Making the Earth’s crust - The generation and destruction of crustal masses through time

Mihai Ducea

University of Arizona,

Department of Geosciences

Tucson, AZ, 85721

Outline

• The paradox of continental formation• Making basalts, the oceanic crust• Plate tectonics, now and in the geologic past;• Continental arcs- the possible solution• The need for a recycling mechanism• Future challenges

Continental crustal paradox

• Continental crust is being generated by several melting stages of the Earth;s mantle; it is essentially “distilled” from the mantle

• All mantle melts are basaltic

• Continents are granitic on average

• WE DON’T QUITE UNDERSTAND HOW CONTINENTS FORM(ED)

Granites, granodiorites, tonalites= continental crust

Basalt and gabbro density-3 g/cm3 Granite-2.7 g/cm3

Oceanic lithosphere- different from continental:Basaltic crust, relatively thin (5-7 km), leads to topographic lows

Heat engine- very efficientEarth differentiation- primarily by magmatism

Mantle convection-Mostly solid stateMelting shallow by adiabatic decompression

Lithosphere- the coldlid at the top

Chondrites- bulk Earth

Iron meteorites=core-like

Mantle xenoliths- PERIDOTITES

T - scale ~ plate motionsLength scales - appear much more complicated than the ridge-trench systems

CONVECTION

Convection cells penetrate to the core-mantle boundary

Making the oceanic crust

Young------- Old

The oceanic crust

• Young (< 200 Ma)

• Forms at mid-ocean ridges, the oceanic crust and lithosphere cools away from ridge

• Made entirely of basalt - which is what we expect to form by partial melting of the mantle

• All other solar system “crusts” are basaltic

In addition- experiments carried out under any possible conditions of mantle melting have demonstrated unambiguously that basaltic melts should be what we get out of the mantle.

Basaltic melts are ~ 50 SiO2, granites are 65% SiO2. If anything (e.g. higher degrees of melting) we’d expect lower than 50% SiO2 melts.

It is instructive to check if this applies to another product of mantle melting - “mantle plumes” or “hot spots” like Hawaii. The answer: YES, they’re also made of basalts.

What happens at subduction zones?

• Oceanic plates are young; older oceanic crust has been subducted

• Obviously it had to be recycled- otherwise the Earth should be increasing its volume

• At subduction zones, magmatism tends to follow the subduction margin - the resulting product is a magmatic “arc”

Island arcs - classic example- the Aleutians arc On average - the composition is also basaltic

But then there are the continents

• When did they form?• How did they form?• Answers: we think they form gradually over

the 4.5 Ga history of the Earth• We also think they form by remelting

basalts• That requires an additional “residual”

reservoir that we don’t see anywhere

Evidence for PT goes back to the Archean. Faster motions, more melt, smaller continents (the continental nuclei known as cratons or “croutons”)

Continents-succession of orogenic events

Ages of continental rocks

• Young ages are determined biostratigraphically

• Older rocks are being determined geochronologically

Zircons - as old as 40- 4.2 Ga; evidence for continental crust

Materials get accreted to the continental margin and become terranes, “exotic terranes”; I.e. blocks of unrelated origin that were once far apart but got assembled by accretion onto a continental margin

Adding mass to a continent

E.g. the Pacific

Classic example: the North American Cordillera

But a factory that transforms basalts into grantioids has to exist• The only reasonable place- the continental

arcs

• If not- we simply don’t have a good enough environment today (or in the recent geologic past) that makes continents

• The key place where all may can be explained: the Sierra Nevada of California

Coast batholith

Idaho batholith

Sierra Nevada

batholith

Peninsular Ranges

batholith

North American batholiths

5-10

km

Igneous Intrusions

????

100-1000000 km3

Typically shallow exposures and no deep “windows” into the arc

Partial melting

Melt

Residue

Melt

Cumulate

Fractionalcrystallization

Source region Magma chamber

melts

solids

cl

c0

=

1

D + F ( 1 − D )

1 part melt requires 1 to 2 parts residue

Sierra granitic thickness is 30-35 km, much larger than previously thought (8-10 km)

three deep crustal exposures (30-35 km)

BIG SUR (Santa Lucia Mts)

Granulites (feldspar +garnet +pyroxenes)

MiocenePliocene

Quaternary

CSNB-Central Sierra Nevada batholith

Xenoliths

Minerals:clinopyroxenegarnetorthopyroxeneamphibole rutile, apatite

Textures1. Large grain sizes2. Cumulate textures

=IGNEOUS textures

Gar

Cpx

Key new observations

• The arc is much thicker than thought

• Requires about 1:1 or 2;1 residue to melt ratio

• Much of the residue is not granulitic as all textbooks argue, but it is “ECLOGITIC”

Garnet- much denser than any other crustal mineral

ECLOGITE = garnet + pyroxene

2.8

2.9

3

3.1

3.2

3.3

3.4

3.5

3.6

3.7

650 850 1050 1250 1450

Temperature (C)

Density (g/cm3)5kbar

10kbar

15kbar

20kbar

25kbar

30kbar

Mantle density

Mafic lower crust

Arc root density vs. temperature

Magmatism intimately linked to thursting of the lithosphere: magmatic flare-ups follow thrusting after 15-25 my

Convective removal of roots

• Don’t see thick roots in N-American arcs. They ought to be removed somehow.

• Most likely process- foundering, or convective removal.

• Buoyancy-driven process that appears to be triggered only in continental arcs and only if the arc column is thick enough to generate eclogitic residues.

Tectonic MOHO

Arc keels- do they founder?Arc keels- do they founder?

? ?150km

100 km

50 km

0 km

foundering

upwelling

remnant crust

Moho

Granitoids

Eclogites

Peridotites

vp=6-6.3 km/s

vp=8.4 km/s

Granulites

Mixed zone

?

??

??

0 4 g/cm3

ρ=3.25-3.3 g/cm3

vp=7.7-7.9 km/s

ρ=2.6-2.7 g/cm3

ρ=3.5-3.6 g/cm3

Eclogite: convectively unstable, will founder? In which tectonic environments

Working hypothesis for convective removal of root

• Should take place in thick arcs (25-30 km thick batholiths) that leave behind an eclogite facies residue

• Should not take place when most residue is in granulite facies;

• Follows periods of flare-ups and might be triggered by extension

So what do we know?

• In the Sierras, we are confident that pre-existing crustal materials have been involved in a second stage melting process that made the granitic “batholith”

• We know now that some 35 km of the crust is granitic

• Since at least that much residue is required, and it had to be located at 35-70 (or more) km depth, much of it had to be dense “eclogitic” residue

• We see these eclogites in 12 Ma volcanic rocks (as xenoliths)- we have direct evidence that they existed!

• They are currently lacking beneath the Sierras; there is strong evidence that this root has foundered (sank!) in the less dense mantle

• If true, we had found an elegant mechanism to making continental crust

Key issues to remember

• Oceanic crust is mafic/basaltic and young - older oceanic materials are subducted

• Continental crust has the average of a granitoid, appears unsubductable

• Continental masses grow over time, some continents have older cratons in their cores separated by mobile belts

More

• Making continents is difficult to explain unless a “residual” mass equivalent to that of the continents is present under the continents;

• We think that these residues have detached and sank in the mantle