volcanism, geochemistry and tectonics at cocos-caribbean

1

Convergent Margin VolcanismThree 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 interestinga. Vents b. Links c. Ba/La (Windows) d. Galapagos

Read everything first (slides and notes) and then select specific slides (by number) to discuss

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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

Spreading rates versus convergence rates:Narrower distribution for convergence rates

0 20 40 60 80 100 120 140 1600

5

10

15

20

25

30

Frequency

"Vc (Km/Ma)

10 Km/Ma =10 mm/yr = 1 cm/yr

MOR CPM

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

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

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).

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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.

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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 15010

20

30

40

50

60

70

Poi

sson

Spa

cing

(K

m)

Plate convergence rate normal to arc (mm/yr)

Uyeda and Kanamori (1979) classification

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Why examine this question?

Because volcano spacings () differ significantly

Northern SumatraCentral AmericaN

500 Km500 Km

N

= 23 Km = 65 Km

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Aleutian volcanoes have spacings intermediate between Central America and northern Sumatra

Aleutians

= 40 Km N

500 Km

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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

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Ignore back-arc volcanoes and volcanoes like these cinder cones

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Why ignore the little volcanoes? Flux derived melts at volcanic frontDecompression melts in back-arc

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A simple composite cone is a Center

Agua volcano in Guatemala

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A cross-arc alignment is a Center

Atitlán-Toliman-Cerro de Oro in Guatemala

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Make decisions defining discrete centers

Central AmericaN

500 Km

Volcanic center

Back-arc cone

Holocene activity doubtfulSecondary cone in a center

Data are from Smithsonian's Global Volcanism Program

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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, ) = xe

-

x!x = 0,1,2,3,…

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Volcano spacing in Central America = 23 Km

0 10 20 30 40 50 60 70 80 90 100 Km0

5

10

15

Fre

quen

cy

Volcano Spacings in 10 Km bins

Poisson distribution

n=36, bin=10

=2.3 or 23 Km

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Volcano spacing in Kuriles-Kamchatka = 17 Km

Volcano Spacings in 10 Km bins

0 20 40 60 80 100 Km0

5

10

15

20F

requ

ency

Poisson distribution

n=62 bin=10

=1.7 or 17 Km

Suggestion of a second mode at 75 Km.

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Volcano spacings determined here agree with those published by d’Bremond d’Ars et al.1995

0 10 20 30 40 50 60 70 800

10

20

30

40

50

60

70

80d'

Ars

et a

l 199

5 sp

acin

g (K

m)

Poisson Spacing (Km)

45º

Cascades - an outlier because d’Ars used Guffanti and Weaver’s list not Smithsonian’s

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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 km0

10

20

30

40

50

60

70

80P

oiss

on S

paci

ng (

Km

)

Plate convergence rate normal to arc (mm/yr)

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Why a negative correlation?

μ1

μ2If 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

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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?

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Volcanic segments based on “Centers”

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Vents <600 ka in Central America

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Vents younger than 600 ka with arcs

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We study the entire volcanic chain. We often plot our volcanological and geochemical data against Distance Distance

3b. To link Volcanology and geochemistry

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Regularities in the Distribution and Geochemistry of Central American Volcanoes

0

50

100

150

Ba/La

Vol

cano

vol

ume

Km

3

0 1000 Km0

100

200

300

400

Guatemala El Salvador Nicaragua Costa Rica

Zr/Nb

010

20

30

40

50

60

70El Salvador Nicaragua Costa Rica

=

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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

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Volume distribution along volcanic front

0 500 10000

100

200

300

400

Guatemala El Salvador Nicaragua Costa Rica

Atitlán Santa Ana

Tecapa

San Cristóbal

Masaya

Irazú

Rincón

Barva

Arenal

Mv

0 500 10000

100

200

300

400

Guatemala El Salvador Nicaragua Costa Rica

Atitlán Santa Ana

Tecapa

San Cristóbal

Masaya

Irazú

RincónVol

can

o vo

lum

e K

m3

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

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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 11000

10

20

30

40

50

60

70

Zr/

Nb

El Salvador Nicaragua Costa Rica

300 500 700 900 1100

0

50

100

150

200

Dep

th to

the

sla

b (

km)

Distance along the arc (km)

El Salvador Nicaragua Costa Rica

Yojoa-back-arc, no slab signal

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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

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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

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A plausible model of Zr/Nb variation: basalt reacts with mantle during ascent

0 50 100 150 2000

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

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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?

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3c. What causes the regional variation in Slab signal (Ba/La)?

0 500 1000 Km0

50

100

150

Ba/La

Distance

Guatemala | El Salvador | Nicaragua | Costa Rica

DSDP 495 DSDP 1039

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Incoming sedimentary sections are similar but substantial unmeasured variation may exist

e

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DSDP 495 sediment and MORBLow variance

maximum in carbonate

maximum in hemipelagic

--------Regional-------- ---------Local---------

10 100Ba/La

100 1000 10000B a/Th

.01 .1 1U/La

De

pth

in m

ete

rs

.1 1 10

0

100

200

300

400

500

U/Th

Mo

rbC

arb

onat

eH

emip

elag

ic

High variance

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See regional variation if sediments are similarSee local variation if sediments differ

Note parallel arrays in local variation

1

10

100

Ba/La

U/Th

EM

DM

CS HS

Yohoa

VFlow-Ti

.01 .110

100

000

10000

1

Ba/Th

U/La

CS

HS

EM DM

Yohoa

20%

W. N ic.

E l S al.

N .C .R .

R eg iona l Va ria tion Loca l Va ria tion

TWO DIFFERENT WINDOWS!!

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La carries the regional signal, not Ba

0.0

0.2

0.4

1/La

SiO2<55wt. %

500 1000 Km0

50

100

150

Ba/La

Distance

Guatemala El Salvador Nicaragua Costa Rica

0 500 1000 Km0

500

1000

Ba

Distance

SiO2< 55wt. %

Black crosses are estimated mantle contributions

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Eiler et al. 2005, strong evidence for a serpentine component in Nicaragua from 18O data

serpentine

carbonate sed

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Irazú-Turrialba volcanic center Costa Rica

594±16 ka

569±6 ka Irazú

Turrialba

136±5 ka

855±6 ka pre Irazú

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Interplay of geology and geochronology improved both age and volume estimates

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Extrusive volcanic flux: all segments the same within error

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Subducted contribution of flux is total flux minus mantle contribution

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Masaya volcano, Nicaraguamantle 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 = 8Lasubducted= 43%

For subduction contributionBa estimate is robust! La is not!

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Constant flux for highly enriched elements (Cs, Ba, K, Pb, Sr)

Segment\Element 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 566SE Nicaragua 1.04 25 1076 1.60 1.75 1.40 7.2 4.48 392Guanacaste 0.73 27 892 1.75 1.14 1.42 9.6 3.55 554Cordillera Central 1.01 45 755 6.88 2.25 1.70 22.5 5.21 523

Element flux in units of 104Kg/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

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The Galapagos is one of the sources

49

Himu

High-μ

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END