226Ra-230Th evidence for multiple dehydration events, rapid melt ascent and the time scales of differentiation beneath the Tonga-Kermadec island arc

Simon Turner, Bernard Bourdon, Chris Hawkesworth, Peter Evans

Research output: Contribution to journalArticleResearchpeer-review

Abstract

U-series disequilibria can be used to constrain the time scales and processes of fluid transfer, partial melting, magma ascent and magma differentiation beneath island arcs. However, maximum information is provided by studies involving more than one parent-daughter pair. Here we present mass spectrometric 226Ra measurements of Tonga-Kermadec arc lavas to complete the first detailed U-Th-Pa-Ra study of island arc lavas. (226Ra/230Th) activity ratios in Tonga-Kermadec lavas range from ∼ 1 up to 6.2, and they are negatively correlated with SiO2 constraining the time scale for differentiation from basalt to dacite and rhyolite to be < 6000 yr. The largest 226Ra-excesses occur in the most depleted rocks which have the highest Ba/Th ratios indicating that the 226Ra-excesses result from fluid addition within the last few 1000 yr. Yet (238U/230Th) and (231Pa/235U) data from Tonga both record U addition by fluids ∼60000 yr ago. A simplified two-stage dehydration model is developed to reconcile these data. Unlike U, 226Ra lost to the mantle wedge during initial dehydration is replenished in the subducting altered oceanic crust by in-growth from residual 230Th. Whereas the U budget was dominated by the first fluid flux, the 226Ra-excesses record the continued addition of fluid to the mantle wedge, probably up until less than 1000 yr ago. The observation of nearly ubiquitous large 226Ra-excesses in the primitive lavas indicates that neither fluids nor melts were produced in the presence of significant amounts of residual amphibole and places tight constraints on the time permitted for melt generation, segregation and ascent.

LanguageEnglish
Pages581-593
Number of pages13
JournalEarth and Planetary Science Letters
Volume179
Issue number3-4
Publication statusPublished - 2000
Externally publishedYes

Fingerprint

island arcs
ascent
Dehydration
dehydration
island arc
melt
timescale
Fluids
fluid
fluids
wedges
magma
Earth mantle
Amphibole Asbestos
mantle
rhyolite
amphiboles
dacite
disequilibrium
basalt

Keywords

  • Equilibrium
  • Fluid phase
  • Island arcs
  • Kermadec Islands
  • Th-230/Ra-226
  • Time scales
  • Tonga

Cite this

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title = "226Ra-230Th evidence for multiple dehydration events, rapid melt ascent and the time scales of differentiation beneath the Tonga-Kermadec island arc",
abstract = "U-series disequilibria can be used to constrain the time scales and processes of fluid transfer, partial melting, magma ascent and magma differentiation beneath island arcs. However, maximum information is provided by studies involving more than one parent-daughter pair. Here we present mass spectrometric 226Ra measurements of Tonga-Kermadec arc lavas to complete the first detailed U-Th-Pa-Ra study of island arc lavas. (226Ra/230Th) activity ratios in Tonga-Kermadec lavas range from ∼ 1 up to 6.2, and they are negatively correlated with SiO2 constraining the time scale for differentiation from basalt to dacite and rhyolite to be < 6000 yr. The largest 226Ra-excesses occur in the most depleted rocks which have the highest Ba/Th ratios indicating that the 226Ra-excesses result from fluid addition within the last few 1000 yr. Yet (238U/230Th) and (231Pa/235U) data from Tonga both record U addition by fluids ∼60000 yr ago. A simplified two-stage dehydration model is developed to reconcile these data. Unlike U, 226Ra lost to the mantle wedge during initial dehydration is replenished in the subducting altered oceanic crust by in-growth from residual 230Th. Whereas the U budget was dominated by the first fluid flux, the 226Ra-excesses record the continued addition of fluid to the mantle wedge, probably up until less than 1000 yr ago. The observation of nearly ubiquitous large 226Ra-excesses in the primitive lavas indicates that neither fluids nor melts were produced in the presence of significant amounts of residual amphibole and places tight constraints on the time permitted for melt generation, segregation and ascent.",
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journal = "Earth and Planetary Science Letters",
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226Ra-230Th evidence for multiple dehydration events, rapid melt ascent and the time scales of differentiation beneath the Tonga-Kermadec island arc. / Turner, Simon; Bourdon, Bernard; Hawkesworth, Chris; Evans, Peter.

In: Earth and Planetary Science Letters, Vol. 179, No. 3-4, 2000, p. 581-593.

Research output: Contribution to journalArticleResearchpeer-review

TY - JOUR

T1 - 226Ra-230Th evidence for multiple dehydration events, rapid melt ascent and the time scales of differentiation beneath the Tonga-Kermadec island arc

AU - Turner, Simon

AU - Bourdon, Bernard

AU - Hawkesworth, Chris

AU - Evans, Peter

PY - 2000

Y1 - 2000

N2 - U-series disequilibria can be used to constrain the time scales and processes of fluid transfer, partial melting, magma ascent and magma differentiation beneath island arcs. However, maximum information is provided by studies involving more than one parent-daughter pair. Here we present mass spectrometric 226Ra measurements of Tonga-Kermadec arc lavas to complete the first detailed U-Th-Pa-Ra study of island arc lavas. (226Ra/230Th) activity ratios in Tonga-Kermadec lavas range from ∼ 1 up to 6.2, and they are negatively correlated with SiO2 constraining the time scale for differentiation from basalt to dacite and rhyolite to be < 6000 yr. The largest 226Ra-excesses occur in the most depleted rocks which have the highest Ba/Th ratios indicating that the 226Ra-excesses result from fluid addition within the last few 1000 yr. Yet (238U/230Th) and (231Pa/235U) data from Tonga both record U addition by fluids ∼60000 yr ago. A simplified two-stage dehydration model is developed to reconcile these data. Unlike U, 226Ra lost to the mantle wedge during initial dehydration is replenished in the subducting altered oceanic crust by in-growth from residual 230Th. Whereas the U budget was dominated by the first fluid flux, the 226Ra-excesses record the continued addition of fluid to the mantle wedge, probably up until less than 1000 yr ago. The observation of nearly ubiquitous large 226Ra-excesses in the primitive lavas indicates that neither fluids nor melts were produced in the presence of significant amounts of residual amphibole and places tight constraints on the time permitted for melt generation, segregation and ascent.

AB - U-series disequilibria can be used to constrain the time scales and processes of fluid transfer, partial melting, magma ascent and magma differentiation beneath island arcs. However, maximum information is provided by studies involving more than one parent-daughter pair. Here we present mass spectrometric 226Ra measurements of Tonga-Kermadec arc lavas to complete the first detailed U-Th-Pa-Ra study of island arc lavas. (226Ra/230Th) activity ratios in Tonga-Kermadec lavas range from ∼ 1 up to 6.2, and they are negatively correlated with SiO2 constraining the time scale for differentiation from basalt to dacite and rhyolite to be < 6000 yr. The largest 226Ra-excesses occur in the most depleted rocks which have the highest Ba/Th ratios indicating that the 226Ra-excesses result from fluid addition within the last few 1000 yr. Yet (238U/230Th) and (231Pa/235U) data from Tonga both record U addition by fluids ∼60000 yr ago. A simplified two-stage dehydration model is developed to reconcile these data. Unlike U, 226Ra lost to the mantle wedge during initial dehydration is replenished in the subducting altered oceanic crust by in-growth from residual 230Th. Whereas the U budget was dominated by the first fluid flux, the 226Ra-excesses record the continued addition of fluid to the mantle wedge, probably up until less than 1000 yr ago. The observation of nearly ubiquitous large 226Ra-excesses in the primitive lavas indicates that neither fluids nor melts were produced in the presence of significant amounts of residual amphibole and places tight constraints on the time permitted for melt generation, segregation and ascent.

KW - Equilibrium

KW - Fluid phase

KW - Island arcs

KW - Kermadec Islands

KW - Th-230/Ra-226

KW - Time scales

KW - Tonga

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M3 - Article

VL - 179

SP - 581

EP - 593

JO - Earth and Planetary Science Letters

T2 - Earth and Planetary Science Letters

JF - Earth and Planetary Science Letters

SN - 0012-821X

IS - 3-4

ER -