Journal article
Magma Source Evolution Following Subduction Initiation: Evidence From the Element Concentrations, Stable Isotope Ratios, and Water Contents of Volcanic Glasses From the Bonin Forearc (IODP Expedition 352)
Geochemistry, geophysics, geosystems : G3, Vol.22(1), e2020GC009054
01/2021
DOI: 10.1029/2020GC009054
Abstract
International Ocean Discovery Program Expedition 352 to the Bonin forearc drilled the sequence of volcanic rocks erupted in the immediate aftermath of subduction initiation along the western margin of the Pacific Plate. Pristine volcanic glasses collected during this expedition were analyzed for major and trace elements, halogens, sulfur, and H and O isotopes with goals of characterizing the fluids and melts of subducted materials that were involved in generating the nascent upper plate crust. Incompatible trace element compositions of the oldest lavas (forearc basalts [FAB]) are similar to those of the most depleted mid‐ocean ridge basalts globally. Most FAB were generated by decompression melting during seafloor spreading in a near‐trench, supra‐subduction zone environment with only minor involvement of diverse and generally dilute water‐rich fluids from the subducting plate. Boninite series glasses are enriched in incompatible trace elements mobilized from the subducting plate, but strongly depleted in other elements, such as the middle‐heavy rare‐earth elements. These traits are attributed to generation of boninites largely by flux melting involving water‐rich melts first derived from the leading edge of subducted basaltic crust and then from both subducted crust and sediment. These melts were generated at low pressures as the shallow, embryonic slab extracted heat from hot asthenosphere near the trench. The progressive depletion of the mantle source for the FAB‐through‐boninite sequence suggests that the asthenospheric mantle remained trapped above the nascent subducting plate for the first several million years of subduction beneath the Philippine Sea Plate.
Plain Language Summary
The origin of crust created along the leading edge of the Philippine Sea Plate after the Pacific Plate began subducting beneath it about 52 million years ago was investigated by analyzing volcanic glasses recovered by drilling the Bonin forearc during International Ocean Discovery Program Expedition 352. The glasses have not been affected by alteration and thus preserve pristine compositions of the erupted lavas. Two deep water drill sites (U1440 and U1441) recovered basalts and rare andesites produced by crystal removal and crust assimilation. The compositions of the basalt glasses suggest that they were produced by depressurization of hot mantle during near‐trench seafloor spreading in the presence of minor amounts of water‐rich fluid from the newly subducting plate. Younger, more silica‐rich lavas termed “boninites” were recovered from two sites drilled in shallower water (U1439 and U1442). Boninite glass compositions demonstrate that these boninites were generated when shallow hot mantle was invaded by water‐rich melts derived first from the subducting basaltic crust and then this crust plus sediment.
Key Points
Forearc basalts formed in response to subduction initiation by melting of depleted mantle after minor inputs of variably saline fluids
Boninites were generated by melting residual mantle after fluxing by water rich melts from altered oceanic crust then crust and sediment
The subducting plate melted at exceedingly shallow depths beneath oceanic crust being generated by seafloor spreading
Details
- Title: Subtitle
- Magma Source Evolution Following Subduction Initiation: Evidence From the Element Concentrations, Stable Isotope Ratios, and Water Contents of Volcanic Glasses From the Bonin Forearc (IODP Expedition 352)
- Creators
- Daniel A Coulthard - Massey UniversityMark K Reagan - University of IowaKenji Shimizu - Japan Agency for Marine-Earth Science and TechnologyIlya N Bindeman - University of OregonMaryjo Brounce - University of California, RiversideRenat R Almeev - Leibniz University HannoverJeffrey Ryan - University of South FloridaTimothy Chapman - University of New EnglandJohn Shervais - Utah State UniversityJulian A Pearce - Cardiff University
- Resource Type
- Journal article
- Publication Details
- Geochemistry, geophysics, geosystems : G3, Vol.22(1), e2020GC009054
- DOI
- 10.1029/2020GC009054
- ISSN
- 1525-2027
- eISSN
- 1525-2027
- Number of pages
- 30
- Language
- English
- Date published
- 01/2021
- Academic Unit
- Earth and Environmental Sciences
- Record Identifier
- 9984229168002771
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