Origin of depleted basalts during subduction initiation and early development of the Izu-Bonin-Mariana island arc; evidence from IODP Expedition 351 Site U1438, Amami-Sankaku Basin

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doi: 10.1016/j.gca.2018.03.007
Author(s): Hickey-Vargas, R.; Yogodzinski, G. M.; Ishizuka, O.; McCarthy, A.; Bizimis, M.; Kusano, Y.; Savov, I. P.; Arculus, R.
Author Affiliation(s): Primary:
Florida International University, Department of Earth and Environment, Miami, FL, United States
University of South Carolina, United States
Japanese Geological Survey, Japan
University of Lausanne, Switzerland
University of Leeds, United Kingdom
Australian National University, Australia
Volume Title: Geochimica et Cosmochimica Acta
Source: Geochimica et Cosmochimica Acta, Vol.229, p.85-111. Publisher: Elsevier, New York, NY, International. ISSN: 0016-7037 CODEN: GCACAK
Note: In English. Includes appendices. 68 refs.; illus., incl. 1 table, sketch map
Summary: The Izu-Bonin-Mariana (IBM) island arc formed following initiation of subduction of the Pacific plate beneath the Philippine Sea plate at about 52 Ma. Site U1438 of IODP Expedition 351 was drilled to sample the oceanic basement on which the IBM arc was constructed, to better understand magmatism prior to and during the subduction initiation event. Site U1438 igneous basement Unit 1 (150 m) was drilled beneath 1460 m of primarily volcaniclastic sediments and sedimentary rock. Basement basalts are microcrystalline to fine-grained flows and form several distinct subunits (1a-1f), all relatively mafic (MgO = 6.5-13.8%; Mg# = 52-83), with Cr = 71-506 ppm and Ni = 62-342 ppm. All subunits are depleted in non-fluid mobile incompatible trace elements. Ratios such as Sm/Nd (0.35-0.44), Lu/Hf (0.19-0.37), and Zr/Nb (55-106) reach the highest values found in MORB, while La/Yb (0.31-0.92), La/Sm (0.43-0.91) and Nb/La (0.39-0.59) reach the lowest values. Abundances of fluid-mobile incompatible elements, K, Rb, Cs and U, vary with rock physical properties, indicating control by post-eruptive seawater alteration, but lowest abundances are typical of fresh, highly depleted MORBs. Mantle sources for the different subunits define a trend of progressive incompatible element depletion. Inferred pressures of magma segregation are 0.6-2.1 GPa with temperatures of 1280-1470 °C. New 40Ar/39Ar dates for Site U1438 basalts averaging 48.7 Ma (Ishizuka et al., 2018) are younger that the inferred age of IBM subduction initiation based on the oldest ages (52 Ma) of IBM forearc basalts (FAB) from the eastern margin of the Philippine Sea plate. FAB are hypothesized to be the first magma type erupted as the Pacific plate subsided, followed by boninites, and ultimately typical arc magmas over a period of about 10 Ma. Site U1438 basalts and IBM FABs are similar, but Site U1438 basalts have lower V contents, higher Ti/V and little geochemical evidence for involvement of slab-derived fluids. We hypothesize that the asthenospheric upwelling and extension expected during subduction initiation occurred over a broad expanse of the upper plate, even as hydrous fluids were introduced near the plate edge to produce FABs and boninites. Site U1438 basalts formed by decompression melting during the first 3 Ma of subduction initiation, and were stranded behind the early IBM arc as mantle conditions shifted to flux melting beneath a well-defined volcanic front.
Year of Publication: 2018
Research Program: IODP Integrated Ocean Drilling Program
IODP2 International Ocean Discovery Program
Key Words: 02 Geochemistry; 05 Petrology, Igneous and Metamorphic; Absolute age; Ar/Ar; Basalts; Cenozoic; Chemical composition; Crust; Dates; Eocene; Expedition 351; IODP Site U1438; Igneous rocks; International Ocean Discovery Program; Island arcs; Isotopes; Izu-Bonin Arc; Major elements; Mariana Islands; Mariana Trench; Marine sediments; Metals; Micronesia; Mid-ocean ridge basalts; North Pacific; Northwest Pacific; Ocean floors; Oceania; Oceanic crust; Pacific Ocean; Paleogene; Philippine Sea Plate; Plate tectonics; Radioactive isotopes; Rare earths; Sediments; Subduction zones; Tertiary; Trace elements; Trenches; Volcanic rocks; Volcaniclastics; West Pacific
Coordinates: N272300 N272301 E1341907 E1341905
Record ID: 2018062567
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands