 | Mикроэлементы |
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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 and 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)
B. Rayleigh fractionation: crystals and melt are separated instantly
Fractional crystallization: C in liquid / Co = F (D - 1). Co = original melt composition, F = fraction of melt remaining
Fractional melting: C in liquid / Co = (1 / D) (1 - F)(1/D - 1), 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 and 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 and Sr in plag
IV. Some applications
A. Trace elements can be used in variation diagrams
If 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. If 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