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Flavors of basalts!
Flavours of basalts!
Figure 16-3. Data compiled by Terry Plank (Plank and Langmuir, 1988) Earth Planet. Sci. Lett., 90, 349-370.
ü Alkaline!ü Sub-alkaline!
ü Tholeiitic !ü Calc-alkaline!
ü Mid Ocean Ridge basalts (MORBs)!
ü Ocean Island basalts (OIBs)!ü Arc basalts!
Ocean islands in the Atlantic
Figure 14.3. After Wilson (1989) Igneous Petrogenesis. Kluwer.
See mechanisms of magma Differentiation on next lecture…
MORBs vs. OIBs!
ü Mid-Ocean Ridge vs. Ocean Island !
Mid-Ocean Ridge System!
Oceanic Intraplate Volcanism!
MORB vs. OIB!
ü Similar major element concentrations!ü Difference in trace element abundance!
increasing incompatibility Figure 10-13b. 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).
Calc-alkaline (arc) basalts!
Crater Lake vs. Skaergaard intrusion!
Volcanic Arc
Magma chamber in
hot spot-rifting environment
Calc-alkaline basalts!
Addition of Fluid-mobile elements
AFM diagram!
Tholeiitic – hot spot Divergent margins
Calc-alkaline Subduction zones (arcs)
Basalts
Alkaline basalts – in every tectonic settings
Figure 14.3. After Wilson (1989) Igneous Petrogenesis. Kluwer.
Questions!
ü Difference between OIBs and MORBs?!ü Difference between calc-alkaline basalts
(arcs) and MORBs?!ü Alkaline basalts?!
Style of mantle melting!
ü CA basalts => Flux melting!ü High water content and higher O concentration (fO2)
from slab dehydration!ü Early crystallization of oxide phases (low Fe, Ti, Nb, Ta in residual
magmas)!
ü OIBs and MORBs => Decompression melting!ü Melting at shallow depths form depleted mantle
(previously melted)!ü MORB!
ü Melting deeper from a more enriched mantle!ü OIB!
Alkaline basalts?!
ü Low degrees of mantle melting!Ø Enrichments in melt-loving elements (incompatible
elements)!
Amount of melting and depth of melting!
% melting (and P) determine the composition of
the basaltic magma produced
Graphite Diamond
solidus
Spinel lherzolite (Ol-opx-cpx-sp)
Garnet lherzolite (Ol-opx-cpx-gar)
20%
1%
20%
10%
1%
10
20
30
40
50
60 P
(kba
rs)
50
100
150
Dep
th (k
m)
1100 1200 1300 1400 1500 (TºC)
Alkaline magmas: Low degrees of
melting
Flavors of basalts!
MORB Tholeiites from shallow Melting of depleted mantle
OIB = Alkaline magmas or tholeiites From deeper melting of more primitive mantle
Continental hot spots and rifts:
Alkaline magmas or tholeiites
Continental Arc basalts (rare) Calc-alkaline From fluxed depleted mantle wedge
Island Arc basalts Calc-alkaline From fluxed depleted mantle wedge