Partial melting processes above subducting plates: Constraints from 231Pa–235U disequilibria

Simon Turner, Marcel Regelous, Christopher John Hawkesworth, Kia Rostami

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    33 Citations (Scopus)

    Abstract

    The processes involved in the formation and transport of partial melts above subducting plates remain poorly constrained relative to those at mid-ocean ridges. In particular, U-238-Th-230-Ra-226 disequilibria, that might normally be used to constrain melting dynamics, tend to be swamped by the effects of fluid addition from the down-going plate. The Pa-231-U-231 system provides an exciting exception to this because the highly incompatible nature of Pa means that fractionation and in-growth during partial melting overwrite the effects of fluid U addition. We present Pa-231-U-235 data on 50 well-characterised lavas from seven subduction zones in order to examine partial melting processes. Measured (Pa-231/U-235) ratios are all > 1 and 15% are > 2. Overall (Pa-231/U-235) shows broad positive correlations with (Th-230/U-211) and La/Yb and negative trends against Ba/Th and (Ra-226/Th-230). These systematics can differ from arc to arc but suggest that (Pa-231/U-231) tends to be higher in sediment-rich arc lavas where the effects of fluid addition are muted and there is less of a Pa-231 deficit for melting to overprint. We have explored the effects of decompression melting, frictional drag dynamic melting with and without ageing subsequent to fluid U addition to the wedge as well as flux melting models. Globally, average (Pa-231/U-235) appears to correlate negatively with convergence rate and so in the numerical models we use the local subduction rate for the rate of matrix flow through the melting zone. Using this assumption and reasonable values for other parameters, the melting models can simulate the overall range of (Pa-231/U-211) and some of the data trends. However, it is clear that local variations in some parameters, especially source composition and extent of melting, exert a major influence on Pa-231-(235) U disequilibria. Some data, which lie at a high angle to the modelled trends, may be explained by mixing between small degree hydrous melts formed near the slab and larger degree, decompression melts produced at shallow depth. (c) 2005 Elsevier Inc. All rights reserved.

    Original languageEnglish
    Pages (from-to)480-503
    Number of pages24
    JournalGeochimica et Cosmochimica Acta
    Volume70
    Issue number2
    DOIs
    Publication statusPublished - 15 Jan 2006

    Keywords

    • U-SERIES DISEQUILIBRIA
    • TONGA-KERMADEC ISLAND
    • STATE RA-226/TH-230 DISEQUILIBRIUM
    • ARC LAVAS
    • LESSER ANTILLES
    • TRANSPORT RATES
    • GEOCHEMICAL EVOLUTION
    • ELEMENT TRANSPORT
    • SPINEL LHERZOLITE
    • GLOBAL VARIATIONS

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