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  1. The objectives of International Ocean Discovery Program (IODP) Expedition 398, Hellenic Arc Volcanic Field (11 December 2022 to 10 February 2023), were to study the volcanic record of the central Hellenic island arc; document the links and feedbacks between volcanism/magmatism, crustal tectonics, and sea level; investigate the processes and products of shallow submarine eruptions of silicic magma; and groundtruth the seismic stratigraphy of Santorini caldera. Reconstructing the subsidence history of the southern Aegean Sea and searching for deep life inside and outside of Santorini caldera were additional objectives. The expedition drilled 10 primary and alternate sites that were originally proposed, in addition to 2 extra sites that were requested during the expedition. Outside of Santorini caldera, drilling penetrated the thick basin fills of the crustal rift system hosting the Christiana-Santorini-Kolumbo volcanic field, identifying numerous pumice and ash layers, some known from on land and others hitherto unknown, pushing back the onset of volcanism in the area into the Early Pleistocene or even Pliocene. Significant events of mass wasting into the basins, accompanied by very high sedimentation rates, were also documented. These basin sites served to groundtruth the seismic stratigraphy of the basins and to open the way to unraveling relationships between volcanic activity and crustal rift pulses. Two sites of condensed sequences on the basin margins served to sample many volcanic layers within the detailed age-depth constraints provided mainly by biostratigraphy, as diagenetic effects complicated the magnetic reversal record significantly. Drilling penetrated the Alpine basement at three basin sites northeast of Santorini, whereas in the Christiana Basin to the southwest it penetrated a thick sequence of Messinian evaporites. Drilling inside Santorini caldera penetrated to ~120 meters below seafloor (mbsf), less than planned due to hole instability issues but deep enough to groundtruth the seismic stratigraphy and to sample the different layers. One intracaldera hole yielded a detailed tephra record of the history of the Kameni Islands, as well as possible evidence for deep bacterial colonies within the caldera. Despite variable recovery in the unstable pumice and ash deposits, the expedition was a significant success that may address almost all the science objectives once the laboratory work has been done. A dense program of preexpedition and shipboard outreach during the expedition gave rise to 59 live ship-to-shore tours, reaching 6,400 people in 7 countries including many school children. A total of 51 journalists were contacted and 9 stories were written about the expedition, with a readership of almost 200,000 people. While in Santorini caldera, the ship hosted 12 documentarians and journalists, the future products of whom should include a 1.5 h documentary and a four-part TV series about Expedition 398. The expedition social media pages were active. Prior to the expedition, an exhibition, “In Search of Earth’s Secrets,” ran for a week on Santorini and was visited by more than 1,800 school children. 
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    Free, publicly-accessible full text available January 16, 2025
  2. We report on a total of 1005 samples analyzed for major and trace element compositions from marine sediments drilled along the Hikurangi subduction zone and within the incoming Pacific plate. The samples are from International Ocean Discovery Program Expeditions 375 and 372; Integrated Ocean Drilling Program Expedition 329; Ocean Drilling Program Leg 181; and Deep Sea Drilling Project Leg 90. All 1005 samples, resulting in a total number of ~20,200 individual measurements, were analyzed for major element compositions with the electron microprobe. A subset of 419 samples, resulting in a total number of ~1820 individual glass shard analyses, were analyzed for trace element compositions using the laser ablation-inductively coupled plasma-mass spectrometer. In total, ~640 samples were identified as primary ash layers based on their homogeneous geochemistry, visual appearance in the core pictures, and high amount of volcanic glass. Based on the biostratigraphy presented in the cruise reports and subsequent work, we can distinguish between Quaternary- and Neogene-derived tephras. The tephra layers of Quaternary age are mostly of rhyolitic composition with occasional andesitic, dacitic, and trachytic glass shards. The Neogene tephras are mostly of basaltic andesite, andesitic, and rhyolitic composition, with a few basaltic and trachytic tephras. Tephras of both age groups follow the calc-alkaline series trend with a tendency to shift into the high-K calc-alkaline series for tephras with >70 wt% SiO2. 
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  3. We report on a total of 310 samples from marine sediments drilled in the Indian Ocean that were analyzed for glass shard compositions. Samples are mainly from International Ocean Discovery Program Expeditions 353 and 362 but are complemented by samples from Expedition 354; Ocean Drilling Program Legs 183, 121, 120, 119, 116, and 115; and Deep Sea Drilling Project Leg 22. We performed 4327 successful single glass shard analyses with the electron microprobe for major element compositions and conducted 937 successful single analyses with laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) for trace element compositions on individual glass shards previously measured with the electron microprobe. In total, we were able to measure glass compositions for 254 samples. Of all the samples, 235 can be classified as tephra layers containing pyroclasts as the predominant component in their clast inventory between the 63 and 125 µm grain size fraction, often exceeding 90 vol%. The compositions of the Indian Ocean marine tephras range from basalt to rhyolite and from basaltic trachyandesite to trachyte and fall into the calc-alkaline, K-rich calc-alkaline, and shoshonitic magmatic series. 
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  4. The understanding of island arc volcanism and associated hazards requires study of the processes that drive such volcanism and how the volcanoes interact with their marine surroundings. What are the links and feedbacks between crustal tectonics, volcanic activity, and magma genesis? What are the dynamics and impacts of submarine explosive volcanism and caldera-forming eruptions? How do calderas collapse during explosive eruptions and then recover to enter new magmatic cycles? What are the reactions of marine ecosystems to volcanic eruptions? The Christiana-Santorini-Kolumbo (CSK) volcanic field on the Hellenic volcanic arc is a unique system for addressing these questions. It consists of three large volcanic centers (Christiana, Santorini, and Kolumbo), and a line of small submarine cones, founded on thinned continental crust in a 100 km long rift zone that cuts across the island arc. The marine rift basins around the CSK field, as well as the Santorini caldera, contain volcano-sedimentary fills up to several hundreds of meters thick, providing rich archives of CSK volcanic products, tectonic evolution, magma genesis and paleoenvironments accessible only by deep drilling backed up by seismic interpretations. We will drill four primary sites in the rift's basins and two additional primary sites inside the Santorini caldera. The expedition science has five main objectives, each with a leading testable hypothesis, and two secondary objectives. Deep ocean drilling will enable us to identify, characterize, and interpret depositional packages visible on seismic images, chemically correlate primary volcaniclastic layers in the rift fills with their source volcanoes, fill in the many gaps in the onshore volcanic records, provide a tight chronostratigraphic framework for rift tectonic and sedimentary histories, and sample deep subsurface microbial life. 
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  5. Abstract

    International Ocean Discovery Program Expedition 352 recovered sedimentary‐volcaniclastic successions and extensional structures (faults and extensional veins) that allow the reconstruction of the Izu‐Bonin forearc tectonic evolution using a combination of shipboard core data, seismic reflection images, and calcite vein microstructure analysis. The oldest recorded biostratigraphic ages within fault‐bounded sedimentary basins (Late Eocene to Early Oligocene) imply a ~15 Ma hiatus between the formation of the igneous basement (52 to 50 Ma) and the onset of sedimentation. At the upslope sites (U1439 and U1442) extension led to the formation of asymmetric basins reflecting regional stretch of ~16–19% at strain rates of ~1.58 × 10−16to 4.62 × 10−16s−1. Downslope Site U1440 (closer to the trench) is characterized by a symmetric graben bounded by conjugate normal faults reflecting regional stretch of ~55% at strain rates of 4.40 × 10−16to 1.43 × 10−15s−1. Mean differential stresses are in the range of ~70–90 MPa. We infer that upper plate extension was triggered by incipient Pacific Plate rollback ~15 Ma after subduction initiation. Extension was accommodated by normal faulting with syntectonic sedimentation during Late Eocene to Early Oligocene times. Backarc extension was assisted by magmatism with related Shikoku and Parece‐Vela Basin spreading at ~25 Ma, so that parts of the arc and rear arc, and the West Philippine backarc Basin were dismembered from the forearc. This was followed by slow‐rift to postrift sedimentation during the transition from forearc to arc rifting to spreading within the Shikoku‐Parece‐Vela Basin system.

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