Back-arc melting

Fluid or source induced?

Christoph Beier, Simon P. Turner, Wolfgang Bach, Dominik Niedermeier, John M. Sinton, Jim Gill

    Research output: Contribution to journalMeeting abstract

    Abstract

    Back-arc melting regimes provide the unique opportunity to enhance our knowledge of the influence of mantle heterogeneity on mantle melting, i.e. of the extent of arc related, fluid-induced melting, vs. decompressional melting. However, few U-Th-Ra disequilibria have been analysed in back-arc lavas so far. Manus back-arc basin lavas range in composition from NMORB/ transitional MORB to back-arc basin basalts (BABB) and some extremely enriched BABBs (XBABB). We have analysed U-Th-Ra disequilibria 100-400 km off the active New Britain trench. While most display (230Th/238U) ratios of 0.77-0.95 a few lavas both closer and more distant to the trench have small Th-excesses with (230Th/238U) <1.06. (226Ra/230Th) ratios are generally >1 implying that the lavas are <8 ka old and that no post-eruptive age correction needs to be applied to U-Th. While (230Th/238U) >1 are generally associated with decompressional melting, (230Th/238U) <0.95 have generally been attributed to the presence of arc-related fluids or unusual melting phases (e.g. apatite , however, we do not find evidence for apatite in these rocks). The Sr-Nd-Pb isotopes imply variable input of an enriched component, i.e. they range from ‘MORB’-like depleted signature to more enriched signatures. There is no evidence for a correlation of arc-related melting signatures and long-term source heterogeneity. Instead the U-Th disequlibria are consistent with fluid-induced melting some 200-400 km distant from the active trench. This U-excess requires a) unusual phases during melting or b) storage of fluids in the mantle and re-melting (e.g. amphibole, however, this would lead to Th-rather than U-excess). We thus favour a much quicker/further transport of the arc-related fluid through the mantle wedge (with or without intermediate storage) inducing melting much further away from the active trench then suggested by dehydration of the subducted slab.
    Original languageEnglish
    Pages (from-to)A104-A104
    Number of pages1
    JournalGeochimica et Cosmochimica Acta
    Volume73
    Issue number13, Supplement
    Publication statusPublished - Jun 2009
    EventGoldschmidt Conference (19th : 2009) - Davos, Switzerland
    Duration: 21 Jun 200926 Jun 2009

    Cite this

    Beier, C., Turner, S. P., Bach, W., Niedermeier, D., Sinton, J. M., & Gill, J. (2009). Back-arc melting: Fluid or source induced? Geochimica et Cosmochimica Acta, 73(13, Supplement), A104-A104.