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.
| Language | English |
|---|---|
| Pages | 581-593 |
| Number of pages | 13 |
| Journal | Earth and Planetary Science Letters |
| Volume | 179 |
| Issue number | 3-4 |
| Publication status | Published - 2000 |
| Externally published | Yes |
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Keywords
- Equilibrium
- Fluid phase
- Island arcs
- Kermadec Islands
- Th-230/Ra-226
- Time scales
- Tonga
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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 journal › Article › Research › peer-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
UR - http://www.scopus.com/inward/record.url?scp=0033915970&partnerID=8YFLogxK
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 -