Characterising the multi-component sources of subduction zone magmas: A geochemical study of submarine volcanoes and active spreading centres, Vanuatu

Zarah Heyworth (2010). Characterising the multi-component sources of subduction zone magmas: A geochemical study of submarine volcanoes and active spreading centres, Vanuatu PhD Thesis, School of Earth Sciences, The University of Queensland.

       
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Author Zarah Heyworth
Thesis Title Characterising the multi-component sources of subduction zone magmas: A geochemical study of submarine volcanoes and active spreading centres, Vanuatu
School, Centre or Institute School of Earth Sciences
Institution The University of Queensland
Publication date 2010-12
Thesis type PhD Thesis
Supervisor Kurt Knesel
Simon Turner
Paulo Vasconcelos
Total pages 125
Total colour pages 3
Total black and white pages 122
Subjects 04 Earth Sciences
Abstract/Summary The currently accepted model for subduction zone magmatism involves metasomatism of the mantle wedge by slab-derived components and then subsequent partial melting of this source. However, there is no consensus on whether the slab-derived component is transferred as a hydrous fluid, a silicate melt, or both. Whether these fluids or melts are derived from the altered oceanic crust, the subducted sediment, or both is also debated. Constraining the nature of the subduction component has important implications for solid-fluid-melt partitioning and slab dehydration depths, particularly elemental budgets and recycling of elements into the deep mantle. Vanuatu is an intra-oceanic subduction zone in the SW Pacific where recent scientific cruises have mapped and dredged submarine lavas that extend the sample coverage of the Vanuatu arc. Major and trace elements and radiogenic isotopes are used to identify the source and transfer mechanism of slab-derived components in the Vanuatu subduction zone. Mixing calculations estimate the mantle wedge, sediment and fluid contributions to Vanuatu arc and backarc lavas. New geochemical results are integrated with the large database of published results for Vanuatu, to investigate the along- and across-arc variation in the slab component and mantle domains. Lavas from the Vanuatu backarc have Indian-like Pb isotope ratios, similar to the Indian-type mantle signatures of central arc volcanoes, but distinct from adjacent southern arc volcanoes with Pacific-like Pb isotope ratios. The Pb isotopic anomaly (∆8/4) in the Vanuatu subduction zone decreases from the central arc segment to the southern backarc, implying trench-parallel flow of Indian-type mantle to the south. The spatial distribution of mantle domains implies that mantle flow patterns in southern Vanuatu are dominated by southward trench-parallel flow, rather than pure corner flow induced by viscous drag along the upper surface of the subducting plate. This southward flow appears to be related to pressure gradients developed during asymmetric rollback of the Australian plate. Despite these unusual mantle dynamics in southern Vanuatu, a subduction component is detectable in the across-arc and backarc lavas. In the southern Vanuatu backarc, the average amount of sediment addition decreases from ~1.2% in the north trough to ~0.3% in the central trough. The average amount of altered oceanic crust fluid addition also decreases from ~0.3% in the northern trough to ~0.1% in the central trough. The southern backarc trough lavas are not enriched in large ion lithophile elements and have light rare earth element depletions, suggesting that these lavas do not have a significant contribution from slab-derived components. In the across-arc volcanoes, the average amount of sediment addition decreases from ~2% in the volcanic front to ~0.5% in the easternmost across-arc volcano. The average amount of altered oceanic crust fluid addition also decreases from ~0.9% in the volcanic front to ~0.3% in the easternmost across-arc volcano. For the backarc and across-arc lavas, sediment and fluid contributions progressively decrease with distance from the trench. Fluid-mobile/fluid-immobile trace element ratios, such as Ba/Yb and Ba/La ratios also decrease with distance from the trench and support progressive distillation of the subducting slab. Enrichment of fluid mobile elements in the southern across-arc volcanoes reflects the addition of hydrous fluids to slab depths of at least 200 km. These constraints on slab dehydration signify that not all fluid mobile elements are recycled at shallow depths. The degree of melting in the across-arc volcanoes progressively decreases with depth to the slab, demonstrating that the degree of melting is not solely controlled by the melt column height. Smaller degree melts from southern Vanuatu have a smaller contribution from slab-derived components. These constraints argue for fluid-fluxed melting, where the extent of melting progressively decreases as the amount of slab-derived fluid decreases. This positive wet melting function contrasts with that observed at mid ocean ridges, where increasing water content of the mantle source is associated with decreasing extents of melting.
Keyword Vanuatu
Subduction zone
Arc
Backarc
Geochemistry
Mantle
Slab components
Melting
Additional Notes Colour pages: 49, 55 and 105.

 
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Created: Fri, 01 Jul 2011, 18:25:46 EST by Ms Zarah Heyworth on behalf of Library - Information Access Service