1 major and trace element geochemistry major and trace element
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Major and Trace Element GeochemistryJust 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
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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)
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RIO DE LA PUENTE
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Vent Flowdirection
Samplelocality
Contour interval = 250 m
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Pella
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ndo
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ero S
a nP
edro
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70o 50' W70o 35' W
36 So
0 1 2 km 3
Younger Holocene Summit Lavas
Younger Holocene Composite Flow
Older Holocene Lavas
Mainly Basaltic Andesite Lavas
Guadal Lavas 500-350 kaVolcan Pellado 190-80 ka
Tatara Dacite 68 ka
Older TSPC lavas 930-220 ka
Huelmul Granite 6.2-6.4 Ma
Volcanics metamorphosed 7-9 Ma
Qt
Qtd
Qoh
Qsp
Qcf
Tvs
Tgh
Alluvium - Colluvium
Neoglacial Moraines
Debris avalanche depositQda
Qal
Qpv
Qpv
Qcg
Qot
Qot Qot
Qot
pre-Volcán Tatara
Basement Rocks
Surficial Deposits
Volcán San Pedro
Geologic MapVolcán San Pedro
Volcán Tatara 120-20 ka
Qm
2750
H70
H16H14
H20
H23
H23
H8
H72
QH2-1
H73
QH1-2
3250
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Qcf
Guadal Lavas
Moraine
Estero Pellado
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2OlderHolocene
OlderHolocene
Volcan San Pedro3621 m
VolcanTatara
VolcanPellado
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H12
H20
H72 H70
PED12
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H161
H12H11i
Volcan San Pedro3621 m
OlderHolocene
H72
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QH2-1
H73
H70
MoraineVolcan Tatara
2750
VolcánSan
Pedro3621 m
PED12
Cerro Pellado
B C
A
Costa and Singer(2002)
Journal of Petrology
Volc<n San PedroSouthern Volcanic
ZoneChilean Andes
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Costa and Singer (2002) Harker Diagrams, Volc<n San Pedro Lavas
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Major and Trace Element Geochemistry• Trace elements
– Partitioning between crystalline and liquid phases• Partition coefficient:
• 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.
Dxtalliq =
concentration in mineralconcentration in liquid
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-free source
Basalt, garnet-bearing source
Basalt, plag fractionationor plag in source
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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
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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 = 0.3so that:
Mg’ = Mg/(Mg+ Fe2+) of basalt in equilibrium with Fo91 is 0.68-0.75
Typically:Ni > 400-500 ppmCr > 1000 ppmSiO2 < 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:
where 8 is the decay constant, i.e., probability that atom will decay in unit time.
ln N/No = -8tN = No e -8t
basic radioactive decay formula. No is initial number of atomsN is number of atoms at time t.
−=
dNdt
Nλ
dNN
tto
t
No
N
= − ∫∫ λ
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Radiogenic Isotopes• The age equation
N = No e -8t
need to realize that daughter atoms D can be expressed as
D = No - NNo = D + N from above
N = (D + N) e -8t
D = N (e -8t - 1)ln(1+D/N) = 8tt = 1/8 ln(1+D/N) need to measure D, daughter atoms present, N parent atoms left.
• Half-life used to determine decay constantst = ln2/8 = 0.693/8
• If some daughter isotope was incorporated into mineral at to , this must be subtracted from the amount measured today:
tD DN
o= +−
11
λln
8
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)
– The 40Ar/39Ar variant of K-Ar dating:
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
tAr ArKec B
ec B
ec
o=+
++
−
−
−11
40 40
40λ λλ λ
λln
t JArArK
= +
11
40
39λln
40Ar/39Ar age spectra and isochrons