Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Island Arc Magmatism Wilson p. 153-225 • In this lecture: – Where & what are island arcs? • Why so important? – Diverse magma sources – Partial melting – Magma segregation, ascent, storage – Magma series – Trace elements in arc magmas – Isotopic compositions • Subducted components • Differentiation mechanisms – Petrologic model – Implications for mantle evolution Island arcs Eleven major active island arcs – Destructive margins – Subduction of oceanic plate beneath adjacent ocean plate Importance – Hazards Explosive volcanism; Earthquakes Millions inhabit arcs (Indonesia, Japan, Phillipines, etc.) – Chemical exchange & recycling Crust ÍÎ Mantle Genesis & evolution of arc magmas – Critical to understand origin of hazards & element recycling 1 Sources of island arc magma JOIDES Resolution Mantle wedge above subducted slab 1. 40-70 km thick oceanic lithosphere depleted mantle = refractory lherzolite+harzburgite 2. Asthenospheric mantle fertile lherzolite; thickness a function of slab dip Ocean crust 1. Metamorphosed basalt, gabbro note: facies vary w/increasing P-T of slab 2. Ocean sediment (clay, CaCO3, clastics) May become involved: Deep: upper part of subducted slab Shallow: base of island arc volcanic sequence Sea water 1. H2O component fundamental to arc magmatism 2. Incorporated during hydrothermal alteration and ocean-floor (low P) metamorphism of ocean crust Thermal structure & partial melting JOIDES Resolution Thermal structure critical magma generation seismicity Numerical Models convection in asthensophere dehydration of subducting crust frictional heating upper slab surface 2 Thermal structure & partial melting JOIDES Resolution Temperature distribution a major control on location of partial melting within the mantle wedge or the subducted slab Enigma cold slab “refrigerates” the mantle the slab is too cold to melt under most circumstances yet, melting and volcanism occur WPS: wet peridotite solidus Thermal structure & partial melting JOIDES Resolution Partial Melting of potential sources Subducted ocean crust 1. Meta-basalt amphibolite or eclogite 2. Meta-sediments 2. Fluid Prograde metamorphism Progressive dehydration at higher P-T H2O-saturated vs. anhydrous melting of basalt Arc Basalt Liquidus Temperature of wet melting does not match arc basalt liquidus range Wet eclogite melting >150 km depth? does not explain arc position (see fig. 6.8) Subducted sediment melting? the jury is……..out. 3 Thermal structure & partial melting JOIDES Resolution Partial Melting of potential sources The mantle wedge Lherzolite fluxed with: 1. H2O-rich fluid 2. H2O-rich partial melt from slab Lowers dry solidus below mantle wedge geotherm Partial melting experiments indicate that basalt can be generated from lherzolite in presence of small % of H2O Basalt is parent magma to spectrum of andesite-rhyolite in island arcs Segregation, ascent, storage of magma Partial melt segregates from asthenosphere polybaric melting & segregation percolative flow vs. fracture transport? Ponding of magma in high-level reservoirs fractional crystallization ground-surface deformation caldera formation S-wave attenuation Low-P fractional crystallization Harker diagrams cumulate-textured plutonic xenoliths: oliv + cpx + opx + plag + amph + mag plagioclase suggests P<10 kbar (<30 km) amphibole implies hydrous melts at depth lack of amphibole in erupted lavas due to resorption 4 Magma series and differentiation JOIDES Resolution K2O vs. SiO2 low-K series (island arc tholeiite series) calc-alkaline series high-K series shoshonitic series (alkaline series) FeO*/MgO vs. SiO2 or AFM tholeiitic series calc-alkaline series Island arcs may contain both CA and TH volcanoes! (see Aleutian examples) Aleutian Island arc Seguam Island: Tholeiitic shield volcano Kanaga Island: calc-alkaline stratovolcano 5 Pyroclastic flows Geologic features at Seguam Older, Pleistocene lavas 5 km 1993 Basalt 1977 Basalt Eastern collapse calderas 93.1 + 9.5 ka Seguam Island, central Aleutians Pleistocene lavas and tephras Holocene basalt 6 Kanaga Volcano Kanaga Island Kanaton Ridge 199.1 + 2.5 ka 198.1 + 2.1 ka Tkb QTb Kana to n R i dg e Photos courtesy of Dörte Mann 7 JOIDES Resolution Major and trace element composition of magmas Major elements Island arc basalts similar to other oceanic basalts except lower in Ti Fractional crystallization produces more SiO2 rich magmas Trace elements (relative to N-MORB) Enriched in low ionic potential elements; LILE + Th fluid mobilized elements added to mantle wedge? Low in high ionic potential elements; Ta, Nb, Zr, Hf, REE larger degree of melting ? residual phase(s) [rutile, zircon, sphene] during melting ? JOIDES Resolution Trace element composition of magmas Island arc basalts distinguished from MORB LILE and REE variations, e.g., Ba/La vs. La/Sm MORB-normalized differences Negative Nb anomaly Nb Slab component 8 JOIDES Resolution Radiogenic isotopes Sr and Nd isotopes mixing between mantle & crustal components (compare to MORB and OIB) mass balance of Sr and Nd: source contamination vs. crustal assimilation terriginous sediment in source of Lesser Antilles & Sunda arcs Ocean basalt array lower older crust upper younger crust JOIDES Resolution Radiogenic isotopes Pb isotopes Pb contents (>20 ppm) and isotopic ratios of sediments very high Pb content (< 1 ppm) and isotopic ratios of mantle are low Thus Pb is a sensitive tracer of sediment involvement in magma source Lesser Antilles arc lavas Pb ratios both higher and lower than Atlantic sediments source contamination and crustal assimilation? 9 JOIDES Resolution Beryllium isotope data The isotope 10Be produced by cosmic ray induced rxns in atmosphere transported to surface pelagic sediments via rain & snow half-life is 1.5 x 106 yr tracer for young marine sediment in arc magma source 10Be contents of basalts below detection in MORB, OIB high in basalt from some arcs thus in some arcs: uppermost, young sediments are not accreted, they subduct and either melt or release 10Be rich fluid to the mantle in < 10 myr General model of island arc magmatism JOIDES Resolution Major element, trace element and isotopic ratios indicate that at most a very small weight %of arc magma comprises subducted crustal elements. Thus most of the subducted crust bypasses the arc and descends into the deeper mantle. Perhaps in cases like the Farallon plate crust travels all the way to the core-mantle boundary 10 Modes of mantle convection The “hybrid” model One layer or two? 1. Neither the 410 or 660 discontinuity seem to act as a barrier to flow One layer: the transition zone phase transitions do not prevent mass flux across the 410/660 discontinuity 2. Still need a chemically distinct source Two layers: There still needs to be chemically distinct regions 3. New boundary: around 1600 km depth with small density contrast Kellogg 1999 Crust generated at ridges is stirred into the deep mantle below island arcs 11