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Inter-element fractionation of highly siderophile elements in the Tonga Arc due to flux melting of a depleted source

Dale, Christopher; Macpherson, Colin, G.; Pearson, D. Graham; Hammond, Samantha; Arculus, Richard

Description

Highly siderophile element concentrations (HSEs: Os, Ir, Ru, Pt, Pd, and Re) have been determined for a suite of fresh, submarine mafic lavas from the northern Tonga Arc front and the nascent backarc Fonualei Spreading Centre (FSC). Prior melt depletion of the Tongan mantle wedge combined with a high degree of fluid fluxed melting is thought to have produced boninitic magmas at several arc and FSC locations. As such, this arc system provides an opportunity to assess the fluid mobility of HSEs...[Show more]

dc.contributor.authorDale, Christopher
dc.contributor.authorMacpherson, Colin, G.
dc.contributor.authorPearson, D. Graham
dc.contributor.authorHammond, Samantha
dc.contributor.authorArculus, Richard
dc.date.accessioned2015-12-10T23:13:53Z
dc.identifier.issn0046-564X
dc.identifier.urihttp://hdl.handle.net/1885/64622
dc.description.abstractHighly siderophile element concentrations (HSEs: Os, Ir, Ru, Pt, Pd, and Re) have been determined for a suite of fresh, submarine mafic lavas from the northern Tonga Arc front and the nascent backarc Fonualei Spreading Centre (FSC). Prior melt depletion of the Tongan mantle wedge combined with a high degree of fluid fluxed melting is thought to have produced boninitic magmas at several arc and FSC locations. As such, this arc system provides an opportunity to assess the fluid mobility of HSEs and to investigate the effects of fluid-induced melting and prior melt depletion on HSE behaviour during both mantle melting and magma evolution. Tongan lavas display extreme enrichment of Pt (2.5-32. ng/g) and Pd over Os (0.002-0.6. ng/g), Ir, and Ru, significantly greater than basalts from mid-ocean ridges. Magma evolution increases the degree of fractionation, resulting in the highest recorded Pt/Ru ratios (>300) in arc front samples with MgO <8 wt.%. This increasing fractionation is due to the mild incompatibility of Pt and Pd, and concurrent compatibility of Ru, during sulphide undersaturated magma evolution. However, the fractionation of Pt and Pd from Os, Ir, and Ru is observed in the highest MgO samples, indicating source inheritance. Prior melt depletion of the mantle and elevated oxygen fugacity both increase the likelihood of complete consumption of sulphide in the source during melting, which typically leads to melts with high concentrations of all the HSE. Indeed, modelling indicates that 25% aggregate partial melting of a depleted MORB-mantle source, proposed for the Tonga Arc, will lead to complete base-metal sulphide consumption unless there is considerable addition of S by the slab flux (at least 200 μg/g). Although source enrichment of Pt, Pd, and Re by slab fluids may take place, the fractionation of Pt and Pd from Os, Ir, and Ru can largely be explained by relatively low-temperature, yet high-degree, melting of fluid-fluxed melt-depleted mantle. The high Pt and Pd contents can be produced by the exhaustion of sulphide in the source, while the presence of Ru-Os-(Ir) alloys or sulphides (e.g. laurite) associated with Cr-spinel can explain Os, Ir, and Ru retention in the source residue. Such phases have been documented in fluid-fluxed sub-arc mantle from ophiolites. Osmium isotopes co-vary negatively with Os abundance and thus appear to be dominated by shallow level contamination. The most Os-rich samples, however, have 187Os/ 188Os ratios (0.126-0.132) which are typical of DMM and MORB, suggesting an indistinguishable flux of radiogenic Os from the slab. The significant fractionation of Pt and Re from Os in arc settings will lead, over time, to elevated 186Os and 187Os which may be relevant to the observed enrichments of these isotopes in some mantle regions. In addition, the differing behaviour of Ru and Ir, and the implication of a mantle source containing Ru-rich microphases, may have consequences for the estimation of the HSE composition of primitive upper mantle.
dc.publisherPergamon Press
dc.sourceGeochimica et Cosmochimica Acta Supplement
dc.subjectKeywords: boninite; concentration (composition); fractionation; fugacity; lava; magma; numerical model; osmium; partial melting; siderophile element; sulfide; Tonga
dc.titleInter-element fractionation of highly siderophile elements in the Tonga Arc due to flux melting of a depleted source
dc.typeJournal article
local.description.notesImported from ARIES
local.identifier.citationvolume89
dc.date.issued2012
local.identifier.absfor040314 - Volcanology
local.identifier.ariespublicationf5625xPUB970
local.type.statusPublished Version
local.contributor.affiliationDale, Christopher, Durham University
local.contributor.affiliationMacpherson, Colin, G., Durham University
local.contributor.affiliationPearson, D. Graham, Durham University
local.contributor.affiliationHammond, Samantha, The Open University
local.contributor.affiliationArculus, Richard, College of Physical and Mathematical Sciences, ANU
local.description.embargo2037-12-31
local.bibliographicCitation.startpage202
local.bibliographicCitation.lastpage225
local.identifier.doi10.1016/j.gca.2012.03.025
local.identifier.absseo970104 - Expanding Knowledge in the Earth Sciences
dc.date.updated2016-02-24T09:46:04Z
local.identifier.scopusID2-s2.0-84861393529
local.identifier.thomsonID000305334100013
CollectionsANU Research Publications

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