IODP Expedition 340 successfully drilled a series of sites offshore Montserrat, Martinique and Dominica in the Lesser Antilles from March to April 2012. These are among the few drill sites gathered around volcanic islands, and the first scientific drilling of large and likely tsunamigenic volcanic island-arc landslide deposits. These cores provide evidence and tests of previous hypotheses for the composition and origin of those deposits. Sites U1394, U1399, and U1400 that penetrated landslide deposits recovered exclusively seafloor sediment, comprising mainly turbidites and hemipelagic deposits, and lacked debris avalanche deposits. This supports the concepts that i/ volcanic debris avalanches tend to stop at the slope break, and ii/ widespread and voluminous failures of preexisting low-gradient seafloor sediment can be triggered by initial emplacement of material from the volcano. Offshore Martinique (U1399 and 1400), the landslide deposits comprised blocks of parallel strata that were tilted or microfaulted, sometimes separated by intervals of homogenized sediment (intense shearing), while Site U1394 offshore Montserrat penetrated a flat-lying block of intact strata. The most likely mechanism for generating these large-scale seafloor sediment failures appears to be propagation of a decollement from proximal areas loaded and incised by a volcanic debris avalanche. These results have implications for the magnitude of tsunami generation. Under some conditions, volcanic island landslide deposits composed of mainly seafloor sediment will tend to form smaller magnitude tsunamis than equivalent volumes of subaerial block-rich mass flows rapidly entering water. Expedition 340 also successfully drilled sites to access the undisturbed record of eruption fallout layers intercalated with marine sediment which provide an outstanding high-resolution data set to analyze eruption and landslides cycles, improve understanding of magmatic evolution as well as offshore sedimentation processes.
The Monviso ophiolite Lago Superiore Unit (LSU; Western Alps) constitutes a well-preserved, largely coherent fragment of eclogitic upper oceanic lithosphere subducted to c. 80 km depth (between 50 and 40 Ma) and exhumed along the subduction interface. Within-slab, 10–100 m thick, eclogite-facies shear zones cut this section; the Intermediate Shear Zone (ISZ) follows the boundary between gabbroic and basaltic eclogites (associated with minor calcschist lenses), and the Lower Shear Zone (LSZ) marks the contact between gabbroic eclogites and the antigorite serpentinite sole. Up to 10 m fragments of mylonitic gabbroic eclogites were transported within serpentinite schists from the LSZ during eclogite-facies deformation. Metasomatic rinds, formed on these fragments during peak to early retrograde lawsonite-eclogite-facies metamorphism (c. 550°C and 2·6 GPa), document episodic, prominent rock–fluid interaction along intra-slab, channelized fluid migration pathways associated with deformation. We present new petrological and geochemical data on hydrous eclogites (talc-, chlorite-, lawsonite- and phengite-bearing eclogites) and serpentinite-derived ultrabasic schists from block rinds. Bulk-rock compositions, laser ablation inductively coupled plasma mass spectrometry mineral analyses and X-ray Cr and Mg maps of garnet and clinopyroxene demonstrate that these samples underwent significant enrichments in Mg, Cr, Ni, ± large ion lithophile elements and prominent depletions in Fe and V during eclogite-facies metasomatism. Boron isotope data for phengite (δ11B = 0 to + 7‰; 80 < B < 130 µg g–1), clinopyroxene and chlorite (δ11B = –7 to + 4‰; B < 3 µg g–1), and antigorite (δ11B = –4 to 0‰; 20 < B < 30 µg g–1) suggest that the metasomatic block rinds formed during interaction with serpentinite-derived fluids. These compositional patterns point to focused, fluid-mediated element transfer through the subducted slab. Serpentinite-derived fluids, via antigorite breakdown some 15–20 km deeper than the maximum depth reached by these eclogites, thereby equilibrate with fluids derived from oceanic crust and/or sedimentary material. Although slab components of diverse flavour have long been recognized to be central in triggering island arc magmatism, this example is among the first to document synkinematic, long-range, large-scale, channelized dehydration fluid flow within subducted oceanic slabs.
