Many arc lavas have two discrete slab-derived components evident from their compositional variations. These components have been interpreted to be melts of subducted sediment and a ‘fluid’ from the mafic oceanic crust. The ‘fluid’ signature is seen most clearly in depleted samples that contain the smallest fraction of the sediment component. The provenance of this ‘fluid’ is reliably pinned by its unradiogenic Pb isotope ratios to be from a MORB-like source. The composition of the ‘fluid’ phase is marked by strong enrichments in the large, divalent cations, notably Ba, Pb and Sr and (238U/230Th)>1. These characteristics have traditionally been explained by the ‘mobility’ of such elements in an aqueous fluid. Such fluids are anticipated to be generated during prograde, subduction-metamorphism and to flux elements into the mantle wedge. Yet no experimental dataset of solid-aqueous fluid partitioning replicates the full set of elemental enrichments described above. The recent realisation of the pivotal role of accessory phases in controlling the mobility of elements from the slab provides a more consistent explanation of the chemical characteristics of the ‘fluid’ phase. Notably rutile and allanite retain the ‘immobile’ HSFE and REE and fractionate Th from U. The enrichments in the ‘fluid’ thus simply reflect the displacement of homeless elements. However, Ba will not be mobile until its preferred host, phengite, is exhausted at the wet solidus of the mafic crust. Thus the ‘fluid’ phase transpires to be a wet melt. The dominant control of accessory phases on the composition of the ‘fluid’ component also helps in understanding the long-standing puzzle of arc lava 235U-231Pa systematics. By analogy with surface aqueous behaviour, many models have assumed that the ‘fluid’ adds only U to the mantle wedge, generating 238U-230Th and 235U-231Pa excesses. Yet many lavas with strong ‘fluid’ signatures and large 238U-230Th excesses have 235U-231Pa deficits. This surprising observation has previously been explained by in-growth, mantle melting models. We have examined the Mariana arc case in detail and show it is difficult to replicate the combined 235U-231Pa-238U-230Th data in a self-consistent manner. In the light of the residual accessory phase model, however, it is apparent that during ‘fluid’ production the only likely host for the large pentavalent Pa ion is zircon. We thus suggest that the highly variable 235U-231Pa disequilibria of depleted arc lavas worldwide reflects the variable role of residual zircon in the subducted slab

The origin of the ‘fluid’ component and its implications for U-Th-Pa disequilibria in island arcs / R. AVANZINELLI; S. SKORA; J. BLUNDY; T. ELLIOTT. - ELETTRONICO. - (2011), pp. V41D-2535-V41D-2535. (Intervento presentato al convegno AGU Fall Meeting 2011 tenutosi a San Francisco (California, US) nel 5-9 Dicembre 2011).

The origin of the ‘fluid’ component and its implications for U-Th-Pa disequilibria in island arcs

AVANZINELLI, RICCARDO;
2011

Abstract

Many arc lavas have two discrete slab-derived components evident from their compositional variations. These components have been interpreted to be melts of subducted sediment and a ‘fluid’ from the mafic oceanic crust. The ‘fluid’ signature is seen most clearly in depleted samples that contain the smallest fraction of the sediment component. The provenance of this ‘fluid’ is reliably pinned by its unradiogenic Pb isotope ratios to be from a MORB-like source. The composition of the ‘fluid’ phase is marked by strong enrichments in the large, divalent cations, notably Ba, Pb and Sr and (238U/230Th)>1. These characteristics have traditionally been explained by the ‘mobility’ of such elements in an aqueous fluid. Such fluids are anticipated to be generated during prograde, subduction-metamorphism and to flux elements into the mantle wedge. Yet no experimental dataset of solid-aqueous fluid partitioning replicates the full set of elemental enrichments described above. The recent realisation of the pivotal role of accessory phases in controlling the mobility of elements from the slab provides a more consistent explanation of the chemical characteristics of the ‘fluid’ phase. Notably rutile and allanite retain the ‘immobile’ HSFE and REE and fractionate Th from U. The enrichments in the ‘fluid’ thus simply reflect the displacement of homeless elements. However, Ba will not be mobile until its preferred host, phengite, is exhausted at the wet solidus of the mafic crust. Thus the ‘fluid’ phase transpires to be a wet melt. The dominant control of accessory phases on the composition of the ‘fluid’ component also helps in understanding the long-standing puzzle of arc lava 235U-231Pa systematics. By analogy with surface aqueous behaviour, many models have assumed that the ‘fluid’ adds only U to the mantle wedge, generating 238U-230Th and 235U-231Pa excesses. Yet many lavas with strong ‘fluid’ signatures and large 238U-230Th excesses have 235U-231Pa deficits. This surprising observation has previously been explained by in-growth, mantle melting models. We have examined the Mariana arc case in detail and show it is difficult to replicate the combined 235U-231Pa-238U-230Th data in a self-consistent manner. In the light of the residual accessory phase model, however, it is apparent that during ‘fluid’ production the only likely host for the large pentavalent Pa ion is zircon. We thus suggest that the highly variable 235U-231Pa disequilibria of depleted arc lavas worldwide reflects the variable role of residual zircon in the subducted slab
2011
AGU Fall Meeting 2011 Abstracts
AGU Fall Meeting 2011
San Francisco (California, US)
R. AVANZINELLI; S. SKORA; J. BLUNDY; T. ELLIOTT
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Utilizza questo identificatore per citare o creare un link a questa risorsa: https://hdl.handle.net/2158/954932
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