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Origin of Basaltic Magma

Origin of Basaltic Magma. Today. Updates: ? Today: Different basalts from the same source Crystallization order Effect on Trace elements. Isochron method. Divide by stable 86 Sr: 87 Sr/ 86 Sr = ( 87 Sr/ 86 Sr) o + ( 87 Rb/ 86 Sr)(e l t -1) l = 1.4 x 10 -11 a -1.

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Origin of Basaltic Magma

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  1. Origin of Basaltic Magma

  2. Today Updates: ? Today: • Different basalts from the same source • Crystallization order • Effect on Trace elements

  3. Isochron method Divide by stable 86Sr: 87Sr/86Sr = (87Sr/86Sr)o + (87Rb/86Sr)(elt -1) l = 1.4 x 10-11 a-1 y = b + x m = equation for a line in 87Sr/86Sr vs. 87Rb/86Sr plot Slope = (elt -1)

  4. ( ) 87Sr 86Sr o 87Sr 87Rb 86Sr 86Sr Begin with 3 rocks plotting at a b c at time to to a b c

  5. t1 c1 b1 a1 ( ) 87Sr to 86Sr a o b c 87Sr 87Rb 86Sr 86Sr After some time increment (t0t1) each sample loses some 87Rb and gains an equivalent amount of 87Sr

  6. t2 c2 t1 b2 c1 ( ) a2 b1 87Sr 86Sr a1 o to a b c 87Rb 87Sr 86Sr 86Sr At time t2 each rock system has evolved  new line Again still linear and steeper line

  7. t2 c2 t1 b2 c1 a2 b1 a1 to a b c 87Rb 87Sr 86Sr 86Sr Isochron Technique

  8. 0.725 Rb-Sr Isochron, Eagle Peak Pluton, Sierra Nevada Batholith x Sr/ Sr = 0.00127 ( Rb/ Sr) + 0.70760 87 86 87 86 0.720 0.715 0.710 0 2 4 6 8 10 12 14 87 S r/ 87 86 Rb/ S r 86 Sr Isochron results Figure 9-9. After Hill et al. (1988). Amer. J. Sci., 288-A, 213-241.

  9. Making Sr isotope reservoirs Figure 9-13. After Wilson (1989). Igneous Petrogenesis. Unwin Hyman/Kluwer.

  10. Geotherm and solidus: how to melt

  11. Melt creation in ocean basin

  12. 2 types in ocean basins Tholeiitic Basalt and Alkaline Basalt Common petrographic differences between tholeiitic and alkaline basalts • Table 10-1 Tholeiitic Basalt Alkaline Basalt Usually fine-grained, intergranular Usually fairly coarse, intergranular to ophitic Groundmass No olivine Olivine common Clinopyroxene = augite (plus possibly pigeonite) Titaniferous augite (reddish) Orthopyroxene (hypersthene) common, may rim ol. Orthopyroxene absent No alkali feldspar Interstitial alkali feldspar or feldspathoid may occur Interstitial glass and/or quartz common Interstitial glass rare, and quartz absent Olivine rare, unzoned, and may be partially resorbed Olivine common and zoned Phenocrysts or show reaction rims of orthopyroxene Orthopyroxene uncommon Orthopyroxene absent Early plagioclase common Plagioclase less common, and later in sequence Clinopyroxene is pale brown augite Clinopyroxene is titaniferous augite, reddish rims after Hughes (1982) and McBirney (1993).

  13. Lherzolite: A type of peridotite with Olivine > Opx + Cpx Olivine Dunite 90 Peridotites Wehrlite Harzburgite Lherzolite 40 Pyroxenites Olivine Websterite Orthopyroxenite 10 Websterite 10 Clinopyroxenite Orthopyroxene Clinopyroxene Figure 2-2 C After IUGS

  14. Examples from the mantle/how we know • Ophiolites • Obducted oceanic crust + upper mantle • Dredge samples from oceanic fracture zones • Xenoliths in some basalts: Local example? • Kimberlite xenoliths • Diamond-bearing pipes blasted up from the mantle

  15. Tholeiite/alkaline basalt based on P&T; big difference: Figure 10-2 After Wyllie, P. J. (1981). Geol. Rundsch. 70, 128-153.

  16. Ne Volatile-free E 3GPa E 2Gpa E 1GPa Ab Highly undersaturated (nepheline - bearing) alkali basalts E 1atm Oversaturated (quartz-bearing) Undersaturated tholeiitic basalts tholeiitic basalts Fo En SiO2 Pressure effects on melting Figure 10-8 After Kushiro (1968), J. Geophys. Res., 73, 619-634.

  17. REE plots 10.00 Modeling the source for different F values 8.00 6.00 sample/chondrite 10.00 4.00 Batch Partial Melting values = F (fraction melted) 8.00 2.00 0.05 0.00 6.00 sample/chondrite 0.1 La Ce Nd Sm Eu Tb Er Yb Lu 4.00 0.2 0.4 2.00 0.6 0.00 La Ce Nd Sm Eu Tb Er Yb Lu atomic number increasing incompatibility Large F Figure 9-3 From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Small F

  18. REE data for oceanic basalts increasing incompatibility Figure 9-3 From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from Sun and McDonough (1989).

  19. Summary • Chemically homogeneous mantle can make tholeiites OR alkali basalts • Alkaline basalts are favored over tholeiites by deeper melting and by low % melting • XL fractionation at moderate to high depths can also create alkaline basalts from tholeiites • At low P there is a thermal divide that separates the two series (earlier lecture)

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