Journal
EARTH AND PLANETARY SCIENCE LETTERS
Volume 432, Issue -, Pages 176-186Publisher
ELSEVIER SCIENCE BV
DOI: 10.1016/j.epsl.2015.10.006
Keywords
Mo isotopes; subduction zones; Mariana arc; alteration; oceanic crust
Categories
Funding
- NERC [NE/H023933/1, NE/D012805/1, NE/J018031/1, NE/L004011/1]
- University of Bristol
- FP7 Marie Curie Fellowship [FP7-PEOPLE-2009-IEF-254260]
- U.S. National Science Foundation (NSF)
- Joint Oceanographic Institutions (JOI), Inc.
- MRC [G0401546] Funding Source: UKRI
- NERC [NE/J018031/1, NE/D012805/1, NE/H023933/1, NE/L004011/1, NE/J018031/2, NE/J009024/1] Funding Source: UKRI
- Medical Research Council [G0401546] Funding Source: researchfish
- Natural Environment Research Council [NE/J018031/1, NE/J018031/2, NE/D012805/1, NE/J009024/1, NE/H023933/1, NE/L004011/1] Funding Source: researchfish
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The fate of crustal material recycled into the convecting mantle by plate tectonics is important for understanding the chemical and physical evolution of the planet. Marked isotopic variability of Mo at the Earth's surface offers the promise of providing distinctive signatures of such recycled material. However, characterisation of the behaviour of Mo during subduction is needed to assess the potential of Mo isotope ratios as tracers for global geochemical cycles. Here we present Mo isotope data for input and output components of the archetypical Mariana arc: Mariana arc lavas, sediments from ODP Sites 800, 801 and 802 near the Mariana trench and the altered mafic, oceanic crust (AOC), from ODP Site 801, together with samples of the deeper oceanic crust from ODP Site 1256. We also report new high precision Pb isotope data for the Mariana arc lavas and a dataset of Pb isotope ratios from sediments from ODP Sites 800, 801 and 802. The Mariana arc lavas are enriched in Mo compared to elements of similar incompatibility during upper mantle melting, and have distinct, isotopically heavy Mo (high Mo-98/Mo-95) relative to the upper mantle, by up to 0.3 parts per thousand. In contrast, the various subducting sediment lithologies dominantly host isotopically light Mo. Coupled Pb and Mo enrichment in the Mariana arc lavas suggests a common source for these elements and we further use Pb isotopes to identify the origin of the isotopically heavy Mo. We infer that an aqueous fluid component with elevated [Mo], [Pb], high Mo-98/Mo-95 and unradiogenic Pb is derived from the subducting, mafic oceanic crust. Although the top few hundred metres of the subducting, mafic crust have a high Mo-98/Mo-95, as a result of seawater alteration, tightly defined Pb isotope arrays of the Mariana arc lavas extrapolate to a fluid component akin to fresh Pacific mid-ocean ridge basalts. This argues against a flux dominantly derived from the highly altered, uppermost mafic crust or indeed from an Indian-like mantle wedge. Thus we infer that the Pb and Mo budgets of the fluid component are dominated by contributions from the deeper, less altered (cooler) portion of the subducting Pacific crust. The high Mo-98/Mo-95 of this flux is likely caused by isotopic fractionation during dehydration and fluid flow in the slab. As a result, the residual mafic crust becomes isotopically lighter than the upper mantle from which it was derived. Our results suggest that the continental crust produced by arc magmatism should have an isotopically heavy Mo composition compared to the mantle, whilst a contribution of deep recycled oceanic crust to the sources of some ocean island basalts might be evident from an isotopically light Mo signature. (C) 2015 Elsevier B.V. All rights reserved.
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