MidOcean Ridge Basalts

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Mid-Ocean Ridge Basalts

• Some of it the rest of it All of it
  
• Using phase diagrams to detect mantle xenoliths.

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Outline

• Introduction
• Basalt Definition
• External Mechanics of Creation
• Internal Mechanics
• Deduction of Xenoliths
• Summary
• Bibliography and Credits

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Using Phase Diagrams to Detect Mantle Xenoliths

• Identifying the composition of the mantle has
  long been a fascin-ation of earth scientists.
  Hard-rock mining (to 14000 feet) and oil well
  drilling (to 20000 feet) do not come close to
  the mantle. Thus, the only evidence other than
  seismic studies for mantle composition has come
  in the form of mantle xenoliths. Xenoliths are
  mineral crystals foreign to the expected
  composition of a magma or lava. They are
  fragments of structures through which the liquid
  passed. Those from the mantle are of particular
  interest since they are readily resorbed during
  transit. A Mid-Ocean Ridge Basalt system is used
  here because it is less complex than a
  continental system.

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BASALT

• Basalt is a fine grained volcanic rock.
• Silica content less than 52 wt.
• Abundant Calcic Plagioclase (An50)
• (An is a ratio of Na and Ca, and equals
  Ca0-100. Therefore, An50 equals 50 Na and 50
  Ca.)
• Abundant Calcic Pyroxene (e.g., Augite)
• N-MORB Normal Mid-Ocean Ridge Basalt
• E-MORB Enriched Mid-Ocean Ridge Basalt

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Basalt Composition
Normal Enriched
Enriched MORB has elevated amounts of alkalis and
mafics at the expense of aluminum and silica.
Fe2O3, P2O5, and K2O almost double in wt.
SiO2 48.77
47.74 Al2O3 15.90
15.12 Alkalis CaO 11.16
11.61 Na2O 2.43
2.04 K2O 0.08
0.19 P2O5 0.09
0.18 Mafics MgO 9.67
8.99 TiO2 1.33
1.59 FeO 8.62
9.74 Fe2O3 1.33
2.31 MnO 0.17
0.20
Composition of basalt is determined X-ray
defraction and/or microprobe analysis. No
specific mineralogy is expressed or implied.
(Blatt and Tracy, 2000)
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SiO2
This is the position of the phase diagrams used
in this presentation.
AREA OF GENESIS
Al2O3
Mafic
Alkali
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How does it do that?

• Steady-State Earth

Temp oC
1050 1700
Not much melting occurs near the surface. For
melting to occur, the peridotite solidus must be
brought nearer to the surface. This is
accomplished by decompression melting, a tactic
whereby the crust is thinned by tension and
lower, hotter strata fill in the void.
Plagioclase
Basalt Melt
Spinel
Peridotite Solidus
Continental Crust
Garnet
Oceanic Crust
After Blatt Tracy, 2000 after Green and
Ringwood, 1970
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How does it do that?

• A steady-state Earth is layered and hotter at
  depth.

Temperature oC
1200 1400 1600
Depth km
Depth km
Solidus
500
Geotherm Conductive
50 100 150
50

Geotherm Convective
1000
Lithosphere
100
1500
After Philpotts, 1990
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How does it do that?
Temp oC 500 1400
TENSION
Melt
Solidus
1000
Geotherm
UPWELLING

• Decompression Melting As tension pulls the
  crust apart, hotter strata are brought nearer to
  the surface as the crust thins. Tension begins
  when one tectonic plate is split. Each part
  moves away from this split with considerable
  momentum if not speed. The crust is then
  stretched (with local volume loss) allowing the
  upwelling of deeper, hotter strata.

After Philpotts, 1990
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How does it do that?
BASALT FLOW
Temp oC 500
Melt
1000 1400
Solidus
UPWELLING
Geotherms

• Melting takes place at the surface because lower
  strata rise tectonically faster than they cool.

After Philpotts, 1990
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How does it do that?

• This is the basalt ternary phase diagram
  positionally identified within the miner-alogical
  pyramid. The minimum tem-perature for the first
  melt is about 1260oC. Given that there is a
  maximum temperature available, (1400 oC), the
  melt composition is limited to this area.
• Therefore, those crystals whose composi-tion
  falls outside of the tinted area are us-ually
  preserved unless chemically reacted. It is these
  that are the fragments of the mantle.

CaAl2Si2O8
1500
1400
1400
1300
1300
1700
Ca(Mg,Fe)Si2O6
(Mg,Fe)SiO4
After Blatt Tracy, 2000 after Osborn and Tate,
1952
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Why does it do that?

• Since the system is open, there is a lot from
  which to draw. Present melt rate is 20 km3 per
  year (Phil-potts, 1990).
• At the contact with water, there is much
  hydro-thermal alteration.

Temp oC
500
1000
1400
UPWELLING
After Philpotts, 1990
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How and why it does that

• MORBs are particularly uniform because of
  formation conditions and mechanism.
• The melt and recrystallization phases present are
  dictated by the temperatures available.
• Mantle xenoliths can be shown to have come from
  depth because they had not melted during transit.

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How and Why we use it

• Some of it the rest of it all of it.
• If we know what is expected, we can detect and
  get information from that which is not expected.
  Xenoliths are those rocks present in the melt
  that came from someplace other than the melt
  itself. For MORBs, this means that they came
  from deeper in the mantle than the melt and were
  along for the ride in the convection currents.
  Its from these xenoliths that information about
  the mantle is known.

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Bibliography

• Blatt, H, and Tracy, R J 2000 Petrology W H
  Freeman and Co.
• Philpotts, A R 1990 Igneous and Metamorphic
  Petrology Prentiss Hall.