1 petrology lecture 6 generation of basaltic magma gly 4310 - spring, 2012
TRANSCRIPT
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Petrology Lecture 6
Generation of Basaltic Magma
GLY 4310 - Spring, 2012
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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
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Major Magma Series
• J.P. Iddings, 1892 Alkaline Sub-alkaline
• C.E. Tilley (1950) split sub-alkaline into: Tholeittic Calc-alkaline
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Relationship of Magma Series to Plate Tectonics
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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
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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
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Basaltic Partial Melt• TiO2 vs, Al2O3 • 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.
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Lherzolite: A type of peridotite with Olivine > Opx + Cpx
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Olivine
ClinopyroxeneOrthopyroxene
Lherzolite
Har
zbur
gite
Wehrlite
Websterite
Orthopyroxenite
Clinopyroxenite
Olivine Websterite
Peridotites
Pyroxenites
90
40
10
10
Dunite
Figure 2.2 C After IUGS
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Aluminous Lherzolite Phase Diagram
• Al – phase Plagioclase
o shallow (< 50 km) Spinel
o 50-80 km Garnet
o 80-400 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, 128-153.
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Mantle Melting
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•Increase in temperature•Problem: No realistic mechanism•Perhaps with local hot spots, with very limited area
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Decompression Melting
• Decompression melting under adiabatic conditions• When adiabat crosses solidus, melting begins• Dashed lines represent approximate % melting• At least 30% melting is realistic
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Addition of Volatiles
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Figure 10.5. Dry peridotite solidus compared to several experiments on H2O-saturated peridotites
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Effect of Pressure on Initial Melting
• Change in eutectic position with increasing pressure
• First melting occurs at the eutectic
• After Kushiro, 1968
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Pyrolite Melting
• Nature of the liquids and refractory residua associated with partial melting of pyrolite
• After Green and Ringwood (1967)
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Fractional Crystallization
of Basaltic Magmas
• Minerals fractionating are listed near arrows
• After Wyllie, 1971
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Partial Melting and Fractional Crystallization
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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
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Basalt Petrogenesis
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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
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Multiple saturationMultiple saturation
Low P Ol then Plag then Cpx as
cool ~70oC T range
Figure 10.13 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, 461-465.
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Low P Ol then Plag then Cpx as
cool 70oC T range
High P Cpx then Plag then Ol
Multiple saturationMultiple saturation
Figure 10.13 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, 461-465.
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Low P Ol then Plag then Cpx as
cool 70oC T range
High P Cpx then Plag then Ol
25 km get all at once = Multiple saturation Suggests that 25 km is the
depth of last equilibrium with the mantle
Multiple saturationMultiple saturation
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OIB and MORB
• Upper, REE diagram
• Lower, Spider diagram
• Data from Sun and McDonough, 1989
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REE Variation
• Chondrite-normalized REE diagrams for spinel and garnet lherzolites
• After Basaltic Vocanism Study Project, 1981
LREE enriched
LREE depletedor unfractionated
LREE depletedor unfractionated
LREE enriched
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143Nd/144Nd vs. 87Sr/86Sr
• Upper, oceanic basalts
• Lower, ultramafic xenoliths from subcontinental mantle
• MAR = Mid-Atlantic Ridge
• EPR = East Pacific Rise
• IR = Indian Ocean Ridge
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Mantle Convection
Models
• After Basaltic Vocanism Study Project, 1981
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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
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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