5. 3 Subduction zones 3 arc magmatism Where can we observe magmatic/volcanic activities along subduction zones? Characteristics of arc magmatism (vs. mid-ocean ridge/intraplate magmatism) Structure of arc crust and how to know the structure. What is the mechanism of arc magmatism? arc magmatism and continental formation 1 Where can we observe magmatic/volcanic activities along subduction zones? Characteristics of arc magmatism (vs. mid-ocean ridge/intraplate magmatism) Structure of arc crust and how to know the structure. What is the mechanism of arc magmatism? arc magmatism and continental formation 2
() subduction zone magmatic activity: arc magmatism 3 Generalized cross-section overriding plate subducted plate down going plate ( An introduction to our dynamic planet, Rogers ed., 2008) 4
outer rise backarc arc trench 5 Tohoku and Izu arcs 6
Practice: Plot locations of arc volcanoes Bathymetry data ETOPO1 Gray contour Slab1.0 model[hayes et al., 2012] volcano locations: Global Volcanism Program database (Smithsonian Institution, National Museum of Natural History) Longitude Latitude Name 139.098 34.9 Izu-Tobu 139.394 34.724 Izu-Oshima 139.279 34.52 Toshima 139.27 34.397 Niijima 139.153 34.219 Kozushima 139.526 34.094 Miyakejima 139.602 33.874 Mikurajima 139.68 33.4 Kurose Hole A: Izu-Bonin B: Mariana C: South America 7 8
9 10
location of volcanic arc and slab dip volcanic arc Steep slab : large distance between trench and volcanic arc 108km+-14km Depth of upper surface of slab beneath volcanic arc is constant Generally speaking, steep slab : narrow volcanic arc, 1994 11 Where can we observe magmatic/volcanic activities along subduction zones? Characteristics of arc magmatism (vs. mid-ocean ridge/intraplate magmatism) Structure of arc crust and how to know the structure. What is the mechanism of arc magmatism? arc magmatism and continental formation 12
Volcanic rock geochemistry @ different tectonic setting Intraplate Mid-ocean ridge 13 Arc Large variation in SiO2 content (Our Dynamic Planet, Rogers, 2008) Izu-Oshima basalt-andesite Kuchinoerabu andesite Unzen andesite-dacite Usu dacite 14
Water content in volcanic glasses Divergent boundary = Mid-ocean ridge Convergent boundary = Arc (Stern, 2002) 15 Trace elements island arc basat (IAB): spiky pattern LILEs= high solubilities, Rb, Cs, Ba, Sr, (U) Th = from subducted sediment (Our Dynamic Planet, Rogers, 2008) 16
Where can we observe magmatic/volcanic activities along subduction zones? Characteristics of arc magmatism (vs. mid-ocean ridge/intraplate magmatism) Structure of arc crust and how to know the structure. What is the mechanism of arc magmatism? arc magmatism and continental formation 17 Acoustic (seismic) surveys reflected wave -<~ source-station distance <~ depth geometry of layers, faults Multi Channel Seismic reflection refracted wave ->> source-station distance >> depth seismic velocity structure Ocean Bottom Seismometers 18
Marine seismic surveys source: air-gun streamer cable Ocean Bottom Seismometer 19 average thickness of crust Discovery of Mohorovičić discontinuity (seismic discontinuity) seismically definition of crust/mantle boundary Depths of Moho discontinuity 39.17±8.52km (Christensen and Mooney, 1995) continent : thick and large regional variation 7.0±0.8km (White et al., 1992) ocean floor: thin and homogeneous attention! Moho can NOT ALWAYS clearly observed. Moho does NOT coincides with petrological boundary. 20
Seismic structure of arc crust oceanic continental? oceanic mid-crust 6km/s (Suyehiro et al., 1996) 21 Variation of crustal structures: around Japan South Ryukyu North Ryukyu Southwest Japan Northeast Japan Izu-Bonin Arai et al., 2016 22
along axis variation Downloaded from geology.gsapubs.org on February 2, 2012 (Kodaira et al., 2007) 23 Where can we observe magmatic/volcanic activities along subduction zones? Characteristics of arc magmatism (vs. mid-ocean ridge/intraplate magmatism) Structure of arc crust and how to know the structure. What is the mechanism of arc magmatism? arc magmatism and continental formation 24
What does this illustration indicate? (Understanding Earth, Sliver&Jordan 2003) 25 influence of H2O on solidus temperature Pressure(Depth) no melt. geotherm solidus temperature (Our Dynamic Planet, Rogers, 2008) H2O reduces the solidus temperature over a range of pressures by as much as 800 C relative to the dry solidus 26
Can subducted oceanic crust melt? Pressure(Depth) no melt. geotherm temperature solidus (Our Dynamic Planet, Rogers, 2008) When young plate subducts slowly, the oceanic crust can melt. H2O saturated basalt 27 Thermal structure of slab and mantle wedge (Peacock, 2003) old and fast subduction young and slow subduction 28
Dehydration from altered oceanic crust H2O saturated basalt (Our Dynamic Planet, Rogers, 2008) The role of altered oceanic crust in most arcs is to give up H2O progressively as the slab dehydrate under the forearm and volcanic front. 29 amphibole hydrous mineral: including OH - Dehydration from sediment / lithospheric mantle red clay Subducted sediment continuously dehydrates as the slab descends. Sediment can melt under certain condition. H2O saturated peridotite (Our Dynamic Planet, Rogers, 2008) It is highly unlikely that hydrous mantle in slab ever melts. Mantle in slab potentially dehydrate and flush H2O. 30
Can mantle wedge receiving a large amount of H2O melt? (Our Dynamic Planet, Rogers, 2008) 2GPa, 1300-1400 C Island arc basalt are produced @ 2Gpa, 1300-1400 C 8%, 30% Amphibole breaks down causes 8% partial melting : arc lavas are generated by at least 30% partial melting? High temperature in mantle wedge? 31 thermal structure of mantle wedge isoviscous H2O-modified viscosity (van Keken et al., 2002) 32 (Zhao et al, 1994:1997, Parsons et al., 1998)
(Our Dynamic Planet, Rogers, 2008) 33 Where can we observe magmatic/volcanic activities along subduction zones? Characteristics of arc magmatism (vs. mid-ocean ridge/intraplate magmatism) Structure of arc crust and how to know the structure. What is the mechanism of arc magmatism? arc magmatism and continental formation 34
chemistry of continental / oceanic crust andesite upper crust lower crust whole crust oceanic SiO2 66.6 53.4 60.6 50.5 TiO2 0.64 0.82 0.72 1.6 Al2O3 15.4 16.9 15.9 15.3 FeO 5.04 8.57 6.7 10.4 MgO 2.48 7.24 4.7 7.6 CaO 3.59 9.59 6.4 11.3 Na2O 3.27 2.65 3.1 2.7 K2O 2.8 0.61 1.8 0.2 (Rudnick and Gao, 2005) (Condie, 1997)) andesitic crust: unique in the solar system mantle-derived basalt 35 Not constant growth rate through earth history continuous episodical? (Hawkesworth and Kemp, 2006) 36
Continent formation today Island arc and Andean margins (convergent boundary) are recognized as the current sites of continental crust formation andesitic rocks in upper and middle layer of arc crust Current estimate of crustal growth rate is 1.6km 3 /year (ranging from 1 to 4 km 3 / year) too small to have generated the total volume of the continent over the age of the Earth Mantle-derived magmas in the modern arc are mostly mafic or basaltic mass balances of current crustal compositions require some crustal material to have been returned to the mantle 37 IODP expeditions Izu-Bonin-Mariana arc crust Exp.350 IBM rear-arc Exp.351 IBM arc-origins geochemistry of the mantle prior to IBM arc inception ~the source of arc foundation history of across-arc variation in magma composition during arc evolution Exp.352 IBM forearc early processes in magmatic evolution associated with subduction initiation 38
Nishinoshima Geologica Sheet Map 1:500,000 Chichijima Islands (GSJ/AIST) 39 1973-74 Nishinoshima eruption 1973-1974 Basic Map of coastal waters 1:500,000 Nishinoshima (Japan Coast Guard) 40
2013.11 2017.7 2 10 41