Major and Trace Element Geochemistry
Just as seismology is an important tool to image the earth’s
interior, so too are chemical and isotopic compositions of
igneous rocks that originate at great depths with the upper
mantle and lower crust.
• Importance of chemical compositions of igneous rocks
– Petrogenesis of primary magmas
• these reflect mineralogy and chemistry of the source rock
– Differentiation of magmas
• need to decipher shallow processes to infer deep source
– Radiogenic isotopes
• allow a time-integrated view of changing composition
Major and Trace Element Geochemistry
•
Major elements
– Comprise most of the rock
– Expressed as weight (wt.) % oxides, each >0.1%
– Analyzed by XRF, ICP-MS
•
Trace elements
– Present in concentrations <0.1%
– Expressed in ppm or ppb
– Analyzed by XRF, ICP-MS, INAA
•
Volatile elements
– H2O, CO2, SO4
– Rare gases: He, Ar, Ne, etc.
– Analyzed by spectroscopy or mass spectrometry
•
Radiogenic isotopes
– Ratios of radiogenic to nonradiogenic isotopes of an element
• recall isotopes of an element have same atomic no., but variable # of neutrons
– Variations in ratios reflect differences produced over time by radioactive decay in source
– Variations are extremely small: analyzed by magnetic sector mass spectrometry
•
Stable Isotopes
– Lighter masses fractionated by geological processes
– Analyzed by magnetic sector mass spectrometry
1
Major and Trace Element Geochemistry
•
Variation Diagrams
– Plot chemical differences and trends among related rocks (lavas = magmas?)
• Only true for liquids (aphyric lavas and tephras)
• Can define and help model products of partial melting and crystallization
• Plot ME, TE or both
Major elements, Harker diagrams
•
•
•
•
Cogenetic lavas = well-defined trends
Lever-rule can quantify fractionating mineral assemblage
Inflected trends = changes in crystallizing mineral assemblage
Simple, yet powerful way to compare/distinguish suites of rocks (magmas)
A
Qot
Qot
Volcán
San
Pedro
Qot
Qt
25
00
•
2750
Qt
3621 m
3213
Qpv
3250
00
30
Qm Neoglacial Moraines
Qda Debris avalanche deposit
275
0
36o S
Qoh
ice
1
30
00
H72
ice
Qtd
PED12
2
H70
ice
Volcán San Pedro
Qsp Younger Holocene Summit Lavas
Qcf
4
H73
Qal
Qt
2
km
5
17
Qcg Guadal Lavas
Qcg
Tgh
P UE
DE LA
Tvs
NTE
Tvs
Qal
B
Volcan San Pedro
Older
Holocene
H72 H70
3621 m
Moraine
QH2-1
Flow
direction
H23
Sample
locality
Volcan San Pedro
C
3621 m
H72
Older
Holocene
H73
500-350 ka
Volcanics metamorphosed 7-9 Ma
Vent
70 o 50' W
70 o 35' W
190-80 ka
Qot Older TSPC lavas 930-220 ka
Basement Rocks
Tgh Huelmul Granite 6.2-6.4 Ma
Qda
R IO
Tatara Dacite 68 ka
pre-Volcán Tatara
Qpv Volcan Pellado
0
00
15
3
Qal
Qcf
Es
te
Pe ro
lla
do
Qoh
QH1-2
Contour interval = 250 m
1
22 50
2000
Younger Holocene Composite Flow
Volcán Tatara 120-20 ka
Qt Mainly Basaltic Andesite Lavas
Qtd
Tgh H23
Tvs
Tvs
H8
El
Tvs
Tvs
QH2-1
do
Hon
Ester o San Pedro
Qt
rada
E. Queb
0
250
0
3
Qt
Tvs
Qcf
Qoh Older Holocene Lavas
Tgh
Qal
Qt
2
H20 H12
H11i
H14 H16
Qm
Qoh
Volc<n San Pedro
Southern Volcanic
Zone
Chilean Andes
Surficial Deposits
Qal Alluvium - Colluvium
Qsp
Tvs
Costa and Singer
(2002)
Journal of Petrology
Geologic Map
Volcán San Pedro
3327
Cerro Pellado
Qal
H70
Older
Holocene
2
Volcan
Tatara
Volcan
Pellado
PED12
Volcan
Tatara
1
Moraine
4
H20
QH1-2
H16
3
2
H12
H14
1
Guadal Lavas
H23
H8
4
E ste
ro P
d
ella
o
2
Costa and Singer (2002) Harker Diagrams, Volc<n San Pedro Lavas
3
Major and Trace Element Geochemistry
•
Trace elements
– Partitioning between crystalline and liquid phases
• Partition coefficient:
liq
Dxtal =
concentration in mineral
concentration in liquid
• D << 1, incompatible elements
– Large Ion Lithophile Elements
(LILE)
» K, Rb, Sr, Ba,
» Zr, U, Th, REE, etc.
• D > 1, compatible elements
» Ni, Cr, Co, etc.
Major and Trace Element Geochemistry
•
Rare Earth Elements (REE)
– 15 elements from mass 57 to 71 (14 occur naturally)
•
•
•
•
La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu
Useful because similar in geochemical behavior
Trivalent except Eu can be Eu3+ or Eu2+, depending on fO2
To eliminate Oddo-Harkins effect, normalize to chondritic meteorites
Basalt, garnet-bearing source
Basalt, plag fractionation
or plag in source
Basalt, garnet-free source
4
Major and Trace Element Geochemistry
•
Rare Earth Elements (REE)
– Particular minerals influence shape of chondrite-normalized REE pattern by
virtue of D values:
•
•
•
•
•
Feldspar:
2+ negative Eu anomaly
Garnet:
high D for Heavy REE (HREE)
Olivine:
D < 0.1 for all REE; uniform effects on magma
Hornblende:
D > 1.0 for middle REE
Zircon, Sphene, Apatite: strong affinity, high D for REE
– Mantle REE: originally flat pattern, 2-3x chondritic
• Partial melting leaves upper mantle depleted in LREE
• Degree of enrichment of REE in melts
– Abundances and mineralogy in source
– Degree (percentage) of melting
– Extent of fractional crystallization
• See Wilson Fig. 2.3 from previous panel
Major and Trace Element Geochemistry
•
Rare Earth Elements (REE)
– Extend normalization approach to several other elements = Spiderdiagrams
• Plot in order of increasing D
• Normalization is arbitrary: to primordial mantle, chondrites, MORB
• Peaks, troughs, slopes, anomalies indicative of specific crystal-liquid equilibria
processes
5
Major and Trace Element Geochemistry
•
Primary Magmas
– Formed by partial melting of upper mantle in equilibrium with olivine+pyroxene
unmodified by fractional xtlln, assimilation/contamination, magma mixing, etc.
• Truly primary magmas are rare to nonexistant
– most basaltic magmas fractionated olivine and assimilated some lithosphere on way up
– Criteria not firm but:
Kd = (Fe2+/ Mg)olivine /(Fe2+/Mg)melt
Kd = 0.3
so that:
Mg’ = Mg/(Mg+ Fe2+) of basalt in equilibrium with Fo91 is 0.68-0.75
Typically:
Ni > 400-500 ppm
Cr > 1000 ppm
SiO2 < 50%
– Metasomatism (addition of fluids + new minerals) of mantle may change
possible primary magma composition
Radiogenic Isotopes
•
Rutherford and Soddy (1902) [Nobel Prize in Physics]
– Experiments indicated that thorium decay to radium is exponential over time.
– Radioactivity is an atomic property. Atoms in radioactive elements are unstable. Within
a given amount of time, a fixed proportion of atoms disintegrate to form new atoms.
– Disintegration accompanied by emission of alpha or beta particles. Activity, or intensity,
of radioactivity is proportional to number of atoms that disintegrate per unit time.
– Thus activity is directly proportional to number of atoms of substance present:
− dN
= λN
dt
where 8 is the decay constant, i.e., probability that atom will decay in unit time.
N
t
dN
∫No N = − to∫ λ t
ln N/No = -8t
N = No e -8t
basic radioactive decay formula.
No is initial number of atoms
N is number of atoms at time t.
6
Radiogenic Isotopes
•
The age equation
N = No e -8t
need to realize that daughter atoms D can be expressed as
D = No - N
No = D + N from above
N = (D + N) e -8t
D = N (e -8t - 1)
ln(1+D/N) = 8t
t = 1/8 ln(1+D/N) need to measure D, daughter atoms present, N parent atoms left.
•
Half-life used to determine decay constants
•
t = ln2/8 = 0.693/8
If some daughter isotope was incorporated into mineral at to , this must be
subtracted from the amount measured today:
t=
D − Do
1
ln 1 +
λ
N
7
Radiogenic Isotopes
•
The K-Ar system
– 40K undergoes branched decay to 40Ar
• half-life of 1.25 x 109 yr
• 8 = 5.81 x 10-11 yr-1
• 40Aro is small or can be corrected for
– System used to date rocks from historical time, 2 ka, to 4.5 Ga (meteorites)
t=
λ ec + λ B −
1
ln 1 +
λ ec + λ B − λ ec
40
Ar − 40Aro
40
K
– The 40Ar/39Ar variant of K-Ar dating:
t=
1
ln 1 + J
λ
Ar
ArK
40
39
J is a constant including a factor for fraction of 39K atoms converted to 39Ar in the
neutron flux of a nuclear reactor
• More powerful than K-Ar dating:
– more precise; all measurements in single mass spectrometer
– smaller samples -- down to single phenocrysts
– incremental-heating; many ages from gas released over range of T in single
sample
– Thermally disturbed samples yield “discordant” release spectrum of ages
40Ar/39Ar
age
spectra and
isochrons
8
0
