Combined 238U/230Th and 235U/231Pa constraints on the transport of slab-derived material beneath the Mariana Islands

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doi: 10.1016/j.gca.2012.06.020
Author(s): Avanzinelli, R.; Prytulak, J.; Skora, S.; Heumann, A.; Koetsier, G.; Elliott, T.
Author Affiliation(s): Primary:
University of Bristol, School of Earth Sciences, Bristol, United Kingdom
Other:
Vrije Universiteit, Netherlands
Volume Title: Geochimica et Cosmochimica Acta
Source: Geochimica et Cosmochimica Acta, Vol.92, p.308-328. Publisher: Elsevier, New York, NY, International. ISSN: 0016-7037 CODEN: GCACAK
Note: In English. Includes appendices. 88 refs.; illus., incl. 2 tables
Summary: We present 235U-231Pa measurements for a suite of well-characterised lavas from the Mariana Islands. There is limited variability in (231Pa/235U), 1.18-1.55, with the lowest values typically found in the incompatible element depleted samples. We also report the complementary 238U-234U-230Th-232Th analyses, which are in good agreement previous measurements on these same samples. In keeping with this earlier work, we interpret the range of (230Th/238U) from 0.65 to 1.06 as the result of variable contributions of an enriched, sediment melt component coupled with a near constant flux of a relatively U-rich aqueous fluid, derived from the subducted mafic oceanic crust to the sub-arc mantle. We reassess the composition of the subducting sedimentary assemblage and find that its (230Th/232Th) was previously under-estimated. We performed quantitative models to reproduce the coupled 238U-230Th-232Th and 235U-231Pa data starting from a mantle source modified by subduction-related components according to traditional assumptions of a Pa (and Th)-free fluid and a sedimentary component in 231Pa-235U and 238U-230Th equilibrium. This scenario does not allow the U-series isotope compositions of the Mariana lavas to be reproduced. Successful solutions are instead possible with a sediment melt component with relatively high (230Th/232Th) ∼0.9 and a modest 230Th- and 231Pa excess, (∼30%). The need for a 230Th- and 231Pa-excess in the sedimentary component implies that the sediment melted <150 ka before eruption of the lavas that carry this signature and that it melted to sufficiently high degree to exhaust monazite. In contrast, the low (230Th/238U) of the aqueous fluid component is explained by its derivation from lower temperature, mafic crust that contained residual allanite. The maximum (230Th/232Th) of this fluid is set by the well-defined array of lavas on an equiline plot and this argues for a fresh, rather than significantly altered MORB protolith. Moreover, in-growth is insufficient to account for the 231Pa excesses observed in the Marianas lavas given a Pa-free fluid. Thus we infer that the combined U-Th-Pa signatures of the Marianas are more strongly controlled by the initial compositions of the slab components than by mantle melting. Creation of U-series disequilibria is strongly favoured by the presence of key accessory phases, such as allanite, monazite and zircon, that are likely to be stable in the slab and we suggest that the variable U-Th-Pa systematics of arc lavas globally may well reflect the contrasting stabilities of accessory phases in subduction zones with different thermal and compositional controls. Abstract Copyright (2012) Elsevier, B.V.
Year of Publication: 2012
Research Program: ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 05 Petrology, Igneous and Metamorphic; Actinides; Alkaline earth metals; Basalts; Chemical composition; Crust; Igneous rocks; Island arcs; Isotope ratios; Isotopes; Izu-Bonin Arc; Jurassic; Leg 129; Lithosphere; Mariana Islands; Mariana Trench; Mariana Trough; Mesozoic; Metals; Micronesia; Mid-ocean ridge basalts; Mineral composition; Nd-144/Nd-143; Neodymium; North Pacific; Northwest Pacific; ODP Site 800; ODP Site 801; ODP Site 802; Ocean Drilling Program; Ocean floors; Oceania; Oceanic crust; Oceanic lithosphere; Ophiolite; Pacific Ocean; Phase equilibria; Philippine Sea Plate; Pigafetta Basin; Plate tectonics; Protactinium; Protoliths; Radioactive isotopes; Rare earths; Slabs; Sr-87/Sr-86; Stable isotopes; Strontium; Subduction zones; Thorium; Trenches; U-235/Pa-231; U-238/Th-230; Uranium; Volcanic rocks; Water-rock interaction; West Pacific
Coordinates: N120546 N120547 E1531238 E1531237
N183831 N183835 E1562136 E1562134
N215522 N215523 E1521920 E1521919
Record ID: 2013013420
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