A primer on magmas and petrology:
or, what the &#$!@ is a MORB?
Tectonics Spring 2012
Results from experiments: Liquids and residuum of melted pyrolite
What is residuum from melting experiments?
After Green and Ringwood (1967). Earth Planet. Sci. Lett. 2, 151-160. From Mary Leech.
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 from which MORBS are derived


Formed at depth and not subsequently modified
by fractional crystallization 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
Were it to be that the mantle is chemically homogeneous….
Are you compatible or incompatible?
Why are trace elements so cool?
Incompatible elements GO TO THE MELT PHASE!

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 STAY IN THE SOLID PHASE!
(small, low valence) include:
Major elements (Fe, Mg) and trace elements
(Ni, Cr, Cu, W, Ru, Rh, Pd, Os, Ir, Pt, and Au)
Trace vs. MAJOR Elements
wt %
ppm
Note magnitude of trace vs. major element changes.
TE’s may vary by > 103! Useful since cuz of sensitivity to distribution & fractionation.
310 analyzed volcanic rocks from Crater Lake (Mt. Mazama), Oregon Cascades. From Mary Leech
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.
After Green (1980). Tectonophys., 63, 367-385. From 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
Ocean Island Basalt (Hawaiian alkaline
basalt) looks like partial melt of ~ typical
mantle
Mid Ocean Ridge Basalt (tholeiite) Has
a positive slope… WTF?
increasing incompatibility
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).
Trace element data for oceanic basalts
Looks like two mantle reservoirs
MORB source is depleted by
melt extraction
OIB source is not depleted
Is it enriched?
increasing incompatibility