Continental Crust

Explain how the average composition of the (Upper) continental crust is established from geochemistry and seismic properties

1. weighted average of the rocks compositions at the surface

2. infer from averages of composition of insoluble elements in fine grained clastic rocks/glacial deposits

Identify the essential compositional characteristics of the upper, middle, and lower crust

upper

• plutons of granite/tonalites

• mica/schists/gneiss

middle

• mafic amphibolite to granulite facies

• felsic tonalitic gneisses (K-poor)

lower

• mafic granulite (crustal relatively strongly K- incompatible- depleted)

• spinel Lherzolites/harzburgites

Describe the behavior of trace elements during weathering and how insoluble trace elements (especially REE) in clastic sediments are used to constrain crustal composition

low solubility goes under less elemental fractionation when weathered = homogenizing effect = nearly constant REE distribution in sed rocks (like shales)

1. must assume clastic rocks REE content = upper crust

2. must calculate average compositions of coarse to fine grained clastic sed rocks worldwide

3. must calculate upper crustal La/element ratios from weighted average of sed proportions

4. determine upper crustal abundances from ratios

5. revise with new/better data

Identify the outstanding problem in the origin of continental crust and possible solutions

1. We are missing/submerged data

2. there is many models

describe the growth of the continental crust on earth including the role of Archean cratonic fragments, juvenile crust and supercontinents, and depleted upper mantle

Explain how U-Pb zircon geochronology works and how the Lu-Hf isotopic systematics of zircon are used to fingerprint the growth of ancient continental crust

identify the role of geochemistry in the exploration of terrestrial planetary bodies to constrain their origin

What does the continental crust tells us

process of differentiation of the upper part of the planet

geologic history of the Earth’s evolution

What is the geochemical earth reference model (GERM)

initiative to establish a community consesnsus on a chemical characterization of

• the earth

• its major reservoirs

• their geochemical histories

• fluxes between them

Why can we infer the upper crust composition by determining averages of the composition of insoluble elements in fine clastic rocks/glacial deposit

weathering = elemental fractionation = high solubilities elements more fractionated than low solubilities elements

• = low solubility elements (REE) provide robust estimates of the average comp of their source regions

How to study the lower crust

1. samples derived from deep crust

   • high grade met terranes

   • tectonically uplifted cross sections

   • deep-crustal xenoliths

2. correlating seismic velocities with rock lithologies

3. surface heat flow measurements

What did the study of surface heat flows suggested about the lower crust, why

Rocks have rocks with low U-Th-K heat productive elements concentrations

• bc heat flow is less than expected

How thick is the global, middle, lower crust

global = 23-40km

middle = 11km

lower = 17km

What processes influenced the upper crust to be largely granitic

intracrustal magmatic differentiation

• partial melting

• crystal fractionation

• mixing processes

What are implications of the crust composition

CC grows by igneous processes = magmatic transport of mass at subduction zones from mantle to crust

Are the continental crust features consistent with derivation of the crust by single stage melting of peridotitic mantle at subduction zones, why or why not

No, otherwise it would be basaltic average comp

• to increase total silica content

  • density foundering/delamination = recycling

  • mixture of silicic melts at subducted oceanic crust/basaltic melts/accreted basaltic oceanic plateaus

What is lithospheric delamination

as a result of density inversion in compressional settings (arcs),

cool lithospheric mantle root becomes denser than the hot asthenosphere due to eclogitized mafic cumulate → detach & sink into the mantle

define craton

segment of continental crust with long-term stability

• thick litho + cool/buoyant Fe depleted upper mantle

Why are zircons a great geochronometer

• highly resistant to weathering/hydrothermal alteration silicate structure

• very little Pb incorporated during crystallization = most Pb is radiogenic → concordia diagram

• Zr+4 → U+4 substitution

When did the first continental crust form and how did we figured this out

Zircons

Progress of U-Pb analyses

1. U-Pb zircon analyses = large sample thermal evaporation

2. isotope dissolution allowed for smaller sample sizes

3. improvement in zircon separation = Frantz

4. improvement pretreatment techniques = Air Abrasion

5. chemical abrasion = CA-ID-TIMS = reduces Pb-loss

comp of old lunar crust

highlands = anorthosite >90% Plag

maria = basaltic lavas 3.2-3.9Ga

What are juvenile basic rocks

basalts derived directly from the mantle with no crustal contamination during ascent = continuous depleted Nd isotopic signature