Volcanism, Geochemistry and Tectonics at Cocos-Caribbean ...


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  • There is a political aspect to this question. Although arc researchers have SOTA (State of the Arc), the RIDGE group has done a better job at synthesis. The logistics and funding patterns of marine geology versus field geology may be the largest part of this problem. Being on foot or in a jeep exposes people like me to too much detail. The marine group is always about 2.2 km above their unseen target (I think the average ridge depth is about 2.2 km) and must look at a wide perspective. In US slang The Ridge workers see the forest rather than the trees and the arc workers see the trees and not the forest. To use another US slang term, I think a lot of low hanging fruit remains in the arc orchard for industrious and bold students.
  • Baker, E.T. and German, C.R. (2004) On the Global Distribution of Hydrothermal Vent Fields. In Mid-Ocean Ridges: Hydrothermal Interactions Between the Lithosphere and Oceans, Geophysical Monograph Series 148, C.R. German, J. Lin, and L.M. Parson (eds.), 245–266
    Vc, convergence normal to the volcanic line, from Syracuse and Abers G-cubed 2006. Global compilation of variations in slab depth beneath arc volcanoes and implications
    The perpendicular component of plate convergence (Vc) has a narrow frequency distribution with 28 out of 50 measured rates between 60 and 80 mm/yr. In contrast, the full spreading rates at ridges have a nearly even distribution from 0 to 80 mm/yr. If the MOR system if 60,000 km, then 80% of its length is in the range of 0 to 80 mm/yr with about 20±7% in each bin. The Vc values are not as complete as the spreading rate values, in part because the Syracuse and Abers paper was not addressing the frequency of convergence rates. Furthermore, the oddly high convergence rates for margins with back-arc spreading are included but only after ignoring the sometimes large nack-arc spreading rate (Tonga especially). This affects Tonga, the Marianas and the Ryukyus.
    The narrower distribution of convergence rates is qualitatively in agreement with studies of plate dynamics that conclude that slab sinking is the most important factor driving plate tectonics. There seems to be a speed limit on slab sinking corresponding to a normal convergence rate of 60 to 80 mm/yr or km/Ma. Areas with rather slow convergence rates bound small plates such as the Caribbean (Lesser Antilles arc) and the Juan de Fuca Plate (Cascades arc). Mexico also has a rather slow rate.
  • The MOR spreading velocity field has a new entrant, ultra slow spreading. Such very slow ridges can have mantle exposed on the surface because the magma production rate is too slow to keep up with even such slow spreading. For most ridges (outside the influence of hot spots) there is a remarkable uniformity of crustal thickness and depth to the ridge axis, all seemingly independent of spreading rate. Is there a rate dependent magma production rate at arcs? No one knows because there are only poor estimates of magma production.
    The closes to the Fast-Slow-Ultraslow paridigm is perhaps the Uyeda and Kanamori (1979) classification of arc into three types, Back-arc spreading, oceanic and continental.
  • Central America and Mexico both have old crust (oldest continental crust is paleozoic in Guatemala, precambrian in Mexico.) Crustal assimilation seems to be a near universal process even if only in small amounts. Nevertheless, old rocks will radically alter the radiogenic isotopes, making opaque the most powerful window into the mantle, the radiogenic isotopes. Island arcs and young arc segments like Nicaragua are less affected by crustal assimilation because whatever crust gets eaten by the magma is young-has not had time to develop enriched radiogenic isotopes.
    In the global array of arcs Central America and Mexico have relatively young ocean floor subducting beneath them. Northern Japan has some of the oldest subducting oceanic crust.
  • Bremond d'Ars, J. de, Jaupart, C., and Sparks, R. S. J. (1995) J. Geophys. Res. 100, 20421-20432.
    Distribution of Quaternary volcanoes in the Northeast Japan arc: geologic and geophysical evidence of hot fingers in the mantle wedge By Yoshihiko TAMURA,*)' t) Yoshiyuki TATSUMI,*) Dapeng ZHAO,**) Yukari KIDO,*) and Hiroshi SHUKUNO Proc. Japan Acad., 77, Ser. B (2001) 135
  • If you do not add in the back-arc spreading rates to the three arcs with blue boxes, they all move left and end up in the array defined by the other arcs. Tonga, the high rate at the lower right, goes all the way back to about 75 mm/yr. The others move less. The lesser Antilles has an unusually low convergent rate, about 23 mm/yr. It is sort of an outlier and the plate motion for the Caribbean is notoriously poorly determined.
  • These two are at the same scale, the next one, slide, the Aleutians, is slightly more compressed. The projections are lambert conical conformal, so there is minimal area distortion.
    Note the nice right stepping volcanic lines in Central America. This is a feature of Central America, that is somewhat present in other arcs but obviously not general.
  • This arc has some bends and wiggles. Some parts of it seem to define short arc segments.
  • Whatever I try to do, at some point I want to cheat. Therefore, it is really important to develop a clear statement of what one is doing or trying to do. I did this work from a Central America perspective. That’s what I have walked on. If other places are really unlike Central America I am in trouble.
  • These are not “Volcanic Centers”
  • One can make the case that back-arc volcanoes are from decompression melting, a rather different type of magmatism that the volcanic front. Thus it is OK to make this division.
    ASIDE: For good of ill, I am a divider. I like to make as many subdivisions as I can to see what organization there is. I classify, categorize, group, sort, and, if needed, pigeonhole. The software I wrote, Igpet, has several tools allowing one to slice and dice geochemical data into different groupings. This can easily be overdone, especially if you get to the point of having a separate explanation for each point!!. Sometimes gems are found this way but sometimes it just adds to the confusion.
  • N=36 mean=29.8 std dev=12.9
  • N=62 mean=35.0 std dev=24.2
  • Always a plus if you do some odd thing and get the same answer as someone else.
  • Slope= -7.962E-01 Int= 8.995E01 n=15 t=-5.14 F=26.5 r=-.82 r'=-.8
  • I have been calling volcanoes in Central America “centers” because there were distinct gaps between the volcanoes in Guatemala. Roads go through these gaps, so I started saying the centers were separate if a decent jeep road separated them. This is not a scientific way to go about the center definition, so I argued vaguely that a center was a focus of a large mass of eruptives, e.g. Atitlan caldera, Agua volcano, the Fuego-Acatenango line of vents, and the Pacaya complex (a complex because it is made of composite cones, cinder cones, calderas, domes, collapse features).
    In an attempt to make a statistical separation of the vents into centers Daniele Dondero and I compiled lists of volcano locations and added new vents in El Salvador and Guatemala. We are in the process of cleaning up the data using goggle earth as the field check. (We found a lot of mistakes).
    The compilation has about 800 vents. By contrast Dick Stoiber and I defined 34 centers in 1973. I have messes with this and have got it as high as 42 and am currently at 39. The slipperiness of this center list was a major motivation to make the vent list. The next two slides should show that
    1. Yes, the large volcanoes define right stepping offset lineaments, making “volcanic segments”
    2. The vent distribution does not generally allow clear definition of a “center.” It works in some places, like Guatemala but it does not work at all in Nicaragua.
    3. The vent distribution is an entirely new way of seeing the volcanoes, with much more structural information that I had imagined.
    4. The centers tells us about the distribution of the great mass of eruptives
    5. The vents reveal structural controls, some recognized before, some not.
  • Note that the segments are less clear here than they were in slides 9 and 16.
  • You will have to zoom in on this to see well. The nice simple lines from the previous slide are gone. The large number of cinder cones, shields and rhyolite flows in southeast Guatemala and northwestern El Salvador are distinctive and make some well formed alignments. Along the volcanic front, there are N-S alignments from Guatemala to Costa Rica. There are a few NE alignments. Do you see anything odd about the vent alignments in Nicaragua, compared to the others countries?
  • The two red arcs highlight a frontward vent limit in Guatemala and a backward vent limit in Nicaragua. The “limits” roughly follow the shape of the coastlines. Frontward limits exist in El Salvador and Costa Rica as well. What are these “structures”?
  • There are other ways to explain the Zr/Nb variation, including decreasing amounts of Nb-holding phase with increasing depth.
  • An earlier version of this slide got Central America picked as a Subduction factory focus area. Practically the entire global range in Ba/La occurs just between Nicaragua and central Costa Rica. Ba/La is a signal or tracer of subduction. As a dimensionless ratio is it worthless for actual measurements or more real things like element fluxes. Most of the other “slab Signals” are also ratios and difficult to make quantitative models from. 10Be is an isotope with a 1.5 Ma half-life, made primarily in the atmosphere by cosmic rays. It concentrates in mud on the sea floor where it gets subducted. A tiny fraction returns to the surface in lavas. Masaya volcano in Nicaragua has the global maximun in 10Be/9Be. The Be isotope in the denominator is the stable Be isotope. Although there are many fewer 10Be measurements, the correlation of the Be isotope ratio and Ba/La is high r2- 0.82. This provides solid confidence that Ba/La in lavas is tracking the subduction of recent (last few million years) sediments. Because of radioactive decay, nearly all the 10Be is in the top 100 m of sediment, so this isotope traces the top of the sediments. Central America is neat because nearly all the sediment is subducted, hardly any accretion (many papers on this by the GEOMAR group- von Huene and others). In fact the wedge of upper plate seems to be eroding, at least in Costa Rica (Papers by Vannouchi and others on subduction erosion).
    For geochemical reasons, especially Be isotopes, Ba/La is an excellent tracer for subducted sediment. But what does it mean quantitatively?
  • The two DSDP holes, offshore Central America on the subducting Cocos Plate, have very similar stratigraphy. Therefore to first order, the variation in Ba/La is probably not due to regional variation in the subducted sediments.
  • There are 3 parts to the subducting section (4 parts actually if we can confirm that water is released from serpentine in the subducting mantle.)
    The two most robust trackers of the regional variation in subduction signal from the volcanoes are Ba/La and U/Th. These 4 elements can be combined in very different ways. On the left, U/Th and Ba/La are roughly constant through the sedimentary section. On the right U/La and Ba/Th are hugely different between the carbonates (lower) and the hemipelagics (upper).
    Element ratios provide very different windows into the melting process. We will later see that U/Th and Ba/La form a simple binary mixing distribution. In that perspective the large variation in U/La and Ba/Th is perpendicular to the view and not seen. A plot of U/La versus Ba/Th opens up a whole new window on variation, sowing “local” mixes between a carbonate component (Ba/Th) and hemipelagic component (U/La)
  • This is the plot I was referring to above. Note the very different story you get from two different combinations of the same four elements. This is an aside to show the importance of understanding what different elements do (their behavior in hypercritical fluids and in melts) and where they are concentrated (their sources)
  • This is from Carr et al. 2007. 1/La – top right- has same shape as Ba/La, so the key variation is in La. La variation is primarily caused by differences in degree of melting. So what is going on, why isn’t Ba following the regional pattern? In a section below I argue that the subducted Ba flux (the amount from the slab) is constant along the four segments in Nicaragua and Costa Rica.
    If the flux is constant, the Ba/La variation is caused by differences in degree of melting. In Nicaragua the flux may be more focused in its delivery (Carr et al, 1990). Thus the same amount of fluid with the same Ba content is delivered everywhere but in Nicaragua, the volume affected is small so the degree of melt is high. Ba is lowered by the diluting effect of higher extent of melting, but raised by the concentration oif flux. These two effects offset and ba id “normal” or just the same as in Guatemala and El Salvador.
    Nicaragua may also have more water or a more dilute Ba concentrati0n in the fluid. The higher water content causes more melting. How this would work quantitatively is not clear, but I expect that the dispersion in Ba and La data is large enough to allow extra water as the main couse for the regional pattern in Ba/La..
  • Eiler measured O isotopes in olivine phenocrysts in the most mafic rocks present. The normal area is marked by NMORB. The higher values are fairly easy to get, add a little marine sediment, especially some carbonate. Getting below NMORB is difficult, only serpentine creates large negative fractionations. This is strong evidence that water from serpentine is involved in Nicaragua. The water may be from the subducted slab, from the mantle section that can be altered either just after formation or at subduction as the plate bends, reactivating old cracks and allowing sea water to penetrate deep into the slab 9Geomar group- Ranero et al. Separately, Abers et al. proposed that the Nicaraguan slab is anomalously “wet” from seismic properties. The “wet Nicaraguan slab” paper in GRL.
    The O-isotope data are an important new WINDOW in to the mantle processes.
  • The oldest lavas of the current pulse are exposed in canyons at the base of the volcanoes (594) or in highly eroded terrain (569). Pink areas are cultivated because they are young and smooth.
    If you want to measure fluxes, you have to measure masses (volume * density) and determine a reasonable time interval. 600 ka is a reasonable time interval for Costa Rica.
  • Ar/Ar geochemistry is giving us a vlear picture of recent volcanic stratigraphy in Costa Rica. A lot left to be done however.
  • How does this volcanic flux match up with Magma fluxes calculated at island arcs? It is really small about 8X lower than the rates for magma flux. 1 unit above = 3.6 km3/km/Ma. So Costa Rica is about 9 km3/km/Ma a factor of nine less that the magmatic flux estimated for the Aleutians ( ). Why so small?, conservative approach, no count of sediments or intrusives.
  • Once you have a mass flux (previous slide) you need a concentration to multiply by to get an element flux.
    Assume the mantle contribution to the lave goes through the heavy REEs and Nb and Ta.
  • The sensitivity of this mantle subtraction procedure varies substantially. Ba, highly enriched, is insensitive to small changes in mantle concentration. La is highly sensitive to changes in the mantle contribution. The log scale obscures how highly enriched some of these elements are.
  • We multiply the mass flux by the subduction derived concentration to get element fluxes. Within error, Cs, Ba, K, Pb and Sr are the same all along Nicaragua and Costa Rica. The large regional variation in Ba/La ratio is not caused by a large variation in Ba flux. There may be a SE-ward increase in La, but this not reliable because of large error in La.
  • There is a clear Galapagos contribution to the arc geochemistry in central Costa Rica. For most elements, isotopes and ratios, the extent of that influence is confined to central Costa Rica and perhaps Ometepe Island in Nicargaua (volcanoes Concepcion and Maderas). However in Pb isotopes Hoernle et al. think they can see Pb from subducted Galapagos material reaching all the way to central Nicaragua or even to the Nicaragua-El Salvador border.
    Is this possible? Can Pb trace a process not seen in other elements. Perhaps, but only time will tell. It is possible because Pb in quite high in Central American lavas and the Pb in the subducted sediments is too low to cause this, so Pb likely comes from subducted oceanic crust. The Galapagos Pb isotopes are distinctive enough and the seafloor with Galapagos crust on it has high Pb contents, thus the Pb signal may stretch all the way to Nicaragua even if other elements won’t show this. Pb isotopes may be a particularly sensitive “window.”
  • From Feigenson et al, showing Pb isotope variation in Central America. Note the Galapagos like Pb ratios behind the arc.
  • Volcanism, Geochemistry and Tectonics at Cocos-Caribbean ...

    1. 1. 1 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
    2. 2. 2 1. Why can’t the arcs* be more like the ridges? • Whenever I think of some possible new tectonic- volcanic/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)
    3. 3. 3 Spreading rates versus convergence rates: Narrower distribution for convergence rates 0 20 40 60 80 100 120 140 160 0 5 10 15 20 25 30 Frequency "Vc (Km/Ma) 10 Km/Ma =10 mm/yr = 1 cm/yr MOR CPM
    4. 4. 4 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.
    5. 5. 5 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
    6. 6. 6 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).
    7. 7. 7 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.
    8. 8. 8 Spacing of volcanic centers at arcs decreases as plate convergence rate increases Michael J. Carr IGC G10.07 August 22, 2004 80 Continental arc Island arc: no active back-arc spreading Island arc: active back-arc spreading 0 50 100 150 10 20 30 40 50 60 70 PoissonSpacing(Km) Plate convergence rate normal to arc (mm/yr) Uyeda and Kanamori (1979) classification
    9. 9. 9 Why examine this question? Because volcano spacings (λ) differ significantly Northern SumatraCentral America N 500 Km 500 Km N λ = 23 Km λ = 65 Km
    10. 10. 10 Aleutian volcanoes have spacings intermediate between Central America and northern Sumatra Aleutians λ = 40 Km N 500 Km
    11. 11. 11 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
    12. 12. 12 Ignore back-arc volcanoes and volcanoes like these cinder cones
    13. 13. 13 Why ignore the little volcanoes? Flux derived melts at volcanic front Decompression melts in back-arc
    14. 14. 14 A simple composite cone is a Center Agua volcano in Guatemala
    15. 15. 15 A cross-arc alignment is a Center Atitlán-Toliman-Cerro de Oro in Guatemala
    16. 16. 16 Make decisions defining discrete centers Central America N 500 Km Volcanic center Back-arc cone Holocene activity doubtful Secondary cone in a center Data are from Smithsonian's Global Volcanism Program
    17. 17. 17 Use Poisson distribution to estimate spacing • Calculate nearest neighbor spacing • Create histogram using 10 Km or 20 Km bins • Vary λ in Poisson equation to fit histogram Poisson is a discrete probability function f(x, λ) = λ x e - λ x! x = 0,1,2,3,…
    18. 18. 18 Volcano spacing in Central America λ = 23 Km 0 10 20 30 40 50 60 70 80 90 100 Km 0 5 10 15 Frequency Volcano Spacings in 10 Km bins Poisson distribution n=36, bin=10 λ =2.3 or 23 Km
    19. 19. 19 Volcano spacing in Kuriles-Kamchatka λ = 17 Km Volcano Spacings in 10 Km bins 0 20 40 60 80 100 Km 0 5 10 15 20 Frequency Poisson distribution n=62 bin=10 λ =1.7 or 17 Km Suggestion of a second mode at 75 Km.
    20. 20. 20 Volcano spacings determined here agree with those published by d’Bremond d’Ars et al.1995 0 10 20 30 40 50 60 70 80 0 10 20 30 40 50 60 70 80 d'Arsetal1995spacing(Km) Poisson Spacing (Km) 45º Cascades - an outlier because d’Ars used Guffanti and Weaver’s list not Smithsonian’s
    21. 21. 21 Negative correlation between plate convergence rate normal to arc and volcano spacing n = 15 r = -0.82 Marianas Ryukyus Tonga ignored in regression 0 50 100 150 km 0 10 20 30 40 50 60 70 80PoissonSpacing(Km) Plate convergence rate normal to arc (mm/yr)
    22. 22. 22 Why a negative correlation? μ1 μ2 If viscosity of lower layer, μ1 << μ2 then wavelength, λ ~ h ( μ2/μ1) 1/3 - Whitehead and Luther (1975)h 1. Raleigh-Taylor gravitational instability and diapirs 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
    23. 23. 23 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 right- stepping lines or volcanic segments. • What if you look at all the volcanoes? That is, ignore size and just plot vent locations?
    24. 24. 24 Volcanic segments based on “Centers”
    25. 25. 25 Vents <600 ka in Central America
    26. 26. 26 Vents younger than 600 ka with arcs
    27. 27. 27 We study the entire volcanic chain. We often plot our volcanological and geochemical data against Distance Distance 3b. To link Volcanology and geochemistry
    28. 28. 28 Regularities in the Distribution and Geochemistry of Central American Volcanoes 0 50 100 150 Ba/La VolcanovolumeKm3 0 1000 Km 0 100 200 300 400 Guatemala El Salvador Nicaragua Costa Rica Zr/Nb 0 10 20 30 40 50 60 70 El Salvador Nicaragua Costa Rica =
    29. 29. 29 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
    30. 30. 30 Volume distribution along volcanic front 0 500 1000 0 100 200 300 400 Guatemala El Salvador Nicaragua Costa Rica Atitlán Santa Ana Tecapa San Cristóbal Masaya Irazú Rincón Barva Arenal Mv 0 500 1000 0 100 200 300 400 Guatemala El Salvador Nicaragua Costa Rica Atitlán Santa Ana Tecapa San Cristóbal Masaya Irazú Rincón VolcanovolumeKm3 Distance Km Carr et al. (2007) modified from Stoiber and Carr (1973). This mostly ignored pattern can now be linked to the volcanic segmentation and aspects of the geochemistry. Volcanic segments
    31. 31. 31 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) Zr/Nb is similar to the saw- tooth pattern of depths to slab beneath volcanoes (from Syracuse and Abers, 2006). Zr/Nb or Nb depletion correlates with volcanic segmentation (Bolge, 2005) Distance along the arc (km) 300 500 700 900 1100 0 10 20 30 40 50 60 70 Zr/Nb El Salvador Nicaragua Costa Rica 300 500 700 900 1100 0 50 100 150 200 Depthtotheslab(km) Distance along the arc (km) El Salvador Nicaragua Costa Rica Yojoa-back-arc, no slab signal
    32. 32. 32 Volcanic segments are oblique to gently curved axis that connects the large volcanoes QSC Axis of volcanic productivity, similar to contours of seismic zone; 150 km in Nicaragua, 90 km contour in Costa Rica
    33. 33. 33 Decompression melt Zoned region of flux melt Within the same segment, magma paths vary, let Zr/Nb = slab signal Water Sed melt Cocos Plate Upper plate stress field controls where the wedge is tapped Lower output with short path, higher slab signal Maximum output, taps everything Lower output with long path, lower slab signal NW SE Variable reactive path lengths Caribbean Plate
    34. 34. 34 A plausible model of Zr/Nb variation: basalt reacts with mantle during ascent 0 50 100 150 200 0 20 40 60 Zr/Nb Ba/La Momotombo-long path Cosigüina - short path DM EM 80 to DM to EM AFC model Part.Coefs. for cpx R=1 Massimilant/Mmagma=2 Mantle compositions
    35. 35. 35 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?
    36. 36. 36 3c. What causes the regional variation in Slab signal (Ba/La)? 0 500 1000 Km 0 50 100 150 Ba/La Distance Guatemala | El Salvador | Nicaragua | Costa Rica DSDP 495 DSDP 1039
    37. 37. 37 Incoming sedimentary sections are similar but substantial unmeasured variation may exist e
    38. 38. 38 DSDP 495 sediment and MORB Low variance maximum in carbonate maximum in hemipelagic --------Regional-------- ---------Local--------- 1 0 1 0 0 B a /L a 1 0 0 1 0 0 0 1 0 0 0 0 B a / T h . 0 1 .1 1 U / L a Depthinmeters .1 1 1 0 0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 U / T h MorbCarbonateHemipelagic High variance
    39. 39. 39 See regional variation if sediments are similar See local variation if sediments differ Note parallel arrays in local variation 1 1 0 1 0 0 B a / L a U / T h E M D M C S H S Y o h o a V F lo w - T i .0 1 .1 1 0 1 0 0 0 0 0 1 0 0 0 0 1 B a / T h U / L a C S H S E M D M Y o h o a 2 0 % W . N ic . E l S a l. N . C . R . R e g io n a l V a r ia tio n L o c a l V a r ia t io n TWO DIFFERENT WINDOWS!!
    40. 40. 40 La carries the regional signal, not Ba 0.0 0.2 0.4 1/La SiO2<55wt. % 500 1000 Km 0 50 100 150 Ba/La Distance Guatemala El Salvador Nicaragua Costa Rica 0 500 1000 Km 0 500 1000 Ba Distance SiO2< 55wt. % Black crosses are estimated mantle contributions
    41. 41. 41 Eiler et al. 2005, strong evidence for a serpentine component in Nicaragua from 18 O data serpentine carbonate sed
    42. 42. 42 Irazú-Turrialba volcanic center Costa Rica 594±16 ka 569±6 ka Irazú Turrialba 136±5 ka 855±6 ka pre Irazú
    43. 43. 43 Interplay of geology and geochronology improved both age and volume estimates
    44. 44. 44 Extrusive volcanic flux: all segments the same within error
    45. 45. 45 Subducted contribution of flux is total flux minus mantle contribution
    46. 46. 46 Masaya volcano, Nicaragua mantle contribution: 7.5% melt of DM 1 10 100 Cs Rb Ba Th U Nb Ta K La Ce Pb Pr Sr P Nd Zr SmEu Ti Dy Y Yb Lu 7.5% melt of DM source Masaya Balava = 100 Bamantle = 4 Basubducted=96% Lalava = 14 Lamantle = 8 Lasubducted= 43% For subduction contribution Ba estimate is robust! La is not!
    47. 47. 47 Constant flux for highly enriched elements (Cs, Ba, K, Pb, Sr) SegmentElement Cs Rb Ba Th U K2O La Pb Sr NW Nicaragua 0.84 20 899 0.90 1.00 1.17 4.2 3.71 566 SE Nicaragua 1.04 25 1076 1.60 1.75 1.40 7.2 4.48 392 Guanacaste 0.73 27 892 1.75 1.14 1.42 9.6 3.55 554 Cordillera Central 1.01 45 755 6.88 2.25 1.70 22.5 5.21 523 Element flux in units of 104 Kg/m/Ma 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. Very weak model of mantle contribution
    48. 48. 48 The Galapagos is one of the sources
    49. 49. 49 Himu High-μ
    50. 50. 50 END