Igenous - Mantle Melting

Question 1

Mantle composition

Answer 1

• Predominantly perioditie

- Abundance of olivine crystals (>40%) makes the rock green

Question 2

Sources of mantle material

Answer 2

Ophiolites
o Slabs of oceanic crust and upper mantle = thrust at subduction zones onto edge of continent
• Abyssal peridotites – exposed at oceanic fracture zones
• Exposed in mountain belts – orogenic peridotites
• Xenoliths

Question 3

Lherzolite:

Answer 3

• Type of peridotite which the upper 400km of the mantle is made of
• Olivine dominated (60%)
• Also contains orthopyroxene (20%) and clinopyroxene (8%) as its second and third phase
• Has an aluminous fourth phase dependent on depth and pressure
o Plagioclase (<50km) – lowest pressure
o Spinel (50-80km)
o Garnet (80-400km) – highest pressure

Question 4

Melting fertile mantle

Answer 4

• Mantle that has experienced little or no melt extraction is referred to as “fertile” and is Lherzolitic
• As you melt fertile mantle at pressures of <= 3GPa the solid residue
o Increase in olivine
o Olivine becomes more Fo-rich (Mg rich)
o Decrease in clinopyroxene – to enter the melt phase
o Decease in aluminous phase - to enter the melt phase
o Orthopyroxene remain = constant or slight decrease

Question 5

Melting fertile mantle results

Answer 5

Forms:
•	Residuum (depleted mantle)
      o	Harzburgite/Dunite
      	Olivine and orthopyroxene 
•	Melt
      o	Basalt
       	Mainly Clinopyroxene + Aluminous phase (+small amount of Ol + OPX)

Mantle rock types related by melting:
• Lherzolite is probably unaltered mantle
• Dunite and harzburgite are refractory residuum
o = after (tholeiitic) basalt has been extracted by partial melting

Question 6

Partial melting techniques

Answer 6

Increase of local temperature:
Move the geotherm up and right
• Solidus and geotherm intersect so that an area of partial melting is possible

Decrease the pressure
Moves the geotherm up
• Adiabatic rise of mantle with no conductive heat
• Upwelling of hot mantle material to areas of lower pressure
• Once melting occurs – lower density drives magma upward by mantle convection
• Dominant mechanism involved at mid-ocean ridges (MORs) and continental rifts

Add volatiles
• Moves the solidus left
• Adding water decreases the melting temperature
• H2O and CO2 (fluid flux melting) in subduction zones

Question 7

Where does partial melting occur on a geotherm plot?

Answer 7

Area of overlap of geotherm and solidus

Question 8

Partial melting in Rift Valleys?

Answer 8

Adiabatic (pressure decrease):
• Lithosphere is thinned and stretched
• Decrease in lithostatic pressure on mantle below
• Asthenosphere flows upwards to fill the space created
• The geotherm has been brought into contact with peridotite melting curve solidus causing a small amount of melt to occur
• Moves to an area of lower pressure = adiabatic partial melting

Question 9

Partial melting in MORB?

Answer 9

(adiabatic/decompression melting):
• Asthenospheric mantle is rising almost to surface
• Adiabatic gradient steeper than geotherm so it intersects solidus
• Decrease in lithostatic pressure on mantle below and increase in temp
• Geotherm crosses the solidus to much greater extent than continental rift valleys – mantle is able to melt and produce huge volumes of magma

Question 10

Partial melting in Mantle Plumes ‘hotspots’ (OIB)?

Answer 10

(increase in local temp):
• Temperature distribution in a convecting plume of anomalously hot mantle
• Deviation from average upper mantle temperature
• Geotherm crosses the solidus by moving towards it
• Small volume of partial melt
• Deeper depths than divergent boundaries

Question 11

Partial melting in Subduction Zones?

Answer 11

Addition of volatiles (fluid flux):
• Geotherm stays the same
• Volatiles are released from dehydration of descending plate
• Water lowers the melting temperature of overlying mantle causing solidus temperature to be reduced allowing for an intersection – partial melting occurs
• Large amount of melt but dependent on volume of volatiles produced

Question 12

Controls on composition of melts:

Answer 12

• TAS - Total alkaline against silica
Different primitive basalts
Magmatic differentiation occurs

Less silica rich to start with = Nepheline way
o More alkaline

More silica rich start = granite way (more quartz)
o Sub alkaline

Question 13

Two types of basalt in the oceans:

Answer 13

Tholeiitic Basalts 
o	Generated in MORs
	Also oceanic islands and subduction zones
o	Large-degree mantle melting (10%)
o	Shallow (>50km)
o	Sub-alkaline

Alkaline Basalts
o	Generated at Ocean Islands
	Also at subduction zones
o	Small-degree melting (1%)
o	Deep (>80 km)

Question 14

Generation of tholeiitic and alkaline basalts from a chemically uniform mantle?

Answer 14

•	Suite of rocks produced from melting the magma
o	Cant be from difference in source
o	Must be another factor
	Temperature/pressure
	Volatile content
	Degree of melting

Question 15

Effect of Pressure, Water, and CO2 on the position of the eutectic in the basalt system?

Answer 15

Increased pressure moves the ternary eutectic (first melt) from silica-saturated to highly undersat alkaline basalts
o More pressure = more alkaline

Water moves the (2 GPa) eutectic toward higher silica
CO2 moves it to more alkaline types

o More water = more silica/ sub-alkaline
o More CO2 = more alkaline

As sub-alkaline magma evolves, it becomes silica-rich.
If SiO2 over-saturated, the silica will react with olivine to produce orthopyroxene or pigeonite
In silica under-saturated melts (i.e. alkaline), this reaction will not occur
o No OPX in alkaline basalts

Question 16

Summary of mantle melting production

Answer 16

• Upper-most mantle = lherzolite
– (Ol-Opx-Cpx + an aluminous phase).
– A chemically homogeneous mantle can yield a variety of basalt types
– Alkaline basalts predominate over tholeiites when melting is deeper, with low % partial melting and low water content.

Question 17

MORB composition

Answer 17

• Typical MORB is an olivine tholeiite
o Low K2O and low TiO2
• Display a (restricted range of compositions
Range:
MgO and FeO
Al2O3 and CaO
SiO2
Na2O, K2O, TiO2, P2O5
• The common crystallization sequence is:
o Olivine (± Mg-Cr spinel)
o Olivine + plagioclase (± Mg-Cr spinel)
o Olivine + plagioclase + clinopyroxene
MORB are not the completely uniform magmas that they were once considered to be.
They show chemical trends consistent with fractional crystallisation of olivine, plagioclase, and perhaps clinopyroxene.
MORB cannot be primary magmas, but are derivative magmas resulting from fractional crystallization (~ 60%).

Question 18

Ridge characteristics

Answer 18

• Fast ridge segments
o A broader range of compositions
o A larger proportion of evolved liquids
• Higher melt fraction (F) the greater the crustal thickness
• The global correlation implies that extent of melting and pressure of melting are correlated, on a global scale
o The correlation of degree of melting with pressure of melting requires that the first-order control on variation among ridge segments is mantle (potential/starting temperature) temperature
If melting continues under the axis to the base of the crust everywhere, then high potential temperature means:
• Long melting column (High pressure) = high mean degree pf melting
• High mean degree of melting = high crustal thickness
• High crustal thickness = shallow axial depth

Question 19

Trace element chemistry of oceanic basalts

Answer 19

Trace elements = shows which tectonic setting it came from
• Enriched (left side of OIB) in incompatible elements = less partial melting
• Depleted (right side of OIB) – higher melt fraction

MORB - depleted in highly incompatible elements
OIB - enriched in highly compatible elements

Question 20

Crust and mantle compared to CHUR

Answer 20

Crust - enriched melt compared to CHUR - below line
Mantle - depleted residue compared to CHUR - above line
• Points to a long-term depletion of Rb>Sr and Nd>Sm in the mantle source.
• Most likely due to the mantle being depleted by melting over much of the age of the Earth.