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1B ESB > Magmatic Settings (L25-30) > Flashcards

Magmatic Settings (L25-30) Flashcards

1
Q

How much magma is produced at mid-ocean ridges globally each year?
Intrusive vs extrusive?

A

21 km^3

18 intrusive, 3 extrusive

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2
Q

What is the range of spreading rates at mid-ocean ridges?

A

8-180mm/yr

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3
Q

What depths do most mid-ocean ridges lie at?

A

2500-3000m

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4
Q

Typical oceanic crust thickness?

Maximum thickness and where?

A

6-8km

40km beneath central Iceland

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5
Q

What is the structure of oceanic crust with the study of seismic velocity?

A 
Layer 1: sediments
Layer 2a: pillow lavas
Layer 2b: sheeted dykes
Layer 3a: gabbro
Layer 3b: layered gabbros
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6
Q

When studying the oceanic crust, what are strong seismic reflectors at 2km depth thought to correspond to?

A

The roof of a shallow magma chamber

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7
Q

How can mid-coean ridge basalt samples be obtained?

A 
Dredging
Wax-coring
Submersible
Ocean drilling
On-land sampling
Ophiolites
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8
Q

What is the petrology of mid-ocean ridge basalts?

A

Often olivine and plag phenocrysts (rarely cpx)

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9
Q

Describe the uniformity of mid-ocean ridge basalts

A

Uniform in major elements (MgO 6-9 wt%), trace elements and isotope ratios

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10
Q

How does Mg# of MORB compare with mantle olivine?

A

Too low to be in equilibrium

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11
Q

What is Mg# often used to characterise?

Why?

A

Composition of basalts

Related to composition of expected equilibrium olivine

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12
Q

What is the Mg# of convecting mantle?

A

~70

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13
Q

Define primary melts

A

Unmodified melts of the mantle

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14
Q

Define primitive melts

A

Melts that have been little modified by fractional crystallisation or other processes after their generation

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15
Q

Define evolved melts

What is the relationship between evolved and primitive melts?

A

Produced after a large extent of crystallisation of a primary melt
More evolved = less primitive

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16
Q

Define parental melts

A

If one melt composition can be related to another by fractional crystallisation, more primitive melt = parent of less primitive melt

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17
Q

Typical MORB are low in what?

A

Alkali: Na2O of 2-3 wt%
H2O: ~0.2 wt%

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18
Q

Olivine tholeiites are normative in what?

A

Forsterite and hypersthene

19
Q

Where do MORB compositions lie in subsections of the basalt tetrahedron?

A

Close to low pressure fractional crystallisation cotectics

20
Q

What is the stable aluminous phase of aluminous lherzolite at various pressures?

A

Plag at low P
Spinel at 10-20kb
Pyrope garnet at high P

21
Q

Where do most MORB compositions lie in the basalt tetrahedron?
What does this mean they are classified as?

A

The Ol-opx-cpx triangle

Olivine-tholeiites

22
Q

What is the order of crystallisation in MORB at low P?

A

Ol -> Ol + opx -> Ol + opx + cpx

23
Q

MORB compositions are unrelated to lherzolite eutectics, what does this indicate in relation to MORB and mantle melts?

A

MORBs are not primary mantle melts

MORBs have undergone cooling and crystallisation before eruption

24
Q

What are ophiolites thought to be?

A

Portions of old oceanic crust exhumed at destructive plate margins

25
Q

What are the potential mechanisms of ophiolite emplacement?

A

Obduction at a passive continental margin
Obduction of oceanic lithosphere
Transfer of a slab of oceanic lithosphere to an accretionary prism

26
Q

What are the two most famous examples of ophiolites?

A

Semail Massif in Oman

Troodos in Cyprus

27
Q

What is currently thought about magma reservoirs beneath MORs?

A

Small sill-like magma bodies
Surrounded by much larger mush and transition zones
Mush zones (dominated by crystals and <30% melt) are continuous with the magma reservoir
Transitional zone between mush zone and surrounding gabbro

28
Q

What techniques can be used to estimate mantle composition underneath MORs?

A

Seismic velocity and density structure
Mantle nodules in kimberlites and alkali basalts
Chondritic Earth model
Harzburgites in ophiolites

29
Q

What does the upper mantle composition correspond to?

A

Aluminous lherzolite

30
Q

What are aluminous lherzolites composed of?

A

Olivine
Orthopyroxene
Clinopyroxene
Aluminuos phase

31
Q

What are the various aluminous phases in aluminous lherzolites at different pressures?

A

Plag at low P
Spinel at 10-20 kbar
Pyrope garnet at high P

32
Q

Outline the low-pressure evolution of eutectic melts of aluminous lherzolite at different initial pressures

A

30kb: ol, ol+plg, ol+plg+cpx
20kb: melt lies in the plane of silica undersaturation and can’t produce MORBs
<15kb: low p fractional xal^n can produce MORB

33
Q

Outline the model of melt generation at mid-ocean ridges

A

Partial melting of aluminous lherzolite to leave a depleted harzburgite residue
Decompression of the primary basalt and fractional crystallisation of olivine
At shallower levels, ol+plg and ol+plg+cpx crystallises
Eruption of fractionated basalt with compositions close to the low-pressure cotectic

34
Q

What is the mantle adiabat?
What is the dry peridotite solidus?
What is the mantle potential temperature?

A

~0.6°C/km
~5°C/km
Extrapolated T of the adiabat at the surface in the absence of melting (1315°C at MOR)

35
Q

When does adiabatic decompression melting occur?

A

If mantle potential T is high enough

Adiabat intersects the solidus during decompression

36
Q

What conclusions can be drawn from the fact that corner flow at MOR allows mantle to decompress and melt over a depth range?

A

Mantle melt is unlikely to be isobaric

Melts produced over a range of P likely mix to form MORB

37
Q

How can oceanic crustal thickness be predicted as a function of potential temperature?
How can Icelandic crustal thickness be explained?

A

Integrating the predicted melt generation over the melting region, and dividing by spreading rate
~30km crust produced by dry peridotite melting if potential temperature ~1500°C

38
Q

Besides crustal thickness at MOR, what helps confirm the melting models with a potential temperature of 1315°C?

A

Involve 0-15% melting over a pressure range from 20-2kb
Predicted to generate melts principally in the olivine tholeiite field
Primary melts are suitable parents for MORB

39
Q

What is the origin of harzburgites in oceanic crust?

A

Unlikely to be low P xal^n as very few MORB are in eq^m with opx at low P
Most likely form as solid residue of fractional melting
When melting is high enough, cpx and aluminous phase get exhausted, leaving ol+opx = harzburgite

40
Q

How fast are melt generation and transport in the mantle at MOR?
How wide is the volcanic zone at MOR?
Which questions arise from this info?

A

Less than 1000 years
Melt moves upwards by >50m/yr
Volcanic zones <2km wide
How can melt move so rapidly in the mantle?
How are melts focused toward the ridge axis?

41
Q

What is the evidence for channelised flow of melts in the convecting mantle?
How well does the evidence hold up?

A

Porous channel models can give 50m/yr melt rise rates
The mantle section of ophiolites show dunite melt channels within harzburgite
Dunites can also easily be generated when high P melts move to lower P and start to cool

42
Q

What evidence in MORB is there for deep mantle melting?

What explains deep mantle melting?

A

Trace element and isotopic data shows some melting occurs in the presence of garnet (~90km)
Small quantities of volatiles (H2O and CO2) in peridotite affect the solidus, low degree melt enriches the erupted melts in incompatible trace elements

43
Q

What are the current possibilities for explaining melt focusing at the ridge axis?

A

Cool boundary at the upper margin of the melting region forms an impermeable barrier to rising melts. Buoyant melts flow upwards along the melting region’s upper edge, focused on the ridge axis.

Corner flow induces a stress field in the mantle which encourages melt channels to align themselves in an orientation that favours the supply of melts towards the ridge axis

1B ESB (9 decks)

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