At convergent margins crust is created by arc magmatism and consumed by subduction. Subducted material is transported into the mantle wedge in H2O-rich fluids, melts or supercritical liquids and sampled back at the surface by arc volcanism. U-series parent-daughter activity ratios, namely (230Th/238U), (231Pa/235U) and (226Ra/230Th) have long been recognised as a potentially powerful tool to constrain both the timescale of the recycling process and melting dynamics of arc magma generation. The widespread occurrence of 238U-excess is commonly interpreted as fluid addition from the subducting slab, and represents one of the key features of the so-called ‘subduction signature’. It is more problematic to assess the fractionation of U and Th in the case of slab melting (sediment and oceanic crust). A general decrease of 238U-excess with increasing Th concentration has suggested that sediment melts are added either in secular equilibrium or enough time has elapsed since melting that any initial disequilibrium has decayed. Protactinium (Pa) is generally assumed to be immobile in subduction zone fluids, but (231Pa/235U) disequilibria in island arcs are ubiquitously greater than unity. This has prompted many authors to apply to subduction related magmatism time-dependent melting models originally designed for MORB and OIB: in such models syn-melting in-growth of 231Pa could explain the transformation from initial fluid-induced 231Pa-deficits to the 231Pa-excesses measured in most arc lavas. Critically, in a successful time-related melting model, both 238U-230Th and 235U-231Pa disequilibria must be explained with the same parameters. In this frame, the combination of U-Th-Pa-Ra measurements on a single volcanic arc, and in particular the variation of (231Pa/235U) with (230Th/238U) allows investigation of the timing of slab dehydration and recycling and its effects on the melting process. The Mariana arc provides the perfect location to study the behaviour of Pa in the arc environment. (230Th/238U) disequilibria suggest that U is fluxed into the mantle during dehydration of the oceanic crust inducing significant 230Th-deficits in the most depleted (least sediment enriched) lavas. The more sediment rich lavas are less affected by this U addition, and show minor 230Th. All the measured samples display (231Pa/235U)>1, requiring sufficient 231Pa in-growth during melting to overcome the initial deficit induced in the source by U-rich fluids. Moreover, the slope of the (231Pa/235U) vs. (230Th/238U) alignment of the erupted lavas, in comparison to that expected for the fluid-fluxed source, suggests that magmas more affected by fluids requires more 231Pa-ingrowth during melting. However combined modelling of 231Pa-235U and 230Th-238U disequilibria suggests that some of the assumptions commonly made in U-series studies might need reconsideration. That also provides interesting constraints on the timing of fluid and melt recycling as well as on the mineralogy and thermal conditions occurring during recycling of subducting material.

Mantle metasomatism and melting in subduction related environments: the message from U-sereis isotopes / R. AVANZINELLI; J. PRYTULAK; T. ELLIOTT. - In: EPITOME. - ISSN 1972-1552. - STAMPA. - 3:(2009), pp. 224-225. (Intervento presentato al convegno Geoitalia 2009, VII Forum Italiano di Scienze della Terra tenutosi a Rimini, Italia nel 9-11 Settembre 2009).

Mantle metasomatism and melting in subduction related environments: the message from U-sereis isotopes.

AVANZINELLI, RICCARDO;
2009

Abstract

At convergent margins crust is created by arc magmatism and consumed by subduction. Subducted material is transported into the mantle wedge in H2O-rich fluids, melts or supercritical liquids and sampled back at the surface by arc volcanism. U-series parent-daughter activity ratios, namely (230Th/238U), (231Pa/235U) and (226Ra/230Th) have long been recognised as a potentially powerful tool to constrain both the timescale of the recycling process and melting dynamics of arc magma generation. The widespread occurrence of 238U-excess is commonly interpreted as fluid addition from the subducting slab, and represents one of the key features of the so-called ‘subduction signature’. It is more problematic to assess the fractionation of U and Th in the case of slab melting (sediment and oceanic crust). A general decrease of 238U-excess with increasing Th concentration has suggested that sediment melts are added either in secular equilibrium or enough time has elapsed since melting that any initial disequilibrium has decayed. Protactinium (Pa) is generally assumed to be immobile in subduction zone fluids, but (231Pa/235U) disequilibria in island arcs are ubiquitously greater than unity. This has prompted many authors to apply to subduction related magmatism time-dependent melting models originally designed for MORB and OIB: in such models syn-melting in-growth of 231Pa could explain the transformation from initial fluid-induced 231Pa-deficits to the 231Pa-excesses measured in most arc lavas. Critically, in a successful time-related melting model, both 238U-230Th and 235U-231Pa disequilibria must be explained with the same parameters. In this frame, the combination of U-Th-Pa-Ra measurements on a single volcanic arc, and in particular the variation of (231Pa/235U) with (230Th/238U) allows investigation of the timing of slab dehydration and recycling and its effects on the melting process. The Mariana arc provides the perfect location to study the behaviour of Pa in the arc environment. (230Th/238U) disequilibria suggest that U is fluxed into the mantle during dehydration of the oceanic crust inducing significant 230Th-deficits in the most depleted (least sediment enriched) lavas. The more sediment rich lavas are less affected by this U addition, and show minor 230Th. All the measured samples display (231Pa/235U)>1, requiring sufficient 231Pa in-growth during melting to overcome the initial deficit induced in the source by U-rich fluids. Moreover, the slope of the (231Pa/235U) vs. (230Th/238U) alignment of the erupted lavas, in comparison to that expected for the fluid-fluxed source, suggests that magmas more affected by fluids requires more 231Pa-ingrowth during melting. However combined modelling of 231Pa-235U and 230Th-238U disequilibria suggests that some of the assumptions commonly made in U-series studies might need reconsideration. That also provides interesting constraints on the timing of fluid and melt recycling as well as on the mineralogy and thermal conditions occurring during recycling of subducting material.
2009
Epitome. Geoitalia 2009
Geoitalia 2009, VII Forum Italiano di Scienze della Terra
Rimini, Italia
R. AVANZINELLI; J. PRYTULAK; T. ELLIOTT
File in questo prodotto:
Non ci sono file associati a questo prodotto.

I documenti in FLORE sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.

Utilizza questo identificatore per citare o creare un link a questa risorsa: https://hdl.handle.net/2158/957177
Citazioni
  • ???jsp.display-item.citation.pmc??? ND
  • Scopus ND
  • ???jsp.display-item.citation.isi??? ND
social impact