origin&evolution of magmas

Textural Interpretations from Phase Textural Interpretations from Phase DiagramsDiagrams

Binary Systems with Binary Systems with Congruent MeltingCongruent Melting

Plagioclase forms before augite– Plagioclase forms before augite– DiabasicDiabasic tecture tectureThis forms on the right side of the eutectic

Binary Systems with Congruent MeltingBinary Systems with Congruent Melting

Textural Interpretations from Phase DiagramsTextural Interpretations from Phase Diagrams

Diabase dike

Left of the eutectic get a similar situation

Textural Interpretations from Phase Diagrams


Augite forms before plagioclase – Augite forms before plagioclase – OphiticOphitic tecture tectureThis forms on the left side of the eutectic

Gabbro of the Stillwater Complex, Montana




Binary Systems with Binary Systems with Incongruent MeltingIncongruent Melting

Olivine cores with pyroxene reaction rims


Binary Systems with Binary Systems with Solid SolutionSolid Solution

CaCa

NaNa


Zoned plagioclaseZoned plagioclase

Diversification of MagmasDiversification of Magmas

Evolution of MagmasEvolution of Magmas

Why do we get so much variation in Why do we get so much variation in igneous rocks?igneous rocks?


Types of Magmas:Types of Magmas:• PrimitivePrimitive• PrimaryPrimary• ParentParent• DerivativeDerivative

Magmatic DifferentiationMagmatic Differentiation

Any process by which a magma is Any process by which a magma is able to diversify and produce a able to diversify and produce a magma or rock of different magma or rock of different compositioncomposition



Ways to produce variation:Ways to produce variation:• different source rocksdifferent source rocks• partial melt fractionationpartial melt fractionation

The Ab-Or-Qtz system with the ternary The Ab-Or-Qtz system with the ternary cotectic curves and eutectic minima from cotectic curves and eutectic minima from 0.1 to 3 GPa. Included is the locus of most 0.1 to 3 GPa. Included is the locus of most granite compositions from Figure 11-2 granite compositions from Figure 11-2 (shaded) and the plotted positions of the (shaded) and the plotted positions of the norms from the analyses in Table 18-2. norms from the analyses in Table 18-2. Note the effects of increasing pressure Note the effects of increasing pressure and the An, B, and F contents on the and the An, B, and F contents on the position of the thermal minima.position of the thermal minima. From From Winter (2001) An Introduction to Igneous Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prenticeand Metamorphic Petrology. Prentice Hall.Hall.


Partial melt Partial melt fractionation:fractionation:

Melting begins at Melting begins at cotectic – more cotectic – more melting means melting means greater divergence greater divergence from cotectic melt.from cotectic melt.


Ways to produce variation:Ways to produce variation:• different source rocksdifferent source rocks• partial melt fractionationpartial melt fractionation• fractional crystallizationfractional crystallization

Evolution of MagmasEvolution of MagmasBowen’s Reaction Bowen’s Reaction

Series:Series:


Types of FX:Types of FX:• gravity settlinggravity settling• filter pressingfilter pressing• convective fractionation convective fractionation • congelation crystallizationcongelation crystallization• flow differentiationflow differentiation


Ways to produce variation:Ways to produce variation:• different source rocksdifferent source rocks• partial melt fractionationpartial melt fractionation• fractional crystallizationfractional crystallization• assimilationassimilation• diffusion/volatile transferdiffusion/volatile transfer• magma mixingmagma mixing• post-solidification alterationpost-solidification alteration

Origin of MagmasOrigin of Magmas

ContinentalContinental

MORBsMORBsSubductionSubduction


Composition of the Mantle:Composition of the Mantle:• ocean crust 6-8km, continental crust ocean crust 6-8km, continental crust

30+ km30+ km• mantle: base of crust to 2900km mantle: base of crust to 2900km • crust+mantle to 70km: lithospherecrust+mantle to 70km: lithosphere• 70 – 145km: asthenosphere70 – 145km: asthenosphere


Composition of the Mantle:Composition of the Mantle:• peridotite: olivine + pyroxeneperidotite: olivine + pyroxene• eclogite: pyroxene + garneteclogite: pyroxene + garnet

• Low pressures (~30km): olivine, Al-poor pyroxene, Low pressures (~30km): olivine, Al-poor pyroxene, plagioclaseplagioclase

• Moderate pressures (30-70km: olivine, Al-rich pyroxene, Moderate pressures (30-70km: olivine, Al-rich pyroxene, spinel (MgAlspinel (MgAl22OO44))

• High pressures (>70km): olivine, Al-poor pyroxene, garnetHigh pressures (>70km): olivine, Al-poor pyroxene, garnet

Lherzolite: A type of peridotite with Olivine > Opx + Cpx

OlivineOlivine

ClinopyroxeneClinopyroxeneOrthopyroxeneOrthopyroxene

LherzoliteLherzoliteH

arzb

urgi

teW

ehrlite

Websterite

OrthopyroxeniteOrthopyroxenite

ClinopyroxeniteClinopyroxenite

Olivine Websterite

PeridotitesPeridotites

PyroxenitesPyroxenites

90

40

10

10

DuniteDunite

After IUGSAfter IUGS



Can the mantle melt under normal heat flow?Can the mantle melt under normal heat flow?

Answer:Answer:

NO!NO!

How does the mantle melt??How does the mantle melt??1) Increase the temperature1) Increase the temperature

Melting by raising the temperature.Melting by raising the temperature.


SolidusSolidusSolidusSolidus

LiquidusLiquidus

May work for May work for Hot Spots,Hot Spots,Unlikely Unlikely

elsewhereelsewhere

2) Lower the pressure2) Lower the pressure– Adiabatic rise of mantle with no conductive heat lossAdiabatic rise of mantle with no conductive heat loss– Decompression melting could melt at least 30%Decompression melting could melt at least 30%

Melting by (adiabatic) pressure reduction. Melting begins when the adiabat crosses the solidus and traverses the shaded melting interval. Dashed lines represent approximate % melting.

Origin of MagmasOrigin of MagmasHow does the mantle melt??How does the mantle melt??

Probably Probably what what happens at happens at spreading spreading centerscenters

3) Add volatiles3) Add volatiles (especially H(especially H22O)O)

Dry peridotite solidus compared to several experiments on H2O-saturated peridotites.


How does the mantle melt??How does the mantle melt??

Probably what Probably what happens at happens at subduction subduction zoneszones


MORBs:MORBs:


MORBs:MORBs:

Primitive melt is olivine Primitive melt is olivine tholeiite; dunites, tholeiite; dunites, pyroxenites, anorthosites pyroxenites, anorthosites and alkaline basalts are and alkaline basalts are differentiatesdifferentiates

Oceanic Crust and Oceanic Crust and Upper Mantle Upper Mantle

StructureStructure

Typical OphioliteTypical Ophiolite

Lithology and thickness of a Lithology and thickness of a typical ophiolite sequence, based typical ophiolite sequence, based on the Samial Ophiolite in Oman. on the Samial Ophiolite in Oman. After Boudier and Nicolas (1985) After Boudier and Nicolas (1985) Earth Planet. Sci. Lett., 76, 84-92. Earth Planet. Sci. Lett., 76, 84-92.


Layer 1

A thin layer of pelagic

sediment

Oceanic Crust and Upper Mantle StructureOceanic Crust and Upper Mantle Structure

Modified after Brown and Modified after Brown and Mussett (1993) Mussett (1993) The Inaccessible The Inaccessible Earth: An Integrated View of Earth: An Integrated View of Its Structure and Composition. Its Structure and Composition. Chapman & Hall. London.Chapman & Hall. London.


Layer 2 is basaltic

Subdivided into two sub-layers

Layer 2A & B = pillow basalts

Layer 2C = vertical sheeted dikes

Oceanic Crust and Upper Mantle StructureOceanic Crust and Upper Mantle Structure

Modified after Brown and Mussett (1993) The Inaccessible Earth: An Integrated View of Its Structure and Composition. Chapman & Hall. London.


Layer 3Layer 3 more complex and controversialmore complex and controversialBelieved to be mostly gabbros, crystallized from a shallowBelieved to be mostly gabbros, crystallized from a shallow axial magma chamberaxial magma chamber (feeds the dikes and basalts)(feeds the dikes and basalts)

Layer 3A = upper isotropic and lower, somewhat foliated (“transitional”) gabbros

Layer 3B is more layered, & may exhibit cumulate textures


Layer 4Layer 4 = ultramafic rocks = ultramafic rocks

Ophiolites: base of 3B grades into layered cumulate wehrlite & gabbro

Wehrlite intruded into layered gabbros

Below cumulate dunite with harzburgite xenoliths

Below this is a tectonite harzburgite and dunite (unmelted residuum of the original mantle)



Massive sulfide depositsMassive sulfide deposits

A more modern concept of the axial magma chamber beneath a fast-spreading ridge

After Perfit et After Perfit et al. (1994) al. (1994) Geology, 22, Geology, 22, 375-379.375-379.


Any model for the origin of magmas at subduction Any model for the origin of magmas at subduction zones must account for:zones must account for:

11. First stages of . First stages of volcanism volcanism tholeiitictholeiitic, then , then changes to changes to calcalkalinecalcalkaline

F

A M

Calc-alkaline

T

ho leiitic


Any model for the origin of magmas at subduction Any model for the origin of magmas at subduction zones must account for:zones must account for:

2. Trend from 2. Trend from tholeiitictholeiitic volcanism nearest trench to volcanism nearest trench to calcalkalinecalcalkaline towards towards continentcontinent

tholeiitictholeiitic calcalkalinecalcalkaline


A model for the origin of magmas at subduction A model for the origin of magmas at subduction zones.zones.

Ringwod (1974)

Early Phases1. Basalt 1. Basalt metamorphoses to metamorphoses to amphibolite; amphibolite; dunite alters to dunite alters to serpentiniteserpentinite

2. Amphibolites 2. Amphibolites dehydrate, H2O dehydrate, H2O triggers melting triggers melting of overlying of overlying mantle producing mantle producing tholeiitic tholeiitic magmas; magmas; dehydrated slab dehydrated slab becomes eclogitebecomes eclogite


A model for the origin of magmas at subduction A model for the origin of magmas at subduction zones.zones.

Ringwod (1974)

Later Phases

1. Dehydration of 1. Dehydration of serpentinite bodies; H2O serpentinite bodies; H2O causes partial melting of causes partial melting of eclogite to form eclogite to form rhyodacite-dacite rhyodacite-dacite magma magma

2. Magma reacts with 2. Magma reacts with overlying mantle –forms overlying mantle –forms less dense garnet less dense garnet pyroxenite; diapiric rise pyroxenite; diapiric rise initiates partial melting; initiates partial melting; fractionation produces fractionation produces calc-alkaline magmascalc-alkaline magmas


Continental Magmas:Continental Magmas:• alkaline rocksalkaline rocks• carbonatitescarbonatites• kimberlites kimberlites • anorthositesanorthosites• gabbroic layered intrusionsgabbroic layered intrusions• anorogenic granitesanorogenic granites

Schematic cross section of an asthenospheric mantle plume beneath a continental rift environment, and the genesis of nephelinite-carbonatites and kimberlite-carbonatites . Numbers correspond to Figure 19-13. After Wyllie (1989, Origin of carbonatites: Evidence from phase equilibrium studies. In K. Bell (ed.), Carbonatites: Genesis and Evolution. Unwin Hyman, London. pp. 500-545) and Wyllie et al., (1990, Lithos, 26, 3-19). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.


Continental Alkaline Magmatism:Carbonatites

Model of an idealized kimberlite system, illustrating the hypabyssal dike-sill complex leading to a diatreme and tuff ring explosive crater. This model is not to scale, as the diatreme portion is expanded to illustrate it better. From Mitchell (1986) Kimberlites: Mineralogy, Geochemistry, and Petrology. Plenum. New York. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Continental Alkaline Magmatism:

Kimberlites


a. Mantle-derived magma underplates the crust as it becomes density a. Mantle-derived magma underplates the crust as it becomes density equilibrated.equilibrated.


A model for the origin of anorthositesA model for the origin of anorthosites

b. Crystallization of mafic phases (which sink), and partial melting of the Crystallization of mafic phases (which sink), and partial melting of the crust above the ponded magma. The melt becomes enriched in Al and crust above the ponded magma. The melt becomes enriched in Al and Fe/Mg.Fe/Mg.



c. Plagioclase forms when the melt is sufficiently enriched. Plagioclase c. Plagioclase forms when the melt is sufficiently enriched. Plagioclase rises to the top of the chamber whereas mafics sink.rises to the top of the chamber whereas mafics sink.



d. Plagioclase accumulations become less dense than the crust above d. Plagioclase accumulations become less dense than the crust above and rise as crystal mush plutons. and rise as crystal mush plutons.



e. Plagioclase plutons coalesce to form massif anorthosite, whereas e. Plagioclase plutons coalesce to form massif anorthosite, whereas granitoid crustal melts rise to shallow levels as well. Mafic cumulates granitoid crustal melts rise to shallow levels as well. Mafic cumulates remain at depth or detach and sink into the mantle.remain at depth or detach and sink into the mantle.


origin&evolution of magmas

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