General

1
Mantle Petrology and the Origin of Basaltic Magma
2
ALL basaltic magmas are partial melts of the
MANTLE
3
Lherzolite A type of peridotite with Olivine gt
Opx Cpx
Olivine
Dunite
90
Peridotites
Wehrlite
Lherzolite
Harzburgite
40
Pyroxenites
Olivine Websterite
Orthopyroxenite
10
Websterite
10
Clinopyroxenite
Orthopyroxene
Clinopyroxene
Figure 2-2 C After IUGS
4
Lherzolite is considered fertile mantle ?
capable of producing basalt Dunite and
harzburgite are refractory residuum after basalt
has been extracted by partial melting ?
infertile, incapable of producing basalt
Tholeiitic basalt
15
Partial Melting
10
Wt. Al2O3
5
Lherzolite
Harzburgite
Residuum
Dunite
0
0.8
0.4
0.6
0.2
0.0
Wt. TiO2
5
Fertile mantle Four-phase lherzolite
(Ol Opx Cpx Al-rich phase)
Al-phase is P sensitive

• Plagioclase
• shallow (lt30 km)
• Spinel
• 30-70 km
• Garnet
• gt70 km

6
(No Transcript)
7
Depth of melting from REE patterns

10.00
60 Ol 15 Opx 15 Cpx 10Plag
8.00
6.00
sample/chondrite
4.00
2.00
0.00
La Ce Nd Sm Eu Tb Er Yb
Lu
10.00
67 Ol 17 Opx 14 Cpx 3 Sp
8.00
6.00
sample/chondrite
4.00
2.00
0.00
56
58
60
62
64
66
68
70
72
La Ce Nd Sm Eu Tb Er Yb
Lu
8
How does the mantle melt??

• 1) Increase the temperature
• 2) Decrease the pressure
• 3) Add volatiles (H2O, CO2)

9
How does the mantle melt??

• 1) Increase the temperature

10
Decompression Melting
(just at melting temp)
(200C below melting temp)
11

• 2) Lower the pressure
• Adiabatic rise of mantle with no conductive heat
  loss
• Decompression melting could melt at least 30

12
Addition of Fluids
Lowers melting T
Causes partial melting
13

• 3) Add volatiles (especially H2O)

14
Basaltic melts can be created under realistic
circumstances

• Decompression melting
• Divergent boundaries plates separate and mantle
  rises at mid-ocean ridges, MORB
• Intraplate hot spots rise of localized mantle
  plumes, OIB
• Fluid fluxing
• important in subduction zones

15
Primary magmas

• Mantle melts with no subsequent modification by
  fractional crystallization or assimilation
• Criteria
• MgO 12 17 wt
• Cr gt 1000 ppm
• Ni gt 400-500 ppm
• Rarely observed, Holy Grail of basalts, very
  important for petrologists and geochemists,
  fingerprint of mantle source!

16
Is the Mantle Chemically Homogeneous??
17
REE data for oceanic basalts
increasing incompatibility
18
Multi-element diagram for oceanic basalts
increasing incompatibility
19
REE data for mantle xenoliths
Figure 10-14 Chondrite-normalized REE diagrams
for spinel (a) and garnet (b) lherzolites. After
Basaltic Volcanism Study Project (1981). Lunar
and Planetary Institute.
20
Is the Mantle Chemically Homogeneous??
NO!!!
21
Mantle Sources and Structure

• Upper depleted mantle MORB source
• Lower undepleted enriched OIB source

22
Two principal types of basalt in the ocean basins
Tholeiitic Basalt and Alkaline Basalt
23

• Tholeiites are lower pressure and larger of
  partial melting

24

• Alkaline basalts are higher pressure and smaller
  of partial melting