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