Trace Elements 


A. Behavior

-Most substitute for major elements into phases containing ions with similar charge and size: Rb in mica with K, Sr in plag with Ca

-Most enter various phases in unequal amounts, i.e., prefer a certain mineral or the liquid = chemical fractionation

B. Partition coefficients

-Def: D = C in solid / C in liquid => element distributes itself between solid & liquid in a constant and measureable proportion.

-Determined by lab measurement = empirical

-For major elements, D is near 1, because they don''t fractionate much.

-For trace elements, D may range over several orders of magnitude

C. Terminology

-Compatible trace elements more readily enter solid phases, and D >> 1: these stay in the solid during melting, and quickly leave the liquid during crystallization.

-Incompatible trace elements prefer the liquid, and D << 1: these are enriched in the liquid during crystallization and are the first to enter the liquid during melting.

-Common trace elements fall into 2 groups: high Field Strength elements: smaller, higher charge; REE, Th, U, Ce, Pb+3, Zr, Hf, Ti, Nb, Ta; large Ion Lithophile: more mobile: K, Rb, Cs, Pb+2, Ba, Sr, Eu+2

D. Bulk partition coefficient

-For an element i, D for whole rock = sum (weight % of i * D of i)

-Consider an olivine gabbro: 40 wt % plagioclase, 25% orthopyroxene, 35% olivine: bulk D for Dy = 0.4*0.023 + 0.25*0.15 + 0.35*0.013 = 0.051 > incompatible

II. Melting models

A. Batch melting

-Solid and liquid remain in contact, in equilibrium, until a "batch" of melt is large enough to segregate (usually assumed to be by buoyancy)

-Let F = melt fraction. Then for a trace element, C in liquid / C in solid = 1 / (D (1 - F) + F)

-CL/Co varies most for low values of F (low degree of melting)/CL/Co varies more for incompatible than compatible elements

B. Rayleigh fractionation: crystals and melt are separated instantly

-Fractional crystallization: C in liquid / Co = F (D - 1) (Where Co = original melt composition /F = fraction of melt remaining )

-Fractional melting: C in liquid / Co = (1 / D) (1 - F)(1/D - 1) (Where Co = original solid composition/F = fraction melted)

C. There are other models that take into account other processes or combinations of processes

III. Spider diagrams

A. Rare earth elements (REE)

-Also called the lanthanide series

-lanthanum La 57
-cerium Ce 58
-praesodumium Pr 59
-neodymium Nd 60
-promethium Pm 61
-samarium Sm 62
-europium Eu 63
-gadolinium Gd 64
-terbium Tb 65
-dysprosium Dy 66
-holmium Ho 67
-erbium Er 68
-thulium Tm 69
-ytterbium Yb 70
-lutetium Lu 71

-Similar chemically and physically => behave similarly

-Typically plotted with atomic number on X-axis, concentration on Y-axis: Y-values are normalized to a standard (~Primitive mantle/Chronditic meteorite )

B. Normalized multi-element diagrams

-Includes other trace elements with REE

-May be normalized to various references

-Generally listed in order of increasing compatibility left to right

-Troughs appear if an element is compatible in a particular phase: Ti in ilmenite; Sr in plag

-incompatibility increases from Sr to Th, then decreases from Th to Yb

IV. Some applications

A. Trace elements can be used in variation diagrams

-Examples: iIf rock melts at depth >70 km (high P), garnet is stable (HREE are compatible in garnet, so pattern will be depleted in HREE (negative slope)/Note negative slope also results from low % partial melting. HREE depletion cannot distinguish between these); iIf rock melts at depth <40 km (lower P), plagioclase may fractionate Eu.

B.Ratios of trace elements

-These are better at identifying a phase involved in melting or crystallization, because ratio doesn''t depend on absolute amount present: low Yb/La could detect garnet in source rock, since Yb and La behave similarly except for compatibility in garnet : K/Rb can detect presence of amphibole

C. Tectonic environments

- how to apply various trace elements to make generalizations

- different plots can distinguish rocks characteristic of different settings (MORB, OIB, etc)

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