1. Dry Mantle Melting and the Origin of Basaltic Magma
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
GEOS 508 Lec

2. Mantle melting Needs special condition to melt, usually solid;
Melt fractions usually low, under 2% at any given time;
Regardless of melting conditions, yields one or another variety of basaltic (low silica) melts - andesites, dacites, do not come out of the mantle;

3. 2 principal types of basalt in the ocean basins
Tholeiitic Basalt and Alkaline Basalt
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).

4. Each is chemically distinct
Evolve via FX as separate series along different paths
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?

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

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
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

8. Phase diagram for aluminous 4-phase lherzolite:
Al-phase =
Plagioclase
shallow (< 30 km)
Spinel
30-80 km
Garnet km
Si ® VI coord.
> 400 km
Note: the mantle will not melt under normal ocean geotherm!
Phase diagram of aluminous lherzolite with melting interval (gray), sub-solidus reactions, and geothermal gradient. After Wyllie, P. J. (1981). Geol. Rundsch. 70,

9. How does the mantle melt??
1) Increase the temperature
No realistic mechanism for the general case
Local hot spots OK
very limited area
Melting by raising the temperature.

10. 2) Lower the pressure
Adiabatic rise of mantle with no conductive heat loss
Decompression melting could melt at least 30%
Adiabatic rise of mantle with no conductive heat loss
Steeper than solidus
Intersects solidus
D slope = heat of fusion as mantle melts
Decompression melting could melt at least 30%
Figure Melting by (adiabatic) pressure reduction. Melting begins when the adiabat crosses the solidus and traverses the shaded melting interval. Dashed lines represent approximate % melting.

11. HW 7
Use pMELTS to determine if the sub SWUS upwelling mantle would melt along an adiabat that contains the PT point of 14500C and 15 kbar. A typical composition of the SWUS mantle is given via a San Carlos peridotite; calculate the fraction of melt and phases in equilibrium with the liquid?
What is the composition and melt fraction at 10 kbar (at the Moho), assume 1390C?

12. 3) Add volatiles (especially H2O)
Remember solid + water = liq(aq) and LeChatelier
Dramatic lowering of melting point of peridotite
Dry peridotite solidus compared to several experiments on H2O-saturated peridotites.

13. Fraction melted is limited by availability of water
15% % % 100%
Fraction melted is limited by availability of water
BUT the only water available is 1-2% contained in amphibole or mica
Albite example above assumed 10 wt% water
From Burnham and Davis (1974). A J Sci., 274,

14. Heating of amphibole-bearing peridotite
1) Ocean geotherm
2) Shield geotherm
Phase diagram (partly schematic) for a hydrous mantle system, including the H2O-saturated lherzolite solidus of Kushiro et al. (1968), the dehydration breakdown curves for amphibole (Millhollen et al., 1974) and phlogopite (Modreski and Boettcher, 1973), plus the ocean and shield geotherms of Clark and Ringwood (1964) and Ringwood (1966). After Wyllie (1979). In H. S. Yoder (ed.), The Evolution of the Igneous Rocks. Fiftieth Anniversary Perspectives. Princeton University Press, Princeton, N. J, pp
Requires T > both 1) dehydration and 2) water-sat melting curves
Can only create 1-2% melt
not sufficient to even separate from the source
may explain low velocity layer at 100 km
hornblende (b) is at 70 km
phlogopite (c) is at 95 km
Uncertainty in curves and geotherms can -> melting of mica or hornblende at 100 km

15. 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

16. 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
Phase diagram of aluminous lherzolite with melting interval (gray), sub-solidus reactions, and geothermal gradient. After Wyllie, P. J. (1981). Geol. Rundsch. 70,

17. Pressure effects:
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,
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

18. 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

19. 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
20 % melting at 60 km ® alkaline basalt
incompatibles (alkalis) ® initial melts
30 % melting at 60 km ® tholeiite

20. Crystal Fractionation of magmas as they rise
Tholeiite ® alkaline
by FX at med to high P
Not at low P
Thermal divide
Al in pyroxenes at Hi P
Low-P FX ® hi-Al
shallow magmas
(“hi-Al” basalt)

21. 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

22. Multiple saturation Low P Ol then Plag then Cpx as cool ~70oC T range
Figure Anhydrous P-T phase relationships for a mid-ocean ridge basalt suspected of being a primary magma. After Fujii and Kushiro (1977). Carnegie Inst. Wash. Yearb., 76,

23. Multiple saturation High P Low P Ol then Plag then Cpx as cool
70oC T range
High P
Cpx then Plag then Ol
Anhydrous P-T phase relationships for a mid-ocean ridge basalt suspected of being a primary magma. After Fujii and Kushiro (1977). Carnegie Inst. Wash. Yearb., 76,

24. Multiple saturation High P 25 km get all at once Low P
Ol then Plag then Cpx as cool
70oC T range
High P
Cpx then Plag then Ol
25 km get all at once
Multiple saturation implies that the liquid corresponding to the melted basalt in the experiment was in equilibrium with Ol + Cpx + Plag at 25 km depth
This is the appropriate mineralogy for a lherzolite at this depth
= Multiple saturation
Suggests that 25 km is the depth of last eqm with the mantle

25. Summary
A chemically homogeneous mantle can yield a variety of basalt types
Alkaline basalts are favored over tholeiites by deeper melting and by low % PM
Fractionation at moderate to high depths can also create alkaline basalts from tholeiites
At low P there is a thermal divide that separates the two series
In spite of this initial success, there is evidence to suggest that such a simple approach is not realistic, and that the mantle is chemically heterogeneous

29. Review of REE Now what happens to partial melts of this mantle??
0.00
2.00
4.00
6.00
8.00
10.00
atomic number
sample/chondrite
La Ce Nd Sm Eu Tb Er Yb Lu
increasing incompatibility
If the mantle is unmodified, it should have the chemistry of a chondrite (we think)
How would it plot on a REE diagram?
Now what happens to partial melts of this mantle??

30. Review of REE Enrich LREE > HREE Greater enrichment for lower % PM
Rare Earth concentrations (normalized to chondrite) for melts produced at various values of F via melting of a hypothetical garnet lherzolite using the batch melting model (equation 9-5). From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
Enrich LREE > HREE
Greater enrichment for lower % PM
increasing incompatibility

31. REE data for oceanic basalts
Ocean Island Basalt (Hawaiian alkaline basalt)
Looks like partial melt of ~ typical mantle
Mid Ocean Ridge Basalt (tholeiite)
How get (+) slope??
increasing incompatibility
REE diagram for a typical alkaline ocean island basalt (OIB) and tholeiitic mid-ocean ridge basalt (MORB). From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from Sun and McDonough (1989).

32. Spider diagram for oceanic basalts
Same approach for larger variety of elements
Still OIB looks like partial melt of ~ typical mantle
MORB still has (+) slope
Looks like two mantle reservoirs
MORB source is depleted by melt extraction
OIB source is not depleted
is it enriched?
increasing incompatibility
Spider diagram for a typical alkaline ocean island basalt (OIB) and tholeiitic mid-ocean ridge basalt (MORB). From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from Sun and McDonough (1989).

33. REE data for UM xenoliths
LREE enriched
LREE depleted
or unfractionated
REE data for UM xenoliths
LREE depleted
or unfractionated
LREE enriched
Depleted types (+) slope
Fertile types (-) slope
Enriched?
Chondrite-normalized REE diagrams for spinel (a) and garnet (b) lherzolites. After Basaltic Volcanism Study Project (1981). Lunar and Planetary Institute.

34. Review of Sr isotopes 87Rb ® 87Sr l = 1.42 x 10-11 a
Rb (parent) conc. in enriched reservoir (incompatible)
Enriched reservoir
develops more
87Sr over time
Depleted reservoir
(less Rb)
develops less
87Sr over time
After Wilson (1989). Igneous Petrogenesis. Unwin Hyman/Kluwer.

35. Review of Nd isotopes 147Sm ® 143Nd l = 6.54 x 10-13 a
Nd Sm
REE diagram
147Sm ® 143Nd l = 6.54 x a
Nd (daughter) ® enriched reservoir > Sm
Enriched res.
develops less
143Nd over time
Depleted res.
(higher Sm/Nd)
develops higher
143Nd/144Nd
over time
After Wilson (1989). Igneous Petrogenesis. Unwin Hyman/Kluwer.

36. mantle model II Upper depleted mantle = MORB source
Lower undepleted & enriched OIB source
Layered mantle
Upper depleted mantle = MORB source
depleted by MORB extraction > 1 Ga
Lower = undepleted & enriched OIB source
Boundary = 670 km phase transition
Sufficient D density to impede convection so they convect independently
It is interesting to note that this concept of a layered mantle was initiated by the REE concentrations of oceanic basalts
Later support came from isotopes and geophysics
After Basaltic Volcanism Study Project (1981). Lunar and Planetary Institute.

37. Mantle convection model I
Homogeneous mantle
Large-scale convection (drives plate tectonics?)
After Basaltic Volcanism Study Project (1981). Lunar and Planetary Institute.

38. Nd and Sr isotopes of Ocean Basalts
“Mantle Array”
MORB at depleted end
Tahiti, Gough, and Kerguelen at enriched end
Truly enriched over Bulk Earth
Array = mixing line?
Two components mixed
How mixed? As liquids?
Initial 143Nd/144Nd vs. 87Sr/86Sr for oceanic basalts. From Wilson (1989). Igneous Petrogenesis. Unwin Hyman/Kluwer. Data from Zindler et al. (1982) and Menzies (1983).

43. OAHU
Pali

46. Significance of the
Koolau component
From Lassiter &Hauri, 1996

47. Koolau component-recycled crust?
Eiler et al., 1996
Lassiter and Hauri, 1998

48. a b d c 2mm (a,b,c), 1mm (d) Plg-lherzolite-a,b,c, Sp lherzolite-d
Take a look at hand specimen too!

52. Sr-Nd isotopes
@Pali, Salt Lake Crater
& Koolau

53. Nd and Sr isotopes of Kimberlite Xenoliths
Much larger variation
Especially Sr
Sub-continental lithospheric mantle may be highly enriched
Especially in Rb?
What does this tell us about the mantle?
Initial 143Nd/144Nd vs. 87Sr/86Sr for mantle xenoliths. From Wilson (1989). Igneous Petrogenesis. Unwin Hyman/Kluwer. Data from Zindler et al. (1982) and Menzies (1983).

54. Dm , bse, em1, em2, himu

55. Chemical dynamics- a word of caution
LAB should be defined based on rheology not chemistry; T=1250 C is where olivine starts behaving ductily;
Asthenosphere becomes lithosphere and viceversa - thus chemistry is not a good indicator;
Small enriched domains in a chemically heterogenous mantle can supply most melts if F is small.
Can be c

57. Experiments on melting enriched vs. depleted mantle samples:
Tholeiite easily created
by 10-30% PM
More silica saturated
at lower P
Grades toward alkalic
at higher P
Results of partial melting experiments on depleted lherzolites. Dashed lines are contours representing percent partial melt produced. Strongly curved lines are contours of the normative olivine content of the melt. “Opx out” and “Cpx out” represent the degree of melting at which these phases are completely consumed in the melt. After Jaques and Green (1980). Contrib. Mineral. Petrol., 73,

58. Experiments on melting enriched vs. depleted mantle samples:
2. Enriched Mantle
Tholeiites extend to
higher P than for DM
Alkaline basalt field
at higher P yet
And lower % PM
Results of partial melting experiments on fertile lherzolites. Dashed lines are contours representing percent partial melt produced. Strongly curved lines are contours of the normative olivine content of the melt. “Opx out” and “Cpx out” represent the degree of melting at which these phases are completely consumed in the melt. The shaded area represents the conditions required for the generation of alkaline basaltic magmas. After Jaques and Green (1980). Contrib. Mineral. Petrol., 73,

59. Need to parametrize melting
Will do this for dry melting only;
Aim to explain major elements;
Assume adiabatic melting;
Need a melting function;
Need a start depth and an end depth;
Assume that SiO2 does not change much;
Use fractionall melting in increments of 1 kbar

60. Parametrization Melting is linear as a function of depth;
Source is only peridotite;
Shape of melting domain is triangular; no extra wings to scavenge traces;
Based on McKenzie and Bickle (1988); Langmuir et al. (1992) and Wang et al. (2002).

63. The important thing to note is that in the triangular region, where melting occurs, the percent of partial melting increases closer to the ridge. So as you move up in the diagram, the mantle left behind (residual mantle column) gets more depleted because more melt was extracted from it. This melting pattern predicts the most depleted mantle in the shallowest oceanic mantle.

64. Depth of melt sources with respect to distance away from a mid ocean spreading center (left side). Petrologists have attempted applying techniques developed from oceanic spreading centers to continental extensional regions such as the western US Basin and Range with mixed results.

65. Assumptions Ti is used as a perfectly incompatible element;
Fe and Na will constrain the depth where melting starts and the length of melting column respectively;
Thickness of melt column is also calculated (e.g. for MORB it should be 6 km);
K influence the calculation - I forget why.

66. Comparing against data
Plot the major elements of your set against MgO (Harker type diagrams);
Find the FeO, Na2O, TiO2 and K2O corresponding to the most primitive composition;
Those are the values to compare against the forward model;
Works for any adiabatic melting assuming that only peridotite is the source. You can mess with fertility (% cpx source), amount of MgO, Na2O, K2O, FeO in source.

67. Na2O=2.8
FeO=9

70. Best match Start at 23 kbar Stop at 15 kbar
8 kbar column of melt, stops exactly at crust -mantle boundary (about 50 km under the Puna);
Predicts 2.5 km of basalt accumulated in the crust; average melting 7%;
Is this any good?

71. Hits solidus at around 1450 C

72. Other constraints Use ol-glass thermometer for magma temp;
Get xenoliths to find out how depleted/fertile a peridotite from under the Puna is;
Crustal and lithospheric thickness constraints from seismo people;

73. HW8
Use Blondes et al major element data for the Papoose flows only to determine the FeO and Na2O corresponding to the most primitive MgO;
Use LPK model to determine the melt starting pressure, ending pressure, melt thickness and average F