Download Island Arc Magmatism

Island Arc Magmatism
Eruption of Anatahan Volcano,
Northern Mariana Is, June 10, 2003
Subduction Factory
Location of arc volcanoes
Open circles = N Pacific Arcs
Syracuse & Abers, 2006
Why does melting occur? Decompression Melting?
This model includes a
backarc spreading center
and shows contours of
decompression melt
production
Conder et al., 2002
Amphibole dehydration model
(Davies & Stevenson 1992)
However: Later experiments suggest Amphibole dehydrates at depths too shallow
Also: Geochemical evidence does not support an amphibole-only origin for arc fluids
Improved model – water input from several
hydrous phases
Hydrous minerals are: lawsonite, zoisite, chloritoid, and talc in the oceanic crust
Chlorite and serpentine in the mantle
They dehydrate over a large depth range, to at least 150 km
Schmidt & Poli 1998
 Modeling water transport and magma production
in the arc
Cagniocle et al., 2007
Arc lavas contain signature of subducted sediments
Be10 has half life of 1.5 Ma, formed by cosmogenic processes in atmosphere
Concentrated in the upper oceanic sediments
High Be10 concentrations in arc magmas show that sediment melts or else
Be10 is transported to the arc magmatic system in subduction fluids
Development of Arc Crust
Comparison with Basalt Geobarometry
= maximum
anomaly
• Si & Mg thermobarometry suggest a final equilibration depth of magma:
21-34 km for the backarc spreading center
34-87 km for the volcanic arc [Kelley et al., in press]
• Corresponds perfectly with the strongest anomalies beneath the backarc and arc
• Shows that velocity and Q anomalies delineate the melt production region
• Anomalies result from small fractions of in-situ melt
TUCAN Broadband Seismic Experiment
Nicaragua vs Costa Rica Attenuation –
More Water?
Rychert et al. [2008]
• Nicaragua shows significantly higher attenuation than Costa Rica
• Geochemical/petrologic constraints suggest similar mantle temperatures
• The greater attenuation is consistent with higher mantle water contents
and extents of melting inferred from geochemistry and melt inclusions.
Nicaragua Attenuation & Vp/Vs Ratio –
evidence of magma ascent?
Attenuation
(from Rychert et al. 2008)
Vp/Vs ratio
(from Syracuse et al. 2008)