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Transcript
Thin section #94
MC-200
Liquids and residuum of melted pyrolite
Figure 10-9 After Green and Ringwood (1967). Earth Planet. Sci. Lett. 2, 151-160.
Initial Conclusions:


Tholeiites favored by shallower melting
 25% melting at <30 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
Primary magmas


Formed at depth and not subsequently modified by
FX or Assimilation
Criteria
 Highest Mg# (100Mg/(Mg+Fe)) really  parental
magma
 Experimental results of lherzolite melts




Mg# = 66-75
Cr > 1000 ppm
Ni > 400-500 ppm
Multiply saturated
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
Ionic charge
vs. radius
Preference
for melt
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).
Preference for
mineral phase

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)
Incompatible elements
HREEs are less incompatible
Relative ionic radii for common valences
and coordination numbers
Compatible elements
Ionic charge
vs. radius
Preference
for melt
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).
Preference for
mineral phase
Note
magnitude
of major
element
changes
wt %
Trace Elements
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.
ppm
Note
magnitude
of trace
element
changes
ppm
Trace Elements
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.
Table 9-6 A brief summary of some particularly useful trace elements in igneous petrology
Element
Use as a petrogenetic indicator
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 Kfeldspar. 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
so to a lesser degree. Sphene and plagioclase accommodates more LREE. Eu 2+ 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.
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
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.
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.
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).
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).
LREE depleted
or unfractionated
LREE enriched
REE data
for UM
xenoliths
LREE depleted
or unfractionated
Figure 10-14 Chondrite-normalized REE diagrams
for spinel (a) and garnet (b) lherzolites. After
Basaltic Volcanism Study Project (1981). Lunar
and Planetary Institute.
LREE enriched