1-Element Partitioning and Chemical Differentiation

What makes an element major, minor or trace?

Major >1 wt%, Na, Mg, Ca, Fe, Al, Si, O

Minor 0.1-1 wt% H, K, Ti, Cr, Mn, C, S

Trace elements <0.1 wt%

Why are trace elements used as tracers and resources?

• not involved in chemical reaction

• passive tracers in earth processes

• relatively easy to measure by mass spectrometry

• ore deposits occur where trace elements are highly concentrated

What is chemical differentiation?

Separation of larger bodies into chemically distinct parts.

Earth and solar system are differentiated.

• trace elements track differentiation

How do we know about solar element abundances?

• estimates from solar spectroscopy and analyses of CI chondrites (same as sun and so same as solar system)

• nucleosynthesis mechanisms, H & He from Big Bang, Li B Be from cosmic ray spallation, C to Fe from stellar fusion, Ni to Bi from neutron capture

What is Odoo-Harkins rule?

Odd-even effect.

Paired protons, even elements are more stable and more abundant

What is siderophile, chalcophile, lithophile, atmophile?

Siderophile = iron liquid, core

Chalcophile = sulfide liquid, core

Lithophile = silicate liquid, mantle and crust

Atmophile = gas, atmosphere

Phile means loving

Partitioning affinity depends on conditions.

What is symbols for partition coefficients, and when is it compatible or incompatible?

What is meant by compatible and incompatible?

When D < 1, element is incompatible. Element prefers the liquid

When D > 1, element is compatible. Element prefers the solid

What factors affect trace element partitioning? What are the general rules of partitioning?

• ionic radius - avoid inducing lattice strain by introducing ions of different sizes

• charge - avoid introducing highly charged ions, and avoid introducing charge imbalance

• large cations are typically incompatible

• small divalent cations are often more compatible than small univalent cations

• ionic radius more important than charge

What is Goldschmidt’s Rules for element substitution?

• ions of one element can extensively substitute for those of another if their radii differ by less than 15%

• two ions have similar radii and the same charge, the smaller ion is preferentially incorporated into the solid

• if two ions have similar radii but different charges, the ion with the higher charge is preferentially incorporated into the solid

What are the types of incompatible trace element?

What are the four different magmatic processes to consider?

• Equilibrium melting ​

• Equilibrium crystallisation​

• Fractional melting​

• Fractional crystallisation

Explain equilbrium melting

Incompatible elements (prefer the liquid) are highly concentrated in the melt, at low F, and are diluted as F increases.

Compatible elements (prefer the solid) have very  low concentrations in melts, especially at low F

What is meant by F and X?

The proportion of liquid is denoted F

Proportion of solid is often expressed as X, where X = 1 - F

What is difference between equilibrium crystallisation, melting, and fractional crystallisation and melting

Equilibrium crystallisation - Occurs when a magma cools and crystals form, but the crystals remain in contact with the melt and can exchange elements freely.

Equilibrium melting - The reverse process — when a solid rock melts and the melt stays in equilibrium with the remaining solid.

• Fractional crystallization: crystals are removed from melt → melt composition changes rapidly.

• Fractional melting: melt is removed from the solid → melt is different from the bulk rock.

What is equilibrium crystallisation?

What is meant by fractional melting?

Incompatible elements (prefer the liquid) are rapidly depleted in the solid residue, so liquids are also rapidly depleted.

Compatible elements (prefer the solid) are initially retained in the solid, so the last liquids are relatively enriched.

What is fractional crystallisation?

Incompatible elements (prefer the liquid) remain in the system, so liquids become highly enriched.

Compatible elements (prefer the solid) are rapidly removed from the system, leaving highly depleted residual liquids

What is the composition of the core like?

• 16% of earth’s volume, 31% of earth’s mass

• Fe 90 and Ni 5wt%, Fe/Ni core is similar to chondrites

• Density is lower than pure alloy due to light elements present, H, C, S, O, Si

• siderophile elements are depleted in the mantle but rich in the core

What is the composition of the mantle like?

• 83% of earth’s volume, 68.3% of earth’s mass

• Bulk silicate earth BSE = CI chondrite - core

• O, Si, Mg, and Fe dominate alongside Ca, Al

• Mantle composition determined by meteorites, xenoliths, and ophiolites

What is the composition of the crust?

• insignificant to earth’s volume, 0.7% of earth’s mass

• composed of O, Si, Al, Fe, Mg, Na, K

• oceanic crust = rich in Ca, Mg, Fe, cover 70% of surface

• continental crust = rich in Si, Na, K, and incompatible elements 

How does the composition of the continental crust change?

Mean composition is andesitic.

Grades from basaltic at the base to granitic at the top. 

What is the oceanic crust like?

Mean composition is basaltic, but stratified from ultramafic at base, to basaltic at top.

How is the continental crust and oceanic crust difference when it comes to having incompatible elements?

Continental crust has a lot more incompatible elements.

Oceanic crust is relatively depleted in high incompatible elements.