Evolution of a mafic volcanic field in the central Great Basin, south central Nevada

This is the published version. Copyright 2012 American Geophysical Union. All Rights Reserved.Evolution of a mafic volcanic field is investigated through a study of Pliocene age rocks in the Reveille Range in south central Nevada. Pliocene activity began with the eruption of relatively abundant hawaiite (episode 1, 5–6 Ma), which was followed by trachytic volcanism (4.3 Ma) and by a second episode of lower-volume hawaiite and basanite (episode 2, 3.0–4.7 Ma). Incompatible elements indicate an asthenospheric source. Isotopically, episode 2 basalts cluster around 87Sr/86Sr = 0.7035 and εNd = +4.2, but episode 1 samples vary to high 87Si/86Sr (up to 0.7060) over a narrow range of εNd (+0.8 to +4.5). Trachytic rocks (MgO ∼ 0.5%) are isotopically akin to the episode 1 basalts. Geochemical variation requires the addition of a crustal component (high 87Sr/86Sr, Sr/Nd, Pb/La, low εNd) to the episode 1 hawaiites and trachytic samples, probably by assimilation of carbonate-rich sedimentary wall rock. The volcanic field developed in at least two eruptive cycles of approximately equal duration. Basanites (deeper and lower percentage melts) appear only in the younger episode. Eruptive episodes were apparently linked to separate melting events in the mantle. Through time, basalts were produced in diminishing volumes by lower percentage melting, magma generation and storage was at greater depths, and magma ascent was at higher velocities. Spatially, the melting anomalies were large in the Pliocene but progressively diminished in size so that by Pleistocene time, volcanism was restricted to a small area near the northern end of the initial outbreak

BERING - A new international marine research project to investigate the magmatic and tectonic evolution of the Bering Sea and its margins

The BERING project is a new large project lead by the GEOMAR institution in Kiel and focused on marine and on-land investigations in Kamchatka, the Kurile and Aleutian Arcs, the Bering Sea, and the NW-Pacific. BERING is funded by the German Ministry of Education and Research with contributions from Russian and U.S. institutions. 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. To achieve this goal BERING will address the following major scientific questions in four major directions of study.............

Tracking along-arc sediment inputs to the Aleutian arc using thallium isotopes.

Sediment transport from the subducted slab to the mantle wedge is an important process in understanding the chemical and physical conditions of arc magma generation. The Aleutian arc offers an excellent opportunity to study sediment transport processes because the subducted sediment flux varies systematically along strike (Kelemen et al., 2003) and many lavas exhibit unambiguous signatures of sediment addition to the sub-arc mantle (Morris et al., 1990). However, the exact sediment contribution to Aleutian lavas and how these sediments are transported from the slab to the surface are still debated. Thallium (Tl) isotope ratios have great potential to distinguish sediment fluxes in subduction zones because pelagic sediments and low-temperature altered oceanic crust are highly enriched in Tl and display heavy and light Tl isotope compositions, respectively, compared with the upper mantle and continental crust. Here, we investigate the Tl isotope composition of lavas covering almost the entire Aleutian arc a well as sediments outboard of both the eastern (DSDP Sites 178 and 183) and central (ODP Hole 886C) portions of the arc. Sediment Tl isotope compositions change systematically from lighter in the Eastern to heavier in the Central Aleutians reflecting a larger proportion of pelagic sediments when distal from the North American continent. Lavas in the Eastern and Central Aleutians mirror this systematic change to heavier Tl isotope compositions to the west, which shows that the subducted sediment composition is directly translated to the arc east of Kanaga Island. Moreover, quantitative mixing models of Tl and Pb, Sr and Nd isotopes reveal that bulk sediment transfer of ∼0.6–1.0% by weight in the Eastern Aleutians and ∼0.2–0.6% by weight in the Central Aleutians can account for all four isotope systems. Bulk mixing models, however, require that fractionation of trace element ratios like Ce/Pb, Cs/Tl, and Sr/Nd in the Central and Eastern Aleutians occurs after the sediment component was mixed with the mantle wedge. Models of Sr and Nd isotopes that involve sediment melting require either high degrees of sediment melting (>50%), in which case trace element ratios like Ce/Pb, Cs/Tl, and Sr/Nd of Aleutian lavas need to be produced after mixing with the mantle, or significant fluid additions from the underlying oceanic crust with Sr and Nd isotope compositions indistinguishable from the mantle wedge as well as high Sr/Nd ratios similar to that of low (<20%) degree sediment melts. Thallium isotope data from Western Aleutian lavas exhibit compositions slightly lighter than the upper mantle, which implies a negligible sediment flux at this location and probably involvement of low-temperature altered oceanic crust in the generation of these lavas. In general, the lightest Tl isotope compositions are observed for the highest Sr/Y ratios and most unradiogenic Sr and Pb isotope compositions, which is broadly consistent with derivation of these lavas via melting of eclogitized altered oceanic crust

New Insights into the Origin of the Bering Sea from SO201 and SO249 cruises

The origin of the Bering Sea Basin remains elusive. It is still not resolved if the basin formed by plate capture or backarc spreading. On the German R/V Sonne cruises SO201/1b-2 KALMAR in 2009 and SO249/1-2 BERING in 2016, combined with fieldwork on the Komandorsky Islands, our studies of the southern (Aleutian) and western(Kamchatka to Chukotka) margins of the Bering Sea and of the Bowers and Shirshov Ridges have provided new insights into the complex origin of the Bering marginal basin..

Sr Isotopes in Western Aleutian Seafloor Lavas: Implications for the Source of Fluids and Geochemical Decoupling of Trace Metals from Water

Sr provides unique constraints on subduction magma source models because it is a fluid-mobile element that is abundant and relatively unradiogenic in arc volcanic rocks. It is common for arc basalts to be 3-4-times more Sr-rich than similarly evolved MORB (Sr/Nd = 30-50 vs 10-15 in MORB) yet Sr isotopes in arc basalts are usually offset from MORB only slightly ( Sr/ Sr ~0.7034 vs 0.7028 for MORB). This is a puzzle because abundant sources of subducted Sr in sediment (GLOSS II Sr/ Sr = 0.712) and altered oceanic crust ( Sr/ Sr = 0.704-0.705) are more radiogenic than average arc basalts globally ( Sr/ Sr ~ 0.7034)..

Exploring the Origin of the Bering Sea: Initial Results of Cruise SO249-2 (17th July – 13th August 2016)

The Bering Sea is one of the largest marginal seas on Earth with still poorly understood origin and evolution. Cruise SO249-2 of the German research vessel Sonne explored the western half of Bering Sea by multibeam mapping, sediment profiling and dredge sampling in the framework of the joint German-Russian-U.S. American project BERING. Focus areas were A) the Chukotka-Beringian margins, once the possible site of Cretaceous arc volcanism prior to Eocene initiation of the Aleutian arc, B) the enigmatic Shirshov Ridge, separating the Komandorsky from the Aleutian Basin, C) Beta Rise, an area of anomalous high heat flow in the Komandorsky basin, D) the Volcanlogists Massif and adjacent volcanic and tectonic structures and E) the Komandorsky block, the westernmost section of the modern arc.....