Description: Highlights: • Marie Byrd Seamounts (MBS) formed off Antarctica at 65-56 Ma in an extensional regime • MBS originate from HIMU-type mantle attached at the base of the Antarctic lithosphere • Continental insulation flow transferred HIMU mantle into the oceanic mantle New radiometric age and geochemical data of volcanic rocks from the guyot-type Marie Byrd Seamounts (MBS) and the De Gerlache Seamounts and Peter I Island (Amundsen Sea) are presented. 40Ar/39Ar ages of the shield phase of three MBS are Early Cenozoic (65 to 56 Ma) and indicate formation well after creation of the Pacific-Antarctic Ridge. A Pliocene age (3.0 Ma) documents a younger phase of volcanism at one MBS and a Pleistocene age (1.8 Ma) for the submarine base of Peter I Island. Together with published data, the new age data imply that Cenozoic intraplate magmatism occurred at distinct time intervals in spatially confined areas of the Amundsen Sea, excluding an origin through a fixed mantle plume. Peter I Island appears strongly influenced by an EMII type mantle component that may reflect shallow mantle recycling of a continental raft during the final breakup of Gondwana. By contrast the Sr-Nd-Pb-Hf isotopic compositions of the MBS display a strong affinity to a HIMU type mantle source. On a regional scale the isotopic signatures overlap with those from volcanics related to the West Antarctic Rift System, and Cretaceous intraplate volcanics in and off New Zealand. We propose reactivation of the HIMU material, initially accreted to the base of continental lithosphere during the pre-rifting stage of Marie Byrd Land/Zealandia to explain intraplate volcanism in the Amundsen Sea in the absence of a long-lived hotspot. We propose continental insulation flow as the most plausible mechanism to transfer the sub-continental accreted plume material into the shallow oceanic mantle. Crustal extension at the southern boundary of the Bellingshausen Plate from about 74 to 62 Ma may have triggered adiabatic rise of the HIMU material from the base of Marie Byrd Land to form the MBS. The De Gerlache Seamounts are most likely related to a preserved zone of lithospheric weakness underneath the De Gerlache Gravity Anomaly.

Description: The Middle Cambrian (~ 540 Ma) Gahcho Kué Kimberlite Field is situated about 275 km ENE of Yellowknife, NWT, Canada. The kimberlites were emplaced into 2.6 Ga Archean granitic rocks of the Yellowknife Supergroup. Four larger kimberlite bodies (5034, Tesla, Tuzo, and Hearne) as well as a number of smaller pipes and associated sheets occur in the field. In plan view, the Tuzo pipe has a circular outline at the surface, and it widens towards deeper levels. The pipe infill consists of several types of coherent and fragmental kimberlite facies. Coherent or apparent coherent (possibly welded) kimberlite facies dominate at depth, but also occur at shallow levels, as dikes intruded late in the eruptive sequence or individual coherent kimberlite clasts. The central and shallower portions of the pipe consist of several fragmental kimberlite varieties that are texturally classified as Tuffisitic Kimberlites. The definition, geometry and extent of the geological units are complex and zones controlled by vertical elements are most significant. The fluidal outlines of some of the coherent kimberlite clasts suggest that at least some are the product of disruption of magma that was in a semi-plastic state or even of welded material. Ragged clasts at low levels are inferred to form part of a complex peperite-like system that intrudes the base of the root zone. A variable, often high abundance of local wall-rock xenoliths between and within the kimberlite phases is observed, varying in size from sub-millimeter to several tens of meters. Wall-rock fragments are common at all locations within the pipe but are especially frequent in a domain with a belt-like geometry between 120 and 200 m depth in the pipe. Steeply outward-dipping bedded deposits made up of wall-rock fragments occur in deep levels of the pipe and are especially common under the downward-widening roof segments. The gradational contact relationships of these deposits with the surrounding kimberlite-bearing rocks as well as their location suggest that they formed more-or-less in situ. Different breccia facies inside the pipe suggest an origin by slumping, grain flows, rock fall or pyroclastic deposition. The shape and facies architecture of the Tuzo pipe suggests that the studied section of the pipe lies at a root zone–diatreme transitional structural level. Composite coherent kimberlite clasts imply that recycling processes were active over time, while reworked wall-rock rich deposits and ductily-deformed clasts of welded kimberlite point to the presence of temporary cavities in the root zone. The emplacement of the Tuzo pipe did not occur in a single, violent explosion, but involved repetitive volcanic explosions alternating with periods of relative quiescence. The observed features are typical of phreatomagmatic processes, which may include phases of less-explosive magmatic activity.

Description: New observations with the submersible ALVIN and deep-tow camera show that bedded sheet-hyaloclastites are common deposits between 2024 and 1723 m depth on the upper flank of Seamount Six, located on the Cocos plate at 12°45′N, 102°35′W. The individual sheets are highly localized and of small areal extent (〈200 m2), though no vent sites were found. Several facies associations of hyaloclastite, with pillow talus, knobbly fist-sized lava fragments and thin sheet lava (〈10 cm) underlying hyaloclastite, are identified. Recovered samples consist of angular, polyhedral sand-sized sideromelane shards and thin, bent, plate-like sideromelane fragments called Limu O'Pelee. Limu are solidified fragments of burst magma bubbles, which formed by vapourization of water entrapped by lava. Analysis of lava and hyaloclastite shards including limu shows three geochemically distinct populations, depleted MORB (N1), more evolved NMORB (N2) and hawaiite (H) of diverse composition. In individual hyaloclastite samples, shards of two or three different types may occur in various proportions, though in samples of hyaloclastite associated with sheet lava the predominant shards are of the same geochemical type as the sheet lava. Deposition of hyaloclastites occurred from lateral density currents formed by transformation from convective suspension settling. Grain size distribution, settling behaviour of different co-deposited shard types and sedimentary structures, together with pelagic ooze in the matrix and geochemically mixed shard populations, indicate some erosion, traction reworking and turbulence during transport. Critical observations are that contorted sheet lava protrudes through hyaloclastite and that sheet lava flow vugs commonly contain pelagic ooze. Facies associations plus consideration of limu formation allow the establishment of a new, multi-component model of hyaloclastite formation. It is inferred that the formation of limu-bearing sheet hyaloclastite involves entrapment of pelagic sediment beneath or within lava. This leads to limu bubble formation and suppressed tephra jetting. Together with convectively rising water heated by the lava flow, these processes loft shards slightly into the water column, from which they settle singly or in vertical sediment gravity flows that are redirected to flow along the seafloor.