Kaj Hoernle

SrNdPb isotopic evolution of Gran Canaria: Evidence for shallow enriched mantle beneath the Canary Islands

We report the Sr, Nd and Pb isotopic compositions (1) of 66 lava flows and dikes spanning the circa 15 Myr subaerial volcanic history of Gran Canaria and (2) of five Miocene through Cretaceous sediment samples from DSDP site 397, located 100 km south of Gran Canaria. The isotope ratios of the Gran Canaria samples vary for 87Sr/86Sr: 0.70302–0.70346, for 143Nd/144Nd: 0.51275–0.51298, and for 206Pb/204Pb: 18.76–20.01. The Miocene and the Pliocene-Recent volcanics form distinct trends on isotope correlation diagrams. The most SiO2-undersaturated volcanics from each group have the least radiogenic Sr and most radiogenic Pb, whereas evolved volcanics from each group have the most radiogenic Sr and least radiogenic Pb. In the Pliocene-Recent group, the most undersaturated basalts also have the most radiogenic Nd, and the evolved volcanics have the least radiogenic Nd. The most SiO2-saturated basalts have intermediate compositions within each age group. Although the two age groups have overlapping Sr and Nd isotope ratios, the Pliocene-Recent volcanics have less radiogenic Pb than the Miocene volcanics. At least four components are required to explain the isotope systematics of Gran Canaria by mixing. There is no evidence for crustal contamination in any of the volcanics. The most undersaturated Miocene volcanics fall within the field for the two youngest and westernmost Canary Islands in all isotope correlation diagrams and thus appear to have the most plume-like (high 238U/204Pb) HIMU-like composition. During the Pliocene-Recent epochs, the plume was located to the west of Gran Canaria. The isotopic composition of the most undersaturated Pliocene-Recent volcanics may reflect entrainment of asthenospheric material (with a depleted mantle (DM)-like composition), as plume material was transported through the upper asthenosphere to the base of the lithosphere beneath Gran Canaria. The shift in isotopic composition with increasing SiO2-saturation in the basalts and degree of differentiation for all volcanics is interpreted to reflect assimilation of enriched mantle (EM1 and EM2) (cf. [1]) in the lithosphere beneath Gran Canaria. This enriched mantle may have been derived from the continental lithospheric mantle beneath the West African Craton by thermal erosion or delamination during rifting of Pangaea. This study suggests that the enriched mantle components (EM1 and EM2) may be stored in the shallow mantle, whereas the HIMU component may have a deeper origin.

RV SONNE Fahrtbericht / Cruise Report SO233 WALVIS II,14.05-21.06.2014,Cape Town, South Africa - Walvis Bay, Namibia

R/V SONNE cruise SO-233 WALVIS II conducted geological, morphological, and biological studies in the area of the aseismic Walvis Ridge and the adjacent ocean floor (South Atlantic). The Walvis Ridge is a textbook example of a hotspot track connected to a continental flood basalt province and represents the Atlantic “type locality” for the enriched mantle one (EM-I) geochemical endmember in intraplate volcanic rocks. Despite its importance in the global hotspot reference frame, endmember geochemical composition, and uncertainties in its formation and evolution, basement sampling of the Walvis Ridge remained poor to date, in particular along its easternmost 1500 km. The geological studies carried out during SO-233 therefore aimed for extensive multi-beam mapping using a SIMRAD EM 120 echo-sounding system, sediment echo-sounding using a ATLAS PARASOUND sub-bottom profiling system, and hard rock sampling by dredging and TV-grab of the Walvis Ridge and associated features. The major targets of the WALVIS II project are (1) to test for age progressive volcanism along the ridge, (2) to differentiate between classical hotspot and plate fracturing models for its formation, and (3) to constrain the origin, temporal and spatial evolution of melting conditions and source compositions (in particular regarding the EM-I endmember and proposed zonation models of mantle plumes). The biology program conducted on SO-233 comprised sampling of benthic organisms and meiofauna using a TV-multi-corer, a TV-grab, sediment traps installed in the geological dredges, and by collecting marine invertebrates from the hard rocks yielded by dredging. The biological investigations of the WALVIS II project intend to describe the benthic diversity of deep-sea invertebrates of the Walvis Ridge and will help to identify proxies of species connectivity and dispersal between the Walvis Ridge and neighboring ridge like structures (e.g. Agulhas Ridge). Another objective is to test whether connectivity of benthic communities in the Angola and Cape Basins is interrupted by the Walvis Ridge. SO-233 multi-beam mapping revealed that the southern bifurcated section of the Walvis Ridge appears to have formed through the coalescence of former volcanic islands. The new bathymetric data also yielded several evidence for large-scale extensional tectonic movements which are most likely related to the separation of the Walvis Ridge and Rio Grande Rise that were rifted apart by the mid Atlantic Ridge. Seventy-one dredge hauls have been conducted during SO-233. Of these, 28 delivered massive lavas, 24 volcaniclastic rocks including breccias containing lava fragments, 22 sedimentary rocks, and 11 Mn-Fe-oxide crusts and nodules. The volcanic rocks comprise a broad variety of lavas as well as epiclastic, hydroclastic, and pyroclastic rocks. Carbonates dominate among the non-volcanic rocks, many of them represent relicts of fossil coral reefs. Despite technical problems with the EM 120 system and difficult weather and seafloor conditions occasionally constraining rock sampling, SO-233 achieved its major goals, i.e. bathymetric mapping and representative hard rock sampling of all major geomorphological units of the Walvis Ridge and of associated features. The set of rock samples recovered during SO-233 represents the by far most detailed sampling of the Walvis Ridge to date. Out of 91 collecting stations, 80 stations yielded the total amount of 80 kg of sediment from sediment traps in the geological dredges, TV-multi-corer tubes and TV-grab. At 44 stations we could collect macrofaunal organisms, partly in large quantities. Ninety specimens of living brachiopods representing 6 genera were found at all depths and will mainly be used for molecular diversity studies. The remaining living macrofauna was largely composed of sponges, octocorals, some deep water hexacorals, molluscs, polychaetes, bryozoans, cirriped crustaceans and a few isopods and amphipods, mainly occurring in small numbers and medium diversity. The most spectacular finding was a fossil cold water reef mound community, which shows similarities in species composition to North Atlantic cold water reefs and proofs the influence of Antarctic benthos communities on the Walvis Ridge fauna mediated by northbound cold water currents. The samples represent the most diverse collection of benthos organisms ever retrieved from the Walvis Ridge region.

RV SONNE Fahrtbericht / Cruise Report SO255: VITIAZ – The Life Cycle ofthe Vitiaz-Kermadec Arc / Backarc System:from Arc Initiation to Splittingand Backarc Basin Formation, Auckland (New Zealand) - Auckland (New Zealand)02.03.-14.04.2017

The R/V SONNE expedition SO255 is part of the research project VITIAZ, conducted in close cooperation with New Zealandic, U.S. American, and Japanese scientists. The overarching goal of VITIAZ is to elucidate the physical and chemical conditions that control the development of subduction zones, including subduction initiation, evolution of mature arc systems, and the transition from arc splitting to backarc basin generation. This approach should also provide basic data for a better evaluation of the impact of subduction volcanism on the environment. Cruise SO255 mapped and sampled magmatic structures at the Kermadec Fore Arc, the Kermadec Ridge, the Havre Trough, and the Colville Ridge (SW-Pacific) using Kongsberg EM122 and EM710 multi-beam echo sounding systems, an Atlas PARASOUND DS P-70 sub-bottom profiler, and chain bag dredges. Besides extensive profiling, a total of 165 dredge hauls in an average water depth of 2,500 m were carried out on SO255. Of these, 137 (= 83%) delivered in situ samples of which 102 obtained lava or subvolcanic rocks, 69 volcaniclastics, and 30 sedimentary rocks. No equipment was lost or seriously damaged. The on shore work program at GEOMAR and cooperating institutions will include volcanological, geochronological, petrological and geochemical studies on igneous samples obtained during the cruise. The results of VITIAZ will be integrated with those of previous campaigns (e.g. SO249 BERING), and work carried out within the GeoPRISMS initiative. The geological program of the cruise has been completed by biological investigations intending (1) to collect planktonic gastropod specimens for morphological and molecular analysis to understand their taxonomy, biogeography and evolution; (2) to use water samples to study degradation processes in the deep sea; and (3) to describe the benthic diversity of deep-sea invertebrates in the working area by collecting macro benthos from the dredged rocks.

RV SONNE Fahrtbericht / Cruise Report SO249 BERING – Origin and Evolution of the Bering Sea:An Integrated Geochronological, Volcanological,Petrological and Geochemical Approach, Leg 1: Dutch Harbor (U.S.A.) - Petropavlovsk-Kamchatsky (Russia),05.06.2016 - 15.07.2016,Leg 2: Petropavlovsk-Kamchatsky (Russia) - Tomakomai (Japan),16.07.2016 - 14.08.2016

The R/V SONNE expedition SO-249 is part of the research project BERING, conducted in the framework of the Russian-German Agreement on Marine and Polar Research and in close cooperation with U.S. American colleagues. The overarching goal of BERING is to elucidate the magmatic and tectonic evolution of the Bering Sea and its margins over the past ≥50 m.y. In particular, BERING investigates the physical and chemical conditions that control the development of subduction zones, including subduction initiation, evolution of mature arc systems, and the impact of subduction volcanism on the environment. R/V SONNE cruise SO- 249 BERING conducted geological, morphological, and biological studies in the in western the Aleutians, the Pacific seafloor subducting beneath the Aleutians and northern Kamchatka, and in the western Bering Sea. Besides extensive multi-beam mapping and sediment echo-sounder profiling, total of 150 dredge hauls have been conducted on the two legs of cruise SO-249. Of these, 91 delivered massive magmatic and/or metamorphic rocks, 34 volcaniclastic rocks including breccias containing lava fragments, 64 sedimentary rocks, and 19 Mn-Fe-Oxide crusts and nodules. No equipment was lost or seriously damaged. SO-249 achieved its major goals and the SO-249 sample set represents the most detailed sampling of the working areas to date. The on shore work program at Russian, German, and U.S.-American institutions includes geochronological, petrological and geochemical studies on igneous samples obtained during the cruise. The results of BERING will be integrated with those of previous campaigns (e.g. KOMEX, KALMAR), and work carried out within the World Oceans and GeoPRISMS initiatives. The main goal of biological sampling was to survey the benthic biodiversity in the study area. In addition, fresh specimens pertaining to specific taxa (Cnidaria, Brachiopoda, Cephalopoda, Echinodermata) were collected to supplement ongoing research projects. Of the 150 dredges taken, 150 (100%) contained sediment and 112 (74.7%) contained macrofauna. In addition to the 150 sediment samples, almost 1,500 single benthic, benthopelagic, and pelagic macrofaunal organisms were obtained. The majority of the objectives of biological sampling were reached, in particular with regard to obtaining fresh tissue for immunohistochemical, genomic, and transcriptomic analysis from various brachiopod and ophiuroid species.

SUBDUCTION INITIATION AND EARLY EVOLUTION OF THE ALEUTIAN ARC: COMBINED ON-LAND AND OFF-SHORE STUDIES

The origin and initiation of the Aleutian Subduction Zone, forming the boundary between the Pacific Ocean and Bering Sea, remains elusive. A continuous belt of arc volcanism extended from mainland Alaska to the Siberian margin with the youngest volcanism along the Beringian margin ranging from 54-50Ma (Davies et al., 1989, CanJEarthSci26). Magnetic anomalies indicate that plate motion changed from a northerly to a westerly direction at ~56-53Ma (chron 25-24). Thereafter volcanism shifted to the Aleutian Arc, presumably trapping a fragment of the Kula plate that formed the basement of the Bering Sea. Interestingly the oldest rocks from the forearcs of the Izu-Bonin-Mariana (IBM) and the Tonga subduction systems yield ages of 50-52Ma (e.g. Reagan et al. 2013, EPSL380; Meffre et al., 2013, G3_13), suggesting subduction initiation as a consequence of a Pacific-wide tectonic reorganization. Did the Aleutian Arc also form during this plate-wide event? The oldest Aleutian ages come from the submarine Murray Canyon in the central Aleutian Arc (46Ma; Jicha et al., 2006, Geology34) and from Medny, the smaller Komandorsky Island, in the westernmost Aleutians (47Ma). The earliest volcanism on Medny (47-21Ma) is tholeiitic with clear subduction-related incompatible element signatures, e.g. relative enrichments in mobile elements such as U, Sr and Pb and depletions in Nb and Ta. Isotopic compositions for Sr-Nd-Pb indicate contributions from subducted sediment and ocean crust, as well as Komandorsky mantle wedge. In the IBM and Tonga Arc systems, boninites have been found among the earliest lavas. In contrast, no boninites have been found in the Aleutians thus far, which could suggest that the oldest lavas have not been found and that subduction may also have initiated earlier, possibly also between 52-50Ma. In order to further constrain the age of arc initiation and the compositions of lavas formed during arc initiation, we have just completed detailed sampling of the lowermost portions of the Adak, Amchitka, Murray and Attu Canyons in the Aleutian forearc and at Kresta Ridge in the rear-arc on R/V SONNE Cruise 249/1. On SO249/2, we will sample forearc canyons in the westernmost Aleutians southeast of Medney Island. Results from the Komandorsky Islands and preliminary results from the cruises will be presented.