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 AFM diagram! Basalts 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 (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! 1100 1200 1300 1400 1500 (TºC) % melting (and P) solidus determine the 10 Spinel lherzolite 20% composition of 50 (Ol-opx-cpx-sp) 1% the basaltic 20% 20 magma produced 10% Garnet lherzolite 1% (Ol-opx-cpx-gar) 30 P (kbars) P Depth(km) 100 40 Alkaline magmas: Graphite Low degrees of Diamond 50 melting 150 60 Flavors of basalts! OIB = Alkaline magmas or tholeiites From deeper melting of more primitive mantle MORB Island Arc basalts Tholeiites from shallow Calc-alkaline Continental Arc basalts Melting of depleted (rare) Calc-alkaline From fluxed depleted mantle mantle wedge From fluxed depleted mantle wedge Continental hot spots and rifts: Alkaline magmas or tholeiites .