Gene M. Yogodzinski

Along‐Strike Variation in the Aleutian Island Arc: Genesis of High Mg# Andesite and Implications for Continental Crust

Based on a compilation of whole rock geochemistry for approximately 1100 lava samples and 200 plutonic rock samples from the Aleutian island arc, we characterize along-strike variation, including data for the western part of the arc which has recently become available. We concentrate on the observation that western Aleutian, high Mg# andesite compositions bracket the composition of the continental crust. Isotope data show that this is not due to recycling of terrigenous sediments. Thus, the western Aleutians can provide insight into genesis of juvenile continental crust. The composition of primitive magmas (molar Mg# > 0.6) varies systematically along the strike of the arc. Concentrations of SiO 2 , Na 2 O and perhaps K 2 O increase from east to west, while MgO, FeO, CaO decrease. Thus, primitive magmas in the central and eastern Aleutians (east of 174°W) are mainly basalts, while those in the western Aleutians are mainly andesites. Along-strike variation in Aleutian magma compositions may be related to a westward decrease in sediment input, and/or to the westward decrease in down-dip subduction velocity. 206 Pb/ 204 Pb, 207 Pb/ 204 Pb, 208 Pb /204 Pb and 87 Sr/ 86 Sr all decrease from east to west, whereas 143 Nd/ 144 Nd increases from east to west. These data, together with analyses of sediment from DSDP Site 183, indicate that the proportion of recycled sediment in Aleutian magmas decreases from east to west. Some proposed trace element signatures of sediment recycling in arc magmas do not vary systematically along the strike of the Aleutians, and do not correlate with radiogenic isotope variations. Thus, for example, Th/Nb and fractionation-corrected K concentration in Aleutian lavas are not related to the flux of subducting sediment. Th/La is strongly correlated with Ba/La, rendering it doubtful that Ba/La is a proxy for an aqueous fluid component derived from subducted basalt. Ce/Pb > 4 is common in Aleutian lavas west of 174°W, in lavas with MORB-like Pb, Sr and Nd isotope ratios, and is also found behind the main arc trend in the central Aleutians. Thus, Ce/Pb in Aleutian lavas with MORB-like isotope ratios is not always low, and may be affected by a component derived from partial melting of subducted basalt in eclogite facies. Enriched, primitive andesites, with high Sr/Y, steep REE patterns, and low Yb and Y, are an important lava type in the Aleutians west of 174°W. High Sr/Y and Dy/Yb, indicative of abundant garnet in the source of melting, are correlated with major element systematics. Lavas with a garnet signature have high SiO 2 , Na 2 O and K 2 O.

Revised age of Aleutian Island Arc formation implies high rate of magma production

Radioisotopic dating of subaerial and submarine volcanic and plutonic rocks from the Aleutian Island Arc provides insight into the timing of arc formation in the middle Eocene. Twenty-eight 40 Ar/ 39 Ar ages constrain the duration of arc magmatism to the last 46 m.y. Basaltic lavas from the Finger Bay volcanics, the oldest exposed rocks in the arc, gave an isochron age of 37.4 ± 0.6 Ma, which is 12-17 m.y. younger than a widely cited age of 55-50 Ma. Three main pulses of arc-wide magmatism occurred at 38-29, 16-11, and 6-0 Ma, which coincide with periods of intense magmatism in other western Pacific island arcs. Using the geochronology and volumetric estimates of crust generated and eroded over the last 46 m.y., we calculate a time-averaged magma production rate for the entire arc that exceeds previous estimates by almost an order of magnitude.

The Cobb hot spot: HIMU‐DMM mixing and melting controlled by a progressively thinning lithospheric lid

The Cobb Seamount Chain in the northeast Pacific basin records the composition of the Cobb hot spot for the past 33 Myr, as the migrating Juan de Fuca Ridge approached and ultimately overran it ca. 0.5 Myr ago. In this first comprehensive geochemical study of the Cobb chain, major and trace element compositions and Sr, Nd, Pb, and Hf isotopic ratios were measured for whole-rock samples from throughout the chain, and He isotopes were acquired for olivine phenocrysts from one seamount. Trace element modeling indicates increased melting along the chain over time, with progressively more depleted lavas as the ridge approached the hot spot. The isotopic data reveal the first evidence of the high µ (µ = 238U/204Pb) (HIMU) mantle component in the north Pacific basin and are consistent with a progressively decreasing mixing proportion of HIMU melts relative to those from depleted mid-ocean ridge basalt mantle (DMM) in the chain over time. Decreasing lithospheric thickness over the Cobb hot spot due to the approach of the migrating Juan de Fuca ridge allowed adiabatic melting to continue to shallower depths, leading to increased melt fractions of the refractory DMM component in the hot spot and more depleted and MORB-like lavas in the younger Cobb seamounts.

High-magnesian andesite from Mount Shasta: A product of magma mixing and contamination, not a primitive melt: COMMENT AND REPLY: COMMENT

[Streck et al. (2007)][1] state that primitive andesite lavas (Mg# > 0.6; 53–63 wt% SiO2) from Mt. Shasta are not primary melts, but instead are mixtures of evolved, high-Sr/Y, low-Mg# dacite with primary, low-Sr/Y, high-Mg# basalt. They propose that this mixing forms primitive andesites worldwide

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.

Sharing Resources for Aleutian Arc Research

The Aleutian arc is arguably the best place on Earth to investigate several fundamental questions about arc magmatism and subduction initiation because the record of arc growth is mostly preserved due to a lack of intra-arc rifting. In December, 94 scientists met in San Francisco, Calif., at a workshop sponsored by the Geodynamic Processes at Rifting and Subduction Margins (GeoPRISMS) program to discuss possibilities for sharing resources for fieldwork in the Aleutian arc so that the cost per project could be reduced.