1 Petrology Lecture 6 Generation of Basaltic Magma GLY Spring, 2012

2 Magma Series Definition A group of rocks that share some chemical, and sometimes mineralogical, characteristics They share patterns on chemical variation diagrams which suggests a genetic relationship – beyond the scope of this course

3 Major Magma Series J.P. Iddings, 1892  Alkaline  Sub-alkaline C.E. Tilley (1950) split sub-alkaline into:  Tholeittic  Calc-alkaline

4 Relationship of Magma Series to Plate Tectonics

5 Geotherms Estimated ranges of oceanic (hatched) and continental (solid) steady-state geotherms to a depth of 100 km using upper and lower limits based on heat flows measured near the surface

6 Sources of Upper Mantle Rocks Ophiolites Dredge Samples from oceanic fracture zones Nodules in basalts  Autoliths  Restites Xenoliths in kimberlites Stony meteorites – analogous to upper mantle of a broken planet

7 Basaltic Partial Melt TiO 2 vs, Al 2 O 3 Extraction of a basaltic partial melt from lherzolite can result in solid refractory harzburgite or dunite Brown and Mussett, A. E. (1993), The Inaccessible Earth: An Integrated View of Its Structure and Composition. Chapman & Hall/Kluwer.

Lherzolite: A type of peridotite with Olivine > Opx + Cpx 8 Olivine Clinopyroxene Orthopyroxene Lherzolite Harzburgite Wehrlite Websterite Orthopyroxenite Clinopyroxenite Olivine Websterite Peridotites Pyroxenites Dunite Figure 2.2 C After IUGS

9 Aluminous Lherzolite Phase Diagram Al – phase  Plagioclase o shallow (< 50 km)  Spinel o km  Garnet o km  Si  VI coord. o > 400 km Figure 10.2 Phase diagram of aluminous lherzolite with melting interval (gray), sub-solidus reactions, and geothermal gradient. After Wyllie, P. J. (1981). Geol. Rundsch. 70,

Mantle Melting 10 Increase in temperature Problem: No realistic mechanism Perhaps with local hot spots, with very limited area

11 Decompression Melting Decompression melting under adiabatic conditions When adiabat crosses solidus, melting begins Dashed lines represent approximate % melting At least 30% melting is realistic

Addition of Volatiles 12 Figure Dry peridotite solidus compared to several experiments on H 2 O-saturated peridotites

13 Effect of Pressure on Initial Melting Change in eutectic position with increasing pressure First melting occurs at the eutectic After Kushiro, 1968

14 Pyrolite Melting Nature of the liquids and refractory residua associated with partial melting of pyrolite After Green and Ringwood (1967)

15 Fractional Crystallization of Basaltic Magmas Minerals fractionating are listed near arrows After Wyllie, 1971

16 Partial Melting and Fractional Crystallization

17 Trends in Partial Melts Spinel lherzolites, from 6 to 35% partial melt Low melt leads to alkaline basalt, higher % melts to more tholeittic compositions After Hirose and Kushiro, 1991

18 Basalt Petrogenesis

19 Magma Types Primary Magma - one that forms by melting at depth, without any later modification Derivative - A primary magma that has been modified by some magma differentiation process on the way to the surface Parental - Most primitive magma type within a given magma series, it may or may not be primary

Multiple saturation l Low P  Ol then Plag then Cpx as cool  ~70 o C T range Figure Anhydrous P-T phase relationships for a mid-ocean ridge basalt suspected of being a primary magma. After Fujii and Kushiro (1977). Carnegie Inst. Wash. Yearb., 76,

l Low P  Ol then Plag then Cpx as cool  70 o C T range l High P  Cpx then Plag then Ol Multiple saturation Figure Anhydrous P-T phase relationships for a mid-ocean ridge basalt suspected of being a primary magma. After Fujii and Kushiro (1977). Carnegie Inst. Wash. Yearb., 76,

l Low P F Ol then Plag then Cpx as cool F 70 o C T range l High P F Cpx then Plag then Ol l 25 km get all at once  = Multiple saturation  Suggests that 25 km is the depth of last equilibrium with the mantle Multiple saturation

23 OIB and MORB Upper, REE diagram Lower, Spider diagram Data from Sun and McDonough, 1989

24 REE Variation Chondrite- normalized REE diagrams for spinel and garnet lherzolites After Basaltic Vocanism Study Project, 1981 LREE enriched LREE depleted or unfractionated LREE depleted or unfractionated LREE enriched

Nd/ 144 Nd vs. 87 Sr/ 86 Sr Upper, oceanic basalts Lower, ultramafic xenoliths from subcontinental mantle MAR = Mid-Atlantic Ridge EPR = East Pacific Rise IR = Indian Ocean Ridge

26 Mantle Convection Models After Basaltic Vocanism Study Project, 1981

27 Partial Melting Experiments Left, depleted lherzolites; Right, fertile lherzolites Dashed lines = % partial melt produced Shaded area = condition required for the generation of alkaline basaltic magmas “Opx out” and “Cpx out” = degree of melting at which these phases are completely consumed

28 Changing ΔV ΔV approaches zero on going from solid to liquid as pressure increases Causes both solidus and liquidus to change slope as depth increases