“Submarine  volcanism  and  hydrothermalism  in  the  western  Pacific”  	  Richard	  J.	  ARCULUS	  Professor,  Research  School  of  Earth  Sciences  The  Australian  National  University  In	   the	   past	   25	   years,	   multinational	   and	   multidisciplinary	   efforts	   using	   multibeam	   sonar	  mapping,	   drilling/dredging/coring	   campaigns,	   hydrothermal	   vent	   surveys,	   and	   mineral	  exploration,	  involving	  ships,	  submersibles,	  and	  various	  remotely	  controlled	  and	  autonomous	  underwater	   vehicles,	   have	   revolutionized	   our	   understanding	   of	   the	   volcanic	   and	  hydrothermal	  activity	  of	  the	  submarine	  portion	  of	  the	  “Ring	  of	  Fire”	  in	  the	  western	  Pacific.	  
The	  discoveries	  include:	  1.	  A	  very	  large	  increase	  in	  the	  number	  of	  known	  volcanoes	  in	  both	  arc	  and	  backarc	  settings.	  In	  the	  case	  of	  the	  Tonga-­‐Kermadec	  (TK)	  system	  for	  example,	  there	  are	   now	   70	   known	   volcanoes	   in	   ~2000km	   of	   arc	   strike	   length,	   of	   which	   about	   30%	   are	  hydrothermally	   active.	   Zones	   of	   widespread	   active	   volcanic	   activity	   and	   hydrothermal	  venting	   are	   occurring	   in	   all	   of	   the	   currently	   extending	   backarc	   basins;	   2.	   The	   particularly	  metal-­‐	   and	   magmatic	   gas-­‐rich	   hydrothermal	   fluids	   associated	   with	   this	   activity	   have	  generated	   volcanogenic	   sulfide	   deposits	   of	   low	   tonnage	   but	   very	   high-­‐grade	   and	   potential	  exploitability,	   and	   constitute	   a	   significant	   fraction	   of	   the	   global	   hydrothermal	   inputs	   into	   the	  Pacific	   Ocean.	   A	   number	   of	   commercial	   companies	   currently	   hold	   exploration	   leases	   for	  much	  of	  the	  arc-­‐back	  systems	  of	  the	  western	  Pacific,	  and	  a	  mining	  license	  has	  been	  issued	  for	  an	  area	  in	  the	  Bismarck	  Sea	  (eastern	  Manus	  backarc	  basin);	  3.	  New	  magma	  types	  have	  been	   identified	   among	   the	   extrusives	   of	   the	   backarc	   basins	   tapped	   from	   a	   remarkable	   range	  of	   mantle	   protoliths,	   spanning	   ultra-­‐depleted	   to	   enriched	   sources.	   For	   example,	   alkali-­‐rich,	  continental	  rift-­‐like	  basalts	  lacking	  the	  characteristic	  high	  field	  strength	  element	  depletions	  associated	  with	  arc-­‐proximal	  backarc	  basin	  basalts,	  are	  being	  erupted	  in	  the	  Nifonea	  Ridge	  of	  the	  Vate	  Trough	  of	  Vanuatu.	  In	  contrast,	  basalts	  of	  the	  Central	  Lau	  Spreading	  Centre	  are	  among	   the	   most	   depleted	   basalt	   types	   known	   in	   the	   spectrum	   of	   global	   backarc	   and	   mid-­‐
ocean	   ridge	   basalts.	   Currently	   active	   boninite	   volcanism	   has	   been	   identified	   at	   the	   Tongan	  volcanic	   front	   and	   reararc.	   Boninites	   are	   generated	   most	   commonly	   only	   in	   subduction	   zone	  settings	   involving	   the	   hydrous	   fluxed	   melting	   of	   refractory	   (e.g.,	   clinopyroxene-­‐poor	  lherzolite	   and	   harzburgite)	   lithologies.	   Boninite	   was	   a	   widely	   erupted	   magma	   type	   closely	  following	  subduction	  zone	  inception	  in	  the	  Izu-­‐Bonin-­‐Mariana	  (IBM)	  arc,	  but	  is	  demonstrably	  involved	  as	  a	  mixing	  component	  of	  active	  magmatism	  in	  both	  the	  TK	  and	  IBM	  arcs;	  4.	  Clear	  links	   have	   been	   established	   between	   backarc	   ridge/rift	   morphologies,	   dissolved	   volatile	  loads	   in	   magmas,	   and	   rates	   of	   spreading;	   5.	   Iron	   isotope	   studies	   have	   demonstrated	   the	  more	   depleted	   (i.e.,	   prior	   melt	   loss)	   character	   of	   the	   mantle	   sources	   involved	   in	   backarc	   and	  arc	   magma	   generation	   than	   those	   associated	   with	   mid-­‐ocean	   ridges;	   6.	   Remarkably	   fresh	  examples	  of	  mantle	  wedge-­‐derived	  peridotites	  (harzburgite-­‐dunite)	  likely	  representative	  of	  these	  mantle	  sources	  have	  been	  recovered	  from	  two	  locations	  in	  Papua	  New	  Guinea.	  These	  peridotites	   are	   strongly	   depleted	   and	   include	   highly	   magnesian	   olivine	   and	   Cr-­‐rich	   spinel,	  correlative	   with	   considerable	   loss	   of	   melt.	   In	   addition	   however,	   their	   orthopyroxene-­‐rich	  nature	   relative	   to	   intraplate	   spinel	   peridotites	   reflects	   enrichment	   in	   silica-­‐rich,	   subducted	  slab-­‐derived	  fluids	  prior	  to	  or	  concurrent	  with	  melting	  in	  the	  mantle	  wedge;	  7.	  Ingress	  of	  the	  Samoan	  (hot-­‐spot)	  plume	  into	  the	  northern	  Lau	  Basin,	  has	  been	  identified	  primarily	  by	  noble	  gas	  (He,	  Ne,	  Xe)	  analysis.	  It	  appears	  the	  primary	  component	  of	  the	  invading	  plume	  comprises	  the	  “FOZO”	  type	  rather	  than	  the	  EM1-­‐EM2-­‐HIMU	  flavours	  also	  identified	  along	  the	  Samoan	  chain.	  Tracking	  of	  the	  geochemically	  distinctive	  Samoan	  plume	  is	  significant	  in	  terms	  of	  both	  toroidal	   and	   poloidal	   mantle	   flow	   patterns	   in	   the	   Tongan	   mantle	   wedge.	   Discoveries	  continue	  to	  be	  made,	  and	  research	  opportunities	  abound.