Transcript ppt
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