Trace Elements in igneous petrology •Abundances of trace elements are used to test petrogenetic hypotheses •No universal definition of TE: Concentration usually less than 100 ppm, often < 10 ppm •Useful trace elements: a)First transition series: Sc Ti V Cr Mn Fe Co Ni Cu Zn Ti and Fe are usually major elements, Cr, Mn, and Ni are minor elements Progressive filling of 3d orbitals Variable crystal field stabilization Commonly multivalent (Sc3, Ti4,3, V2,3,4,5, Cr2,3,6, Mn2,3, Fe2,3, Co2, Ni2 b) Lanthanides (REE): La Ce Pr Nd (Pm) Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Light REE and heavy REE (Y behaves like a HREE) normalization factors (chondrites) 1

Eu2+ 1.2

Ionic radius (Å)

Abundance (ppm)

.8

REE Chondritic abundances odd vs. even abundances

.6

Lanthanide Contraction

1.1

.4 .2

1.0

0 La Ce Pr Nd

Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Eu3+ Based on 8-fold coordination La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

(c) Large Ion Lithophile Elements (LILE): may also be partitioned into fluid phase Alkalis: K Rb Cs (monovalent) Alkaline earths: Ba Sr (divalent) Actinides: U, Th, Ra, Pa (multiple valency) (d) High field strength elements (HFSE): small, highly-charged ions Zr, Hf (4 valent) Nb, Ta (4 and 5 valent) (e) Chalcophile elements: Cu, Zn, Pb, Ag, Hg, PGE, (Fe, Co, Ni) (f) Siderophile elements: Fe, Ni, Co, Ge, P, Ga, Au (PGE)… • Decoupled from major elements: lack of stoichiometric constraints (not strictly true) • Goldschmidt’s Rules • Generalities: Incompatible elements are elements that tend to be excluded from common minerals (olivines, pyroxenes, garnets, feldspars, oxides…) in equilibrium with a melt, i.e., they have low D values. •Numerous exceptions, e.g., Sr, Eu in plag, Cr, Sc in pyroxene, Ni in olivine, HREE in garnet.. •Empirical (not thermodynamic) definition of D (see relevant definitions and equations posted on class website) C i C/L

Di

C/L

=

C

C iL C

where Di is the weight distribution coefficient, C i is concentration (ppm) of trace element i in liquid, and L is concentration (ppm) of trace element i in the liquid i

C

For multiphase crystalline assemblages:

D

C/L = Bulk distribution coefficient of i between crystals and liquid = i where

Wj

∑D W ij

j

j

is the weight fraction of mineral j in the solid assemblage

Trace elements are used to model processes of melting (equilibrium and fractional) and crystallization (equilibrium and fractional). To model melting processes, two types of melting are considered (1) modal melting where the minerals melt in the same proportions that they are present in the crystal assemblage and (2) non-modal melting in which the minerals melt in proportions controlled by the stoichiometry of the melting reaction, which usually has to be determined experimentally or from a known phase diagram. The equations for all types of crystallization and melting are listed on the class website under: “Trace element definitions and equations”. Distribution coefficients •

DiC / L

Attempts have been made over the past 3 decades to determine the appropriate values of distribution coefficients to be used in modeling. The earliest attempts simply separated phenocrysts from matrix in volcanic rocks and analyzed each to obtain an empirical set of D values. Some of these data are still used today.

• More recently experiments have been carried out over a wide range of T, P and X using a variety of methods. As the techniques of microanalysis of trace element abundances improved (SIMS, LA-ICPMS…), this approach has been popular.

D values are functions of T, P and composition of both crystalline and melt phases so the problem becomes one of controlling these variables and trying to establish a theoretical basis that would allow one to predict D values based on a limited number of experiments. Only limited success to date so a wide range of D values exist in the literature. Caveat: Be careful about choice of D values used—make sure they are appropriate. This figure on left shows D values for REE that have been used to model processes in mafic magmas. While the D values may vary as functions of T, P, XL and Xxal, the figure clearly show the major differences among common minerals. Note the log scale on the Y-axis. The figure below shows an example of the effect on D values of changing crystal composition

Good source of D values: Table 7.5 in the online book by W. M. White “Geochemistry”

Rb Sr Ba Ni Cr La Ce Nd Sm Eu Dy Er Yb Lu

Rare Earth Elements

Table 9-1. Partition Coefficients (CS/CL) for Some Commonly Used Trace Elements in Basaltic and Andesitic Rocks Olivine 0.010 0.014 0.010 14 0.70 0.007 0.006 0.006 0.007 0.007 0.013 0.026 0.049 0.045

Opx 0.022 0.040 0.013 5 10 0.03 0.02 0.03 0.05 0.05 0.15 0.23 0.34 0.42

Cpx Garnet 0.031 0.042 0.060 0.012 0.026 0.023 7 0.955 34 1.345 0.056 0.001 0.092 0.007 0.230 0.026 0.445 0.102 0.474 0.243 0.582 1.940 0.583 4.700 0.542 6.167 0.506 6.950

Data from Rollinson (1993).

Plag Amph Magnetite 0.071 0.29 1.830 0.46 0.23 0.42 0.01 6.8 29 0.01 2.00 7.4 0.148 0.544 2 0.082 0.843 2 0.055 1.340 2 0.039 1.804 1 0.1/1.5* 1.557 1 0.023 2.024 1 0.020 1.740 1.5 0.023 1.642 1.4 0.019 1.563 * Eu3+/Eu2+

D values > 1 compatible element D values < 1 incompatible element

Table 9-1: from Winter (2001) An introduction to Igneous and metamorphic petrology. Prentice Hall

Italics are estimated

Tables such as this one are useful but they can also be misleading in that they imply that D values are constant for a given element in a given mineral 30

Olivine/melt D values

20 DNI 10

0 0

10 20 Wt. % MgO in liquid

This figure shows the range of values measured experimentally for the distribution coefficient of Ni in olivine showing the effect on D value of variable MgO content of the melt. There is also a temperature dependence which is “hidden” in these data. The experiments (>10 sets) were carried out at T ranging from 1600ºC to 1100ºC. Liquid compositions ranged from ultramafic (komatiites) to mafic (basalts).

30

10

“Onuma” diagram for plagioclase Sr2+

Ca2+

1

In Onuma diagrams, partition coefficients plotted against ionic radius define smooth convexupward curves with a different curve for each valence. The maximum in the curves predicts the best fit ionic radius.

D Ba2+

0.1 3+ Sm

Mg2+

0.01 0.8

Ce

Lu YbDy

1.0

1.2

1.4

Ionic radius (Å)

1.6

Normalization factors, spider diagrams, etc. 1. REE: REE abundances in minerals and rocks are normalized by dividing the abundances by abundances in C1 chondrites (see table below for typical values) La Ce Nd Sm Eu Gd Tb Dy Er Yb Lu

Ch (ppm) 0.325 0.798 0.567 0.186 0.0692 0.255 0.047 0.305 0.209 0.209 0.0349

This figure shows a typical REE normalized plot for basalts. In this example, basalts from the CRB flood basalt province. A pattern like this is said to show a moderate degree of LREE enrichment (La/Yb)N > 1 Data from Hooper and Hawkesworth (1993) J. Petrol., 34, 1203-1246. Reproduced in Winter (2001) An introduction to Igneous and metamorphic petrology. Prentice Hall

2. Spidergrams: abundances of trace elements in minerals and rocks are normalized by dividing by abundances in the mantle (or MORB or…). Plotted in order of decreasing incompatibility Commonly used normalization factors for spider diagrams are provided by Sun and McDonough (1989)

MORB-normalized spider diagram for some representative analyses from the CRB flood basalts Data from Hooper and Hawkesworth (1993) J. Petrol., 34, 1203-1246. Reproduced in Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall

Example of an equilibrium partial modal melting calculation involving Rb and Sr Suppose we are melting a lower crustal granulite containing 50% plagioclase + 25% cpx + 20% opx + 5% garnet and we want to track how the Rb, Sr and Rb/Sr concentrations in the melt vary as the melting progresses. Assuming modal batch melting, equation to use is:

C iL 1 = C iO ( FL (1 − D i ) + D i ) First, we need to calculate D

C/L i

for the crystalline assemblage using White’s D values

DSr = (0.5x2.7) + (0.25x0.157) + (0.2x0.0068) + (0.05x.0099) = 1.39 DRb = (0.5x0.025) + (0.25x0.033) + (0.2x0.022) + (0.05x0.007) = 0.025

FL 0 0.02 0.05 0.1 0.2 0.3 0.4

C SrL C Sro .72 .723 .73 .74 .76 .79 .81

L C Rb o C Rb

40.0 22.5 13.6 8.2 4.6 3.1 2.4

o L C Rb C Rb / o L C Sr C Sr

55.6 31.1 18.6 11.0 6.0 4.0 3.0

This calculation assumes constant D values

Note: (1) dramatic decrease in Rb and Rb/Sr as melting progresses.

60 50 40 30

L C Rb o C Rb

20

L o C Rb C Rb / C SrL C Sro

10 C SrL 0

(2) Essentially constant Sr during progressive melting.

C Sro

0

0.1

0.2 F 0.3 L

0.4

(3) Is it justified to assume constant D values and modal melting?