Mass-related U isotope fractionation during alteration of oceanic crust and release of U in subduction zones; implications for deep recycling of oceanic crust

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doi: 10.1180/minmag.2013.077.5.6
Author(s): Freymuth, Heye; Andersen, Morten B.; Elliott, Tim
Author Affiliation(s): Primary:
University of Bristol, Bristol Isotope Group, Bristol, United Kingdom
Eidgenössische Technische Hochschule Zürich, Switzerland
Volume Title: Goldschmidt abstracts 2013
Source: Mineralogical Magazine, 77(5), p.1112; Goldschmidt 2013, Florence, Italy, Aug. 25-30, 2013. Publisher: Mineralogical Society, London, United Kingdom. ISSN: 0026-461X CODEN: MNLMBB
Note: In English
Summary: The modification of material during seafloor alteration and subsequent processing within subduction zones is of primary importance for assessing the composition of deeply subducted material and it's influence on mantle heterogeneity. It has long been known that U is added to the oceanic crust during both low and high temperature alteration of the oceanic crust, whilst some of the subducted inventory of U is returned to the surface in arc lavas. The advent of techniques to measure mass-related 238U/235U isotope fractionation with sufficient precision allows us to investigate the effect of alteration and subduction zones processes on the isotopic composition of U. This is a key step in evaluating the potential of U isotope measurements to trace deep crustal recycling. Thus we have measured the U isotopic compositions of samples from the altered, mafic, oceanic crust (AOC) at ODP site 801 as well as lavas erupted at the volcanic front of the Mariana arc. The former represents a reference site for studying the time-integrated influence of seafloor alteration and the latter constitute a well characterised sample set for which the role of slab-derived 'fluid' and sediment components can be separately recognised. The altered oceanic crust is compositionally variable with 238U/235U similar to seawater in the top ∼100 m and isotopically heavier in deeper parts. These differences are likely to be caused by oxidizing conditions in the top part of the AOC and reducing conditions in deeper parts of the AOC and isotopic fractionation occurring during the alteration of the oceanic crust. The Mariana arc lavas span a range of ∼100 pm in 238U/235U and vary systematically between seawater-like compositions in samples that have been previously identified as 'fluid-rich' and heavier values similar to fresh mantle basalts in the more sediment-rich samples. These systematics indicate that either the light U in the upper mafic crust is preferentially lost to the arc lavas or that during slab dehydration of the AOC, U is fractionated to generate an isotopically light fluid and heavy residue. In either scenario the deep-subducted material is left distinctively, isotopically heavy and should be an effective tracer of deep recycled material.
Year of Publication: 2013
Research Program: ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 18 Geophysics, Solid-Earth; Actinides; Crust; Geochemistry; Isotope fractionation; Isotope ratios; Isotopes; Leg 129; Mariana Islands; Metals; Micronesia; North Pacific; Northwest Pacific; ODP Site 801; Ocean Drilling Program; Oceania; Oceanic crust; Pacific Ocean; Pigafetta Basin; Plate tectonics; Radioactive isotopes; Sampling; Subduction; U-238/U-235; Uranium; West Pacific
Coordinates: N183831 N183835 E1562136 E1562134
Record ID: 2014010816
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Abstract, Copyright, Mineralogical Society of Great Britain and Ireland