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Petrological & geochemical insights into subduction initiation- the case of Izu-Bonin-Mariana volcanic arc

Dr Ivan Savov (SEE), Dr. David Ferguson

Contact email: i.savov@see.leeds.ac.uk

Summary

Convergent margins mark sites of plate destruction and are unique to Earth among the terrestrial planets. However, currently we lack clear understanding of why and how they are initiated. The so-called “spontaneous subduction initiation” model (Stern, 2004; Arculus et al., 2015) appears particularly relevant to the initiation of one of the largest, nominally intra-oceanic subduction zones in the W. Pacific-the Izu-Bonin-Mariana (IBM) system, i.e., spontaneous subduction initiation occurs when a change in plate motion allows the gravitationally unstable lithosphere to founder along an existing plate boundary. The IBM system represents an example of arc initiation wherein subsidence of relatively old Pacific lithosphere began along a system of transform faults/fracture zones adjacent to relatively buoyant lithosphere. Following subduction initiation, further igneous development is fundamental to the creation of arc volcanic chains, which in turn is essential to the formation and evolution of continental crust. 

IODP Exp. 351 site IBM-1 penetrated the pre-arc IBM basement (Early Eocene) and the overlying 1400 m of volcaniclastic sediments. Studying the sedimentary cover is crucial to decipher the geochemical and petrological evolution of the post-arc inception volcanism in the entire IBM region. The primary target of this PhD studentship is to study the arc- derived sediments directly overlaying the basement at this site. To date, the geochemical data available for such materials recovered from the fore-arc regions of the IBM are concentrated in the Neogene and only limited Paleogene record of volcaniclastics and ash material that is well-dated AND appropriate for geochemical study is available from any of the existing DSDP/ODP sites. Increasing the resolution of the post-IBM arc inception records will allow geochemical modelling aiding the quantification of the volumes of sediment, crust and mantle involved in the early arc volcanism. 

In detail, the PhD student will construct geochemical profiles across the entire Cenozoic, with emphasis on the Paleogene volcaniclastics. The student will analyse the volcanic ash and the most unaltered volcaniclasts for Sr, Nd, Nd and B isotopes. These isotope systems, in addition to major and trace element variations, have been shown to be a powerful tool for resolving provenance (Pb, Sr), as well as uniquely trace the sedimentary and/or crustal (slab) and mantle contributions (Sr, B and Nd) to VOLCANIC arcs (Straub, 2004). Using a combination of elemental and isotope tracers will allow the PhD student to uniquely quantify the volumes of fluids/melts involved in the sources of the earliest volcanic eruptions that took place at the IBM arc (see Savov et al., 2006; Brandl et al, 2017; Hickey-Vargas et al., 2018). This task is critical for subduction initiation modelling and also for better understanding of the causes of the modern variations in major element and isotope variations along the IBM arc. Unaltered volcaniclasts and ash samples have already been Ar-Ar dated in collaboration with the IODP Expedition co-chief scientist Prof. Ishizuka. This, in combination with the ages already derived from the nanofossil and paleomag records, will allow the student to constrain very precisely the temporal intervals that the selected IBM-1 volcanics represent. Once this is known, the student will have the opportunity to connect the anticipated results with the existing (mostly Neogene) ash and volcaniclasts datasets from other parts of the IBM and other W. Pacific arc-basin systems. Together, the combined age and isotope results are expected to be a major step toward the more complete understanding of the temporal chemical and petrological evolution of arc volcanism after subduction initiation, which is an issue remaining unknown.

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Related undergraduate subjects:

  • Chemistry
  • Earth science
  • Earth system science
  • Geochemistry
  • Geological science
  • Geology
  • Geoscience
  • Natural sciences
  • Oceanography