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1
Convergent Margin Volcanism
Three topics
1. MORs versus ARCs, a fruitful comparison
2. What is the global population of ...
2
1. Why can’t the arcs* be more like the ridges?
• Whenever I think of some possible new tectonic-
volcanic/geochemical r...
3
Spreading rates versus convergence rates:
Narrower distribution for convergence rates
0 20 40 60 80 100 120 140 160
0
5
...
4
Structures depending on rates
• The MOR morphology, structure and gravity field has
an interesting dependence on spreadi...
5
Magma chemistry and crustal thickness
• MOR depths/crustal thickness reflect magma
chemistry. The thicker the crust, the...
6
Age/history
• MOR - what history? The axis is zero age.
Plate geometry causes ridges to form and
jump. Hotspots influenc...
7
2. What is the global population of arcs like?
• There are relatively few global compilations of
arc properties. The rec...
8
Spacing of volcanic centers at arcs decreases
as plate convergence rate increases
Michael J. Carr IGC G10.07 August 22, ...
9
Why examine this question?
Because volcano spacings (λ) differ significantly
Northern SumatraCentral America
N
500 Km
50...
10
Aleutian volcanoes have spacings intermediate between Central
America and northern Sumatra
Aleutians
λ = 40 Km N
500 Km
11
Defining volcano spacings
• Use Central America
as a guide
• Ignore the back-arc
• Focus on the volcanic
front
• Define...
12
Ignore back-arc volcanoes and volcanoes like these cinder cones
13
Why ignore the little volcanoes?
Flux derived melts at volcanic front
Decompression melts in back-arc
14
A simple composite cone is a Center
Agua volcano in
Guatemala
15
A cross-arc alignment is a Center
Atitlán-Toliman-Cerro de Oro in Guatemala
16
Make decisions defining discrete centers
Central America
N
500 Km
Volcanic center
Back-arc cone
Holocene activity doubt...
17
Use Poisson distribution to estimate spacing
• Calculate nearest neighbor spacing
• Create histogram using 10 Km or 20 ...
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 i...
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
Frequen...
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
1...
21
Negative correlation between plate convergence rate normal to
arc and volcano spacing
n = 15
r = -0.82
Marianas
Ryukyus...
22
Why a negative correlation?
μ1
μ2
If viscosity of lower layer, μ1 << μ2 then
wavelength, λ ~ h ( μ2/μ1)
1/3
- Whitehead...
23
3. Central America is interesting.
a. The volcano distribution
• Stoiber and Carr 1973, after Sapper (1897) and
Dollfus...
24
Volcanic segments based on “Centers”
25
Vents <600 ka in Central America
26
Vents younger than 600 ka with arcs
27
We study the entire
volcanic chain. We
often plot our
volcanological and
geochemical data
against Distance Distance
3b....
28
Regularities in the Distribution and
Geochemistry of Central American Volcanoes
0
50
100
150
Ba/La
VolcanovolumeKm3
0 1...
29
Volcanic front consists of right stepping lines
Stoiber and Carr
(1973) suggested the
subducting slab was
segmented but...
30
Volume distribution along volcanic front
0 500 1000
0
100
200
300
400
Guatemala El Salvador Nicaragua Costa Rica
Atitlá...
31
Zr/Nb decreases along each
segment then steps up at the
beginning of the next
segment (except for Central
Costa Rica, w...
32
Volcanic segments are oblique to gently curved
axis that connects the large volcanoes
QSC
Axis of volcanic
productivity...
33
Decompression
melt
Zoned
region of
flux melt
Within the same segment, magma paths vary, let Zr/Nb = slab signal
Water
S...
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
M...
35
New insights on volcanic segmentation
• Zr/Nb saw-tooth requires the smooth slab
imaged in modern seismicity studies
• ...
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...
37
Incoming sedimentary sections are similar
but substantial unmeasured variation may exist
e
38
DSDP 495 sediment and MORB
Low
variance
maximum in
carbonate
maximum in
hemipelagic
--------Regional-------- ---------L...
39
See regional variation if sediments are similar
See local variation if sediments differ
Note parallel arrays in local v...
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 ...
41
Eiler et al. 2005, strong evidence for a serpentine
component in Nicaragua from 18
O data
serpentine
carbonate sed
42
Irazú-Turrialba volcanic center Costa Rica
594±16 ka
569±6 ka Irazú
Turrialba
136±5 ka
855±6 ka pre Irazú
43
Interplay of geology and geochronology improved both age and
volume estimates
44
Extrusive volcanic flux: all segments the
same within error
45
Subducted contribution of flux is total flux minus mantle
contribution
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 Sm...
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...
48
The Galapagos is one of the sources
49
Himu
High-μ
50
END
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Volcanism, Geochemistry and Tectonics at Cocos-Caribbean ...

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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

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