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Convergent Margin Volcanism Three topics 1. MORs versus ARCs, a fruitful comparison 2. What is the global population of arcs like? a. I add a wrinkle I have been trying to become comfortable enough with to publish. Volcano spacing decreases as plate convergence rate increases. 3. Central America is interesting a. Vents b. Links c. Ba/La (Windows) d. Galapagos Read everything first (slides and notes) and then select specific slides (by number) to discuss 1 1. Why can’t the arcs* be more like the ridges? • Whenever I think of some possible new tectonicvolcanic/geochemical relationship for Central America, I check the RIDGE site and/or review the extensive literature on Mid-ocean ridges. The global set of convergent plate margins (CPMs) or arcs seems to be more complicated than the ridges, or do the arc groups just not talk to each other enough? • *arcs (sensu lato - because many convergent plate margins do not have an arc shape) 2 Spreading rates versus convergence rates: Narrower distribution for convergence rates MOR CPM Frequency 30 25 20 15 10 5 0 0 20 40 60 "Vc 80 100 120 140 160 (Km/Ma) 10 Km/Ma =10 mm/yr = 1 cm/yr 3 Structures depending on rates • The MOR morphology, structure and gravity field has an interesting dependence on spreading rate. Slow spreading (mid Atlantic) has rugged topography and an axial graben. Fast spreading (EPR) has smooth topography and an axial high or crest. • At ARCs there is nothing like the MOR systematics with rate. There is some dependence of volcano spacing and convergence (see below). Oblique subduction may eventually define some global patterns. 4 Magma chemistry and crustal thickness • MOR depths/crustal thickness reflect magma chemistry. The thicker the crust, the higher the degree of melting and the lower the Na2O content (Klein and Langmuir and a whole host of papers) • ARC crust may affect magma chemistry in a similar way but the community does not seem impressed (Plank and Langmuir proposed this using Central America as an example that works pretty well, but the community resisted this idea.) I think it is a reasonable idea 5 Age/history • MOR - what history? The axis is zero age. Plate geometry causes ridges to form and jump. Hotspots influence ridge locations and ridge geochemistry. • ARC - history is vital on both plates (e.g. Hotspot chains on subducting plate commonly indent CPMs and/or shut off volcanism for a period of time). 6 2. What is the global population of arcs like? • There are relatively few global compilations of arc properties. The recent G-Cubed paper by Syracuse and Abers is a good start. It refers to Jarrad (1986?) who made a global compilation of arc parameters. Another useful paper is d’Bremond d’Ars et al. 1995 in JGR. They looked globally at volcano spacing and found it random, not periodic. 7 Spacing of volcanic centers at arcs decreases as plate convergence rate increases Poisson Spacing (Km) 80 70 Uyeda and Kanamori (1979) classification 60 Island arc: no active back-arc spreading 50 Continental arc 40 Island arc: active back-arc spreading 30 20 10 0 50 100 150 Plate convergence rate normal to arc (mm/yr) 8 Michael J. Carr IGC G10.07 August 22, 2004 Why examine this question? Because volcano spacings () differ significantly Central America Northern Sumatra = 23 Km = 65 Km 500 Km 500 Km 9 Aleutian volcanoes have spacings intermediate between Central America and northern Sumatra Aleutians = 40 Km 500 Km 10 Defining volcano spacings • Use Central America as a guide • Ignore the back-arc • Focus on the volcanic front • Define Volcanic centers • Use Smithsonian’s GVP reference list 11 Ignore back-arc volcanoes and volcanoes like these cinder cones 12 Why ignore the little volcanoes? Flux derived melts at volcanic front Decompression melts in back-arc 13 A simple composite cone is a Center Agua volcano in Guatemala 14 A cross-arc alignment is a Center Atitlán-Toliman-Cerro de Oro in Guatemala 15 Make decisions defining discrete centers Volcanic center Central America Secondary cone in a center Holocene activity doubtful Back-arc cone 500 Km Data are from Smithsonian's Global Volcanism Program 16 Use Poisson distribution to estimate spacing • Calculate nearest neighbor spacing • Create histogram using 10 Km or 20 Km bins • Vary in Poisson equation to fit histogram Poisson is a discrete probability function f(x, ) = x - e x = 0,1,2,3,… x! 17 Volcano spacing in Central America = 23 Km 15 Poisson distribution n=36, bin=10 =2.3 or 23 Km Frequency 10 5 0 0 10 20 30 40 50 60 70 80 90 100 Km Volcano Spacings in 10 Km bins 18 Volcano spacing in Kuriles-Kamchatka = 17 Km 20 Poisson distribution Frequency 15 n=62 bin=10 =1.7 or 17 Km 10 5 Suggestion of a second mode at 75 Km. 0 0 20 40 60 80 100 Km Volcano Spacings in 10 Km bins 19 Volcano spacings determined here agree with those published by d’Bremond d’Ars et al.1995 d'Ars et al 1995 spacing (Km) 80 70 60 50 40 30 Cascades - an outlier because d’Ars used Guffanti and Weaver’s list not Smithsonian’s 20 10 0 0 10 20 30 40 50 60 70 Poisson Spacing (Km) 80 20 Negative correlation between plate convergence rate normal to arc and volcano spacing 80 Poisson Spacing (Km) 70 n = 15 60 r = -0.82 Marianas 50 Ryukyus 40 Tonga 30 ignored in regression 20 10 0 0 50 100 150 km Plate convergence rate normal to arc (mm/yr) 21 Why a negative correlation? 1. Raleigh-Taylor gravitational instability and diapirs μ2 μ1 If viscosity of lower layer, μ1 << μ2 then h wavelength, λ ~ h ( μ2/μ1) 1/3 - Whitehead and Luther (1975) Higher convergence rate could increase the thickness of the buoyant layer (h) or lowers its viscosity, μ1 Unlikely: a. effect of μ1 has to be > than effect of h b. distributions of spacings are random 2. Multiple generations of cavity plumes – d’Bremond d’Ars et al. (1995) Higher convergence rate increases the rate of cavity plume production, resulting in closer spacings 22 3. Central America is interesting. a. The volcano distribution • Stoiber and Carr 1973, after Sapper (1897) and Dollfus and Montserrat (1868), showed that the large volcanoes define several rightstepping lines or volcanic segments. • What if you look at all the volcanoes? That is, ignore size and just plot vent locations? 23 Volcanic segments based on “Centers” 24 Vents <600 ka in Central America 25 Vents younger than 600 ka with arcs 26 3b. To link Volcanology and geochemistry We study the entire volcanic chain. We often plot our volcanological and geochemical data against Distance 27 Regularities in the Distribution and 150 Geochemistry of Central American Volcanoes 100 Ba/La 50 0 = Zr/Nb 70 60 50 40 30 20 10 0 El Salvador Nicaragua Guatemala El Salvador Nicaragua Costa Rica Costa Rica Volcano volume Km3 400 300 200 100 0 0 1000 Km 28 Volcanic front consists of right stepping lines Stoiber and Carr (1973) suggested the subducting slab was segmented but the Zr/Nb result of Bolge (2006) requires a smooth slab (e.g. Syracuse and Abers, Protti, etc) thus volcanic segments are an upper plate phenomenon 29 Volume distribution along volcanic front Guatemala El Salvador Nicaragua Carr et al. (2007) modified from Stoiber and Carr (1973). 400 Atitlán Santa Ana Volcano volume Km3 This mostly ignored pattern can now be linked to the volcanic segmentation and aspects of the geochemistry. Costa Rica Irazú 300 Masaya Tecapa Barva 200 San Cristóbal Rincón 100 Mv Arenal 0 0 Volcanic segments 500 1000 Distance Km 30 El Salvador 60 Costa Rica 40 30 20 Zr/Nb decreases along each segment then steps up at the beginning of the next segment (except for Central Costa Rica, where there is no step in the volcanic line) Yojoa-back-arc, no slab signal 10 0 300 500 700 900 Distance along the arc (km) 1100 0 Depth to the slab (km) El Salvador Zr/Nb is similar to the sawtooth pattern of depths to slab beneath volcanoes (from Syracuse and Abers, 2006). Nicaragua 50 Zr/Nb Zr/Nb or Nb depletion correlates with volcanic segmentation (Bolge, 2005) 70 Nicaragua Costa Rica 50 100 150 200 300 500 700 900 Distance along the arc (km) 1100 31 Volcanic segments are oblique to gently curved axis that connects the large volcanoes Axis of volcanic productivity, similar to contours of seismic zone; 150 km in Nicaragua, 90 km contour in Costa Rica QSC 32 Within the same segment, magma paths vary, let Zr/Nb = slab signal NW SE Variable reactive path lengths Caribbean Plate Decompression melt Cocos Plate Zoned region of Upper plate stress field controls where the wedge is tapped Lower output with short path, higher slab signal Maximum output, taps everything flux melt Sed melt Water Lower output with long path, lower 33 slab signal A plausible model of Zr/Nb variation: basalt reacts with mantle during ascent 80 AFC model Part.Coefs. for cpx Cosigüina - short path 60 R=1 Massimilant/Mmagma=2 to DM Zr/Nb 40 to EM DM Mantle compositions 20 0 Momotombo-long path EM 0 50 100 Ba/La 150 200 34 New insights on volcanic segmentation • Zr/Nb saw-tooth requires the smooth slab imaged in modern seismicity studies • Volcanic segments are upper plate structures • A volcano’s size depends on its location relative to melt zone • Nb depletion is sensitive to depth to the slab • Need to know: What causes the segments? 35 3c. What causes the regional variation in Slab signal (Ba/La)? Guatemala | El Salvador | Nicaragua | Costa Rica 150 100 Ba/La 50 0 0 500 DSDP 495 Distance 1000 Km DSDP 1039 36 Incoming sedimentary sections are similar but substantial unmeasured variation may exist e 37 DSDP 495 sediment and MORB Low variance High variance maximum in carbonate maximum in hemipelagic Hemipelagic 0 Carbonate 200 300 400 Morb Depth in meters 100 500 .1 1 10 U/Th 10 100 Ba/La --------Regional-------- 100 1000 10000 Ba/Th .01 .1 U/La 1 ---------Local--------38 See regional variation if sediments are similar See local variation if sediments differ Regional Variation Local Variation Ba/Th Ba/La CS HS CS 10000 VF low-Ti 100 20% HS 1000 N.C.R. W. Nic. Yohoa 10 EM El Sal. Yohoa 100 EM DM DM 1 U/Th TWO DIFFERENT WINDOWS!! 10 .01 .1 U/La Note parallel arrays in local variation 39 La carries the regional signal, not Ba SiO2< 55wt. % 0.4 1/La Guatemala El Salvador Nicaragua Costa Rica 0.2 150 0.0 100 Ba/La 50 SiO2< 55wt. % 1000 Ba 0 500 Distance 1000 Km Black crosses are estimated mantle contributions 500 0 0 500 Distance 1000 Km 40 Eiler et al. 2005, strong evidence for a serpentine component in Nicaragua from 18O data carbonate sed serpentine 41 Irazú-Turrialba volcanic center Costa Rica Turrialba 569±6 ka Irazú 136±5 ka 594±16 ka 42 855±6 ka pre Irazú Interplay of geology and geochronology improved both age and volume estimates 43 Extrusive volcanic flux: all segments the same within error 44 Subducted contribution of flux is total flux minus mantle contribution 45 For subduction contribution Ba estimate is robust! La is not! Balava = 100 Bamantle = 4 Basubducted=96% Lalava = 14 Lamantle = 8 Lasubducted= 43% Masaya 100 10 7.5% melt of DM source 1 Cs Rb Ba Th U Nb Ta K La Ce Pb Pr Sr P Nd Zr Sm Eu Ti Dy Y Yb Lu Masaya volcano, Nicaragua mantle contribution: 7.5% melt of DM 46 Constant flux for highly enriched elements (Cs, Ba, K, Pb, Sr) Segment\Element NW Nicaragua SE Nicaragua Guanacaste Cordillera Central Cs Rb Ba Th Element flux in units of 10 0.84 20 899 0.90 1.04 25 1076 1.60 0.73 27 892 1.75 1.01 45 755 6.88 U K2O 4 La Pb Sr Kg/m/Ma 1.00 1.17 4.2 1.75 1.40 7.2 1.14 1.42 9.6 2.25 1.70 22.5 3.71 4.48 3.55 5.21 566 392 554 523 Very weak model of mantle contribution If a variable flux of subducted fluids occurs, then highly enriched elements, like Ba, should decrease from NW to SE. They do not. La increases from NW to SE but has high error. 47 The Galapagos is one of the sources 48 Himu High-μ 49 END 50