Thorium isotope evidence for melting of the mafic oceanic crust beneath the Izu Arc

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doi: 10.1016/j.gca.2016.04.034
Author(s): Freymuth, Heye; Ivko, Ben; Gill, James B.; Tamura, Yoshihiko; Elliott, Tim
Author Affiliation(s): Primary:
University of Bristol, School of Earth Sciences, Bristol, United Kingdom
University of California at Santa Cruz, United States
Japan Agency for Marine-Earth Science and Technology, Japan
Volume Title: Geochimica et Cosmochimica Acta
Source: Geochimica et Cosmochimica Acta, Vol.186, p.49-70. Publisher: Elsevier, New York, NY, International. ISSN: 0016-7037 CODEN: GCACAK
Note: In English. 121 refs.; illus., incl. 6 tables, sketch map
Summary: We address the question of whether melting of the mafic oceanic crust occurs beneath ordinary volcanic arcs using constraints from U-Series (238U/232Th, 230Th/232Th and 226Ra/230Th) measurements. Alteration of the top few hundred meters of the mafic crust leads to strong U enrichment. Via decay of 238U to 230Th, this results in elevated (230Th/232Th) (where brackets indicate activity ratios) over time-scales of ∼350 ka. This process leads to the high (230Th/232Th), between 2.6 and 11.0 in the mafic altered oceanic crust (AOC) sampled at ODP Sites 801 and 1149 near the Izu-Bonin-Mariana arc. Th activity ratios in the Izu arc lavas range from (230Th/232Th) = 1.2-2.0. These values are substantially higher than those in bulk sediment subducting at the Izu trench and also extend to higher values than in mid-ocean ridge basalts and the Mariana arc. We show that the range in Th isotope ratios in the Izu arc lavas is consistent with the presence of a slab melt from a mixed source consisting of AOC and subducted sediments with an AOC mass fraction of up to approximately 80 wt.% in the component added to the arc lava source. The oceanic plate subducting at the Izu arc is comparatively cold which therefore indicates that temperatures high enough for fluid-saturated melting of the AOC are commonly achieved beneath volcanic arcs. The high ratio of AOC/sediments of the slab melt component suggested for the Izu arc lavas requires preferential melting of the AOC. This can be achieved when fluid-saturated melting of the slab is triggered by fluids derived from underlying subducted serpentinites. Dehydration of serpentinites and migration of the fluid into the overlying crust causes melting to start within the AOC. The absence of a significant sediment melt component suggests there was insufficient water to flux both AOC and overlying sediments.
Year of Publication: 2016
Research Program: ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 05 Petrology, Igneous and Metamorphic; Actinides; Alkaline earth metals; Chemical composition; Chromatograms; Crust; Dehydration; Fluid phase; ICP mass spectra; Igneous rocks; Island arcs; Isotope ratios; Isotopes; Izu-Bonin Arc; Leg 185; Mafic composition; Magmas; Marine sediments; Mass spectra; Melting; Melts; Metaigneous rocks; Metals; Metamorphic rocks; Metasomatic rocks; North Pacific; Northwest Pacific; ODP Site 1149; ODP Site 801; Ocean Drilling Program; Ocean floors; Oceanic crust; Pacific Ocean; Pigafetta Basin; Radioactive isotopes; Radium; Sediments; Serpentinite; Slabs; Spectra; Subduction zones; Th-230/Ra-226; Th-232/Th-230; Thorium; Trenches; U-238/Th-232; Uranium; Volcanic rocks; Water-rock interaction; West Pacific
Coordinates: N183831 N183835 E1562136 E1562134
N183000 N320000 E1570000 E1420000
N312030 N312030 E1432100 E1432100
Record ID: 2016069250
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands