A primer on magmas and petrology:A primer on magmas and petrology:or, what the &#$!@ is a MORB?or, what the &#$!@ is a MORB?

Tectonics Spring 2012Tectonics Spring 2012

Results from experiments: Liquids and residuum of melted pyroliteResults from experiments: Liquids and residuum of melted pyrolite

After Green and Ringwood (1967).After Green and Ringwood (1967). Earth Planet. Sci. Lett.Earth Planet. Sci. Lett. 2, 151-160. 2, 151-160. From Mary Leech. From Mary Leech.

Incompatible alkalis

concentrate in first PM’s.

What is residuum from melting experiments?What is residuum from melting experiments?

Initial Conclusions:Initial Conclusions: Tholeiites favored by shallower meltingTholeiites favored by shallower melting

25% melting at 25% melting at <<30 km 30 km tholeiite tholeiite 25% melting at 60 km 25% melting at 60 km olivine basalt olivine basalt

Tholeiites favored by greater % partial meltingTholeiites favored by greater % partial melting 20 % melting at 60 km 20 % melting at 60 km alkaline basalt alkaline basalt

incompatibles (alkalis) incompatibles (alkalis) initial melts initial melts 30 % melting at 60 km 30 % melting at 60 km tholeiite tholeiite

Primary magmas from which MORBS are derivedPrimary magmas from which MORBS are derived

Formed at depth and not subsequently modified Formed at depth and not subsequently modified by fractional crystallization or assimilationby fractional crystallization or assimilation

CriteriaCriteria Highest Mg# Highest Mg# (100Mg/(Mg+Fe))(100Mg/(Mg+Fe)) really really ®®

parentalparental magma magma Experimental results of lherzolite meltsExperimental results of lherzolite melts

Mg# = 66-75Mg# = 66-75 Cr > 1000 ppmCr > 1000 ppm Ni > 400-500 ppmNi > 400-500 ppm Multiply saturatedMultiply saturated

SummarySummary A chemically homogeneous mantle can A chemically homogeneous mantle can

yield a variety of basalt typesyield a variety of basalt types Alkaline basalts are favored over tholeiites Alkaline basalts are favored over tholeiites

by deeper melting and by low % PMby deeper melting and by low % PM Fractionation at moderate to high depths Fractionation at moderate to high depths

can also create alkaline basalts from can also create alkaline basalts from tholeiitestholeiites

Were it to be that the mantle is chemically homogeneous….Were it to be that the mantle is chemically homogeneous….

Are you compatible or incompatible?Are you compatible or incompatible?

Why are trace elements so cool?Why are trace elements so cool?

IncompatibleIncompatible elements GO TO THE MELT PHASE! elements GO TO THE MELT PHASE! Commonly Commonly two subgroups based on the ratio of valence to two subgroups based on the ratio of valence to

ionic radius:ionic radius: Smaller, highly charged Smaller, highly charged high field strength (HFS)high field strength (HFS) elementselements

(REE, Th, U, Ce, Pb(REE, Th, U, Ce, Pb4+4+, Zr, Hf, Ti, Nb, Ta), Zr, Hf, Ti, Nb, Ta) Low field strength Low field strength large ion lithophile (LIL)large ion lithophile (LIL) elements elements (K, Rb, Cs, (K, Rb, Cs,

Ba, PbBa, Pb2+2+, Sr, Eu, Sr, Eu2+2+)) are more mobile, particularly if a fluid phase are more mobile, particularly if a fluid phase is involvedis involved

CompatibleCompatible elements STAY IN THE SOLID PHASE! elements STAY IN THE SOLID PHASE!(small, low valence) include:(small, low valence) include:

Major elements (Fe, Mg) and trace elements Major elements (Fe, Mg) and trace elements (Ni, Cr, Cu, W, Ru, Rh, Pd, Os, Ir, Pt, and Au)(Ni, Cr, Cu, W, Ru, Rh, Pd, Os, Ir, Pt, and Au)

Trace vs. MAJOR ElementsTrace vs. MAJOR Elements

310 analyzed volcanic rocks from Crater Lake (Mt. Mazama), Oregon Cascades. From Mary Leech

wt

%w

t %

Note magnitude of Note magnitude of tracetrace vs. major element changes. vs. major element changes.

ppm

ppm

TE’s may vary by > 103! Useful since cuz of sensitivity to distribution & fractionation.

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 andclinopyroxene. High concentrations indicate a mantle source.

V, Ti Both show strong fractionation into Fe-Ti oxides (ilmenite or titanomagnetite). If they behavedifferently, 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 mayreplace Ti in sphene or rutile).

Ba, Rb Incompatible element that substitutes for K in K-feldspar, micas, or hornblende. Rb substitutesless readily in hornblende than K-spar and micas, such that the K/Ba ratio may distinguish thesephases.

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, butas an incompatible at higher pressure where plagioclase is no longer stable.

REE Garnet accommodates the HREE more than the LREE, and orthopyroxene and hornblende doso to a lesser degree. Sphene and plagioclase accommodates more LREE. Eu2+

is stronglypartitioned into plagioclase.

Y Commonly incompatible (like HREE). Strongly partitioned into garnet and amphibole. Spheneand apatite also concentrate Y, so the presence of these as accessories could have asignificant effect.

After Green (1980). Tectonophys., After Green (1980). Tectonophys., 6363, 367-385. From Winter (2001) An Introduction to Igneous and , 367-385. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.Metamorphic Petrology. Prentice Hall.

Figure 9-8.Figure 9-8. (a)(a) after Pearce and Cann (1973), after Pearce and Cann (1973), Earth Planet, Sci. Lett., Earth Planet, Sci. Lett., 1919, 290-300, 290-300. . (b)(b) after Pearce (1982) after Pearce (1982) in Thorpe (ed.), in Thorpe (ed.), Andesites: Orogenic andesites and related rocks. Wiley. Chichester. pp. 525-548Andesites: Orogenic andesites and related rocks. Wiley. Chichester. pp. 525-548 , Coish et al. (1986), , Coish et al. (1986), Amer. J. Sci., Amer. J. Sci., 286286, 1-28, 1-28.. (c)(c) after Mullen (1983), after Mullen (1983), Earth Planet. Sci. Lett., Earth Planet. Sci. Lett., 6262, 53-62., 53-62.

REE data for oceanic basaltsREE data for oceanic basalts

REE diagram for a typical alkaline ocean island basalt (OIB) and tholeiitic mid-ocean ridge basalt (MORB). From Winter 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. (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from Sun and McDonough (1989).Data from Sun and McDonough (1989).

increasing incompatibilityincreasing incompatibility

Ocean Island Basalt (Hawaiian alkaline Ocean Island Basalt (Hawaiian alkaline basalt) looks like partial melt of ~ typical basalt) looks like partial melt of ~ typical mantlemantle

Mid Ocean Ridge Basalt (tholeiite) Has Mid Ocean Ridge Basalt (tholeiite) Has a positive slope… WTF?a positive slope… WTF?

Trace element data for oceanic basalts Trace element data for oceanic basalts

Looks like two mantle reservoirsLooks like two mantle reservoirs

MORB source is depleted by MORB source is depleted by melt extractionmelt extraction

OIB source is not depletedOIB source is not depleted

Is it enriched?Is it enriched?

increasing incompatibilityincreasing incompatibility