1. Mantle Melting, Generation of Basaltic Magma and Magmatic diversification
Seismic evidence -> basalts are generated in the mantle
Partial melting of mantle material probably can derive most other magmas from this primary magma by fractional crystallization, assimilation, etc.
Basalt is the most common magma. If we are going to understand the origin of igneous rocks, it’s best to start with the generation of basalt from the mantle

2. 2 principal types of basalt in the ocean basins
Tholeiitic Basalt and Alkaline Basalt
Table 10.1
Common petrographic differences between tholeiitic and alkaline basalts
Tholeiitic Basalt
Alkaline Basalt
Usually fine-grained, intergranular
Usually fairly coarse, intergranular to ophitic
Groundmass
No olivine
Olivine common
Clinopyroxene = augite (plus possibly pigeonite)
Titaniferous augite (reddish)
Orthopyroxene (hypersthene) common, may rim ol.
Orthopyroxene absent
(a third, minor, one is hi-Al, or calc-alk basalt & will be discussed later)
No alkali feldspar
Interstitial alkali feldspar or feldspathoid may occur
Interstitial glass and/or quartz common
Interstitial glass rare, and quartz absent
Olivine rare, unzoned, and may be partially resorbed
Olivine common and zoned
Phenocrysts
or show reaction rims of orthopyroxene
Orthopyroxene uncommon
Orthopyroxene absent
Early plagioclase common
Plagioclase less common, and later in sequence
Clinopyroxene is pale brown augite
Clinopyroxene is titaniferous augite, reddish rims
after Hughes (1982) and McBirney (1993).

3. Tectonic settings Tholeiites are generated at mid-ocean ridges
Also generated at oceanic islands, subduction zones
Alkaline basalts generated at ocean islands
Also at subduction zones
How are they generated? And why two major types?
Source is the mantle
1. What comprises the mantle?
2. What do we get when we melt it?

4. Samples from the mantle
Ophiolites
Slabs of oceanic crust and upper mantle
Thrust at subduction zones onto edge of continent
Dredge samples from oceanic crust
Nodules and xenoliths in some basalts
Kimberlite xenoliths
Diamond-bearing pipes blasted up from the mantle carrying numerous xenoliths from depth

5. Mantle rocks Figure 2.2 C After IUGS Olivine Peridotites Lherzolite
Dunite 90
Peridotites
Wehrlite
Harzburgite
Lherzolite 40
Olivine Websterite
Pyroxenites
Orthopyroxenite 10
Websterite 10
Clinopyroxenite
Orthopyroxene
Clinopyroxene
Figure 2.2 C After IUGS

6. Lherzolite is probably fertile unaltered mantle
Dunite and harzburgite are refractory residuum after basalt has been extracted by partial melting 15
Tholeiitic basalt 10
Partial Melting
Wt.% Al2O3 5
Figure 10-1 Brown and Mussett, A. E. (1993), The Inaccessible Earth: An Integrated View of Its Structure and Composition. Chapman & Hall/Kluwer.
Lherzolite
Harzburgite
Residuum
Dunite
0.0
0.2
0.4
0.6
0.8
Wt.% TiO2

7. Lherzolite: A type of peridotite with Olivine > Opx + Cpx
Dunite 90
Peridotites
Wehrlite
Harzburgite
Lherzolite 40
Olivine Websterite
Pyroxenites
Orthopyroxenite 10
Websterite 10
Clinopyroxenite
Orthopyroxene
Clinopyroxene
Figure 2.2 C After IUGS

8. Phase diagram for aluminous 4-phase lherzolite:
Al-phase =
Plagioclase
shallow (< 50 km)
Spinel
50-80 km
Garnet km
Si ® VI coord.
> 400 km
Note: the mantle will not melt under normal ocean geotherm!
Figure Phase diagram of aluminous lherzolite with melting interval (gray), sub-solidus reactions, and geothermal gradient. After Wyllie, P. J. (1981). Geol. Rundsch. 70,

9. Melts can be created under realistic circumstances
Plates separate and mantle rises at mid-ocean ridges
Adibatic rise ® decompression melting
Hot spots ® localized plumes of melt
Fluid fluxing may give LVL
Also important in subduction zones and other settings

10. Generation of tholeiitic and alkaline basalts from a chemically uniform mantle
Variables (other than X)
Temperature
Pressure
Variables (other than X)
Temperature
= % partial melting
Pressure
Fig indicates that, although the chemistry may be the same, the mineralogy varies
Pressure effects on eutectic shift
Figure Phase diagram of aluminous lherzolite with melting interval (gray), sub-solidus reactions, and geothermal gradient. After Wyllie, P. J. (1981). Geol. Rundsch. 70,

11. Temperature: % of partial melting
No realistic mechanism for the general case
Local hot spots OK
very limited area
Figure Melting by raising the temperature.

12. Pressure effects:
Increased pressure moves the ternary eutectic minimum from the oversaturated tholeiite field to the under-saturated alkaline basalt field
Alkaline basalts are thus favored by greater depth of melting In
Figure 10.8 Change in the eutectic (first melt) composition with increasing pressure from 1 to 3 GPa projected onto the base of the basalt tetrahedron. After Kushiro (1968), J. Geophys. Res., 73,

13. Liquids and residuum of melted pyrolite
Tholeiite produced at < 30 km depth by 25% PM
60 km
Alkalis are incompatible so tend to concentrate in first low % partial melts
20% PM -> alkaline basalt
30% PM -> tholeiite (only 25% or less at 30 km so looks like tholeiitic nature suppressed with depth)
Note that residuum is Ol + Opx (harzburgite)
Note also the thermal divide between thol and alk at low pressure for FX
Figure 10.9 After Green and Ringwood (1967). Earth Planet. Sci. Lett. 2,

14. Initial Conclusions: Tholeiites favored by shallower melting
25% melting at <30 km ® tholeiite
25% melting at 60 km ® olivine basalt
Tholeiites favored by greater % partial melting (F)
20 % melting at 60 km ® alkaline basalt
incompatibles (alkalis) ® initial melts
30 % melting at 60 km ® tholeiite

15. Primary magmas
Formed at depth and not subsequently modified by FX or Assimilation
Criteria
Highest Mg# (100Mg/(Mg+Fe)) really ® parental magma
Experimental results of lherzolite melts
Mg# = 66-75
Cr > 1000 ppm
Ni > ppm
Multiply saturated

16. Magmatic Differentiation
Any process by which a magma is able to diversify and produce a magma or rock of different composition

17. Magmatic Differentiation
Two essential processes
1. Creates a compositional difference in one or more phases
2. Preserves the chemical difference by segregating (or fractionating) the chemically distinct portions
1. Creates a compositional difference in one or more phases as elements partition themselves in response to a change in an intensive variable, such as pressure, temperature, or composition
This will determine the trend of the differentiation process
2. Preserves the chemical difference by segregating (or fractionating) the chemically distinct portions so that they may either form a rock, or continue to evolve as separate systems
The effectiveness of the fractionation process determines extent of differentiation proceeds along a particular trend
Most common: differentiation involving the physical separation of phases in multi- phase systems
The effectiveness depends upon contrasts in physical properties such as density, viscosity, diffusivity, and size/shape
The energy is usually thermal or gravitational
The phases that are fractionated in magmatic systems can be either liquid-solid, liquid-liquid, or liquid-vapor

18. Partial Melting
Separation of a partially melted liquid from the solid residue
Involves the partitioning of chemical constituents
Can produce a variety of melt compositions from a single source
Discussed already, only recap here

19. Effects of removing liquid at various stages of melting
Eutectic systems
First melt always = eutectic composition
Major element composition of eutectic melt is constant until one of the source mineral phases is consumed (trace elements differ)
Once a phase is consumed, the next increment of melt will be different X and T
Trace element behavior (Chapter 9): several models for crystallization and melting (batch melting, Rayleigh fractional melting...)
X of a melt produced by partial melting of a particular source is a function of the pressure, temperature (the fraction of the source that is melted)

20. Sufficient melt must be produced for it to
Separation of a partially melted liquid from the solid residue requires a critical melt %
Sufficient melt must be produced for it to
Form a continuous, interconnected film
Have enough interior volume that it is not all of it is adsorbed to the crystal surfaces
Partial melting may be responsible for the generation of a variety of magmas. But for now we have created a basalt (or 2) via partial melting of mantle lherzolite.
How might this basalt diversify to create a broader spectrum of rock types?

21. Crystal Fractionation
Dominant mechanism by which most magmas, once formed, differentiate
Chapters 6 and 7: effects of removing crystals as they formed (compared to equilibrium crystallization) on the liquid line of descent

22. Kilauea Iki lava lake, Hawaii:
A textbook example of magma differentiation
The Hawaiian Islands
Kilauea Iki lava lake

23. Kilauea Iki lava lake, Hawaii, USA
Before eruption
After eruption
Teng et al. 08 Science

24. Crystallization sequence of Kilauea Iki lavas
5%> MgO: oxides
7.5% > MgO > 5%: augite+ plag
11% > MgO > 7.5%: olivine
MgO = 11%: primitive magma
Helz (1987)
MgO > 11%:
Olivine + primitive magma
Teng et al. 08 Science

25. Gravity settling
The differential motion of crystals and liquid under the influence of gravity due to their differences in density
Observations
Sinking of crystals in experiments
Natural occurrences: plutons and cumulates

26. Gravity settling
Cool point a  olivine layer at base of pluton if first olivine sinks
Next get ol+cpx layer
finally get ol+cpx+plag
Figure 7-2. After Bowen (1915), A. J. Sci., and Morse (1994), Basalts and Phase Diagrams. Krieger Publishers.
Cumulate texture:
Mutually touching phenocrysts with interstitial crystallized residual melt

27. Model settling velocities for spherical particles in Newtonian fluid
Stoke’s Law
V = the settling velocity (cm/sec)
g = the acceleration due to gravity (980 cm/sec2)
r = the radius of a spherical particle (cm)
rs = the density of the solid spherical particle (g/cm3)
rl = the density of the liquid (g/cm3)
h = the viscosity of the liquid (1 c/cm sec = 1 poise) V
2gr ( ) 9 2 = - r h s l
Model settling velocities for spherical particles in Newtonian fluid
Model settling velocities for spherical particles in Newtonian fluid (no yield stress)

28. Olivine in basalt Olivine (rs = 3.3 g/cm3, r = 0.1 cm)
Basaltic liquid (rl = 2.65 g/cm3, h = 1000 poise)
V = 2·980·0.12 ( )/9·1000 = cm/sec
= 4.7 cm/hr, or over a meter per day
In the 5 years that the cooling of the Makaopuhi lava lake was studied (and it was largely liquid at the end of that period), olivines could have settled over 2 km!
Plutons solidify over time periods of 104 to 106 years, permitting considerable gravity settling if Stokes’ Law is any proper measure
= 4.7 cm/hr, or over a meter per day
In the 5 years that the cooling of the Makaopuhi lava lake was studied (and it was largely liquid at the end of that period), olivines could have settled over 2 km!
Plutons solidify over time periods of 104 to 106 years, permitting considerable gravity settling if Stokes’ Law is any proper measure

29. Rhyolitic melt h = 107 poise and rl = 2.3 g/cm3
hornblende crystal (rs = 3.2 g/cm3, r = 0.1 cm)
V = 2 x 10-7 cm/sec, or 6 cm/year
feldspars (rl = 2.7 g/cm3)
V = 2 cm/year
= 200 m in the 104 years that a stock might cool
If 0.5 cm in radius (1 cm diameter) settle at meters/year, or 6.5 km in 104 year cooling of stock
Gravity settling of crystals is more effective in basaltic liquids, but also possible in granitics.
Mafic plutons show more obvious textural features of the process
Notice also that plagioclase crystals (r = 2.7 g/cm3) would not sink in a slightly Fe- rich basaltic melt (r = 2.7 g/cm3), and would even float if the Fe enrichment were greater
Notice also that plagioclase crystals (r = 2.7 g/cm3) would not sink in a slightly Fe-rich basaltic melt (r = 2.7 g/cm3), and would even float if the Fe enrichment were greater

30. Stokes’ Law is overly simplified 1. Crystals are not spherical
2. Only basaltic magmas very near their liquidus temperatures behave as Newtonian fluids
1. Crystals are not spherical
Tabular, accicular, and platy minerals will settle with slower velocities, but it is difficult to determine exactly how much slower
2. Only basaltic magmas very near their liquidus temperatures behave as Newtonian fluids
Once even these begin to crystallize they develop a significant yield strength, that must be overcome before any motion is possible
CRB at 1195oC had a yield strength of 60 Pa
In order to overcome this resistance an olivine crystal must have been several centimeters in diameter!
Yield strength considerably higher for cooler and more silicic liquids
Gravity settling viable only in a mafic magma within a few degrees of the liquidus?
Next slide is Kfs-Plag-Q of Toulumne

31. Late-stage fractional crystallization
Fractional crystallization enriches late melt in incompatible, LIL, and non-lithophile elements
Many concentrate further in the vapor
Particularly enriched with resurgent boiling (melt already evolved when vapor phase released)
Get a silicate-saturated vapor + a vapor-saturated late derivative silicate liquid
Mineral segregation from a melt will enrich the melt in volatile phases
Even fractionation of hydrous minerals removes less water from the melt than is concentrated by the separation of other minerals
Eventually the magma reaches the saturation point and a hydrous vapor phase is produced
This somewhat paradoxical “boiling off” of water as a magma cools has been called retrograde or resurgent boiling
The vapor phase concentrates volatile constituents such as H2O, CO2, S, Cl, F, B, and P, as well as a wide range of incompatible and chalcophile elements (esp LILs)

32. 8 cm tourmaline crystals from pegmatite
5 mm gold from a hydrothermal deposit

33. Pegmatites