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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.

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

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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.

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

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

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.

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.

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