General

1
Thin section 94
2
MC-200
3
Liquids and residuum of melted pyrolite
Figure 10-9 After Green and Ringwood (1967).
Earth Planet. Sci. Lett. 2, 151-160.
4
Initial Conclusions

• Tholeiites favored by shallower melting
• 25 melting at lt30 km tholeiite
• 25 melting at 60 km olivine basalt
• Tholeiites favored by greater partial melting
• 20 melting at 60 km alkaline basalt
• incompatibles (alkalis) initial melts
• 30 melting at 60 km tholeiite

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Primary magmas

• Formed at depth and not subsequently modified by
  FX or Assimilation
• Criteria
• Highest Mg (100Mg/(MgFe)) really parental
  magma
• Experimental results of lherzolite melts
• Mg 66-75
• Cr gt 1000 ppm
• Ni gt 400-500 ppm
• Multiply saturated

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Summary

• A chemically homogeneous mantle can yield a
  variety of basalt types
• Alkaline basalts are favored over tholeiites by
  deeper melting and by low PM
• Fractionation at moderate to high depths can also
  create alkaline basalts from tholeiites

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Ionic charge vs. radius
Preference for melt
Preference for mineral phase
Plot of ionic radius vs. ionic charge for trace
elements of geological interest. Ionic radii are
quoted for eight-fold coordination to allow for
comparison between elements. From
Rollinson (1993).
8

• Incompatible elements commonly ? two subgroups
  based on the ratio of valence to ionic radius
• Smaller, highly charged high field strength (HFS)
  elements (REE, Th, U, Ce, Pb4, Zr, Hf, Ti, Nb,
  Ta)
• Low field strength large ion lithophile (LIL)
  elements (K, Rb, Cs, Ba, Pb2, Sr, Eu2) are more
  mobile, particularly if a fluid phase is involved

Compatible elements (small, low valence)
include Major elements (Fe, Mg) and trace
elements (Ni, Cr, Cu, W, Ru, Rh, Pd, Os, Ir,
Pt, and Au)
9
Incompatible elements
HREEs are less incompatible
10
Relative ionic radii for common valences and
coordination numbers
Compatible elements
11
Ionic charge vs. radius
Preference for melt
Preference for mineral phase
Plot of ionic radius vs. ionic charge for trace
elements of geological interest. Ionic radii are
quoted for eight-fold coordination to allow for
comparison between elements. From
Rollinson (1993).
12
Trace Elements
Note magnitude of major element changes
wt
Figure 8-2. Harker variation diagram for 310
analyzed volcanic rocks from Crater Lake (Mt.
Mazama), Oregon Cascades. Data compiled by Rick
Conrey (personal communication). From Winter
(2001) An Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
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Trace Elements
Note magnitude of trace element changes
ppm
ppm
Figure 9-1. Harker Diagram for Crater Lake. From
data compiled by Rick Conrey. From Winter (2001)
An Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
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Table 9-6 A brief summary of some particularly
useful trace elements in igneous petrology
Use as a petrogenetic indicator
Element
Ni, Co, Cr
Highly compatible elements. Ni (and Co) are
concentrated in olivine, and Cr in spinel and
clinopyroxene. High concentrations indicate a
mantle source.
V, Ti
Both show strong fractionation into Fe-Ti oxides
(ilmenite or titanomagnetite). If they behave
differently, Ti probably fractionates into an
accessory phase, such as sphene or rutile.
Zr, Hf
Very incompatible elements that do not substitute
into major silicate phases (although they may
replace Ti in sphene or rutile).
Ba, Rb
Incompatible element that substitutes for K in
K-feldspar, micas, or hornblende. Rb substitutes
less readily in hornblende than K-spar and micas,
such that the K/Ba ratio may distinguish these
phases.
Sr
Substitutes for Ca in plagioclase (but not in
pyroxene), and, to a lesser extent, for K in K-
feldspar. Behaves as a compatible element at low
pressure where plagioclase forms early, but
as an incompatible at higher pressure where
plagioclase is no longer stable.
REE
Garnet accommodates the HREE more than the LREE,
and orthopyroxene and hornblende do
2
so to a lesser degree. Sphene and plagioclase
accommodates more LREE. Eu
is strongly
partitioned into plagioclase.
Y
Commonly incompatible (like HREE). Strongly
partitioned into garnet and amphibole. Sphene
and apatite also concentrate Y, so the presence
of these as accessories could have a
significant effect.
Table 9-6. After Green (1980). Tectonophys., 63,
367-385. From Winter (2001) An Introduction to
Igneous and Metamorphic Petrology. Prentice Hall.

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Trace elements as a tool to determine
paleotectonic environment

• Useful for rocks in mobile belts that are no
  longer recognizably in their original setting
• Can trace elements be discriminators of igneous
  environment?
• Approach is empirical on modern occurrences
• Concentrate on elements that are immobile during
  low/medium grade metamorphism

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Table 18-4. A Classification of Granitoid Rocks
Based on Tectonic Setting. After Pitcher (1983)
in K. J. Hsü (ed.), Mountain Building Processes,
Academic Press, London Pitcher (1993), The
Nature and Origin of Granite, Blackie, London
and Barbarin (1990) Geol. Journal, 25, 227-238.
Winter (2001) An Introduction to Igneous and
Metamorphic Petrology. Prentice Hall.
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Figure 9-8. (a) after Pearce and Cann (1973),
Earth Planet, Sci. Lett., 19, 290-300. (b) after
Pearce (1982) in Thorpe (ed.), Andesites
Orogenic andesites and related rocks. Wiley.
Chichester. pp. 525-548, Coish et al. (1986),
Amer. J. Sci., 286, 1-28. (c) after Mullen
(1983), Earth Planet. Sci. Lett., 62, 53-62.
18
REE data for oceanic basalts
increasing incompatibility
Figure 10-13a. REE diagram for a typical alkaline
ocean island basalt (OIB) and tholeiitic
mid-ocean ridge basalt (MORB). From Winter (2001)
An Introduction to Igneous and Metamorphic
Petrology. Prentice Hall. Data from Sun and
McDonough (1989).
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Spider diagram for oceanic basalts
increasing incompatibility
Figure 10-13b. Spider diagram for a typical
alkaline ocean island basalt (OIB) and tholeiitic
mid-ocean ridge basalt (MORB). From Winter (2001)
An Introduction to Igneous and Metamorphic
Petrology. Prentice Hall. Data from Sun and
McDonough (1989).
20
REE datafor UM xenoliths
Figure 10-14 Chondrite-normalized REE diagrams
for spinel (a) and garnet (b) lherzolites. After
Basaltic Volcanism Study Project (1981). Lunar
and Planetary Institute.