive decreases of the effective friction along the megathrust are required to form landward-vergent frontal thrusts. These decreases are most likely caused by dynamic weakening mechanisms, such as thermal pressurization of the pore fluids related to the propagation of earthquakes to the seafloor. Therefore, landward vergence in accretionary prisms is indicative of past seafloor frontal ruptures and consequent tsunamis. The presence of landward-verging structures in the Cascadia and Sumatra accretionary prisms might indicate future frontal rupture of the shallowest portion of the megathrust, resulting in large tsunamis.

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Sikuliaq week 2 recap

We’ve done a lot of science this week! Since the last update, we’ve successfully towed the super sucker, started multi-coring, and upped our CTD tally to a whopping 87 casts, plus all the continuous surface underway data we’ve collected while steaming between sites. The scientists have some preliminary results and ideas about where they’d like to visit again (the beginning of the Wainwright line is of particular interest).

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Human activities rattle natural rock of Utah’s Rainbow Bridge

Utah’s iconic Rainbow Bridge hums with natural and man-made vibrations, according to a new study accepted for publication today in Geophysical Research Letters, a journal of the American Geophysical Union. The study found both natural waves in Lake Powell and induced earthquakes in Oklahoma cause the rock bridge to vibrate at different resonant frequencies.

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LA‐ICP‐MS U–Pb geochronology of zircons from granitoids and modern river sands in Guyang, North China Craton and their tectonic significance

The Palaeo‐Asian Ocean (PAO) was located between Siberia and the North China Craton (NCC). The time of its closure, as well as the crustal architecture of the surrounding region, are still uncertain. In this paper, we address these problems using LA‐ICP‐MS U‐Pb ages of zircon grains in three magmatic rocks and three river sand samples of the Guyang area at the northern margin of the NCC. The magmatic rocks yield early Carboniferous crystallization ages of 340.3 ± 7.5 Ma, 331.7 ± 2.3 Ma and 341.1 ± 1.9 Ma for the main granitic pluton, a granodiorite dyke and a diorite enclave, respectively. They were the product of subduction of the PAO underneath the NCC. The detrital zircon U‐Pb ages in the river sand samples are dominated by two groups at 2550–2400 and 290–260 Ma, with subordinate groups at 2700–2600, 2000–1800 and 320–300 Ma, dispersed Early Palaeoproterozoic grains and a minor but important group of Cretaceous grains in sands of the Aibugai River. These results, combined with previous geochronological data, permit a re‐assessment of the tectonic evolution of the northern margin of the NCC. The events at ~2.5 Ga reflect major crustal reworking and cratonization of the northern margin of the NCC, which then became a stable platform lasting until the Carboniferous. The period between 2.0 and 1.8 Ga was characterized by major collisions related to formation of the Khondalite Belt and the Trans‐North China Orogen. In the Carboniferous, subduction of the PAO underneath the northern margin of the NCC developed a zoned distribution of magmatic rocks. The PAO is now constrained to have closed in the Early Permian by the collision of the NCC and Siberia. The youngest group of Cretaceous zircons reflects magmatic events related to lithosphere thinning beneath the Eastern Block of the NCC. Copyright © 2016 John Wiley & Sons, Ltd.

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A MONOGRAPH OF THE AMMONITES OF THE INFERIOR OOLITE SERIES by Sydney Savory Buckman (in two volumes, including supplement). Cambridge University Press, Cambridge, 2015. No of pages: 1‐312 + 313‐456 + 1‐262. Price: Volume 1 UK£33‐00; Volume 2 UK£40‐00. ISBN 978-1-108-08433-8 (volume 1) and 978‐1‐108‐08432‐1 (volume 2) (paperback).

Book Review Authors Robert B. Chandler Department of Earth Sciences, The Natural History Museum, London, United Kingdom Search for more

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Ms. Callaghan’s Classroom: Sampling from the Sea

This is the latest in a series of dispatches from scientists and education officers aboard the National Science Foundation’s R/V Sikuliaq. Jil Callaghan is a 6th grade science teacher at Houck Middle School in Salem, Oregon. She is posting blogs for her students while aboard the Sikuliaq as part of a teacher at sea program through Oregon State University. Read more posts here. Track the Sikuliaq’s progress here. By Jil …

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