PetrologyPetrologyIgneous and Metamorphic

Convection demo “Lava Lamp”Density displacement demo: oil and water are immiscible.Marble Demo :Fractionation

The Texts

• An Introduction to Igneous and

Metamorphic Petrology 1st ed by J. D. Winter• or Principles of Igneous and Metamorphic

Petrology J.D. Winter 2nd ed.

The Earth’s InteriorThe Earth’s InteriorCrust:Oceanic crust

Usually < 10 km

ophiolite suite: list

Continental CrustThicker: 20-90 km average ~35 km

Variable composition but average a granodiorite

O2 -

O2 -

O2 -

O2 -

Si4+

2_25

The Silicate Tetrahedron

The basis of most rock-forming minerals, charge - 4

The Mantle is mostly SilicatesThe Mantle is mostly Silicates

The Earth’s InteriorThe Earth’s InteriorMantle:Mantle:Peridotite (ultramafic)Peridotite (ultramafic)

Upper Mantle Upper Mantle to 410 kmto 410 km

olivine, pyroxenes, spinel - structure minerals, olivine, pyroxenes, spinel - structure minerals, and garnet and garnet

Low Velocity Layer Low Velocity Layer 60-220 km 60-220 km AesthenosphereAesthenosphere

Transition Zone Transition Zone as velocity increases 410 - as velocity increases 410 - 660 km , olivine not stable, replaced by 660 km , olivine not stable, replaced by high P polymorphs with ~ same high P polymorphs with ~ same composition: wadsleyite (beta-spinel composition: wadsleyite (beta-spinel structure), and ringwoodite (gamma-spinel structure), and ringwoodite (gamma-spinel structure) structure)

Lower Mantle Lower Mantle 660 Upper minerals unstable, 660 Upper minerals unstable, perovskite-type structure perovskite-type structure SiSiIVIV Si SiVIVI

Seismic Tomography

The Earth’s InteriorThe Earth’s Interior

Core: Core: Fe-Ni metallic alloyFe-Ni metallic alloy

SulfurSulfur

Outer CoreOuter Core is liquidis liquid No S-wavesNo S-waves

Inner CoreInner Core is solidis solid

Discussions: Differentiation Iron Meteorites, ImpactorDensity and Buoyancy

Source: Recommended Text Kearey and Vine (1990), Global Tectonics.

Note how S-wave velocities drop to zero in the Liquid outer core

LVZ

Upper Mantle SamplesUpper Mantle Samples• Samples of the upper mantle occasionally appear Samples of the upper mantle occasionally appear

where faulting has exposed it in oceanic fracture where faulting has exposed it in oceanic fracture zones, thrust it up in collision zones, or where brought zones, thrust it up in collision zones, or where brought up in diatreme and basalt eruptions. The rock revealed up in diatreme and basalt eruptions. The rock revealed is usually is usually PeridotitePeridotite, which is three-quarters Dunite , which is three-quarters Dunite (pure olivine) and one-quarter basalt. The Basalt forms (pure olivine) and one-quarter basalt. The Basalt forms by the partial melting of this peridotite, which drives off by the partial melting of this peridotite, which drives off the basaltic melt, leaving behind the solid “the basaltic melt, leaving behind the solid “depleteddepleted “ “ dunite (basaltic components removed). dunite (basaltic components removed).

• The original (fertile) mantle has more Al, Ca, Ti, Na, and K and The original (fertile) mantle has more Al, Ca, Ti, Na, and K and lower Mg# = Mg/(Mg +Fe) than Dunitelower Mg# = Mg/(Mg +Fe) than Dunite

So some of the above go into the basalt.

Molten- Molten- VERYVERY Hot HotNo solids

Molten- Not so hotMolten- Not so hot

100% Solid

First mineral to crystallize outIndependent TetrahedraIndependent Tetrahedra

SingleSinglechainschains

DoubleDoublechainschains

sheetssheets

3-D3-D

3-D3-D3-D3-D

“Basaltic”

“Andesitic”

“Granitic”

Fo Mg++ 1900C Fa Fe++ 1500C

1900 oC

1553 oC

3-D3-D

sheets

Fine crystalsNeed a microscope

Course crystalsEasily seen

Low silica, HOT, fluid High silica, warm, viscousIntermediate

Dark Green Gray Pink to SalmonGray

If crystals are left in contact with melt …If crystals are left in contact with melt …

• Ultramafic to Basaltic• Gray needles are

Plagioclase (Plag) Feldspar, Yellow-brown crystals are Pyroxene (Py), brightly colored crystals are Olivine (Ol). At lower Temps, the Olivine xtals have been partially resorbed by the melt, their atoms reused to make Py & Plag.

http://www4.nau.edu/meteorite/Meteorite/Eucrite.html

Plagioclase Feldspar

Zoned feldspar (plagioclase) showing change in composition with time in magma chamber (calcium-rich in core to sodium-rich at rim)

If the first formed crystals of Calcium-rich (Ca) Plagioclase Feldspar are left in contact with the melt , as the melt cools more stable sodium-rich layers will be deposited on their outer rims

Stable composition varies with Temperature

Isolated Olivine crystalsIsolated Olivine crystals

• Early formed Olivine crystals can sink to the bottom of a magma chamber, so they are isolated from the very reactive ions in the melt.

If early crystals are removed (isolated), the If early crystals are removed (isolated), the melt becomes richer in Silicamelt becomes richer in Silica

A melt will crystallize its mafic components first, and the remaining melt may be granitic

Remove Fe, Mg, CaSome Si

Left withK and AlMost of Si

You can start with aMafic (silica-poor) magmaand end up with some Felsic (silica-rich)Granites.

Marble Demo

Pressure GradientPressure Gradient

• P increases = gh

1 GPa at base of crust

• Linear increase mantle~ 30 MPa/km

• Core: increases more rapidly since Fe-Ni alloy more dense

We need to be able to estimate pressures

Pressure Calcs Pressure Calcs

• To calculate pressures at the base of a stack of layers with different densities, start from the top layer, calculate the pressure at the base as

• P0-1 = 0-1gh0-1

• For the second layer,

• P2 = P0-1 + 1-2gh1-2

Etc.

Multi-layer Pressure Calc Example Multi-layer Pressure Calc Example • Upper crust 25 km thick, density 2.75 Mg/m3

0-1 = 2.75 Mg/m3 x 1000 kg/1Mg = 2.75 x 103 kg/m3

• P1 = 0-1gh0-1

• = 2.75 x 103kg/m3 2 x 9.81 m/s2 x 25 x 103 m• = 6.744 x 108 kg . m/s2 x 1/m2 (aka “Pascals”)• Next layer down, 10 km basalt 1-2 = 3 x 103 kg/m3

• P2 = P1 + 1-2gh1-2

Etc. See the handout, after the lecture

Mg3Al2(SiO4)3

Olivine ExampleOlivine Example• At high TP, the At high TP, the olivine structure is no longer stable. olivine structure is no longer stable.

• Below depths of about 410 km olivine undergoes an Below depths of about 410 km olivine undergoes an exothermicexothermic phase phase transition to the sorosilicate, wadsleyite , the transition to the sorosilicate, wadsleyite , the Olivine Olivine

• At about 520 km depth, wadsleyite transforms exothermically into ringwoodite, At about 520 km depth, wadsleyite transforms exothermically into ringwoodite, the the Olivine, which has the spinel structure. Olivine, which has the spinel structure.

• At a depth of about 670 – 700 km ringwoodite decomposes into silicate At a depth of about 670 – 700 km ringwoodite decomposes into silicate

perovskite ((Mg,Fe)SiO3) and ferropericlase ((Mg,Fe)O) in an perovskite ((Mg,Fe)SiO3) and ferropericlase ((Mg,Fe)O) in an endothermicendothermic reaction. reaction.

• These phase transitions lead to a discontinuous increase in the density of the These phase transitions lead to a discontinuous increase in the density of the Earth's mantle that can be observed by seismic methods. They are also thought Earth's mantle that can be observed by seismic methods. They are also thought to influence the dynamics of mantle convection in that the exothermic transitions to influence the dynamics of mantle convection in that the exothermic transitions reinforce flow across the phase boundary, whereas the endothermic reaction reinforce flow across the phase boundary, whereas the endothermic reaction hampers it.hampers it.

• This leads some workers to believe that the 700 km boundary blocks convection This leads some workers to believe that the 700 km boundary blocks convection from the core mantle boundary, and upper mantle convection cells are distinct.from the core mantle boundary, and upper mantle convection cells are distinct.

Exothermic materials heat, expand, more buoyant

Phase diagram for aluminous Phase diagram for aluminous 4-phase Lherzolite:4-phase Lherzolite:

Ca++ PlagioclaseCa++ Plagioclase shallow (< 50 km)shallow (< 50 km)

Spinel Spinel Lherzolite Spinel is MgAl2O4

50-80 km50-80 km Garnet Garnet Lherzolite

80-400 km80-400 km Si[4] Si[4] Si[6] coord. Si[6] coord.

> 400 km> 400 km

Al-phase =Al-phase =

Figure 10-2 Figure 10-2 Phase diagram of aluminous Lherzolite with melting interval (gray), sub-solidus Phase diagram of aluminous Lherzolite with melting interval (gray), sub-solidus reactions, and geothermal gradient. After reactions, and geothermal gradient. After Wyllie, P. J. (1981). Geol. Rundsch. 70, 128-153.Wyllie, P. J. (1981). Geol. Rundsch. 70, 128-153.

Notice the mantle will not melt under normal ocean geotherm!

Si [4] => Si [6]

Heat Sources Heat Sources in the Earthin the Earth

• Impact heat from the early accretion and differentiation of the Earth

– Convection cells redistribute heat to cold surface

Heat Sources Heat Sources in the Earthin the Earth

1.1. Heat from the early accretion and Heat from the early accretion and differentiation of the differentiation of the

EarthEarth still slowly reaching surfacestill slowly reaching surface

2.2. Heat released by the radioactive Heat released by the radioactive breakdown of unstable nuclidesbreakdown of unstable nuclides

Heat TransferHeat Transfer1. Radiation

Requires transparent mediumRocks aren’t (except perhaps at great depth)

2. ConductionRocks are poor conductorsVery slow

3. ConvectionMaterial movement (requires ductility)Heat-induced expansion and buoyancyMuch more efficient than conduction

Geothermal Geothermal

GradientGradient Silica-rich rocks (with Quartz, K-feldspar) melt at cooler temperatures. Melts are viscous

Silica-poor rocks (with Olivine, Pyroxene, Ca-feldspar) melt at higher temperatures Melts are very fluid Hot

Cool

Lithosphere BuoyancyLithosphere Buoyancy

Ocean and Continental Ocean and Continental Lithosphere Thermal GradientsLithosphere Thermal Gradients

Melting depths vary w\ volcanic provinceMelting depths vary w\ volcanic provinceMost within upper few hundred kilometersMost within upper few hundred kilometers

Highest at MORs

Heat highest at MOR, suggests rising convection cells there

Origin of Basaltic Magma - MOROrigin of Basaltic Magma - MOR

• Role of Role of Pressure in divergent marginPressure in divergent margin– Reducing the pressure lowers the melting Reducing the pressure lowers the melting

temperature – the mantle partially meltstemperature – the mantle partially melts– Mid-ocean ridge and rift valley: called Mid-ocean ridge and rift valley: called

decompression meltingdecompression melting

Harry Hess’ Seafloor Spreading

http://volcanoes.usgs.gov/about/edu/dynamicplanet/nutshell.php

Mantle loses heat at Mantle loses heat at surface, becomes surface, becomes denser. Pulls denser. Pulls lithosphere down into lithosphere down into “Subduction Zone”“Subduction Zone”

Origin of Basaltic Magma 2 Origin of Basaltic Magma 2 Subduction ZoneSubduction Zone

• Role of volatiles - WATERRole of volatiles - WATER

INITIALLY BASALTIC

Origin of Basaltic Magma 3 Origin of Basaltic Magma 3 Plumes, also basalticPlumes, also basaltic

Assimilation and magmatic Assimilation and magmatic differentiation differentiation

Show Samples

Why are the continents so silica rich? Weathering dissolves high-temp. minerals, but also:

Fractionation: if early crystals settle out, remaining melt is relatively richer in silica

Origin of Andesite & Diorite:Origin of Andesite & Diorite: intermediate silica contentintermediate silica content

Good diagram for the Andes Mountains

Small blobs, not much heat in themAssimilate some crust, fractionate

Basaltic here

Origin of Granitic RocksOrigin of Granitic Rocks

Can also get small amounts of granites from deep felsic rock passed by ascending magma

Huge blobs w/ low temps but lots of magma, fractionation & assimilation => Granite Batholiths

Magma rises further distance, more fractionation. Passes through thicker crust, more assimilation.

Plate Tectonic - Igneous GenesisPlate Tectonic - Igneous Genesis

1. Mid-ocean Ridges

2. Intracontinental Rifts

3. Island Arcs

4. Active Continental

Margins

5.5. Back-arc BasinsBack-arc Basins

6.6. Ocean Island BasaltsOcean Island Basalts

7.7. Miscellaneous Intra-Miscellaneous Intra-Continental Continental

ActivityActivity kimberlites, carbonatites, kimberlites, carbonatites,

anorthosites...anorthosites...

Or, for Kimberlites (7)Or, for Kimberlites (7)

Many workers think plumes from the core-mantle boundary can punch through the endothermic 670-700 km transition. Diamonds formed from subducted organic carbon are lifted by rising plumes that happen to hit a subducted slab of ocean lithosphere.

Isotope SignaturesIsotope Signatures

• Plate tectonic provinces have a characteristic stable isotope signature