1. A primer on magmas and petrology: or, what the &#$. @ is a MORB
A primer on magmas and petrology: or, what the is a MORB? Tectonics Spring 2012
Tholeiite produced at < 30 km depth by 25% PM
60 km
Alkalis are incompatible so tend to concentrate in first low % partial melts
20% PM -> alkaline basalt
30% PM -> tholeiite (only 25% or less at 30 km so looks like tholeiitic nature suppressed with depth)
Note that residuum is Ol + Opx (harzburgite)
Note also the thermal divide between thol and alk at low pressure for FX

2. Results from experiments: Liquids and residuum of melted pyrolite
Incompatible alkalis concentrate in first PM’s.
Tholeiite produced at < 30 km depth by 25% PM
60 km
Alkalis are incompatible so tend to concentrate in first low % partial melts
20% PM -> alkaline basalt
30% PM -> tholeiite (only 25% or less at 30 km so looks like tholeiitic nature suppressed with depth)
Note that residuum is Ol + Opx (harzburgite)
Note also the thermal divide between thol and alk at low pressure for fractional crystallization
What is residuum from melting experiments?
After Green and Ringwood (1967). Earth Planet. Sci. Lett. 2, From Mary Leech.

3. 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

4. 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 > ppm
Multiply saturated

5. 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
In spite of this initial success, there is evidence to suggest that such a simple approach is not realistic, and that the mantle is chemically heterogeneous
Were it to be that the mantle is chemically homogeneous….

6. Are you compatible or incompatible?
Why are trace elements so cool?

7. 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)
Depends on the minerals involved!
Sr -> melt as ol & px separate -> plag (Ca) & not melt if plag is phenocryst phase
Commonly standardized to mantle compositions (olivine, pyroxenes, and perhaps garnet)
Thus the major elements Mg and Fe would usually be referred to as compatible, while K and Na as incompatible

8. Trace vs. MAJOR Elements
Note magnitude of trace vs. major element changes.
ppm
wt %
Element Distribution...as a tool in the interpretation of the history of igneous rocks
Different elements have diff. affinities for environments to reside: Si later melts - Mg early xls.
Remember from Chapter 1: A. Goldschmidt:
some elements metals "Siderophile" Fe, Pt, Mo
some elements sulfides "Chalcophile" S,Cu,Zn
some elements silicates "Lithophile" Si,K,Ca,REE
SIMPLISTIC Xl Field Theory best results. Study of elec. envir. in lattice & melt...
Trace Elements: very low conc.
TE's don't govern the appearance of a phase (as K req. Ksp or Bi), but enter various phases by substitution.
Compare Harkers
Major E usually vary by < 101
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

9. Use as a petrogenetic indicator
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 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
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, From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

10. Figure 9-8. (a) after Pearce and Cann (1973), Earth Planet, Sci. Lett
Figure 9-8. (a) after Pearce and Cann (1973), Earth Planet, Sci. Lett., 19, (b) after Pearce (1982) in Thorpe (ed.), Andesites: Orogenic andesites and related rocks. Wiley. Chichester. pp , Coish et al. (1986), Amer. J. Sci., 286, (c) after Mullen (1983), Earth Planet. Sci. Lett., 62,

11. 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?
Ocean Island Basalt (Hawaiian alkaline basalt)
Looks like partial melt of ~ typical mantle
Mid Ocean Ridge Basalt (tholeiite)
How get (+) slope??
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).

12. 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?
Same approach for larger variety of elements
Still OIB looks like partial melt of ~ typical mantle
MORB still has (+) slope
Looks like two mantle reservoirs
MORB source is depleted by melt extraction
OIB source is not depleted
is it enriched?
increasing incompatibility