using geochemical data in igneous petrology l.
Download
Skip this Video
Loading SlideShow in 5 Seconds..
Using geochemical data in igneous petrology PowerPoint Presentation
Download Presentation
Using geochemical data in igneous petrology

Loading in 2 Seconds...

play fullscreen
1 / 58

Using geochemical data in igneous petrology - PowerPoint PPT Presentation


  • 780 Views
  • Uploaded on

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?. Trace elements

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'Using geochemical data in igneous petrology' - aida


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
using geochemical data in igneous petrology

Using geochemical data in igneous petrology

Trace elements: spidergrams, ratios and magical diagrams

(or – presenting and using trace elements data)

slide5
Trace elements
    • Representing trace element compositions: the use of spidergrams
    • Spidergrams and ratios
    • Main families of trace elements
    • Some diagrams using trace elements
4 1 spidergrams
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
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
building a spidergram recipe
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
multi elements diagrams
Multi-elements diagrams

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

various normalizations
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!

slide17

La/Sm

E-MORB

OIB

N-MORB

Gd/Yb

anomalies
« Anomalies »

Granites from the Cape Granite Suite

Darling-Vredenburg area

eu anomaly
Eu anomaly
  • Eu anomaly is supposed to reflect the implication of plagioclase
slide20

… because :

Kd’s for REE in basaltic liquids

eu anomaly21
Eu anomaly
  • Can you invent a « magical number » showing the implication of plagioclase?
slide22
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
oib vs island arcs lil and hfs elements
OIB vs. Island-arcs: LIL and HFS elements

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.

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.

slide25

Arcs

Back-arc

Mid-ocean ridges

Within plate

What will happen if you plot HFS vs. LIL?

(eg Nb & Rb)

ratios of incompatible elements less affected by differentiation
Ratios (of incompatible elements)Less affected by differentiation

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

slide28
Use a diagram showing ratios:
    • HFS/Reference
    • LIL/Reference
slide30

Dark arts – Geotectonic 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
Why does it work?
  • Well, this is going to be (part of) Jaco’s seminar – stay tuned.
commonly used trace 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
slide36
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
slide37
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)
oib vs island arcs lil and hfs elements38
OIB vs. Island-arcs: LIL and HFS elements

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.

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.

slide39
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
REE: the case of Eu
  • REEs are normally 3+ (La3+, etc.)
  • Eu can be Eu3+ or Eu2+
  • Eu2+ strongly compatible
  • Especially in reducing environments

Reducing (Eu2+)

Oxydizing (Eu3+)

slide41
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
slide42
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
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…
slide44
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
Specific minerals
  • Garnet implication in OIB genesis
different sources
Different sources
  • N-, E- and T-MORB
describing different groups
Describing different groups

N-MORB

E-MORB

N-MORB

E-MORB

groups of rocks potential spurious correlations
Groups of rocks: Potential spurious correlations

Continental arcs

Back-arc

Is this a useful diagram?

slide49
Our nice diagram just tells us that back-arcs are basalts and cont. arcs. dacites to rhyolites – we knew that already!
geotectonic

Arcs

Back-arc

Mid-ocean ridges

Within plate

Geotectonic

WPB

MORB

IAT

IAT

CAB

CAB

Pearce & Cann 1973

geotectonic51

Arcs

Back-arc

Mid-ocean ridges

Within plate

Geotectonic

N-MORB

IAT

E-MORB

WPT

CAB

WPA

Wood 1980

geotectonic diagrams
Geotectonic diagrams
  • A specific site = combination of sources + processes (in terms of PT and therefore of minerals)
trace elements ratios how
Trace elements ratiosHow?
  • Element-Element diagrams with linear scale
trace elements ratios how56
Trace elements ratiosHow?
  • Element-ratio diagrams with linear scale
trace elements ratios how57
Trace elements ratiosHow?
  • Element-element diagrams with log scale

Nb/Ta=15

Nb/Ta=20

Nb/Ta=50

Nb/Ta=10

Nb/Ta=5

Nb/Ta=1

trace elements ratios be careful
Trace elements ratiosBe careful!
  • Dividing by a common value yields spurious correlations…