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

Flavors of basalts �

Flavours of basalts � ü Alkaline � ü Sub-alkaline � ü Tholeiitic � ü Calc-alkaline � ü Mid Ocean Ridge basalts (MORBs) � ü Ocean Island basalts (OIBs) � ü Arc basalts � Figure 16-3. Data compiled by Terry Plank (Plank and Langmuir, 1988) Earth Planet. Sci. Lett. , 90, 349-370.

Ocean islands in the Atlantic See mechanisms of magma Differentiation on next lecture … Figure 14.3. After Wilson (1989) Igneous Petrogenesis. Kluwer.

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 � Magma chamber in hot spot-rifting environment Volcanic Arc

Calc-alkaline basalts � Addition of Fluid-mobile elements

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

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 (f O2 ) 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 � 1500 (TºC) 1100 1200 1300 1400 % melting (and P) solidus determine the 10 composition of Spinel lherzolite 20% (Ol-opx-cpx-sp) the basaltic 50 1% Depth (km) 20 20% magma produced P (kbars) 10% Garnet lherzolite 1% (Ol-opx-cpx-gar) 30 100 Alkaline magmas: 40 Low degrees of Graphite melting Diamond 50 150 60

Flavors of basalts � OIB = Alkaline magmas or tholeiites From deeper melting of more primitive mantle MORB Island Arc basalts Tholeiites from shallow Continental Arc basalts Calc-alkaline Melting of depleted (rare) Calc-alkaline From fluxed depleted mantle From fluxed depleted mantle wedge mantle wedge Continental hot spots and rifts: Alkaline magmas or tholeiites