Drivers of the Episodic life-cycle of Intra-oceanic Arc-Backarc Hydrothermal Systems
Convergent margins, where oceanic lithosphere is subducted into the mantle, form the key interface for large–scale chemical and thermal recycling amongst the ocean, crust, mantle, and atmosphere. Intra-oceanic convergent margins typically produce a forearc-island arc-backarc sequence within an extensional stress regime and contain a disproportionate number of hydrothermal vents per unit area. The evolutionary cycle of a typical intra-oceanic island arc-backarc sequence is intricately linked to the evolution of subduction and crustal accretion, from the initiation of subduction to the episodic eruption at island arcs and backarc basin rifting and spreading to eventual termination of subduction and backarc closure. The episodic nature of this system leaves behind a series of remnant and active island arcs, spreading ridges and hydrothermal vents, most clearly observed in the seafloor fabric of the Western Pacific. Backarc closure leads to the accretion of entire (and often multiple) island arc-backarc sequences to the continental margins, producing one of the fundamental components of continental crust. The contact between cold seawater and hot volcanic or magmatic rocks at hydrothermal vents forms seafloor massive sulphide deposits, providing an active laboratory in which to study the formation of onshore massive sulphide deposits that were accreted to the margin during back-arc closure.
The thermal, geochemical, and structural characteristics of intra-oceanic island arc-backarc systems during their active phase are predominately influenced by complex, slab-derived fluids, the mantle wedge, and overlying lithosphere. Rock compositions and mineralization hosted by intra-oceanic island arc volcanics and backarc basin basalts have distinct geochemical signatures initially influenced by the subducted oceanic lithosphere, sediments, and seamounts on the down-going slab. In addition, the processes of magmatic differentiation within arc-backarc crust is fundamentally different compared with mid-ocean ridges, leading to dramatically different styles of noble and base metal concentration in evolving magmas. The concentration of gases and some elements in hydrothermal fluids expelled at arc-backarc vent sites can be many times higher than those seen at mid-ocean ridge sites, and result from the interaction of evolving hydrous magmas with crustal lithologies and fluids. The mode of extension and the crustal accretion pattern found in backarc basins may be driven by the small-scale convection within the mantle wedge, its temperature and viscosity and/or changes in surface kinematics between the down-going and over-riding plates.
Understanding the dominant role that the subducting slab and mantle wedge appear to play in governing the dynamics of the entire subduction system requires a renewed focus on the relationship between the surface expression of arc volcanism and backarc seafloor spreading, mantle markers, and the processes occurring in the underlying mantle. Additionally, the working group hopes to foster a holistic approach to address fundamental questions about intra-oceanic arc and backarc basin processes by examining the long-term and short-term evolutionary cycles of these systems using geochemical, hydrothermal, tectonics and subduction dynamics approaches. The working group seeks to bring together experts from both the observational and modeling communities to facilitate the linkage between surface processes and the deep earth.
Science Questions
1. What role does the mantle wedge play in the distribution and composition of magmatism and hydrothermal activity in both the arc and backarc? What is the mode of convection within and around the mantle wedge?
2. Is backarc extension driven by the rollback of the subduction hinge, purely driven by the absolute motion of the over-riding plate or independent of the surface kinematics and instead dominated by magmatism in the mantle wedge by rising diapirs or secondary convection cells?
3. Can we predict what type of mineral deposit might form in the backarc and arc knowing what components are being subducted with the down-going slab (i.e., contributions made by subducted sediments, oceanic plateau, seamounts, etc)? What degree of partial melting is occurring in the underlying mantle wedge, and how are the metals transported to the seafloor (i.e., as magmatic volatiles or simply water/rock reactions)?
4. What is the role of slab-derived fluxes (fluids and melts) in altering the physical and thermodynamic nature of the mantle such as its temperature and redox state? How are lithophile elements that constitute new crust, volatiles (e.g. H2O, S2 and CO2) that drive volcanism and affect climate and life on Earth, and ore-forming metals (e.g. Cu, Pb, Au) concentrated in hydrothermal vents and sulfide deposits, recycled between the Earth’s surface and interior?
5. What interdisciplinary methods can be applied to deconvolve fluxes from slab-derived elements from the ambient mantle wedge?
6. Analogue and numerical experiments of subduction have predicted the initiation of backarc extension but what drives the episodic nature of backarc basin opening?
7. Accreted margins contain evidence of obducted forearc-arc-backarc sequences through ophiolitic suites. These ophiolites are often said to have formed via “backarc closure” but what are the driving forces required to close a backarc sequence? How does this relate to the termination of subduction? Backarc opening works on time-scales of tens of millions of years. Does the closure of backarc basins work on similar timescales?
Timeline
2011: Proposed working group presented to the InterRidge Committee, apply for UNESCO-IGCP funding to setup a complementary 5-year arc-backarc basin project
2012: Initial working group planning meeting before/after the International Geological Convention (ICG), Brisbane, July 2012 or before/after a proposed IODP workshop on the SW Pacific scheduled for Sydney in October 2012. All working group members will be invited (extra accommodation costs will be covered, but not airfares)
2013: Second working group meeting to plan workshop before/after the EGU or AGU
2014: Intra-oceanic Arc and Backarc Systems workshop to be held in Sydney or NZ
Chair: Maria Seton (USYD, Australia); Co-chair: Cornel de Ronde (GNS Science, New Zealand)
Group Members: Richard Wysoczanski (NIWA/U. Victoria), Richard Arculus (ANU, Australia), Michael Gurnis (Caltech, USA), Jo Whittaker (USYD, Australia), Dietmar Müller (USYD, Australia), Colin Macpherson (Durham, UK), Erin Todd (Munster, Germany), Jim Gill (UCSC, USA), Sven Petersen (GEOMAR, Germany), Jonny Wu (NUT, Taiwan), Yoshihiko Tamura (JAMSTEC, Japan), Hiromi Watanabe (JAMSTEC, Japan), Jonathan Aitchison (USYD, Australia)
