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Crystal mush is rapidly emerging as a new paradigm for the evolution of igneous systems. Mid-ocean ridges provide a unique opportunity to study mush processes: geophysical data indicate that, even at the most magmatically robust fast-spreading ridges, the magma plumbing system typically comprises crystal mush. In this paper, we describe some of the consequences of crystal mush for the evolution of the mid-ocean ridge magmatic system. One of these is that melt migration by porous flow plays an important role, in addition to rapid, channelized flow. Facilitated by both buoyancy and (deformation-enhanced) compaction, porous flow leads to reactions between the mush and migrating melts. Reactions between melt and the surrounding crystal framework are also likely to occur upon emplacement of primitive melts into the mush. Furthermore, replenishment facilitates mixing between the replenishing melt and interstitial melts of the mush. Hence, crystal mushes facilitate reaction and mixing, which leads to significant homogenization, and which may account for the geochemical systematics of mid-ocean ridge basalt (MORB). A second consequence is cryptic fractionation. At mid-ocean ridges, a plagioclase framework may already have formed when clinopyroxene saturates. As a result, clinopyroxene phenocrysts are rare, despite the fact that the vast majority of MORB records clinopyroxene fractionation. Hence, melts extracted from crystal mush may show a cryptic fractionation signature. Another consequence of a mush-dominated plumbing system is that channelized flow of melts through the crystal mush leads to the occurrence of vertical magmatic fabrics in oceanic gabbros, as well as the entrainment of diverse populations of phenocrysts. Overall, we conclude that the occurrence of crystal mush has a number of fundamental implications for the behaviour and evolution of magmatic systems, and that mid-ocean ridges can serve as a useful template for trans-crustal mush columns elsewhere. This article is part of the Theo Murphy meeting issue ‘Magma reservoir architecture and dynamics'.

\"Longing for the Bomb traces the unusual story of the first atomic city and the emergence of American nuclear culture. Tucked into the folds of Appalachia and kept off all commercial maps, Oak Ridge, Tennessee, was created for the Manhattan Project by the U.S. government in the 1940s. Its workers labored at a breakneck pace, most aware only that their jobs were helping 'the war effort.' The city has experienced the entire lifespan of the Atomic Age, from the fevered wartime enrichment of the uranium that fueled Little Boy, through a brief period of atomic utopianism after World War II when it began to brand itself as 'The Atomic City,' to the anxieties of the Cold War, to the contradictory contemporary period of nuclear unease and atomic nostalgia. Oak Ridge's story deepens our understanding of the complex relationship between America and its bombs. Blending historiography and ethnography, Lindsey Freeman shows how a once-secret city is visibly caught in an uncertain present, no longer what it was historically yet still clinging to the hope of a nuclear future. It is a place where history, memory, and myth compete and conspire to tell the story of America's atomic past and to explain the nuclear present\"-- Provided by publisher.

Analysis of new and existing geophysical data for the Central Indian and Wharton Basins of the Indian Ocean were used to understand the formation and evolution of the Ninetyeast Ridge (NER), especially its relationship to the Kerguelen hot spot and the Wharton spreading ridge. Satellite gravity data and magnetic anomalies 34 through 19 define crustal isochrons and show fracture zones striking ∼N5°E. One of these, at 89°E, crosses the ∼N10°E trending NER, impacting the NER morphology. From 77 to 43 Ma the NER lengthened at a rate of ∼118 km/Myr, twice that of the ∼48–58 km/Myr accretion rate of adjacent oceanic crust. This difference can be explained by southward jumps of the Wharton spreading ridge toward the hot spot, which transferred portions of crust from the Antarctic plate to the Indian plate, lengthening the NER. Magnetic anomalies document a small number of large spreading ridge jumps in the ocean crust immediately to the west of the NER, especially two leaving observable 65 and 42 Ma fossil spreading ridges. In contrast, complex magnetic anomaly progressions and morphology imply that smaller spreading ridge jumps occurred at more frequent intervals beneath the NER. Comparison of the NER dates and magnetic anomaly ages implies that the hot spot first emplaced NER volcanoes on the Indian plate at a distance from the Wharton Ridge, but as the northward drifting spreading ridge approached the hot spot, the two interacted, keeping later NER volcanism near the spreading ridge crest by spreading center jumps. Key Points Compiled entire magnetic data of the Ninetyeast Ridge, analyzed and interpreted Discussed the structural and magnetic pattern of the region Models proposed for Ninetyeast Ridge accretion during ridge‐hot spot interactions

American Wild: it can kill you, or exhilarate you. It's always there, a character in its own right in the great unfolding narrative of American writing. This issue of Granta is dedicated to stories of the wild, from MELINDA MOUSTAKIS on gutting fish in Alaska to CLAIRE VAYE WATKINS on a lost child in a dystopian California. Also: ANTHONY DOERR on a family of pioneers in Idaho, ADAM NICOLSON on tracking wolves in New Mexico and DAVID TREUER on cage fighting and his Ojibwe heritage.

Modern oceans contain large bathymetric highs (spreading oceanic ridges, aseismic ridges or oceanic plateaus and inactive arc ridges) that, in total, constitute more than 20–30% of the total area of the world’s ocean floor. These bathymetric highs may be subducted, and such processes are commonly referred to as ridge subduction. Such ridge subduction events are not only very common and important geodynamic processes in modern oceanic plate tectonics, they also play an important role in the generation of arc magmatism, material recycling, the growth and evolution of continental crust, the deformation and modification of the overlying plates, and metallogenesis at convergent plate boundaries. Therefore, these events have attracted widespread attention. The perpendicular or high-angle subduction of mid-ocean spreading ridges is commonly characterized by the occurrence of a slab window, and the formation of a distinctive adakite-high-Mg andesite-Nb-enriched basalt-oceanic island basalt (OIB) or a mid-oceanic ridge basalt (MORB)-type rock suite, and is closely associated with Au mineralization. Aseismic ridges or oceanic plateaus are traditionally considered to be difficult to subduct, to typically collide with arcs or continents or to induce flat subduction (low angle of less than 10°) due to the thickness of their underlying normal oceanic crust (>6–7 km) and high topography. However, the subduction of aseismic ridges and oceanic plateaus occurred on both the western and eastern sides of the Pacific Ocean during the Cenozoic. On the eastern side of the Pacific Ocean, aseismic ridges or oceanic plateaus are being subducted flatly or at low angles beneath South and Central American continents, which may cause a magmatic gap. But slab melting can occur and adakites, or an adakite-high-Mg andesite-adakitic andesite-Nb-enriched basalt suite may be formed during the slab rollback or tearing. Cu-Au mineralization is commonly associated with such flat subduction events. On the western side of the Pacific Ocean, however, aseismic ridges and oceanic plateaus are subducted at relatively high angles (>30°). These subduction processes can generate large scale eruptions of basalts, basaltic andesites and andesites, which may be derived from fractional crystallization of magmas originating from the subduction zone fluid-metasomatized mantle wedge. In addition, some inactive arc ridges are subducted beneath Southwest Japan, and these subduction processes are commonly associated with the production of basalts, high-Mg andesites and adakites and Au mineralization. Besides magmatism and Cu-Au mineralization, ridge subduction may also trigger subduction erosion in subduction zones. Future frontiers of research will include characterizing the spatial and temporal patterns of ridge subduction events, clarifying the associated geodynamic mechanisms, quantifying subduction zone material recycling, establishing the associated deep crustal and mantle events that generate or influence magmatism and Cu-Au mineralization, establishing criteria to recognize pre-Cenozoic ridge subduction, the onset of modern-style plate tectonics and the growth mechanisms for Archean continental crust.

Mapping and sampling four sections of the slow‐spreading Reykjanes Ridge provide insight into how tectonic and volcanic activity varies with distance from the Iceland plume. The studied areas are characterized by significant variations in water depth, lava chemistry, crustal thickness, thermal structure, and ridge morphology. For each study area, fault pattern and dimension, tectonic strain, seamount morphology, and density are inferred from 15 m‐resolution bathymetry. These observations are combined with geochemical analysis from glass samples and sediment thickness estimations along Remotely Operated Vehicle‐dive videos. They reveal that (a) tectonic and volcanic activity along the Reykjanes Ridge, do not systematically vary with distance from the plume center. (b) The tectonic geometry appears directly related to the deepening of the brittle/ductile transition and the maximum change in tectonic strain related to the rapid change in crustal thickness and the transition between axial‐high and axial valley (∼59.5°N). (c) Across‐axis variations in the fault density and sediment thickness provide similar widths for the neo‐volcanic zone except in regions of increased seamount emplacement. (d) The variations in seamount density (especially strong for flat‐topped seamounts) are not related to the distance from the plume but appear to be correlated with the interaction between the V‐shape ridges (VSR) flanking the ridge and the ridge axis. These observations are more compatible with the buoyant upwelling melting instability hypothesis for VSR formation and suggest that buoyant melting instabilities create many small magma batches which by‐pass the normal subaxial magmatic plumbing system, erupting over a wider‐than‐normal area. Plain Language Summary Volcanic eruptions and faults growth are two important geologic processes taking place along seafloor spreading centers. Their variations in space and time are displayed in the morphology of the spreading centers. Investigating these morphological variations is key to understanding the deeper processes of the oceanic crust formation. South of Iceland, the Reykjanes Ridge is the location of increased volcanism due to the interaction between the mid‐ocean ridge and the Iceland hotspot. Using high‐resolution seafloor topographic data, chemical analyses of volcanic rock, and videos of the seafloor from a remotely operated vehicle, we investigated how volcanism and faulting change along the ridge. The increase in fault dimensions (height, length) with distance from the plume center is probably the result of the crust and mantle becoming cooler and stiffer and thus able to support larger faults. Fault density and thickness of the sediment covering the lava flows near the ridge axis are used to delimit the region of young volcanism. Seamounts often emplaced beyond that region. A peak in seamount abundance near 60°N suggests that the thick crust here is generated from numerous small batches of magma possibly resulting from a migrating instability in the melt production process beneath the axis. Key Points The distance from the plume center is not the only factor controlling tectonic and volcanic activity along the Reykjanes Ridge Fault dimensions are primarily controlled by the variation of crustal thermal structure with distance from the hotspot Flat‐topped seamount abundances peak where a V‐shaped ridge intersects the axis, consistent with a buoyant upwelling melting instability

\"Sixteen-year-old Sam McKenna discovers that becoming one of the first girls to attend the revered Denmark Military Academy means living with a target on her back\"-- Provided by publisher.

Mid‐ocean ridge basalts reflect the mantle’s composition and reveal processes from melting to eruption. The Mohns and Knipovich Ridges have ultraslow spreading rates, low magma budgets and erupted lavas indicating various mantle domains. Here, we use geochemistry and isotope systematics of in situ samples from two axial volcanic ridges (AVRs) to study mantle heterogeneity and melt production. By linking chemical variations to high‐resolution bathymetry and age data, we document systematic changes over time in the mantle source of the volcanic sequence. At Mohns Ridge AVR‐M10 (72.3°N), we observed significant variations in chemistry (e.g., (La/Sm)N from 0.7 to 2.9) and isotope systematics in basaltic samples from a small area (∼1 km2), suggesting the emplacement of multiple small‐volume lava flows. Pb isotope variations, for example, 206Pb/204Pb (17.91–18.76), are comparable with the observed range along the entire Mohns and Knipovich Ridges. Temporal constraints document that erupted basalts have changed from highly radiogenic Pb compositions to a more depleted signature within 30 ka. To explain the extreme variations in the erupted lavas at the Mohns Ridge, the mantle would need to be highly heterogeneous in composition with effective melt extraction and limited mixing prior to eruption. We use the highly heterogenous mantle underneath the Mohns Ridge to understand the melt extraction processes and mixing of melts and propose a two‐stage melting model: continuous generation of enriched melts from a deep and fertile source in the first stage, while depleted melts from a shallower and more refractory mantle occur sporadically and simultaneously with the intermittent ascent of diapirs. Key Points Significant variations in chemistry within a small area suggest the emplacement of multiple small‐volume lava flows Temporal constraints document that erupted basalts have changed from highly radiogenic Pb compositions to a depleted signature within 30 ka Extreme mantle heterogeneity is recorded in on‐axis lavas when melt extraction is efficient and homogenization in magma chambers is limited