Julie Prytulak

No change in the neodymium isotope composition of deep water exported from the North Atlantic on glacial-interglacial time scales

Quantifying past circulation is a vital part of testing our understanding of the modern and future climate system. The isotopic composition of neodymium (Nd) in marine precipitates has considerable promise as a recorder of past circulation patterns, but its robust application requires knowledge of the end-member compositions in order to correctly deconvolute a downstream signal. We show here, using in situ, high temporal resolution analyses of ferro-manganese crusts from the North Atlantic, that the Nd isotopic composition of deep water during times of much more extensive Northern Hemisphere ice cover was no different than the modern-day interglacial value. This result is surprising, but greatly simplifies the use of Nd isotopes as tracers of the strength and patterns of circulation in the Atlantic in the past.

Geology, geochronology, and geochemistry of the Miocene–Pliocene Ancestral Cascades arc, northern Sierra Nevada, California and Nevada: The roles of the upper mantle, subducting slab, and the Sierra Nevada lithosphere

The assemblage of ca. 28–3 Ma volcanic rocks exposed in the Lake Tahoe–Reno region of the northern Sierra Nevada, United States, is interpreted to be part of the Ancestral Cascades volcanic arc. The volcanic rocks are commonly highly porphyritic, including abundant plagioclase with clinopyroxene, amphibole, and rare biotite, and range from basaltic andesite to dacite in composition. Less common are poorly phyric, olivine- and clinopyroxene-bearing basalts and basaltic andesites. Porphyritic lavas dominate composite volcanic centers, whereas the poorly phyric lavas form isolated cinder cone and lava flow complexes. Tahoe-Reno arc lavas are calc-alkaline, enriched in the large ion lithophile elements but depleted in Nb and Ta relative to the light rare earth elements, and have highly variable radiogenic isotopic compositions. Compared to the modern south Cascade arc, Tahoe-Reno region basalts are enriched in the light rare earth and large ion lithophile elements and have higher 87 Sr/ 86 Sr and lower 143 Nd/ 144 Nd that are consistent with an old, subduction-modified lithospheric mantle source, such as that proposed for lavas of the Western Great Basin. Melting of the lithospheric mantle may be enhanced by fluid flux from the subducting slab if the Juan de Fuca slab dip is shallow. Andesites and dacites evolved from basaltic magmas by a combination of fractional crystallization and assimilation of lower crustal melts. Available geochronological data indicate that the westward sweep of Cenozoic volcanism through Nevada was associated with steepening of the slab dip, but the dip angle was lower during Miocene–Pliocene arc volcanism than it is today beneath the modern south Cascades.

Subduction initiation and ophiolite crust: new insights from IODP drilling

ABSTRACT International Ocean Discovery Program (IODP) Expedition 352 recovered a high-fidelity record of volcanism related to subduction initiation in the Bonin fore-arc. Two sites (U1440 and U1441) located in deep water nearer to the trench recovered basalts and related rocks; two sites (U1439 and U1442) located in shallower water further from the trench recovered boninites and related rocks. Drilling in both areas ended in dolerites inferred to be sheeted intrusive rocks. The basalts apparently erupted immediately after subduction initiation and have compositions similar to those of the most depleted basalts generated by rapid sea-floor spreading at mid-ocean ridges, with little or no slab input. Subsequent melting to generate boninites involved more depleted mantle and hotter and deeper subducted components as subduction progressed and volcanism migrated away from the trench. This volcanic sequence is akin to that recorded by many ophiolites, supporting a direct link between subduction initiation, fore-arc spreading, and ophiolite genesis.

Reconciling mantle wedge thermal structure with arc lava thermobarometric determinations in oceanic subduction zones

Subduction zone mantle wedge temperatures impact plate interaction, melt generation, and chemical recycling. However, it has been challenging to reconcile geophysical and geochemical constraints on wedge thermal structure. Here we chemically determine the equilibration pressures and temperatures of primitive arc lavas from worldwide intra-oceanic subduction zones and compare them to kinematically driven thermal wedge models. We find that equilibration pressures are typically located in the lithosphere, starting just below the Moho, and spanning a wide depth range of ∼25 km. Equilibration temperatures are high for these depths, averaging ∼1300°C. We test for correlations with subduction parameters and find that equilibration pressures correlate with upper plate age, indicating overriding lithosphere thickness plays a role in magma equilibration. We suggest that most, if not all, thermobarometric pressure and temperature conditions reflect magmatic re-equilibration at a mechanical boundary, rather than reflecting the conditions of major melt generation. The magma re-equilibration conditions are difficult to reconcile, to a first order, with any of the conditions predicted by our dynamic models, with the exception of subduction zones with very young, thin upper plates. For most zones, a mechanism for substantially thinning the overriding plate is required. Most likely thinning is localised below the arc, as kinematic thinning above the wedge corner would lead to a hot forearc, incompatible with forearc surface heat flow and seismic properties. Localised sub-arc thermal erosion is consistent with seismic imaging and exhumed arc structures. Furthermore, such thermal erosion can serve as a weakness zone and affect subsequent plate evolution. This article is protected by copyright. All rights reserved.