Ocean Ridges and Transforms Flashcards

1
Q

Ocean Ridges and Transforms

A
  • Oceanic lithosphere
  • Hydrothermal circulation at ridges
  • Axial magma chambers
  • Transform faults and ridge segmentation
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2
Q

Heat flow, q

A
  • Heat flow density, mW/m^2
  • Energy per unit time (watts) flowing through a unit area
  • Fourier’s Law
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3
Q

Fourier’s Law

A

Heat flow, q = -k (dT/dz)

- Where dT = temp change, dz = thickness, k = thermal conductivity

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4
Q

Heat flow at Earth’s surface

A
  • dT/dz = 20-30 degrees K/km, k = 2-3W/m/degree K
  • q = 40-90 mW/m^2
  • Continents = 55mW/m^2
  • Oceans = 80-90mW/m^2
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5
Q

q at continents

A

55mW/m^2

- approximately 1/2 is crustal radioactivity

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6
Q

q at oceans

A

80-90mW/m^2

- approximately 75 percent of Earth’s heat flow

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7
Q

Heat Flow Probe

A
  • Temperature gradient measured over known distances (drill hole up to a few km, sediment probe 3m length)
  • Conductivity measured in lab, or in situ using decay of a heat pulse from the probe
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8
Q

What are the 2 plate models?

A
  • GDH1

- PSM

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9
Q

Half-space

A
  • Boundary layer cooling model
  • Material cools and contracts as it moves away from ridge
  • Surface layer cools from top down
  • Lithospheric thickness can be calculated
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10
Q

Lithosphere, HS model

A
  • Defined as region w/ temp below certain value
  • eg. base of lithosphere = 1100C or 1300C
  • Thickness increases away from ridge
  • Can calculate thickness using model
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11
Q

HS Lithospheric thickness calculation

A
  • L = 11 x sq.root t
  • q = 1/sq.root t
  • Where L is thickness in km, t is age in Ma, q is heat flow
  • Thickness increases with age
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12
Q

HS cooling model, seafloor depth d

A
  • From age or distance from ridge
  • As material cools, density increases
  • Isostasy leads to calculation
  • d = 2.5 plus 0.35 x sq.root t
  • Implies typical ridge depth is 2.5km
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13
Q

What are the exceptions to the HS model for depth of ridge?

A
  • Typical depth is 2.5km
  • Iceland = 0km
  • Pacific-Antarctic Discordance zone = 3km
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14
Q

HS boundary layer cooling model comparison with observations

A
  • Heat flow is too low for ages greater than 120Ma
  • Depths are too large for ages greater than approximately 70Ma
  • Model lithosphere continues to cool w/out limit but a constant rate of cooling must be reached (about 70Ma)
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15
Q

Plate model

A
  • Assumes fixed lithospheric thickness L of 95km
  • Assumes fixed temperature at base of lithosphere and vertical boundary below ridge (1450C, Stein model)
  • Far from ridge the plate is far from high T influence and equilibrium is reached, constant heat loss
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16
Q

GDH model

A
  • Relationships for depth and heat flow
  • Different eons for different age ranges
  • T< or > 20, T< or> 55
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17
Q

Problem with plate models

A
  • Seismic evidence suggests lithosphere is thinner under ridge
  • Therefore thickness, L, cannot be constant
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18
Q

Both models (HS and GDH) vs. Heat flow

A
  • Plate models fit depth observations better than heat flow
  • Both models over-predict heat flow in young lithosphere
  • Large data scatter near ridge
  • Therefore hydrothermal circulation has an influence
19
Q

Hydrothermal flow at ridges

A

Seawater near ridges:

  • Penetrates and cools new ocean crust through cracks
  • Heated and driven out at hydrothermal vents
  • Carries away heat by convection rather than conduction
20
Q

Black smokers

A
  • Leach things out of rocks and then precipitate metal sulphides at vent and change minerals in basalt to be more hydrous
  • Organisms use chemosynthesis to survive in this environment w/ no light
21
Q

Seismic data from Juan de Fuca ridge

A
  • Regimes for hydrothermal flow
  • W/ distance from ridge, open to sediment sealed circulation, changes in heat flow, fluids, seismic velocity
  • Effect of basement highs, forced fluid flow
22
Q

Transition from open to sediment-sealed hydrothermal circulation

A
  • Heat flow and basement temperature increase away from ridge
  • Hydrothermal circulation nearer ridge cools younger rock more than expected
  • Increasing seds covering and filling cracks away from ridge decrease hydrothermal circulation (hemipelagics settling out from column, turbidites and mixing/continental influence further from ridge)
23
Q

Near ridge envr

A
  • Seds: Very thin hemipelagics
  • Heat flow much lower than expected
  • Basement Temp 10C
  • Pore fluids like seawater
  • Seismic layer 2A velocity 3.0-3.5km/s
24
Q

20km from exposed basement envr

A
  • Seds: Turbidites, provide a hydrologic barrier
  • Heat flow approaches expected value
  • Basement Temp 40-50C
  • Pore fluids depleted in Mg, enriched in Ca, elevated chlorinity
  • Seismic layer 2A velocity >5km/s
25
Implications of seds, heat flow, and basement temps for near ridge
- Near-ridge hydrothermal circulation - Open fissures, cracks - Carries heat away
26
Implications of pore fluids and seismic velocities near ridge vs. 20km away from exposed basement
- Alteration due to hydration reaction in crust | - Porosity, permeability decreases due to cracks and fissures infilling further from ridge
27
Seismic layer 2A
- Pillow basalt layer of ophiolite - Velocity increases further from ridge - b/c rock more altered and precipitated minerals have potentially filled cracks - Therefore less pores and less impedance for seismic waves
28
Basement highs (high heat flow)
- Forced fluid flow - Rugged basement topography, 300-500m relief - Ridges mostly sediment covered, some exposed - Thinner sed cover = increased heat flow - Fluid's leak through sed seal - Massive discharge through outcrop and form precipitates
29
Expression of ridge depending on speed
- Fast spreading ridge, E. Pacific rise, Smooth bathymetry | - Slow spreading ridge, Mid-Atlantic ridge, Rugged bathymetry
30
Slow spreading ridge
- Rugged bathymetry - More broken up looking - More discernable transforms - Median valley w/ discontinuous axial high - Coalescence of small volcanoes, axial volcanic ridge
31
Fast spreading ridge
- Smooth bathymetry - Relatively linear - Less discernible transform faults and valleys - Axial high continuous, buoyant hot shallow rock
32
AMC
Axial Magma Chamber
33
AMC: Fast spreading ridges
- High magma supply - Melt lens: 10's - 100's m thick, 1-2km wide - Crystal mush zone, partially solid, surrounded by transition zone to solid rock - Differentiation of lavas - Variations along-axis, pockets of melt, tapped in eruptions
34
AMC: Slow-spreading ridges
- Low magma supply - No long-term melt lens - Dike-like mush zones, crystallize to ocean crust - Eruptions related to injection of new magma from mantle - Undifferentiated lavas, injected magma mixes with crystal mush, no melt separation - Tilted fault blocks, rift valley, volcanoes in back valley
35
AMC: East Pacific Rise, approx. 14 degrees S
- Seismic reflection along ridge | - AMC is continuous for 10's of km, width 250-4500m
36
How does percent melt affect basalt seismic velocity
- 0 percent melt (eg 1000C): P-vel = 6.2km/s, S-vel=3.4km/s - 100 percent melt (pure basalt melt): P-vel=3.0-3.4km/s, S-vel=0 - Partial melt: P-vel decreases w/ increasing melt, S-vel to 0 once crystals lose inter-connections (variable percentages of melt for S-vel to become 0)
37
Roof and base of AMC
- Observations: P-vel = 6.0km/s, S-vel = 3.2km/s | - Interpretation: mostly solid, approx. 2 percent melt, gabbros (cooling and crystallization w/in AMC)
38
Within AMC
- At 1625, P-vel = 4 km/s, S-vel = 2.3km/s, mush 40-60 percent melt - At 2488, P-vel = 3.4km/s, S-vel = 0km/s, Pure melt 90-95 percent melt
39
Above AMC roof
- At 2488 (More melt area) - In a approx. 200m thick region above roof P-vel, S-vel are 0.3-0.5km/s lower than solid basalt - Interpretation: Reduced velocities from hydrothermal fracturing (fracture porosity approx. 7 percent)
40
Magma chamber vs. above roof
- Magma chamber has high velocity floor (150-200m) and roof (50-60m) - Above roof 150-200m thick low-velocity zone likely results from hydrothermal fracturing
41
Melt lenses
- only 2-4km in length | - Steady-state at fast ridges over 100's yrs
42
Hydrothermal plumes
- Associated w/ melt lenses in crust | - Fresh supply of magma from mantle
43
Observations of Melt vs. Hydrothermal plumes
- Melt lenses steady-state at fast ridges over 100's years - Hydrothermal plumes have fresh supply of magma from mantle - It would take approx. 50 yrs to solidify a 50m thick melt if not replenished