Flavors of basalts Flavours of basalts Alkaline Sub-alkaline - - PowerPoint PPT Presentation

▶

Sep 23, 2022 15 likes •185 views

Flavors of basalts Flavours of basalts Alkaline Sub-alkaline Tholeiitic Calc-alkaline Mid Ocean Ridge basalts (MORBs) Ocean Island basalts (OIBs) Arc basalts Figure 16-3.

SLIDE 1

Flavors of basalts

SLIDE 2

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

SLIDE 3

Ocean islands in the Atlantic

Figure 14.3. After Wilson (1989) Igneous Petrogenesis. Kluwer.

See mechanisms of magma Differentiation on next lecture…

SLIDE 4

MORBs vs. OIBs

ü Mid-Ocean Ridge vs. Ocean Island

SLIDE 5

Mid-Ocean Ridge System

SLIDE 6

Oceanic Intraplate Volcanism

SLIDE 7

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).

SLIDE 8

Calc-alkaline (arc) basalts

SLIDE 9

Crater Lake vs. Skaergaard intrusion

Volcanic Arc Magma chamber in hot spot-rifting environment

SLIDE 10

Calc-alkaline basalts

Addition of Fluid-mobile elements

SLIDE 11

AFM diagram

Tholeiitic – hot spot Divergent margins Calc-alkaline Subduction zones (arcs) Basalts

SLIDE 12

Alkaline basalts – in every tectonic settings

Figure 14.3. After Wilson (1989) Igneous Petrogenesis. Kluwer.

SLIDE 13

Questions

ü Difference between OIBs and MORBs? ü Difference between calc-alkaline basalts (arcs) and MORBs? ü Alkaline basalts?

SLIDE 14

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

SLIDE 15

Alkaline basalts?

ü Low degrees of mantle melting

Ø Enrichments in melt-loving elements (incompatible elements)

SLIDE 16

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 (kbars) 50 100 150 Depth (km) 1100 1200 1300 1400 1500 (TºC)

Alkaline magmas: Low degrees of melting

SLIDE 17

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

r tholeiites

Continental Arc basalts (rare) Calc-alkaline From fluxed depleted mantle wedge Island Arc basalts Calc-alkaline From fluxed depleted mantle wedge