Thin section #94

MC-200

Liquids and residuum of melted pyrolite 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 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 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

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).

• Compatible elements (small, low valence) include: 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 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 • 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 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).

Trace Elements Note magnitude of major element changes 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 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.

Trace Elements Note magnitude of trace element changes ppm ppm TE often vary by > 103 very useful since so sensitive to distr. & fractionation 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.

Use as a petrogenetic indicator 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 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. 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 A bit ambiguous, since many variables (country rx, %PM, %FX)   Need immobile TE's, so met has no effect. Ti, Zr, Y, Hf & apply to basic volc's to eliminate high %FX

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 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) How get (+) slope?? 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 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 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).

REE data for UM xenoliths LREE enriched LREE depleted or unfractionated REE data for UM xenoliths LREE depleted or unfractionated LREE enriched Depleted types (+) slope Fertile types (-) slope Enriched? Figure 10-14 Chondrite-normalized REE diagrams for spinel (a) and garnet (b) lherzolites. After Basaltic Volcanism Study Project (1981). Lunar and Planetary Institute.