Magmatic Settings (L25-30)

Question 1

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

Answer 1

21 km^3

18 intrusive, 3 extrusive

Question 2

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

Answer 2

8-180mm/yr

Question 3

What depths do most mid-ocean ridges lie at?

Answer 3

2500-3000m

Question 4

Typical oceanic crust thickness?

Maximum thickness and where?

Answer 4

6-8km

40km beneath central Iceland

Question 5

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

Answer 5

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

Question 6

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

Answer 6

The roof of a shallow magma chamber

Question 7

How can mid-coean ridge basalt samples be obtained?

Answer 7

Dredging
Wax-coring
Submersible
Ocean drilling
On-land sampling
Ophiolites

Question 8

What is the petrology of mid-ocean ridge basalts?

Answer 8

Often olivine and plag phenocrysts (rarely cpx)

Question 9

Describe the uniformity of mid-ocean ridge basalts

Answer 9

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

Question 10

How does Mg# of MORB compare with mantle olivine?

Answer 10

Too low to be in equilibrium

Question 11

What is Mg# often used to characterise?

Why?

Answer 11

Composition of basalts

Related to composition of expected equilibrium olivine

Question 12

What is the Mg# of convecting mantle?

Answer 12

~70

Question 13

Define primary melts

Answer 13

Unmodified melts of the mantle

Question 14

Define primitive melts

Answer 14

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

Question 15

Define evolved melts

What is the relationship between evolved and primitive melts?

Answer 15

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

Question 16

Define parental melts

Answer 16

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

Question 17

Typical MORB are low in what?

Answer 17

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

Question 18

Olivine tholeiites are normative in what?

Answer 18

Forsterite and hypersthene

Question 19

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

Answer 19

Close to low pressure fractional crystallisation cotectics

Question 20

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

Answer 20

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

Question 21

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

Answer 21

The Ol-opx-cpx triangle

Olivine-tholeiites

Question 22

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

Answer 22

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

Question 23

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

Answer 23

MORBs are not primary mantle melts

MORBs have undergone cooling and crystallisation before eruption

Question 24

What are ophiolites thought to be?

Answer 24

Portions of old oceanic crust exhumed at destructive plate margins

Question 25

What are the potential mechanisms of ophiolite emplacement?

Answer 25

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

Question 26

What are the two most famous examples of ophiolites?

Answer 26

Semail Massif in Oman | Troodos in Cyprus

Question 27

What is currently thought about magma reservoirs beneath MORs?

Answer 27

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

Question 28

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

Answer 28

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

Question 29

What does the upper mantle composition correspond to?

Answer 29

Aluminous lherzolite

Question 30

What are aluminous lherzolites composed of?

Answer 30

Olivine Orthopyroxene Clinopyroxene Aluminuos phase

Question 31

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

Answer 31

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

Question 32

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

Answer 32

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

Question 33

Outline the model of melt generation at mid-ocean ridges

Answer 33

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

Question 34

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

Answer 34

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

Question 35

When does adiabatic decompression melting occur?

Answer 35

If mantle potential T is high enough | Adiabat intersects the solidus during decompression

Question 36

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

Answer 36

Mantle melt is unlikely to be isobaric | Melts produced over a range of P likely mix to form MORB

Question 37

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

Answer 37

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

Question 38

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

Answer 38

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

Question 39

What is the origin of harzburgites in oceanic crust?

Answer 39

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

Question 40

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?

Answer 40

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?

Question 41

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

Answer 41

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

Question 42

What evidence in MORB is there for deep mantle melting? | What explains deep mantle melting?

Answer 42

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

Question 43

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

Answer 43

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