Using geochemical data in igneous petrology

Trace elements: spidergrams, ratios and magical diagrams

(or – presenting and using trace elements data)

A slide of a recent presentation by Julian Pearce

And therefore…

Why is it magical?

4. Trace elements1. Representing trace element compositions:

the use of spidergrams

2. Spidergrams and ratios

3. Main families of trace elements

4. Some diagrams using trace elements

4.1 Spidergrams

• Also (better) known as multi-elements diagram• Allow to represent the whole composition of a

sample on a single diagram• Allow to compare the concentration in elements

in different ranges• Allow to get rid of the effects of primordial

abundances

Elements abundance patterns in Earth are a product of

• Nucleosynthesis– Lights > Heavies– Even > Odd– Abundance peak close to Fe (n=56)

• Differenciation– Lithophile mantle (+ crust)– Siderophile core

Solar system abundance

Chondrites

Concentration of REE in a sample

Building a spidergram (Recipe)

• Arrange the elements in given order (generally the more incompatible on the left)

• Divide each element’s concentration in the sample by the concentration in a reference material (chondrite, primitive mantle, MORB…)

• Plot using a log scale

Contrasted REE patternsGranites

Basalts

Multi-elements diagrams

Normalized to the PRImitive Mantle (close to chondrites) (Wood version)

Various normalizations:To MORB (Mid-Oceanic Ridge Basalts – the most common type of basalt!)Meaningful for basalts and co.

Look how the elements on the left-hand side behave in a different way as those on the right-hand side!

4.2 Using ratiosExample: MORBs and OIB

N-MORBs OIBs

La/Sm

Gd/Yb

OIB

N-MORB

E-MORB

« Anomalies »

Granites from the Cape Granite SuiteDarling-Vredenburg area

Eu anomaly

• Eu anomaly is supposed to reflect the implication of plagioclase

Kd’s for REE in basaltic liquids

… because :

Eu anomaly

• Can you invent a « magical number » showing the implication of plagioclase?

• REE ratios– Eu/Eu* is a measure of the size of the Eu anomaly

– La/Yb (or LaN/YbN, also written (La/Yb)N ) is an indication of the slope of the REE pattern

)(21*

NN

N

GdSm

Eu

Eu

Eu

Why can a Eu/Eu* vs. La/Yb diagram be read as a plagioclase vs. garnet diagram?

Figure 16-11a. MORB-normalized spider diagrams for selected island arc basalts. Using the normalization and ordering scheme of Pearce (1983) with LIL on the left and HFS on the right and compatibility increasing outward from Ba-Th. Data from BVTP. Composite OIB from Fig 14-3 in yellow.

Figure 14-3. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from Sun and McDonough (1989) In A. D. Saunders and M. J. Norry (eds.), Magmatism in the Ocean Basins. Geol. Soc. London Spec. Publ., 42. pp. 313-345.

OIB vs. Island-arcs: LIL and HFS elements

What will happen if you plot HFS vs. LIL?

(eg Nb & Rb)

Arcs

Back-arc

Mid-ocean ridges

Within plate

Why is it not magical?

Ratios (of incompatible elements)Less affected by differentiation

Differences in Nb/Yb reflect (mostly) different primitive magmas; mostly preserved during differentiation

• Use a diagram showing ratios:– HFS/Reference– LIL/Reference

• Once you’ve understood the trick, you can build many similar diagrams!

Wood diagrams (for basalts)

Dark arts – Geotectonic diagrams

Dark arts – Geotectonic diagrams

Another diagram by You-know-Who (Pearce et al. 1984)

(With all due respect for J. Pearce, who is a nice person and one of the best living geochemists !)

Why does it work?

• Well, this is going to be (part of) Jaco’s seminar – stay tuned.

4.3 Families of elements

Commonly used trace elements

• LILE= Large Ion Lithophile Elements– Cs, Rb, K, Ba, Sr, Pb– Large atoms with a small charge– Tend to be incompatible to very incompatible– Some exceptions (Rb in Biotite, Sr in plag…)– Typically fluid mobile (and therefore can be

subject to weathering)– Interesting to use but some caution should be

exercised

• HFSE= High Field Strength Elements– Sc, Y, Th, U, Pb, Zr, Hf, Ti, Nb, Ta– Variable behaviours, generally incompatible

except in some specific phases (Y in Grt, Nb in Hbl…)

– Normally fluid immobile, insensible to weathering

– Regarded as good petrogenetic indicators

• HFSE: some interesting « pairs » with very similar behaviours– Nb and Ta (Nb/Ta chondritic ≈ 15-20, less for

crustal rocks)– Zr and Hf (Zr/Hf chondritic ≈ 30-35)– Values largely departing from this call for

explanation (phases able to fractionnate Nb from Ta or Zr from Hf)

Figure 16-11a. MORB-normalized spider diagrams for selected island arc basalts. Using the normalization and ordering scheme of Pearce (1983) with LIL on the left and HFS on the right and compatibility increasing outward from Ba-Th. Data from BVTP. Composite OIB from Fig 14-3 in yellow.

Figure 14-3. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from Sun and McDonough (1989) In A. D. Saunders and M. J. Norry (eds.), Magmatism in the Ocean Basins. Geol. Soc. London Spec. Publ., 42. pp. 313-345.

OIB vs. Island-arcs: LIL and HFS elements

• REE= Rare Earth Elements– La Ce Pr Nd (Pm) Sm Eu Gd Tb Dy Ho Er Tm

Yb Lu– Technically they are HFS– Rather incompatible, except in specific

phases– For a given mineral phases, different REE

have different behaviours– Nearly insensible to weathering– Excellent petrogenetic indicators!

REE: the case of Eu• REEs are normally 3+ (La3+, etc.)• Eu can be Eu3+ or Eu2+

• Eu2+ strongly compatible• Especially in reducing

environmentsReducing (Eu2+)

Oxydizing (Eu3+)

• Transition elements– Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn– All compatible, no huge differences– Low abundances in felsic or intermediate

rocks, useful for basic or ultrabasic systems, or for some mineral deposits (chromite)

– Fluid immobile

• PGE= Platinum Group Elements– Ru, Rh, Pd, Os, Ir, Pt, Au– Not that well-known, large uncertainities on

Kd’s– Low abudances, commonly below detection

limit (bdl) with usual mehods– Economic importance, especially in

chromitites and sulphides– Marginal petrologic use, could become more

significant in the future

4.4 Some trace element diagrams

• In general, far greater diversity than for majors

• You can plot anything against anything else, and then start again with ratios

• It’s easy to get confused…

• Diagrams showing different types or groups of rocks

• Diagrams showing differentiation and implication of specific minerals (during melting or differentiation)

• Diagrams reflecting different sources

• Geotectonic diagrams ?

Specific minerals• Garnet

implication in OIB genesis

Different sources• N-, E- and T-MORB

Describing different groups

N-MORB E-MORB N-MORB E-MORB

Groups of rocks: Potential spurious correlations

Continental arcs

Back-arc

Is this a useful diagram?

Our nice diagram just tells us that back-arcs are basalts and cont. arcs. dacites to rhyolites – we knew that already!

Arcs

Back-arc

Mid-ocean ridges

Within plate

Pearce & Cann 1973

WPB

CAB

IAT MORB

IAT

CAB

Geotectonic

Arcs

Back-arc

Mid-ocean ridges

Within plate

CAB

IAT

N-MORB

E-MORBWPT

WPA

Geotectonic

Wood 1980

Geotectonic diagrams

• A specific site = combination of sources + processes (in terms of PT and therefore of minerals)

Trace elements ratiosHow?

• Element-Element diagrams with linear scale

Trace elements ratiosHow?

• Element-ratio diagrams with linear scale

Trace elements ratiosHow?

• Element-element diagrams with log scale

Nb/Ta=1

Nb/Ta=5

Nb/Ta=10Nb/Ta=15

Nb/Ta=20

Nb/Ta=50

Trace elements ratiosBe careful!

• Dividing by a common value yields spurious correlations…