Trace Elements Francis, 2013
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Transcript of Trace Elements Francis, 2013
Trace ElementsFrancis, 2013
What is a trace element?
Cxyli / Cliq
i = Ki
For our purposes, a trace element is one which obeys Henry’s law:
Its partition coefficient is not a function of its concentration
Trace Elements
At equilibrium:
Coi = Cxyl
i × (1-F) + Cliqi × F
Coi = Ki × Cliq
i × (1-F) + Cliqi × F
Whole = ∑ Parts
F = fraction liquid
Ciliq = Ci
o / ((F + Ki(1-F))
Cixyl = Ki×Ci
o / ((F + Ki(1-F))
liquid
solid
Prediction of traceelement partitioning
Ideal mixing
Mixing of networkmodifiers
Ideally trace elements are those elements whose concentration is so low that they obey Henry’s law.
Cisolid / Ci
liq = K constant
In practice, many trace element partition coefficients vary with the composition of the silicate melt. Using a two lattice activity model one can sometimes reduce this dependence
Ni2+ substitutes for Mg2+ in olivine
KdNi = 3.346 × XNMMg - 3.665
GERM Website: earthref.org/GERM/index.html?main.htm
Trace Element PartitioningCpx M2 site
DI = conc. Isolid / conc. Iliquid
Partition coefficients for trace elements of the same valence are essentially a function of ionic radius versus the ideal size for the crystallographic site to be occupied.
Two Kinds of Trace Elements
Incompatible Compatible
Ki = Cxyli / Cliq
i < 1 Ki = Cxyli / Cliq
i > 1
Incompatible trace elements can not achieve satisfactory coordination in the crystallographic sites of the minerals that are present, commonly because they are too large or have too high a valence state.
Incompatible trace elements preferentially partition into the liquid phase.
Compatible trace elements can achieve satisfactory coordination in the mineral phases that are present by substituting for a major element with similar valence state and ionic radius.
Compatible trace elements preferentially partition into the solid phase.
La, Ce, Nb, Zr, Rb, Ba
Sc Ca in Cpx M2 siteCr Al in spinel in Y siteNi Mg in olivine in M1 & M2 sites
Rare Earth Elements - REE
First Inner Transition Series Elements (4f orbitals) :
La3+, Ce3-4+, Pr3+, Nd3+, Pm3+, Sm3+, Eu2-3+, Gd3+, Tb3+, Dy3+, Ho3+, Er3+, Tm3+, Yb3+, Lu3+
Ionic radius decreases and compatibility increases with atomic number for most minerals.
Although not officially a Rare Earth, Y3+ behaves vary similarly to Yb3+ and is usually considered with the REE
Typically presented in a spider-diagram in which actual abundances are normalized to the values for chondritic meteorite to eliminate the odd-even effect reflected in the Oddo-Harkins rule
Nd143 Sm147
[Xe] 4fn 6s2
Ciliq = Ci
o / ((F + Ki(1-F))
Cixyl = Ki×Ci
o / ((F + Ki(1-F))
Cixyl / Ci
liq = Ki
Coi = Ci
xyl × (1-F) + Ci
liq × F
The overall chemical similarity of the REE, but increasing compatiblity with increasing atomic number, makes them sensitive measures of partial melting. Furthermore, their relative insolubility in aqueous fluids (except Ce4+ ) makes them relatively immobile during metamorphism and alteration compared to major elements.
High field Strength Elements - HFSE
Field Strength = charge / ionic radius
Zr4+, Nb5+, Hf4+, Ta5+
Typically highly incompatible because of absence of suitable crystallographic sites in the common rock forming minerals. They become compatible only when sufficiently high levels are reached to stabilize their own phase – such as zircon in the case of Zr.
Typically insoluble, not transported by water, and thus they are resistant to alteration and metamorphic effects.
Characteristically high in alkaline and hot-spot magmas, but relatively depleted in calc-alkaline arc magmas.
Large Ion Lithophile Elements - LIL
Trace elements that have low field strengths; large size combined with low charge
Rb+, Sr2+, Cs+, Ba2+, & K+
These elements substitute for Na and K in feldspars, but are incompatible in mafic minerals that do not contain large 12-coordinated sites.
They are also relatively soluble in water, and thus very sensitive to alteration and metasomatic processes
Characteristically high in calc-alkaline lavas whose petrogenesis involves the dehydration of subducted slabs and metasomatic enrichment of the overlying mantle wedge.
LIL trace elements are also sensitive indicators of the involvement of mica or amphibole, which accept LIL elements substituting for K.
Increasing degree of melting ?
Compatible trace elements
Elements whose charge and ionic radius enables them to substitute for major elements in common minerals
Ni2+ substitutes for Mg2+ in olivine
KdNi = 3.346 × XNMMg - 3.665
Compatible trace elements – con’t
Cr3+ substitutes for Al3+ in spinel and clinopyroxene
In addition to being important sources of heat and isotopic tracers, Th and U, as well as their daughter Pb, are sensitive elemental tracers of crustal contamination. Their large size and charge make them incompatible in most common minerals and as a result they are highly concentrated in the Earth’s crust. For example, unlike OIB and MORB basalts, continental flood basalts are commonly characterized by positive Pb, U, and Th anomalies. U and Pb are relatively mobile and untrustworthy in altered or metamorphosed rocks. Th, however, appears more robust and is often used as a tracer of crustal contamination in old rocks.
Oceanic plateau, in contrast, have relatively flat, unfractionated trace element profiles, with relative depletions in LIL elements, and lack Nb and Pb anomalies.
U3-6+, Th4+, Pb2-4+Radiogenic Elements
Trace element Composition of the Continental Crust
The continental crust is highly enriched in incompatible trace elements compared to chondrite meteorites. Despite its small proportional mass (< 0.2 wt.%), the continental crust remains an important reservoir for the large ion lithophile elements, as well as the important heat producing elements U, Th, and K, but shows relative depletions in HFSE elements such as Nb, with respect to trace elements with similar degrees of incompatibility (e.g., K, Th, La). Although the lower crust is relatively poorly constrained, there appears to be an exponential decrease in the concentrations of heat producing elements, such as K, Th, and U, with depth, along with a change from granodioritic to gabbroic composition.
Extended Spider-Diagram
There is a remarkable similarity between the trace element profiles of the Earth’s continental crust and MORB source and that predicted for a 1 - 2% partial melt of the primitive mantle.
There is a systematic anti-correlation between degree of incompatible trace element enrichment and degree of Si saturation. Going from tholeiite to AOB to basanite and then olivine nephelinite corresponding to a systematic increase in the degree of enrichment in LREE, Nb, and Ta, with little change or a slight decrease in the levels of HREE.
olivine
Cxyli / Cliq
i = KiAt equilibrium:
Coi = Cxyl
i × (1-F) + Cliqi × F
Coi = Ki × Cliq
i × (1-F) + Cliqi × F
Whole = ∑ Parts
F = fraction liquid
Ciliq = Ci
o / ((F + Ki(1-F))
Cixyl = Ki×Ci
o / ((F + Ki(1-F))
liquid
solid
The fundamental equations:
Equilibrium Crystallization and/or Batch Melting
Ciliq = Ci
o / ((F + Ki(1-F))
Cixyl = Ki×Ci
o / ((F + Ki(1-F))
F = fraction liquid
If more than 1 mineral is involved, then the weighted-average partition coefficient can be calculated from the individual mineral partition coefficients and substituted for Ki:
∑n Xn = 1
Coi = Ci
xyl × (1-F) + Ci
liq × F
Cixyl / Ci
liq = Ki
Di = Xα× Kiα + Xβ × Ki
β + Xγ × Kiδ + ……..
Ciliq
Coi = Ci
xyl × (1-F) + Ci
liq × F
Cixyl / Ci
liq = KiFractional Crystallization
F × Ciliq1 = Ci
xyl × δF + (F-δF) × Ciliq2
δCiliq = Ci
liq2 × ((K-1) / F) × δF
F = fraction liquid
Ciliq = Ci
o × F(Ki -1)
Bulk Solid = Cio × (1-F)(Ki-1)
Liquid composition:
Ciliq1- Ci
liq2 = ((Cixyl –Ci
liq)/F)× δF
δCiliq / Ci
liq2 = ((K-1) / F) × δFCi
liq
Crystallization
Cieliq = Ci
o / ((F + Ki(1-F))
Cio
Ciliq
Cifliq = Ci
o × F(Ki -1)
The concentration of highly incompatible elements (K 0) in a residual liquid is:
Cio = Ciliq(1-f) + Cisolid(f)
Ciliq =Co / (1-f)when Cisolid =
0.0
As a result, plotting other elements against a highly incompatible element gives a good representation of the liquid line of descent produced by crystal fractionation.
Binary plots of 2 highly incompatible elements define straight lines passing through the origin
Highly Incompatible Trace Elements
Moderately Incompatible TraceElements
Compatible Trace Elements
Modal Fractional Melting
Cixyl = Ci
o × (1-F)(1-Ki)/Ki
Coi = Ci
xyl × (1-F) + Ci
liq × F
Cixyl / Ci
liq = Ki
δCixyl = ((Ci
liq – Cixy2) /(1-F)) × δF
Solid residue composition:
Instantaneous liquid composition:
δCixyl / Ci
xyl2 = (((1-Ki) / Ki) / (1-F))× δF
(1-F) × Cixyl1 = Ci
liq × δF + (1-F-δF) × Cixyl2
Cixyl1 – Ci
xyl2 = ((Ciliq - Ci
xyl2) / (1-F)) × δF
Ciiliq = (Ci
o / Ki) × (1-F)(1-Ki)/Ki
Cialiq = (Ci
o × (1 - (1-F)1/Ki) / F
Modal Fractional Melting – con’t
A more useful parameter than the instantaneous melt for fractional partial melting is the composition of the aggregate liquid that would be made by blending all the instantaneous melt fractions together over some interval of melting:
Aggregate Fractional Melt:
Modal Fractional Melting –con’t
CrystallizationPartial Melting
Plotting ratios of trace elements enhances the difference between partial melting and crystal fractionation and reduces the effects of changing K’s with pressure, temperature, and melt or mineral composition.
Partial melting is best at fractionating incompatible elements in silicate melts.
Fractional crystallization is best at fractionating compatible elements in silicate melts.
Cio/Ck
o = 1
Ci/Ckliq
Ciliq = Ci
o × F(Ki -1)
Cisoli = Ki × Cliq
i
For Fractional Crystallization:
Log – Log Diagrams
Modified after Cocherie, 1986
Cisol = Ci
o × (1-F)(1-Ki)/Ki
Ciiliq = (Ci
o / Ki) × (1-F)(1-Ki)/Ki
For Fractional Fusion:
Fractional crystallization is best at fractionating compatible elements, whereas fractional fusion is best at fractionating incompatible elements.
Log C1 versus log C2 is a straight line if K’sare constant
Slope = (K1-1) / (K2-1)
P
P
KNi ~ 5
KAl ~ 0.2
Compositional variation in solids
Oxidation State and Trace Element Partitioning
A number of trace elements have variable oxidation states that affect their partitioning between liquid and solid phases.
Ce3+ Ce4+
incompat ible soluble, mobile
Eu 2+ Eu3+
compatible in Feldspar relatively incompatible
Cr2+ Cr3+
incompatible on Moon compatible in Spinel & Cpx
V2+ V3+ V4+ V5+
compatible in silicates incompatible in silicates compatible in oxides
Reducing Oxidizing
Oxidation State of the Cordilleran Mantle
2×Fe2+Fe23+O4 + 6×FeSiO3 = 6×Fe2
2+SiO4 + O2
spinel opx oliv
Most likely oxygen buffer in the spinel lherzolite field:
P
Non-Modal Batch Melting
The previous trace element melting equations assumed that the proportion of phases going into the melt is the same as the proportion of minerals (mode) in the initial source. From our knowledge of the olivine-cliopyroxene-silica liquidus projection we know this is far from true.
The first melt is highly enriched in clinopyoxene (e1-P) compared to the mode of the mantle source. The accurate calculation of trace element behaviour requires a knowledge of both the starting mode of the source and the mode of the phases going into the melt.
Doi = Xα× Ki
α + Xβ × Kiβ + Xγ × Ki
δ + ……..
Pi = pα× Kiα + pβ × Ki
β + pγ × Kiδ + ……..
∑n Xn = 1
∑n pn = 1
The Batch Melting equation becomes:
Source Mode
Melt mode
Ciliq = Ci
o / (Doi + F × (1-Pi)) Ci
liq = Cio / ((F + Ki(1-F))
Modal melting
Non-Modal Fractional Melting
Cialiq = Ci
o × (1 - (1-(PiF / Doi))1/Pi) / F
Ciiliq = (Ci
o / Doi) × (1- (PiF) / Do
i)(1-Pi)/Pi
Instantaneous Liquid:
Aggregate Fractional Liquid:
The effects of incomplete melt extraction during partial melting or of the presence of interstitial melt in the crystal cumulates of crystal fractionation can be accounted for by including the melt as fictive mineral phase in the solid assemblage with a K i = 1 in the calculation of Do
i and Pi
Doi = Xα× Ki
α + Xβ × Kiβ + Xliq × 1
Pi = pα× Kiα + pβ × Ki
β + pliq × 1
∑n Xn = 1
∑n pn = 1
Cialiq = Ci
o × (1 - (1-F)1/Do) / F
modal fractional melting
There is typically de-coupling between major elements or compatible trace elements and incompatible trace elements, even in apparently co-magmatic suites. This can be attributed to at least two factors:
Element Decoupling
Mixing between liquids representing different melt fractions
Mixing during Crystal Fractionation
Increasing degree of melting ?
2 – component mixing ?
Increasing degree of melting ?
Mixing?
Mantle Sources for
Magmatic End-Members Hy-Norm
Basalt
Ol- Neph
Miller’s Ridge Transition
1mm
oliv
cpx
cpx
oliv
cpx cpxoliv
oliv
ankaramitic flows
ankaramitic flows
basaltic flows
basaltic flows
Ciliq1 = ΔF × Ci
xyl + (1-ΔF) × Ciliq2
Ciliq2 = (Ci
liq1× (1–ΔF × Ki)) / (1-ΔF)
Coi = Ci
xyl × (1-F) + Ci
liq × F
Cixyl / Ci
liq = Ki
Finite - Difference Computer Models
Mg
Mg
CpxFe
Olivine
Fe
Mg
Mg
Contrasting Olivine – Cpx Zoning
2 mm
cpx
oliv
oliv
cpx
M1 site
REECpx M2 site
Mg
Mg
CpxFe
Olivine
Fe
Mg
Mg
Contrasting Olivine – Cpx Zoning
2 mm
cpx
oliv
oliv
cpx